Write a Script That Connects to 100 Hosts,

Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

10 Mar 2014

Revision History
Revision 6.5	05 Apr 2012	Revised by: mc
'TUNGSTENBERRY' release
Revision 6.6	27 Nov 2012	Revised by: mc
'YTTERBIUMBERRY' release
Revision 10	10 Mar 2014	Revised by: mc
'PUBLICDOMAIN' release

This tutorial assumes no previous knowledge of scripting or programming, yet progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX® wisdom and lore. It serves as a textbook, a manual for self-study, and as a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts .

This book is suitable for classroom use as a general introduction to programming concepts.

This document is herewith granted to the Public Domain. No copyright!

Dedication

For Anita, the source of all the magic

List of Examples
2-1. cleanup: A script to clean up log files in /var/log
2-2. cleanup: An improved clean-up script
2-3. cleanup: An enhanced and generalized version of above scripts.
3-1. Code blocks and I/O redirection
3-2. Saving the output of a code block to a file
3-3. Running a loop in the background
3-4. Backup of all files changed in last day
4-1. Variable assignment and substitution
4-2. Plain Variable Assignment
4-3. Variable Assignment, plain and fancy
4-4. Integer or string?
4-5. Positional Parameters
4-6. wh, whois domain name lookup
4-7. Using shift
5-1. Echoing Weird Variables
5-2. Escaped Characters
5-3. Detecting key-presses
6-1. exit / exit status
6-2. Negating a condition using !
7-1. What is truth?
7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[
7-3. Arithmetic Tests using (( ))
7-4. Testing for broken links
7-5. Arithmetic and string comparisons
7-6. Testing whether a string is null
7-7. zmore
8-1. Greatest common divisor
8-2. Using Arithmetic Operations
8-3. Compound Condition Tests Using && and ||
8-4. Representation of numerical constants
8-5. C-style manipulation of variables
9-1. $IFS and whitespace
9-2. Timed Input
9-3. Once more, timed input
9-4. Timed read
9-5. Am I root?
9-6. arglist: Listing arguments with $* and $@
9-7. Inconsistent $* and $@ behavior
9-8. $* and $@ when $IFS is empty
9-9. Underscore variable
9-10. Using declare to type variables
9-11. Generating random numbers
9-12. Picking a random card from a deck
9-13. Brownian Motion Simulation
9-14. Random between values
9-15. Rolling a single die with RANDOM
9-16. Reseeding RANDOM
9-17. Pseudorandom numbers, using awk
10-1. Inserting a blank line between paragraphs in a text file
10-2. Generating an 8-character "random" string
10-3. Converting graphic file formats, with filename change
10-4. Converting streaming audio files to ogg
10-5. Emulating getopt
10-6. Alternate ways of extracting and locating substrings
10-7. Using parameter substitution and error messages
10-8. Parameter substitution and "usage" messages
10-9. Length of a variable
10-10. Pattern matching in parameter substitution
10-11. Renaming file extensions:
10-12. Using pattern matching to parse arbitrary strings
10-13. Matching patterns at prefix or suffix of string
11-1. Simple for loops
11-2. for loop with two parameters in each [list] element
11-3. Fileinfo: operating on a file list contained in a variable
11-4. Operating on a parameterized file list
11-5. Operating on files with a for loop
11-6. Missing in [list] in a for loop
11-7. Generating the [list] in a for loop with command substitution
11-8. A grep replacement for binary files
11-9. Listing all users on the system
11-10. Checking all the binaries in a directory for authorship
11-11. Listing the symbolic links in a directory
11-12. Symbolic links in a directory, saved to a file
11-13. A C-style for loop
11-14. Using efax in batch mode
11-15. Simple while loop
11-16. Another while loop
11-17. while loop with multiple conditions
11-18. C-style syntax in a while loop
11-19. until loop
11-20. Nested Loop
11-21. Effects of break and continue in a loop
11-22. Breaking out of multiple loop levels
11-23. Continuing at a higher loop level
11-24. Using continue N in an actual task
11-25. Using case
11-26. Creating menus using case
11-27. Using command substitution to generate the case variable
11-28. Simple string matching
11-29. Checking for alphabetic input
11-30. Creating menus using select
11-31. Creating menus using select in a function
12-1. Stupid script tricks
12-2. Generating a variable from a loop
12-3. Finding anagrams
15-1. A script that spawns multiple instances of itself
15-2. printf in action
15-3. Variable assignment, using read
15-4. What happens when read has no variable
15-5. Multi-line input to read
15-6. Detecting the arrow keys
15-7. Using read with file redirection
15-8. Problems reading from a pipe
15-9. Changing the current working directory
15-10. Letting let do arithmetic.
15-11. Showing the effect of eval
15-12. Using eval to select among variables
15-13. Echoing the command-line parameters
15-14. Forcing a log-off
15-15. A version of rot13
15-16. Using set with positional parameters
15-17. Reversing the positional parameters
15-18. Reassigning the positional parameters
15-19. "Unsetting" a variable
15-20. Using export to pass a variable to an embedded awk script
15-21. Using getopts to read the options/arguments passed to a script
15-22. "Including" a data file
15-23. A (useless) script that sources itself
15-24. Effects of exec
15-25. A script that exec's itself
15-26. Waiting for a process to finish before proceeding
15-27. A script that kills itself
16-1. Using ls to create a table of contents for burning a CDR disk
16-2. Hello or Good-bye
16-3. Badname, eliminate file names in current directory containing bad characters and whitespace.
16-4. Deleting a file by its inode number
16-5. Logfile: Using xargs to monitor system log
16-6. Copying files in current directory to another
16-7. Killing processes by name
16-8. Word frequency analysis using xargs
16-9. Using expr
16-10. Using date
16-11. Date calculations
16-12. Word Frequency Analysis
16-13. Which files are scripts?
16-14. Generating 10-digit random numbers
16-15. Using tail to monitor the system log
16-16. Printing out the From lines in stored e-mail messages
16-17. Emulating grep in a script
16-18. Crossword puzzle solver
16-19. Looking up definitions in Webster's 1913 Dictionary
16-20. Checking words in a list for validity
16-21. toupper: Transforms a file to all uppercase.
16-22. lowercase: Changes all filenames in working directory to lowercase.
16-23. du: DOS to UNIX text file conversion.
16-24. rot13: ultra-weak encryption.
16-25. Generating "Crypto-Quote" Puzzles
16-26. Formatted file listing.
16-27. Using column to format a directory listing
16-28. nl: A self-numbering script.
16-29. manview: Viewing formatted manpages
16-30. Using cpio to move a directory tree
16-31. Unpacking an rpm archive
16-32. Stripping comments from C program files
16-33. Exploring /usr/X11R6/bin
16-34. An "improved" strings command
16-35. Using cmp to compare two files within a script.
16-36. basename and dirname
16-37. A script that copies itself in sections
16-38. Checking file integrity
16-39. Uudecoding encoded files
16-40. Finding out where to report a spammer
16-41. Analyzing a spam domain
16-42. Getting a stock quote
16-43. Updating FC4
16-44. Using ssh
16-45. A script that mails itself
16-46. Generating prime numbers
16-47. Monthly Payment on a Mortgage
16-48. Base Conversion
16-49. Invoking bc using a here document
16-50. Calculating PI
16-51. Converting a decimal number to hexadecimal
16-52. Factoring
16-53. Calculating the hypotenuse of a triangle
16-54. Using seq to generate loop arguments
16-55. Letter Count"
16-56. Using getopt to parse command-line options
16-57. A script that copies itself
16-58. Exercising dd
16-59. Capturing Keystrokes
16-60. Preparing a bootable SD card for the Raspberry Pi
16-61. Securely deleting a file
16-62. Filename generator
16-63. Converting meters to miles
16-64. Using m4
17-1. Setting a new password
17-2. Setting an erase character
17-3. secret password: Turning off terminal echoing
17-4. Keypress detection
17-5. Checking a remote server for identd
17-6. pidof helps kill a process
17-7. Checking a CD image
17-8. Creating a filesystem in a file
17-9. Adding a new hard drive
17-10. Using umask to hide an output file from prying eyes
17-11. Backlight: changes the brightness of the (laptop) screen backlight
17-12. killall, from /etc/rc.d/init.d
19-1. broadcast: Sends message to everyone logged in
19-2. dummyfile: Creates a 2-line dummy file
19-3. Multi-line message using cat
19-4. Multi-line message, with tabs suppressed
19-5. Here document with replaceable parameters
19-6. Upload a file pair to Sunsite incoming directory
19-7. Parameter substitution turned off
19-8. A script that generates another script
19-9. Here documents and functions
19-10. "Anonymous" Here Document
19-11. Commenting out a block of code
19-12. A self-documenting script
19-13. Prepending a line to a file
19-14. Parsing a mailbox
20-1. Redirecting stdin using exec
20-2. Redirecting stdout using exec
20-3. Redirecting both stdin and stdout in the same script with exec
20-4. Avoiding a subshell
20-5. Redirected while loop
20-6. Alternate form of redirected while loop
20-7. Redirected until loop
20-8. Redirected for loop
20-9. Redirected for loop (both stdin and stdout redirected)
20-10. Redirected if/then test
20-11. Data file names.data for above examples
20-12. Logging events
21-1. Variable scope in a subshell
21-2. List User Profiles
21-3. Running parallel processes in subshells
22-1. Running a script in restricted mode
23-1. Code block redirection without forking
23-2. Redirecting the output of process substitution into a loop.
24-1. Simple functions
24-2. Function Taking Parameters
24-3. Functions and command-line args passed to the script
24-4. Passing an indirect reference to a function
24-5. Dereferencing a parameter passed to a function
24-6. Again, dereferencing a parameter passed to a function
24-7. Maximum of two numbers
24-8. Converting numbers to Roman numerals
24-9. Testing large return values in a function
24-10. Comparing two large integers
24-11. Real name from username
24-12. Local variable visibility
24-13. Demonstration of a simple recursive function
24-14. Another simple demonstration
24-15. Recursion, using a local variable
24-16. The Fibonacci Sequence
24-17. The Towers of Hanoi
25-1. Aliases within a script
25-2. unalias: Setting and unsetting an alias
26-1. Using an and list to test for command-line arguments
26-2. Another command-line arg test using an and list
26-3. Using or lists in combination with an and list
27-1. Simple array usage
27-2. Formatting a poem
27-3. Various array operations
27-4. String operations on arrays
27-5. Loading the contents of a script into an array
27-6. Some special properties of arrays
27-7. Of empty arrays and empty elements
27-8. Initializing arrays
27-9. Copying and concatenating arrays
27-10. More on concatenating arrays
27-11. The Bubble Sort
27-12. Embedded arrays and indirect references
27-13. The Sieve of Eratosthenes
27-14. The Sieve of Eratosthenes, Optimized
27-15. Emulating a push-down stack
27-16. Complex array application: Exploring a weird mathematical series
27-17. Simulating a two-dimensional array, then tilting it
28-1. Indirect Variable References
28-2. Passing an indirect reference to awk
29-1. Using /dev/tcp for troubleshooting
29-2. Playing music
29-3. Finding the process associated with a PID
29-4. On-line connect status
30-1. Print the server environment
30-2. IP addresses
31-1. Hiding the cookie jar
31-2. Setting up a swapfile using /dev/zero
31-3. Creating a ramdisk
32-1. A buggy script
32-2. Missing keyword
32-3. test24: another buggy script
32-4. Testing a condition with an assert
32-5. Trapping at exit
32-6. Cleaning up after Control-C
32-7. A Simple Implementation of a Progress Bar
32-8. Tracing a variable
32-9. Running multiple processes (on an SMP box)
34-1. Numerical and string comparison are not equivalent
34-2. Subshell Pitfalls
34-3. Piping the output of echo to a read
36-1. shell wrapper
36-2. A slightly more complex shell wrapper
36-3. A generic shell wrapper that writes to a logfile
36-4. A shell wrapper around an awk script
36-5. A shell wrapper around another awk script
36-6. Perl embedded in a Bash script
36-7. Bash and Perl scripts combined
36-8. Python embedded in a Bash script
36-9. A script that speaks
36-10. A (useless) script that recursively calls itself
36-11. A (useful) script that recursively calls itself
36-12. Another (useful) script that recursively calls itself
36-13. A "colorized" address database
36-14. Drawing a box
36-15. Echoing colored text
36-16. A "horserace" game
36-17. A Progress Bar
36-18. Return value trickery
36-19. Even more return value trickery
36-20. Passing and returning arrays
36-21. Fun with anagrams
36-22. Widgets invoked from a shell script
36-23. Test Suite
37-1. String expansion
37-2. Indirect variable references - the new way
37-3. Simple database application, using indirect variable referencing
37-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards
37-5. A simple address database
37-6. A somewhat more elaborate address database
37-7. Testing characters
37-8. Reading N characters
37-9. Using a here document to set a variable
37-10. Piping input to a read
37-11. Negative array indices
37-12. Negative parameter in string-extraction construct
A-1. mailformat: Formatting an e-mail message
A-2. rn: A simple-minded file renaming utility
A-3. blank-rename: Renames filenames containing blanks
A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted password
A-5. copy-cd: Copying a data CD
A-6. Collatz series
A-7. days-between: Days between two dates
A-8. Making a dictionary
A-9. Soundex conversion
A-10. Game of Life
A-11. Data file for Game of Life
A-12. behead: Removing mail and news message headers
A-13. password: Generating random 8-character passwords
A-14. fifo: Making daily backups, using named pipes
A-15. Generating prime numbers using the modulo operator
A-16. tree: Displaying a directory tree
A-17. tree2: Alternate directory tree script
A-18. string functions: C-style string functions
A-19. Directory information
A-20. Library of hash functions
A-21. Colorizing text using hash functions
A-22. More on hash functions
A-23. Mounting USB keychain storage devices
A-24. Converting to HTML
A-25. Preserving weblogs
A-26. Protecting literal strings
A-27. Unprotecting literal strings
A-28. Spammer Identification
A-29. Spammer Hunt
A-30. Making wget easier to use
A-31. A podcasting script
A-32. Nightly backup to a firewire HD
A-33. An expanded cd command
A-34. A soundcard setup script
A-35. Locating split paragraphs in a text file
A-36. Insertion sort
A-37. Standard Deviation
A-38. A pad file generator for shareware authors
A-39. A man page editor
A-40. Petals Around the Rose
A-41. Quacky: a Perquackey-type word game
A-42. Nim
A-43. A command-line stopwatch
A-44. An all-purpose shell scripting homework assignment solution
A-45. The Knight's Tour
A-46. Magic Squares
A-47. Fifteen Puzzle
A-48. The Towers of Hanoi, graphic version
A-49. The Towers of Hanoi, alternate graphic version
A-50. An alternate version of the getopt-simple.sh script
A-51. The version of the UseGetOpt.sh example used in the Tab Expansion appendix
A-52. Cycling through all the possible color backgrounds
A-53. Morse Code Practice
A-54. Base64 encoding/decoding
A-55. Inserting text in a file using sed
A-56. The Gronsfeld Cipher
A-57. Bingo Number Generator
A-58. Basics Reviewed
A-59. Testing execution times of various commands
A-60. Associative arrays vs. conventional arrays (execution times)
C-1. Counting Letter Occurrences
J-1. Completion script for UseGetOpt.sh
M-1. Sample .bashrc file
M-2. .bash_profile file
N-1. VIEWDATA.BAT: DOS Batch File
N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
T-1. A script that generates an ASCII table
T-2. Another ASCII table script
T-3. A third ASCII table script, using awk

Part 1. Introduction

Script: A writing; a written document. [Obs.]

--Webster's Dictionary, 1913 ed.

The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by "gluing together" system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, lend additional power and flexibility to scripts. Shell scripts are especially well suited for administrative system tasks and other routine repetitive tasks not requiring the bells and whistles of a full-blown tightly structured programming language.

Chapter 1. Shell Programming!

No programming language is perfect. There is not even a single best language; there are only languages well suited or perhaps poorly suited for particular purposes.

--Herbert Mayer

A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it.

The craft of scripting is not hard to master, since scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options [1] to learn. The syntax is simple -- even austere -- similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" governing their use. Most short scripts work right the first time, and debugging even the longer ones is straightforward.

In the early days of personal computing, the BASIC language enabled
anyone reasonably computer proficient to write programs on an early
generation of microcomputers. Decades later, the Bash scripting
language enables anyone with a rudimentary knowledge of Linux or
UNIX to do the same on modern machines.

We now have miniaturized single-board computers with amazing
capabilities, such as the Raspberry Pi.
Bash scripting provides a way to explore the capabilities of these
fascinating devices.

A shell script is a quick-and-dirty method of prototyping a complex application. Getting even a limited subset of the functionality to work in a script is often a useful first stage in project development. In this way, the structure of the application can be tested and tinkered with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, Perl, or Python.

Shell scripting hearkens back to the classic UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high-powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool.

According to Herbert Mayer, "a useful language needs arrays, pointers, and a generic mechanism for building data structures." By these criteria, shell scripting falls somewhat short of being "useful." Or, perhaps not. . . .

We will be using Bash, an acronym [3] for "Bourne-Again shell" and a pun on Stephen Bourne's now classic Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, [4] and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.)

What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work -- they've been tested, insofar as possible -- and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (scriptname.sh or scriptname.bash), [5] give them execute permission ( chmod u+rx scriptname ), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the HTML or pdf rendered versions. Be aware that some of the scripts presented here introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment.

Unless otherwise noted, the author of this book wrote the example scripts that follow.

His countenance was bold and bashed not.

--Edmund Spenser

Chapter 2. Starting Off With a Sha-Bang

Shell programming is a 1950s juke box . . .

--Larry Wall

In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked.

Example 2-1. cleanup: A script to clean up log files in /var/log

# Cleanup # Run as root, of course.  cd /var/log cat /dev/null > messages cat /dev/null > wtmp echo "Log files cleaned up."

There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one from the command-line on the console or in a terminal window. The advantages of placing the commands in a script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it can easily be modified or customized for a particular application.

Example 2-2. cleanup: An improved clean-up script

#!/bin/bash # Proper header for a Bash script.  # Cleanup, version 2  # Run as root, of course. # Insert code here to print error message and exit if not root.  LOG_DIR=/var/log # Variables are better than hard-coded values. cd $LOG_DIR  cat /dev/null > messages cat /dev/null > wtmp   echo "Logs cleaned up."  exit #  The right and proper method of "exiting" from a script.      #  A bare "exit" (no parameter) returns the exit status      #+ of the preceding command.

Now that's beginning to look like a real script. But we can go even farther . . .

Example 2-3. cleanup: An enhanced and generalized version of above scripts.

#!/bin/bash # Cleanup, version 3  #  Warning: #  ------- #  This script uses quite a number of features that will be explained #+ later on. #  By the time you've finished the first half of the book, #+ there should be nothing mysterious about it.    LOG_DIR=/var/log ROOT_UID=0     # Only users with $UID 0 have root privileges. LINES=50       # Default number of lines saved. E_XCD=86       # Can't change directory? E_NOTROOT=87   # Non-root exit error.   # Run as root, of course. if [ "$UID" -ne "$ROOT_UID" ] then   echo "Must be root to run this script."   exit $E_NOTROOT fi    if [ -n "$1" ] # Test whether command-line argument is present (non-empty). then   lines=$1 else     lines=$LINES # Default, if not specified on command-line. fi     #  Stephane Chazelas suggests the following, #+ as a better way of checking command-line arguments, #+ but this is still a bit advanced for this stage of the tutorial. # #    E_WRONGARGS=85  # Non-numerical argument (bad argument format). # #    case "$1" in #    ""      ) lines=50;; #    *[!0-9]*) echo "Usage: `basename $0` lines-to-cleanup"; #     exit $E_WRONGARGS;; #    *       ) lines=$1;; #    esac # #* Skip ahead to "Loops" chapter to decipher all this.   cd $LOG_DIR  if [ `pwd` != "$LOG_DIR" ]  # or   if [ "$PWD" != "$LOG_DIR" ]                             # Not in /var/log? then   echo "Can't change to $LOG_DIR."   exit $E_XCD fi  # Doublecheck if in right directory before messing with log file.  # Far more efficient is: # # cd /var/log || { #   echo "Cannot change to necessary directory." >&2 #   exit $E_XCD; # }     tail -n $lines messages > mesg.temp # Save last section of message log file. mv mesg.temp messages               # Rename it as system log file.   #  cat /dev/null > messages #* No longer needed, as the above method is safer.  cat /dev/null > wtmp  #  ': > wtmp' and '> wtmp'  have the same effect. echo "Log files cleaned up." #  Note that there are other log files in /var/log not affected #+ by this script.  exit 0 #  A zero return value from the script upon exit indicates success #+ to the shell.

Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of the message log intact. You will constantly discover ways of fine-tuning previously written scripts for increased effectiveness.

* * *

The sha-bang ( #!) [6] at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte [7] magic number, a special marker that designates a file type, or in this case an executable shell script (type man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (the line following the sha-bang line), and ignoring comments. [8]

#!/bin/sh #!/bin/bash #!/usr/bin/perl #!/usr/bin/tcl #!/bin/sed -f #!/bin/awk -f

Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. [9] Using #!/bin/sh , the default Bourne shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific features. The script will, however, conform to the POSIX [10] sh standard.

Note that the path given at the "sha-bang" must be correct, otherwise an error message -- usually "Command not found." -- will be the only result of running the script. [11]

#! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50 , uses a shell-specific construct. [12] Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine.

This tutorial encourages a modular approach to constructing a script. Make note of and collect "boilerplate" code snippets that might be useful in future scripts. Eventually you will build quite an extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters.

E_WRONG_ARGS=85 script_parameters="-a -h -m -z" #                  -a = all, -h = help, etc.  if [ $# -ne $Number_of_expected_args ] then   echo "Usage: `basename $0` $script_parameters"   # `basename $0` is the script's filename.   exit $E_WRONG_ARGS fi

Many times, you will write a script that carries out one particular task. The first script in this chapter is an example. Later, it might occur to you to generalize the script to do other, similar tasks. Replacing the literal ("hard-wired") constants by variables is a step in that direction, as is replacing repetitive code blocks by functions.

2.1. Invoking the script

Having written the script, you can invoke it by sh scriptname , [13] or alternatively bash scriptname . (Not recommended is using sh <scriptname , since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod.

Either:

chmod 555 scriptname (gives everyone read/execute permission) [14]

or

chmod +rx scriptname (gives everyone read/execute permission)

chmod u+rx scriptname (gives only the script owner read/execute permission)

Having made the script executable, you may now test it by ./scriptname . [15] If it begins with a "sha-bang" line, invoking the script calls the correct command interpreter to run it.

As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a systemwide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command-line.

2.2. Preliminary Exercises

System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful.
Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile.

Part 2. Basics

Chapter 3. Special Characters

What makes a character special? If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character. Along with commands and keywords, special characters are building blocks of Bash scripts.

Special Characters Found In Scripts and Elsewhere

#

Comments. Lines beginning with a # (with the exception of #!) are comments and will not be executed.

# This line is a comment.

Comments may also occur following the end of a command.

echo "A comment will follow." # Comment here. #                            ^ Note whitespace before #

Comments may also follow whitespace at the beginning of a line.

                          # A tab precedes this comment.

Comments may even be embedded within a pipe.

initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\ # Delete lines containing '#' comment character.            sed -e 's/\./\. /g' -e 's/_/_ /g'` ) # Excerpted from life.sh script

A command may not follow a comment on the same line. There is no method of terminating the comment, in order for "live code" to begin on the same line. Use a new line for the next command.

Of course, a quoted or an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter-substitution constructs and in numerical constant expressions.

echo "The # here does not begin a comment." echo 'The # here does not begin a comment.' echo The \# here does not begin a comment. echo The # here begins a comment.  echo ${PATH#*:}       # Parameter substitution, not a comment. echo $(( 2#101011 ))  # Base conversion, not a comment.  # Thanks, S.C.

The standard quoting and escape characters (" ' \) escape the #.

Certain pattern matching operations also use the #.

;

Command separator [semicolon]. Permits putting two or more commands on the same line.

echo hello; echo there   if [ -x "$filename" ]; then    #  Note the space after the semicolon. #+                   ^^   echo "File $filename exists."; cp $filename $filename.bak else   #                       ^^   echo "File $filename not found."; touch $filename fi; echo "File test complete."

Note that the ";" sometimes needs to be escaped.

;;

Terminator in a case option [double semicolon].

case "$variable" in   abc)  echo "\$variable = abc" ;;   xyz)  echo "\$variable = xyz" ;; esac

;;&, ;&

Terminators in a case option (version 4+ of Bash).

.

"dot" command [period]. Equivalent to source (see Example 15-22). This is a bash builtin.

.

"dot", as a component of a filename. When working with filenames, a leading dot is the prefix of a "hidden" file, a file that an ls will not normally show.

                          bash$                                                                                touch .hidden-file                                                    bash$                                                                                ls -l                                                    total 10  -rw-r--r--    1 bozo      4034 Jul 18 22:04 data1.addressbook  -rw-r--r--    1 bozo      4602 May 25 13:58 data1.addressbook.bak  -rw-r--r--    1 bozo       877 Dec 17  2000 employment.addressbook                          bash$                                                                                ls -al                                                    total 14  drwxrwxr-x    2 bozo  bozo      1024 Aug 29 20:54 ./  drwx------   52 bozo  bozo      3072 Aug 29 20:51 ../  -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.addressbook  -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.addressbook.bak  -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.addressbook  -rw-rw-r--    1 bozo  bozo         0 Aug 29 20:54 .hidden-file

When considering directory names, a single dot represents the current working directory, and two dots denote the parent directory.

                          bash$                                                                                pwd                                                    /home/bozo/projects                          bash$                                                                                cd .                                                    bash$                                                                                pwd                                                    /home/bozo/projects                          bash$                                                                                cd ..                                                    bash$                                                                                pwd                                                    /home/bozo/

The dot often appears as the destination (directory) of a file movement command, in this context meaning current directory.

                          bash$                                                                                cp /home/bozo/current_work/junk/* .

Copy all the "junk" files to $PWD.

.

"dot" character match. When matching characters, as part of a regular expression, a "dot" matches a single character.

"

partial quoting [double quote]. "STRING" preserves (from interpretation) most of the special characters within STRING. See Chapter 5.

'

full quoting [single quote]. 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than "STRING". See Chapter 5.

,

comma operator. The comma operator [16] links together a series of arithmetic operations. All are evaluated, but only the last one is returned.

let "t2 = ((a = 9, 15 / 3))" # Set "a = 9" and "t2 = 15 / 3"

The comma operator can also concatenate strings.

for file in /{,usr/}bin/*calc #             ^    Find all executable files ending in "calc" #+                 in /bin and /usr/bin directories. do         if [ -x "$file" ]         then           echo $file         fi done  # /bin/ipcalc # /usr/bin/kcalc # /usr/bin/oidcalc # /usr/bin/oocalc   # Thank you, Rory Winston, for pointing this out.

,, ,

Lowercase conversion in parameter substitution (added in version 4 of Bash).

\

escape [backslash]. A quoting mechanism for single characters.

\X escapes the character X. This has the effect of "quoting" X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally.

See Chapter 5 for an in-depth explanation of escaped characters.

/

Filename path separator [forward slash]. Separates the components of a filename (as in /home/bozo/projects/Makefile).

This is also the division arithmetic operator.

`

command substitution. The `command` construct makes available the output of command for assignment to a variable. This is also known as backquotes or backticks.

:

null command [colon]. This is the shell equivalent of a "NOP" ( no op , a do-nothing operation). It may be considered a synonym for the shell builtin true. The ":" command is itself a Bash builtin, and its exit status is true (0).

Endless loop:

while : do    operation-1    operation-2    ...    operation-n done  # Same as: #    while true #    do #      ... #    done

Placeholder in if/then test:

if condition then :   # Do nothing and branch ahead else     # Or else ...    take-some-action fi

Provide a placeholder where a binary operation is expected, see Example 8-2 and default parameters.

: ${username=`whoami`} # ${username=`whoami`}   Gives an error without the leading : #                        unless "username" is a command or builtin...  : ${1?"Usage: $0 ARGUMENT"}     # From "usage-message.sh example script.

Provide a placeholder where a command is expected in a here document. See Example 19-10.

Evaluate string of variables using parameter substitution (as in Example 10-7).

: ${HOSTNAME?} ${USER?} ${MAIL?} #  Prints error message #+ if one or more of essential environmental variables not set.

Variable expansion / substring replacement.

In combination with the > redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it.

: > data.xxx   # File "data.xxx" now empty.	        # Same effect as   cat /dev/null >data.xxx # However, this does not fork a new process, since ":" is a builtin.

Chapter 4. Introduction to Variables and Parameters

Variables are how programming and scripting languages represent data. A variable is nothing more than a label, a name assigned to a location or set of locations in computer memory holding an item of data.

Variables appear in arithmetic operations and manipulation of quantities, and in string parsing.

4.1. Variable Substitution

The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is called variable substitution.

$

Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. [25]

                            bash$                                                                                      variable1=23                                                        bash$                                                                                      echo variable1                                                        variable1                            bash$                                                                                      echo $variable1                                                        23

The only times a variable appears "naked" -- without the $ prefix -- is when declared or assigned, when unset, when exported, in an arithmetic expression within double parentheses (( ... )), or in the special case of a variable representing a signal (see Example 32-5). Assignment may be with an = (as in var1=27 ), in a read statement, and at the head of a loop ( for var2 in 1 2 3 ).

Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as "weak quoting." Using single quotes (' ... ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as 'strong quoting.' See Chapter 5 for a detailed discussion.

Note that $variable is actually a simplified form of ${variable} . In contexts where the $variable syntax causes an error, the longer form may work (see Section 10.2, below).

Example 4-1. Variable assignment and substitution

#!/bin/bash # ex9.sh  # Variables: assignment and substitution  a=375 hello=$a #   ^ ^  #------------------------------------------------------------------------- # No space permitted on either side of = sign when initializing variables. # What happens if there is a space?  #  "VARIABLE =value" #           ^ #% Script tries to run "VARIABLE" command with one argument, "=value".  #  "VARIABLE= value" #            ^ #% Script tries to run "value" command with #+ the environmental variable "VARIABLE" set to "". #-------------------------------------------------------------------------   echo hello    # hello # Not a variable reference, just the string "hello" ...  echo $hello   # 375 #    ^          This *is* a variable reference. echo ${hello} # 375 #               Likewise a variable reference, as above.  # Quoting . . . echo "$hello"    # 375 echo "${hello}"  # 375  echo  hello="A B  C   D" echo $hello   # A B C D echo "$hello" # A B  C   D # As we see, echo $hello   and   echo "$hello"   give different results. # ======================================= # Quoting a variable preserves whitespace. # =======================================  echo  echo '$hello'  # $hello #    ^      ^ #  Variable referencing disabled (escaped) by single quotes, #+ which causes the "$" to be interpreted literally.  # Notice the effect of different types of quoting.   hello=    # Setting it to a null value. echo "\$hello (null value) = $hello"      # $hello (null value) = #  Note that setting a variable to a null value is not the same as #+ unsetting it, although the end result is the same (see below).  # --------------------------------------------------------------  #  It is permissible to set multiple variables on the same line, #+ if separated by white space. #  Caution, this may reduce legibility, and may not be portable.  var1=21  var2=22  var3=$V3 echo echo "var1=$var1   var2=$var2   var3=$var3"  # May cause problems with legacy versions of "sh" . . .  # --------------------------------------------------------------  echo; echo  numbers="one two three" #           ^   ^ other_numbers="1 2 3" #               ^ ^ #  If there is whitespace embedded within a variable, #+ then quotes are necessary. #  other_numbers=1 2 3                  # Gives an error message. echo "numbers = $numbers" echo "other_numbers = $other_numbers"   # other_numbers = 1 2 3 #  Escaping the whitespace also works. mixed_bag=2\ ---\ Whatever #           ^    ^ Space after escape (\).  echo "$mixed_bag"         # 2 --- Whatever  echo; echo  echo "uninitialized_variable = $uninitialized_variable" # Uninitialized variable has null value (no value at all!). uninitialized_variable=   #  Declaring, but not initializing it --                           #+ same as setting it to a null value, as above. echo "uninitialized_variable = $uninitialized_variable"                           # It still has a null value.  uninitialized_variable=23       # Set it. unset uninitialized_variable    # Unset it. echo "uninitialized_variable = $uninitialized_variable"                                 # uninitialized_variable =                                 # It still has a null value. echo  exit 0

An uninitialized variable has a "null" value -- no assigned value at all (not zero!).

if [ -z "$unassigned" ] then   echo "\$unassigned is NULL." fi     # $unassigned is NULL.

Using a variable before assigning a value to it may cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.

echo "$uninitialized"                                # (blank line) let "uninitialized += 5"                             # Add 5 to it. echo "$uninitialized"                                # 5  #  Conclusion: #  An uninitialized variable has no value, #+ however it evaluates as 0 in an arithmetic operation.

4.2. Variable Assignment

=

the assignment operator (no space before and after)

Do not confuse this with = and -eq, which test, rather than assign!

Note that = can be either an assignment or a test operator, depending on context.

Example 4-2. Plain Variable Assignment

#!/bin/bash # Naked variables  echo  # When is a variable "naked", i.e., lacking the '$' in front? # When it is being assigned, rather than referenced.  # Assignment a=879 echo "The value of \"a\" is $a."  # Assignment using 'let' let a=16+5 echo "The value of \"a\" is now $a."  echo  # In a 'for' loop (really, a type of disguised assignment): echo -n "Values of \"a\" in the loop are: " for a in 7 8 9 11 do   echo -n "$a " done  echo echo  # In a 'read' statement (also a type of assignment): echo -n "Enter \"a\" " read a echo "The value of \"a\" is now $a."  echo  exit 0

Example 4-3. Variable Assignment, plain and fancy

#!/bin/bash  a=23              # Simple case echo $a b=$a echo $b  # Now, getting a little bit fancier (command substitution).  a=`echo Hello!`   # Assigns result of 'echo' command to 'a' ... echo $a #  Note that including an exclamation mark (!) within a #+ command substitution construct will not work from the command-line, #+ since this triggers the Bash "history mechanism." #  Inside a script, however, the history functions are disabled by default.  a=`ls -l`         # Assigns result of 'ls -l' command to 'a' echo $a           # Unquoted, however, it removes tabs and newlines. echo echo "$a"         # The quoted variable preserves whitespace.                   # (See the chapter on "Quoting.")  exit 0

Variable assignment using the $(...) mechanism (a newer method than backquotes). This is likewise a form of command substitution.

# From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m)

4.3. Bash Variables Are Untyped

Unlike many other programming languages, Bash does not segregate its variables by "type." Essentially, Bash variables are character strings, but, depending on context, Bash permits arithmetic operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.

Example 4-4. Integer or string?

#!/bin/bash # int-or-string.sh  a=2334                   # Integer. let "a += 1" echo "a = $a "           # a = 2335 echo                     # Integer, still.   b=${a/23/BB}             # Substitute "BB" for "23".                          # This transforms $b into a string. echo "b = $b"            # b = BB35 declare -i b             # Declaring it an integer doesn't help. echo "b = $b"            # b = BB35  let "b += 1"             # BB35 + 1 echo "b = $b"            # b = 1 echo                     # Bash sets the "integer value" of a string to 0.  c=BB34 echo "c = $c"            # c = BB34 d=${c/BB/23}             # Substitute "23" for "BB".                          # This makes $d an integer. echo "d = $d"            # d = 2334 let "d += 1"             # 2334 + 1 echo "d = $d"            # d = 2335 echo   # What about null variables? e=''                     # ... Or e="" ... Or e= echo "e = $e"            # e = let "e += 1"             # Arithmetic operations allowed on a null variable? echo "e = $e"            # e = 1 echo                     # Null variable transformed into an integer.  # What about undeclared variables? echo "f = $f"            # f = let "f += 1"             # Arithmetic operations allowed? echo "f = $f"            # f = 1 echo                     # Undeclared variable transformed into an integer. # # However ... let "f /= $undecl_var"   # Divide by zero? #   let: f /= : syntax error: operand expected (error token is " ") # Syntax error! Variable $undecl_var is not set to zero here! # # But still ... let "f /= 0" #   let: f /= 0: division by 0 (error token is "0") # Expected behavior.   #  Bash (usually) sets the "integer value" of null to zero #+ when performing an arithmetic operation. #  But, don't try this at home, folks! #  It's undocumented and probably non-portable behavior.   # Conclusion: Variables in Bash are untyped, #+ with all attendant consequences.  exit $?

Untyped variables are both a blessing and a curse. They permit more flexibility in scripting and make it easier to grind out lines of code (and give you enough rope to hang yourself!). However, they likewise permit subtle errors to creep in and encourage sloppy programming habits.

To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables.

4.4. Special Variable Types

Local variables

Variables visible only within a code block or function (see also local variables in functions)

Environmental variables

Variables that affect the behavior of the shell and user interface

In a more general context, each process has an "environment", that is, a group of variables that the process may reference. In this sense, the shell behaves like any other process.

Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment.

The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

                                      bash$                                                                                                                    eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"                                                                            bash$                                                                                                                    du                                                                            bash: /usr/bin/du: Argument list too long

Note: this "error" has been fixed, as of kernel version 2.6.23.

(Thank you, St�phane Chazelas for the clarification, and for providing the above example.)

If a script sets environmental variables, they need to be "exported," that is, reported to the environment local to the script. This is the function of the export command.

A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command-line cannot export variables back to the command-line environment. Child processes cannot export variables back to the parent processes that spawned them.

Definition: A child process is a subprocess launched by another process, its parent.

Positional parameters

Arguments passed to the script from the command line [26] : $0, $1, $2, $3 . . .

$0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. [27] After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}.

The special variables $* and $@ denote all the positional parameters.

Example 4-5. Positional Parameters

#!/bin/bash  # Call this script with at least 10 parameters, for example # ./scriptname 1 2 3 4 5 6 7 8 9 10 MINPARAMS=10  echo  echo "The name of this script is \"$0\"." # Adds ./ for current directory echo "The name of this script is \"`basename $0`\"." # Strips out path name info (see 'basename')  echo  if [ -n "$1" ]              # Tested variable is quoted. then  echo "Parameter #1 is $1"  # Need quotes to escape # fi   if [ -n "$2" ] then  echo "Parameter #2 is $2" fi   if [ -n "$3" ] then  echo "Parameter #3 is $3" fi   # ...   if [ -n "${10}" ]  # Parameters > $9 must be enclosed in {brackets}. then  echo "Parameter #10 is ${10}" fi   echo "-----------------------------------" echo "All the command-line parameters are: "$*""  if [ $# -lt "$MINPARAMS" ] then   echo   echo "This script needs at least $MINPARAMS command-line arguments!" fi    echo  exit 0

Bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command-line. This also requires indirect referencing.

args=$#           # Number of args passed. lastarg=${!args} # Note: This is an *indirect reference* to $args ...   # Or:       lastarg=${!#}             (Thanks, Chris Monson.) # This is an *indirect reference* to the $# variable. # Note that lastarg=${!$#} doesn't work.

Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. [28] There must also exist symbolic links to all the alternate names of the script. See Example 16-2.

If a script expects a command-line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

variable1_=$1_  # Rather than variable1=$1 # This will prevent an error, even if positional parameter is absent.  critical_argument01=$variable1_  # The extra character can be stripped off later, like so. variable1=${variable1_/_/} # Side effects only if $variable1_ begins with an underscore. # This uses one of the parameter substitution templates discussed later. # (Leaving out the replacement pattern results in a deletion.)  #  A more straightforward way of dealing with this is #+ to simply test whether expected positional parameters have been passed. if [ -z $1 ] then   exit $E_MISSING_POS_PARAM fi   #  However, as Fabian Kreutz points out, #+ the above method may have unexpected side-effects. #  A better method is parameter substitution: #         ${1:-$DefaultVal} #  See the "Parameter Substition" section #+ in the "Variables Revisited" chapter.

---

Example 4-6. wh, whois domain name lookup

#!/bin/bash # ex18.sh  # Does a 'whois domain-name' lookup on any of 3 alternate servers: #                    ripe.net, cw.net, radb.net  # Place this script -- renamed 'wh' -- in /usr/local/bin  # Requires symbolic links: # ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe # ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic # ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows  E_NOARGS=75   if [ -z "$1" ] then   echo "Usage: `basename $0` [domain-name]"   exit $E_NOARGS fi  # Check script name and call proper server. case `basename $0` in    # Or:    case ${0##*/} in     "wh"       ) whois $1@whois.tucows.com;;     "wh-ripe"  ) whois $1@whois.ripe.net;;     "wh-apnic" ) whois $1@whois.apnic.net;;     "wh-cw"    ) whois $1@whois.cw.net;;     *          ) echo "Usage: `basename $0` [domain-name]";; esac   exit $?

---

The shift command reassigns the positional parameters, in effect shifting them to the left one notch.

$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.

The old $1 disappears, but $0 (the script name) does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10, although {bracket} notation also permits this.

Example 4-7. Using shift

#!/bin/bash # shft.sh: Using 'shift' to step through all the positional parameters.  #  Name this script something like shft.sh, #+ and invoke it with some parameters. #+ For example: #             sh shft.sh a b c def 83 barndoor  until [ -z "$1" ]  # Until all parameters used up . . . do   echo -n "$1 "   shift done  echo               # Extra linefeed.  # But, what happens to the "used-up" parameters? echo "$2" #  Nothing echoes! #  When $2 shifts into $1 (and there is no $3 to shift into $2) #+ then $2 remains empty. #  So, it is not a parameter *copy*, but a *move*.  exit  #  See also the echo-params.sh script for a "shiftless" #+ alternative method of stepping through the positional params.

The shift command can take a numerical parameter indicating how many positions to shift.

#!/bin/bash # shift-past.sh  shift 3    # Shift 3 positions. #  n=3; shift $n #  Has the same effect.  echo "$1"  exit 0  # ======================== #   $ sh shift-past.sh 1 2 3 4 5 4  #  However, as Eleni Fragkiadaki, points out, #+ attempting a 'shift' past the number of #+ positional parameters ($#) returns an exit status of 1, #+ and the positional parameters themselves do not change. #  This means possibly getting stuck in an endless loop. . . . #  For example: #      until [ -z "$1" ] #      do #         echo -n "$1 " #         shift 20    #  If less than 20 pos params, #      done           #+ then loop never ends! # # When in doubt, add a sanity check. . . . #           shift 20 || break #                    ^^^^^^^^

The shift command works in a similar fashion on parameters passed to a function. See Example 36-18.

Chapter 5. Quoting

Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an interpretation other than its literal meaning. For example, the asterisk * represents a wild card character in globbing and Regular Expressions).

                    bash$                                                              ls -l [Vv]*                                        -rw-rw-r--    1 bozo  bozo       324 Apr  2 15:05 VIEWDATA.BAT  -rw-rw-r--    1 bozo  bozo       507 May  4 14:25 vartrace.sh  -rw-rw-r--    1 bozo  bozo       539 Apr 14 17:11 viewdata.sh                    bash$                                                              ls -l '[Vv]*'                                        ls: [Vv]*: No such file or directory

Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it.

                    bash$                                                              grep '[Ff]irst' *.txt                                        file1.txt:This is the first line of file1.txt.  file2.txt:This is the First line of file2.txt.

Note that the unquoted grep [Ff]irst *.txt works under the Bash shell. [29]

Quoting can also suppress echo's "appetite" for newlines.

                    bash$                                                              echo $(ls -l)                                        total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh                    bash$                                                              echo "$(ls -l)"                                        total 8  -rw-rw-r--  1 bo bo  13 Aug 21 12:57 t.sh  -rw-rw-r--  1 bo bo  78 Aug 21 12:57 u.sh

5.1. Quoting Variables

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). [30] Keeping $ as a special character within double quotes permits referencing a quoted variable ( "$variable" ), that is, replacing the variable with its value (see Example 4-1, above).

Use double quotes to prevent word splitting. [31] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

List="one two three"  for a in $List     # Splits the variable in parts at whitespace. do   echo "$a" done # one # two # three  echo "---"  for a in "$List"   # Preserves whitespace in a single variable. do #     ^     ^   echo "$a" done # one two three

A more elaborate example:

variable1="a variable containing five words" COMMAND This is $variable1    # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words"  COMMAND "This is $variable1"  # Executes COMMAND with 1 argument: # "This is a variable containing five words"   variable2=""    # Empty.  COMMAND $variable2 $variable2 $variable2                 # Executes COMMAND with no arguments.  COMMAND "$variable2" "$variable2" "$variable2"                 # Executes COMMAND with 3 empty arguments.  COMMAND "$variable2 $variable2 $variable2"                 # Executes COMMAND with 1 argument (2 spaces).   # Thanks, St�phane Chazelas.

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

Example 5-1. Echoing Weird Variables

#!/bin/bash # weirdvars.sh: Echoing weird variables.  echo  var="'(]\\{}\$\"" echo $var        # '(]\{}$" echo "$var"      # '(]\{}$"     Doesn't make a difference.  echo  IFS='\' echo $var        # '(] {}$"     \ converted to space. Why? echo "$var"      # '(]\{}$"  # Examples above supplied by Stephane Chazelas.  echo  var2="\\\\\"" echo $var2       #   " echo "$var2"     # \\" echo # But ... var2="\\\\"" is illegal. Why? var3='\\\\' echo "$var3"     # \\\\ # Strong quoting works, though.   # ************************************************************ # # As the first example above shows, nesting quotes is permitted.  echo "$(echo '"')"           # " #    ^           ^   # At times this comes in useful.  var1="Two bits" echo "\$var1 = "$var1""      # $var1 = Two bits #    ^                ^  # Or, as Chris Hiestand points out ...  if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]] #     ^     ^         ^ ^     ^     ^         ^ ^ then   ... fi # ************************************************************ #

Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting").

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.

echo "Why can't I write 's between single quotes"  echo  # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' #    |-------|  |----------|   |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between.  # This example courtesy of St�phane Chazelas.

5.2. Escaping

Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.

With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character.

Special meanings of certain escaped characters

used with echo and sed

\n

means newline

\r

means return

\t

means tab

\v

means vertical tab

\b

means backspace

\a

means alert (beep or flash)

\0xx

translates to the octal ASCII equivalent of 0nn , where nn is a string of digits

The $' ... ' quoted string-expansion construct is a mechanism that uses escaped octal or hex values to assign ASCII characters to variables, e.g., quote=$'\042'.

Example 5-2. Escaped Characters

#!/bin/bash # escaped.sh: escaped characters  ############################################################# ### First, let's show some basic escaped-character usage. ### #############################################################  # Escaping a newline. # ------------------  echo ""  echo "This will print as two lines." # This will print # as two lines.  echo "This will print \ as one line." # This will print as one line.  echo; echo  echo "============="   echo "\v\v\v\v"      # Prints \v\v\v\v literally. # Use the -e option with 'echo' to print escaped characters. echo "=============" echo "VERTICAL TABS" echo -e "\v\v\v\v"   # Prints 4 vertical tabs. echo "=============="  echo "QUOTATION MARK" echo -e "\042"       # Prints " (quote, octal ASCII character 42). echo "=============="    # The $'\X' construct makes the -e option unnecessary.  echo; echo "NEWLINE and (maybe) BEEP" echo $'\n'           # Newline. echo $'\a'           # Alert (beep).                      # May only flash, not beep, depending on terminal.  # We have seen $'\nnn" string expansion, and now . . .  # =================================================================== # # Version 2 of Bash introduced the $'\nnn' string expansion construct. # =================================================================== #  echo "Introducing the \$\' ... \' string-expansion construct . . . " echo ". . . featuring more quotation marks."  echo $'\t \042 \t'   # Quote (") framed by tabs. # Note that  '\nnn' is an octal value.  # It also works with hexadecimal values, in an $'\xhhh' construct. echo $'\t \x22 \t'  # Quote (") framed by tabs. # Thank you, Greg Keraunen, for pointing this out. # Earlier Bash versions allowed '\x022'.  echo   # Assigning ASCII characters to a variable. # ---------------------------------------- quote=$'\042'        # " assigned to a variable. echo "$quote Quoted string $quote and this lies outside the quotes."  echo  # Concatenating ASCII chars in a variable. triple_underline=$'\137\137\137'  # 137 is octal ASCII code for '_'. echo "$triple_underline UNDERLINE $triple_underline"  echo  ABC=$'\101\102\103\010'           # 101, 102, 103 are octal A, B, C. echo $ABC  echo  escape=$'\033'                    # 033 is octal for escape. echo "\"escape\" echoes as $escape" #                                   no visible output.  echo  exit 0

A more elaborate example:

Example 5-3. Detecting key-presses

#!/bin/bash # Author: Sigurd Solaas, 20 Apr 2011 # Used in ABS Guide with permission. # Requires version 4.2+ of Bash.  key="no value yet" while true; do   clear   echo "Bash Extra Keys Demo. Keys to try:"   echo   echo "* Insert, Delete, Home, End, Page_Up and Page_Down"   echo "* The four arrow keys"   echo "* Tab, enter, escape, and space key"   echo "* The letter and number keys, etc."   echo   echo "    d = show date/time"   echo "    q = quit"   echo "================================"   echo   # Convert the separate home-key to home-key_num_7:  if [ "$key" = $'\x1b\x4f\x48' ]; then   key=$'\x1b\x5b\x31\x7e'   #   Quoted string-expansion construct.   fi   # Convert the separate end-key to end-key_num_1.  if [ "$key" = $'\x1b\x4f\x46' ]; then   key=$'\x1b\x5b\x34\x7e'  fi   case "$key" in   $'\x1b\x5b\x32\x7e')  # Insert    echo Insert Key   ;;   $'\x1b\x5b\x33\x7e')  # Delete    echo Delete Key   ;;   $'\x1b\x5b\x31\x7e')  # Home_key_num_7    echo Home Key   ;;   $'\x1b\x5b\x34\x7e')  # End_key_num_1    echo End Key   ;;   $'\x1b\x5b\x35\x7e')  # Page_Up    echo Page_Up   ;;   $'\x1b\x5b\x36\x7e')  # Page_Down    echo Page_Down   ;;   $'\x1b\x5b\x41')  # Up_arrow    echo Up arrow   ;;   $'\x1b\x5b\x42')  # Down_arrow    echo Down arrow   ;;   $'\x1b\x5b\x43')  # Right_arrow    echo Right arrow   ;;   $'\x1b\x5b\x44')  # Left_arrow    echo Left arrow   ;;   $'\x09')  # Tab    echo Tab Key   ;;   $'\x0a')  # Enter    echo Enter Key   ;;   $'\x1b')  # Escape    echo Escape Key   ;;   $'\x20')  # Space    echo Space Key   ;;   d)    date   ;;   q)   echo Time to quit...   echo   exit 0   ;;   *)    echo You pressed: \'"$key"\'   ;;  esac   echo  echo "================================"   unset K1 K2 K3  read -s -N1 -p "Press a key: "  K1="$REPLY"  read -s -N2 -t 0.001  K2="$REPLY"  read -s -N1 -t 0.001  K3="$REPLY"  key="$K1$K2$K3"  done  exit $?

Chapter 6. Exit and Exit Status

... there are dark corners in the Bourne shell, and people use all of them.

--Chet Ramey

The exit command terminates a script, just as in a C program. It can also return a value, which is available to the script's parent process.

Every command returns an exit status (sometimes referred to as a return status or exit code). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually can be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.

Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an exit nnn command may be used to deliver an nnn exit status to the shell ( nnn must be an integer in the 0 - 255 range).

When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the last command executed in the script (previous to the exit).

#!/bin/bash  COMMAND_1  . . .  COMMAND_LAST  # Will exit with status of last command.  exit

The equivalent of a bare exit is exit $? or even just omitting the exit.

#!/bin/bash  COMMAND_1  . . .  COMMAND_LAST  # Will exit with status of last command.  exit $?

#!/bin/bash  COMMAND1  . . .   COMMAND_LAST  # Will exit with status of last command.

$? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a "return value." [32]

Following the execution of a pipe, a $? gives the exit status of the last command executed.

After a script terminates, a $? from the command-line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, 0 on success or an integer in the range 1 - 255 on error.

Example 6-1. exit / exit status

#!/bin/bash  echo hello echo $?    # Exit status 0 returned because command executed successfully.  lskdf      # Unrecognized command. echo $?    # Non-zero exit status returned -- command failed to execute.  echo  exit 113   # Will return 113 to shell.            # To verify this, type "echo $?" after script terminates.  #  By convention, an 'exit 0' indicates success, #+ while a non-zero exit value means an error or anomalous condition. #  See the "Exit Codes With Special Meanings" appendix.

$? is especially useful for testing the result of a command in a script (see Example 16-35 and Example 16-20).

The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status.

Example 6-2. Negating a condition using !

true    # The "true" builtin. echo "exit status of \"true\" = $?"     # 0  ! true echo "exit status of \"! true\" = $?"   # 1 # Note that the "!" needs a space between it and the command. #    !true   leads to a "command not found" error # # The '!' operator prefixing a command invokes the Bash history mechanism.  true !true # No error this time, but no negation either. # It just repeats the previous command (true).   # =========================================================== # # Preceding a _pipe_ with ! inverts the exit status returned. ls | bogus_command     # bash: bogus_command: command not found echo $?                # 127  ! ls | bogus_command   # bash: bogus_command: command not found echo $?                # 0 # Note that the ! does not change the execution of the pipe. # Only the exit status changes. # =========================================================== #  # Thanks, St�phane Chazelas and Kristopher Newsome.

Certain exit status codes have reserved meanings and should not be user-specified in a script.

Chapter 7. Tests

Every reasonably complete programming language can test for a condition, then act according to the result of the test. Bash has the test command, various bracket and parenthesis operators, and the if/then construct.

7.1. Test Constructs

An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means "success" by UNIX convention), and if so, executes one or more commands.
There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false).
With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command.

Bash sees [[ $a -lt $b ]] as a single element, which returns an exit status.

The (( ... )) and let ... constructs return an exit status, according to whether the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic-expansion constructs may therefore be used to perform arithmetic comparisons.

(( 0 && 1 ))                 # Logical AND echo $?     # 1     *** # And so ... let "num = (( 0 && 1 ))" echo $num   # 0 # But ... let "num = (( 0 && 1 ))" echo $?     # 1     ***   (( 200 || 11 ))              # Logical OR echo $?     # 0     *** # ... let "num = (( 200 || 11 ))" echo $num   # 1 let "num = (( 200 || 11 ))" echo $?     # 0     ***   (( 200 | 11 ))               # Bitwise OR echo $?                      # 0     *** # ... let "num = (( 200 | 11 ))" echo $num                    # 203 let "num = (( 200 | 11 ))" echo $?                      # 0     ***  # The "let" construct returns the same exit status #+ as the double-parentheses arithmetic expansion.

Again, note that the exit status of an arithmetic expression is not an error value.

var=-2 && (( var+=2 )) echo $?                   # 1  var=-2 && (( var+=2 )) && echo $var                           # Will not echo $var!

An if can test any command, not just conditions enclosed within brackets.

if cmp a b &> /dev/null  # Suppress output. then echo "Files a and b are identical." else echo "Files a and b differ." fi  # The very useful "if-grep" construct: # -----------------------------------  if grep -q Bash file   then echo "File contains at least one occurrence of Bash." fi  word=Linux letter_sequence=inu if echo "$word" | grep -q "$letter_sequence" # The "-q" option to grep suppresses output. then   echo "$letter_sequence found in $word" else   echo "$letter_sequence not found in $word" fi   if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED   then echo "Command succeeded."   else echo "Command failed." fi

These last two examples courtesy of St�phane Chazelas.

Example 7-1. What is truth?

#!/bin/bash  #  Tip: #  If you're unsure how a certain condition might evaluate, #+ test it in an if-test.  echo  echo "Testing \"0\"" if [ 0 ]      # zero then   echo "0 is true." else          # Or else ...   echo "0 is false." fi            # 0 is true.  echo  echo "Testing \"1\"" if [ 1 ]      # one then   echo "1 is true." else   echo "1 is false." fi            # 1 is true.  echo  echo "Testing \"-1\"" if [ -1 ]     # minus one then   echo "-1 is true." else   echo "-1 is false." fi            # -1 is true.  echo  echo "Testing \"NULL\"" if [ ]        # NULL (empty condition) then   echo "NULL is true." else   echo "NULL is false." fi            # NULL is false.  echo  echo "Testing \"xyz\"" if [ xyz ]    # string then   echo "Random string is true." else   echo "Random string is false." fi            # Random string is true.  echo  echo "Testing \"\$xyz\"" if [ $xyz ]   # Tests if $xyz is null, but...               # it's only an uninitialized variable. then   echo "Uninitialized variable is true." else   echo "Uninitialized variable is false." fi            # Uninitialized variable is false.  echo  echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ]            # More pedantically correct. then   echo "Uninitialized variable is true." else   echo "Uninitialized variable is false." fi            # Uninitialized variable is false.  echo   xyz=          # Initialized, but set to null value.  echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ] then   echo "Null variable is true." else   echo "Null variable is false." fi            # Null variable is false.   echo   # When is "false" true?  echo "Testing \"false\"" if [ "false" ]              #  It seems that "false" is just a string ... then   echo "\"false\" is true." #+ and it tests true. else   echo "\"false\" is false." fi            # "false" is true.  echo  echo "Testing \"\$false\""  # Again, uninitialized variable. if [ "$false" ] then   echo "\"\$false\" is true." else   echo "\"\$false\" is false." fi            # "$false" is false.               # Now, we get the expected result.  #  What would happen if we tested the uninitialized variable "$true"?  echo  exit 0

Exercise. Explain the behavior of Example 7-1, above.

if [ condition-true ] then    command 1    command 2    ... else  # Or else ...       # Adds default code block executing if original condition tests false.    command 3    command 4    ... fi

When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate.

if [ -x "$filename" ]; then

Else if and elif

elif

elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.

if [ condition1 ] then    command1    command2    command3 elif [ condition2 ] # Same as else if then    command4    command5 else    default-command fi

The if test condition-true construct is the exact equivalent of if [ condition-true ] . As it happens, the left bracket, [ , is a token [33] which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it.

The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test.

                                bash$                                                                                                  type test                                                                test is a shell builtin                                bash$                                                                                                  type '['                                                                [ is a shell builtin                                bash$                                                                                                  type '[['                                                                [[ is a shell keyword                                bash$                                                                                                  type ']]'                                                                ]] is a shell keyword                                bash$                                                                                                  type ']'                                                                bash: type: ]: not found

If, for some reason, you wish to use /usr/bin/test in a Bash script, then specify it by full pathname.

Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[

#!/bin/bash  echo  if test -z "$1" then   echo "No command-line arguments." else   echo "First command-line argument is $1." fi  echo  if /usr/bin/test -z "$1"      # Equivalent to "test" builtin. #  ^^^^^^^^^^^^^              # Specifying full pathname. then   echo "No command-line arguments." else   echo "First command-line argument is $1." fi  echo  if [ -z "$1" ]                # Functionally identical to above code blocks. #   if [ -z "$1"                should work, but... #+  Bash responds to a missing close-bracket with an error message. then   echo "No command-line arguments." else   echo "First command-line argument is $1." fi  echo   if /usr/bin/[ -z "$1" ]       # Again, functionally identical to above. # if /usr/bin/[ -z "$1"       # Works, but gives an error message. #                             # Note: #                               This has been fixed in Bash, version 3.x. then   echo "No command-line arguments." else   echo "First command-line argument is $1." fi  echo  exit 0

Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary.

dir=/home/bozo  if cd "$dir" 2>/dev/null; then   # "2>/dev/null" hides error message.   echo "Now in $dir." else   echo "Can't change to $dir." fi

The "if COMMAND" construct returns the exit status of COMMAND.

Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct.

var1=20 var2=22 [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"  home=/home/bozo [ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or "false". A non-zero expression returns an exit status of 0, or "true". This is in marked contrast to using the test and [ ] constructs previously discussed.

Example 7-3. Arithmetic Tests using (( ))

#!/bin/bash # arith-tests.sh # Arithmetic tests.  # The (( ... )) construct evaluates and tests numerical expressions. # Exit status opposite from [ ... ] construct!  (( 0 )) echo "Exit status of \"(( 0 ))\" is $?."         # 1  (( 1 )) echo "Exit status of \"(( 1 ))\" is $?."         # 0  (( 5 > 4 ))                                      # true echo "Exit status of \"(( 5 > 4 ))\" is $?."     # 0  (( 5 > 9 ))                                      # false echo "Exit status of \"(( 5 > 9 ))\" is $?."     # 1  (( 5 == 5 ))                                     # true echo "Exit status of \"(( 5 == 5 ))\" is $?."    # 0 # (( 5 = 5 ))  gives an error message.  (( 5 - 5 ))                                      # 0 echo "Exit status of \"(( 5 - 5 ))\" is $?."     # 1  (( 5 / 4 ))                                      # Division o.k. echo "Exit status of \"(( 5 / 4 ))\" is $?."     # 0  (( 1 / 2 ))                                      # Division result < 1. echo "Exit status of \"(( 1 / 2 ))\" is $?."     # Rounded off to 0.                                                  # 1  (( 1 / 0 )) 2>/dev/null                          # Illegal division by 0. #           ^^^^^^^^^^^ echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1  # What effect does the "2>/dev/null" have? # What would happen if it were removed? # Try removing it, then rerunning the script.  # ======================================= #  # (( ... )) also useful in an if-then test.  var1=5 var2=4  if (( var1 > var2 )) then #^      ^      Note: Not $var1, $var2. Why?   echo "$var1 is greater than $var2" fi     # 5 is greater than 4  exit 0

7.2. File test operators

Returns true if...

-e

file exists

-a

file exists

This is identical in effect to -e. It has been "deprecated," [34] and its use is discouraged.

-f

file is a regular file (not a directory or device file)

-s

file is not zero size

-d

file is a directory

-b

file is a block device

-c

file is a character device

device0="/dev/sda2"    # /   (root directory) if [ -b "$device0" ] then   echo "$device0 is a block device." fi  # /dev/sda2 is a block device.    device1="/dev/ttyS1"   # PCMCIA modem card. if [ -c "$device1" ] then   echo "$device1 is a character device." fi  # /dev/ttyS1 is a character device.

-p

file is a pipe

function show_input_type() {    [ -p /dev/fd/0 ] && echo PIPE || echo STDIN }  show_input_type "Input"                           # STDIN echo "Input" | show_input_type                    # PIPE  # This example courtesy of Carl Anderson.

-h

file is a symbolic link

-L

file is a symbolic link

-S

file is a socket

-t

file (descriptor) is associated with a terminal device

This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a given script is a terminal.

-r

file has read permission (for the user running the test)

-w

file has write permission (for the user running the test)

-x

file has execute permission (for the user running the test)

-g

set-group-id (sgid) flag set on file or directory

If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup.

-u

set-user-id (suid) flag set on file

A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. [35] This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user.

                            -rwsr-xr-t    1 root       178236 Oct  2  2000 /usr/sbin/pppd

A file with the suid flag set shows an s in its permissions.

-k

sticky bit set

Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. [36] If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files.

                            drwxrwxrwt    7 root         1024 May 19 21:26 tmp/

If a user does not own a directory that has the sticky bit set, but has write permission in that directory, she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. (The owner of the directory or root can, of course, delete or rename files there.)

-O

you are owner of file

-G

group-id of file same as yours

-N

file modified since it was last read

f1 -nt f2

file f1 is newer than f2

f1 -ot f2

file f1 is older than f2

f1 -ef f2

files f1 and f2 are hard links to the same file

!

"not" -- reverses the sense of the tests above (returns true if condition absent).

Example 7-4. Testing for broken links

#!/bin/bash # broken-link.sh # Written by Lee bigelow <ligelowbee@yahoo.com> # Used in ABS Guide with permission.  #  A pure shell script to find dead symlinks and output them quoted #+ so they can be fed to xargs and dealt with :) #+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm # #  This, however, is a better method: # #  find "somedir" -type l -print0|\ #  xargs -r0 file|\ #  grep "broken symbolic"| #  sed -e 's/^\|: *broken symbolic.*$/"/g' # #+ but that wouldn't be pure Bash, now would it. #  Caution: beware the /proc file system and any circular links! ################################################################   #  If no args are passed to the script set directories-to-search  #+ to current directory.  Otherwise set the directories-to-search  #+ to the args passed. ######################  [ $# -eq 0 ] && directorys=`pwd` || directorys=$@   #  Setup the function linkchk to check the directory it is passed  #+ for files that are links and don't exist, then print them quoted. #  If one of the elements in the directory is a subdirectory then  #+ send that subdirectory to the linkcheck function. ##########  linkchk () {     for element in $1/*; do       [ -h "$element" -a ! -e "$element" ] && echo \"$element\"       [ -d "$element" ] && linkchk $element     # Of course, '-h' tests for symbolic link, '-d' for directory.     done }  #  Send each arg that was passed to the script to the linkchk() function #+ if it is a valid directoy.  If not, then print the error message #+ and usage info. ################## for directory in $directorys; do     if [ -d $directory ] 	then linkchk $directory 	else  	    echo "$directory is not a directory" 	    echo "Usage: $0 dir1 dir2 ..."     fi done  exit $?

Example 31-1, Example 11-8, Example 11-3, Example 31-3, and Example A-1 also illustrate uses of the file test operators.

7.3. Other Comparison Operators

A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.

integer comparison

-eq

is equal to

if [ "$a" -eq "$b" ]

-ne

is not equal to

if [ "$a" -ne "$b" ]

-gt

is greater than

if [ "$a" -gt "$b" ]

-ge

is greater than or equal to

if [ "$a" -ge "$b" ]

-lt

is less than

if [ "$a" -lt "$b" ]

-le

is less than or equal to

if [ "$a" -le "$b" ]

<

is less than (within double parentheses)

(("$a" < "$b"))

<=

is less than or equal to (within double parentheses)

(("$a" <= "$b"))

>

is greater than (within double parentheses)

(("$a" > "$b"))

>=

is greater than or equal to (within double parentheses)

(("$a" >= "$b"))

string comparison

=

is equal to

if [ "$a" = "$b" ]

Note the whitespace framing the =.

if [ "$a"="$b" ] is not equivalent to the above.

==

is equal to

if [ "$a" == "$b" ]

This is a synonym for =.

The == comparison operator behaves differently within a double-brackets test than within single brackets.

[[ $a == z* ]]   # True if $a starts with an "z" (pattern matching). [[ $a == "z*" ]] # True if $a is equal to z* (literal matching).  [ $a == z* ]     # File globbing and word splitting take place. [ "$a" == "z*" ] # True if $a is equal to z* (literal matching).  # Thanks, St�phane Chazelas

!=

is not equal to

if [ "$a" != "$b" ]

This operator uses pattern matching within a [[ ... ]] construct.

<

is less than, in ASCII alphabetical order

if [[ "$a" < "$b" ]]

if [ "$a" \< "$b" ]

Note that the "<" needs to be escaped within a [ ] construct.

>

is greater than, in ASCII alphabetical order

if [[ "$a" > "$b" ]]

if [ "$a" \> "$b" ]

Note that the ">" needs to be escaped within a [ ] construct.

See Example 27-11 for an application of this comparison operator.

-z

string is null, that is, has zero length

                            String=''   # Zero-length ("null") string variable.  if [ -z "$String" ] then   echo "\$String is null." else   echo "\$String is NOT null." fi     # $String is null.

-n

string is not null.

The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [37]

Example 7-5. Arithmetic and string comparisons

#!/bin/bash  a=4 b=5  #  Here "a" and "b" can be treated either as integers or strings. #  There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed.  #  Bash permits integer operations and comparisons on variables #+ whose value consists of all-integer characters. #  Caution advised, however.  echo  if [ "$a" -ne "$b" ] then   echo "$a is not equal to $b"   echo "(arithmetic comparison)" fi  echo  if [ "$a" != "$b" ] then   echo "$a is not equal to $b."   echo "(string comparison)"   #     "4"  != "5"   # ASCII 52 != ASCII 53 fi  # In this particular instance, both "-ne" and "!=" work.  echo  exit 0

Example 7-6. Testing whether a string is null

#!/bin/bash #  str-test.sh: Testing null strings and unquoted strings, #+ but not strings and sealing wax, not to mention cabbages and kings . . .  # Using   if [ ... ]  # If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero!).  if [ -n $string1 ]    # string1 has not been declared or initialized. then   echo "String \"string1\" is not null." else     echo "String \"string1\" is null." fi                    # Wrong result. # Shows $string1 as not null, although it was not initialized.  echo  # Let's try it again.  if [ -n "$string1" ]  # This time, $string1 is quoted. then   echo "String \"string1\" is not null." else     echo "String \"string1\" is null." fi                    # Quote strings within test brackets!  echo  if [ $string1 ]       # This time, $string1 stands naked. then   echo "String \"string1\" is not null." else     echo "String \"string1\" is null." fi                    # This works fine. # The [ ... ] test operator alone detects whether the string is null. # However it is good practice to quote it (if [ "$string1" ]). # # As Stephane Chazelas points out, #    if [ $string1 ]    has one argument, "]" #    if [ "$string1" ]  has two arguments, the empty "$string1" and "]"    echo   string1=initialized  if [ $string1 ]       # Again, $string1 stands unquoted. then   echo "String \"string1\" is not null." else     echo "String \"string1\" is null." fi                    # Again, gives correct result. # Still, it is better to quote it ("$string1"), because . . .   string1="a = b"  if [ $string1 ]       # Again, $string1 stands unquoted. then   echo "String \"string1\" is not null." else     echo "String \"string1\" is null." fi                    # Not quoting "$string1" now gives wrong result!  exit 0   # Thank you, also, Florian Wisser, for the "heads-up".

Example 7-7. zmore

#!/bin/bash # zmore  # View gzipped files with 'more' filter.  E_NOARGS=85 E_NOTFOUND=86 E_NOTGZIP=87  if [ $# -eq 0 ] # same effect as:  if [ -z "$1" ] # $1 can exist, but be empty:  zmore "" arg2 arg3 then   echo "Usage: `basename $0` filename" >&2   # Error message to stderr.   exit $E_NOARGS   # Returns 85 as exit status of script (error code). fi    filename=$1  if [ ! -f "$filename" ]   # Quoting $filename allows for possible spaces. then   echo "File $filename not found!" >&2   # Error message to stderr.   exit $E_NOTFOUND fi    if [ ${filename##*.} != "gz" ] # Using bracket in variable substitution. then   echo "File $1 is not a gzipped file!"   exit $E_NOTGZIP fi    zcat $1 | more  # Uses the 'more' filter. # May substitute 'less' if desired.  exit $?   # Script returns exit status of pipe. #  Actually "exit $?" is unnecessary, as the script will, in any case, #+ return the exit status of the last command executed.

compound comparison

-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 is true.

These are similar to the Bash comparison operators && and ||, used within double brackets.

[[ condition1 && condition2 ]]

The -o and -a operators work with the test command or occur within single test brackets.

if [ "$expr1" -a "$expr2" ] then   echo "Both expr1 and expr2 are true." else   echo "Either expr1 or expr2 is false." fi

But, as rihad points out:

[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ]   # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ]   # (no output) # ^^^^^^^ False condition. So far, everything as expected.  # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ]       # true # ^^^^^^^ False condition. So, why "true" output?  # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]]     # (no output) # No, that's not it.  # Apparently && and || "short-circuit" while -a and -o do not.

Refer to Example 8-3, Example 27-17, and Example A-29 to see compound comparison operators in action.

7.4. Nested `if/then` Condition Tests

Condition tests using the if/then construct may be nested. The net result is equivalent to using the && compound comparison operator.

a=3  if [ "$a" -gt 0 ] then   if [ "$a" -lt 5 ]   then     echo "The value of \"a\" lies somewhere between 0 and 5."   fi fi  # Same result as:  if [ "$a" -gt 0 ] && [ "$a" -lt 5 ] then   echo "The value of \"a\" lies somewhere between 0 and 5." fi

Example 37-4 and Example 17-11 demonstrate nested if/then condition tests.

7.5. Testing Your Knowledge of Tests

The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests. The following is excerpted from an "ancient" version of xinitrc (Red Hat 7.1, or thereabouts).

if [ -f $HOME/.Xclients ]; then   exec $HOME/.Xclients elif [ -f /etc/X11/xinit/Xclients ]; then   exec /etc/X11/xinit/Xclients else      # failsafe settings.  Although we should never get here      # (we provide fallbacks in Xclients as well) it can't hurt.      xclock -geometry 100x100-5+5 &      xterm -geometry 80x50-50+150 &      if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then              netscape /usr/share/doc/HTML/index.html &      fi fi

Explain the test constructs in the above snippet, then examine an updated version of the file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions.

Chapter 8. Operations and Related Topics

8.1. Operators

assignment

variable assignment

Initializing or changing the value of a variable

=

All-purpose assignment operator, which works for both arithmetic and string assignments.

var=27 category=minerals  # No spaces allowed after the "=".

Do not confuse the "=" assignment operator with the = test operator.

#   =  as a test operator  if [ "$string1" = "$string2" ] then    command fi  #  if [ "X$string1" = "X$string2" ] is safer, #+ to prevent an error message should one of the variables be empty. #  (The prepended "X" characters cancel out.)

arithmetic operators

+

plus

-

minus

*

multiplication

/

division

exponentiation

# Bash, version 2.02, introduced the "**" exponentiation operator.  let "z=5**3"    # 5 * 5 * 5 echo "z = $z"   # z = 125

%

modulo, or mod (returns the remainder of an integer division operation)

5/3 = 1, with remainder 2

This operator finds use in, among other things, generating numbers within a specific range (see Example 9-11 and Example 9-15) and formatting program output (see Example 27-16 and Example A-6). It can even be used to generate prime numbers, (see Example A-15). Modulo turns up surprisingly often in numerical recipes.

Example 8-1. Greatest common divisor

#!/bin/bash # gcd.sh: greatest common divisor #         Uses Euclid's algorithm  #  The "greatest common divisor" (gcd) of two integers #+ is the largest integer that will divide both, leaving no remainder.  #  Euclid's algorithm uses successive division. #    In each pass, #+      dividend <---  divisor #+      divisor  <---  remainder #+   until remainder = 0. #    The gcd = dividend, on the final pass. # #  For an excellent discussion of Euclid's algorithm, see #+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm.   # ------------------------------------------------------ # Argument check ARGS=2 E_BADARGS=85  if [ $# -ne "$ARGS" ] then   echo "Usage: `basename $0` first-number second-number"   exit $E_BADARGS fi # ------------------------------------------------------   gcd () {    dividend=$1             #  Arbitrary assignment.   divisor=$2              #! It doesn't matter which of the two is larger.                           #  Why not?    remainder=1             #  If an uninitialized variable is used inside                           #+ test brackets, an error message results.    until [ "$remainder" -eq 0 ]   do    #  ^^^^^^^^^^  Must be previously initialized!     let "remainder = $dividend % $divisor"     dividend=$divisor     # Now repeat with 2 smallest numbers.     divisor=$remainder   done                    # Euclid's algorithm  }                         # Last $dividend is the gcd.   gcd $1 $2  echo; echo "GCD of $1 and $2 = $dividend"; echo   # Exercises : # --------- # 1) Check command-line arguments to make sure they are integers, #+   and exit the script with an appropriate error message if not. # 2) Rewrite the gcd () function to use local variables.  exit 0

+=

plus-equal (increment variable by a constant) [38]

let "var += 5" results in var being incremented by 5.

-=

minus-equal (decrement variable by a constant)

*=

times-equal (multiply variable by a constant)

let "var *= 4" results in var being multiplied by 4.

/=

slash-equal (divide variable by a constant)

%=

mod-equal (remainder of dividing variable by a constant)

Arithmetic operators often occur in an expr or let expression.

Example 8-2. Using Arithmetic Operations

#!/bin/bash # Counting to 11 in 10 different ways.  n=1; echo -n "$n "  let "n = $n + 1"   # let "n = n + 1"  also works. echo -n "$n "   : $((n = $n + 1)) #  ":" necessary because otherwise Bash attempts #+ to interpret "$((n = $n + 1))" as a command. echo -n "$n "  (( n = n + 1 )) #  A simpler alternative to the method above. #  Thanks, David Lombard, for pointing this out. echo -n "$n "  n=$(($n + 1)) echo -n "$n "  : $[ n = $n + 1 ] #  ":" necessary because otherwise Bash attempts #+ to interpret "$[ n = $n + 1 ]" as a command. #  Works even if "n" was initialized as a string. echo -n "$n "  n=$[ $n + 1 ] #  Works even if "n" was initialized as a string. #* Avoid this type of construct, since it is obsolete and nonportable. #  Thanks, Stephane Chazelas. echo -n "$n "  # Now for C-style increment operators. # Thanks, Frank Wang, for pointing this out.  let "n++"          # let "++n"  also works. echo -n "$n "  (( n++ ))          # (( ++n ))  also works. echo -n "$n "  : $(( n++ ))       # : $(( ++n )) also works. echo -n "$n "  : $[ n++ ]         # : $[ ++n ] also works echo -n "$n "  echo  exit 0

Integer variables in older versions of Bash were signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that took a variable outside these limits gave an erroneous result.

echo $BASH_VERSION   # 1.14  a=2147483646 echo "a = $a"        # a = 2147483646 let "a+=1"           # Increment "a". echo "a = $a"        # a = 2147483647 let "a+=1"           # increment "a" again, past the limit. echo "a = $a"        # a = -2147483648                      #      ERROR: out of range,                      # +    and the leftmost bit, the sign bit,                      # +    has been set, making the result negative.

As of version >= 2.05b, Bash supports 64-bit integers.

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.

a=1.5  let "b = $a + 1.3"  # Error. # t2.sh: let: b = 1.5 + 1.3: syntax error in expression #                            (error token is ".5 + 1.3")  echo "b = $b"       # b=1

Use bc in scripts that that need floating point calculations or math library functions.

bitwise operators. The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to be manipulating and testing values read from ports or sockets. "Bit flipping" is more relevant to compiled languages, such as C and C++, which provide direct access to system hardware. However, see vladz's ingenious use of bitwise operators in his base64.sh (Example A-54) script.

bitwise operators

<<

bitwise left shift (multiplies by 2 for each shift position)

<<=

left-shift-equal

let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4)

>>

bitwise right shift (divides by 2 for each shift position)

>>=

right-shift-equal (inverse of <<=)

&

bitwise AND

&=

bitwise AND-equal

|

bitwise OR

|=

bitwise OR-equal

~

bitwise NOT

^

bitwise XOR

^=

bitwise XOR-equal

logical (boolean) operators

!

NOT

if [ ! -f $FILENAME ] then   ...

&&

AND

if [ $condition1 ] && [ $condition2 ] #  Same as:  if [ $condition1 -a $condition2 ] #  Returns true if both condition1 and condition2 hold true...  if [[ $condition1 && $condition2 ]]    # Also works. #  Note that && operator not permitted                            inside brackets                            #+ of [ ... ] construct.

&& may also be used, depending on context, in an and list to concatenate commands.

||

if [ $condition1 ] || [ $condition2 ] # Same as:  if [ $condition1 -o $condition2 ] # Returns true if either condition1 or condition2 holds true...  if [[ $condition1 || $condition2 ]]    # Also works. #  Note that || operator not permitted                            inside brackets                            #+ of a [ ... ] construct.

Bash tests the exit status of each statement linked with a logical operator.

Example 8-3. Compound Condition Tests Using && and ||

#!/bin/bash  a=24 b=47  if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] then   echo "Test #1 succeeds." else   echo "Test #1 fails." fi  # ERROR:   if [ "$a" -eq 24 && "$b" -eq 47 ] #+         attempts to execute  ' [ "$a" -eq 24 ' #+         and fails to finding matching ']'. # #  Note:  if [[ $a -eq 24 && $b -eq 24 ]]  works. #  The double-bracket if-test is more flexible #+ than the single-bracket version.        #    (The "&&" has a different meaning in line 17 than in line 6.) #    Thanks, Stephane Chazelas, for pointing this out.   if [ "$a" -eq 98 ] || [ "$b" -eq 47 ] then   echo "Test #2 succeeds." else   echo "Test #2 fails." fi   #  The -a and -o options provide #+ an alternative compound condition test. #  Thanks to Patrick Callahan for pointing this out.   if [ "$a" -eq 24 -a "$b" -eq 47 ] then   echo "Test #3 succeeds." else   echo "Test #3 fails." fi   if [ "$a" -eq 98 -o "$b" -eq 47 ] then   echo "Test #4 succeeds." else   echo "Test #4 fails." fi   a=rhino b=crocodile if [ "$a" = rhino ] && [ "$b" = crocodile ] then   echo "Test #5 succeeds." else   echo "Test #5 fails." fi  exit 0

The && and || operators also find use in an arithmetic context.

                            bash$                                                                                      echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))                                                        1 0 1 0

miscellaneous operators

,

Comma operator

The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects. [39]

let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1"           ^^^^^^  # t1 = 11 # Here t1 is set to the result of the last operation. Why?  let "t2 = ((a = 9, 15 / 3))"      # Set "a" and calculate "t2". echo "t2 = $t2    a = $a"         # t2 = 5    a = 9

The comma operator finds use mainly in for loops. See Example 11-13.

8.2. Numerical Constants

A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions).

Example 8-4. Representation of numerical constants

#!/bin/bash # numbers.sh: Representation of numbers in different bases.  # Decimal: the default let "dec = 32" echo "decimal number = $dec"             # 32 # Nothing out of the ordinary here.   # Octal: numbers preceded by '0' (zero) let "oct = 032" echo "octal number = $oct"               # 26 # Expresses result in decimal. # --------- ------ -- -------   # Hexadecimal: numbers preceded by '0x' or '0X' let "hex = 0x32" echo "hexadecimal number = $hex"         # 50  echo $((0x9abc))                         # 39612 #     ^^      ^^   double-parentheses arithmetic expansion/evaluation # Expresses result in decimal.    # Other bases: BASE#NUMBER # BASE between 2 and 64. # NUMBER must use symbols within the BASE range, see below.   let "bin = 2#111100111001101" echo "binary number = $bin"              # 31181  let "b32 = 32#77" echo "base-32 number = $b32"             # 231  let "b64 = 64#@_" echo "base-64 number = $b64"             # 4031 # This notation only works for a limited range (2 - 64) of ASCII characters. # 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _   echo  echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))                                          # 1295 170 44822 3375   #  Important note: #  -------------- #  Using a digit out of range of the specified base notation #+ gives an error message.  let "bad_oct = 081" # (Partial) error message output: #  bad_oct = 081: value too great for base (error token is "081") #              Octal numbers use only digits in the range 0 - 7.  exit $?   # Exit value = 1 (error)  # Thanks, Rich Bartell and Stephane Chazelas, for clarification.

8.3. The Double-Parentheses Construct

Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3 , or 8 . However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ )).

Example 8-5. C-style manipulation of variables

#!/bin/bash # c-vars.sh # Manipulating a variable, C-style, using the (( ... )) construct.   echo  (( a = 23 ))  #  Setting a value, C-style,               #+ with spaces on both sides of the "=". echo "a (initial value) = $a"   # 23  (( a++ ))     #  Post-increment 'a', C-style. echo "a (after a++) = $a"       # 24  (( a-- ))     #  Post-decrement 'a', C-style. echo "a (after a--) = $a"       # 23   (( ++a ))     #  Pre-increment 'a', C-style. echo "a (after ++a) = $a"       # 24  (( --a ))     #  Pre-decrement 'a', C-style. echo "a (after --a) = $a"       # 23  echo  ######################################################## #  Note that, as in C, pre- and post-decrement operators #+ have different side-effects.  n=1; let --n && echo "True" || echo "False"  # False n=1; let n-- && echo "True" || echo "False"  # True  #  Thanks, Jeroen Domburg. ########################################################  echo  (( t = a<45?7:11 ))   # C-style trinary operator. #       ^  ^ ^ echo "If a < 45, then t = 7, else t = 11."  # a = 23 echo "t = $t "                              # t = 7  echo   # ----------------- # Easter Egg alert! # ----------------- #  Chet Ramey seems to have snuck a bunch of undocumented C-style #+ constructs into Bash (actually adapted from ksh, pretty much). #  In the Bash docs, Ramey calls (( ... )) shell arithmetic, #+ but it goes far beyond that. #  Sorry, Chet, the secret is out.  # See also "for" and "while" loops using the (( ... )) construct.  # These work only with version 2.04 or later of Bash.  exit

8.4. Operator Precedence

In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones. [40]

Table 8-1. Operator Precedence

Operator	Meaning	Comments
		HIGHEST PRECEDENCE
`var++ var--`	post-increment, post-decrement	C-style operators
`++var --var`	pre-increment, pre-decrement

`! ~`	negation	logical / bitwise, inverts sense of following operator

`**`	exponentiation	arithmetic operation
`* / %`	multiplication, division, modulo	arithmetic operation
`+ -`	addition, subtraction	arithmetic operation

`<< >>`	left, right shift	bitwise

`-z -n`	unary comparison	string is/is-not null
`-e -f -t -x, etc.`	unary comparison	file-test
`< -lt > -gt <= -le >= -ge`	compound comparison	string and integer
`-nt -ot -ef`	compound comparison	file-test
`== -eq != -ne`	equality / inequality	test operators, string and integer

`&`	AND	bitwise
`^`	XOR	exclusive OR, bitwise
`\|`	OR	bitwise

`&& -a`	AND	logical, compound comparison
`\|\| -o`	OR	logical, compound comparison

`?:`	trinary operator	C-style
`=`	assignment	(do not confuse with equality test)
`*= /= %= += -= <<= >>= &=`	combination assignment	times-equal, divide-equal, mod-equal, etc.

`,`	comma	links a sequence of operations
		LOWEST PRECEDENCE

In practice, all you really need to remember is the following:

The "My Dear Aunt Sally" mantra (multiply, divide, add, subtract) for the familiar arithmetic operations.
The compound logical operators, &&, ||, -a, and -o have low precedence.
The order of evaluation of equal-precedence operators is usually left-to-right.

Now, let's utilize our knowledge of operator precedence to analyze a couple of lines from the /etc/init.d/functions file, as found in the Fedora Core Linux distro.

while [ -n "$remaining" -a "$retry" -gt 0 ]; do  # This looks rather daunting at first glance.   # Separate the conditions: while [ -n "$remaining" -a "$retry" -gt 0 ]; do #       --condition 1-- ^^ --condition 2-  #  If variable "$remaining" is not zero length #+      AND (-a) #+ variable "$retry" is greater-than zero #+ then #+ the [ expresion-within-condition-brackets ] returns success (0) #+ and the while-loop executes an iteration. #  ============================================================== #  Evaluate "condition 1" and "condition 2" ***before*** #+ ANDing them. Why? Because the AND (-a) has a lower precedence #+ than the -n and -gt operators, #+ and therefore gets evaluated *last*.  #################################################################  if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then   # Again, separate the conditions: if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then #    --condition 1--------- ^^ --condition 2-----  #  If file "/etc/sysconfig/i18n" exists #+      AND (-a) #+ variable $NOLOCALE is zero length #+ then #+ the [ test-expresion-within-condition-brackets ] returns success (0) #+ and the commands following execute. # #  As before, the AND (-a) gets evaluated *last* #+ because it has the lowest precedence of the operators within #+ the test brackets. #  ============================================================== #  Note: #  ${NOLOCALE:-} is a parameter expansion that seems redundant. #  But, if $NOLOCALE has not been declared, it gets set to *null*, #+ in effect declaring it. #  This makes a difference in some contexts.

To avoid confusion or error in a complex sequence of test operators, break up the sequence into bracketed sections.

if [ "$v1" -gt "$v2"  -o  "$v1" -lt "$v2"  -a  -e "$filename" ] # Unclear what's going on here...  if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] && [[ -e "$filename" ]] # Much better -- the condition tests are grouped in logical sections.

Part 3. Beyond the Basics

Chapter 9. Another Look at Variables

Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and nuances.

9.1. Internal Variables

Builtin variables:

variables affecting bash script behavior

$BASH

The path to the Bash binary itself

                            bash$                                                                                      echo $BASH                                                        /bin/bash

$BASH_ENV

An environmental variable pointing to a Bash startup file to be read when a script is invoked

$BASH_SUBSHELL

A variable indicating the subshell level. This is a new addition to Bash, version 3.

See Example 21-1 for usage.

$BASHPID

Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same result.

                            bash4$                                                                                      echo $$                                                        11015                            bash4$                                                                                      echo $BASHPID                                                        11015                            bash4$                                                                                      ps ax | grep bash4                                                        11015 pts/2    R      0:00 bash4

But ...

#!/bin/bash4  echo "\$\$ outside of subshell = $$"                              # 9602 echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0 echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602  echo  ( echo "\$\$ inside of subshell = $$"                             # 9602   echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1   echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603   # Note that $$ returns PID of parent process.

$BASH_VERSINFO[n]

A 6-element array containing version information about the installed release of Bash. This is similar to $BASH_VERSION, below, but a bit more detailed.

# Bash version info:  for n in 0 1 2 3 4 5 do   echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}" done    # BASH_VERSINFO[0] = 3                      # Major version no. # BASH_VERSINFO[1] = 00                     # Minor version no. # BASH_VERSINFO[2] = 14                     # Patch level. # BASH_VERSINFO[3] = 1                      # Build version. # BASH_VERSINFO[4] = release                # Release status. # BASH_VERSINFO[5] = i386-redhat-linux-gnu  # Architecture                                             # (same as $MACHTYPE).

$BASH_VERSION

The version of Bash installed on the system

                            bash$                                                                                      echo $BASH_VERSION                                                        3.2.25(1)-release

                            tcsh%                                                                                      echo $BASH_VERSION                                                        BASH_VERSION: Undefined variable.

Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer.

$CDPATH

A colon-separated list of search paths available to the cd command, similar in function to the $PATH variable for binaries. The $CDPATH variable may be set in the local ~/.bashrc file.

                            bash$                                                                                      cd bash-doc                                                        bash: cd: bash-doc: No such file or directory                            bash$                                                                                      CDPATH=/usr/share/doc                                                        bash$                                                                                      cd bash-doc                                                        /usr/share/doc/bash-doc                            bash$                                                                                      echo $PWD                                                        /usr/share/doc/bash-doc

$DIRSTACK

The top value in the directory stack [41] (affected by pushd and popd)

This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the directory stack.

$EDITOR

The default editor invoked by a script, usually vi or emacs.

$EUID

"effective" user ID number

Identification number of whatever identity the current user has assumed, perhaps by means of su.

The $EUID is not necessarily the same as the $UID.

$FUNCNAME

Name of the current function

xyz23 () {   echo "$FUNCNAME now executing."  # xyz23 now executing. }  xyz23  echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =                                    # Null value outside a function.

9.2. Typing variables: declare or typeset

The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing [46] available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.

declare/typeset options

-r readonly

( declare -r var1 works the same as readonly var1 )

This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message.

declare -r var1=1 echo "var1 = $var1"   # var1 = 1  (( var1++ ))          # x.sh: line 4: var1: readonly variable

-i integer

declare -i number # The script will treat subsequent occurrences of "number" as an integer.		  number=3 echo "Number = $number"     # Number = 3  number=three echo "Number = $number"     # Number = 0 # Tries to evaluate the string "three" as an integer.

Certain arithmetic operations are permitted for declared integer variables without the need for expr or let.

n=6/3 echo "n = $n"       # n = 6/3  declare -i n n=6/3 echo "n = $n"       # n = 2

-a array

The variable indices will be treated as an array.

-f function(s)

A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script.

A declare -f function_name in a script lists just the function named.

-x export

This declares a variable as available for exporting outside the environment of the script itself.

-x var=$value

The declare command permits assigning a value to a variable in the same statement as setting its properties.

Example 9-10. Using declare to type variables

#!/bin/bash  func1 () {   echo This is a function. }  declare -f        # Lists the function above.  echo  declare -i var1   # var1 is an integer. var1=2367 echo "var1 declared as $var1" var1=var1+1       # Integer declaration eliminates the need for 'let'. echo "var1 incremented by 1 is $var1." # Attempt to change variable declared as integer. echo "Attempting to change var1 to floating point value, 2367.1." var1=2367.1       # Results in error message, with no change to variable. echo "var1 is still $var1"  echo  declare -r var2=13.36         # 'declare' permits setting a variable property                               #+ and simultaneously assigning it a value. echo "var2 declared as $var2" # Attempt to change readonly variable. var2=13.37                    # Generates error message, and exit from script.  echo "var2 is still $var2"    # This line will not execute.  exit 0                        # Script will not exit here.

Using the declare builtin restricts the scope of a variable.

foo () { FOO="bar" }  bar () { foo echo $FOO }  bar   # Prints bar.

However . . .

foo (){ declare FOO="bar" }  bar () { foo echo $FOO }  bar  # Prints nothing.   # Thank you, Michael Iatrou, for pointing this out.

9.2.1. Another use for declare

The declare command can be helpful in identifying variables, environmental or otherwise. This can be especially useful with arrays.

                        bash$                                                                          declare | grep HOME                                                HOME=/home/bozo                        bash$                                                                          zzy=68                                                bash$                                                                          declare | grep zzy                                                zzy=68                        bash$                                                                          Colors=([0]="purple" [1]="reddish-orange" [2]="light green")                                                bash$                                                                          echo ${Colors[@]}                                                purple reddish-orange light green                        bash$                                                                          declare | grep Colors                                                Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

9.3. $RANDOM: generate random integer

Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.

--John von Neumann

$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom [47] integer in the range 0 - 32767. It should not be used to generate an encryption key.

Example 9-11. Generating random numbers

#!/bin/bash  # $RANDOM returns a different random integer at each invocation. # Nominal range: 0 - 32767 (signed 16-bit integer).  MAXCOUNT=10 count=1  echo echo "$MAXCOUNT random numbers:" echo "-----------------" while [ "$count" -le $MAXCOUNT ]      # Generate 10 ($MAXCOUNT) random integers. do   number=$RANDOM   echo $number   let "count += 1"  # Increment count. done echo "-----------------"  # If you need a random int within a certain range, use the 'modulo' operator. # This returns the remainder of a division operation.  RANGE=500  echo  number=$RANDOM let "number %= $RANGE" #           ^^ echo "Random number less than $RANGE  ---  $number"  echo    #  If you need a random integer greater than a lower bound, #+ then set up a test to discard all numbers below that.  FLOOR=200  number=0   #initialize while [ "$number" -le $FLOOR ] do   number=$RANDOM done echo "Random number greater than $FLOOR ---  $number" echo     # Let's examine a simple alternative to the above loop, namely    #       let "number = $RANDOM + $FLOOR"    # That would eliminate the while-loop and run faster.    # But, there might be a problem with that. What is it?    # Combine above two techniques to retrieve random number between two limits. number=0   #initialize while [ "$number" -le $FLOOR ] do   number=$RANDOM   let "number %= $RANGE"  # Scales $number down within $RANGE. done echo "Random number between $FLOOR and $RANGE ---  $number" echo    # Generate binary choice, that is, "true" or "false" value. BINARY=2 T=1 number=$RANDOM  let "number %= $BINARY" #  Note that    let "number >>= 14"    gives a better random distribution #+ (right shifts out everything except last binary digit). if [ "$number" -eq $T ] then   echo "TRUE" else   echo "FALSE" fi    echo   # Generate a toss of the dice. SPOTS=6   # Modulo 6 gives range 0 - 5.           # Incrementing by 1 gives desired range of 1 - 6.           # Thanks, Paulo Marcel Coelho Aragao, for the simplification. die1=0 die2=0 # Would it be better to just set SPOTS=7 and not add 1? Why or why not?  # Tosses each die separately, and so gives correct odds.      let "die1 = $RANDOM % $SPOTS +1" # Roll first one.     let "die2 = $RANDOM % $SPOTS +1" # Roll second one.     #  Which arithmetic operation, above, has greater precedence --     #+ modulo (%) or addition (+)?   let "throw = $die1 + $die2" echo "Throw of the dice = $throw" echo   exit 0

Example 9-12. Picking a random card from a deck

#!/bin/bash # pick-card.sh  # This is an example of choosing random elements of an array.   # Pick a card, any card.  Suites="Clubs Diamonds Hearts Spades"  Denominations="2 3 4 5 6 7 8 9 10 Jack Queen King Ace"  # Note variables spread over multiple lines.   suite=($Suites)                # Read into array variable. denomination=($Denominations)  num_suites=${#suite[*]}        # Count how many elements. num_denominations=${#denomination[*]}  echo -n "${denomination[$((RANDOM%num_denominations))]} of " echo ${suite[$((RANDOM%num_suites))]}   # $bozo sh pick-cards.sh # Jack of Clubs   # Thank you, "jipe," for pointing out this use of $RANDOM. exit 0

Example 9-13. Brownian Motion Simulation

#!/bin/bash # brownian.sh # Author: Mendel Cooper # Reldate: 10/26/07 # License: GPL3  #  ---------------------------------------------------------------- #  This script models Brownian motion: #+ the random wanderings of tiny particles in a fluid, #+ as they are buffeted by random currents and collisions. #+ This is colloquially known as the "Drunkard's Walk."  #  It can also be considered as a stripped-down simulation of a #+ Galton Board, a slanted board with a pattern of pegs, #+ down which rolls a succession of marbles, one at a time. #+ At the bottom is a row of slots or catch basins in which #+ the marbles come to rest at the end of their journey. #  Think of it as a kind of bare-bones Pachinko game. #  As you see by running the script, #+ most of the marbles cluster around the center slot. #+ This is consistent with the expected binomial distribution. #  As a Galton Board simulation, the script #+ disregards such parameters as #+ board tilt-angle, rolling friction of the marbles, #+ angles of impact, and elasticity of the pegs. #  To what extent does this affect the accuracy of the simulation? #  ----------------------------------------------------------------  PASSES=500            #  Number of particle interactions / marbles. ROWS=10               #  Number of "collisions" (or horiz. peg rows). RANGE=3               #  0 - 2 output range from $RANDOM. POS=0                 #  Left/right position. RANDOM=$$             #  Seeds the random number generator from PID                       #+ of script.  declare -a Slots      # Array holding cumulative results of passes. NUMSLOTS=21           # Number of slots at bottom of board.   Initialize_Slots () { # Zero out all elements of the array. for i in $( seq $NUMSLOTS ) do   Slots[$i]=0 done  echo                  # Blank line at beginning of run.   }   Show_Slots () { echo; echo echo -n " " for i in $( seq $NUMSLOTS )   # Pretty-print array elements. do   printf "%3d" ${Slots[$i]}   # Allot three spaces per result. done  echo # Row of slots: echo " |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|" echo "                                ||" echo #  Note that if the count within any particular slot exceeds 99,      #+ it messes up the display.      #  Running only(!) 500 passes usually avoids this.   }   Move () {              # Move one unit right / left, or stay put.   Move=$RANDOM         # How random is $RANDOM? Well, let's see ...   let "Move %= RANGE"  # Normalize into range of 0 - 2.   case "$Move" in     0 ) ;;                   # Do nothing, i.e., stay in place.     1 ) ((POS--));;          # Left.     2 ) ((POS++));;          # Right.     * ) echo -n "Error ";;   # Anomaly! (Should never occur.)   esac   }   Play () {                    # Single pass (inner loop). i=0 while [ "$i" -lt "$ROWS" ]   # One event per row. do   Move   ((i++)); done  SHIFT=11                     # Why 11, and not 10? let "POS += $SHIFT"          # Shift "zero position" to center. (( Slots[$POS]++ ))          # DEBUG: echo $POS  # echo -n "$POS "    }   Run () {                     # Outer loop. p=0 while [ "$p" -lt "$PASSES" ] do   Play   (( p++ ))   POS=0                      # Reset to zero. Why? done   }   # -------------- # main () Initialize_Slots Run Show_Slots # --------------  exit $?  #  Exercises: #  --------- #  1) Show the results in a vertical bar graph, or as an alternative, #+    a scattergram. #  2) Alter the script to use /dev/urandom instead of $RANDOM. #     Will this make the results more random? #  3) Provide some sort of "animation" or graphic output #     for each marble played.

Jipe points out a set of techniques for generating random numbers within a range.

#  Generate random number between 6 and 30.    rnumber=$((RANDOM%25+6))	  #  Generate random number in the same 6 - 30 range, #+ but the number must be evenly divisible by 3.    rnumber=$(((RANDOM%30/3+1)*3))  #  Note that this will not work all the time. #  It fails if $RANDOM%30 returns 0.  #  Frank Wang suggests the following alternative:    rnumber=$(( RANDOM%27/3*3+6 ))

Bill Gradwohl came up with an improved formula that works for positive numbers.

rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))

Here Bill presents a versatile function that returns a random number between two specified values.

Example 9-14. Random between values

#!/bin/bash # random-between.sh # Random number between two specified values.  # Script by Bill Gradwohl, with minor modifications by the document author. # Corrections in lines 187 and 189 by Anthony Le Clezio. # Used with permission.   randomBetween() {    #  Generates a positive or negative random number    #+ between $min and $max    #+ and divisible by $divisibleBy.    #  Gives a "reasonably random" distribution of return values.    #    #  Bill Gradwohl - Oct 1, 2003     syntax() {    # Function embedded within function.       echo       echo    "Syntax: randomBetween [min] [max] [multiple]"       echo       echo -n "Expects up to 3 passed parameters, "       echo    "but all are completely optional."       echo    "min is the minimum value"       echo    "max is the maximum value"       echo -n "multiple specifies that the answer must be "       echo     "a multiple of this value."       echo    "    i.e. answer must be evenly divisible by this number."       echo           echo    "If any value is missing, defaults area supplied as: 0 32767 1"       echo -n "Successful completion returns 0, "       echo     "unsuccessful completion returns"       echo    "function syntax and 1."       echo -n "The answer is returned in the global variable "       echo    "randomBetweenAnswer"       echo -n "Negative values for any passed parameter are "       echo    "handled correctly."    }     local min=${1:-0}    local max=${2:-32767}    local divisibleBy=${3:-1}    # Default values assigned, in case parameters not passed to function.     local x    local spread     # Let's make sure the divisibleBy value is positive.    [ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy))     # Sanity check.    if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o  ${min} -eq ${max} ]; then        syntax       return 1    fi     # See if the min and max are reversed.    if [ ${min} -gt ${max} ]; then       # Swap them.       x=${min}       min=${max}       max=${x}    fi     #  If min is itself not evenly divisible by $divisibleBy,    #+ then fix the min to be within range.    if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then        if [ ${min} -lt 0 ]; then          min=$((min/divisibleBy*divisibleBy))       else          min=$((((min/divisibleBy)+1)*divisibleBy))       fi    fi     #  If max is itself not evenly divisible by $divisibleBy,    #+ then fix the max to be within range.    if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then        if [ ${max} -lt 0 ]; then          max=$((((max/divisibleBy)-1)*divisibleBy))       else          max=$((max/divisibleBy*divisibleBy))       fi    fi     #  ---------------------------------------------------------------------    #  Now, to do the real work.     #  Note that to get a proper distribution for the end points,    #+ the range of random values has to be allowed to go between    #+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1.     #  The slight increase will produce the proper distribution for the    #+ end points.     #  Changing the formula to use abs(max-min)+1 will still produce    #+ correct answers, but the randomness of those answers is faulty in    #+ that the number of times the end points ($min and $max) are returned    #+ is considerably lower than when the correct formula is used.    #  ---------------------------------------------------------------------     spread=$((max-min))    #  Omair Eshkenazi points out that this test is unnecessary,    #+ since max and min have already been switched around.    [ ${spread} -lt 0 ] && spread=$((0-spread))    let spread+=divisibleBy    randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min))        return 0     #  However, Paulo Marcel Coelho Aragao points out that    #+ when $max and $min are not divisible by $divisibleBy,    #+ the formula fails.    #    #  He suggests instead the following formula:    #    rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy))  }  # Let's test the function. min=-14 max=20 divisibleBy=3   #  Generate an array of expected answers and check to make sure we get #+ at least one of each answer if we loop long enough.  declare -a answer minimum=${min} maximum=${max}    if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then        if [ ${minimum} -lt 0 ]; then          minimum=$((minimum/divisibleBy*divisibleBy))       else          minimum=$((((minimum/divisibleBy)+1)*divisibleBy))       fi    fi      #  If max is itself not evenly divisible by $divisibleBy,    #+ then fix the max to be within range.     if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then        if [ ${maximum} -lt 0 ]; then          maximum=$((((maximum/divisibleBy)-1)*divisibleBy))       else          maximum=$((maximum/divisibleBy*divisibleBy))       fi    fi   #  We need to generate only positive array subscripts, #+ so we need a displacement that that will guarantee #+ positive results.  disp=$((0-minimum)) for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do    answer[i+disp]=0 done   # Now loop a large number of times to see what we get. loopIt=1000   #  The script author suggests 100000,               #+ but that takes a good long while.  for ((i=0; i<${loopIt}; ++i)); do     #  Note that we are specifying min and max in reversed order here to    #+ make the function correct for this case.     randomBetween ${max} ${min} ${divisibleBy}     # Report an error if an answer is unexpected.    [ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ] \    && echo MIN or MAX error - ${randomBetweenAnswer}!    [ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] \    && echo DIVISIBLE BY error - ${randomBetweenAnswer}!     # Store the answer away statistically.    answer[randomBetweenAnswer+disp]=$((answer[randomBetweenAnswer+disp]+1)) done    # Let's check the results  for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do    [ ${answer[i+disp]} -eq 0 ] \    && echo "We never got an answer of $i." \    || echo "${i} occurred ${answer[i+disp]} times." done   exit 0

Just how random is $RANDOM? The best way to test this is to write a script that tracks the distribution of "random" numbers generated by $RANDOM. Let's roll a $RANDOM die a few times . . .

Example 9-15. Rolling a single die with RANDOM

#!/bin/bash # How random is RANDOM?  RANDOM=$$       # Reseed the random number generator using script process ID.  PIPS=6          # A die has 6 pips. MAXTHROWS=600   # Increase this if you have nothing better to do with your time. throw=0         # Number of times the dice have been cast.  ones=0          #  Must initialize counts to zero, twos=0          #+ since an uninitialized variable is null, NOT zero. threes=0 fours=0 fives=0 sixes=0  print_result () { echo echo "ones =   $ones" echo "twos =   $twos" echo "threes = $threes" echo "fours =  $fours" echo "fives =  $fives" echo "sixes =  $sixes" echo }  update_count() { case "$1" in   0) ((ones++));;   # Since a die has no "zero", this corresponds to 1.   1) ((twos++));;   # And this to 2.   2) ((threes++));; # And so forth.   3) ((fours++));;   4) ((fives++));;   5) ((sixes++));; esac }  echo   while [ "$throw" -lt "$MAXTHROWS" ] do   let "die1 = RANDOM % $PIPS"   update_count $die1   let "throw += 1" done    print_result  exit $?  #  The scores should distribute evenly, assuming RANDOM is random. #  With $MAXTHROWS at 600, all should cluster around 100, #+ plus-or-minus 20 or so. # #  Keep in mind that RANDOM is a ***pseudorandom*** generator, #+ and not a spectacularly good one at that.  #  Randomness is a deep and complex subject. #  Sufficiently long "random" sequences may exhibit #+ chaotic and other "non-random" behavior.  # Exercise (easy): # --------------- # Rewrite this script to flip a coin 1000 times. # Choices are "HEADS" and "TAILS."

As we have seen in the last example, it is best to reseed the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. [48] (This mirrors the behavior of the random() function in C.)

Example 9-16. Reseeding RANDOM

#!/bin/bash # seeding-random.sh: Seeding the RANDOM variable. # v 1.1, reldate 09 Feb 2013  MAXCOUNT=25       # How many numbers to generate. SEED=  random_numbers () { local count=0 local number  while [ "$count" -lt "$MAXCOUNT" ] do   number=$RANDOM   echo -n "$number "   let "count++" done   }  echo; echo  SEED=1 RANDOM=$SEED      # Setting RANDOM seeds the random number generator. echo "Random seed = $SEED" random_numbers   RANDOM=$SEED      # Same seed for RANDOM . . . echo; echo "Again, with same random seed ..." echo "Random seed = $SEED" random_numbers    # . . . reproduces the exact same number series.                   #                   # When is it useful to duplicate a "random" series?  echo; echo  SEED=2 RANDOM=$SEED      # Trying again, but with a different seed . . . echo "Random seed = $SEED" random_numbers    # . . . gives a different number series.  echo; echo  # RANDOM=$$  seeds RANDOM from process id of script. # It is also possible to seed RANDOM from 'time' or 'date' commands.  # Getting fancy... SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }'| sed s/^0*//) #  Pseudo-random output fetched #+ from /dev/urandom (system pseudo-random device-file), #+ then converted to line of printable (octal) numbers by "od", #+ then "awk" retrieves just one number for SEED, #+ finally "sed" removes any leading zeros. RANDOM=$SEED echo "Random seed = $SEED" random_numbers  echo; echo  exit 0

The /dev/urandom pseudo-device file provides a method of generating much more "random" pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example, Example 16-14, and Example A-36), or even piping to md5sum (see Example 36-16).

There are also other ways to generate pseudorandom numbers in a script. Awk provides a convenient means of doing this.

Example 9-17. Pseudorandom numbers, using awk

#!/bin/bash #  random2.sh: Returns a pseudorandom number in the range 0 - 1, #+ to 6 decimal places. For example: 0.822725 #  Uses the awk rand() function.  AWKSCRIPT=' { srand(); print rand() } ' #           Command(s)/parameters passed to awk # Note that srand() reseeds awk's random number generator.   echo -n "Random number between 0 and 1 = "  echo | awk "$AWKSCRIPT" # What happens if you leave out the 'echo'?  exit 0   # Exercises: # ---------  # 1) Using a loop construct, print out 10 different random numbers. #      (Hint: you must reseed the srand() function with a different seed #+     in each pass through the loop. What happens if you omit this?)  # 2) Using an integer multiplier as a scaling factor, generate random numbers  #+   in the range of 10 to 100.  # 3) Same as exercise #2, above, but generate random integers this time.

The date command also lends itself to generating pseudorandom integer sequences.

Chapter 10. Manipulating Variables

10.1. Manipulating Strings

Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion.

String Length

${#string}

expr length $string

These are the equivalent of strlen() in C.

expr "$string" : '.*'

stringZ=abcABC123ABCabc  echo ${#stringZ}                 # 15 echo `expr length $stringZ`      # 15 echo `expr "$stringZ" : '.*'`    # 15

Example 10-1. Inserting a blank line between paragraphs in a text file

#!/bin/bash # paragraph-space.sh # Ver. 2.1, Reldate 29Jul12 [fixup]  # Inserts a blank line between paragraphs of a single-spaced text file. # Usage: $0 <FILENAME  MINLEN=60        # Change this value? It's a judgment call. #  Assume lines shorter than $MINLEN characters ending in a period #+ terminate a paragraph. See exercises below.  while read line  # For as many lines as the input file has ... do   echo "$line"   # Output the line itself.    len=${#line}   if [[ "$len" -lt "$MINLEN" && "$line" =~ [*{\.}]$ ]] # if [[ "$len" -lt "$MINLEN" && "$line" =~ \[*\.\] ]] # An update to Bash broke the previous version of this script. Ouch! # Thank you, Halim Srama, for pointing this out and suggesting a fix.     then echo    #  Add a blank line immediately   fi             #+ after a short line terminated by a period. done  exit  # Exercises: # --------- #  1) The script usually inserts a blank line at the end #+    of the target file. Fix this. #  2) Line 17 only considers periods as sentence terminators. #     Modify this to include other common end-of-sentence characters, #+    such as ?, !, and ".

Length of Matching Substring at Beginning of String

expr match "$string" '$substring'

$substring is a regular expression.

expr "$string" : '$substring'

$substring is a regular expression.

stringZ=abcABC123ABCabc #       |------| #       12345678  echo `expr match "$stringZ" 'abc[A-Z]*.2'`   # 8 echo `expr "$stringZ" : 'abc[A-Z]*.2'`       # 8

Index

expr index $string $substring

Numerical position in $string of first character in $substring that matches.

stringZ=abcABC123ABCabc #       123456 ... echo `expr index "$stringZ" C12`             # 6                                              # C position.  echo `expr index "$stringZ" 1c`              # 3 # 'c' (in #3 position) matches before '1'.

This is the near equivalent of strchr() in C.

Substring Extraction

${string:position}

Extracts substring from $string at $position .

If the $string parameter is "*" or "@", then this extracts the positional parameters, [49] starting at $position.

${string:position:length}

Extracts $length characters of substring from $string at $position .

stringZ=abcABC123ABCabc #       0123456789..... #       0-based indexing.  echo ${stringZ:0}                            # abcABC123ABCabc echo ${stringZ:1}                            # bcABC123ABCabc echo ${stringZ:7}                            # 23ABCabc  echo ${stringZ:7:3}                          # 23A                                              # Three characters of substring.    # Is it possible to index from the right end of the string?      echo ${stringZ:-4}                           # abcABC123ABCabc # Defaults to full string, as in ${parameter:-default}. # However . . .  echo ${stringZ:(-4)}                         # Cabc  echo ${stringZ: -4}                          # Cabc # Now, it works. # Parentheses or added space "escape" the position parameter.  # Thank you, Dan Jacobson, for pointing this out.

The position and length arguments can be "parameterized," that is, represented as a variable, rather than as a numerical constant.

Example 10-2. Generating an 8-character "random" string

#!/bin/bash # rand-string.sh # Generating an 8-character "random" string.  if [ -n "$1" ]  #  If command-line argument present, then            #+ then set start-string to it.   str0="$1" else            #  Else use PID of script as start-string.   str0="$$" fi  POS=2  # Starting from position 2 in the string. LEN=8  # Extract eight characters.  str1=$( echo "$str0" | md5sum | md5sum ) #  Doubly scramble     ^^^^^^   ^^^^^^ #+ by piping and repiping to md5sum.  randstring="${str1:$POS:$LEN}" # Can parameterize ^^^^ ^^^^  echo "$randstring"  exit $?  # bozo$ ./rand-string.sh my-password # 1bdd88c4  #  No, this is is not recommended #+ as a method of generating hack-proof passwords.

If the $string parameter is "*" or "@", then this extracts a maximum of $length positional parameters, starting at $position.

echo ${*:2}          # Echoes second and following positional parameters. echo ${@:2}          # Same as above.  echo ${*:2:3}        # Echoes three positional parameters, starting at second.

expr substr $string $position $length

Extracts $length characters from $string starting at $position .

stringZ=abcABC123ABCabc #       123456789...... #       1-based indexing.  echo `expr substr $stringZ 1 2`              # ab echo `expr substr $stringZ 4 3`              # ABC

expr match "$string" '$$substring$'

Extracts $substring at beginning of $string , where $substring is a regular expression.

expr "$string" : '$$substring$'

Extracts $substring at beginning of $string , where $substring is a regular expression.

stringZ=abcABC123ABCabc #       =======	      echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'`   # abcABC1 echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'`       # abcABC1 echo `expr "$stringZ" : '\(.......\)'`                   # abcABC1 # All of the above forms give an identical result.

expr match "$string" '.*$$substring$'

Extracts $substring at end of $string , where $substring is a regular expression.

expr "$string" : '.*$$substring$'

Extracts $substring at end of $string , where $substring is a regular expression.

stringZ=abcABC123ABCabc #                ======  echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'`    # ABCabc echo `expr "$stringZ" : '.*\(......\)'`                       # ABCabc

Substring Removal

${string#substring}

Deletes shortest match of $substring from front of $string .

${string##substring}

Deletes longest match of $substring from front of $string .

stringZ=abcABC123ABCabc #       |----|          shortest #       |----------|    longest  echo ${stringZ#a*C}      # 123ABCabc # Strip out shortest match between 'a' and 'C'.  echo ${stringZ##a*C}     # abc # Strip out longest match between 'a' and 'C'.    # You can parameterize the substrings.  X='a*C'  echo ${stringZ#$X}      # 123ABCabc echo ${stringZ##$X}     # abc                         # As above.

${string%substring}

Deletes shortest match of $substring from back of $string .

For example:

# Rename all filenames in $PWD with "TXT" suffix to a "txt" suffix. # For example, "file1.TXT" becomes "file1.txt" . . .  SUFF=TXT suff=txt  for i in $(ls *.$SUFF) do   mv -f $i ${i%.$SUFF}.$suff   #  Leave unchanged everything *except* the shortest pattern match   #+ starting from the right-hand-side of the variable $i . . . done ### This could be condensed into a "one-liner" if desired.  # Thank you, Rory Winston.

${string%%substring}

Deletes longest match of $substring from back of $string .

stringZ=abcABC123ABCabc #                    ||     shortest #        |------------|     longest  echo ${stringZ%b*c}      # abcABC123ABCa # Strip out shortest match between 'b' and 'c', from back of $stringZ.  echo ${stringZ%%b*c}     # a # Strip out longest match between 'b' and 'c', from back of $stringZ.

This operator is useful for generating filenames.

Example 10-3. Converting graphic file formats, with filename change

#!/bin/bash #  cvt.sh: #  Converts all the MacPaint image files in a directory to "pbm" format.  #  Uses the "macptopbm" binary from the "netpbm" package, #+ which is maintained by Brian Henderson (bryanh@giraffe-data.com). #  Netpbm is a standard part of most Linux distros.  OPERATION=macptopbm SUFFIX=pbm          # New filename suffix.   if [ -n "$1" ] then   directory=$1      # If directory name given as a script argument... else   directory=$PWD    # Otherwise use current working directory. fi      #  Assumes all files in the target directory are MacPaint image files, #+ with a ".mac" filename suffix.  for file in $directory/*    # Filename globbing. do   filename=${file%.*c}      #  Strip ".mac" suffix off filename                             #+ ('.*c' matches everything 			    #+ between '.' and 'c', inclusive).   $OPERATION $file > "$filename.$SUFFIX"                             # Redirect conversion to new filename.   rm -f $file               # Delete original files after converting.      echo "$filename.$SUFFIX"  # Log what is happening to stdout. done  exit 0  # Exercise: # -------- #  As it stands, this script converts *all* the files in the current #+ working directory. #  Modify it to work *only* on files with a ".mac" suffix.    # *** And here's another way to do it. *** #  #!/bin/bash # Batch convert into different graphic formats. # Assumes imagemagick installed (standard in most Linux distros).  INFMT=png   # Can be tif, jpg, gif, etc. OUTFMT=pdf  # Can be tif, jpg, gif, pdf, etc.  for pic in *"$INFMT" do   p2=$(ls "$pic" | sed -e s/\.$INFMT//)   # echo $p2     convert "$pic" $p2.$OUTFMT     done  exit $?

Example 10-4. Converting streaming audio files to ogg

#!/bin/bash # ra2ogg.sh: Convert streaming audio files (*.ra) to ogg.  # Uses the "mplayer" media player program: #      http://www.mplayerhq.hu/homepage # Uses the "ogg" library and "oggenc": #      http://www.xiph.org/ # # This script may need appropriate codecs installed, such as sipr.so ... # Possibly also the compat-libstdc++ package.   OFILEPREF=${1%%ra}      # Strip off the "ra" suffix. OFILESUFF=wav           # Suffix for wav file. OUTFILE="$OFILEPREF""$OFILESUFF" E_NOARGS=85  if [ -z "$1" ]          # Must specify a filename to convert. then   echo "Usage: `basename $0` [filename]"   exit $E_NOARGS fi   ########################################################################## mplayer "$1" -ao pcm:file=$OUTFILE oggenc "$OUTFILE"  # Correct file extension automatically added by oggenc. ##########################################################################  rm "$OUTFILE"      # Delete intermediate *.wav file.                    # If you want to keep it, comment out above line.  exit $?  #  Note: #  ---- #  On a Website, simply clicking on a *.ram streaming audio file #+ usually only downloads the URL of the actual *.ra audio file. #  You can then use "wget" or something similar #+ to download the *.ra file itself.   #  Exercises: #  --------- #  As is, this script converts only *.ra filenames. #  Add flexibility by permitting use of *.ram and other filenames. # #  If you're really ambitious, expand the script #+ to do automatic downloads and conversions of streaming audio files. #  Given a URL, batch download streaming audio files (using "wget") #+ and convert them on the fly.

A simple emulation of getopt using substring-extraction constructs.

Example 10-5. Emulating getopt

#!/bin/bash # getopt-simple.sh # Author: Chris Morgan # Used in the ABS Guide with permission.   getopt_simple() {     echo "getopt_simple()"     echo "Parameters are '$*'"     until [ -z "$1" ]     do       echo "Processing parameter of: '$1'"       if [ ${1:0:1} = '/' ]       then           tmp=${1:1}               # Strip off leading '/' . . .           parameter=${tmp%%=*}     # Extract name.           value=${tmp##*=}         # Extract value.           echo "Parameter: '$parameter', value: '$value'"           eval $parameter=$value       fi       shift     done }  # Pass all options to getopt_simple(). getopt_simple $*  echo "test is '$test'" echo "test2 is '$test2'"  exit 0  # See also, UseGetOpt.sh, a modified version of this script.  ---  sh getopt_example.sh /test=value1 /test2=value2  Parameters are '/test=value1 /test2=value2' Processing parameter of: '/test=value1' Parameter: 'test', value: 'value1' Processing parameter of: '/test2=value2' Parameter: 'test2', value: 'value2' test is 'value1' test2 is 'value2'

Substring Replacement

${string/substring/replacement}

Replace first match of $substring with $replacement . [50]

${string//substring/replacement}

Replace all matches of $substring with $replacement .

stringZ=abcABC123ABCabc  echo ${stringZ/abc/xyz}       # xyzABC123ABCabc                               # Replaces first match of 'abc' with 'xyz'.  echo ${stringZ//abc/xyz}      # xyzABC123ABCxyz                               # Replaces all matches of 'abc' with # 'xyz'.  echo  --------------- echo "$stringZ"               # abcABC123ABCabc echo  ---------------                               # The string itself is not altered!  # Can the match and replacement strings be parameterized? match=abc repl=000 echo ${stringZ/$match/$repl}  # 000ABC123ABCabc #              ^      ^         ^^^ echo ${stringZ//$match/$repl} # 000ABC123ABC000 # Yes!          ^      ^        ^^^         ^^^  echo  # What happens if no $replacement string is supplied? echo ${stringZ/abc}           # ABC123ABCabc echo ${stringZ//abc}          # ABC123ABC # A simple deletion takes place.

${string/#substring/replacement}

If $substring matches front end of $string , substitute $replacement for $substring .

${string/%substring/replacement}

If $substring matches back end of $string , substitute $replacement for $substring .

stringZ=abcABC123ABCabc  echo ${stringZ/#abc/XYZ}          # XYZABC123ABCabc                                   # Replaces front-end match of 'abc' with 'XYZ'.  echo ${stringZ/%abc/XYZ}          # abcABC123ABCXYZ                                   # Replaces back-end match of 'abc' with 'XYZ'.

10.1.1. Manipulating strings using awk

A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in operations.

Example 10-6. Alternate ways of extracting and locating substrings

#!/bin/bash # substring-extraction.sh  String=23skidoo1 #      012345678    Bash #      123456789    awk # Note different string indexing system: # Bash numbers first character of string as 0. # Awk  numbers first character of string as 1.  echo ${String:2:4} # position 3 (0-1-2), 4 characters long                                          # skid  # The awk equivalent of ${string:pos:length} is substr(string,pos,length). echo | awk ' { print substr("'"${String}"'",3,4)      # skid } ' #  Piping an empty "echo" to awk gives it dummy input, #+ and thus makes it unnecessary to supply a filename.  echo "----"  # And likewise:  echo | awk ' { print index("'"${String}"'", "skid")      # 3 }                                           # (skid starts at position 3) '   # The awk equivalent of "expr index" ...  exit 0

10.1.2. Further Reference

For more on string manipulation in scripts, refer to Section 10.2 and the relevant section of the expr command listing.

Script examples:

Example 16-9
Example 10-9
Example 10-10
Example 10-11
Example 10-13
Example A-36
Example A-41

10.2. Parameter Substitution

Manipulating and/or expanding variables

${parameter}

Same as $parameter , i.e., value of the variable parameter . In certain contexts, only the less ambiguous ${parameter} form works.

May be used for concatenating variables with strings.

your_id=${USER}-on-${HOSTNAME} echo "$your_id" # echo "Old \$PATH = $PATH" PATH=${PATH}:/opt/bin  # Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH"

${parameter-default} , ${parameter:-default}

If parameter not set, use default.

var1=1 var2=2 # var3 is unset.  echo ${var1-$var2}   # 1 echo ${var3-$var2}   # 2 #           ^          Note the $ prefix.    echo ${username-`whoami`} # Echoes the result of `whoami`, if variable $username is still unset.

${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null.

#!/bin/bash # param-sub.sh  #  Whether a variable has been declared #+ affects triggering of the default option #+ even if the variable is null.  username0= echo "username0 has been declared, but is set to null." echo "username0 = ${username0-`whoami`}" # Will not echo.  echo  echo username1 has not been declared. echo "username1 = ${username1-`whoami`}" # Will echo.  username2= echo "username2 has been declared, but is set to null." echo "username2 = ${username2:-`whoami`}" #                            ^ # Will echo because of :- rather than just - in condition test. # Compare to first instance, above.   #  # Once again:  variable= # variable has been declared, but is set to null.  echo "${variable-0}"    # (no output) echo "${variable:-1}"   # 1 #               ^  unset variable  echo "${variable-2}"    # 2 echo "${variable:-3}"   # 3  exit 0

The default parameter construct finds use in providing "missing" command-line arguments in scripts.

DEFAULT_FILENAME=generic.data filename=${1:-$DEFAULT_FILENAME} #  If not otherwise specified, the following command block operates #+ on the file "generic.data". #  Begin-Command-Block #  ... #  ... #  ... #  End-Command-Block    #  From "hanoi2.bash" example: DISKS=${1:-E_NOPARAM}   # Must specify how many disks. #  Set $DISKS to $1 command-line-parameter, #+ or to $E_NOPARAM if that is unset.

See also Example 3-4, Example 31-2, and Example A-6.

Compare this method with using an and list to supply a default command-line argument.

${parameter=default} , ${parameter:=default}

If parameter not set, set it to default.

Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, [51] as above.

echo ${var=abc}   # abc echo ${var=xyz}   # abc # $var had already been set to abc, so it did not change.

${parameter+alt_value} , ${parameter:+alt_value}

If parameter set, use alt_value , else use null string.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below.

echo "###### \${parameter+alt_value} ########" echo  a=${param1+xyz} echo "a = $a"      # a =  param2= a=${param2+xyz} echo "a = $a"      # a = xyz  param3=123 a=${param3+xyz} echo "a = $a"      # a = xyz  echo echo "###### \${parameter:+alt_value} ########" echo  a=${param4:+xyz} echo "a = $a"      # a =  param5= a=${param5:+xyz} echo "a = $a"      # a = # Different result from   a=${param5+xyz}  param6=123 a=${param6:+xyz} echo "a = $a"      # a = xyz

${parameter?err_msg} , ${parameter:?err_msg}

If parameter set, use it, else print err_msg and abort the script with an exit status of 1.

Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above.

Example 10-7. Using parameter substitution and error messages

#!/bin/bash  #  Check some of the system's environmental variables. #  This is good preventative maintenance. #  If, for example, $USER, the name of the person at the console, is not set, #+ the machine will not recognize you.  : ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}   echo   echo "Name of the machine is $HOSTNAME."   echo "You are $USER."   echo "Your home directory is $HOME."   echo "Your mail INBOX is located in $MAIL."   echo   echo "If you are reading this message,"   echo "critical environmental variables have been set."   echo   echo  # ------------------------------------------------------  #  The ${variablename?} construction can also check #+ for variables set within the script.  ThisVariable=Value-of-ThisVariable #  Note, by the way, that string variables may be set #+ to characters disallowed in their names. : ${ThisVariable?} echo "Value of ThisVariable is $ThisVariable".  echo; echo   : ${ZZXy23AB?"ZZXy23AB has not been set."} #  Since ZZXy23AB has not been set, #+ then the script terminates with an error message.  # You can specify the error message. # : ${variablename?"ERROR MESSAGE"}   # Same result with:   dummy_variable=${ZZXy23AB?} #                     dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."} # #                     echo ${ZZXy23AB?} >/dev/null  #  Compare these methods of checking whether a variable has been set #+ with "set -u" . . .    echo "You will not see this message, because script already terminated."  HERE=0 exit $HERE   # Will NOT exit here.  # In fact, this script will return an exit status (echo $?) of 1.

Example 10-8. Parameter substitution and "usage" messages

#!/bin/bash # usage-message.sh  : ${1?"Usage: $0 ARGUMENT"} #  Script exits here if command-line parameter absent, #+ with following error message. #    usage-message.sh: 1: Usage: usage-message.sh ARGUMENT  echo "These two lines echo only if command-line parameter given." echo "command-line parameter = \"$1\""  exit 0  # Will exit here only if command-line parameter present.  # Check the exit status, both with and without command-line parameter. # If command-line parameter present, then "$?" is 0. # If not, then "$?" is 1.

Parameter substitution and/or expansion. The following expressions are the complement to the match in expr string operations (see Example 16-9). These particular ones are used mostly in parsing file path names.

Variable length / Substring removal

${#var}

String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array.

Exceptions:

${#*} and ${#@} give the number of positional parameters.
For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array.

Example 10-9. Length of a variable

#!/bin/bash # length.sh  E_NO_ARGS=65  if [ $# -eq 0 ]  # Must have command-line args to demo script. then   echo "Please invoke this script with one or more command-line arguments."   exit $E_NO_ARGS fi    var01=abcdEFGH28ij echo "var01 = ${var01}" echo "Length of var01 = ${#var01}" # Now, let's try embedding a space. var02="abcd EFGH28ij" echo "var02 = ${var02}" echo "Length of var02 = ${#var02}"  echo "Number of command-line arguments passed to script = ${#@}" echo "Number of command-line arguments passed to script = ${#*}"  exit 0

${var#Pattern} , ${var##Pattern}

${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end of $var.

${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end of $var.

A usage illustration from Example A-7:

# Function from "days-between.sh" example. # Strips leading zero(s) from argument passed.  strip_leading_zero () #  Strip possible leading zero(s) {                     #+ from argument passed.   return=${1#0}       #  The "1" refers to "$1" -- passed arg. }                     #  The "0" is what to remove from "$1" -- strips zeros.

Manfred Schwarb's more elaborate variation of the above:

strip_leading_zero2 () # Strip possible leading zero(s), since otherwise {                      # Bash will interpret such numbers as octal values.   shopt -s extglob     # Turn on extended globbing.   local val=${1##+(0)} # Use local variable, longest matching series of 0's.   shopt -u extglob     # Turn off extended globbing.   _strip_leading_zero2=${val:-0}                        # If input was 0, return 0 instead of "". }

Another usage illustration:

echo `basename $PWD`        # Basename of current working directory. echo "${PWD##*/}"           # Basename of current working directory. echo echo `basename $0`          # Name of script. echo $0                     # Name of script. echo "${0##*/}"             # Name of script. echo filename=test.data echo "${filename##*.}"      # data                             # Extension of filename.

${var%Pattern} , ${var%%Pattern}

${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end of $var.

${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end of $var.

Version 2 of Bash added additional options.

Example 10-10. Pattern matching in parameter substitution

#!/bin/bash # patt-matching.sh  # Pattern matching  using the # ## % %% parameter substitution operators.  var1=abcd12345abc6789 pattern1=a*c  # * (wild card) matches everything between a - c.  echo echo "var1 = $var1"           # abcd12345abc6789 echo "var1 = ${var1}"         # abcd12345abc6789                               # (alternate form) echo "Number of characters in ${var1} = ${#var1}" echo  echo "pattern1 = $pattern1"   # a*c  (everything between 'a' and 'c') echo "--------------" echo '${var1#$pattern1}  =' "${var1#$pattern1}"    #         d12345abc6789 # Shortest possible match, strips out first 3 characters  abcd12345abc6789 #                                     ^^^^^               |-| echo '${var1##$pattern1} =' "${var1##$pattern1}"   #                  6789       # Longest possible match, strips out first 12 characters  abcd12345abc6789 #                                    ^^^^^                |----------|  echo; echo; echo  pattern2=b*9            # everything between 'b' and '9' echo "var1 = $var1"     # Still  abcd12345abc6789 echo echo "pattern2 = $pattern2" echo "--------------" echo '${var1%pattern2}  =' "${var1%$pattern2}"     #     abcd12345a # Shortest possible match, strips out last 6 characters  abcd12345abc6789 #                                     ^^^^                         |----| echo '${var1%%pattern2} =' "${var1%%$pattern2}"    #     a # Longest possible match, strips out last 12 characters  abcd12345abc6789 #                                    ^^^^                 |-------------|  # Remember, # and ## work from the left end (beginning) of string, #           % and %% work from the right end.  echo  exit 0

Example 10-11. Renaming file extensions:

#!/bin/bash # rfe.sh: Renaming file extensions. # #         rfe old_extension new_extension # # Example: # To rename all *.gif files in working directory to *.jpg, #          rfe gif jpg   E_BADARGS=65  case $# in   0|1)             # The vertical bar means "or" in this context.   echo "Usage: `basename $0` old_file_suffix new_file_suffix"   exit $E_BADARGS  # If 0 or 1 arg, then bail out.   ;; esac   for filename in *.$1 # Traverse list of files ending with 1st argument. do   mv $filename ${filename%$1}$2   #  Strip off part of filename matching 1st argument,   #+ then append 2nd argument. done  exit 0

Variable expansion / Substring replacement

These constructs have been adopted from ksh.

${var:pos}

Variable var expanded, starting from offset pos .

${var:pos:len}

Expansion to a max of len characters of variable var , from offset pos . See Example A-13 for an example of the creative use of this operator.

${var/Pattern/Replacement}

First match of Pattern , within var replaced with Replacement .

If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted.

${var//Pattern/Replacement}

Global replacement. All matches of Pattern , within var replaced with Replacement .

As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted.

Example 10-12. Using pattern matching to parse arbitrary strings

#!/bin/bash  var1=abcd-1234-defg echo "var1 = $var1"  t=${var1#*-*} echo "var1 (with everything, up to and including first - stripped out) = $t" #  t=${var1#*-}  works just the same, #+ since # matches the shortest string, #+ and * matches everything preceding, including an empty string. # (Thanks, Stephane Chazelas, for pointing this out.)  t=${var1##*-*} echo "If var1 contains a \"-\", returns empty string...   var1 = $t"   t=${var1%*-*} echo "var1 (with everything from the last - on stripped out) = $t"  echo  # ------------------------------------------- path_name=/home/bozo/ideas/thoughts.for.today # ------------------------------------------- echo "path_name = $path_name" t=${path_name##/*/} echo "path_name, stripped of prefixes = $t" # Same effect as   t=`basename $path_name` in this particular case. #  t=${path_name%/}; t=${t##*/}   is a more general solution, #+ but still fails sometimes. #  If $path_name ends with a newline, then `basename $path_name` will not work, #+ but the above expression will. # (Thanks, S.C.)  t=${path_name%/*.*} # Same effect as   t=`dirname $path_name` echo "path_name, stripped of suffixes = $t" # These will fail in some cases, such as "../", "/foo////", # "foo/", "/". #  Removing suffixes, especially when the basename has no suffix, #+ but the dirname does, also complicates matters. # (Thanks, S.C.)  echo  t=${path_name:11} echo "$path_name, with first 11 chars stripped off = $t" t=${path_name:11:5} echo "$path_name, with first 11 chars stripped off, length 5 = $t"  echo  t=${path_name/bozo/clown} echo "$path_name with \"bozo\" replaced  by \"clown\" = $t" t=${path_name/today/} echo "$path_name with \"today\" deleted = $t" t=${path_name//o/O} echo "$path_name with all o's capitalized = $t" t=${path_name//o/} echo "$path_name with all o's deleted = $t"  exit 0

${var/#Pattern/Replacement}

If prefix of var matches Pattern , then substitute Replacement for Pattern .

${var/%Pattern/Replacement}

If suffix of var matches Pattern , then substitute Replacement for Pattern .

Example 10-13. Matching patterns at prefix or suffix of string

#!/bin/bash # var-match.sh: # Demo of pattern replacement at prefix / suffix of string.  v0=abc1234zip1234abc    # Original variable. echo "v0 = $v0"         # abc1234zip1234abc echo  # Match at prefix (beginning) of string. v1=${v0/#abc/ABCDEF}    # abc1234zip1234abc                         # |-| echo "v1 = $v1"         # ABCDEF1234zip1234abc                         # |----|  # Match at suffix (end) of string. v2=${v0/%abc/ABCDEF}    # abc1234zip123abc                         #              |-| echo "v2 = $v2"         # abc1234zip1234ABCDEF                         #               |----|  echo  #  ---------------------------------------------------- #  Must match at beginning / end of string, #+ otherwise no replacement results. #  ---------------------------------------------------- v3=${v0/#123/000}       # Matches, but not at beginning. echo "v3 = $v3"         # abc1234zip1234abc                         # NO REPLACEMENT. v4=${v0/%123/000}       # Matches, but not at end. echo "v4 = $v4"         # abc1234zip1234abc                         # NO REPLACEMENT.  exit 0

${!varprefix*} , ${!varprefix@}

Matches names of all previously declared variables beginning with varprefix .

# This is a variation on indirect reference, but with a * or @. # Bash, version 2.04, adds this feature.  xyz23=whatever xyz24=  a=${!xyz*}         #  Expands to *names* of declared variables # ^ ^   ^           + beginning with "xyz". echo "a = $a"      #  a = xyz23 xyz24 a=${!xyz@}         #  Same as above. echo "a = $a"      #  a = xyz23 xyz24  echo "---"  abc23=something_else b=${!abc*} echo "b = $b"      #  b = abc23 c=${!b}            #  Now, the more familiar type of indirect reference. echo $c            #  something_else

Chapter 11. Loops and Branches

What needs this iteration, woman?

--Shakespeare, Othello

Operations on code blocks are the key to structured and organized shell scripts. Looping and branching constructs provide the tools for accomplishing this.

11.1. Loops

A loop is a block of code that iterates [52] a list of commands as long as the loop control condition is true.

for loops

for arg in [list]

This is the basic looping construct. It differs significantly from its C counterpart.

for arg in [ list ]
do
�command(s) ...
done

During each pass through the loop, arg takes on the value of each successive variable in the list .

for arg in "$var1" "$var2" "$var3" ... "$varN"   # In pass 1 of the loop, arg = $var1	     # In pass 2 of the loop, arg = $var2	     # In pass 3 of the loop, arg = $var3	     # ... # In pass N of the loop, arg = $varN  # Arguments in [list] quoted to prevent possible word splitting.

The argument list may contain wild cards.

If do is on same line as for, there needs to be a semicolon after list.

for arg in [ list ] ; do

Example 11-1. Simple for loops

#!/bin/bash # Listing the planets.  for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto do   echo $planet  # Each planet on a separate line. done  echo; echo  for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"     # All planets on same line.     # Entire 'list' enclosed in quotes creates a single variable.     # Why? Whitespace incorporated into the variable. do   echo $planet done  echo; echo "Whoops! Pluto is no longer a planet!"  exit 0

Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see Example 15-16) to force parsing of each [list] element and assignment of each component to the positional parameters.

Example 11-2. for loop with two parameters in each [list] element

#!/bin/bash # Planets revisited.  # Associate the name of each planet with its distance from the sun.  for planet in "Mercury 36" "Venus 67" "Earth 93"  "Mars 142" "Jupiter 483" do   set -- $planet  #  Parses variable "planet"                   #+ and sets positional parameters.   #  The "--" prevents nasty surprises if $planet is null or   #+ begins with a dash.    #  May need to save original positional parameters,   #+ since they get overwritten.   #  One way of doing this is to use an array,   #         original_params=("$@")    echo "$1		$2,000,000 miles from the sun"   #-------two  tabs---concatenate zeroes onto parameter $2 done  # (Thanks, S.C., for additional clarification.)  exit 0

A variable may supply the [list] in a for loop.

Example 11-3. Fileinfo: operating on a file list contained in a variable

#!/bin/bash # fileinfo.sh  FILES="/usr/sbin/accept /usr/sbin/pwck /usr/sbin/chroot /usr/bin/fakefile /sbin/badblocks /sbin/ypbind"     # List of files you are curious about.                   # Threw in a dummy file, /usr/bin/fakefile.  echo  for file in $FILES do    if [ ! -e "$file" ]       # Check if file exists.   then     echo "$file does not exist."; echo     continue                # On to next.    fi    ls -l $file | awk '{ print $8 "         file size: " $5 }'  # Print 2 fields.   whatis `basename $file`   # File info.   # Note that the whatis database needs to have been set up for this to work.   # To do this, as root run /usr/bin/makewhatis.   echo done    exit 0

The [list] in a for loop may be parameterized.

Example 11-4. Operating on a parameterized file list

#!/bin/bash  filename="*txt"  for file in $filename do  echo "Contents of $file"  echo "---"  cat "$file"  echo done

If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing takes place.

Example 11-5. Operating on files with a for loop

#!/bin/bash # list-glob.sh: Generating [list] in a for-loop, using "globbing" ... # Globbing = filename expansion.  echo  for file in * #           ^  Bash performs filename expansion #+             on expressions that globbing recognizes. do   ls -l "$file"  # Lists all files in $PWD (current directory).   #  Recall that the wild card character "*" matches every filename,   #+ however, in "globbing," it doesn't match dot-files.    #  If the pattern matches no file, it is expanded to itself.   #  To prevent this, set the nullglob option   #+   (shopt -s nullglob).   #  Thanks, S.C. done  echo; echo  for file in [jx]* do   rm -f $file    # Removes only files beginning with "j" or "x" in $PWD.   echo "Removed file \"$file\"". done  echo  exit 0

Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is Example A-15. See also Example 15-17.

Example 11-6. Missing in [list] in a for loop

#!/bin/bash  #  Invoke this script both with and without arguments, #+ and see what happens.  for a do  echo -n "$a " done  #  The 'in list' missing, therefore the loop operates on '$@' #+ (command-line argument list, including whitespace).  echo  exit 0

It is possible to use command substitution to generate the [list] in a for loop. See also Example 16-54, Example 11-11 and Example 16-48.

Example 11-7. Generating the [list] in a for loop with command substitution

#!/bin/bash #  for-loopcmd.sh: for-loop with [list] #+ generated by command substitution.  NUMBERS="9 7 3 8 37.53"  for number in `echo $NUMBERS`  # for number in 9 7 3 8 37.53 do   echo -n "$number " done  echo  exit 0

Here is a somewhat more complex example of using command substitution to create the [list] .

Example 11-8. A grep replacement for binary files

#!/bin/bash # bin-grep.sh: Locates matching strings in a binary file.  # A "grep" replacement for binary files. # Similar effect to "grep -a"  E_BADARGS=65 E_NOFILE=66  if [ $# -ne 2 ] then   echo "Usage: `basename $0` search_string filename"   exit $E_BADARGS fi  if [ ! -f "$2" ] then   echo "File \"$2\" does not exist."   exit $E_NOFILE fi     IFS=$'\012'       # Per suggestion of Anton Filippov.                   # was:  IFS="\n" for word in $( strings "$2" | grep "$1" ) # The "strings" command lists strings in binary files. # Output then piped to "grep", which tests for desired string. do   echo $word done  # As S.C. points out, lines 23 - 30 could be replaced with the simpler #    strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'   #  Try something like  "./bin-grep.sh mem /bin/ls" #+ to exercise this script.  exit 0

More of the same.

Example 11-9. Listing all users on the system

#!/bin/bash # userlist.sh  PASSWORD_FILE=/etc/passwd n=1           # User number  for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" ) # Field separator = :    ^^^^^^ # Print first field              ^^^^^^^^ # Get input from password file  /etc/passwd  ^^^^^^^^^^^^^^^^^ do   echo "USER #$n = $name"   let "n += 1" done     # USER #1 = root # USER #2 = bin # USER #3 = daemon # ... # USER #33 = bozo  exit $?  #  Discussion: #  ---------- #  How is it that an ordinary user, or a script run by same, #+ can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.) #  Is this a security hole? Why or why not?

Yet another example of the [list] resulting from command substitution.

Example 11-10. Checking all the binaries in a directory for authorship

#!/bin/bash # findstring.sh: # Find a particular string in the binaries in a specified directory.  directory=/usr/bin/ fstring="Free Software Foundation"  # See which files come from the FSF.  for file in $( find $directory -type f -name '*' | sort ) do   strings -f $file | grep "$fstring" | sed -e "s%$directory%%"   #  In the "sed" expression,   #+ it is necessary to substitute for the normal "/" delimiter   #+ because "/" happens to be one of the characters filtered out.   #  Failure to do so gives an error message. (Try it.) done    exit $?  #  Exercise (easy): #  --------------- #  Convert this script to take command-line parameters #+ for $directory and $fstring.

A final example of [list] / command substitution, but this time the "command" is a function.

generate_list () {   echo "one two three" }  for word in $(generate_list)  # Let "word" grab output of function. do   echo "$word" done  # one # two # three

The output of a for loop may be piped to a command or commands.

Example 11-11. Listing the symbolic links in a directory

#!/bin/bash # symlinks.sh: Lists symbolic links in a directory.   directory=${1-`pwd`} #  Defaults to current working directory, #+ if not otherwise specified. #  Equivalent to code block below. # ---------------------------------------------------------- # ARGS=1                 # Expect one command-line argument. # # if [ $# -ne "$ARGS" ]  # If not 1 arg... # then #   directory=`pwd`      # current working directory # else #   directory=$1 # fi # ----------------------------------------------------------  echo "symbolic links in directory \"$directory\""  for file in "$( find $directory -type l )"   # -type l = symbolic links do   echo "$file" done | sort                                  # Otherwise file list is unsorted. #  Strictly speaking, a loop isn't really necessary here, #+ since the output of the "find" command is expanded into a single word. #  However, it's easy to understand and illustrative this way.  #  As Dominik 'Aeneas' Schnitzer points out, #+ failing to quote  $( find $directory -type l ) #+ will choke on filenames with embedded whitespace. #  containing whitespace.   exit 0   # -------------------------------------------------------- # Jean Helou proposes the following alternative:  echo "symbolic links in directory \"$directory\"" # Backup of the current IFS. One can never be too cautious. OLDIFS=$IFS IFS=:  for file in $(find $directory -type l -printf "%p$IFS") do     #                              ^^^^^^^^^^^^^^^^        echo "$file"        done|sort  # And, James "Mike" Conley suggests modifying Helou's code thusly:  OLDIFS=$IFS IFS='' # Null IFS means no word breaks for file in $( find $directory -type l ) do   echo $file   done | sort  #  This works in the "pathological" case of a directory name having #+ an embedded colon. #  "This also fixes the pathological case of the directory name having #+  a colon (or space in earlier example) as well."

The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows.

Example 11-12. Symbolic links in a directory, saved to a file

#!/bin/bash # symlinks.sh: Lists symbolic links in a directory.  OUTFILE=symlinks.list                         # save-file  directory=${1-`pwd`} #  Defaults to current working directory, #+ if not otherwise specified.   echo "symbolic links in directory \"$directory\"" > "$OUTFILE" echo "---------------------------" >> "$OUTFILE"  for file in "$( find $directory -type l )"    # -type l = symbolic links do   echo "$file" done | sort >> "$OUTFILE"                     # stdout of loop #           ^^^^^^^^^^^^^                       redirected to save file.  # echo "Output file = $OUTFILE"  exit $?

There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.

Example 11-13. A C-style for loop

#!/bin/bash # Multiple ways to count up to 10.  echo  # Standard syntax. for a in 1 2 3 4 5 6 7 8 9 10 do   echo -n "$a " done    echo; echo  # +==========================================+  # Using "seq" ... for a in `seq 10` do   echo -n "$a " done    echo; echo  # +==========================================+  # Using brace expansion ... # Bash, version 3+. for a in {1..10} do   echo -n "$a " done    echo; echo  # +==========================================+  # Now, let's do the same, using C-like syntax.  LIMIT=10  for ((a=1; a <= LIMIT ; a++))  # Double parentheses, and naked "LIMIT" do   echo -n "$a " done                           # A construct borrowed from ksh93.  echo; echo  # +=========================================================================+  # Let's use the C "comma operator" to increment two variables simultaneously.  for ((a=1, b=1; a <= LIMIT ; a++, b++)) do  # The comma concatenates operations.   echo -n "$a-$b " done  echo; echo  exit 0

See also Example 27-16, Example 27-17, and Example A-6.

---

Now, a for loop used in a "real-life" context.

Example 11-14. Using efax in batch mode

#!/bin/bash # Faxing (must have 'efax' package installed).  EXPECTED_ARGS=2 E_BADARGS=85 MODEM_PORT="/dev/ttyS2"   # May be different on your machine. #                ^^^^^      PCMCIA modem card default port.  if [ $# -ne $EXPECTED_ARGS ] # Check for proper number of command-line args. then    echo "Usage: `basename $0` phone# text-file"    exit $E_BADARGS fi   if [ ! -f "$2" ] then   echo "File $2 is not a text file."   #     File is not a regular file, or does not exist.   exit $E_BADARGS fi     fax make $2              #  Create fax-formatted files from text files.  for file in $(ls $2.0*)  #  Concatenate the converted files.                          #  Uses wild card (filename "globbing") 			 #+ in variable list. do   fil="$fil $file" done    efax -d "$MODEM_PORT"  -t "T$1" $fil   # Finally, do the work. # Trying adding  -o1  if above line fails.   #  As S.C. points out, the for-loop can be eliminated with #     efax -d /dev/ttyS2 -o1 -t "T$1" $2.0* #+ but it's not quite as instructive [grin].  exit $?   # Also, efax sends diagnostic messages to stdout.

The keywords do and done delineate the for-loop command block. However, these may, in certain contexts, be omitted by framing the command block within curly brackets

for((n=1; n<=10; n++))  # No do! {   echo -n "* $n *" } # No done!   # Outputs: # * 1 ** 2 ** 3 ** 4 ** 5 ** 6 ** 7 ** 8 ** 9 ** 10 * # And, echo $? returns 0, so Bash does not register an error.   echo   #  But, note that in a classic for-loop:    for n in [list] ... #+ a terminal semicolon is required.  for n in 1 2 3 {  echo -n "$n "; } #               ^   # Thank you, YongYe, for pointing this out.

while

This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand.

while [ condition ]
do
�command(s) ...
done

The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]).

As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

while [ condition ] ; do

Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.

Example 11-15. Simple while loop

#!/bin/bash  var0=0 LIMIT=10  while [ "$var0" -lt "$LIMIT" ] #      ^                    ^ # Spaces, because these are "test-brackets" . . . do   echo -n "$var0 "        # -n suppresses newline.   #             ^           Space, to separate printed out numbers.    var0=`expr $var0 + 1`   # var0=$(($var0+1))  also works.                           # var0=$((var0 + 1)) also works.                           # let "var0 += 1"    also works. done                      # Various other methods also work.  echo  exit 0

Example 11-16. Another while loop

#!/bin/bash  echo                                # Equivalent to: while [ "$var1" != "end" ]     # while test "$var1" != "end" do   echo "Input variable #1 (end to exit) "   read var1                    # Not 'read $var1' (why?).   echo "variable #1 = $var1"   # Need quotes because of "#" . . .   # If input is 'end', echoes it here.   # Does not test for termination condition until top of loop.   echo done    exit 0

A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however.

Example 11-17. while loop with multiple conditions

#!/bin/bash  var1=unset previous=$var1  while echo "previous-variable = $previous"       echo       previous=$var1       [ "$var1" != end ] # Keeps track of what $var1 was previously.       # Four conditions on *while*, but only the final one controls loop.       # The *last* exit status is the one that counts. do echo "Input variable #1 (end to exit) "   read var1   echo "variable #1 = $var1" done    # Try to figure out how this all works. # It's a wee bit tricky.  exit 0

As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct (see also Example 8-5).

Example 11-18. C-style syntax in a while loop

#!/bin/bash # wh-loopc.sh: Count to 10 in a "while" loop.  LIMIT=10                 # 10 iterations. a=1  while [ "$a" -le $LIMIT ] do   echo -n "$a "   let "a+=1" done                     # No surprises, so far.  echo; echo  # +=================================================================+  # Now, we'll repeat with C-like syntax.  ((a = 1))      # a=1 # Double parentheses permit space when setting a variable, as in C.  while (( a <= LIMIT ))   #  Double parentheses, do                       #+ and no "$" preceding variables.   echo -n "$a "   ((a += 1))             # let "a+=1"   # Yes, indeed.   # Double parentheses permit incrementing a variable with C-like syntax. done  echo  # C and Java programmers can feel right at home in Bash.  exit 0

Inside its test brackets, a while loop can call a function.

t=0  condition () {   ((t++))    if [ $t -lt 5 ]   then     return 0  # true   else     return 1  # false   fi }  while condition #     ^^^^^^^^^ #     Function call -- four loop iterations. do   echo "Still going: t = $t" done  # Still going: t = 1 # Still going: t = 2 # Still going: t = 3 # Still going: t = 4

By coupling the power of the read command with a while loop, we get the handy while read construct, useful for reading and parsing files.

cat $filename |   # Supply input from a file. while read line   # As long as there is another line to read ... do   ... done  # =========== Snippet from "sd.sh" example script ========== #    while read value   # Read one data point at a time.   do     rt=$(echo "scale=$SC; $rt + $value" | bc)     (( ct++ ))   done    am=$(echo "scale=$SC; $rt / $ct" | bc)    echo $am; return $ct   # This function "returns" TWO values!   #  Caution: This little trick will not work if $ct > 255!   #  To handle a larger number of data points,   #+ simply comment out the "return $ct" above. } <"$datafile"   # Feed in data file.

A while loop may have its stdin redirected to a file by a < at its end.

A while loop may have its stdin supplied by a pipe.

until

This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop).

until [ condition-is-true ]
do
�command(s) ...
done

Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages.

As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

until [ condition-is-true ] ; do

Example 11-19. until loop

#!/bin/bash  END_CONDITION=end  until [ "$var1" = "$END_CONDITION" ] # Tests condition here, at top of loop. do   echo "Input variable #1 "   echo "($END_CONDITION to exit)"   read var1   echo "variable #1 = $var1"   echo done    #                     ---                        #  #  As with "for" and "while" loops, #+ an "until" loop permits C-like test constructs.  LIMIT=10 var=0  until (( var > LIMIT )) do  # ^^ ^     ^     ^^   No brackets, no $ prefixing variables.   echo -n "$var "   (( var++ )) done    # 0 1 2 3 4 5 6 7 8 9 10    exit 0

How to choose between a for loop or a while loop or until loop? In C, you would typically use a for loop when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore, feel free to use whatever type of loop gets the job done in the simplest way.

11.2. Nested Loops

A nested loop is a loop within a loop, an inner loop within the body of an outer one. How this works is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break within either the inner or outer loop would interrupt this process.

Example 11-20. Nested Loop

#!/bin/bash # nested-loop.sh: Nested "for" loops.  outer=1             # Set outer loop counter.  # Beginning of outer loop. for a in 1 2 3 4 5 do   echo "Pass $outer in outer loop."   echo "---------------------"   inner=1           # Reset inner loop counter.    # ===============================================   # Beginning of inner loop.   for b in 1 2 3 4 5   do     echo "Pass $inner in inner loop."     let "inner+=1"  # Increment inner loop counter.   done   # End of inner loop.   # ===============================================    let "outer+=1"    # Increment outer loop counter.    echo              # Space between output blocks in pass of outer loop. done                # End of outer loop.  exit 0

See Example 27-11 for an illustration of nested while loops, and Example 27-13 to see a while loop nested inside an until loop.

11.3. Loop Control

Tournez cent tours, tournez mille tours,

Tournez souvent et tournez toujours . . .

--Verlaine, "Chevaux de bois"

Commands affecting loop behavior

break, continue

The break and continue loop control commands [53] correspond exactly to their counterparts in other programming languages. The break command terminates the loop (breaks out of it), while continue causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle.

Example 11-21. Effects of break and continue in a loop

#!/bin/bash  LIMIT=19  # Upper limit  echo echo "Printing Numbers 1 through 20 (but not 3 and 11)."  a=0  while [ $a -le "$LIMIT" ] do  a=$(($a+1))   if [ "$a" -eq 3 ] || [ "$a" -eq 11 ]  # Excludes 3 and 11.  then    continue      # Skip rest of this particular loop iteration.  fi   echo -n "$a "   # This will not execute for 3 and 11. done   # Exercise: # Why does the loop print up to 20?  echo; echo  echo Printing Numbers 1 through 20, but something happens after 2.  ##################################################################  # Same loop, but substituting 'break' for 'continue'.  a=0  while [ "$a" -le "$LIMIT" ] do  a=$(($a+1))   if [ "$a" -gt 2 ]  then    break  # Skip entire rest of loop.  fi   echo -n "$a " done  echo; echo; echo  exit 0

The break command may optionally take a parameter. A plain break terminates only the innermost loop in which it is embedded, but a break N breaks out of N levels of loop.

Example 11-22. Breaking out of multiple loop levels

#!/bin/bash # break-levels.sh: Breaking out of loops.  # "break N" breaks out of N level loops.  for outerloop in 1 2 3 4 5 do   echo -n "Group $outerloop:   "    # --------------------------------------------------------   for innerloop in 1 2 3 4 5   do     echo -n "$innerloop "      if [ "$innerloop" -eq 3 ]     then       break  # Try   break 2   to see what happens.              # ("Breaks" out of both inner and outer loops.)     fi   done   # --------------------------------------------------------    echo done    echo  exit 0

The continue command, similar to break, optionally takes a parameter. A plain continue cuts short the current iteration within its loop and begins the next. A continue N terminates all remaining iterations at its loop level and continues with the next iteration at the loop, N levels above.

Example 11-23. Continuing at a higher loop level

#!/bin/bash # The "continue N" command, continuing at the Nth level loop.  for outer in I II III IV V           # outer loop do   echo; echo -n "Group $outer: "    # --------------------------------------------------------------------   for inner in 1 2 3 4 5 6 7 8 9 10  # inner loop   do      if [[ "$inner" -eq 7 && "$outer" = "III" ]]     then       continue 2  # Continue at loop on 2nd level, that is "outer loop".                   # Replace above line with a simple "continue"                   # to see normal loop behavior.     fi        echo -n "$inner "  # 7 8 9 10 will not echo on "Group III."   done     # --------------------------------------------------------------------  done  echo; echo  # Exercise: # Come up with a meaningful use for "continue N" in a script.  exit 0

Example 11-24. Using continue N in an actual task

# Albert Reiner gives an example of how to use "continue N": # ---------------------------------------------------------  #  Suppose I have a large number of jobs that need to be run, with #+ any data that is to be treated in files of a given name pattern #+ in a directory. There are several machines that access #+ this directory, and I want to distribute the work over these #+ different boxen. #  Then I usually nohup something like the following on every box:  while true do   for n in .iso.*   do     [ "$n" = ".iso.opts" ] && continue     beta=${n#.iso.}     [ -r .Iso.$beta ] && continue     [ -r .lock.$beta ] && sleep 10 && continue     lockfile -r0 .lock.$beta || continue     echo -n "$beta: " `date`     run-isotherm $beta     date     ls -alF .Iso.$beta     [ -r .Iso.$beta ] && rm -f .lock.$beta     continue 2   done   break done  exit 0  #  The details, in particular the sleep N, are particular to my #+ application, but the general pattern is:  while true do   for job in {pattern}   do     {job already done or running} && continue     {mark job as running, do job, mark job as done}     continue 2   done   break        # Or something like `sleep 600' to avoid termination. done  #  This way the script will stop only when there are no more jobs to do #+ (including jobs that were added during runtime). Through the use #+ of appropriate lockfiles it can be run on several machines #+ concurrently without duplication of calculations [which run a couple #+ of hours in my case, so I really want to avoid this]. Also, as search #+ always starts again from the beginning, one can encode priorities in #+ the file names. Of course, one could also do this without `continue 2', #+ but then one would have to actually check whether or not some job #+ was done (so that we should immediately look for the next job) or not #+ (in which case we terminate or sleep for a long time before checking #+ for a new job).

The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided.

11.4. Testing and Branching

The case and select constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block.

Controlling program flow in a code block

case (in) / esac

The case construct is the shell scripting analog to switch in C/C++. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus.

case "$

variable

" in

�"$

condition1

" )
�

command

...
�;;

�"$

condition2

" )
�

command

...
�;;

esac

Quoting the variables is not mandatory, since word splitting does not take place.
Each test line ends with a right paren ). [54]
Each condition block ends with a double semicolon ;;.
If a condition tests true, then the associated commands execute and the case block terminates.
The entire case block ends with an esac (case spelled backwards).

Example 11-25. Using case

#!/bin/bash # Testing ranges of characters.  echo; echo "Hit a key, then hit return." read Keypress  case "$Keypress" in   [[:lower:]]   ) echo "Lowercase letter";;   [[:upper:]]   ) echo "Uppercase letter";;   [0-9]         ) echo "Digit";;   *             ) echo "Punctuation, whitespace, or other";; esac      #  Allows ranges of characters in [square brackets],           #+ or POSIX ranges in [[double square brackets.  #  In the first version of this example, #+ the tests for lowercase and uppercase characters were #+ [a-z] and [A-Z]. #  This no longer works in certain locales and/or Linux distros. #  POSIX is more portable. #  Thanks to Frank Wang for pointing this out.  #  Exercise: #  -------- #  As the script stands, it accepts a single keystroke, then terminates. #  Change the script so it accepts repeated input, #+ reports on each keystroke, and terminates only when "X" is hit. #  Hint: enclose everything in a "while" loop.  exit 0

Example 11-26. Creating menus using case

#!/bin/bash  # Crude address database  clear # Clear the screen.  echo "          Contact List" echo "          ------- ----" echo "Choose one of the following persons:"  echo echo "[E]vans, Roland" echo "[J]ones, Mildred" echo "[S]mith, Julie" echo "[Z]ane, Morris" echo  read person  case "$person" in # Note variable is quoted.    "E" | "e" )   # Accept upper or lowercase input.   echo   echo "Roland Evans"   echo "4321 Flash Dr."   echo "Hardscrabble, CO 80753"   echo "(303) 734-9874"   echo "(303) 734-9892 fax"   echo "revans@zzy.net"   echo "Business partner & old friend"   ;; # Note double semicolon to terminate each option.    "J" | "j" )   echo   echo "Mildred Jones"   echo "249 E. 7th St., Apt. 19"   echo "New York, NY 10009"   echo "(212) 533-2814"   echo "(212) 533-9972 fax"   echo "milliej@loisaida.com"   echo "Ex-girlfriend"   echo "Birthday: Feb. 11"   ;;  # Add info for Smith & Zane later.            * )    # Default option.	      # Empty input (hitting RETURN) fits here, too.    echo    echo "Not yet in database."   ;;  esac  echo  #  Exercise: #  -------- #  Change the script so it accepts multiple inputs, #+ instead of terminating after displaying just one address.  exit 0

An exceptionally clever use of case involves testing for command-line parameters.

#! /bin/bash  case "$1" in   "") echo "Usage: ${0##*/} <filename>"; exit $E_PARAM;;                       # No command-line parameters,                       # or first parameter empty. # Note that ${0##*/} is ${var##pattern} param substitution.                       # Net result is $0.    -*) FILENAME=./$1;;   #  If filename passed as argument ($1)                       #+ starts with a dash,                       #+ replace it with ./$1                       #+ so further commands don't interpret it                       #+ as an option.    * ) FILENAME=$1;;     # Otherwise, $1. esac

Here is a more straightforward example of command-line parameter handling:

#! /bin/bash   while [ $# -gt 0 ]; do    # Until you run out of parameters . . .   case "$1" in     -d|--debug)               # "-d" or "--debug" parameter?               DEBUG=1               ;;     -c|--conf)               CONFFILE="$2"               shift               if [ ! -f $CONFFILE ]; then                 echo "Error: Supplied file doesn't exist!"                 exit $E_CONFFILE     # File not found error.               fi               ;;   esac   shift       # Check next set of parameters. done  #  From Stefano Falsetto's "Log2Rot" script, #+ part of his "rottlog" package. #  Used with permission.

Example 11-27. Using command substitution to generate the case variable

#!/bin/bash # case-cmd.sh: Using command substitution to generate a "case" variable.  case $( arch ) in   # $( arch ) returns machine architecture.                     # Equivalent to 'uname -m' ...   i386 ) echo "80386-based machine";;   i486 ) echo "80486-based machine";;   i586 ) echo "Pentium-based machine";;   i686 ) echo "Pentium2+-based machine";;   *    ) echo "Other type of machine";; esac  exit 0

A case construct can filter strings for globbing patterns.

Example 11-28. Simple string matching

#!/bin/bash # match-string.sh: Simple string matching #                  using a 'case' construct.  match_string () { # Exact string match.   MATCH=0   E_NOMATCH=90   PARAMS=2     # Function requires 2 arguments.   E_BAD_PARAMS=91    [ $# -eq $PARAMS ] || return $E_BAD_PARAMS    case "$1" in   "$2") return $MATCH;;   *   ) return $E_NOMATCH;;   esac  }     a=one b=two c=three d=two   match_string $a     # wrong number of parameters echo $?             # 91  match_string $a $b  # no match echo $?             # 90  match_string $b $d  # match echo $?             # 0   exit 0

Example 11-29. Checking for alphabetic input

#!/bin/bash # isalpha.sh: Using a "case" structure to filter a string.  SUCCESS=0 FAILURE=1   #  Was FAILURE=-1,             #+ but Bash no longer allows negative return value.  isalpha ()  # Tests whether *first character* of input string is alphabetic. { if [ -z "$1" ]                # No argument passed? then   return $FAILURE fi  case "$1" in   [a-zA-Z]*) return $SUCCESS;;  # Begins with a letter?   *        ) return $FAILURE;; esac }             # Compare this with "isalpha ()" function in C.   isalpha2 ()   # Tests whether *entire string* is alphabetic. {   [ $# -eq 1 ] || return $FAILURE    case $1 in   *[!a-zA-Z]*|"") return $FAILURE;;                *) return $SUCCESS;;   esac }  isdigit ()    # Tests whether *entire string* is numerical. {             # In other words, tests for integer variable.   [ $# -eq 1 ] || return $FAILURE    case $1 in     *[!0-9]*|"") return $FAILURE;;               *) return $SUCCESS;;   esac }    check_var ()  # Front-end to isalpha (). { if isalpha "$@" then   echo "\"$*\" begins with an alpha character."   if isalpha2 "$@"   then        # No point in testing if first char is non-alpha.     echo "\"$*\" contains only alpha characters."   else     echo "\"$*\" contains at least one non-alpha character."   fi   else   echo "\"$*\" begins with a non-alpha character."               # Also "non-alpha" if no argument passed. fi  echo  }  digit_check ()  # Front-end to isdigit (). { if isdigit "$@" then   echo "\"$*\" contains only digits [0 - 9]." else   echo "\"$*\" has at least one non-digit character." fi  echo  }  a=23skidoo b=H3llo c=-What? d=What? e=$(echo $b)   # Command substitution. f=AbcDef g=27234 h=27a34 i=27.34  check_var $a check_var $b check_var $c check_var $d check_var $e check_var $f check_var     # No argument passed, so what happens? # digit_check $g digit_check $h digit_check $i   exit 0        # Script improved by S.C.  # Exercise: # -------- #  Write an 'isfloat ()' function that tests for floating point numbers. #  Hint: The function duplicates 'isdigit ()', #+ but adds a test for a mandatory decimal point.

select

The select construct, adopted from the Korn Shell, is yet another tool for building menus.

select variable [in list ]
do
� command ...
�break
done

This prompts the user to enter one of the choices presented in the variable list. Note that select uses the $PS3 prompt (#? ) by default, but this may be changed.

Example 11-30. Creating menus using select

#!/bin/bash  PS3='Choose your favorite vegetable: ' # Sets the prompt string.                                        # Otherwise it defaults to #? .  echo  select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas" do   echo   echo "Your favorite veggie is $vegetable."   echo "Yuck!"   echo   break  # What happens if there is no 'break' here? done  exit  # Exercise: # -------- #  Fix this script to accept user input not specified in #+ the "select" statement. #  For example, if the user inputs "peas," #+ the script would respond "Sorry. That is not on the menu."

If in list is omitted, then select uses the list of command line arguments ($@) passed to the script or the function containing the select construct.

Compare this to the behavior of a

for variable [in list ]

construct with the in list omitted.

Example 11-31. Creating menus using select in a function

#!/bin/bash  PS3='Choose your favorite vegetable: '  echo  choice_of() { select vegetable # [in list] omitted, so 'select' uses arguments passed to function. do   echo   echo "Your favorite veggie is $vegetable."   echo "Yuck!"   echo   break done }  choice_of beans rice carrots radishes rutabaga spinach #         $1    $2   $3      $4       $5       $6 #         passed to choice_of() function  exit 0

Chapter 12. Command Substitution

Command substitution reassigns the output of a command [55] or even multiple commands; it literally plugs the command output into another context. [56]

The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command-line text.

script_name=`basename $0` echo "The name of this script is $script_name."

The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for loop.

rm `cat filename`   #                    "filename"                    contains a list of files to delete. # # S. C. points out that "arg list too long" error might result. # Better is              xargs rm -- < filename  # ( -- covers those cases where                    "filename"                    begins with a                    "-"                    )  textfile_listing=`ls *.txt` # Variable contains names of all *.txt files in current working directory. echo $textfile_listing  textfile_listing2=$(ls *.txt)   # The alternative form of command substitution. echo $textfile_listing2 # Same result.  # A possible problem with putting a list of files into a single string # is that a newline may creep in. # # A safer way to assign a list of files to a parameter is with an array. #      shopt -s nullglob    # If no match, filename expands to nothing. #      textfile_listing=( *.txt ) # # Thanks, S.C.

Command substitution invokes a subshell.

Command substitution may result in word splitting.

COMMAND `echo a b`     # 2 args: a and b  COMMAND "`echo a b`"   # 1 arg: "a b"  COMMAND `echo`         # no arg  COMMAND "`echo`"       # one empty arg   # Thanks, S.C.

Even when there is no word splitting, command substitution can remove trailing newlines.

# cd "`pwd`"  # This should always work. # However...  mkdir 'dir with trailing newline '  cd 'dir with trailing newline '  cd "`pwd`"  # Error message: # bash: cd: /tmp/file with trailing newline: No such file or directory  cd "$PWD"   # Works fine.      old_tty_setting=$(stty -g)   # Save old terminal setting. echo "Hit a key " stty -icanon -echo           # Disable "canonical" mode for terminal.                              # Also, disable *local* echo. key=$(dd bs=1 count=1 2> /dev/null)   # Using 'dd' to get a keypress. stty "$old_tty_setting"      # Restore old setting.  echo "You hit ${#key} key."  # ${#variable} = number of characters in $variable # # Hit any key except RETURN, and the output is "You hit 1 key." # Hit RETURN, and it's "You hit 0 key." # The newline gets eaten in the command substitution.  #Code snippet by St�phane Chazelas.

Using echo to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises.

dir_listing=`ls -l` echo $dir_listing     # unquoted  # Expecting a nicely ordered directory listing.  # However, what you get is: # total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo # bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh  # The newlines disappeared.   echo "$dir_listing"   # quoted # -rw-rw-r--    1 bozo       30 May 13 17:15 1.txt # -rw-rw-r--    1 bozo       51 May 15 20:57 t2.sh # -rwxr-xr-x    1 bozo      217 Mar  5 21:13 wi.sh

Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command.

variable1=`<file1`      #  Set "variable1" to contents of "file1". variable2=`cat file2`   #  Set "variable2" to contents of "file2".                         #  This, however, forks a new process,                         #+ so the line of code executes slower than the above version.  #  Note that the variables may contain embedded whitespace, #+ or even (horrors), control characters.  #  It is not necessary to explicitly assign a variable. echo "` <$0`"           # Echoes the script itself to stdout.

#  Excerpts from system file, /etc/rc.d/rc.sysinit #+ (on a Red Hat Linux installation)   if [ -f /fsckoptions ]; then         fsckoptions=`cat /fsckoptions` ... fi # # if [ -e "/proc/ide/${disk[$device]}/media" ] ; then              hdmedia=`cat /proc/ide/${disk[$device]}/media` ... fi # # if [ ! -n "`uname -r | grep -- "-"`" ]; then        ktag="`cat /proc/version`" ... fi # # if [ $usb = "1" ]; then     sleep 5     mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"`     kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"` ... fi

Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke.

Example 12-1. Stupid script tricks

#!/bin/bash # stupid-script-tricks.sh: Don't try this at home, folks. # From "Stupid Script Tricks," Volume I.  exit 99  ### Comment out this line if you dare.  dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.  echo "string-length of \$dangerous_variable = ${#dangerous_variable}" # string-length of $dangerous_variable = 794151 # (Newer kernels are bigger.) # Does not give same count as 'wc -c /boot/vmlinuz'.  # echo "$dangerous_variable" # Don't try this! It would hang the script.   #  The document author is aware of no useful applications for #+ setting a variable to the contents of a binary file.  exit 0

Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as Bash, provides more protection from programmer mistakes than a compiled language.

Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop.

Example 12-2. Generating a variable from a loop

#!/bin/bash # csubloop.sh: Setting a variable to the output of a loop.  variable1=`for i in 1 2 3 4 5 do   echo -n "$i"                 #  The 'echo' command is critical done`                          #+ to command substitution here.  echo "variable1 = $variable1"  # variable1 = 12345   i=0 variable2=`while [ "$i" -lt 10 ] do   echo -n "$i"                 # Again, the necessary 'echo'.   let "i += 1"                 # Increment. done`  echo "variable2 = $variable2"  # variable2 = 0123456789  #  Demonstrates that it's possible to embed a loop #+ inside a variable declaration.  exit 0

The $(...) form has superseded backticks for command substitution.

output=$(sed -n /"$1"/p $file)   # From "grp.sh"	example. 	       # Setting a variable to the contents of a text file. File_contents1=$(cat $file1)       File_contents2=$(<$file2)        # Bash permits this also.

The $(...) form of command substitution treats a double backslash in a different way than `...`.

                              bash$                                                                                            echo `echo \\`                                                                                          bash$                                                                                            echo $(echo \\)                                                            \

The $(...) form of command substitution permits nesting. [57]

word_count=$( wc -w $(echo * | awk '{print $8}') )

Or, for something a bit more elaborate . . .

Example 12-3. Finding anagrams

#!/bin/bash # agram2.sh # Example of nested command substitution.  #  Uses "anagram" utility #+ that is part of the author's "yawl" word list package. #  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz #  http://bash.deta.in/yawl-0.3.2.tar.gz  E_NOARGS=86 E_BADARG=87 MINLEN=7  if [ -z "$1" ] then   echo "Usage $0 LETTERSET"   exit $E_NOARGS         # Script needs a command-line argument. elif [ ${#1} -lt $MINLEN ] then   echo "Argument must have at least $MINLEN letters."   exit $E_BADARG fi    FILTER='.......'         # Must have at least 7 letters. #       1234567 Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) ) #          $(     $(  nested command sub.    ) ) #        (              array assignment         )  echo echo "${#Anagrams[*]}  7+ letter anagrams found" echo echo ${Anagrams[0]}      # First anagram. echo ${Anagrams[1]}      # Second anagram.                          # Etc.  # echo "${Anagrams[*]}"  # To list all the anagrams in a single line . . .  #  Look ahead to the Arrays chapter for enlightenment on #+ what's going on here.  # See also the agram.sh script for an exercise in anagram finding.  exit $?

Examples of command substitution in shell scripts:

Example 11-8
Example 11-27
Example 9-16
Example 16-3
Example 16-22
Example 16-17
Example 16-54
Example 11-14
Example 11-11
Example 16-32
Example 20-8
Example A-16
Example 29-3
Example 16-47
Example 16-48
Example 16-49

Chapter 13. Arithmetic Expansion

Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let.

Variations

Arithmetic expansion with backticks (often used in conjunction with expr)

z=`expr $z + 3`          # The 'expr' command performs the expansion.

Arithmetic expansion with double parentheses, and using let

The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses -- ((...)) and $((...)) -- and also by the very convenient let construction.

z=$(($z+3)) z=$((z+3))                                  #  Also correct.                                             #  Within double parentheses,                                             #+ parameter dereferencing                                             #+ is optional.  # $((EXPRESSION)) is arithmetic expansion.  #  Not to be confused with                                             #+ command substitution.    # You may also use operations within double parentheses without assignment.    n=0   echo "n = $n"                             # n = 0    (( n += 1 ))                              # Increment. # (( $n += 1 )) is incorrect!   echo "n = $n"                             # n = 1   let z=z+3 let "z += 3"  #  Quotes permit the use of spaces in variable assignment.               #  The 'let' operator actually performs arithmetic evaluation,               #+ rather than expansion.

Examples of arithmetic expansion in scripts:

Example 16-9
Example 11-15
Example 27-1
Example 27-11
Example A-16

Chapter 14. Recess Time

This bizarre little intermission gives the reader a chance to relax and maybe laugh a bit.

  Fellow Linux user, greetings! You are reading something which
will bring you luck and good fortune. Just e-mail a copy of
this document to 10 of your friends. Before making the copies,
send a 100-line Bash script to the first person on the list
at the bottom of this letter. Then delete their name and add
yours to the bottom of the list.

  Don't break the chain! Make the copies within 48 hours.
Wilfred P. of Brooklyn failed to send out his ten copies and
woke the next morning to find his job description changed
to "COBOL programmer." Howard L. of Newport News sent
out his ten copies and within a month had enough hardware
to build a 100-node Beowulf cluster dedicated to playing
Tuxracer. Amelia V. of Chicago laughed at this letter
and broke the chain. Shortly thereafter, a fire broke out
in her terminal and she now spends her days writing
documentation for MS Windows.

  Don't break the chain!  Send out your ten copies today!

Courtesy 'NIX "fortune cookies", with some alterations and many apologies

Part 4. Commands

Chapter 15. Internal Commands and Builtins

A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off [58] a separate process -- or because a particular builtin needs direct access to the shell internals.

A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For example, the Bash echo command is not the same as /bin/echo, although their behavior is almost identical.

#!/bin/bash  echo "This line uses the \"echo\" builtin." /bin/echo "This line uses the /bin/echo system command."

A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a subunit of a command construct. [59]

I/O

echo

prints (to stdout) an expression or variable (see Example 4-1).

An echo requires the -e option to print escaped characters. See Example 5-2.

Normally, each echo command prints a terminal newline, but the -n option suppresses this.

An echo can be used to feed a sequence of commands down a pipe.

if echo "$VAR" | grep -q txt   # if [[ $VAR = *txt* ]] then   echo "$VAR contains the substring sequence \"txt\"" fi

An echo, in combination with command substitution can set a variable.

a=`echo "HELLO" | tr A-Z a-z`

See also Example 16-22, Example 16-3, Example 16-47, and Example 16-48.

Be aware that echo `command` deletes any linefeeds that the output of command generates.

The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to echo. Then echo outputs these arguments, separated by spaces.

                          bash$                                                                                ls -l /usr/share/apps/kjezz/sounds                                                    -rw-r--r--    1 root     root         1407 Nov  7  2000 reflect.au  -rw-r--r--    1 root     root          362 Nov  7  2000 seconds.au                          bash$                                                                                echo `ls -l /usr/share/apps/kjezz/sounds`                                                    total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root ...

So, how can we embed a linefeed within an echoed character string?

# Embedding a linefeed? echo "Why doesn't this string \n split on two lines?" # Doesn't split.  # Let's try something else.  echo 	      echo $"A line of text containing a linefeed." # Prints as two distinct lines (embedded linefeed). # But, is the "$" variable prefix really necessary?  echo  echo "This string splits on two lines." # No, the "$" is not needed.  echo echo "---------------" echo  echo -n $"Another line of text containing a linefeed." # Prints as two distinct lines (embedded linefeed). # Even the -n option fails to suppress the linefeed here.  echo echo echo "---------------" echo echo  # However, the following doesn't work as expected. # Why not? Hint: Assignment to a variable. string1=$"Yet another line of text containing a linefeed (maybe)."  echo $string1 # Yet another line of text containing a linefeed (maybe). #                                    ^ # Linefeed becomes a space.  # Thanks, Steve Parker, for pointing this out.

This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar.

                                    bash$                                                                                                              type -a echo                                                                        echo is a shell builtin  echo is /bin/echo

printf

The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language printf() library function, and its syntax is somewhat different.

printf format-string ... parameter ...

This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the printf manpage (of the system command) for in-depth coverage.

Older versions of Bash may not support printf.

Example 15-2. printf in action

#!/bin/bash # printf demo  declare -r PI=3.14159265358979     # Read-only variable, i.e., a constant. declare -r DecimalConstant=31373  Message1="Greetings," Message2="Earthling."  echo  printf "Pi to 2 decimal places = %1.2f" $PI echo printf "Pi to 9 decimal places = %1.9f" $PI  # It even rounds off correctly.  printf "\n"                                  # Prints a line feed,                                              # Equivalent to 'echo' . . .  printf "Constant = \t%d\n" $DecimalConstant  # Inserts tab (\t).  printf "%s %s \n" $Message1 $Message2  echo  # ==========================================# # Simulation of C function, sprintf(). # Loading a variable with a formatted string.  echo   Pi12=$(printf "%1.12f" $PI) echo "Pi to 12 decimal places = $Pi12"      # Roundoff error!  Msg=`printf "%s %s \n" $Message1 $Message2` echo $Msg; echo $Msg  #  As it happens, the 'sprintf' function can now be accessed #+ as a loadable module to Bash, #+ but this is not portable.  exit 0

Formatting error messages is a useful application of printf

E_BADDIR=85  var=nonexistent_directory  error() {   printf "$@" >&2   # Formats positional params passed, and sends them to stderr.   echo   exit $E_BADDIR }  cd $var || error $"Can't cd to %s." "$var"  # Thanks, S.C.

15.1. Job Control Commands

Certain of the following job control commands take a job identifier as an argument. See the table at end of the chapter.

jobs

Lists the jobs running in the background, giving the job number. Not as useful as ps.

It is all too easy to confuse jobs and processes. Certain builtins, such as kill, disown, and wait accept either a job number or a process number as an argument. The fg, bg and jobs commands accept only a job number.

                                      bash$                                                                                                                    sleep 100 &                                                                            [1] 1384                                      bash $                                                                                                                    jobs                                                                            [1]+  Running                 sleep 100 &

"1" is the job number (jobs are maintained by the current shell). "1384" is the PID or process ID number (processes are maintained by the system). To kill this job/process, either a kill %1 or a kill 1384 works.

Thanks, S.C.

disown

Remove job(s) from the shell's table of active jobs.

fg, bg

The fg command switches a job running in the background into the foreground. The bg command restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg command acts upon the currently running job.

wait

Suspend script execution until all jobs running in background have terminated, or until the job number or process ID specified as an option terminates. Returns the exit status of waited-for command.

You may use the wait command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process).

Example 15-26. Waiting for a process to finish before proceeding

#!/bin/bash  ROOT_UID=0   # Only users with $UID 0 have root privileges. E_NOTROOT=65 E_NOPARAMS=66  if [ "$UID" -ne "$ROOT_UID" ] then   echo "Must be root to run this script."   # "Run along kid, it's past your bedtime."   exit $E_NOTROOT fi    if [ -z "$1" ] then   echo "Usage: `basename $0` find-string"   exit $E_NOPARAMS fi   echo "Updating 'locate' database..." echo "This may take a while." updatedb /usr &     # Must be run as root.  wait # Don't run the rest of the script until 'updatedb' finished. # You want the the database updated before looking up the file name.  locate $1  #  Without the 'wait' command, in the worse case scenario, #+ the script would exit while 'updatedb' was still running, #+ leaving it as an orphan process.  exit 0

Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID . [67] See the job id table.

Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to stdout. It can be a major annoyance.

#!/bin/bash # test.sh		    ls -l & echo "Done."

                                      bash$                                                                                                                    ./test.sh                                                                            Done.  [bozo@localhost test-scripts]$ total 1  -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh  _

    As Walter Brameld IV explains it:

    As far as I can tell, such scripts don't actually hang. It just
seems that they do because the background command writes text to
the console after the prompt. The user gets the impression that
the prompt was never displayed. Here's the sequence of events:

    1. Script launches background command.
2. Script exits.
3. Shell displays the prompt.
4. Background command continues running and writing text to the
console.
5. Background command finishes.
6. User doesn't see a prompt at the bottom of the output, thinks script
is hanging.

Placing a wait after the background command seems to remedy this.

#!/bin/bash # test.sh		    ls -l & echo "Done." wait

                                      bash$                                                                                                                    ./test.sh                                                                            Done.  [bozo@localhost test-scripts]$ total 1  -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh

Redirecting the output of the command to a file or even to /dev/null also takes care of this problem.

suspend

This has a similar effect to Control-Z, but it suspends the shell (the shell's parent process should resume it at an appropriate time).

logout

Exit a login shell, optionally specifying an exit status.

times

Gives statistics on the system time elapsed when executing commands, in the following form:

This capability is of relatively limited value, since it is not common to profile and benchmark shell scripts.

kill

Forcibly terminate a process by sending it an appropriate terminate signal (see Example 17-6).

Example 15-27. A script that kills itself

#!/bin/bash # self-destruct.sh  kill $$  # Script kills its own process here.          # Recall that "$$" is the script's PID.  echo "This line will not echo." # Instead, the shell sends a "Terminated" message to stdout.  exit 0   # Normal exit? No!  #  After this script terminates prematurely, #+ what exit status does it return? # # sh self-destruct.sh # echo $? # 143 # # 143 = 128 + 15 #             TERM signal

kill -l lists all the signals (as does the file /usr/include/asm/signal.h). A kill -9 is a sure kill, which will usually terminate a process that stubbornly refuses to die with a plain kill. Sometimes, a kill -15 works. A zombie process, that is, a child process that has terminated, but that the parent process has not (yet) killed, cannot be killed by a logged-on user -- you can't kill something that is already dead -- but init will generally clean it up sooner or later.

killall

The killall command kills a running process by name, rather than by process ID. If there are multiple instances of a particular command running, then doing a killall on that command will terminate them all.

This refers to the killall command in /usr/bin, not the killall script in /etc/rc.d/init.d.

command

The command directive disables aliases and functions for the command immediately following it.

This is one of three shell directives that effect script command processing. The others are builtin and enable.

builtin

Invoking builtin BUILTIN_COMMAND runs the command BUILTIN_COMMAND as a shell builtin, temporarily disabling both functions and external system commands with the same name.

enable

This either enables or disables a shell builtin command. As an example, enable -n kill disables the shell builtin kill, so that when Bash subsequently encounters kill, it invokes the external command /bin/kill.

The -a option to enable lists all the shell builtins, indicating whether or not they are enabled. The -f filename option lets enable load a builtin as a shared library (DLL) module from a properly compiled object file. [68].

autoload

This is a port to Bash of the ksh autoloader. With autoload in place, a function with an autoload declaration will load from an external file at its first invocation. [69] This saves system resources.

Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable -f (see above).

Table 15-1. Job identifiers

Notation	Meaning
`%N`	Job number [N]
`%S`	Invocation (command-line) of job begins with string S
`%?S`	Invocation (command-line) of job contains within it string S
`%%`	"current" job (last job stopped in foreground or started in background)
`%+`	"current" job (last job stopped in foreground or started in background)
`%-`	Last job
`$!`	Last background process

Chapter 16. External Filters, Programs and Commands

Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling system commands and shell directives with simple programming constructs.

16.1. Basic Commands

The first commands a novice learns

ls

The basic file "list" command. It is all too easy to underestimate the power of this humble command. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Other useful options are -S, sort listing by file size, -t, sort by file modification time, -v, sort by (numerical) version numbers embedded in the filenames, [70] -b, show escape characters, and -i, show file inodes (see Example 16-4).

                            bash$                                                                                      ls -l                                                        -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt                            bash$                                                                                      ls -lv                                                                                      total 0  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt  -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt

The ls command returns a non-zero exit status when attempting to list a non-existent file.

                                      bash$                                                                                                                    ls abc                                                                            ls: abc: No such file or directory                                      bash$                                                                                                                    echo $?                                                                            2

Example 16-1. Using ls to create a table of contents for burning a CDR disk

#!/bin/bash # ex40.sh (burn-cd.sh) # Script to automate burning a CDR.   SPEED=10         # May use higher speed if your hardware supports it. IMAGEFILE=cdimage.iso CONTENTSFILE=contents # DEVICE=/dev/cdrom     For older versions of cdrecord DEVICE="1,0,0" DEFAULTDIR=/opt  # This is the directory containing the data to be burned.                  # Make sure it exists.                  # Exercise: Add a test for this.  # Uses Joerg Schilling's "cdrecord" package: # http://www.fokus.fhg.de/usr/schilling/cdrecord.html  #  If this script invoked as an ordinary user, may need to suid cdrecord #+ chmod u+s /usr/bin/cdrecord, as root. #  Of course, this creates a security hole, though a relatively minor one.  if [ -z "$1" ] then   IMAGE_DIRECTORY=$DEFAULTDIR   # Default directory, if not specified on command-line. else     IMAGE_DIRECTORY=$1 fi  # Create a "table of contents" file. ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE # The "l" option gives a "long" file listing. # The "R" option makes the listing recursive. # The "F" option marks the file types (directories get a trailing /). echo "Creating table of contents."  # Create an image file preparatory to burning it onto the CDR. mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY echo "Creating ISO9660 file system image ($IMAGEFILE)."  # Burn the CDR. echo "Burning the disk." echo "Please be patient, this will take a while." wodim -v -isosize dev=$DEVICE $IMAGEFILE #  In newer Linux distros, the "wodim" utility assumes the #+ functionality of "cdrecord." exitcode=$? echo "Exit code = $exitcode"  exit $exitcode

cat, tac

cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files.

# Uses of 'cat' cat filename                          # Lists the file.  cat file.1 file.2 file.3 > file.123   # Combines three files into one.

The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line.

See also Example 16-28 and Example 16-24.

In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat the file.

cat filename | tr a-z A-Z  tr a-z A-Z < filename   #  Same effect, but starts one less process,                         #+ and also dispenses with the pipe.

tac, is the inverse of cat, listing a file backwards from its end.

rev

reverses each line of a file, and outputs to stdout. This does not have the same effect as tac, as it preserves the order of the lines, but flips each one around (mirror image).

                            bash$                                                                                      cat file1.txt                                                        This is line 1.  This is line 2.                            bash$                                                                                      tac file1.txt                                                        This is line 2.  This is line 1.                            bash$                                                                                      rev file1.txt                                                        .1 enil si sihT  .2 enil si sihT

cp

This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see Example 16-6).

Particularly useful are the -a archive flag (for copying an entire directory tree), the -u update flag (which prevents overwriting identically-named newer files), and the -r and -R recursive flags.

cp -u source_dir/* dest_dir #  "Synchronize" dest_dir to source_dir #+  by copying over all newer and not previously existing files.

mv

This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see Example 10-11 and Example A-2.

When used in a non-interactive script, mv takes the -f (force) option to bypass user input.

When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory.

                                      bash$                                                                                                                    mv source_directory target_directory                                                                            bash$                                                                                                                    ls -lF target_directory                                                                            total 1  drwxrwxr-x    2 bozo  bozo      1024 May 28 19:20 source_directory/

rm

Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for bypassing user input in a script.

The rm command will, by itself, fail to remove filenames beginning with a dash. Why? Because rm sees a dash-prefixed filename as an option.

                                      bash$                                                                                                                    rm -badname                                                                            rm: invalid option -- b  Try `rm --help' for more information.

One clever workaround is to precede the filename with a " -- " (the end-of-options flag).

Another method to is to preface the filename to be removed with a dot-slash .

When used with the recursive flag -r, this command removes files all the way down the directory tree from the current directory. A careless rm -rf * can wipe out a big chunk of a directory structure.

rmdir

Remove directory. The directory must be empty of all files -- including "invisible" dotfiles [71] -- for this command to succeed.

mkdir

Make directory, creates a new directory. For example, mkdir -p project/programs/December creates the named directory. The -p option automatically creates any necessary parent directories.

chmod

Changes the attributes of an existing file or directory (see Example 15-14).

chmod +x filename # Makes "filename" executable for all users.  chmod u+s filename # Sets "suid" bit on "filename" permissions. # An ordinary user may execute "filename" with same privileges as the file's owner. # (This does not apply to shell scripts.)

chmod 644 filename #  Makes "filename" readable/writable to owner, readable to others #+ (octal mode).  chmod 444 filename #  Makes "filename" read-only for all. #  Modifying the file (for example, with a text editor) #+ not allowed for a user who does not own the file (except for root), #+ and even the file owner must force a file-save #+ if she modifies the file. #  Same restrictions apply for deleting the file.

chmod 1777 directory-name #  Gives everyone read, write, and execute permission in directory, #+ however also sets the "sticky bit". #  This means that only the owner of the directory, #+ owner of the file, and, of course, root #+ can delete any particular file in that directory.  chmod 111 directory-name #  Gives everyone execute-only permission in a directory. #  This means that you can execute and READ the files in that directory #+ (execute permission necessarily includes read permission #+ because you can't execute a file without being able to read it). #  But you can't list the files or search for them with the "find" command. #  These restrictions do not apply to root.  chmod 000 directory-name #  No permissions at all for that directory. #  Can't read, write, or execute files in it. #  Can't even list files in it or "cd" to it. #  But, you can rename (mv) the directory #+ or delete it (rmdir) if it is empty. #  You can even symlink to files in the directory, #+ but you can't read, write, or execute the symlinks. #  These restrictions do not apply to root.

chattr

Change file attributes. This is analogous to chmod above, but with different options and a different invocation syntax, and it works only on ext2/ext3 filesystems.

One particularly interesting chattr option is i. A chattr +i filename marks the file as immutable. The file cannot be modified, linked to, or deleted, not even by root. This file attribute can be set or removed only by root. In a similar fashion, the a option marks the file as append only.

                            root#                                                                                      chattr +i file1.txt                                                        root#                                                                                      rm file1.txt                                                        rm: remove write-protected regular file `file1.txt'? y  rm: cannot remove `file1.txt': Operation not permitted

If a file has the s (secure) attribute set, then when it is deleted its block is overwritten with binary zeroes. [72]

If a file has the u (undelete) attribute set, then when it is deleted, its contents can still be retrieved (undeleted).

If a file has the c (compress) attribute set, then it will automatically be compressed on writes to disk, and uncompressed on reads.

The file attributes set with chattr do not show in a file listing (ls -l).

ln

Creates links to pre-existings files. A "link" is a reference to a file, an alternate name for it. The ln command permits referencing the linked file by more than one name and is a superior alternative to aliasing (see Example 4-6).

The ln creates only a reference, a pointer to the file only a few bytes in size.

The ln command is most often used with the -s, symbolic or "soft" link flag. Advantages of using the -s flag are that it permits linking across file systems or to directories.

The syntax of the command is a bit tricky. For example: ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile.

If a file named newfile has previously existed, an error message will result.

Links give the ability to invoke a script (or any other type of executable) with multiple names, and having that script behave according to how it was invoked.

Example 16-2. Hello or Good-bye

#!/bin/bash # hello.sh: Saying "hello" or "goodbye" #+          depending on how script is invoked.  # Make a link in current working directory ($PWD) to this script: #    ln -s hello.sh goodbye # Now, try invoking this script both ways: # ./hello.sh # ./goodbye   HELLO_CALL=65 GOODBYE_CALL=66  if [ $0 = "./goodbye" ] then   echo "Good-bye!"   # Some other goodbye-type commands, as appropriate.   exit $GOODBYE_CALL fi  echo "Hello!" # Some other hello-type commands, as appropriate. exit $HELLO_CALL

man, info

These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain more detailed descriptions than do the man pages.

There have been various attempts at "automating" the writing of man pages. For a script that makes a tentative first step in that direction, see Example A-39.

16.2. Complex Commands

Commands for more advanced users

find

-exec COMMAND \;

Carries out COMMAND on each file that find matches. The command sequence terminates with ; (the ";" is escaped to make certain the shell passes it to find literally, without interpreting it as a special character).

                            bash$                                                                                      find ~/ -name '*.txt'                                                        /home/bozo/.kde/share/apps/karm/karmdata.txt  /home/bozo/misc/irmeyc.txt  /home/bozo/test-scripts/1.txt

If COMMAND contains {}, then find substitutes the full path name of the selected file for "{}".

find ~/ -name 'core*' -exec rm {} \; # Removes all core dump files from user's home directory.

find /home/bozo/projects -mtime -1 #                               ^   Note minus sign! #  Lists all files in /home/bozo/projects directory tree #+ that were modified within the last day (current_day - 1). # find /home/bozo/projects -mtime 1 #  Same as above, but modified *exactly* one day ago. # #  mtime = last modification time of the target file #  ctime = last status change time (via 'chmod' or otherwise) #  atime = last access time  DIR=/home/bozo/junk_files find "$DIR" -type f -atime +5 -exec rm {} \; #                          ^           ^^ #  Curly brackets are placeholder for the path name output by "find." # #  Deletes all files in "/home/bozo/junk_files" #+ that have not been accessed in *at least* 5 days (plus sign ... +5). # #  "-type filetype", where #  f = regular file #  d = directory #  l = symbolic link, etc. # #  (The 'find' manpage and info page have complete option listings.)

find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \;  # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. # There a few extraneous hits. Can they be filtered out?  # Possibly by:  find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \ | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' # #  [:digit:] is one of the character classes #+ introduced with the POSIX 1003.2 standard.   # Thanks, St�phane Chazelas.

The -exec option to find should not be confused with the exec shell builtin.

Example 16-3. Badname, eliminate file names in current directory containing bad characters and whitespace.

#!/bin/bash # badname.sh # Delete filenames in current directory containing bad characters.  for filename in * do   badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p` # badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'`  also works. # Deletes files containing these nasties:     + { ; " \ = ? ~ ( ) < > & * | $ #   rm $badname 2>/dev/null #             ^^^^^^^^^^^ Error messages deep-sixed. done  # Now, take care of files containing all manner of whitespace. find . -name "* *" -exec rm -f {} \; # The path name of the file that _find_ finds replaces the "{}". # The '\' ensures that the ';' is interpreted literally, as end of command.  exit 0  #--------------------------------------------------------------------- # Commands below this line will not execute because of _exit_ command.  # An alternative to the above script: find . -name '*[+{;"\\=?~()<>&*|$ ]*' -maxdepth 0 \ -exec rm -f '{}' \; #  The "-maxdepth 0" option ensures that _find_ will not search #+ subdirectories below $PWD.  # (Thanks, S.C.)

Example 16-4. Deleting a file by its inode number

#!/bin/bash # idelete.sh: Deleting a file by its inode number.  #  This is useful when a filename starts with an illegal character, #+ such as ? or -.  ARGCOUNT=1                      # Filename arg must be passed to script. E_WRONGARGS=70 E_FILE_NOT_EXIST=71 E_CHANGED_MIND=72  if [ $# -ne "$ARGCOUNT" ] then   echo "Usage: `basename $0` filename"   exit $E_WRONGARGS fi    if [ ! -e "$1" ] then   echo "File \""$1"\" does not exist."   exit $E_FILE_NOT_EXIST fi    inum=`ls -i | grep "$1" | awk '{print $1}'` # inum = inode (index node) number of file # ----------------------------------------------------------------------- # Every file has an inode, a record that holds its physical address info. # -----------------------------------------------------------------------  echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? " # The '-v' option to 'rm' also asks this. read answer case "$answer" in [nN]) echo "Changed your mind, huh?"       exit $E_CHANGED_MIND       ;; *)    echo "Deleting file \"$1\".";; esac  find . -inum $inum -exec rm {} \; #                           ^^ #        Curly brackets are placeholder #+       for text output by "find." echo "File "\"$1"\" deleted!"  exit 0

The find command also works without the -exec option.

#!/bin/bash #  Find suid root files. #  A strange suid file might indicate a security hole, #+ or even a system intrusion.  directory="/usr/sbin" # Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc. permissions="+4000"  # suid root (dangerous!)   for file in $( find "$directory" -perm "$permissions" ) do   ls -ltF --author "$file" done

See Example 16-30, Example 3-4, and Example 11-10 for scripts using find. Its manpage provides more detail on this complex and powerful command.

xargs

A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where command substitution fails with a too many arguments error, substituting xargs often works. [73] Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file.

The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out.

                            bash$                                                                                      ls -l                                                        total 0  -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file1  -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file2                            bash$                                                                                      ls -l | xargs                                                        total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan...                            bash$                                                                                      find ~/mail -type f | xargs grep "Linux"                                                        ./misc:User-Agent: slrn/0.9.8.1 (Linux)  ./sent-mail-jul-2005: hosted by the Linux Documentation Project.  ./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)  ./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article  ./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem  . . .

ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation.

Note that xargs processes the arguments passed to it sequentially, one at a time.

                                      bash$                                                                                                                    find /usr/bin | xargs file                                                                            /usr/bin:          directory  /usr/bin/foomatic-ppd-options:          perl script text executable  . . .

An interesting xargs option is -n NN , which limits to NN the number of arguments passed.

ls | xargs -n 8 echo lists the files in the current directory in 8 columns.

Another useful option is -0, in combination with find -print0 or grep -lZ . This allows handling arguments containing whitespace or quotes.

find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f

grep -rliwZ GUI / | xargs -0 rm -f

Either of the above will remove any file containing "GUI". (Thanks, S.C.)

Or:

cat /proc/"$pid"/"$OPTION" | xargs -0 echo #  Formats output:         ^^^^^^^^^^^^^^^ #  From Han Holl's fixup of "get-commandline.sh" #+ script in "/dev and /proc" chapter.

The -P option to xargs permits running processes in parallel. This speeds up execution in a machine with a multicore CPU.

#!/bin/bash  ls *gif | xargs -t -n1 -P2 gif2png # Converts all the gif images in current directory to png.  # Options: # ======= # -t    Print command to stderr. # -n1   At most 1 argument per command line. # -P2   Run up to 2 processes simultaneously.  # Thank you, Roberto Polli, for the inspiration.

Example 16-5. Logfile: Using xargs to monitor system log

#!/bin/bash  # Generates a log file in current directory # from the tail end of /var/log/messages.  # Note: /var/log/messages must be world readable # if this script invoked by an ordinary user. #         #root chmod 644 /var/log/messages  LINES=5  ( date; uname -a ) >>logfile # Time and machine name echo ---------------------------------------------------------- >>logfile tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile echo >>logfile echo >>logfile  exit 0  #  Note: #  ---- #  As Frank Wang points out, #+ unmatched quotes (either single or double quotes) in the source file #+ may give xargs indigestion. # #  He suggests the following substitution for line 15: #  tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile    #  Exercise: #  -------- #  Modify this script to track changes in /var/log/messages at intervals #+ of 20 minutes. #  Hint: Use the "watch" command.

As in find, a curly bracket pair serves as a placeholder for replacement text.

Example 16-6. Copying files in current directory to another

#!/bin/bash # copydir.sh  #  Copy (verbose) all files in current directory ($PWD) #+ to directory specified on command-line.  E_NOARGS=85  if [ -z "$1" ]   # Exit if no argument given. then   echo "Usage: `basename $0` directory-to-copy-to"   exit $E_NOARGS fi    ls . | xargs -i -t cp ./{} $1 #            ^^ ^^      ^^ #  -t is "verbose" (output command-line to stderr) option. #  -i is "replace strings" option. #  {} is a placeholder for output text. #  This is similar to the use of a curly-bracket pair in "find." # #  List the files in current directory (ls .), #+ pass the output of "ls" as arguments to "xargs" (-i -t options), #+ then copy (cp) these arguments ({}) to new directory ($1).   # #  The net result is the exact equivalent of #+   cp * $1 #+ unless any of the filenames has embedded "whitespace" characters.  exit 0

Example 16-7. Killing processes by name

#!/bin/bash # kill-byname.sh: Killing processes by name. # Compare this script with kill-process.sh.  #  For instance, #+ try "./kill-byname.sh xterm" -- #+ and watch all the xterms on your desktop disappear.  #  Warning: #  ------- #  This is a fairly dangerous script. #  Running it carelessly (especially as root) #+ can cause data loss and other undesirable effects.  E_BADARGS=66  if test -z "$1"  # No command-line arg supplied? then   echo "Usage: `basename $0` Process(es)_to_kill"   exit $E_BADARGS fi   PROCESS_NAME="$1" ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/null #                                                       ^^      ^^  # --------------------------------------------------------------- # Notes: # -i is the "replace strings" option to xargs. # The curly brackets are the placeholder for the replacement. # 2&>/dev/null suppresses unwanted error messages. # # Can  grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"? # ---------------------------------------------------------------  exit $?  #  The "killall" command has the same effect as this script, #+ but using it is not quite as educational.

Example 16-8. Word frequency analysis using xargs

#!/bin/bash # wf2.sh: Crude word frequency analysis on a text file.  # Uses 'xargs' to decompose lines of text into single words. # Compare this example to the "wf.sh" script later on.   # Check for input file on command-line. ARGS=1 E_BADARGS=85 E_NOFILE=86  if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script? then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi  if [ ! -f "$1" ]       # Does file exist? then   echo "File \"$1\" does not exist."   exit $E_NOFILE fi    ##################################################### cat "$1" | xargs -n1 | \ #  List the file, one word per line.  tr A-Z a-z | \ #  Shift characters to lowercase. sed -e 's/\.//g'  -e 's/\,//g' -e 's/ /\ /g' | \ #  Filter out periods and commas, and #+ change space between words to linefeed, sort | uniq -c | sort -nr #  Finally remove duplicates, prefix occurrence count #+ and sort numerically. #####################################################  #  This does the same job as the "wf.sh" example, #+ but a bit more ponderously, and it runs more slowly (why?).  exit $?

expr

All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical.

expr 3 + 5

returns 8

expr 5 % 3

returns 2

expr 1 / 0

returns the error message, expr: division by zero

Illegal arithmetic operations not allowed.

expr 5 \* 3

returns 15

The multiplication operator must be escaped when used in an arithmetic expression with expr.

y=`expr $y + 1`

Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)) . This is an example of arithmetic expansion.

z=`expr substr $string $position $length`

Extract substring of $length characters, starting at $position.

Example 16-9. Using expr

#!/bin/bash  # Demonstrating some of the uses of 'expr' # =======================================  echo  # Arithmetic Operators # ---------- ---------  echo "Arithmetic Operators" echo a=`expr 5 + 3` echo "5 + 3 = $a"  a=`expr $a + 1` echo echo "a + 1 = $a" echo "(incrementing a variable)"  a=`expr 5 % 3` # modulo echo echo "5 mod 3 = $a"  echo echo  # Logical Operators # ------- ---------  #  Returns 1 if true, 0 if false, #+ opposite of normal Bash convention.  echo "Logical Operators" echo  x=24 y=25 b=`expr $x = $y`         # Test equality. echo "b = $b"            # 0  ( $x -ne $y ) echo  a=3 b=`expr $a \> 10` echo 'b=`expr $a \> 10`, therefore...' echo "If a > 10, b = 0 (false)" echo "b = $b"            # 0  ( 3 ! -gt 10 ) echo  b=`expr $a \< 10` echo "If a < 10, b = 1 (true)" echo "b = $b"            # 1  ( 3 -lt 10 ) echo # Note escaping of operators.  b=`expr $a \<= 3` echo "If a <= 3, b = 1 (true)" echo "b = $b"            # 1  ( 3 -le 3 ) # There is also a "\>=" operator (greater than or equal to).   echo echo    # String Operators # ------ ---------  echo "String Operators" echo  a=1234zipper43231 echo "The string being operated upon is \"$a\"."  # length: length of string b=`expr length $a` echo "Length of \"$a\" is $b."  # index: position of first character in substring #        that matches a character in string b=`expr index $a 23` echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."  # substr: extract substring, starting position & length specified b=`expr substr $a 2 6` echo "Substring of \"$a\", starting at position 2,\ and 6 chars long is \"$b\"."   #  The default behavior of the 'match' operations is to #+ search for the specified match at the BEGINNING of the string. # #       Using Regular Expressions ... b=`expr match "$a" '[0-9]*'`               #  Numerical count. echo Number of digits at the beginning of \"$a\" is $b. b=`expr match "$a" '\([0-9]*\)'`           #  Note that escaped parentheses #                   ==      ==             #+ trigger substring match. echo "The digits at the beginning of \"$a\" are \"$b\"."  echo  exit 0

The : (null) operator can substitute for match. For example, b=`expr $a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in the above listing.

#!/bin/bash  echo echo "String operations using \"expr \$string : \" construct" echo "===================================================" echo  a=1234zipper5FLIPPER43231  echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"." #     Escaped parentheses grouping operator.            ==  ==  #       *************************** #+          Escaped parentheses #+           match a substring #       ***************************   #  If no escaped parentheses ... #+ then 'expr' converts the string operand to an integer.  echo "Length of \"$a\" is `expr "$a" : '.*'`."   # Length of string  echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."  # ------------------------------------------------------------------------- #  echo  echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`." #                                                             ==      == echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`." #         =====                                          ==       == # Again, escaped parentheses force a substring match. # echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`." #         ====                  end of string operator  ^^ #  (In fact, means skip over one or more of any characters until specified #+  substring found.)  echo  exit 0

The above script illustrates how expr uses the escaped parentheses -- $ ... $ -- grouping operator in tandem with regular expression parsing to match a substring. Here is a another example, this time from "real life."

# Strip the whitespace from the beginning and end. LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'`  #  From Peter Knowles' "booklistgen.sh" script #+ for converting files to Sony Librie/PRS-50X format. #  (http://booklistgensh.peterknowles.com)

Perl, sed, and awk have far superior string parsing facilities. A short sed or awk "subroutine" within a script (see Section 36.2) is an attractive alternative to expr.

See Section 10.1 for more on using expr in string operations.

16.3. Time / Date Commands

Time/date and timing

date

Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options.

Example 16-10. Using date

#!/bin/bash # Exercising the 'date' command  echo "The number of days since the year's beginning is `date +%j`." # Needs a leading '+' to invoke formatting. # %j gives day of year.  echo "The number of seconds elapsed since 01/01/1970 is `date +%s`." #  %s yields number of seconds since "UNIX epoch" began, #+ but how is this useful?  prefix=temp suffix=$(date +%s)  # The "+%s" option to 'date' is GNU-specific. filename=$prefix.$suffix echo "Temporary filename = $filename" #  It's great for creating "unique and random" temp filenames, #+ even better than using $$.  # Read the 'date' man page for more formatting options.  exit 0

The -u option gives the UTC (Universal Coordinated Time).

                            bash$                                                                                      date                                                        Fri Mar 29 21:07:39 MST 2002                            bash$                                                                                      date -u                                                        Sat Mar 30 04:07:42 UTC 2002

This option facilitates calculating the time between different dates.

Example 16-11. Date calculations

#!/bin/bash # date-calc.sh # Author: Nathan Coulter # Used in ABS Guide with permission (thanks!).  MPHR=60    # Minutes per hour. HPD=24     # Hours per day.  diff () {         printf '%s' $(( $(date -u -d"$TARGET" +%s) -                         $(date -u -d"$CURRENT" +%s))) #                       %d = day of month. }   CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z') TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z') # %F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.  printf '\nIn 2007, %s ' \        "$(date -d"$CURRENT +         $(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"  #       %B = name of month                ^ halfway printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')" printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"  printf '\nOn %s at %s, there were\n' \         $(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T) DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD )) CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z') HOURS=$(( $(diff) / $MPHR / $MPHR )) CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z') MINUTES=$(( $(diff) / $MPHR )) CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z') printf '%s days, %s hours, ' "$DAYS" "$HOURS" printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)" printf 'until Christmas Dinner!\n\n'  #  Exercise: #  -------- #  Rewrite the diff () function to accept passed parameters, #+ rather than using global variables.

The date command has quite a number of output options. For example %N gives the nanosecond portion of the current time. One interesting use for this is to generate random integers.

date +%N | sed -e 's/000$//' -e 's/^0//'            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #  Strip off leading and trailing zeroes, if present. #  Length of generated integer depends on #+ how many zeroes stripped off.  # 115281032 # 63408725 # 394504284

There are many more options (try man date).

date +%j # Echoes day of the year (days elapsed since January 1).  date +%k%M # Echoes hour and minute in 24-hour format, as a single digit string.    # The 'TZ' parameter permits overriding the default time zone. date                 # Mon Mar 28 21:42:16 MST 2005 TZ=EST date          # Mon Mar 28 23:42:16 EST 2005 # Thanks, Frank Kannemann and Pete Sjoberg, for the tip.   SixDaysAgo=$(date --date='6 days ago') OneMonthAgo=$(date --date='1 month ago')  # Four weeks back (not a month!) OneYearAgo=$(date --date='1 year ago')

16.4. Text Processing Commands

Commands affecting text and text files

sort

File sort utility, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the -m option, it merges presorted input files. The info page lists its many capabilities and options. See Example 11-10, Example 11-11, and Example A-8.

tsort

Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns. The original purpose of tsort was to sort a list of dependencies for an obsolete version of the ld linker in an "ancient" version of UNIX.

The results of a tsort will usually differ markedly from those of the standard sort command, above.

uniq

This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort.

cat list-1 list-2 list-3 | sort | uniq > final.list # Concatenates the list files, # sorts them, # removes duplicate lines, # and finally writes the result to an output file.

The useful -c option prefixes each line of the input file with its number of occurrences.

                            bash$                                                                                      cat testfile                                                        This line occurs only once.  This line occurs twice.  This line occurs twice.  This line occurs three times.  This line occurs three times.  This line occurs three times.                            bash$                                                                                      uniq -c testfile                                                                                      1 This line occurs only once.        2 This line occurs twice.        3 This line occurs three times.                            bash$                                                                                      sort testfile | uniq -c | sort -nr                                                                                      3 This line occurs three times.        2 This line occurs twice.        1 This line occurs only once.

The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined.

Example 16-12. Word Frequency Analysis

#!/bin/bash # wf.sh: Crude word frequency analysis on a text file. # This is a more efficient version of the "wf2.sh" script.   # Check for input file on command-line. ARGS=1 E_BADARGS=85 E_NOFILE=86  if [ $# -ne "$ARGS" ]  # Correct number of arguments passed to script? then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi  if [ ! -f "$1" ]       # Check if file exists. then   echo "File \"$1\" does not exist."   exit $E_NOFILE fi    ######################################################## # main () sed -e 's/\.//g'  -e 's/\,//g' -e 's/ /\ /g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr #                           ========================= #                            Frequency of occurrence  #  Filter out periods and commas, and #+ change space between words to linefeed, #+ then shift characters to lowercase, and #+ finally prefix occurrence count and sort numerically.  #  Arun Giridhar suggests modifying the above to: #  . . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr #  This adds a secondary sort key, so instances of #+ equal occurrence are sorted alphabetically. #  As he explains it: #  "This is effectively a radix sort, first on the #+ least significant column #+ (word or string, optionally case-insensitive) #+ and last on the most significant column (frequency)." # #  As Frank Wang explains, the above is equivalent to #+       . . . | sort | uniq -c | sort +0 -nr #+ and the following also works: #+       . . . | sort | uniq -c | sort -k1nr -k ########################################################  exit 0  # Exercises: # --------- # 1) Add 'sed' commands to filter out other punctuation, #+   such as semicolons. # 2) Modify the script to also filter out multiple spaces and #+   other whitespace.

                            bash$                                                                                      cat testfile                                                        This line occurs only once.  This line occurs twice.  This line occurs twice.  This line occurs three times.  This line occurs three times.  This line occurs three times.                            bash$                                                                                      ./wf.sh testfile                                                                                      6 this        6 occurs        6 line        3 times        3 three        2 twice        1 only        1 once

expand, unexpand

The expand filter converts tabs to spaces. It is often used in a pipe.

The unexpand filter converts spaces to tabs. This reverses the effect of expand.

cut

A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter) and -f (field specifier) options.

Using cut to obtain a listing of the mounted filesystems:

cut -d ' ' -f1,2 /etc/mtab

Using cut to list the OS and kernel version:

uname -a | cut -d" " -f1,3,11,12

Using cut to extract message headers from an e-mail folder:

                            bash$                                                                                      grep '^Subject:' read-messages | cut -c10-80                                                        Re: Linux suitable for mission-critical apps?  MAKE MILLIONS WORKING AT HOME!!!  Spam complaint  Re: Spam complaint

Using cut to parse a file:

# List all the users in /etc/passwd.  FILENAME=/etc/passwd  for user in $(cut -d: -f1 $FILENAME) do   echo $user done  # Thanks, Oleg Philon for suggesting this.

cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }' filename

It is even possible to specify a linefeed as a delimiter. The trick is to actually embed a linefeed (RETURN) in the command sequence.

                                      bash$                                                                                                                    cut -d'  ' -f3,7,19 testfile                                                                            This is line 3 of testfile.  This is line 7 of testfile.  This is line 19 of testfile.

Thank you, Jaka Kranjc, for pointing this out.

16.5. File and Archiving Commands

Archiving

tar

The standard UNIX archiving utility. [75] Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even stdout (see Example 3-4). GNU tar has been patched to accept various compression filters, for example: tar czvf archive_name.tar.gz *, which recursively archives and gzips all files in a directory tree except dotfiles in the current working directory ($PWD). [76]

Some useful tar options:

-c create (a new archive)
-x extract (files from existing archive)

--delete delete (files from existing archive)

This option will not work on magnetic tape devices.

-r append (files to existing archive)
-A append (tar files to existing archive)
-t list (contents of existing archive)
-u update archive
-d compare archive with specified filesystem
--after-date only process files with a date stamp after specified date
-z gzip the archive

(compress or uncompress, depending on whether combined with the -c or -x) option
-j bzip2 the archive

It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups.

shar

Shell archiving utility. The text and/or binary files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, containing all the necessary unarchiving commands, as well as the files themselves. Unprintable binary characters in the target file(s) are converted to printable ASCII characters in the output shar file. Shar archives still show up in Usenet newsgroups, but otherwise shar has been replaced by tar/gzip. The unshar command unpacks shar archives.

The mailshar command is a Bash script that uses shar to concatenate multiple files into a single one for e-mailing. This script supports compression and uuencoding.

ar

Creation and manipulation utility for archives, mainly used for binary object file libraries.

rpm

The Red Hat Package Manager, or rpm utility provides a wrapper for source or binary archives. It includes commands for installing and checking the integrity of packages, among other things.

A simple rpm -i package_name.rpm usually suffices to install a package, though there are many more options available.

rpm -qf identifies which package a file originates from.

                                      bash$                                                                                                                    rpm -qf /bin/ls                                                                            coreutils-5.2.1-31

rpm -qa gives a complete list of all installed rpm packages on a given system. An rpm -qa package_name lists only the package(s) corresponding to package_name.

                                      bash$                                                                                                                    rpm -qa                                                                            redhat-logos-1.1.3-1  glibc-2.2.4-13  cracklib-2.7-12  dosfstools-2.7-1  gdbm-1.8.0-10  ksymoops-2.4.1-1  mktemp-1.5-11  perl-5.6.0-17  reiserfs-utils-3.x.0j-2  ...                                      bash$                                                                                                                    rpm -qa docbook-utils                                                                            docbook-utils-0.6.9-2                                      bash$                                                                                                                    rpm -qa docbook | grep docbook                                                                            docbook-dtd31-sgml-1.0-10  docbook-style-dsssl-1.64-3  docbook-dtd30-sgml-1.0-10  docbook-dtd40-sgml-1.0-11  docbook-utils-pdf-0.6.9-2  docbook-dtd41-sgml-1.0-10  docbook-utils-0.6.9-2

cpio

This specialized archiving copy command (copy input and output) is rarely seen any more, having been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. With an appropriate block size (for copying) specified, it can be appreciably faster than tar.

Example 16-30. Using cpio to move a directory tree

#!/bin/bash  # Copying a directory tree using cpio.  # Advantages of using 'cpio': #   Speed of copying. It's faster than 'tar' with pipes. #   Well suited for copying special files (named pipes, etc.) #+  that 'cp' may choke on.  ARGS=2 E_BADARGS=65  if [ $# -ne "$ARGS" ] then   echo "Usage: `basename $0` source destination"   exit $E_BADARGS fi    source="$1" destination="$2"  ################################################################### find "$source" -depth | cpio -admvp "$destination" #               ^^^^^         ^^^^^ #  Read the 'find' and 'cpio' info pages to decipher these options. #  The above works only relative to $PWD (current directory) . . . #+ full pathnames are specified. ###################################################################   # Exercise: # --------  #  Add code to check the exit status ($?) of the 'find | cpio' pipe #+ and output appropriate error messages if anything went wrong.  exit $?

rpm2cpio

This command extracts a cpio archive from an rpm one.

Example 16-31. Unpacking an rpm archive

#!/bin/bash # de-rpm.sh: Unpack an 'rpm' archive  : ${1?"Usage: `basename $0` target-file"} # Must specify 'rpm' archive name as an argument.   TEMPFILE=$$.cpio                         #  Tempfile with "unique" name.                                          #  $$ is process ID of script.  rpm2cpio < $1 > $TEMPFILE                #  Converts rpm archive into                                          #+ cpio archive. cpio --make-directories -F $TEMPFILE -i  #  Unpacks cpio archive. rm -f $TEMPFILE                          #  Deletes cpio archive.  exit 0  #  Exercise: #  Add check for whether 1) "target-file" exists and #+                       2) it is an rpm archive. #  Hint:                    Parse output of 'file' command.

pax

The pax portable archive exchange toolkit facilitates periodic file backups and is designed to be cross-compatible between various flavors of UNIX. It was designed to replace tar and cpio.

pax -wf daily_backup.pax ~/linux-server/files  #  Creates a tar archive of all files in the target directory. #  Note that the options to pax must be in the correct order -- #+ pax -fw     has an entirely different effect.  pax -f daily_backup.pax #  Lists the files in the archive.  pax -rf daily_backup.pax ~/bsd-server/files #  Restores the backed-up files from the Linux machine #+ onto a BSD one.

Note that pax handles many of the standard archiving and compression commands.

Compression

gzip

The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The corresponding decompression command is gunzip, which is the equivalent of gzip -d.

The -c option sends the output of gzip to stdout. This is useful when piping to other commands.

The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This is, in effect, a cat command that works on compressed files (including files processed with the older compress utility). The zcat command is equivalent to gzip -dc.

On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will not work on gzipped files.

16.6. Communications Commands

Certain of the following commands find use in network data transfer and analysis, as well as in chasing spammers.

Information and Statistics

host

Searches for information about an Internet host by name or IP address, using DNS.

                            bash$                                                                                      host surfacemail.com                                                        surfacemail.com. has address 202.92.42.236

ipcalc

Displays IP information for a host. With the -h option, ipcalc does a reverse DNS lookup, finding the name of the host (server) from the IP address.

                            bash$                                                                                      ipcalc -h 202.92.42.236                                                        HOSTNAME=surfacemail.com

nslookup

Do an Internet "name server lookup" on a host by IP address. This is essentially equivalent to ipcalc -h or dig -x . The command may be run either interactively or noninteractively, i.e., from within a script.

The nslookup command has allegedly been "deprecated," but it is still useful.

                            bash$                                                                                      nslookup -sil 66.97.104.180                                                        nslookup kuhleersparnis.ch  Server:         135.116.137.2  Address:        135.116.137.2#53   Non-authoritative answer:  Name:   kuhleersparnis.ch

dig

Domain Information Groper. Similar to nslookup, dig does an Internet name server lookup on a host. May be run from the command-line or from within a script.

Some interesting options to dig are +time=N for setting a query timeout to N seconds, +nofail for continuing to query servers until a reply is received, and -x for doing a reverse address lookup.

Compare the output of dig -x with ipcalc -h and nslookup.

                            bash$                                                                                      dig -x 81.9.6.2                                                        ;; Got answer:  ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649  ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0   ;; QUESTION SECTION:  ;2.6.9.81.in-addr.arpa.         IN      PTR   ;; AUTHORITY SECTION:  6.9.81.in-addr.arpa.    3600    IN      SOA     ns.eltel.net. noc.eltel.net.  2002031705 900 600 86400 3600   ;; Query time: 537 msec  ;; SERVER: 135.116.137.2#53(135.116.137.2)  ;; WHEN: Wed Jun 26 08:35:24 2002  ;; MSG SIZE  rcvd: 91

Example 16-40. Finding out where to report a spammer

#!/bin/bash # spam-lookup.sh: Look up abuse contact to report a spammer. # Thanks, Michael Zick.  # Check for command-line arg. ARGCOUNT=1 E_WRONGARGS=85 if [ $# -ne "$ARGCOUNT" ] then   echo "Usage: `basename $0` domain-name"   exit $E_WRONGARGS fi   dig +short $1.contacts.abuse.net -c in -t txt # Also try: #     dig +nssearch $1 #     Tries to find "authoritative name servers" and display SOA records.  # The following also works: #     whois -h whois.abuse.net $1 #           ^^ ^^^^^^^^^^^^^^^  Specify host.   #     Can even lookup multiple spammers with this, i.e." #     whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .   #  Exercise: #  -------- #  Expand the functionality of this script #+ so that it automatically e-mails a notification #+ to the responsible ISP's contact address(es). #  Hint: use the "mail" command.  exit $?  # spam-lookup.sh chinatietong.com #                A known spam domain.  # "crnet_mgr@chinatietong.com" # "crnet_tec@chinatietong.com" # "postmaster@chinatietong.com"   #  For a more elaborate version of this script, #+ see the SpamViz home page, http://www.spamviz.net/index.html.

Example 16-41. Analyzing a spam domain

#! /bin/bash # is-spammer.sh: Identifying spam domains  # $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $ # Above line is RCS ID info. # #  This is a simplified version of the "is_spammer.bash #+ script in the Contributed Scripts appendix.  # is-spammer <domain.name>  # Uses an external program: 'dig' # Tested with version: 9.2.4rc5  # Uses functions. # Uses IFS to parse strings by assignment into arrays. # And even does something useful: checks e-mail blacklists.  # Use the domain.name(s) from the text body: # http://www.good_stuff.spammer.biz/just_ignore_everything_else #                       ^^^^^^^^^^^ # Or the domain.name(s) from any e-mail address: # Really_Good_Offer@spammer.biz # # as the only argument to this script. #(PS: have your Inet connection running) # # So, to invoke this script in the above two instances: #       is-spammer.sh spammer.biz   # Whitespace == :Space:Tab:Line Feed:Carriage Return: WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'  # No Whitespace == Line Feed:Carriage Return No_WSP=$'\x0A'$'\x0D'  # Field separator for dotted decimal ip addresses ADR_IFS=${No_WSP}'.'  # Get the dns text resource record. # get_txt <error_code> <list_query> get_txt() {      # Parse $1 by assignment at the dots.     local -a dns     IFS=$ADR_IFS     dns=( $1 )     IFS=$WSP_IFS     if [ "${dns[0]}" == '127' ]     then         # See if there is a reason.         echo $(dig +short $2 -t txt)     fi }  # Get the dns address resource record. # chk_adr <rev_dns> <list_server> chk_adr() {     local reply     local server     local reason      server=${1}${2}     reply=$( dig +short ${server} )      # If reply might be an error code . . .     if [ ${#reply} -gt 6 ]     then         reason=$(get_txt ${reply} ${server} )         reason=${reason:-${reply}}     fi     echo ${reason:-' not blacklisted.'} }  # Need to get the IP address from the name. echo 'Get address of: '$1 ip_adr=$(dig +short $1) dns_reply=${ip_adr:-' no answer '} echo ' Found address: '${dns_reply}  # A valid reply is at least 4 digits plus 3 dots. if [ ${#ip_adr} -gt 6 ] then     echo     declare query      # Parse by assignment at the dots.     declare -a dns     IFS=$ADR_IFS     dns=( ${ip_adr} )     IFS=$WSP_IFS      # Reorder octets into dns query order.     rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'  # See: http://www.spamhaus.org (Conservative, well maintained)     echo -n 'spamhaus.org says: '     echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')  # See: http://ordb.org (Open mail relays)     echo -n '   ordb.org  says: '     echo $(chk_adr ${rev_dns} 'relays.ordb.org')  # See: http://www.spamcop.net/ (You can report spammers here)     echo -n ' spamcop.net says: '     echo $(chk_adr ${rev_dns} 'bl.spamcop.net')  # # # other blacklist operations # # #  # See: http://cbl.abuseat.org.     echo -n ' abuseat.org says: '     echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')  # See: http://dsbl.org/usage (Various mail relays)     echo     echo 'Distributed Server Listings'     echo -n '       list.dsbl.org says: '     echo $(chk_adr ${rev_dns} 'list.dsbl.org')      echo -n '   multihop.dsbl.org says: '     echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')      echo -n 'unconfirmed.dsbl.org says: '     echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')  else     echo     echo 'Could not use that address.' fi  exit 0  # Exercises: # --------  # 1) Check arguments to script, #    and exit with appropriate error message if necessary.  # 2) Check if on-line at invocation of script, #    and exit with appropriate error message if necessary.  # 3) Substitute generic variables for "hard-coded" BHL domains.  # 4) Set a time-out for the script using the "+time=" option      to the 'dig' command.

For a much more elaborate version of the above script, see Example A-28.

traceroute

Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe.

                            bash$                                                                                      traceroute 81.9.6.2                                                        traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets  1  tc43.xjbnnbrb.com (136.30.178.8)  191.303 ms  179.400 ms  179.767 ms  2  or0.xjbnnbrb.com (136.30.178.1)  179.536 ms  179.534 ms  169.685 ms  3  192.168.11.101 (192.168.11.101)  189.471 ms  189.556 ms *  ...

ping

Broadcast an ICMP ECHO_REQUEST packet to another machine, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution.

                            bash$                                                                                      ping localhost                                                        PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.  64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec  64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec   --- localhost.localdomain ping statistics ---  2 packets transmitted, 2 packets received, 0% packet loss  round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms

A successful ping returns an exit status of 0. This can be tested for in a script.

                            HNAME=news-15.net  # Notorious spammer. # HNAME=$HOST     # Debug: test for localhost.   count=2  # Send only two pings.  if [[ `ping -c $count "$HNAME"` ]] then   echo ""$HNAME" still up and broadcasting spam your way." else   echo ""$HNAME" seems to be down. Pity." fi

whois

Perform a DNS (Domain Name System) lookup. The -h option permits specifying which particular whois server to query. See Example 4-6 and Example 16-40.

finger

Retrieve information about users on a network. Optionally, this command can display a user's ~/.plan, ~/.project, and ~/.forward files, if present.

                            bash$                                                                                      finger                                                        Login  Name           Tty      Idle  Login Time   Office     Office Phone  bozo   Bozo Bozeman   tty1        8  Jun 25 16:59                (:0)  bozo   Bozo Bozeman   ttyp0          Jun 25 16:59                (:0.0)  bozo   Bozo Bozeman   ttyp1          Jun 25 17:07                (:0.0)                            bash$                                                                                      finger bozo                                                        Login: bozo                             Name: Bozo Bozeman  Directory: /home/bozo                   Shell: /bin/bash  Office: 2355 Clown St., 543-1234  On since Fri Aug 31 20:13 (MST) on tty1    1 hour 38 minutes idle  On since Fri Aug 31 20:13 (MST) on pts/0   12 seconds idle  On since Fri Aug 31 20:13 (MST) on pts/1  On since Fri Aug 31 20:31 (MST) on pts/2   1 hour 16 minutes idle  Mail last read Tue Jul  3 10:08 2007 (MST)   No Plan.

Out of security considerations, many networks disable finger and its associated daemon. [81]

chfn

Change information disclosed by the finger command.

vrfy

Verify an Internet e-mail address.

This command seems to be missing from newer Linux distros.

Remote Host Access

sx, rx

The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom.

sz, rz

The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package.

ftp

Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see Example 19-6 and Example A-4).

uucp, uux, cu

uucp: UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence.

Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate. The advantage of uucp is that it is fault-tolerant, so even if there is a service interruption the copy operation will resume where it left off when the connection is restored.

---

uux: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the uucp package.

---

cu: Call Up a remote system and connect as a simple terminal. It is a sort of dumbed-down version of telnet. This command is part of the uucp package.

telnet

Utility and protocol for connecting to a remote host.

The telnet protocol contains security holes and should therefore probably be avoided. Its use within a shell script is not recommended.

wget

The wget utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in a script.

wget -p http://www.xyz23.com/file01.html #  The -p or --page-requisite option causes wget to fetch all files #+ required to display the specified page.  wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE #  The -r option recursively follows and retrieves all links #+ on the specified site.  wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2 #  The -c option lets wget resume an interrupted download. #  This works with ftp servers and many HTTP sites.

Example 16-42. Getting a stock quote

#!/bin/bash # quote-fetch.sh: Download a stock quote.   E_NOPARAMS=86  if [ -z "$1" ]  # Must specify a stock (symbol) to fetch.   then echo "Usage: `basename $0` stock-symbol"   exit $E_NOPARAMS fi  stock_symbol=$1  file_suffix=.html # Fetches an HTML file, so name it appropriately. URL='http://finance.yahoo.com/q?s=' # Yahoo finance board, with stock query suffix.  # ----------------------------------------------------------- wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}" # -----------------------------------------------------------   # To look up stuff on http://search.yahoo.com: # ----------------------------------------------------------- # URL="http://search.yahoo.com/search?fr=ush-news&p=${query}" # wget -O "$savefilename" "${URL}" # ----------------------------------------------------------- # Saves a list of relevant URLs.  exit $?  # Exercises: # --------- # # 1) Add a test to ensure the user running the script is on-line. #    (Hint: parse the output of 'ps -ax' for "ppp" or "connect." # # 2) Modify this script to fetch the local weather report, #+   taking the user's zip code as an argument.

16.7. Terminal Control Commands

Command affecting the console or terminal

tput

Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain terminal operations: tput clear is the equivalent of clear; tput reset is the equivalent of reset.

                            bash$                                                                                      tput longname                                                        xterm terminal emulator (X Window System)

Issuing a tput cup X Y moves the cursor to the (X,Y) coordinates in the current terminal. A clear to erase the terminal screen would normally precede this.

Some interesting options to tput are:

bold, for high-intensity text
smul, to underline text in the terminal
smso, to render text in reverse
sgr0, to reset the terminal parameters (to normal), without clearing the screen

Example scripts using tput:

Example 36-15
Example 36-13
Example A-44
Example A-42
Example 27-2

Note that stty offers a more powerful command set for controlling a terminal.

infocmp

This command prints out extensive information about the current terminal. It references the terminfo database.

                            bash$                                                                                      infocmp                                                        #       Reconstructed via infocmp from file:  /usr/share/terminfo/r/rxvt  rxvt|rxvt terminal emulator (X Window System),           am, bce, eo, km, mir, msgr, xenl, xon,           colors#8, cols#80, it#8, lines#24, pairs#64,           acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,           bel=^G, blink=\E[5m, bold=\E[1m,          civis=\E[?25l,           clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,           ...

reset

Reset terminal parameters and clear text screen. As with clear, the cursor and prompt reappear in the upper lefthand corner of the terminal.

clear

The clear command simply clears the text screen at the console or in an xterm. The prompt and cursor reappear at the upper lefthand corner of the screen or xterm window. This command may be used either at the command line or in a script. See Example 11-26.

resize

Echoes commands necessary to set $TERM and $TERMCAP to duplicate the size (dimensions) of the current terminal.

                            bash$                                                                                      resize                                                        set noglob;  setenv COLUMNS '80';  setenv LINES '24';  unset noglob;

script

This utility records (saves to a file) all the user keystrokes at the command-line in a console or an xterm window. This, in effect, creates a record of a session.

16.8. Math Commands

"Doing the numbers"

factor

Decompose an integer into prime factors.

                            bash$                                                                                      factor 27417                                                        27417: 3 13 19 37

Example 16-46. Generating prime numbers

#!/bin/bash # primes2.sh  #  Generating prime numbers the quick-and-easy way, #+ without resorting to fancy algorithms.  CEILING=10000   # 1 to 10000 PRIME=0 E_NOTPRIME=  is_prime () {   local factors   factors=( $(factor $1) )  # Load output of `factor` into array.  if [ -z "${factors[2]}" ] #  Third element of "factors" array: #+ ${factors[2]} is 2nd factor of argument. #  If it is blank, then there is no 2nd factor, #+ and the argument is therefore prime. then   return $PRIME             # 0 else   return $E_NOTPRIME        # null fi }  echo for n in $(seq $CEILING) do   if is_prime $n   then     printf %5d $n   fi   #    ^  Five positions per number suffices. done   #       For a higher $CEILING, adjust upward, as necessary.  echo  exit

bc

Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, bc gallops to the rescue.

Not just a versatile, arbitrary precision calculation utility, bc offers many of the facilities of a programming language. It has a syntax vaguely resembling C.

Since it is a fairly well-behaved UNIX utility, and may therefore be used in a pipe, bc comes in handy in scripts.

Here is a simple template for using bc to calculate a script variable. This uses command substitution.

                                                          variable=$(echo "OPTIONS; OPERATIONS" | bc)

Example 16-47. Monthly Payment on a Mortgage

#!/bin/bash # monthlypmt.sh: Calculates monthly payment on a mortgage.   #  This is a modification of code in the #+ "mcalc" (mortgage calculator) package, #+ by Jeff Schmidt #+ and #+ Mendel Cooper (yours truly, the ABS Guide author). #   http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz  echo echo "Given the principal, interest rate, and term of a mortgage," echo "calculate the monthly payment."  bottom=1.0  echo echo -n "Enter principal (no commas) " read principal echo -n "Enter interest rate (percent) "  # If 12%, enter "12", not ".12". read interest_r echo -n "Enter term (months) " read term    interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.                  #           ^^^^^^^^^^^^^^^^^  Divide by 100.                   # "scale" determines how many decimal places.   interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)     top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)           #           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^           #           Standard formula for figuring interest.   echo; echo "Please be patient. This may take a while."   let "months = $term - 1" # ====================================================================   for ((x=$months; x > 0; x--))  do    bot=$(echo "scale=9; $interest_rate^$x" | bc)    bottom=$(echo "scale=9; $bottom+$bot" | bc) #  bottom = $(($bottom + $bot"))  done # ====================================================================   # --------------------------------------------------------------------  #  Rick Boivie pointed out a more efficient implementation #+ of the above loop, which decreases computation time by 2/3.  # for ((x=1; x <= $months; x++)) # do #   bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc) # done   #  And then he came up with an even more efficient alternative, #+ one that cuts down the run time by about 95%!  # bottom=`{ #     echo "scale=9; bottom=$bottom; interest_rate=$interest_rate" #     for ((x=1; x <= $months; x++)) #     do #          echo 'bottom = bottom * interest_rate + 1' #     done #     echo 'bottom' #     } | bc`       # Embeds a 'for loop' within command substitution. # -------------------------------------------------------------------------- #  On the other hand, Frank Wang suggests: #  bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc)  #  Because . . . #  The algorithm behind the loop #+ is actually a sum of geometric proportion series. #  The sum formula is e0(1-q^n)/(1-q), #+ where e0 is the first element and q=e(n+1)/e(n) #+ and n is the number of elements. # --------------------------------------------------------------------------    # let "payment = $top/$bottom"  payment=$(echo "scale=2; $top/$bottom" | bc)  # Use two decimal places for dollars and cents.    echo  echo "monthly payment = \$$payment"  # Echo a dollar sign in front of amount.  echo    exit 0    # Exercises:  #   1) Filter input to permit commas in principal amount.  #   2) Filter input to permit interest to be entered as percent or decimal.  #   3) If you are really ambitious,  #+     expand this script to print complete amortization tables.

Example 16-48. Base Conversion

#!/bin/bash ########################################################################### # Shellscript:	base.sh - print number to different bases (Bourne Shell) # Author     :	Heiner Steven (heiner.steven@odn.de) # Date       :	07-03-95 # Category   :	Desktop # $Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $ # ==> Above line is RCS ID info. ########################################################################### # Description # # Changes # 21-03-95 stv	fixed error occuring with 0xb as input (0.2) ###########################################################################  # ==> Used in ABS Guide with the script author's permission. # ==> Comments added by ABS Guide author.  NOARGS=85 PN=`basename "$0"`			       # Program name VER=`echo '$Revision: 1.2 $' | cut -d' ' -f2`  # ==> VER=1.2  Usage () {     echo "$PN - print number to different bases, $VER (stv '95) usage: $PN [number ...]  If no number is given, the numbers are read from standard input. A number may be     binary (base 2)		starting with 0b (i.e. 0b1100)     octal (base 8)		starting with 0  (i.e. 014)     hexadecimal (base 16)	starting with 0x (i.e. 0xc)     decimal			otherwise (i.e. 12)" >&2     exit $NOARGS  }   # ==> Prints usage message.  Msg () {     for i   # ==> in [list] missing. Why?     do echo "$PN: $i" >&2     done }  Fatal () { Msg "$@"; exit 66; }  PrintBases () {     # Determine base of the number     for i      # ==> in [list] missing...     do         # ==> so operates on command-line arg(s). 	case "$i" in 	    0b*)		ibase=2;;	# binary 	    0x*|[a-f]*|[A-F]*)	ibase=16;;	# hexadecimal 	    0*)			ibase=8;;	# octal 	    [1-9]*)		ibase=10;;	# decimal 	    *) 		Msg "illegal number $i - ignored" 		continue;; 	esac  	# Remove prefix, convert hex digits to uppercase (bc needs this). 	number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'` 	# ==> Uses ":" as sed separator, rather than "/".  	# Convert number to decimal 	dec=`echo "ibase=$ibase; $number" | bc`  # ==> 'bc' is calculator utility. 	case "$dec" in 	    [0-9]*)	;;			 # number ok 	    *)		continue;;		 # error: ignore 	esac  	# Print all conversions in one line. 	# ==> 'here document' feeds command list to 'bc'. 	echo `bc <<! 	    obase=16; "hex="; $dec 	    obase=10; "dec="; $dec 	    obase=8;  "oct="; $dec 	    obase=2;  "bin="; $dec !     ` | sed -e 's: :	:g'      done }  while [ $# -gt 0 ] # ==>  Is a "while loop" really necessary here, # ==>+ since all the cases either break out of the loop # ==>+ or terminate the script. # ==> (Above comment by Paulo Marcel Coelho Aragao.) do     case "$1" in 	--)     shift; break;; 	-h)     Usage;;                 # ==> Help message. 	-*)     Usage;;          *)     break;;                 # First number     esac   # ==> Error checking for illegal input might be appropriate.     shift done  if [ $# -gt 0 ] then     PrintBases "$@" else					# Read from stdin.     while read line     do 	PrintBases $line     done fi   exit

An alternate method of invoking bc involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to bc.

variable=`bc << LIMIT_STRING options statements operations LIMIT_STRING `  ...or...   variable=$(bc << LIMIT_STRING options statements operations LIMIT_STRING )

Example 16-49. Invoking bc using a here document

#!/bin/bash # Invoking 'bc' using command substitution # in combination with a 'here document'.   var1=`bc << EOF 18.33 * 19.78 EOF ` echo $var1       # 362.56   #  $( ... ) notation also works. v1=23.53 v2=17.881 v3=83.501 v4=171.63  var2=$(bc << EOF scale = 4 a = ( $v1 + $v2 ) b = ( $v3 * $v4 ) a * b + 15.35 EOF ) echo $var2       # 593487.8452   var3=$(bc -l << EOF scale = 9 s ( 1.7 ) EOF ) # Returns the sine of 1.7 radians. # The "-l" option calls the 'bc' math library. echo $var3       # .991664810   # Now, try it in a function... hypotenuse ()    # Calculate hypotenuse of a right triangle. {                # c = sqrt( a^2 + b^2 ) hyp=$(bc -l << EOF scale = 9 sqrt ( $1 * $1 + $2 * $2 ) EOF ) # Can't directly return floating point values from a Bash function. # But, can echo-and-capture: echo "$hyp" }  hyp=$(hypotenuse 3.68 7.31) echo "hypotenuse = $hyp"    # 8.184039344   exit 0

Example 16-50. Calculating PI

#!/bin/bash # cannon.sh: Approximating PI by firing cannonballs.  # Author: Mendel Cooper # License: Public Domain # Version 2.2, reldate 13oct08.  # This is a very simple instance of a "Monte Carlo" simulation: #+ a mathematical model of a real-life event, #+ using pseudorandom numbers to emulate random chance.  #  Consider a perfectly square plot of land, 10000 units on a side. #  This land has a perfectly circular lake in its center, #+ with a diameter of 10000 units. #  The plot is actually mostly water, except for land in the four corners. #  (Think of it as a square with an inscribed circle.) # #  We will fire iron cannonballs from an old-style cannon #+ at the square. #  All the shots impact somewhere on the square, #+ either in the lake or on the dry corners. #  Since the lake takes up most of the area, #+ most of the shots will SPLASH! into the water. #  Just a few shots will THUD! into solid ground #+ in the four corners of the square. # #  If we take enough random, unaimed shots at the square, #+ Then the ratio of SPLASHES to total shots will approximate #+ the value of PI/4. # #  The simplified explanation is that the cannon is actually #+ shooting only at the upper right-hand quadrant of the square, #+ i.e., Quadrant I of the Cartesian coordinate plane. # # #  Theoretically, the more shots taken, the better the fit. #  However, a shell script, as opposed to a compiled language #+ with floating-point math built in, requires some compromises. #  This decreases the accuracy of the simulation.   DIMENSION=10000  # Length of each side of the plot.                  # Also sets ceiling for random integers generated.  MAXSHOTS=1000    # Fire this many shots.                  # 10000 or more would be better, but would take too long. PMULTIPLIER=4.0  # Scaling factor.  declare -r M_PI=3.141592654                  # Actual 9-place value of PI, for comparison purposes.  get_random () { SEED=$(head -n 1 /dev/urandom | od -N 1 | awk '{ print $2 }') RANDOM=$SEED                                  #  From "seeding-random.sh"                                               #+ example script. let "rnum = $RANDOM % $DIMENSION"             #  Range less than 10000. echo $rnum }  distance=        # Declare global variable. hypotenuse ()    # Calculate hypotenuse of a right triangle. {                # From "alt-bc.sh" example. distance=$(bc -l << EOF scale = 0 sqrt ( $1 * $1 + $2 * $2 ) EOF ) #  Setting "scale" to zero rounds down result to integer value, #+ a necessary compromise in this script. #  It decreases the accuracy of this simulation. }   # ========================================================== # main() { # "Main" code block, mimicking a C-language main() function.  # Initialize variables. shots=0 splashes=0 thuds=0 Pi=0 error=0  while [ "$shots" -lt  "$MAXSHOTS" ]           # Main loop. do    xCoord=$(get_random)                        # Get random X and Y coords.   yCoord=$(get_random)   hypotenuse $xCoord $yCoord                  #  Hypotenuse of                                               #+ right-triangle = distance.   ((shots++))    printf "#%4d   " $shots   printf "Xc = %4d  " $xCoord   printf "Yc = %4d  " $yCoord   printf "Distance = %5d  " $distance         #   Distance from                                               #+  center of lake                                               #+  -- the "origin" --                                               #+  coordinate (0,0).    if [ "$distance" -le "$DIMENSION" ]   then     echo -n "SPLASH!  "     ((splashes++))   else     echo -n "THUD!    "     ((thuds++))   fi    Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc)   # Multiply ratio by 4.0.   echo -n "PI ~ $Pi"   echo  done  echo echo "After $shots shots, PI looks like approximately   $Pi" #  Tends to run a bit high, #+ possibly due to round-off error and imperfect randomness of $RANDOM. #  But still usually within plus-or-minus 5% . . . #+ a pretty fair rough approximation. error=$(echo "scale=9; $Pi - $M_PI" | bc) pct_error=$(echo "scale=2; 100.0 * $error / $M_PI" | bc) echo -n "Deviation from mathematical value of PI =        $error" echo " ($pct_error% error)" echo  # End of "main" code block. # } # ==========================================================  exit 0  #  One might well wonder whether a shell script is appropriate for #+ an application as complex and computation-intensive as a simulation. # #  There are at least two justifications. #  1) As a proof of concept: to show it can be done. #  2) To prototype and test the algorithms before rewriting #+    it in a compiled high-level language.

16.9. Miscellaneous Commands

Command that fit in no special category

jot, seq

These utilities emit a sequence of integers, with a user-selectable increment.

The default separator character between each integer is a newline, but this can be changed with the -s option.

                            bash$                                                                                      seq 5                                                        1  2  3  4  5                            bash$                                                                                      seq -s : 5                                                        1:2:3:4:5

Both jot and seq come in handy in a for loop.

Example 16-54. Using seq to generate loop arguments

#!/bin/bash # Using "seq"  echo  for a in `seq 80`  # or   for a in $( seq 80 ) # Same as   for a in 1 2 3 4 5 ... 80   (saves much typing!). # May also use 'jot' (if present on system). do   echo -n "$a " done      # 1 2 3 4 5 ... 80 # Example of using the output of a command to generate  # the [list] in a "for" loop.  echo; echo   COUNT=80  # Yes, 'seq' also accepts a replaceable parameter.  for a in `seq $COUNT`  # or   for a in $( seq $COUNT ) do   echo -n "$a " done      # 1 2 3 4 5 ... 80  echo; echo  BEGIN=75 END=80  for a in `seq $BEGIN $END` #  Giving "seq" two arguments starts the count at the first one, #+ and continues until it reaches the second. do   echo -n "$a " done      # 75 76 77 78 79 80  echo; echo  BEGIN=45 INTERVAL=5 END=80  for a in `seq $BEGIN $INTERVAL $END` #  Giving "seq" three arguments starts the count at the first one, #+ uses the second for a step interval, #+ and continues until it reaches the third. do   echo -n "$a " done      # 45 50 55 60 65 70 75 80  echo; echo  exit 0

A simpler example:

#  Create a set of 10 files, #+ named file.1, file.2 . . . file.10. COUNT=10 PREFIX=file  for filename in `seq $COUNT` do   touch $PREFIX.$filename   #  Or, can do other operations,   #+ such as rm, grep, etc. done

Example 16-55. Letter Count"

#!/bin/bash # letter-count.sh: Counting letter occurrences in a text file. # Written by Stefano Palmeri. # Used in ABS Guide with permission. # Slightly modified by document author.  MINARGS=2          # Script requires at least two arguments. E_BADARGS=65 FILE=$1  let LETTERS=$#-1   # How many letters specified (as command-line args).                    # (Subtract 1 from number of command-line args.)   show_help(){ 	   echo            echo Usage: `basename $0` file letters              echo Note: `basename $0` arguments are case sensitive.            echo Example: `basename $0` foobar.txt G n U L i N U x. 	   echo }  # Checks number of arguments. if [ $# -lt $MINARGS ]; then    echo    echo "Not enough arguments."    echo    show_help    exit $E_BADARGS fi     # Checks if file exists. if [ ! -f $FILE ]; then     echo "File \"$FILE\" does not exist."     exit $E_BADARGS fi    # Counts letter occurrences . for n in `seq $LETTERS`; do       shift       if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then             #  Checks arg.              echo "$1" -\> `cat $FILE | tr -cd  "$1" | wc -c` #  Counting.       else              echo "$1 is not a  single char."       fi   done  exit $?  #  This script has exactly the same functionality as letter-count2.sh, #+ but executes faster. #  Why?

Somewhat more capable than seq, jot is a classic UNIX utility that is not normally included in a standard Linux distro. However, the source rpm is available for download from the MIT repository.

Unlike seq, jot can generate a sequence of random numbers, using the -r option.

                                      bash$                                                                                                                    jot -r 3 999                                                                            1069  1272  1428

getopt

The getopt command parses command-line options preceded by a dash. This external command corresponds to the getopts Bash builtin. Using getopt permits handling long options by means of the -l flag, and this also allows parameter reshuffling.

Example 16-56. Using getopt to parse command-line options

#!/bin/bash # Using getopt  # Try the following when invoking this script: #   sh ex33a.sh -a #   sh ex33a.sh -abc #   sh ex33a.sh -a -b -c #   sh ex33a.sh -d #   sh ex33a.sh -dXYZ #   sh ex33a.sh -d XYZ #   sh ex33a.sh -abcd #   sh ex33a.sh -abcdZ #   sh ex33a.sh -z #   sh ex33a.sh a # Explain the results of each of the above.  E_OPTERR=65  if [ "$#" -eq 0 ] then   # Script needs at least one command-line argument.   echo "Usage $0 -[options a,b,c]"   exit $E_OPTERR fi    set -- `getopt "abcd:" "$@"` # Sets positional parameters to command-line arguments. # What happens if you use "$*" instead of "$@"?  while [ ! -z "$1" ] do   case "$1" in     -a) echo "Option \"a\"";;     -b) echo "Option \"b\"";;     -c) echo "Option \"c\"";;     -d) echo "Option \"d\" $2";;      *) break;;   esac    shift done  #  It is usually better to use the 'getopts' builtin in a script. #  See "ex33.sh."  exit 0

As Peggy Russell points out:

It is often necessary to include an eval to correctly process whitespace and quotes.

args=$(getopt -o a:bc:d -- "$@") eval set -- "$args"

See Example 10-5 for a simplified emulation of getopt.

run-parts

The run-parts command [82] executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission.

The cron daemon invokes run-parts to run the scripts in the /etc/cron.* directories.

yes

In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A control-C terminates the run. A different output string may be specified, as in yes different string , which would continually output different string to stdout.

One might well ask the purpose of this. From the command-line or in a script, the output of yes can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect.

yes | fsck /dev/hda1 runs fsck non-interactively (careful!).

yes | rm -r dirname has same effect as rm -rf dirname (careful!).

Caution advised when piping yes to a potentially dangerous system command, such as fsck or fdisk. It might have unintended consequences.

The yes command parses variables, or more accurately, it echoes parsed variables. For example:

                                      bash$                                                                                                                    yes $BASH_VERSION                                                                            3.1.17(1)-release  3.1.17(1)-release  3.1.17(1)-release  3.1.17(1)-release  3.1.17(1)-release  . . .

This particular "feature" may be used to create a very large ASCII file on the fly:

                                      bash$                                                                                                                    yes $PATH > huge_file.txt                                                                                                                    Ctl-C

Hit Ctl-C very quickly, or you just might get more than you bargained for. . . .

The yes command may be emulated in a very simple script function.

yes () { # Trivial emulation of "yes" ...   local DEFAULT_TEXT="y"   while [ true ]   # Endless loop.   do     if [ -z "$1" ]     then       echo "$DEFAULT_TEXT"     else           # If argument ...       echo "$1"    # ... expand and echo it.     fi   done             #  The only things missing are the }                  #+ --help and --version options.

banner

Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy.

Note that banner has been dropped from many Linux distros, presumably because it is no longer considered useful.

printenv

Show all the environmental variables set for a particular user.

                            bash$                                                                                      printenv | grep HOME                                                        HOME=/home/bozo

lp

The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. [83] These commands trace the origin of their names to the line printers of another era. [84]

bash$ lp file1.txt or bash lp <file1.txt

It is often useful to pipe the formatted output from pr to lp.

bash$ pr -options file1.txt | lp

Formatting packages, such as groff and Ghostscript may send their output directly to lp.

bash$ groff -Tascii file.tr | lp

bash$ gs -options | lp file.ps

Related commands are lpq, for viewing the print queue, and lprm, for removing jobs from the print queue.

tee

[UNIX borrows an idea from the plumbing trade.]

This is a redirection operator, but with a difference. Like the plumber's tee, it permits "siphoning off" to a file the output of a command or commands within a pipe, but without affecting the result. This is useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging purposes.

                          (redirection)                             |----> to file                             |   ==========================|====================   command ---> command ---> |tee ---> command ---> ---> output of pipe   ===============================================

cat listfile* | sort | tee check.file | uniq > result.file #                      ^^^^^^^^^^^^^^   ^^^^      #  The file "check.file" contains the concatenated sorted "listfiles," #+ before the duplicate lines are removed by 'uniq.'

mkfifo

This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. [85] Typically, one process writes to the FIFO, and the other reads from it. See Example A-14.

#!/bin/bash # This short script by Omair Eshkenazi. # Used in ABS Guide with permission (thanks!).  mkfifo pipe1   # Yes, pipes can be given names. mkfifo pipe2   # Hence the designation "named pipe."  (cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2 <pipe1 & ls -l | tr -s ' ' | cut -d' ' -f3,9- | tee pipe1 | cut -d' ' -f2 | paste - pipe2  rm -f pipe1 rm -f pipe2  # No need to kill background processes when script terminates (why not?).  exit $?  Now, invoke the script and explain the output: sh mkfifo-example.sh  4830.tar.gz          BOZO pipe1   BOZO pipe2   BOZO mkfifo-example.sh    BOZO Mixed.msg BOZO

pathchk

This command checks the validity of a filename. If the filename exceeds the maximum allowable length (255 characters) or one or more of the directories in its path is not searchable, then an error message results.

Unfortunately, pathchk does not return a recognizable error code, and it is therefore pretty much useless in a script. Consider instead the file test operators.

dd

Though this somewhat obscure and much feared data duplicator command originated as a utility for exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, it still has its uses. The dd command simply copies a file (or stdin/stdout), but with conversions. Possible conversions include ASCII/EBCDIC, [86] upper/lower case, swapping of byte pairs between input and output, and skipping and/or truncating the head or tail of the input file.

# Converting a file to all uppercase:  dd if=$filename conv=ucase > $filename.uppercase #                    lcase   # For lower case conversion

Some basic options to dd are:

if=INFILE

INFILE is the source file.
of=OUTFILE

OUTFILE is the target file, the file that will have the data written to it.
bs=BLOCKSIZE

This is the size of each block of data being read and written, usually a power of 2.
skip=BLOCKS

How many blocks of data to skip in INFILE before starting to copy. This is useful when the INFILE has "garbage" or garbled data in its header or when it is desirable to copy only a portion of the INFILE.
seek=BLOCKS

How many blocks of data to skip in OUTFILE before starting to copy, leaving blank data at beginning of OUTFILE.
count=BLOCKS

Copy only this many blocks of data, rather than the entire INFILE.
conv=CONVERSION

Type of conversion to be applied to INFILE data before copying operation.

A dd --help lists all the options this powerful utility takes.

Example 16-57. A script that copies itself

#!/bin/bash # self-copy.sh  # This script copies itself.  file_subscript=copy  dd if=$0 of=$0.$file_subscript 2>/dev/null # Suppress messages from dd:   ^^^^^^^^^^^  exit $?  #  A program whose only output is its own source code #+ is called a "quine" per Willard Quine. #  Does this script qualify as a quine?

Example 16-58. Exercising dd

#!/bin/bash # exercising-dd.sh  # Script by Stephane Chazelas. # Somewhat modified by ABS Guide author.  infile=$0           # This script. outfile=log.txt     # Output file left behind. n=8 p=11  dd if=$infile of=$outfile bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null # Extracts characters n to p (8 to 11) from this script ("bash").  # ----------------------------------------------------------------  echo -n "hello vertical world" | dd cbs=1 conv=unblock 2> /dev/null # Echoes "hello vertical world" vertically downward. # Why? A newline follows each character dd emits.  exit $?

To demonstrate just how versatile dd is, let's use it to capture keystrokes.

Example 16-59. Capturing Keystrokes

#!/bin/bash # dd-keypress.sh: Capture keystrokes without needing to press ENTER.   keypresses=4                      # Number of keypresses to capture.   old_tty_setting=$(stty -g)        # Save old terminal settings.  echo "Press $keypresses keys." stty -icanon -echo                # Disable canonical mode.                                   # Disable local echo. keys=$(dd bs=1 count=$keypresses 2> /dev/null) # 'dd' uses stdin, if "if" (input file) not specified.  stty "$old_tty_setting"           # Restore old terminal settings.  echo "You pressed the \"$keys\" keys."  # Thanks, Stephane Chazelas, for showing the way. exit 0

The dd command can do random access on a data stream.

echo -n . | dd bs=1 seek=4 of=file conv=notrunc #  The "conv=notrunc" option means that the output file #+ will not be truncated.  # Thanks, S.C.

The dd command can copy raw data and disk images to and from devices, such as floppies and tape drives (Example A-5). A common use is creating boot floppies.

dd if=kernel-image of=/dev/fd0H1440

Similarly, dd can copy the entire contents of a floppy, even one formatted with a "foreign" OS, to the hard drive as an image file.

dd if=/dev/fd0 of=/home/bozo/projects/floppy.img

Likewise, dd can create bootable flash drives and SD cards.

dd if=image.iso of=/dev/sdb

Example 16-60. Preparing a bootable SD card for the Raspberry Pi

#!/bin/bash # rp.sdcard.sh # Preparing an SD card with a bootable image for the Raspberry Pi.  # $1 = imagefile name # $2 = sdcard (device file) # Otherwise defaults to the defaults, see below.  DEFAULTbs=4M                                 # Block size, 4 mb default. DEFAULTif="2013-07-26-wheezy-raspbian.img"   # Commonly used distro. DEFAULTsdcard="/dev/mmcblk0"                 # May be different. Check! ROOTUSER_NAME=root                           # Must run as root! E_NOTROOT=81 E_NOIMAGE=82  username=$(id -nu)                           # Who is running this script? if [ "$username" != "$ROOTUSER_NAME" ] then   echo "This script must run as root or with root privileges."   exit $E_NOTROOT fi  if [ -n "$1" ] then   imagefile="$1" else   imagefile="$DEFAULTif" fi  if [ -n "$2" ] then   sdcard="$2" else   sdcard="$DEFAULTsdcard" fi  if [ ! -e $imagefile ] then   echo "Image file \"$imagefile\" not found!"   exit $E_NOIMAGE fi  echo "Last chance to change your mind!"; echo read -s -n1 -p "Hit a key to write $imagefile to $sdcard [Ctl-c to exit]." echo; echo  echo "Writing $imagefile to $sdcard ..." dd bs=$DEFAULTbs if=$imagefile of=$sdcard  exit $?  # Exercises: # --------- # 1) Provide additional error checking. # 2) Have script autodetect device file for SD card (difficult!). # 3) Have script sutodetect image file (*img) in $PWD.

Other applications of dd include initializing temporary swap files (Example 31-2) and ramdisks (Example 31-3). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended.

People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of dd.

Example 16-61. Securely deleting a file

#!/bin/bash # blot-out.sh: Erase "all" traces of a file.  #  This script overwrites a target file alternately #+ with random bytes, then zeros before finally deleting it. #  After that, even examining the raw disk sectors by conventional methods #+ will not reveal the original file data.  PASSES=7         #  Number of file-shredding passes.                  #  Increasing this slows script execution,                  #+ especially on large target files. BLOCKSIZE=1      #  I/O with /dev/urandom requires unit block size,                  #+ otherwise you get weird results. E_BADARGS=70     #  Various error exit codes. E_NOT_FOUND=71 E_CHANGED_MIND=72  if [ -z "$1" ]   # No filename specified. then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi  file=$1  if [ ! -e "$file" ] then   echo "File \"$file\" not found."   exit $E_NOT_FOUND fi    echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? " read answer case "$answer" in [nN]) echo "Changed your mind, huh?"       exit $E_CHANGED_MIND       ;; *)    echo "Blotting out file \"$file\".";; esac   flength=$(ls -l "$file" | awk '{print $5}')  # Field 5 is file length. pass_count=1  chmod u+w "$file"   # Allow overwriting/deleting the file.  echo  while [ "$pass_count" -le "$PASSES" ] do   echo "Pass #$pass_count"   sync         # Flush buffers.   dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength                # Fill with random bytes.   sync         # Flush buffers again.   dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength                # Fill with zeros.   sync         # Flush buffers yet again.   let "pass_count += 1"   echo done     rm -f $file    # Finally, delete scrambled and shredded file. sync           # Flush buffers a final time.  echo "File \"$file\" blotted out and deleted."; echo   exit 0  #  This is a fairly secure, if inefficient and slow method #+ of thoroughly "shredding" a file. #  The "shred" command, part of the GNU "fileutils" package, #+ does the same thing, although more efficiently.  #  The file cannot not be "undeleted" or retrieved by normal methods. #  However . . . #+ this simple method would *not* likely withstand #+ sophisticated forensic analysis.  #  This script may not play well with a journaled file system. #  Exercise (difficult): Fix it so it does.    #  Tom Vier's "wipe" file-deletion package does a much more thorough job #+ of file shredding than this simple script. #     http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2  #  For an in-depth analysis on the topic of file deletion and security, #+ see Peter Gutmann's paper, #+     "Secure Deletion of Data From Magnetic and Solid-State Memory". #       http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html

See also the dd thread entry in the bibliography.

od

The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as /dev/urandom, and as a filter for binary data.

head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p' # Sample output: 1324725719, 3918166450, 2989231420, etc.  # From rnd.sh example script, by St�phane Chazelas

Chapter 17. System and Administrative Commands

The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused.

Users and Groups

users

Show all logged on users. This is the approximate equivalent of who -q.

groups

Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers.

                        bash$                                                                          groups                                                bozita cdrom cdwriter audio xgrp                        bash$                                                                          echo $GROUPS                                                501

chown, chgrp

The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. [88]

The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation.

chgrp --recursive dunderheads *.data #  The "dunderheads" group will now own all the "*.data" files #+ all the way down the $PWD directory tree (that's what "recursive" means).

useradd, userdel

The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system [89] and deletes associated files.

The adduser command is a synonym for useradd and is usually a symbolic link to it.

usermod

Modify a user account. Changes may be made to the password, group membership, expiration date, and other attributes of a given user's account. With this command, a user's password may be locked, which has the effect of disabling the account.

groupmod

Modify a given group. The group name and/or ID number may be changed using this command.

id

The id command lists the real and effective user IDs and the group IDs of the user associated with the current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.

                        bash$                                                                          id                                                uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)                        bash$                                                                          echo $UID                                                501

The id command shows the effective IDs only when they differ from the real ones.

Also see Example 9-5.

lid

The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users belonging to a given group. May be invoked only by root.

                          root#                                                                                lid bozo                                                                                bozo(gid=500)                          root#                                                                                lid daemon                                                                                bin(gid=1)   daemon(gid=2)   adm(gid=4)   lp(gid=7)

who

Show all users logged on to the system.

                          bash$                                                                                who                                                    bozo  tty1     Apr 27 17:45  bozo  pts/0    Apr 27 17:46  bozo  pts/1    Apr 27 17:47  bozo  pts/2    Apr 27 17:49

The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man.

                          bash$                                                                                who -m                                                    localhost.localdomain!bozo  pts/2    Apr 27 17:49

whoami is similar to who -m, but only lists the user name.

w

Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process.

                        bash$                                                                          w | grep startx                                                bozo  tty1     -                 4:22pm  6:41   4.47s  0.45s  startx

logname

Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above.

                        bash$                                                                          logname                                                bozo                        bash$                                                                          whoami                                                bozo

However . . .

                          bash$                                                                                su                                                    Password: ......                          bash#                                                                                whoami                                                    root                          bash#                                                                                logname                                                    bozo

While logname prints the name of the logged in user, whoami gives the name of the user attached to the current process. As we have just seen, sometimes these are not the same.

su

Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See Example A-14.

sudo

Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script.

#!/bin/bash  # Some commands. sudo cp /root/secretfile /home/bozo/secret # Some more commands.

The file /etc/sudoers holds the names of users permitted to invoke sudo.

passwd

Sets, changes, or manages a user's password.

The passwd command can be used in a script, but probably should not be.

Example 17-1. Setting a new password

#!/bin/bash #  setnew-password.sh: For demonstration purposes only. #                      Not a good idea to actually run this script. #  This script must be run as root.  ROOT_UID=0         # Root has $UID 0. E_WRONG_USER=65    # Not root?  E_NOSUCHUSER=70 SUCCESS=0   if [ "$UID" -ne "$ROOT_UID" ] then   echo; echo "Only root can run this script."; echo   exit $E_WRONG_USER else   echo   echo "You should know better than to run this script, root."   echo "Even root users get the blues... "   echo fi     username=bozo NEWPASSWORD=security_violation  # Check if bozo lives here. grep -q "$username" /etc/passwd if [ $? -ne $SUCCESS ] then   echo "User $username does not exist."   echo "No password changed."   exit $E_NOSUCHUSER fi    echo "$NEWPASSWORD" | passwd --stdin "$username" #  The '--stdin' option to 'passwd' permits #+ getting a new password from stdin (or a pipe).  echo; echo "User $username's password changed!"  # Using the 'passwd' command in a script is dangerous.  exit 0

The passwd command's -l, -u, and -d options permit locking, unlocking, and deleting a user's password. Only root may use these options.

ac

Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities.

last

List last logged in users, as read from /var/log/wtmp. This command can also show remote logins.

For example, to show the last few times the system rebooted:

                        bash$                                                                          last reboot                                                reboot   system boot  2.6.9-1.667      Fri Feb  4 18:18          (00:02)      reboot   system boot  2.6.9-1.667      Fri Feb  4 15:20          (01:27)      reboot   system boot  2.6.9-1.667      Fri Feb  4 12:56          (00:49)      reboot   system boot  2.6.9-1.667      Thu Feb  3 21:08          (02:17)      . . .   wtmp begins Tue Feb  1 12:50:09 2005

newgrp

Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds only limited use.

Kurt Glaesemann points out that the newgrp command could prove helpful in setting the default group permissions for files a user writes. However, the chgrp command might be more convenient for this purpose.

Terminals

tty

Echoes the name (filename) of the current user's terminal. Note that each separate xterm window counts as a different terminal.

stty

Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully.

Example 17-2. Setting an erase character

#!/bin/bash # erase.sh: Using "stty" to set an erase character when reading input.  echo -n "What is your name? " read name                      #  Try to backspace                                #+ to erase characters of input.                                #  Problems? echo "Your name is $name."  stty erase '#'                 #  Set "hashmark" (#) as erase character. echo -n "What is your name? " read name                      #  Use # to erase last character typed. echo "Your name is $name."  exit 0  # Even after the script exits, the new key value remains set. # Exercise: How would you reset the erase character to the default value?

Example 17-3. secret password: Turning off terminal echoing

#!/bin/bash # secret-pw.sh: secret password  echo echo -n "Enter password " read passwd echo "password is $passwd" echo -n "If someone had been looking over your shoulder, " echo "your password would have been compromised."  echo && echo  # Two line-feeds in an "and list."   stty -echo    # Turns off screen echo. #   May also be done with #   read -sp passwd #   A big Thank You to Leigh James for pointing this out.  echo -n "Enter password again " read passwd echo echo "password is $passwd" echo  stty echo     # Restores screen echo.  exit 0  # Do an 'info stty' for more on this useful-but-tricky command.

A creative use of stty is detecting a user keypress (without hitting ENTER).

Example 17-4. Keypress detection

#!/bin/bash # keypress.sh: Detect a user keypress ("hot keys").  echo  old_tty_settings=$(stty -g)   # Save old settings (why?). stty -icanon Keypress=$(head -c1)          # or $(dd bs=1 count=1 2> /dev/null)                               # on non-GNU systems  echo echo "Key pressed was \""$Keypress"\"." echo  stty "$old_tty_settings"      # Restore old settings.  # Thanks, Stephane Chazelas.  exit 0

Also see Example 9-3 and Example A-43.

setterm

Set certain terminal attributes. This command writes to its terminal's stdout a string that changes the behavior of that terminal.

                          bash$                                                                                setterm -cursor off                                                    bash$

The setterm command can be used within a script to change the appearance of text written to stdout, although there are certainly better tools available for this purpose.

setterm -bold on echo bold hello  setterm -bold off echo normal hello

tset

Show or initialize terminal settings. This is a less capable version of stty.

                          bash$                                                                                tset -r                                                    Terminal type is xterm-xfree86.  Kill is control-U (^U).  Interrupt is control-C (^C).

setserial

Set or display serial port parameters. This command must be run by root and is usually found in a system setup script.

# From /etc/pcmcia/serial script:  IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

getty, agetty

The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty.

mesg

Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal.

It can be quite annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions.

wall

This is an acronym for "write all," i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see Example 19-1).

                          bash$                                                                                wall System going down for maintenance in 5 minutes!                                                    Broadcast message from bozo (pts/1) Sun Jul  8 13:53:27 2001...   System going down for maintenance in 5 minutes!

If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to that terminal.

Information and Statistics

uname

Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see Example 16-5). The -s option shows only the OS type.

                        bash$                                                                          uname                                                Linux                        bash$                                                                          uname -s                                                Linux                        bash$                                                                          uname -a                                                Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006  i686 i686 i386 GNU/Linux

arch

Show system architecture. Equivalent to uname -m. See Example 11-27.

                        bash$                                                                          arch                                                i686                        bash$                                                                          uname -m                                                i686

lastcomm

Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities.

lastlog

List the last login time of all system users. This references the /var/log/lastlog file.

                          bash$                                                                                lastlog                                                    root          tty1                      Fri Dec  7 18:43:21 -0700 2001  bin                                     **Never logged in**  daemon                                  **Never logged in**  ...  bozo          tty1                      Sat Dec  8 21:14:29 -0700 2001                          bash$                                                                                lastlog | grep root                                                    root          tty1                      Fri Dec  7 18:43:21 -0700 2001

This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file.

lsof

List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results.

                          bash$                                                                                lsof                                                    COMMAND    PID    USER   FD   TYPE     DEVICE    SIZE     NODE NAME  init         1    root  mem    REG        3,5   30748    30303 /sbin/init  init         1    root  mem    REG        3,5   73120     8069 /lib/ld-2.1.3.so  init         1    root  mem    REG        3,5  931668     8075 /lib/libc-2.1.3.so  cardmgr    213    root  mem    REG        3,5   36956    30357 /sbin/cardmgr  ...

The lsof command is a useful, if complex administrative tool. If you are unable to dismount a filesystem and get an error message that it is still in use, then running lsof helps determine which files are still open on that filesystem. The -i option lists open network socket files, and this can help trace intrusion or hack attempts.

                          bash$                                                                                lsof -an -i tcp                                                    COMMAND  PID USER  FD  TYPE DEVICE SIZE NODE NAME  firefox 2330 bozo  32u IPv4   9956       TCP 66.0.118.137:57596->67.112.7.104:http ...  firefox 2330 bozo  38u IPv4  10535       TCP 66.0.118.137:57708->216.79.48.24:http ...

See Example 30-2 for an effective use of lsof.

strace

System trace: diagnostic and debugging tool for tracing system calls and signals. This command and ltrace, following, are useful for diagnosing why a given program or package fails to run . . . perhaps due to missing libraries or related causes.

                          bash$                                                                                strace df                                                    execve("/bin/df", ["df"], [/* 45 vars */]) = 0  uname({sys="Linux", node="bozo.localdomain", ...}) = 0  brk(0)                                  = 0x804f5e4   ...

This is the Linux equivalent of the Solaris truss command.

ltrace

Library trace: diagnostic and debugging tool that traces library calls invoked by a given command.

                          bash$                                                                                ltrace df                                                    __libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished ...>:  setlocale(6, "")                                 = "en_US.UTF-8" bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale" textdomain("coreutils")                          = "coreutils" __cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0 getenv("DF_BLOCK_SIZE")                          = NULL   ...

nc

The nc (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and servers.

                          bash$                                                                                nc localhost.localdomain 25                                                    220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;  Thu, 31 Mar 2005 15:41:35 -0700

A real-life usage example.

Example 17-5. Checking a remote server for identd

#! /bin/sh ## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed. ## Args: target port [port port port ...] ## Hose stdout _and_ stderr together. ## ##  Advantages: runs slower than ident-scan, giving remote inetd less cause ##+ for alarm, and only hits the few known daemon ports you specify. ##  Disadvantages: requires numeric-only port args, the output sleazitude, ##+ and won't work for r-services when coming from high source ports. # Script author: Hobbit <hobbit@avian.org> # Used in ABS Guide with permission.  # --------------------------------------------------- E_BADARGS=65       # Need at least two args. TWO_WINKS=2        # How long to sleep. THREE_WINKS=3 IDPORT=113         # Authentication "tap ident" port. RAND1=999 RAND2=31337 TIMEOUT0=9 TIMEOUT1=8 TIMEOUT2=4 # ---------------------------------------------------  case "${2}" in   "" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;; esac  # Ping 'em once and see if they *are* running identd. nc -z -w $TIMEOUT0 "$1" $IDPORT || \ { echo "Oops, $1 isn't running identd." ; exit 0 ; } #  -z scans for listening daemons. #     -w $TIMEOUT = How long to try to connect.  # Generate a randomish base port. RP=`expr $$ % $RAND1 + $RAND2`  TRG="$1" shift  while test "$1" ; do   nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &   PROC=$!   sleep $THREE_WINKS   echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1   sleep $TWO_WINKS  # Does this look like a lamer script or what . . . ? # ABS Guide author comments: "Ain't really all that bad . . . #+                            kinda clever, actually."    kill -HUP $PROC   RP=`expr ${RP} + 1`   shift done  exit $?  #  Notes: #  -----  #  Try commenting out line 30 and running this script #+ with "localhost.localdomain 25" as arguments.  #  For more of Hobbit's 'nc' example scripts, #+ look in the documentation: #+ the /usr/share/doc/nc-X.XX/scripts directory.

And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair:

echo clone | nc thunk.org 5000 > e2fsprogs.dat

free

Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more.

                        bash$                                                free                                                  total       used       free     shared    buffers     cached    Mem:         30504      28624       1880      15820       1608       16376    -/+ buffers/cache:      10640      19864    Swap:        68540       3128      65412

To show unused RAM memory:

                        bash$                                                free | grep Mem | awk '{ print $4 }'                        1880

procinfo

Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing.

                        bash$                                                                          procinfo | grep Bootup                                                Bootup: Wed Mar 21 15:15:50 2001    Load average: 0.04 0.21 0.34 3/47 6829

lsdev

List devices, that is, show installed hardware.

                          bash$                                                                                lsdev                                                    Device            DMA   IRQ  I/O Ports  ------------------------------------------------  cascade             4     2   dma                          0080-008f  dma1                         0000-001f  dma2                         00c0-00df  fpu                          00f0-00ff  ide0                     14  01f0-01f7 03f6-03f6  ...

du

Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.

                        bash$                                                du -ach                        1.0k    ./wi.sh  1.0k    ./tst.sh  1.0k    ./random.file  6.0k    .  6.0k    total

df

Shows filesystem usage in tabular form.

                        bash$                                                df                        Filesystem           1k-blocks      Used Available Use% Mounted on  /dev/hda5               273262     92607    166547  36% /  /dev/hda8               222525    123951     87085  59% /home  /dev/hda7              1408796   1075744    261488  80% /usr

dmesg

Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script.

                          bash$                                                                                dmesg | grep hda                                                    Kernel command line: ro root=/dev/hda2  hda: IBM-DLGA-23080, ATA DISK drive  hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63  hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4

stat

Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.

                          bash$                                                                                stat test.cru                                                                                File: "test.cru"    Size: 49970        Allocated Blocks: 100          Filetype: Regular File    Mode: (0664/-rw-rw-r--)         Uid: (  501/ bozo)  Gid: (  501/ bozo)  Device:  3,8   Inode: 18185     Links: 1      Access: Sat Jun  2 16:40:24 2001  Modify: Sat Jun  2 16:40:24 2001  Change: Sat Jun  2 16:40:24 2001

If the target file does not exist, stat returns an error message.

                          bash$                                                                                stat nonexistent-file                                                    nonexistent-file: No such file or directory

In a script, you can use stat to extract information about files (and filesystems) and set variables accordingly.

#!/bin/bash # fileinfo2.sh  # Per suggestion of Jo�l Bourquard and . . . # http://www.linuxquestions.org/questions/showthread.php?t=410766   FILENAME=testfile.txt file_name=$(stat -c%n "$FILENAME")   # Same as "$FILENAME" of course. file_owner=$(stat -c%U "$FILENAME") file_size=$(stat -c%s "$FILENAME") #  Certainly easier than using "ls -l $FILENAME" #+ and then parsing with sed. file_inode=$(stat -c%i "$FILENAME") file_type=$(stat -c%F "$FILENAME") file_access_rights=$(stat -c%A "$FILENAME")  echo "File name:          $file_name" echo "File owner:         $file_owner" echo "File size:          $file_size" echo "File inode:         $file_inode" echo "File type:          $file_type" echo "File access rights: $file_access_rights"  exit 0  sh fileinfo2.sh  File name:          testfile.txt File owner:         bozo File size:          418 File inode:         1730378 File type:          regular file File access rights: -rw-rw-r--

vmstat

Display virtual memory statistics.

                          bash$                                                                                vmstat                                                                                procs                      memory    swap          io system         cpu  r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us  sy id  0  0  0      0  11040   2636  38952   0   0    33     7  271    88   8   3 89

uptime

Shows how long the system has been running, along with associated statistics.

                          bash$                                                                                uptime                                                    10:28pm  up  1:57,  3 users,  load average: 0.17, 0.34, 0.27

A load average of 1 or less indicates that the system handles processes immediately. A load average greater than 1 means that processes are being queued. When the load average gets above 3 (on a single-core processor), then system performance is significantly degraded.

hostname

Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable.

                        bash$                                                                          hostname                                                localhost.localdomain                        bash$                                                                          echo $HOSTNAME                                                localhost.localdomain

Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to hostname also perform these functions.

hostid

Echo a 32-bit hexadecimal numerical identifier for the host machine.

This command allegedly fetches a "unique" serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed.

The network address of a typical non-networked Linux machine, is found in /etc/hosts.

                                    bash$                                                                                                              cat /etc/hosts                                                                        127.0.0.1               localhost.localdomain localhost

As it happens, transposing the bytes of 127.0.0.1 , we get 0.127.1.0 , which translates in hex to 007f0100 , the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid.

sar

Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation ("Old" SCO) released sar as Open Source in June, 1999.

This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard.

                        bash$                                                                          sar                                                Linux 2.4.9 (brooks.seringas.fr) 	09/26/03  10:30:00          CPU     %user     %nice   %system   %iowait     %idle 10:40:00          all      2.21     10.90     65.48      0.00     21.41 10:50:00          all      3.36      0.00     72.36      0.00     24.28 11:00:00          all      1.12      0.00     80.77      0.00     18.11 Average:          all      2.23      3.63     72.87      0.00     21.27  14:32:30          LINUX RESTART  15:00:00          CPU     %user     %nice   %system   %iowait     %idle 15:10:00          all      8.59      2.40     17.47      0.00     71.54 15:20:00          all      4.07      1.00     11.95      0.00     82.98 15:30:00          all      0.79      2.94      7.56      0.00     88.71 Average:          all      6.33      1.70     14.71      0.00     77.26

readelf

Show information and statistics about a designated elf binary. This is part of the binutils package.

                        bash$                                                                          readelf -h /bin/bash                                                ELF Header:    Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00     Class:                             ELF32    Data:                              2's complement, little endian    Version:                           1 (current)    OS/ABI:                            UNIX - System V    ABI Version:                       0    Type:                              EXEC (Executable file)    . . .

size

The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers.

                          bash$                                                                                size /bin/bash                                                                                text    data     bss     dec     hex filename   495971   22496   17392  535859   82d33 /bin/bash

System Logs

logger

Appends a user-generated message to the system log (/var/log/messages). You do not have to be root to invoke logger.

logger Experiencing instability in network connection at 23:10, 05/21. # Now, do a 'tail /var/log/messages'.

By embedding a logger command in a script, it is possible to write debugging information to /var/log/messages.

logger -t $0 -i Logging at line "$LINENO". # The "-t" option specifies the tag for the logger entry. # The "-i" option records the process ID.  # tail /var/log/message # ... # Jul  7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

logrotate

This utility manages the system log files, rotating, compressing, deleting, and/or e-mailing them, as appropriate. This keeps the /var/log from getting cluttered with old log files. Usually cron runs logrotate on a daily basis.

Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log files, as well as system-wide ones.

Stefano Falsetto has created rottlog, which he considers to be an improved version of logrotate.

Job Control

ps

P rocess S tatistics: lists currently executing processes by owner and PID (process ID). This is usually invoked with ax or aux options, and may be piped to grep or sed to search for a specific process (see Example 15-14 and Example 29-3).

                        bash$                                                                                                      ps ax | grep sendmail                                                295 ?	   S	  0:00 sendmail: accepting connections on port 25

To display system processes in graphical "tree" format: ps afjx or ps ax --forest.

pgrep, pkill

Combining the ps command with grep or kill.

                          bash$                                                                                ps a | grep mingetty                                                    2212 tty2     Ss+    0:00 /sbin/mingetty tty2  2213 tty3     Ss+    0:00 /sbin/mingetty tty3  2214 tty4     Ss+    0:00 /sbin/mingetty tty4  2215 tty5     Ss+    0:00 /sbin/mingetty tty5  2216 tty6     Ss+    0:00 /sbin/mingetty tty6  4849 pts/2    S+     0:00 grep mingetty                          bash$                                                                                pgrep mingetty                                                    2212 mingetty  2213 mingetty  2214 mingetty  2215 mingetty  2216 mingetty

Compare the action of pkill with killall.

pstree

Lists currently executing processes in "tree" format. The -p option shows the PIDs, as well as the process names.

top

Continuously updated display of most cpu-intensive processes. The -b option displays in text mode, so that the output may be parsed or accessed from a script.

                          bash$                                                                                top -b                                                                                8:30pm  up 3 min,  3 users,  load average: 0.49, 0.32, 0.13  45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped  CPU states: 13.6% user,  7.3% system,  0.0% nice, 78.9% idle  Mem:    78396K av,   65468K used,   12928K free,       0K shrd,    2352K buff  Swap:  157208K av,       0K used,  157208K free                   37244K cached     PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND    848 bozo      17   0   996  996   800 R     5.6  1.2   0:00 top      1 root       8   0   512  512   444 S     0.0  0.6   0:04 init      2 root       9   0     0    0     0 SW    0.0  0.0   0:00 keventd    ...

nice

Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are renice and snice, which change the priority of a running process or processes, and skill, which sends a kill signal to a process or processes.

nohup

Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid creating an orphan or zombie process.

pidof

Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act on the PID of a process (not its name), it is sometimes necessary to identify that PID. The pidof command is the approximate counterpart to the $PPID internal variable.

Example 17-6. pidof helps kill a process

#!/bin/bash # kill-process.sh  NOPROCESS=2  process=xxxyyyzzz  # Use nonexistent process. # For demo purposes only... # ... don't want to actually kill any actual process with this script. # # If, for example, you wanted to use this script to logoff the Internet, #     process=pppd  t=`pidof $process`       # Find pid (process id) of $process. # The pid is needed by 'kill' (can't 'kill' by program name).  if [ -z "$t" ]           # If process not present, 'pidof' returns null. then   echo "Process $process was not running."   echo "Nothing killed."   exit $NOPROCESS fi    kill $t                  # May need 'kill -9' for stubborn process.  # Need a check here to see if process allowed itself to be killed. # Perhaps another " t=`pidof $process` " or ...   # This entire script could be replaced by #        kill $(pidof -x process_name) # or #        killall process_name # but it would not be as instructive.  exit 0

fuser

Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be invoked with the -k option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services.

                          bash$                                                                                fuser -u /usr/bin/vim                                                    /usr/bin/vim:         3207e(bozo)                          bash$                                                                                fuser -u /dev/null                                                    /dev/null:            3009(bozo)  3010(bozo)  3197(bozo)  3199(bozo)

One important application for fuser is when physically inserting or removing storage media, such as CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error message. This means that some user(s) and/or process(es) are accessing the device. An fuser -um /dev/device_name will clear up the mystery, so you can kill any relevant processes.

                          bash$                                                                                umount /mnt/usbdrive                                                    umount: /mnt/usbdrive: device is busy                          bash$                                                                                fuser -um /dev/usbdrive                                                    /mnt/usbdrive:        1772c(bozo)                          bash$                                                                                kill -9 1772                                                    bash$                                                                                umount /mnt/usbdrive

The fuser command, invoked with the -n option identifies the processes accessing a port. This is especially useful in combination with nmap.

                          root#                                                                                nmap localhost.localdomain                                                    PORT     STATE SERVICE  25/tcp   open  smtp                          root#                                                                                fuser -un tcp 25                                                    25/tcp:               2095(root)                          root#                                                                                ps ax | grep 2095 | grep -v grep                                                    2095 ?        Ss     0:00 sendmail: accepting connections

cron

Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own crontab file which can be changed with the crontab command). It runs as a daemon and executes scheduled entries from /etc/crontab.

Some flavors of Linux run crond, Matthew Dillon's version of cron.

Process Control and Booting

init

The init command is the parent of all processes. Called in the final step of a bootup, init determines the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only.

telinit

Symlinked to init, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous -- be certain you understand it well before using!

runlevel

Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses the /var/run/utmp file.

halt, shutdown, reboot

Command set to shut the system down, usually just prior to a power down.

On some Linux distros, the halt command has 755 permissions, so it can be invoked by a non-root user. A careless halt in a terminal or a script may shut down the system!

service

Starts or stops a system service. The startup scripts in /etc/init.d and /etc/rc.d use this command to start services at bootup.

                          root#                                                                                /sbin/service iptables stop                                                    Flushing firewall rules:                                   [  OK  ]  Setting chains to policy ACCEPT: filter                    [  OK  ]  Unloading iptables modules:                                [  OK  ]

Network

nmap

Network mapper and port scanner. This command scans a server to locate open ports and the services associated with those ports. It can also report information about packet filters and firewalls. This is an important security tool for locking down a network against hacking attempts.

#!/bin/bash  SERVER=$HOST                           # localhost.localdomain (127.0.0.1). PORT_NUMBER=25                         # SMTP port.  nmap $SERVER | grep -w "$PORT_NUMBER"  # Is that particular port open? #              grep -w matches whole words only, #+             so this wouldn't match port 1025, for example.  exit 0  # 25/tcp     open        smtp

ifconfig

Network interface configuration and tuning utility.

                        bash$                                                                          ifconfig -a                                                lo        Link encap:Local Loopback            inet addr:127.0.0.1  Mask:255.0.0.0            UP LOOPBACK RUNNING  MTU:16436  Metric:1            RX packets:10 errors:0 dropped:0 overruns:0 frame:0            TX packets:10 errors:0 dropped:0 overruns:0 carrier:0            collisions:0 txqueuelen:0             RX bytes:700 (700.0 b)  TX bytes:700 (700.0 b)

The ifconfig command is most often used at bootup to set up the interfaces, or to shut them down when rebooting.

# Code snippets from /etc/rc.d/init.d/network  # ...  # Check that networking is up. [ ${NETWORKING} = "no" ] && exit 0  [ -x /sbin/ifconfig ] || exit 0  # ...  for i in $interfaces ; do   if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then     action "Shutting down interface $i: " ./ifdown $i boot   fi #  The GNU-specific "-q" option to "grep" means "quiet", i.e., #+ producing no output. #  Redirecting output to /dev/null is therefore not strictly necessary.         # ...  echo "Currently active devices:" echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'` #                            ^^^^^  should be quoted to prevent globbing. #  The following also work. #    echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })' #    echo $(/sbin/ifconfig | sed -e 's/ .*//') #  Thanks, S.C., for additional comments.

17.1. Analyzing a System Script

Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is "killall," [97] used to suspend running processes at system shutdown.

Example 17-12. killall, from /etc/rc.d/init.d

#!/bin/sh  # --> Comments added by the author of this document marked by "# -->".  # --> This is part of the 'rc' script package # --> by Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>.  # --> This particular script seems to be Red Hat / FC specific # --> (may not be present in other distributions).  #  Bring down all unneeded services that are still running #+ (there shouldn't be any, so this is just a sanity check)  for i in /var/lock/subsys/*; do         # --> Standard for/in loop, but since "do" is on same line,         # --> it is necessary to add ";".         # Check if the script is there.         [ ! -f $i ] && continue         # --> This is a clever use of an "and list", equivalent to:         # --> if [ ! -f "$i" ]; then continue          # Get the subsystem name.         subsys=${i#/var/lock/subsys/}         # --> Match variable name, which, in this case, is the file name.         # --> This is the exact equivalent of subsys=`basename $i`. 	         # -->  It gets it from the lock file name         # -->+ (if there is a lock file,         # -->+ that's proof the process has been running).         # -->  See the "lockfile" entry, above.           # Bring the subsystem down.         if [ -f /etc/rc.d/init.d/$subsys.init ]; then            /etc/rc.d/init.d/$subsys.init stop         else            /etc/rc.d/init.d/$subsys stop         # -->  Suspend running jobs and daemons.         # -->  Note that "stop" is a positional parameter,         # -->+ not a shell builtin.         fi done

That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there.

Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the -vn flags ( sh -vn scriptname ). Add extensive comments. Change the commands to echos.

Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. Try to understand at least portions of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the "initscripts" documentation.

Part 5. Advanced Topics

At this point, we are ready to delve into certain of the difficult and unusual aspects of scripting. Along the way, we will attempt to "push the envelope" in various ways and examine boundary conditions (what happens when we move into uncharted territory?).

Chapter 18. Regular Expressions

. . . the intellectual activity associated with software development is largely one of gaining insight.

--Stowe Boyd

To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and utilities commonly used in scripts, such as grep, expr, sed and awk, interpret and use REs. As of version 3, Bash has acquired its own RE-match operator: =~.

18.1. A Brief Introduction to Regular Expressions

An expression is a string of characters. Those characters having an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning [98] for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that match (or specify) patterns.

A Regular Expression contains one or more of the following:

A character set. These are the characters retaining their literal meaning. The simplest type of Regular Expression consists only of a character set, with no metacharacters.
An anchor. These designate (anchor) the position in the line of text that the RE is to match. For example, ^, and $ are anchors.
Modifiers. These expand or narrow (modify) the range of text the RE is to match. Modifiers include the asterisk, brackets, and the backslash.

The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters -- a string or a part of a string.

The asterisk -- * -- matches any number of repeats of the character string or RE preceding it, including zero instances.

"1133*" matches 11 + one or more 3's : 113 , 1133 , 1133333 , and so forth.
The dot -- . -- matches any one character, except a newline. [99]

"13." matches 13 + at least one of any character (including a space) : 1133 , 11333 , but not 13 (additional character missing).

See Example 16-18 for a demonstration of dot single-character matching.
The caret -- ^ -- matches the beginning of a line, but sometimes, depending on context, negates the meaning of a set of characters in an RE.
The dollar sign -- $ -- at the end of an RE matches the end of a line.

"XXX$" matches XXX at the end of a line.

"^$" matches blank lines.
Brackets -- [...] -- enclose a set of characters to match in a single RE.

"[xyz]" matches any one of the characters x , y , or z .

"[c-n]" matches any one of the characters in the range c to n .

"[B-Pk-y]" matches any one of the characters in the ranges B to P and k to y .

"[a-z0-9]" matches any single lowercase letter or any digit.

"[^b-d]" matches any character except those in the range b to d . This is an instance of ^ negating or inverting the meaning of the following RE (taking on a role similar to ! in a different context).

Combined sequences of bracketed characters match common word patterns. "[Yy][Ee][Ss]" matches yes , Yes , YES , yEs , and so forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches any Social Security number.
The backslash -- \ -- escapes a special character, which means that character gets interpreted literally (and is therefore no longer special).

A "\$" reverts back to its literal meaning of "$", rather than its RE meaning of end-of-line. Likewise a "\\" has the literal meaning of "\".

Escaped "angle brackets" -- \<...\> -- mark word boundaries.

The angle brackets must be escaped, since otherwise they have only their literal character meaning.

"\<the\>" matches the word "the," but not the words "them," "there," "other," etc.

                          bash$                                                                                cat textfile                                                    This is line 1, of which there is only one instance.  This is the only instance of line 2.  This is line 3, another line.  This is line 4.                          bash$                                                                                grep 'the' textfile                                                    This is line 1, of which there is only one instance.  This is the only instance of line 2.  This is line 3, another line.                          bash$                                                                                grep '\<the\>' textfile                                                    This is the only instance of line 2.

Extended REs. Additional metacharacters added to the basic set. Used in egrep, awk, and Perl.
The question mark -- ? -- matches zero or one of the previous RE. It is generally used for matching single characters.

The plus -- + -- matches one or more of the previous RE. It serves a role similar to the *, but does not match zero occurrences.

# GNU versions of sed and awk can use "+", # but it needs to be escaped.  echo a111b | sed -ne '/a1\+b/p' echo a111b | grep 'a1\+b' echo a111b | gawk '/a1+b/' # All of above are equivalent.  # Thanks, S.C.

Escaped "curly brackets" -- \{ \} -- indicate the number of occurrences of a preceding RE to match.

It is necessary to escape the curly brackets since they have only their literal character meaning otherwise. This usage is technically not part of the basic RE set.

"[0-9]\{5\}" matches exactly five digits (characters in the range of 0 to 9).

Curly brackets are not available as an RE in the "classic" (non-POSIX compliant) version of awk. However, the GNU extended version of awk, gawk, has the --re-interval option that permits them (without being escaped).

                                    bash$                                                                                                              echo 2222 | gawk --re-interval '/2{3}/'                                                                        2222

Perl and some egrep versions do not require escaping the curly brackets.

Parentheses -- ( ) -- enclose a group of REs. They are useful with the following "|" operator and in substring extraction using expr.

The -- | -- "or" RE operator matches any of a set of alternate characters.

                          bash$                                                                                egrep 're(a|e)d' misc.txt                                                    People who read seem to be better informed than those who do not.  The clarinet produces sound by the vibration of its reed.

Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions described above, as do the GNU utilities.

POSIX Character Classes. [:class:]

This is an alternate method of specifying a range of characters to match.
[:alnum:] matches alphabetic or numeric characters. This is equivalent to A-Za-z0-9 .
[:alpha:] matches alphabetic characters. This is equivalent to A-Za-z .
[:blank:] matches a space or a tab.
[:cntrl:] matches control characters.
[:digit:] matches (decimal) digits. This is equivalent to 0-9 .
[:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This is the same as [:print:] , below, but excluding the space character.
[:lower:] matches lowercase alphabetic characters. This is equivalent to a-z .
[:print:] (printable characters). Matches characters in the range of ASCII 32 - 126. This is the same as [:graph:] , above, but adding the space character.
[:space:] matches whitespace characters (space and horizontal tab).
[:upper:] matches uppercase alphabetic characters. This is equivalent to A-Z .

[:xdigit:] matches hexadecimal digits. This is equivalent to 0-9A-Fa-f .

POSIX character classes generally require quoting or double brackets ([[ ]]).

                          bash$                                                                                grep [[:digit:]] test.file                                                    abc=723

# ... if [[ $arow =~ [[:digit:]] ]]   #  Numerical input? then       #  POSIX char class   if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal! # ... # From ktour.sh example script.

These character classes may even be used with globbing, to a limited extent.

                          bash$                                                                                ls -l ?[[:digit:]][[:digit:]]?                                                    -rw-rw-r--    1 bozo  bozo         0 Aug 21 14:47 a33b

POSIX character classes are used in Example 16-21 and Example 16-22.

Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See Example A-12 and Example A-16 for illustrations of this.

The standard reference on this complex topic is Friedl's Mastering Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives a very lucid treatment of REs. See the Bibliography for more information on these books.

18.2. Globbing

Bash itself cannot recognize Regular Expressions. Inside scripts, it is commands and utilities -- such as sed and awk -- that interpret RE's.

Bash does carry out filename expansion [100] -- a process known as globbing -- but this does not use the standard RE set. Instead, globbing recognizes and expands wild cards. Globbing interprets the standard wild card characters [101] -- * and ?, character lists in square brackets, and certain other special characters (such as ^ for negating the sense of a match). There are important limitations on wild card characters in globbing, however. Strings containing * will not match filenames that start with a dot, as, for example, .bashrc. [102] Likewise, the ? has a different meaning in globbing than as part of an RE.

                      bash$                                                                    ls -l                                            total 2  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1  -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh  -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt                      bash$                                                                    ls -l t?.sh                                            -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh                      bash$                                                                    ls -l [ab]*                                            -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1                      bash$                                                                    ls -l [a-c]*                                            -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1                      bash$                                                                    ls -l [^ab]*                                            -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1  -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh  -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt                      bash$                                                                    ls -l {b*,c*,*est*}                                            -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1  -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1  -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt

Bash performs filename expansion on unquoted command-line arguments. The echo command demonstrates this.

                      bash$                                                                    echo *                                            a.1 b.1 c.1 t2.sh test1.txt                      bash$                                                                    echo t*                                            t2.sh test1.txt                      bash$                                                                    echo t?.sh                                            t2.sh

It is possible to modify the way Bash interprets special characters in globbing. A set -f command disables globbing, and the nocaseglob and nullglob options to shopt change globbing behavior.

Chapter 19. Here Documents

Here and now, boys.

--Aldous Huxley, Island

A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

COMMAND <<InputComesFromHERE ... ... ... InputComesFromHERE

A limit string delineates (frames) the command list. The special symbol << precedes the limit string. This has the effect of redirecting the output of a command block into the stdin of the program or command. It is similar to interactive-program < command-file , where command-file contains

command #1 command #2 ...

The here document equivalent looks like this:

interactive-program <<LimitString command #1 command #2 ... LimitString

Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters.

Note that here documents may sometimes be used to good effect with non-interactive utilities and commands, such as, for example, wall.

Example 19-1. broadcast: Sends message to everyone logged in

#!/bin/bash  wall <<zzz23EndOfMessagezzz23 E-mail your noontime orders for pizza to the system administrator.     (Add an extra dollar for anchovy or mushroom topping.) # Additional message text goes here. # Note: 'wall' prints comment lines. zzz23EndOfMessagezzz23  # Could have been done more efficiently by #         wall <message-file #  However, embedding the message template in a script #+ is a quick-and-dirty one-off solution.  exit

Even such unlikely candidates as the vi text editor lend themselves to here documents.

Example 19-2. dummyfile: Creates a 2-line dummy file

#!/bin/bash  # Noninteractive use of 'vi' to edit a file. # Emulates 'sed'.  E_BADARGS=85  if [ -z "$1" ] then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi  TARGETFILE=$1  # Insert 2 lines in file, then save. #--------Begin here document-----------# vi $TARGETFILE <<x23LimitStringx23 i This is line 1 of the example file. This is line 2 of the example file. ^[ ZZ x23LimitStringx23 #----------End here document-----------#  #  Note that ^[ above is a literal escape #+ typed by Control-V <Esc>.  #  Bram Moolenaar points out that this may not work with 'vim' #+ because of possible problems with terminal interaction.  exit

The above script could just as effectively have been implemented with ex, rather than vi. Here documents containing a list of ex commands are common enough to form their own category, known as ex scripts.

#!/bin/bash #  Replace all instances of "Smith" with "Jones" #+ in files with a ".txt" filename suffix.   ORIGINAL=Smith REPLACEMENT=Jones  for word in $(fgrep -l $ORIGINAL *.txt) do   # -------------------------------------   ex $word <<EOF   :%s/$ORIGINAL/$REPLACEMENT/g   :wq EOF   # :%s is the "ex" substitution command.   # :wq is write-and-quit.   # ------------------------------------- done

Analogous to "ex scripts" are cat scripts.

Example 19-3. Multi-line message using cat

#!/bin/bash  #  'echo' is fine for printing single line messages, #+  but somewhat problematic for for message blocks. #   A 'cat' here document overcomes this limitation.  cat <<End-of-message ------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. ------------------------------------- End-of-message  #  Replacing line 7, above, with #+   cat > $Newfile <<End-of-message #+       ^^^^^^^^^^ #+ writes the output to the file $Newfile, rather than to stdout.  exit 0   #-------------------------------------------- # Code below disabled, due to "exit 0" above.  # S.C. points out that the following also works. echo "------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. -------------------------------------" # However, text may not include double quotes unless they are escaped.

The - option to mark a here document limit string ( <<-LimitString ) suppresses leading tabs (but not spaces) in the output. This may be useful in making a script more readable.

Example 19-4. Multi-line message, with tabs suppressed

#!/bin/bash # Same as previous example, but...  #  The - option to a here document <<- #+ suppresses leading tabs in the body of the document, #+ but *not* spaces.  cat <<-ENDOFMESSAGE 	This is line 1 of the message. 	This is line 2 of the message. 	This is line 3 of the message. 	This is line 4 of the message. 	This is the last line of the message. ENDOFMESSAGE # The output of the script will be flush left. # Leading tab in each line will not show.  # Above 5 lines of "message" prefaced by a tab, not spaces. # Spaces not affected by   <<-  .  # Note that this option has no effect on *embedded* tabs.  exit 0

A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly.

Example 19-5. Here document with replaceable parameters

#!/bin/bash # Another 'cat' here document, using parameter substitution.  # Try it with no command-line parameters,   ./scriptname # Try it with one command-line parameter,   ./scriptname Mortimer # Try it with one two-word quoted command-line parameter, #                           ./scriptname "Mortimer Jones"  CMDLINEPARAM=1     #  Expect at least command-line parameter.  if [ $# -ge $CMDLINEPARAM ] then   NAME=$1          #  If more than one command-line param,                    #+ then just take the first. else   NAME="John Doe"  #  Default, if no command-line parameter. fi    RESPONDENT="the author of this fine script"       cat <<Endofmessage  Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT.  # This comment shows up in the output (why?).  Endofmessage  # Note that the blank lines show up in the output. # So does the comment.  exit

This is a useful script containing a here document with parameter substitution.

Example 19-6. Upload a file pair to Sunsite incoming directory

#!/bin/bash # upload.sh  #  Upload file pair (Filename.lsm, Filename.tar.gz) #+ to incoming directory at Sunsite/UNC (ibiblio.org). #  Filename.tar.gz is the tarball itself. #  Filename.lsm is the descriptor file. #  Sunsite requires "lsm" file, otherwise will bounce contributions.   E_ARGERROR=85  if [ -z "$1" ] then   echo "Usage: `basename $0` Filename-to-upload"   exit $E_ARGERROR fi     Filename=`basename $1`           # Strips pathname out of file name.  Server="ibiblio.org" Directory="/incoming/Linux" #  These need not be hard-coded into script, #+ but may instead be changed to command-line argument.  Password="your.e-mail.address"   # Change above to suit.  ftp -n $Server <<End-Of-Session # -n option disables auto-logon  user anonymous "$Password"       #  If this doesn't work, then try:                                  #  quote user anonymous "$Password" binary bell                             # Ring 'bell' after each file transfer. cd $Directory put "$Filename.lsm" put "$Filename.tar.gz" bye End-Of-Session  exit 0

Quoting or escaping the "limit string" at the head of a here document disables parameter substitution within its body. The reason for this is that quoting/escaping the limit string effectively escapes the $, `, and \ special characters, and causes them to be interpreted literally. (Thank you, Allen Halsey, for pointing this out.)

Example 19-7. Parameter substitution turned off

#!/bin/bash #  A 'cat' here-document, but with parameter substitution disabled.  NAME="John Doe" RESPONDENT="the author of this fine script"    cat <<'Endofmessage'  Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT.  Endofmessage  #   No parameter substitution when the "limit string" is quoted or escaped. #   Either of the following at the head of the here document would have #+  the same effect. #   cat <<"Endofmessage" #   cat <<\Endofmessage    #   And, likewise:  cat <<"SpecialCharTest"  Directory listing would follow if limit string were not quoted. `ls -l`  Arithmetic expansion would take place if limit string were not quoted. $((5 + 3))  A a single backslash would echo if limit string were not quoted. \\  SpecialCharTest   exit

Disabling parameter substitution permits outputting literal text. Generating scripts or even program code is one use for this.

Example 19-8. A script that generates another script

#!/bin/bash # generate-script.sh # Based on an idea by Albert Reiner.  OUTFILE=generated.sh         # Name of the file to generate.   # ----------------------------------------------------------- # 'Here document containing the body of the generated script. ( cat <<'EOF' #!/bin/bash  echo "This is a generated shell script." #  Note that since we are inside a subshell, #+ we can't access variables in the "outside" script.  echo "Generated file will be named: $OUTFILE" #  Above line will not work as normally expected #+ because parameter expansion has been disabled. #  Instead, the result is literal output.  a=7 b=3  let "c = $a * $b" echo "c = $c"  exit 0 EOF ) > $OUTFILE # -----------------------------------------------------------  #  Quoting the 'limit string' prevents variable expansion #+ within the body of the above 'here document.' #  This permits outputting literal strings in the output file.  if [ -f "$OUTFILE" ] then   chmod 755 $OUTFILE   # Make the generated file executable. else   echo "Problem in creating file: \"$OUTFILE\"" fi  #  This method also works for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like.  exit 0

It is possible to set a variable from the output of a here document. This is actually a devious form of command substitution.

variable=$(cat <<SETVAR This variable runs over multiple lines. SETVAR )  echo "$variable"

A here document can supply input to a function in the same script.

Example 19-9. Here documents and functions

#!/bin/bash # here-function.sh  GetPersonalData () {   read firstname   read lastname   read address   read city    read state    read zipcode } # This certainly appears to be an interactive function, but . . .   # Supply input to the above function. GetPersonalData <<RECORD001 Bozo Bozeman 2726 Nondescript Dr. Bozeman MT 21226 RECORD001   echo echo "$firstname $lastname" echo "$address" echo "$city, $state $zipcode" echo  exit 0

It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an "anonymous" here document.

Example 19-10. "Anonymous" Here Document

#!/bin/bash  : <<TESTVARIABLES ${HOSTNAME?}${USER?}${MAIL?}  # Print error message if one of the variables not set. TESTVARIABLES  exit $?

A variation of the above technique permits "commenting out" blocks of code.

Example 19-11. Commenting out a block of code

#!/bin/bash # commentblock.sh  : <<COMMENTBLOCK echo "This line will not echo." This is a comment line missing the "#" prefix. This is another comment line missing the "#" prefix.  &*@!!++= The above line will cause no error message, because the Bash interpreter will ignore it. COMMENTBLOCK  echo "Exit value of above \"COMMENTBLOCK\" is $?."   # 0 # No error shown. echo   #  The above technique also comes in useful for commenting out #+ a block of working code for debugging purposes. #  This saves having to put a "#" at the beginning of each line, #+ then having to go back and delete each "#" later. #  Note that the use of of colon, above, is optional.  echo "Just before commented-out code block." #  The lines of code between the double-dashed lines will not execute. #  =================================================================== : <<DEBUGXXX for file in * do  cat "$file" done DEBUGXXX #  =================================================================== echo "Just after commented-out code block."  exit 0    ###################################################################### #  Note, however, that if a bracketed variable is contained within #+ the commented-out code block, #+ then this could cause problems. #  for example:   #/!/bin/bash    : <<COMMENTBLOCK   echo "This line will not echo."   &*@!!++=   ${foo_bar_bazz?}   $(rm -rf /tmp/foobar/)   $(touch my_build_directory/cups/Makefile) COMMENTBLOCK   $ sh commented-bad.sh commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set  # The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above.    : <<'COMMENTBLOCK'  # Thank you, Kurt Pfeifle, for pointing this out.

Yet another twist of this nifty trick makes "self-documenting" scripts possible.

Example 19-12. A self-documenting script

#!/bin/bash # self-document.sh: self-documenting script # Modification of "colm.sh".  DOC_REQUEST=70  if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help. then   echo; echo "Usage: $0 [directory-name]"; echo   sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" |   sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi   : <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command-line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory.  DOCUMENTATIONXX  if [ -z "$1" -o ! -r "$1" ] then   directory=. else   directory="$1" fi    echo "Listing of "$directory":"; echo (printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \ ; ls -l "$directory" | sed 1d) | column -t  exit 0

Using a cat script is an alternate way of accomplishing this.

DOC_REQUEST=70  if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help. then                                       # Use a "cat script" . . .   cat <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command-line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory.  DOCUMENTATIONXX exit $DOC_REQUEST fi

See also Example A-28, Example A-40, Example A-41, and Example A-42 for more examples of self-documenting scripts.

Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process.

                              bash$                                                                                            bash -c 'lsof -a -p $$ -d0' << EOF                                                            >                                                                                            EOF                                                            lsof    1213 bozo    0r   REG    3,5    0 30386 /tmp/t1213-0-sh (deleted)

Some utilities will not work inside a here document.

The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized. [103]

#!/bin/bash  echo "----------------------------------------------------------------------"  cat <<LimitString echo "This is line 1 of the message inside the here document." echo "This is line 2 of the message inside the here document." echo "This is the final line of the message inside the here document."      LimitString #^^^^Indented limit string. Error! This script will not behave as expected.  echo "----------------------------------------------------------------------"  #  These comments are outside the 'here document', #+ and should not echo.  echo "Outside the here document."  exit 0  echo "This line had better not echo."  # Follows an 'exit' command.

Some people very cleverly use a single ! as a limit string. But, that's not necessarily a good idea.

# This works. cat <<! Hello! ! Three more exclamations !!! !   # But . . . cat <<! Hello! Single exclamation point follows! ! ! # Crashes with an error message.   # However, the following will work. cat <<EOF Hello! Single exclamation point follows! ! EOF # It's safer to use a multi-character limit string.

For those tasks too complex for a here document, consider using the expect scripting language, which was specifically designed for feeding input into interactive programs.

19.1. Here Strings

Ahere string can be considered as a stripped-down form of ahere document.
It consists of nothing more thanCOMMAND <<< $WORD,
where$WORD is expanded and fed to thestdin ofCOMMAND.

As a simple example, consider this alternative to the echo-grep construction.

# Instead of: if echo "$VAR" | grep -q txt   # if [[ $VAR = *txt* ]] # etc.  # Try: if grep -q "txt" <<< "$VAR" then   #         ^^^    echo "$VAR contains the substring sequence \"txt\"" fi # Thank you, Sebastian Kaminski, for the suggestion.

Or, in combination with read:

String="This is a string of words."  read -r -a Words <<< "$String" #  The -a option to "read" #+ assigns the resulting values to successive members of an array.  echo "First word in String is:    ${Words[0]}"   # This echo "Second word in String is:   ${Words[1]}"   # is echo "Third word in String is:    ${Words[2]}"   # a echo "Fourth word in String is:   ${Words[3]}"   # string echo "Fifth word in String is:    ${Words[4]}"   # of echo "Sixth word in String is:    ${Words[5]}"   # words. echo "Seventh word in String is:  ${Words[6]}"   # (null)                                                  # Past end of $String.  # Thank you, Francisco Lobo, for the suggestion.

It is, of course, possible to feed the output of a here string into the stdin of a loop.

# As Seamus points out . . .  ArrayVar=( element0 element1 element2 {A..D} )  while read element ; do   echo "$element" 1>&2 done <<< $(echo ${ArrayVar[*]})  # element0 element1 element2 A B C D

Example 19-13. Prepending a line to a file

#!/bin/bash # prepend.sh: Add text at beginning of file. # #  Example contributed by Kenny Stauffer, #+ and slightly modified by document author.   E_NOSUCHFILE=85  read -p "File: " file   # -p arg to 'read' displays prompt. if [ ! -e "$file" ] then   # Bail out if no such file.   echo "File $file not found."   exit $E_NOSUCHFILE fi  read -p "Title: " title cat - $file <<<$title > $file.new  echo "Modified file is $file.new"  exit  # Ends script execution.    from 'man bash':   Here Strings   	A variant of here documents, the format is:      		<<<word      	The word is expanded and supplied to the command on its standard input.     Of course, the following also works:    sed -e '1i\    Title: ' $file

Example 19-14. Parsing a mailbox

#!/bin/bash #  Script by Francisco Lobo, #+ and slightly modified and commented by ABS Guide author. #  Used in ABS Guide with permission. (Thank you!)  # This script will not run under Bash versions -lt 3.0.   E_MISSING_ARG=87 if [ -z "$1" ] then   echo "Usage: $0 mailbox-file"   exit $E_MISSING_ARG fi  mbox_grep()  # Parse mailbox file. {     declare -i body=0 match=0     declare -a date sender     declare mail header value       while IFS= read -r mail #         ^^^^                 Reset $IFS. #  Otherwise "read" will strip leading & trailing space from its input.     do        if [[ $mail =~ ^From  ]]   # Match "From" field in message.        then           (( body  = 0 ))           # "Zero out" variables.           (( match = 0 ))           unset date         elif (( body ))        then             (( match ))             #  echo "$mail"             #  Uncomment above line if you want entire body             #+ of message to display.     elif [[ $mail ]]; then       IFS=: read -r header value <<< "$mail"       #                          ^^^  "here string"        case "$header" in       [Ff][Rr][Oo][Mm] ) [[ $value =~ "$2" ]] && (( match++ )) ;;       # Match "From" line.       [Dd][Aa][Tt][Ee] ) read -r -a date <<< "$value" ;;       #                                  ^^^       # Match "Date" line.       [Rr][Ee][Cc][Ee][Ii][Vv][Ee][Dd] ) read -r -a sender <<< "$value" ;;       #                                                    ^^^       # Match IP Address (may be spoofed).       esac         else           (( body++ ))           (( match  )) &&           echo "MESSAGE ${date:+of: ${date[*]} }"        #    Entire $date array             ^           echo "IP address of sender: ${sender[1]}"        #    Second field of "Received" line    ^         fi       done < "$1" # Redirect stdout of file into loop. }   mbox_grep "$1"  # Send mailbox file to function.  exit $?  # Exercises: # --------- # 1) Break the single function, above, into multiple functions, #+   for the sake of readability. # 2) Add additional parsing to the script, checking for various keywords.    $ mailbox_grep.sh scam_mail   MESSAGE of Thu, 5 Jan 2006 08:00:56 -0500 (EST)    IP address of sender: 196.3.62.4

Exercise: Find other uses for here strings, such as, for example, feeding input to dc.

Chapter 20. I/O Redirection

There are always three default files [104] open, stdin (the keyboard), stdout (the screen), and stderr (error messages output to the screen). These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script (see Example 3-1 and Example 3-2) and sending it as input to another file, command, program, or script.

Each open file gets assigned a file descriptor. [105] The file descriptors for stdin, stdout, and stderr are 0, 1, and 2, respectively. For opening additional files, there remain descriptors 3 to 9. It is sometimes useful to assign one of these additional file descriptors to stdin, stdout, or stderr as a temporary duplicate link. [106] This simplifies restoration to normal after complex redirection and reshuffling (see Example 20-1).

                  COMMAND_OUTPUT >       # Redirect stdout to a file.       # Creates the file if not present, otherwise overwrites it.        ls -lR > dir-tree.list       # Creates a file containing a listing of the directory tree.     : > filename       # The > truncates file "filename" to zero length.       # If file not present, creates zero-length file (same effect as 'touch').       # The : serves as a dummy placeholder, producing no output.     > filename           # The > truncates file "filename" to zero length.       # If file not present, creates zero-length file (same effect as 'touch').       # (Same result as ": >", above, but this does not work with some shells.)     COMMAND_OUTPUT >>       # Redirect stdout to a file.       # Creates the file if not present, otherwise appends to it.         # Single-line redirection commands (affect only the line they are on):       # --------------------------------------------------------------------     1>filename       # Redirect stdout to file "filename."    1>>filename       # Redirect and append stdout to file "filename."    2>filename       # Redirect stderr to file "filename."    2>>filename       # Redirect and append stderr to file "filename."    &>filename       # Redirect both stdout and stderr to file "filename."       # This operator is now functional, as of Bash 4, final release.     M>N      # "M" is a file descriptor, which defaults to 1, if not explicitly set.      # "N" is a filename.      # File descriptor "M" is redirect to file "N."    M>&N      # "M" is a file descriptor, which defaults to 1, if not set.      # "N" is another file descriptor.        #==============================================================================        # Redirecting stdout, one line at a time.       LOGFILE=script.log        echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE       echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE       echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE       echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."       # These redirection commands automatically "reset" after each line.          # Redirecting stderr, one line at a time.       ERRORFILE=script.errors        bad_command1 2>$ERRORFILE       #  Error message sent to $ERRORFILE.       bad_command2 2>>$ERRORFILE      #  Error message appended to $ERRORFILE.       bad_command3                    #  Error message echoed to stderr,                                       #+ and does not appear in $ERRORFILE.       # These redirection commands also automatically "reset" after each line.       #=======================================================================

                  2>&1       # Redirects stderr to stdout.       # Error messages get sent to same place as standard output.         >>filename 2>&1             bad_command >>filename 2>&1             # Appends both stdout and stderr to the file "filename" ...         2>&1 | [command(s)]             bad_command 2>&1 | awk '{print $5}'   # found             # Sends stderr through a pipe.             # |& was added to Bash 4 as an abbreviation for 2>&1 |.     i>&j       # Redirects file descriptor                  i                  to                  j.       # All output of file pointed to by                  i                  gets sent to file pointed to by                  j.     >&j       # Redirects, by default, file descriptor                  1                  (stdout) to                  j.       # All stdout gets sent to file pointed to by                  j.

                  0< FILENAME     < FILENAME       # Accept input from a file.       # Companion command to                  ">", and often used in combination with it.       #       # grep search-word <filename      [j]<>filename       #  Open file "filename" for reading and writing,       #+ and assign file descriptor "j" to it.       #  If "filename" does not exist, create it.       #  If file descriptor "j" is not specified, default to fd 0, stdin.       #       #  An application of this is writing at a specified place in a file.        echo 1234567890 > File    # Write string to "File".       exec 3<> File             # Open "File" and assign fd 3 to it.       read -n 4 <&3             # Read only 4 characters.       echo -n . >&3             # Write a decimal point there.       exec 3>&-                 # Close fd 3.       cat File                  # ==> 1234.67890       #  Random access, by golly.       |       # Pipe.       # General purpose process and command chaining tool.       # Similar to                  ">", but more general in effect.       # Useful for chaining commands, scripts, files, and programs together.       cat *.txt | sort | uniq > result-file       # Sorts the output of all the .txt files and deletes duplicate lines,       # finally saves results to                  "result-file".

Multiple instances of input and output redirection and/or pipes can be combined in a single command line.

command < input-file > output-file # Or the equivalent: < input-file command > output-file   # Although this is non-standard.  command1 | command2 | command3 > output-file

See Example 16-31 and Example A-14.

Multiple output streams may be redirected to one file.

ls -yz >> command.log 2>&1 #  Capture result of illegal options "yz" in file "command.log." #  Because stderr is redirected to the file, #+ any error messages will also be there.  #  Note, however, that the following does *not* give the same result. ls -yz 2>&1 >> command.log #  Outputs an error message, but does not write to file. #  More precisely, the command output (in this case, null) #+ writes to the file, but the error message goes only to stdout.  #  If redirecting both stdout and stderr, #+ the order of the commands makes a difference.

Closing File Descriptors

n<&-: Close input file descriptor n .
0<&-, <&-: Close stdin.
n>&-: Close output file descriptor n .
1>&-, >&-: Close stdout.

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.

# Redirecting only stderr to a pipe.  exec 3>&1                              # Save current "value" of stdout. ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls'). #              ^^^^   ^^^^ exec 3>&-                              # Now close it for the remainder of the script.  # Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix F.

20.1. Using exec

An exec <filename command redirects stdin to a file. From that point on, all stdin comes from that file, rather than its normal source (usually keyboard input). This provides a method of reading a file line by line and possibly parsing each line of input using sed and/or awk.

Example 20-1. Redirecting stdin using exec

#!/bin/bash # Redirecting stdin using 'exec'.   exec 6<&0          # Link file descriptor #6 with stdin.                    # Saves stdin.  exec < data-file   # stdin replaced by file "data-file"  read a1            # Reads first line of file "data-file". read a2            # Reads second line of file "data-file."  echo echo "Following lines read from file." echo "-------------------------------" echo $a1 echo $a2  echo; echo; echo  exec 0<&6 6<&- #  Now restore stdin from fd #6, where it had been saved, #+ and close fd #6 ( 6<&- ) to free it for other processes to use. # # <&6 6<&-    also works.  echo -n "Enter data  " read b1  # Now "read" functions as expected, reading from normal stdin. echo "Input read from stdin." echo "----------------------" echo "b1 = $b1"  echo  exit 0

Similarly, an exec >filename command redirects stdout to a designated file. This sends all command output that would normally go to stdout to that file.

exec N > filename affects the entire script or current shell. Redirection in the PID of the script or shell from that point on has changed. However . . .

N > filename affects only the newly-forked process, not the entire script or shell.

Thank you, Ahmed Darwish, for pointing this out.

Example 20-2. Redirecting stdout using exec

#!/bin/bash # reassign-stdout.sh  LOGFILE=logfile.txt  exec 6>&1           # Link file descriptor #6 with stdout.                     # Saves stdout.  exec > $LOGFILE     # stdout replaced with file "logfile.txt".  # ----------------------------------------------------------- # # All output from commands in this block sent to file $LOGFILE.  echo -n "Logfile: " date echo "-------------------------------------" echo  echo "Output of \"ls -al\" command" echo ls -al echo; echo echo "Output of \"df\" command" echo df  # ----------------------------------------------------------- #  exec 1>&6 6>&-      # Restore stdout and close file descriptor #6.  echo echo "== stdout now restored to default == " echo ls -al echo  exit 0

Example 20-3. Redirecting both stdin and stdout in the same script with exec

#!/bin/bash # upperconv.sh # Converts a specified input file to uppercase.  E_FILE_ACCESS=70 E_WRONG_ARGS=71  if [ ! -r "$1" ]     # Is specified input file readable? then   echo "Can't read from input file!"   echo "Usage: $0 input-file output-file"   exit $E_FILE_ACCESS fi                   #  Will exit with same error                      #+ even if input file ($1) not specified (why?).  if [ -z "$2" ] then   echo "Need to specify output file."   echo "Usage: $0 input-file output-file"   exit $E_WRONG_ARGS fi   exec 4<&0 exec < $1            # Will read from input file.  exec 7>&1 exec > $2            # Will write to output file.                      # Assumes output file writable (add check?).  # -----------------------------------------------     cat - | tr a-z A-Z   # Uppercase conversion. #   ^^^^^                # Reads from stdin. #           ^^^^^^^^^^   # Writes to stdout. # However, both stdin and stdout were redirected. # Note that the 'cat' can be omitted. # -----------------------------------------------  exec 1>&7 7>&-       # Restore stout. exec 0<&4 4<&-       # Restore stdin.  # After restoration, the following line prints to stdout as expected. echo "File \"$1\" written to \"$2\" as uppercase conversion."  exit 0

I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem.

Example 20-4. Avoiding a subshell

#!/bin/bash # avoid-subshell.sh # Suggested by Matthew Walker.  Lines=0  echo  cat myfile.txt | while read line;                  do {                    echo $line                    (( Lines++ ));  #  Incremented values of this variable                                    #+ inaccessible outside loop.                                    #  Subshell problem.                  }                  done  echo "Number of lines read = $Lines"     # 0                                          # Wrong!  echo "------------------------"   exec 3<> myfile.txt while read line <&3 do {   echo "$line"   (( Lines++ ));                   #  Incremented values of this variable                                    #+ accessible outside loop.                                    #  No subshell, no problem. } done exec 3>&-  echo "Number of lines read = $Lines"     # 8  echo  exit 0  # Lines below not seen by script.  $ cat myfile.txt  Line 1. Line 2. Line 3. Line 4. Line 5. Line 6. Line 7. Line 8.

20.2. Redirecting Code Blocks

Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of stdin. Even a function may use this form of redirection (see Example 24-11). The < operator at the end of the code block accomplishes this.

Example 20-5. Redirected while loop

#!/bin/bash # redir2.sh  if [ -z "$1" ] then   Filename=names.data       # Default, if no filename specified. else   Filename=$1 fi   #+ Filename=${1:-names.data} #  can replace the above test (parameter substitution).  count=0  echo  while [ "$name" != Smith ]  # Why is variable $name in quotes? do   read name                 # Reads from $Filename, rather than stdin.   echo $name   let "count += 1" done <"$Filename"           # Redirects stdin to file $Filename.  #    ^^^^^^^^^^^^  echo; echo "$count names read"; echo  exit 0  #  Note that in some older shell scripting languages, #+ the redirected loop would run as a subshell. #  Therefore, $count would return 0, the initialized value outside the loop. #  Bash and ksh avoid starting a subshell *whenever possible*, #+ so that this script, for example, runs correctly. #  (Thanks to Heiner Steven for pointing this out.)  #  However . . . #  Bash *can* sometimes start a subshell in a PIPED "while-read" loop, #+ as distinct from a REDIRECTED "while" loop.  abc=hi echo -e "1\n2\n3" | while read l      do abc="$l"         echo $abc      done echo $abc  #  Thanks, Bruno de Oliveira Schneider, for demonstrating this #+ with the above snippet of code. #  And, thanks, Brian Onn, for correcting an annotation error.

Example 20-6. Alternate form of redirected while loop

#!/bin/bash  # This is an alternate form of the preceding script.  #  Suggested by Heiner Steven #+ as a workaround in those situations when a redirect loop #+ runs as a subshell, and therefore variables inside the loop # +do not keep their values upon loop termination.   if [ -z "$1" ] then   Filename=names.data     # Default, if no filename specified. else   Filename=$1 fi     exec 3<&0                 # Save stdin to file descriptor 3. exec 0<"$Filename"        # Redirect standard input.  count=0 echo   while [ "$name" != Smith ] do   read name               # Reads from redirected stdin ($Filename).   echo $name   let "count += 1" done                      #  Loop reads from file $Filename                           #+ because of line 20.  #  The original version of this script terminated the "while" loop with #+      done <"$Filename"  #  Exercise: #  Why is this unnecessary?   exec 0<&3                 # Restore old stdin. exec 3<&-                 # Close temporary fd 3.  echo; echo "$count names read"; echo  exit 0

Example 20-7. Redirected until loop

#!/bin/bash # Same as previous example, but with "until" loop.  if [ -z "$1" ] then   Filename=names.data         # Default, if no filename specified. else   Filename=$1 fi    # while [ "$name" != Smith ] until [ "$name" = Smith ]     # Change  !=  to =. do   read name                   # Reads from $Filename, rather than stdin.   echo $name done <"$Filename"             # Redirects stdin to file $Filename.  #    ^^^^^^^^^^^^  # Same results as with "while" loop in previous example.  exit 0

Example 20-8. Redirected for loop

#!/bin/bash  if [ -z "$1" ] then   Filename=names.data          # Default, if no filename specified. else   Filename=$1 fi    line_count=`wc $Filename | awk '{ print $1 }'` #           Number of lines in target file. # #  Very contrived and kludgy, nevertheless shows that #+ it's possible to redirect stdin within a "for" loop... #+ if you're clever enough. # # More concise is     line_count=$(wc -l < "$Filename")   for name in `seq $line_count`  # Recall that "seq" prints sequence of numbers. # while [ "$name" != Smith ]   --   more complicated than a "while" loop   -- do   read name                    # Reads from $Filename, rather than stdin.   echo $name   if [ "$name" = Smith ]       # Need all this extra baggage here.   then     break   fi   done <"$Filename"              # Redirects stdin to file $Filename.  #    ^^^^^^^^^^^^  exit 0

We can modify the previous example to also redirect the output of the loop.

Example 20-9. Redirected for loop (both stdin and stdout redirected)

#!/bin/bash  if [ -z "$1" ] then   Filename=names.data          # Default, if no filename specified. else   Filename=$1 fi    Savefile=$Filename.new         # Filename to save results in. FinalName=Jonah                # Name to terminate "read" on.  line_count=`wc $Filename | awk '{ print $1 }'`  # Number of lines in target file.   for name in `seq $line_count` do   read name   echo "$name"   if [ "$name" = "$FinalName" ]   then     break   fi   done < "$Filename" > "$Savefile"     # Redirects stdin to file $Filename, #    ^^^^^^^^^^^^^^^^^^^^^^^^^^^       and saves it to backup file.  exit 0

Example 20-10. Redirected if/then test

#!/bin/bash  if [ -z "$1" ] then   Filename=names.data   # Default, if no filename specified. else   Filename=$1 fi    TRUE=1  if [ "$TRUE" ]          # if true    and   if :   also work. then  read name  echo $name fi <"$Filename" #  ^^^^^^^^^^^^  # Reads only first line of file. # An "if/then" test has no way of iterating unless embedded in a loop.  exit 0

Example 20-11. Data file names.data for above examples

Aristotle Arrhenius Belisarius Capablanca Dickens Euler Goethe Hegel Jonah Laplace Maroczy Purcell Schmidt Schopenhauer Semmelweiss Smith Steinmetz Tukhashevsky Turing Venn Warshawski Znosko-Borowski  #  This is a data file for #+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".

Redirecting the stdout of a code block has the effect of saving its output to a file. See Example 3-2.

Here documents are a special case of redirected code blocks. That being the case, it should be possible to feed the output of a here document into the stdin for a while loop.

# This example by Albert Siersema # Used with permission (thanks!).  function doesOutput()  # Could be an external command too, of course.  # Here we show you can use a function as well. {   ls -al *.jpg | awk '{print $5,$9}' }   nr=0          #  We want the while loop to be able to manipulate these and totalSize=0   #+ to be able to see the changes after the 'while' finished.  while read fileSize fileName ; do   echo "$fileName is $fileSize bytes"   let nr++   totalSize=$((totalSize+fileSize))   # Or: "let totalSize+=fileSize" done<<EOF $(doesOutput) EOF  echo "$nr files totaling $totalSize bytes"

20.3. Applications

Clever use of I/O redirection permits parsing and stitching together snippets of command output (see Example 15-7). This permits generating report and log files.

Example 20-12. Logging events

#!/bin/bash # logevents.sh # Author: Stephane Chazelas. # Used in ABS Guide with permission.  # Event logging to a file. # Must be run as root (for write access in /var/log).  ROOT_UID=0     # Only users with $UID 0 have root privileges. E_NOTROOT=67   # Non-root exit error.   if [ "$UID" -ne "$ROOT_UID" ] then   echo "Must be root to run this script."   exit $E_NOTROOT fi     FD_DEBUG1=3 FD_DEBUG2=4 FD_DEBUG3=5  # === Uncomment one of the two lines below to activate script. === # LOG_EVENTS=1 # LOG_VARS=1   log()  # Writes time and date to log file. { echo "$(date)  $*" >&7     # This *appends* the date to the file. #     ^^^^^^^  command substitution                            # See below. }    case $LOG_LEVEL in  1) exec 3>&2         4> /dev/null 5> /dev/null;;  2) exec 3>&2         4>&2         5> /dev/null;;  3) exec 3>&2         4>&2         5>&2;;  *) exec 3> /dev/null 4> /dev/null 5> /dev/null;; esac  FD_LOGVARS=6 if [[ $LOG_VARS ]] then exec 6>> /var/log/vars.log else exec 6> /dev/null                     # Bury output. fi  FD_LOGEVENTS=7 if [[ $LOG_EVENTS ]] then   # exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)   # Above line fails in versions of Bash more recent than 2.04. Why?   exec 7>> /var/log/event.log              # Append to "event.log".   log                                      # Write time and date. else exec 7> /dev/null                     # Bury output. fi  echo "DEBUG3: beginning" >&${FD_DEBUG3}  ls -l >&5 2>&4                             # command1 >&5 2>&4  echo "Done"                                # command2   echo "sending mail" >&${FD_LOGEVENTS} # Writes "sending mail" to file descriptor #7.   exit 0

Chapter 21. Subshells

Running a shell script launches a new process, a subshell.

A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child process) of the parent shell.

A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.

#!/bin/bash # subshell-test.sh  ( # Inside parentheses, and therefore a subshell . . . while [ 1 ]   # Endless loop. do   echo "Subshell running . . ." done )  #  Script will run forever, #+ or at least until terminated by a Ctl-C.  exit $?  # End of script (but will never get here).    Now, run the script: sh subshell-test.sh  And, while the script is running, from a different xterm: ps -ef | grep subshell-test.sh  UID       PID   PPID  C STIME TTY      TIME     CMD 500       2698  2502  0 14:26 pts/4    00:00:00 sh subshell-test.sh 500       2699  2698 21 14:26 pts/4    00:00:24 sh subshell-test.sh            ^^^^  Analysis: PID 2698, the script, launched PID 2699, the subshell.  Note: The "UID ..." line would be filtered out by the "grep" command, but is shown here for illustrative purposes.

In general, an external command in a script forks off a subprocess, [107] whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents.

Command List within Parentheses

( command1; command2; command3; ... ): A command list embedded between parentheses runs as a subshell.

Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, variables local to the child process.

Example 21-1. Variable scope in a subshell

#!/bin/bash # subshell.sh  echo  echo "We are outside the subshell." echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL" # Bash, version 3, adds the new         $BASH_SUBSHELL variable. echo; echo  outer_variable=Outer global_variable= #  Define global variable for "storage" of #+ value of subshell variable.  ( echo "We are inside the subshell." echo "Subshell level INSIDE subshell = $BASH_SUBSHELL" inner_variable=Inner  echo "From inside subshell, \"inner_variable\" = $inner_variable" echo "From inside subshell, \"outer\" = $outer_variable"  global_variable="$inner_variable"   #  Will this allow "exporting"                                     #+ a subshell variable? )  echo; echo echo "We are outside the subshell." echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL" echo  if [ -z "$inner_variable" ] then   echo "inner_variable undefined in main body of shell" else   echo "inner_variable defined in main body of shell" fi  echo "From main body of shell, \"inner_variable\" = $inner_variable" #  $inner_variable will show as blank (uninitialized) #+ because variables defined in a subshell are "local variables". #  Is there a remedy for this? echo "global_variable = "$global_variable""  # Why doesn't this work?  echo  # =======================================================================  # Additionally ...  echo "-----------------"; echo  var=41                                                 # Global variable.  ( let "var+=1"; echo "\$var INSIDE subshell = $var" )  # 42  echo "\$var OUTSIDE subshell = $var"                   # 41 #  Variable operations inside a subshell, even to a GLOBAL variable #+ do not affect the value of the variable outside the subshell!   exit 0  #  Question: #  -------- #  Once having exited a subshell, #+ is there any way to reenter that very same subshell #+ to modify or access the subshell variables?

Chapter 22. Restricted Shells

Disabled commands in restricted shells

: . Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script.

The following commands and actions are disabled:

Using cd to change the working directory.
Changing the values of the $PATH , $SHELL , $BASH_ENV , or $ENV environmental variables.
Reading or changing the $SHELLOPTS , shell environmental options.
Output redirection.
Invoking commands containing one or more /'s.
Invoking exec to substitute a different process for the shell.
Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose.
Getting out of restricted mode within the script.

Example 22-1. Running a script in restricted mode

#!/bin/bash  #  Starting the script with "#!/bin/bash -r" #+ runs entire script in restricted mode.  echo  echo "Changing directory." cd /usr/local echo "Now in `pwd`" echo "Coming back home." cd echo "Now in `pwd`" echo  # Everything up to here in normal, unrestricted mode.  set -r # set --restricted    has same effect. echo "==> Now in restricted mode. <=="  echo echo  echo "Attempting directory change in restricted mode." cd .. echo "Still in `pwd`"  echo echo  echo "\$SHELL = $SHELL" echo "Attempting to change shell in restricted mode." SHELL="/bin/ash" echo echo "\$SHELL= $SHELL"  echo echo  echo "Attempting to redirect output in restricted mode." ls -l /usr/bin > bin.files ls -l bin.files    # Try to list attempted file creation effort.  echo  exit 0

Chapter 23. Process Substitution

Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in.

Process substitution feeds the output of a process (or processes) into the stdin of another process.

Template

Command list enclosed within parentheses

>(command_list)

<(command_list)

Process substitution uses /dev/fd/<n> files to send the results of the process(es) within parentheses to another process. [108]

There is no space between the the "<" or ">" and the parentheses. Space there would give an error message.

                    bash$                                                              echo >(true)                                        /dev/fd/63                    bash$                                                              echo <(true)                                        /dev/fd/63                    bash$                                                              echo >(true) <(true)                                        /dev/fd/63 /dev/fd/62                    bash$                                                              wc <(cat /usr/share/dict/linux.words)                                                              483523  483523 4992010 /dev/fd/63                    bash$                                                              grep script /usr/share/dict/linux.words | wc                                                              262     262    3601                    bash$                                                              wc <(grep script /usr/share/dict/linux.words)                                                              262     262    3601 /dev/fd/63

Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking /dev/fd/<n> files, Bash may use temporary files. (Thanks, S.C.)

Process substitution can compare the output of two different commands, or even the output of different options to the same command.

                  bash$                                                        comm <(ls -l) <(ls -al)                                    total 12 -rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0 -rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2 -rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh         total 20         drwxrwxrwx    2 bozo bozo     4096 Mar 10 18:10 .         drwx------   72 bozo bozo     4096 Mar 10 17:58 ..         -rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0         -rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2         -rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh

Process substitution can compare the contents of two directories -- to see which filenames are in one, but not the other.

diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) #  Read a list of random numbers from /dev/urandom, #+ process with "od" #+ and feed into stdin of "read" . . .  #  From "insertion-sort.bash" example script. #  Courtesy of JuanJo Ciarlante.

PORT=6881   # bittorrent  # Scan the port to make sure nothing nefarious is going on. netcat -l $PORT | tee>(md5sum ->mydata-orig.md5) | gzip | tee>(md5sum - | sed 's/-$/mydata.lz2/'>mydata-gz.md5)>mydata.gz  # Check the decompression:   gzip -d<mydata.gz | md5sum -c mydata-orig.md5) # The MD5sum of the original checks stdin and detects compression issues.  #  Bill Davidsen contributed this example #+ (with light edits by the ABS Guide author).

cat <(ls -l) # Same as     ls -l | cat  sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin) # Lists all the files in the 3 main 'bin' directories, and sorts by filename. # Note that three (count 'em) distinct commands are fed to 'sort'.    diff <(command1) <(command2)    # Gives difference in command output.  tar cf >(bzip2 -c > file.tar.bz2) $directory_name # Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2". # # Because of the /dev/fd/<n> system feature, # the pipe between both commands does not need to be named. # # This can be emulated. # bzip2 -c < pipe > file.tar.bz2& tar cf pipe $directory_name rm pipe #        or exec 3>&1 tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&- exec 3>&-   # Thanks, St�phane Chazelas

Here is a method of circumventing the problem of an echo piped to a while-read loop running in a subshell.

Example 23-1. Code block redirection without forking

#!/bin/bash # wr-ps.bash: while-read loop with process substitution.  # This example contributed by Tomas Pospisek. # (Heavily edited by the ABS Guide author.)  echo  echo "random input" | while read i do   global=3D": Not available outside the loop."   # ... because it runs in a subshell. done  echo "\$global (from outside the subprocess) = $global" # $global (from outside the subprocess) =  echo; echo "--"; echo  while read i do   echo $i   global=3D": Available outside the loop."   # ... because it does NOT run in a subshell. done < <( echo "random input" ) #    ^ ^  echo "\$global (using process substitution) = $global" # Random input # $global (using process substitution) = 3D: Available outside the loop.   echo; echo "##########"; echo    # And likewise . . .  declare -a inloop index=0 cat $0 | while read line do   inloop[$index]="$line"   ((index++))   # It runs in a subshell, so ... done echo "OUTPUT = " echo ${inloop[*]}           # ... nothing echoes.   echo; echo "--"; echo   declare -a outloop index=0 while read line do   outloop[$index]="$line"   ((index++))   # It does NOT run in a subshell, so ... done < <( cat $0 ) echo "OUTPUT = " echo ${outloop[*]}          # ... the entire script echoes.  exit $?

This is a similar example.

Example 23-2. Redirecting the output of process substitution into a loop.

#!/bin/bash # psub.bash  # As inspired by Diego Molina (thanks!).  declare -a array0 while read do   array0[${#array0[@]}]="$REPLY" done < <( sed -e 's/bash/CRASH-BANG!/' $0 | grep bin | awk '{print $1}' ) #  Sets the default 'read' variable, $REPLY, by process substitution, #+ then copies it into an array.  echo "${array0[@]}"  exit $?  # ====================================== #  bash psub.bash  #!/bin/CRASH-BANG! done #!/bin/CRASH-BANG!

A reader sent in the following interesting example of process substitution.

# Script fragment taken from SuSE distribution:  # --------------------------------------------------------------# while read  des what mask iface; do # Some commands ... done < <(route -n)   #    ^ ^  First < is redirection, second is process substitution.  # To test it, let's make it do something. while read  des what mask iface; do   echo $des $what $mask $iface done < <(route -n)    # Output: # Kernel IP routing table # Destination Gateway Genmask Flags Metric Ref Use Iface # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo # --------------------------------------------------------------#  #  As St�phane Chazelas points out, #+ an easier-to-understand equivalent is: route -n |   while read des what mask iface; do   # Variables set from output of pipe.     echo $des $what $mask $iface   done  #  This yields the same output as above.         #  However, as Ulrich Gayer points out . . .         #+ this simplified equivalent uses a subshell for the while loop,         #+ and therefore the variables disappear when the pipe terminates. 	 # --------------------------------------------------------------# 	 #  However, Filip Moritz comments that there is a subtle difference #+ between the above two examples, as the following shows.  ( route -n | while read x; do ((y++)); done echo $y # $y is still unset  while read x; do ((y++)); done < <(route -n) echo $y # $y has the number of lines of output of route -n )  More generally spoken ( : | x=x # seems to start a subshell like : | ( x=x ) # while x=x < <(:) # does not )  # This is useful, when parsing csv and the like. # That is, in effect, what the original SuSE code fragment does.

Chapter 24. Functions

Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function.

function function_name {
command ...
}

function_name () {
command ...
}

This second form will cheer the hearts of C programmers (and is more portable).

As in C, the function's opening bracket may optionally appear on the second line.

function_name ()
{
command ...
}

A function may be "compacted" into a single line.

fun () { echo "This is a function"; echo; } #                                 ^     ^

In this case, however, a semicolon must follow the final command in the function.

fun () { echo "This is a function"; echo } # Error! #                                       ^  fun2 () { echo "Even a single-command function? Yes!"; } #                                                    ^

Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.

Example 24-1. Simple functions

#!/bin/bash # ex59.sh: Exercising functions (simple).  JUST_A_SECOND=1  funky () { # This is about as simple as functions get.   echo "This is a funky function."   echo "Now exiting funky function." } # Function declaration must precede call.   fun () { # A somewhat more complex function.   i=0   REPEATS=30    echo   echo "And now the fun really begins."   echo    sleep $JUST_A_SECOND    # Hey, wait a second!   while [ $i -lt $REPEATS ]   do     echo "----------FUNCTIONS---------->"     echo "<------------ARE-------------"     echo "<------------FUN------------>"     echo     let "i+=1"   done }    # Now, call the functions.  funky fun  exit $?

The function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C.

f1 # Will give an error message, since function "f1" not yet defined.  declare -f f1      # This doesn't help either. f1                 # Still an error message.  # However...  	   f1 () {   echo "Calling function \"f2\" from within function \"f1\"."   f2 }  f2 () {   echo "Function \"f2\"." }  f1  #  Function "f2" is not actually called until this point,     #+ although it is referenced before its definition.     #  This is permissible.          # Thanks, S.C.

Functions may not be empty!

#!/bin/bash # empty-function.sh  empty () { }  exit 0  # Will not exit here!  # $ sh empty-function.sh # empty-function.sh: line 6: syntax error near unexpected token `}' # empty-function.sh: line 6: `}'  # $ echo $? # 2   # Note that a function containing only comments is empty.  func () {   # Comment 1.   # Comment 2.   # This is still an empty function.   # Thank you, Mark Bova, for pointing this out. } # Results in same error message as above.   # However ...  not_quite_empty () {   illegal_command } #  A script containing this function will *not* bomb   #+ as long as the function is not called.  not_empty () {   : } # Contains a : (null command), and this is okay.   # Thank you, Dominick Geyer and Thiemo Kellner.

It is even possible to nest a function within another function, although this is not very useful.

f1 () {    f2 () # nested   {     echo "Function \"f2\", inside \"f1\"."   }  }    f2  #  Gives an error message.     #  Even a preceding "declare -f f2" wouldn't help.  echo      f1  #  Does nothing, since calling "f1" does not automatically call "f2". f2  #  Now, it's all right to call "f2",     #+ since its definition has been made visible by calling "f1".      # Thanks, S.C.

Function declarations can appear in unlikely places, even where a command would otherwise go.

ls -l | foo() { echo "foo"; }  # Permissible, but useless.    if [ "$USER" = bozo ] then   bozo_greet ()   # Function definition embedded in an if/then construct.   {     echo "Hello, Bozo."   } fi    bozo_greet        # Works only for Bozo, and other users get an error.    # Something like this might be useful in some contexts. NO_EXIT=1   # Will enable function definition below.  [[ $NO_EXIT -eq 1 ]] && exit() { true; }     # Function definition in an "and-list". # If $NO_EXIT is 1, declares "exit ()". # This disables the "exit" builtin by aliasing it to "true".  exit  # Invokes "exit ()" function, not "exit" builtin.    # Or, similarly: filename=file1  [ -f "$filename" ] && foo () { rm -f "$filename"; echo "File "$filename" deleted."; } || foo () { echo "File "$filename" not found."; touch bar; }  foo  # Thanks, S.C. and Christopher Head

Function names can take strange forms.

                    _(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; } # ^^^         No space between function name and parentheses. #             This doesn't always work. Why not?  # Now, let's invoke the function.   _         # __________ #             ^^^^^^^^^^   10 underscores (10 x function name)!   # A "naked" underscore is an acceptable function name.   # In fact, a colon is likewise an acceptable function name.  :(){ echo ":"; }; :  # Of what use is this? # It's a devious way to obfuscate the code in a script.

24.1. Complex Functions and Function Complexities

Functions may process arguments passed to them and return an exit status to the script for further processing.

function_name $arg1 $arg2

The function refers to the passed arguments by position (as if they were positional parameters), that is, $1, $2, and so forth.

Example 24-2. Function Taking Parameters

#!/bin/bash # Functions and parameters  DEFAULT=default                             # Default param value.  func2 () {    if [ -z "$1" ]                           # Is parameter #1 zero length?    then      echo "-Parameter #1 is zero length.-"  # Or no parameter passed.    else      echo "-Parameter #1 is \"$1\".-"    fi     variable=${1-$DEFAULT}                   #  What does    echo "variable = $variable"              #+ parameter substitution show?                                             #  ---------------------------                                             #  It distinguishes between                                             #+ no param and a null param.     if [ "$2" ]    then      echo "-Parameter #2 is \"$2\".-"    fi     return 0 }  echo     echo "Nothing passed."    func2                          # Called with no params echo   echo "Zero-length parameter passed." func2 ""                       # Called with zero-length param echo  echo "Null parameter passed." func2 "$uninitialized_param"   # Called with uninitialized param echo  echo "One parameter passed."    func2 first           # Called with one param echo  echo "Two parameters passed."    func2 first second    # Called with two params echo  echo "\"\" \"second\" passed." func2 "" second       # Called with zero-length first parameter echo                  # and ASCII string as a second one.  exit 0

The shift command works on arguments passed to functions (see Example 36-18).

But, what about command-line arguments passed to the script? Does a function see them? Well, let's clear up the confusion.

Example 24-3. Functions and command-line args passed to the script

#!/bin/bash # func-cmdlinearg.sh #  Call this script with a command-line argument, #+ something like $0 arg1.   func ()  { echo "$1"   # Echoes first arg passed to the function. }           # Does a command-line arg qualify?  echo "First call to function: no arg passed." echo "See if command-line arg is seen." func # No! Command-line arg not seen.  echo "============================================================" echo echo "Second call to function: command-line arg passed explicitly." func $1 # Now it's seen!  exit 0

In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals. Functions interpret their arguments literally.

Indirect variable references (see Example 37-2) provide a clumsy sort of mechanism for passing variable pointers to functions.

Example 24-4. Passing an indirect reference to a function

#!/bin/bash # ind-func.sh: Passing an indirect reference to a function.  echo_var () { echo "$1" }  message=Hello Hello=Goodbye  echo_var "$message"        # Hello # Now, let's pass an indirect reference to the function. echo_var "${!message}"     # Goodbye  echo "-------------"  # What happens if we change the contents of "hello" variable? Hello="Hello, again!" echo_var "$message"        # Hello echo_var "${!message}"     # Hello, again!  exit 0

The next logical question is whether parameters can be dereferenced after being passed to a function.

Example 24-5. Dereferencing a parameter passed to a function

#!/bin/bash # dereference.sh # Dereferencing parameter passed to a function. # Script by Bruce W. Clare.  dereference () {      y=\$"$1"   # Name of variable (not value!).      echo $y    # $Junk       x=`eval "expr \"$y\" "`      echo $1=$x      eval "$1=\"Some Different Text \""  # Assign new value. }  Junk="Some Text" echo $Junk "before"    # Some Text before  dereference Junk echo $Junk "after"     # Some Different Text after  exit 0

Example 24-6. Again, dereferencing a parameter passed to a function

#!/bin/bash # ref-params.sh: Dereferencing a parameter passed to a function. #                (Complex Example)  ITERATIONS=3  # How many times to get input. icount=1  my_read () {   #  Called with my_read varname,   #+ outputs the previous value between brackets as the default value,   #+ then asks for a new value.    local local_var    echo -n "Enter a value "   eval 'echo -n "[$'$1'] "'  #  Previous value. # eval echo -n "[\$$1] "     #  Easier to understand,                              #+ but loses trailing space in user prompt.   read local_var   [ -n "$local_var" ] && eval $1=\$local_var    # "And-list": if "local_var" then set "$1" to its value. }  echo  while [ "$icount" -le "$ITERATIONS" ] do   my_read var   echo "Entry #$icount = $var"   let "icount += 1"   echo done     # Thanks to Stephane Chazelas for providing this instructive example.  exit 0

Exit and Return

exit status

Functions return a value, called an exit status. This is analogous to the exit status returned by a command. The exit status may be explicitly specified by a return statement, otherwise it is the exit status of the last command in the function (0 if successful, and a non-zero error code if not). This exit status may be used in the script by referencing it as $?. This mechanism effectively permits script functions to have a "return value" similar to C functions.

return

Terminates a function. A return command [109] optionally takes an integer argument, which is returned to the calling script as the "exit status" of the function, and this exit status is assigned to the variable $?.

Example 24-7. Maximum of two numbers

#!/bin/bash # max.sh: Maximum of two integers.  E_PARAM_ERR=250    # If less than 2 params passed to function. EQUAL=251          # Return value if both params equal. #  Error values out of range of any #+ params that might be fed to the function.  max2 ()             # Returns larger of two numbers. {                   # Note: numbers compared must be less than 250. if [ -z "$2" ] then   return $E_PARAM_ERR fi  if [ "$1" -eq "$2" ] then   return $EQUAL else   if [ "$1" -gt "$2" ]   then     return $1   else     return $2   fi fi }  max2 33 34 return_val=$?  if [ "$return_val" -eq $E_PARAM_ERR ] then   echo "Need to pass two parameters to the function." elif [ "$return_val" -eq $EQUAL ]   then     echo "The two numbers are equal." else     echo "The larger of the two numbers is $return_val." fi       exit 0  #  Exercise (easy): #  --------------- #  Convert this to an interactive script, #+ that is, have the script ask for input (two numbers).

For a function to return a string or array, use a dedicated variable.

count_lines_in_etc_passwd() {   [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))   #  If /etc/passwd is readable, set REPLY to line count.   #  Returns both a parameter value and status information.   #  The 'echo' seems unnecessary, but . . .   #+ it removes excess whitespace from the output. }  if count_lines_in_etc_passwd then   echo "There are $REPLY lines in /etc/passwd." else   echo "Cannot count lines in /etc/passwd." fi    # Thanks, S.C.

Example 24-8. Converting numbers to Roman numerals

#!/bin/bash  # Arabic number to Roman numeral conversion # Range: 0 - 200 # It's crude, but it works.  # Extending the range and otherwise improving the script is left as an exercise.  # Usage: roman number-to-convert  LIMIT=200 E_ARG_ERR=65 E_OUT_OF_RANGE=66  if [ -z "$1" ] then   echo "Usage: `basename $0` number-to-convert"   exit $E_ARG_ERR fi    num=$1 if [ "$num" -gt $LIMIT ] then   echo "Out of range!"   exit $E_OUT_OF_RANGE fi    to_roman ()   # Must declare function before first call to it. { number=$1 factor=$2 rchar=$3 let "remainder = number - factor" while [ "$remainder" -ge 0 ] do   echo -n $rchar   let "number -= factor"   let "remainder = number - factor" done    return $number        # Exercises:        # ---------        # 1) Explain how this function works.        #    Hint: division by successive subtraction.        # 2) Extend to range of the function.        #    Hint: use "echo" and command-substitution capture. }      to_roman $num 100 C num=$? to_roman $num 90 LXXXX num=$? to_roman $num 50 L num=$? to_roman $num 40 XL num=$? to_roman $num 10 X num=$? to_roman $num 9 IX num=$? to_roman $num 5 V num=$? to_roman $num 4 IV num=$? to_roman $num 1 I # Successive calls to conversion function! # Is this really necessary??? Can it be simplified?  echo  exit

24.2. Local Variables

What makes a variable local?

local variables

A variable declared as local is one that is visible only within the block of code in which it appears. It has local scope. In a function, a local variable has meaning only within that function block. [110]

Example 24-12. Local variable visibility

#!/bin/bash # ex62.sh: Global and local variables inside a function.  func () {   local loc_var=23       # Declared as local variable.   echo                   # Uses the 'local' builtin.   echo "\"loc_var\" in function = $loc_var"   global_var=999         # Not declared as local.                          # Therefore, defaults to global.    echo "\"global_var\" in function = $global_var" }    func  # Now, to see if local variable "loc_var" exists outside the function.  echo echo "\"loc_var\" outside function = $loc_var"                                       # $loc_var outside function =                                        # No, $loc_var not visible globally. echo "\"global_var\" outside function = $global_var"                                       # $global_var outside function = 999                                       # $global_var is visible globally. echo				        exit 0 #  In contrast to C, a Bash variable declared inside a function #+ is local ONLY if declared as such.

Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local.

#!/bin/bash  func () { global_var=37    #  Visible only within the function block                  #+ before the function has been called.  }                #  END OF FUNCTION  echo "global_var = $global_var"  # global_var =                                  #  Function "func" has not yet been called,                                  #+ so $global_var is not visible here.  func echo "global_var = $global_var"  # global_var = 37                                  # Has been set by function call.

As Evgeniy Ivanov points out, when declaring and setting a local variable in a single command, apparently the order of operations is to first set the variable, and only afterwards restrict it to local scope. This is reflected in the return value.

#!/bin/bash  echo "==OUTSIDE Function (global)==" t=$(exit 1) echo $?      # 1              # As expected. echo  function0 () {  echo "==INSIDE Function==" echo "Global" t0=$(exit 1) echo $?      # 1              # As expected.  echo echo "Local declared & assigned in same command." local t1=$(exit 1) echo $?      # 0              # Unexpected! #  Apparently, the variable assignment takes place before #+ the local declaration. #+ The return value is for the latter.  echo echo "Local declared, then assigned (separate commands)." local t2 t2=$(exit 1) echo $?      # 1              # As expected.  }  function0

24.2.1. Local variables and recursion.

Local variables are a useful tool for writing recursive code, but this practice generally involves a great deal of computational overhead and is definitely not recommended in a shell script. [115]

Example 24-15. Recursion, using a local variable

#!/bin/bash  #               factorial #               ---------   # Does bash permit recursion? # Well, yes, but... # It's so slow that you gotta have rocks in your head to try it.   MAX_ARG=5 E_WRONG_ARGS=85 E_RANGE_ERR=86   if [ -z "$1" ] then   echo "Usage: `basename $0` number"   exit $E_WRONG_ARGS fi  if [ "$1" -gt $MAX_ARG ] then   echo "Out of range ($MAX_ARG is maximum)."   #  Let's get real now.   #  If you want greater range than this,   #+ rewrite it in a Real Programming Language.   exit $E_RANGE_ERR fi    fact () {   local number=$1   #  Variable "number" must be declared as local,   #+ otherwise this doesn't work.   if [ "$number" -eq 0 ]   then     factorial=1    # Factorial of 0 = 1.   else     let "decrnum = number - 1"     fact $decrnum  # Recursive function call (the function calls itself).     let "factorial = $number * $?"   fi    return $factorial }  fact $1 echo "Factorial of $1 is $?."  exit 0

Also see Example A-15 for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate in a script.

24.3. Recursion Without Local Variables

A function may recursively call itself even without use of local variables.

Example 24-16. The Fibonacci Sequence

#!/bin/bash # fibo.sh : Fibonacci sequence (recursive) # Author: M. Cooper # License: GPL3  # ----------algorithm-------------- # Fibo(0) = 0 # Fibo(1) = 1 # else #   Fibo(j) = Fibo(j-1) + Fibo(j-2) # ---------------------------------  MAXTERM=15       # Number of terms (+1) to generate. MINIDX=2         # If idx is less than 2, then Fibo(idx) = idx.  Fibonacci () {   idx=$1   # Doesn't need to be local. Why not?   if [ "$idx" -lt "$MINIDX" ]   then     echo "$idx"  # First two terms are 0 1 ... see above.   else     (( --idx ))  # j-1     term1=$( Fibonacci $idx )   #  Fibo(j-1)      (( --idx ))  # j-2     term2=$( Fibonacci $idx )   #  Fibo(j-2)      echo $(( term1 + term2 ))   fi   #  An ugly, ugly kludge.   #  The more elegant implementation of recursive fibo in C   #+ is a straightforward translation of the algorithm in lines 7 - 10. }  for i in $(seq 0 $MAXTERM) do  # Calculate $MAXTERM+1 terms.   FIBO=$(Fibonacci $i)   echo -n "$FIBO " done # 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 # Takes a while, doesn't it? Recursion in a script is slow.  echo  exit 0

Example 24-17. The Towers of Hanoi

#! /bin/bash # # The Towers Of Hanoi # Bash script # Copyright (C) 2000 Amit Singh. All Rights Reserved. # http://hanoi.kernelthread.com # # Tested under Bash version 2.05b.0(13)-release. # Also works under Bash version 3.x. # #  Used in "Advanced Bash Scripting Guide" #+ with permission of script author. #  Slightly modified and commented by ABS author.  #=================================================================# #  The Tower of Hanoi is a mathematical puzzle attributed to #+ Edouard Lucas, a nineteenth-century French mathematician. # #  There are three vertical posts set in a base. #  The first post has a set of annular rings stacked on it. #  These rings are disks with a hole drilled out of the center, #+ so they can slip over the posts and rest flat. #  The rings have different diameters, and they stack in ascending #+ order, according to size. #  The smallest ring is on top, and the largest on the bottom. # #  The task is to transfer the stack of rings #+ to one of the other posts. #  You can move only one ring at a time to another post. #  You are permitted to move rings back to the original post. #  You may place a smaller ring atop a larger one, #+ but *not* vice versa. #  Again, it is forbidden to place a larger ring atop a smaller one. # #  For a small number of rings, only a few moves are required. #+ For each additional ring, #+ the required number of moves approximately doubles, #+ and the "strategy" becomes increasingly complicated. # #  For more information, see http://hanoi.kernelthread.com #+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney. # # #         ...                   ...                    ... #         | |                   | |                    | | #        _|_|_                  | |                    | | #       |_____|                 | |                    | | #      |_______|                | |                    | | #     |_________|               | |                    | | #    |___________|              | |                    | | #   |             |             | |                    | | # .--------------------------------------------------------------. # |**************************************************************| #          #1                   #2                      #3 # #=================================================================#   E_NOPARAM=66  # No parameter passed to script. E_BADPARAM=67 # Illegal number of disks passed to script. Moves=        # Global variable holding number of moves.               # Modification to original script.  dohanoi() {   # Recursive function.     case $1 in     0)         ;;     *)         dohanoi "$(($1-1))" $2 $4 $3         echo move $2 "-->" $3         ((Moves++))          # Modification to original script.         dohanoi "$(($1-1))" $4 $3 $2         ;;     esac }  case $# in     1) case $(($1>0)) in     # Must have at least one disk.        1)  # Nested case statement.            dohanoi $1 1 3 2            echo "Total moves = $Moves"   # 2^n - 1, where n = # of disks.            exit 0;            ;;        *)            echo "$0: illegal value for number of disks";            exit $E_BADPARAM;            ;;        esac     ;;     *)        echo "usage: $0 N"        echo "       Where \"N\" is the number of disks."        exit $E_NOPARAM;        ;; esac  # Exercises: # --------- # 1) Would commands beyond this point ever be executed? #    Why not? (Easy) # 2) Explain the workings of the workings of the "dohanoi" function. #    (Difficult -- see the Dewdney reference, above.)

Chapter 25. Aliases

A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence. If, for example, we include alias lm="ls -l | more" in the ~/.bashrc file, then each lm [116] typed at the command-line will automatically be replaced by a ls -l | more. This can save a great deal of typing at the command-line and avoid having to remember complex combinations of commands and options. Setting alias rm="rm -i" (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently deleting important files.

In a script, aliases have very limited usefulness. It would be nice if aliases could assume some of the functionality of the C preprocessor, such as macro expansion, but unfortunately Bash does not expand arguments within the alias body. [117] Moreover, a script fails to expand an alias itself within "compound constructs," such as if/then statements, loops, and functions. An added limitation is that an alias will not expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function.

Example 25-1. Aliases within a script

#!/bin/bash # alias.sh  shopt -s expand_aliases # Must set this option, else script will not expand aliases.   # First, some fun. alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."' Jesse_James  echo; echo; echo;  alias ll="ls -l" # May use either single (') or double (") quotes to define an alias.  echo "Trying aliased \"ll\":" ll /usr/X11R6/bin/mk*   #* Alias works.  echo  directory=/usr/X11R6/bin/ prefix=mk*  # See if wild card causes problems. echo "Variables \"directory\" + \"prefix\" = $directory$prefix" echo  alias lll="ls -l $directory$prefix"  echo "Trying aliased \"lll\":" lll         # Long listing of all files in /usr/X11R6/bin stating with mk. # An alias can handle concatenated variables -- including wild card -- o.k.     TRUE=1  echo  if [ TRUE ] then   alias rr="ls -l"   echo "Trying aliased \"rr\" within if/then statement:"   rr /usr/X11R6/bin/mk*   #* Error message results!   # Aliases not expanded within compound statements.   echo "However, previously expanded alias still recognized:"   ll /usr/X11R6/bin/mk* fi    echo  count=0 while [ $count -lt 3 ] do   alias rrr="ls -l"   echo "Trying aliased \"rrr\" within \"while\" loop:"   rrr /usr/X11R6/bin/mk*   #* Alias will not expand here either.                            #  alias.sh: line 57: rrr: command not found   let count+=1 done   echo; echo  alias xyz='cat $0'   # Script lists itself.                      # Note strong quotes. xyz #  This seems to work, #+ although the Bash documentation suggests that it shouldn't. # #  However, as Steve Jacobson points out, #+ the "$0" parameter expands immediately upon declaration of the alias.  exit 0

The unalias command removes a previously set alias.

Example 25-2. unalias: Setting and unsetting an alias

#!/bin/bash # unalias.sh  shopt -s expand_aliases  # Enables alias expansion.  alias llm='ls -al | more' llm  echo  unalias llm              # Unset alias. llm # Error message results, since 'llm' no longer recognized.  exit 0

                  bash$                                                        ./unalias.sh                                    total 6 drwxrwxr-x    2 bozo     bozo         3072 Feb  6 14:04 . drwxr-xr-x   40 bozo     bozo         2048 Feb  6 14:04 .. -rwxr-xr-x    1 bozo     bozo          199 Feb  6 14:04 unalias.sh  ./unalias.sh: llm: command not found

Chapter 26. List Constructs

The and list and or list constructs provide a means of processing a number of commands consecutively. These can effectively replace complex nested if/then or even case statements.

Chaining together commands

and list

command-1 && command-2 && command-3 && ... command-n

Each command executes in turn, provided that the previous command has given a return value of true (zero). At the first false (non-zero) return, the command chain terminates (the first command returning false is the last one to execute).

An interesting use of a two-condition and list from an early version of YongYe's Tetris game script:

equation()  {  # core algorithm used for doubling and halving the coordinates    [[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2))    eval ${1}+=\"${x} ${y} \" }

Example 26-1. Using an and list to test for command-line arguments

#!/bin/bash # and list  if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \ #                ^^                         ^^               ^^ echo "Argument #2 = $2" then   echo "At least 2 arguments passed to script."   # All the chained commands return true. else   echo "Fewer than 2 arguments passed to script."   # At least one of the chained commands returns false. fi   # Note that "if [ ! -z $1 ]" works, but its alleged equivalent, #   "if [ -n $1 ]" does not. #     However, quoting fixes this. #  if "[ -n "$1" ]" works. #           ^  ^    Careful! # It is always best to QUOTE the variables being tested.   # This accomplishes the same thing, using "pure" if/then statements. if [ ! -z "$1" ] then   echo "Argument #1 = $1" fi if [ ! -z "$2" ] then   echo "Argument #2 = $2"   echo "At least 2 arguments passed to script." else   echo "Fewer than 2 arguments passed to script." fi # It's longer and more ponderous than using an "and list".   exit $?

Example 26-2. Another command-line arg test using an and list

#!/bin/bash  ARGS=1        # Number of arguments expected. E_BADARGS=85  # Exit value if incorrect number of args passed.  test $# -ne $ARGS && \ #    ^^^^^^^^^^^^ condition #1 echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS #                                             ^^ #  If condition #1 tests true (wrong number of args passed to script), #+ then the rest of the line executes, and script terminates.  # Line below executes only if the above test fails. echo "Correct number of arguments passed to this script."  exit 0  # To check exit value, do a "echo $?" after script termination.

Of course, an and list can also set variables to a default value.

arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT 		               # Set $arg1 to command-line arguments, if any.               # But . . . set to DEFAULT if not specified on command-line.

or list

command-1 || command-2 || command-3 || ... command-n

Each command executes in turn for as long as the previous command returns false. At the first true return, the command chain terminates (the first command returning true is the last one to execute). This is obviously the inverse of the "and list".

Example 26-3. Using or lists in combination with an and list

#!/bin/bash  #  delete.sh, a not-so-cunning file deletion utility. #  Usage: delete filename  E_BADARGS=85  if [ -z "$1" ] then   echo "Usage: `basename $0` filename"   exit $E_BADARGS  # No arg? Bail out. else     file=$1          # Set filename. fi     [ ! -f "$file" ] && echo "File \"$file\" not found. \ Cowardly refusing to delete a nonexistent file." # AND LIST, to give error message if file not present. # Note echo message continuing on to a second line after an escape.  [ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.") # OR LIST, to delete file if present.  # Note logic inversion above. # AND LIST executes on true, OR LIST on false.  exit $?

If the first command in an or list returns true, it will execute.

# ==> The following snippets from the /etc/rc.d/init.d/single #+==> script by Miquel van Smoorenburg #+==> illustrate use of "and" and "or" lists. # ==> "Arrowed" comments added by document author.  [ -x /usr/bin/clear ] && /usr/bin/clear   # ==> If /usr/bin/clear exists, then invoke it.   # ==> Checking for the existence of a command before calling it   #+==> avoids error messages and other awkward consequences.    # ==> . . .  # If they want to run something in single user mode, might as well run it... for i in /etc/rc1.d/S[0-9][0-9]* ; do         # Check if the script is there.         [ -x "$i" ] || continue   # ==> If corresponding file in $PWD *not* found,   #+==> then "continue" by jumping to the top of the loop.          # Reject backup files and files generated by rpm.         case "$1" in                 *.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)                         continue;;         esac         [ "$i" = "/etc/rc1.d/S00single" ] && continue   # ==> Set script name, but don't execute it yet.         $i start done    # ==> . . .

The exit status of an and list or an or list is the exit status of the last command executed.

Clever combinations of and and or lists are possible, but the logic may easily become convoluted and require close attention to operator precedence rules, and possibly extensive debugging.

false && true || echo false         # false  # Same result as ( false && true ) || echo false     # false # But NOT false && ( true || echo false )     # (nothing echoed)  #  Note left-to-right grouping and evaluation of statements.  #  It's usually best to avoid such complexities.  #  Thanks, S.C.

See Example A-7 and Example 7-4 for illustrations of using and / or list constructs to test variables.

Chapter 27. Arrays

Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the variable[xx] notation. Alternatively, a script may introduce the entire array by an explicit declare -a variable statement. To dereference (retrieve the contents of) an array element, use curly bracket notation, that is, ${element[xx]} .

Example 27-1. Simple array usage

#!/bin/bash   area[11]=23 area[13]=37 area[51]=UFOs  #  Array members need not be consecutive or contiguous.  #  Some members of the array can be left uninitialized. #  Gaps in the array are okay. #  In fact, arrays with sparse data ("sparse arrays") #+ are useful in spreadsheet-processing software.   echo -n "area[11] = " echo ${area[11]}    #  {curly brackets} needed.  echo -n "area[13] = " echo ${area[13]}  echo "Contents of area[51] are ${area[51]}."  # Contents of uninitialized array variable print blank (null variable). echo -n "area[43] = " echo ${area[43]} echo "(area[43] unassigned)"  echo  # Sum of two array variables assigned to third area[5]=`expr ${area[11]} + ${area[13]}` echo "area[5] = area[11] + area[13]" echo -n "area[5] = " echo ${area[5]}  area[6]=`expr ${area[11]} + ${area[51]}` echo "area[6] = area[11] + area[51]" echo -n "area[6] = " echo ${area[6]} # This fails because adding an integer to a string is not permitted.  echo; echo; echo  # ----------------------------------------------------------------- # Another array, "area2". # Another way of assigning array variables... # array_name=( XXX YYY ZZZ ... )  area2=( zero one two three four )  echo -n "area2[0] = " echo ${area2[0]} # Aha, zero-based indexing (first element of array is [0], not [1]).  echo -n "area2[1] = " echo ${area2[1]}    # [1] is second element of array. # -----------------------------------------------------------------  echo; echo; echo  # ----------------------------------------------- # Yet another array, "area3". # Yet another way of assigning array variables... # array_name=([xx]=XXX [yy]=YYY ...)  area3=([17]=seventeen [24]=twenty-four)  echo -n "area3[17] = " echo ${area3[17]}  echo -n "area3[24] = " echo ${area3[24]} # -----------------------------------------------  exit 0

As we have seen, a convenient way of initializing an entire array is the array=( element1 element2 ... elementN ) notation.

base64_charset=( {A..Z} {a..z} {0..9} + / = )                #  Using extended brace expansion                #+ to initialize the elements of the array.                                #  Excerpted from vladz's "base64.sh" script                #+ in the "Contributed Scripts" appendix.

Example 27-2. Formatting a poem

#!/bin/bash # poem.sh: Pretty-prints one of the ABS Guide author's favorite poems.  # Lines of the poem (single stanza). Line[1]="I do not know which to prefer," Line[2]="The beauty of inflections" Line[3]="Or the beauty of innuendoes," Line[4]="The blackbird whistling" Line[5]="Or just after." # Note that quoting permits embedding whitespace.  # Attribution. Attrib[1]=" Wallace Stevens" Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\"" # This poem is in the Public Domain (copyright expired).  echo  tput bold   # Bold print.  for index in 1 2 3 4 5    # Five lines. do   printf "     %s\n" "${Line[index]}" done  for index in 1 2          # Two attribution lines. do   printf "          %s\n" "${Attrib[index]}" done  tput sgr0   # Reset terminal.             # See 'tput' docs.  echo  exit 0  # Exercise: # -------- # Modify this script to pretty-print a poem from a text data file.

Array variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use.

Example 27-3. Various array operations

#!/bin/bash # array-ops.sh: More fun with arrays.   array=( zero one two three four five ) # Element 0   1   2    3     4    5  echo ${array[0]}       #  zero echo ${array:0}        #  zero                        #  Parameter expansion of first element,                        #+ starting at position # 0 (1st character). echo ${array:1}        #  ero                        #  Parameter expansion of first element,                        #+ starting at position # 1 (2nd character).  echo "--------------"  echo ${#array[0]}      #  4                        #  Length of first element of array. echo ${#array}         #  4                        #  Length of first element of array.                        #  (Alternate notation)  echo ${#array[1]}      #  3                        #  Length of second element of array.                        #  Arrays in Bash have zero-based indexing.  echo ${#array[*]}      #  6                        #  Number of elements in array. echo ${#array[@]}      #  6                        #  Number of elements in array.  echo "--------------"  array2=( [0]="first element" [1]="second element" [3]="fourth element" ) #            ^     ^       ^     ^      ^       ^     ^      ^       ^ # Quoting permits embedding whitespace within individual array elements.  echo ${array2[0]}      # first element echo ${array2[1]}      # second element echo ${array2[2]}      #                        # Skipped in initialization, and therefore null. echo ${array2[3]}      # fourth element echo ${#array2[0]}     # 13    (length of first element) echo ${#array2[*]}     # 3     (number of elements in array)  exit

Many of the standard string operations work on arrays.

Example 27-4. String operations on arrays

#!/bin/bash # array-strops.sh: String operations on arrays.  # Script by Michael Zick. # Used in ABS Guide with permission. # Fixups: 05 May 08, 04 Aug 08.  #  In general, any string operation using the ${name ... } notation #+ can be applied to all string elements in an array, #+ with the ${name[@] ... } or ${name[*] ...} notation.   arrayZ=( one two three four five five )  echo  # Trailing Substring Extraction echo ${arrayZ[@]:0}     # one two three four five five #                ^        All elements.  echo ${arrayZ[@]:1}     # two three four five five #                ^        All elements following element[0].  echo ${arrayZ[@]:1:2}   # two three #                  ^      Only the two elements after element[0].  echo "---------"   # Substring Removal  # Removes shortest match from front of string(s).  echo ${arrayZ[@]#f*r}   # one two three five five #               ^       # Applied to all elements of the array.                         # Matches "four" and removes it.  # Longest match from front of string(s) echo ${arrayZ[@]##t*e}  # one two four five five #               ^^      # Applied to all elements of the array.                         # Matches "three" and removes it.  # Shortest match from back of string(s) echo ${arrayZ[@]%h*e}   # one two t four five five #               ^       # Applied to all elements of the array.                         # Matches "hree" and removes it.  # Longest match from back of string(s) echo ${arrayZ[@]%%t*e}  # one two four five five #               ^^      # Applied to all elements of the array.                         # Matches "three" and removes it.  echo "----------------------"   # Substring Replacement  # Replace first occurrence of substring with replacement. echo ${arrayZ[@]/fiv/XYZ}   # one two three four XYZe XYZe #               ^           # Applied to all elements of the array.  # Replace all occurrences of substring. echo ${arrayZ[@]//iv/YY}    # one two three four fYYe fYYe                             # Applied to all elements of the array.  # Delete all occurrences of substring. # Not specifing a replacement defaults to 'delete' ... echo ${arrayZ[@]//fi/}      # one two three four ve ve #               ^^          # Applied to all elements of the array.  # Replace front-end occurrences of substring. echo ${arrayZ[@]/#fi/XY}    # one two three four XYve XYve #                ^          # Applied to all elements of the array.  # Replace back-end occurrences of substring. echo ${arrayZ[@]/%ve/ZZ}    # one two three four fiZZ fiZZ #                ^          # Applied to all elements of the array.  echo ${arrayZ[@]/%o/XX}     # one twXX three four five five #                ^          # Why?  echo "-----------------------------"   replacement() {     echo -n "!!!" }  echo ${arrayZ[@]/%e/$(replacement)} #                ^  ^^^^^^^^^^^^^^ # on!!! two thre!!! four fiv!!! fiv!!! # The stdout of replacement() is the replacement string. # Q.E.D: The replacement action is, in effect, an 'assignment.'  echo "------------------------------------"  #  Accessing the "for-each": echo ${arrayZ[@]//*/$(replacement optional_arguments)} #                ^^ ^^^^^^^^^^^^^ # !!! !!! !!! !!! !!! !!!  #  Now, if Bash would only pass the matched string #+ to the function being called . . .  echo  exit 0  #  Before reaching for a Big Hammer -- Perl, Python, or all the rest -- #  recall: #    $( ... ) is command substitution. #    A function runs as a sub-process. #    A function writes its output (if echo-ed) to stdout. #    Assignment, in conjunction with "echo" and command substitution, #+   can read a function's stdout. #    The name[@] notation specifies (the equivalent of) a "for-each" #+   operation. #  Bash is more powerful than you think!

Command substitution can construct the individual elements of an array.

Example 27-5. Loading the contents of a script into an array

#!/bin/bash # script-array.sh: Loads this script into an array. # Inspired by an e-mail from Chris Martin (thanks!).  script_contents=( $(cat "$0") )  #  Stores contents of this script ($0)                                  #+ in an array.  for element in $(seq 0 $((${#script_contents[@]} - 1)))   do                #  ${#script_contents[@]}                     #+ gives number of elements in the array.                     #                     #  Question:                     #  Why is  seq 0  necessary?                     #  Try changing it to seq 1.   echo -n "${script_contents[$element]}"                     # List each field of this script on a single line. # echo -n "${script_contents[element]}" also works because of ${ ... }.   echo -n " -- "    # Use " -- " as a field separator. done  echo  exit 0  # Exercise: # -------- #  Modify this script so it lists itself #+ in its original format, #+ complete with whitespace, line breaks, etc.

In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array elements, or even an entire array.

Example 27-6. Some special properties of arrays

#!/bin/bash  declare -a colors #  All subsequent commands in this script will treat #+ the variable "colors" as an array.  echo "Enter your favorite colors (separated from each other by a space)."  read -a colors    # Enter at least 3 colors to demonstrate features below. #  Special option to 'read' command, #+ allowing assignment of elements in an array.  echo  element_count=${#colors[@]} # Special syntax to extract number of elements in array. #     element_count=${#colors[*]} works also. # #  The "@" variable allows word splitting within quotes #+ (extracts variables separated by whitespace). # #  This corresponds to the behavior of "$@" and "$*" #+ in positional parameters.   index=0  while [ "$index" -lt "$element_count" ] do    # List all the elements in the array.   echo ${colors[$index]}   #    ${colors[index]} also works because it's within ${ ... } brackets.   let "index = $index + 1"   # Or:   #    ((index++)) done # Each array element listed on a separate line. # If this is not desired, use  echo -n "${colors[$index]} " # # Doing it with a "for" loop instead: #   for i in "${colors[@]}" #   do #     echo "$i" #   done # (Thanks, S.C.)  echo  # Again, list all the elements in the array, but using a more elegant method.   echo ${colors[@]}          # echo ${colors[*]} also works.  echo  # The "unset" command deletes elements of an array, or entire array. unset colors[1]              # Remove 2nd element of array.                              # Same effect as   colors[1]= echo  ${colors[@]}           # List array again, missing 2nd element.  unset colors                 # Delete entire array.                              #  unset colors[*] and                              #+ unset colors[@] also work. echo; echo -n "Colors gone."			    echo ${colors[@]}            # List array again, now empty.  exit 0

As seen in the previous example, either ${array_name[@]} or ${array_name[*]} refers to all the elements of the array. Similarly, to get a count of the number of elements in an array, use either ${#array_name[@]} or ${#array_name[*]}. ${#array_name} is the length (number of characters) of ${array_name[0]}, the first element of the array.

Example 27-7. Of empty arrays and empty elements

#!/bin/bash # empty-array.sh  #  Thanks to Stephane Chazelas for the original example, #+ and to Michael Zick and Omair Eshkenazi, for extending it. #  And to Nathan Coulter for clarifications and corrections.   # An empty array is not the same as an array with empty elements.    array0=( first second third )   array1=( '' )   # "array1" consists of one empty element.   array2=( )      # No elements . . . "array2" is empty.   array3=(   )    # What about this array?   echo ListArray() { echo echo "Elements in array0:  ${array0[@]}" echo "Elements in array1:  ${array1[@]}" echo "Elements in array2:  ${array2[@]}" echo "Elements in array3:  ${array3[@]}" echo echo "Length of first element in array0 = ${#array0}" echo "Length of first element in array1 = ${#array1}" echo "Length of first element in array2 = ${#array2}" echo "Length of first element in array3 = ${#array3}" echo echo "Number of elements in array0 = ${#array0[*]}"  # 3 echo "Number of elements in array1 = ${#array1[*]}"  # 1  (Surprise!) echo "Number of elements in array2 = ${#array2[*]}"  # 0 echo "Number of elements in array3 = ${#array3[*]}"  # 0 }  # ===================================================================  ListArray  # Try extending those arrays.  # Adding an element to an array. array0=( "${array0[@]}" "new1" ) array1=( "${array1[@]}" "new1" ) array2=( "${array2[@]}" "new1" ) array3=( "${array3[@]}" "new1" )  ListArray  # or array0[${#array0[*]}]="new2" array1[${#array1[*]}]="new2" array2[${#array2[*]}]="new2" array3[${#array3[*]}]="new2"  ListArray  # When extended as above, arrays are 'stacks' ... # Above is the 'push' ... # The stack 'height' is: height=${#array2[@]} echo echo "Stack height for array2 = $height"  # The 'pop' is: unset array2[${#array2[@]}-1]   #  Arrays are zero-based, height=${#array2[@]}            #+ which means first element has index 0. echo echo "POP" echo "New stack height for array2 = $height"  ListArray  # List only 2nd and 3rd elements of array0. from=1		    # Zero-based numbering. to=2 array3=( ${array0[@]:1:2} ) echo echo "Elements in array3:  ${array3[@]}"  # Works like a string (array of characters). # Try some other "string" forms.  # Replacement: array4=( ${array0[@]/second/2nd} ) echo echo "Elements in array4:  ${array4[@]}"  # Replace all matching wildcarded string. array5=( ${array0[@]//new?/old} ) echo echo "Elements in array5:  ${array5[@]}"  # Just when you are getting the feel for this . . . array6=( ${array0[@]#*new} ) echo # This one might surprise you. echo "Elements in array6:  ${array6[@]}"  array7=( ${array0[@]#new1} ) echo # After array6 this should not be a surprise. echo "Elements in array7:  ${array7[@]}"  # Which looks a lot like . . . array8=( ${array0[@]/new1/} ) echo echo "Elements in array8:  ${array8[@]}"  #  So what can one say about this?  #  The string operations are performed on #+ each of the elements in var[@] in succession. #  Therefore : Bash supports string vector operations. #  If the result is a zero length string, #+ that element disappears in the resulting assignment. #  However, if the expansion is in quotes, the null elements remain.  #  Michael Zick:    Question, are those strings hard or soft quotes? #  Nathan Coulter:  There is no such thing as "soft quotes." #!    What's really happening is that #!+   the pattern matching happens after #!+   all the other expansions of [word] #!+   in cases like ${parameter#word}.   zap='new*' array9=( ${array0[@]/$zap/} ) echo echo "Number of elements in array9:  ${#array9[@]}" array9=( "${array0[@]/$zap/}" ) echo "Elements in array9:  ${array9[@]}" # This time the null elements remain. echo "Number of elements in array9:  ${#array9[@]}"   # Just when you thought you were still in Kansas . . . array10=( ${array0[@]#$zap} ) echo echo "Elements in array10:  ${array10[@]}" # But, the asterisk in zap won't be interpreted if quoted. array10=( ${array0[@]#"$zap"} ) echo echo "Elements in array10:  ${array10[@]}" # Well, maybe we _are_ still in Kansas . . . # (Revisions to above code block by Nathan Coulter.)   #  Compare array7 with array10. #  Compare array8 with array9.  #  Reiterating: No such thing as soft quotes! #  Nathan Coulter explains: #  Pattern matching of 'word' in ${parameter#word} is done after #+ parameter expansion and *before* quote removal. #  In the normal case, pattern matching is done *after* quote removal.   exit

The relationship of ${array_name[@]} and ${array_name[*]} is analogous to that between $@ and $*. This powerful array notation has a number of uses.

# Copying an array. array2=( "${array1[@]}" ) # or array2="${array1[@]}" # #  However, this fails with "sparse" arrays, #+ arrays with holes (missing elements) in them, #+ as Jochen DeSmet points out. # ------------------------------------------   array1[0]=0 # array1[1] not assigned   array1[2]=2   array2=( "${array1[@]}" )       # Copy it?  echo ${array2[0]}      # 0 echo ${array2[2]}      # (null), should be 2 # ------------------------------------------    # Adding an element to an array. array=( "${array[@]}" "new element" ) # or array[${#array[*]}]="new element"  # Thanks, S.C.

The array=( element1 element2 ... elementN ) initialization operation, with the help of command substitution, makes it possible to load the contents of a text file into an array.

#!/bin/bash  filename=sample_file  #            cat sample_file # #            1 a b c #            2 d e fg   declare -a array1  array1=( `cat "$filename"`)                #  Loads contents #         List file to stdout              #+ of $filename into array1. # #  array1=( `cat "$filename" | tr '\n' ' '`) #                            change linefeeds in file to spaces.  #  Not necessary because Bash does word splitting, #+ changing linefeeds to spaces.  echo ${array1[@]}            # List the array. #                              1 a b c 2 d e fg # #  Each whitespace-separated "word" in the file #+ has been assigned to an element of the array.  element_count=${#array1[*]} echo $element_count          # 8

Clever scripting makes it possible to add array operations.

Example 27-8. Initializing arrays

#! /bin/bash # array-assign.bash  #  Array operations are Bash-specific, #+ hence the ".bash" in the script name.  # Copyright (c) Michael S. Zick, 2003, All rights reserved. # License: Unrestricted reuse in any form, for any purpose. # Version: $ID$ # # Clarification and additional comments by William Park.  #  Based on an example provided by Stephane Chazelas #+ which appeared in an earlier version of the #+ Advanced Bash Scripting Guide.  # Output format of the 'times' command: # User CPU <space> System CPU # User CPU of dead children <space> System CPU of dead children  #  Bash has two versions of assigning all elements of an array #+ to a new array variable. #  Both drop 'null reference' elements #+ in Bash versions 2.04 and later. #  An additional array assignment that maintains the relationship of #+ [subscript]=value for arrays may be added to newer versions.  #  Constructs a large array using an internal command, #+ but anything creating an array of several thousand elements #+ will do just fine.  declare -a bigOne=( /dev/* )  # All the files in /dev . . . echo echo 'Conditions: Unquoted, default IFS, All-Elements-Of' echo "Number of elements in array is ${#bigOne[@]}"  # set -vx    echo echo '- - testing: =( ${array[@]} ) - -' times declare -a bigTwo=( ${bigOne[@]} ) # Note parens:    ^              ^ times   echo echo '- - testing: =${array[@]} - -' times declare -a bigThree=${bigOne[@]} # No parentheses this time. times  #  Comparing the numbers shows that the second form, pointed out #+ by Stephane Chazelas, is faster. # #  As William Park explains: #+ The bigTwo array assigned element by element (because of parentheses), #+ whereas bigThree assigned as a single string. #  So, in essence, you have: #                   bigTwo=( [0]="..." [1]="..." [2]="..." ... ) #                   bigThree=( [0]="... ... ..." ) # #  Verify this by:  echo ${bigTwo[0]} #                   echo ${bigThree[0]}   #  I will continue to use the first form in my example descriptions #+ because I think it is a better illustration of what is happening.  #  The reusable portions of my examples will actual contain #+ the second form where appropriate because of the speedup.  # MSZ: Sorry about that earlier oversight folks.   #  Note: #  ---- #  The "declare -a" statements in lines 32 and 44 #+ are not strictly necessary, since it is implicit #+ in the  Array=( ... )  assignment form. #  However, eliminating these declarations slows down #+ the execution of the following sections of the script. #  Try it, and see.  exit 0

Adding a superfluous declare -a statement to an array declaration may speed up execution of subsequent operations on the array.

Example 27-9. Copying and concatenating arrays

#! /bin/bash # CopyArray.sh # # This script written by Michael Zick. # Used here with permission.  #  How-To "Pass by Name & Return by Name" #+ or "Building your own assignment statement".   CpArray_Mac() {  # Assignment Command Statement Builder      echo -n 'eval '     echo -n "$2"                    # Destination name     echo -n '=( ${'     echo -n "$1"                    # Source name     echo -n '[@]} )'  # That could all be a single command. # Matter of style only. }  declare -f CopyArray                # Function "Pointer" CopyArray=CpArray_Mac               # Statement Builder  Hype() {  # Hype the array named $1. # (Splice it together with array containing "Really Rocks".) # Return in array named $2.      local -a TMP     local -a hype=( Really Rocks )      $($CopyArray $1 TMP)     TMP=( ${TMP[@]} ${hype[@]} )     $($CopyArray TMP $2) }  declare -a before=( Advanced Bash Scripting ) declare -a after  echo "Array Before = ${before[@]}"  Hype before after  echo "Array After = ${after[@]}"  # Too much hype?  echo "What ${after[@]:3:2}?"  declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} ) #                    ---- substring extraction ----  echo "Array Modest = ${modest[@]}"  # What happened to 'before' ?  echo "Array Before = ${before[@]}"  exit 0

Example 27-10. More on concatenating arrays

#! /bin/bash # array-append.bash  # Copyright (c) Michael S. Zick, 2003, All rights reserved. # License: Unrestricted reuse in any form, for any purpose. # Version: $ID$ # # Slightly modified in formatting by M.C.   # Array operations are Bash-specific. # Legacy UNIX /bin/sh lacks equivalents.   #  Pipe the output of this script to 'more' #+ so it doesn't scroll off the terminal. #  Or, redirect output to a file.   declare -a array1=( zero1 one1 two1 ) # Subscript packed. declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 ) # Subscript sparse -- [1] is not defined.  echo echo '- Confirm that the array is really subscript sparse. -' echo "Number of elements: 4"        # Hard-coded for illustration. for (( i = 0 ; i < 4 ; i++ )) do     echo "Element [$i]: ${array2[$i]}" done # See also the more general code example in basics-reviewed.bash.   declare -a dest  # Combine (append) two arrays into a third array. echo echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator' echo '- Undefined elements not present, subscripts not maintained. -' # # The undefined elements do not exist; they are not being dropped.  dest=( ${array1[@]} ${array2[@]} ) # dest=${array1[@]}${array2[@]}     # Strange results, possibly a bug.  # Now, list the result. echo echo '- - Testing Array Append - -' cnt=${#dest[@]}  echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do     echo "Element [$i]: ${dest[$i]}" done  # Assign an array to a single array element (twice). dest[0]=${array1[@]} dest[1]=${array2[@]}  # List the result. echo echo '- - Testing modified array - -' cnt=${#dest[@]}  echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do     echo "Element [$i]: ${dest[$i]}" done  # Examine the modified second element. echo echo '- - Reassign and list second element - -'  declare -a subArray=${dest[1]} cnt=${#subArray[@]}  echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do     echo "Element [$i]: ${subArray[$i]}" done  #  The assignment of an entire array to a single element #+ of another array using the '=${ ... }' array assignment #+ has converted the array being assigned into a string, #+ with the elements separated by a space (the first character of IFS).  # If the original elements didn't contain whitespace . . . # If the original array isn't subscript sparse . . . # Then we could get the original array structure back again.  # Restore from the modified second element. echo echo '- - Listing restored element - -'  declare -a subArray=( ${dest[1]} ) cnt=${#subArray[@]}  echo "Number of elements: $cnt" for (( i = 0 ; i < cnt ; i++ )) do     echo "Element [$i]: ${subArray[$i]}" done echo '- - Do not depend on this behavior. - -' echo '- - This behavior is subject to change - -' echo '- - in versions of Bash newer than version 2.05b - -'  # MSZ: Sorry about any earlier confusion folks.  exit 0

Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left for the reader to decide.

Example 27-11. The Bubble Sort

#!/bin/bash # bubble.sh: Bubble sort, of sorts.  # Recall the algorithm for a bubble sort. In this particular version...  #  With each successive pass through the array to be sorted, #+ compare two adjacent elements, and swap them if out of order. #  At the end of the first pass, the "heaviest" element has sunk to bottom. #  At the end of the second pass, the next "heaviest" one has sunk next to bottom. #  And so forth. #  This means that each successive pass needs to traverse less of the array. #  You will therefore notice a speeding up in the printing of the later passes.   exchange() {   # Swaps two members of the array.   local temp=${Countries[$1]} #  Temporary storage                               #+ for element getting swapped out.   Countries[$1]=${Countries[$2]}   Countries[$2]=$temp      return }    declare -a Countries  #  Declare array,                       #+ optional here since it's initialized below.  #  Is it permissable to split an array variable over multiple lines #+ using an escape (\)? #  Yes.  Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \ Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \ Israel Peru Canada Oman Denmark Wales France Kenya \ Xanadu Qatar Liechtenstein Hungary)  # "Xanadu" is the mythical place where, according to Coleridge, #+ Kubla Khan did a pleasure dome decree.   clear                      # Clear the screen to start with.   echo "0: ${Countries[*]}"  # List entire array at pass 0.  number_of_elements=${#Countries[@]} let "comparisons = $number_of_elements - 1"  count=1 # Pass number.  while [ "$comparisons" -gt 0 ]          # Beginning of outer loop do    index=0  # Reset index to start of array after each pass.    while [ "$index" -lt "$comparisons" ] # Beginning of inner loop   do     if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ]     #  If out of order...     #  Recalling that \> is ASCII comparison operator     #+ within single brackets.      #  if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]     #+ also works.     then       exchange $index `expr $index + 1`  # Swap.     fi       let "index += 1"  # Or,   index+=1   on Bash, ver. 3.1 or newer.   done # End of inner loop  # ---------------------------------------------------------------------- # Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative. # # for (( last = $number_of_elements - 1 ; last > 0 ; last-- )) ##                     Fix by C.Y. Hunt          ^   (Thanks!) # do #     for (( i = 0 ; i < last ; i++ )) #     do #         [[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \ #             && exchange $i $((i+1)) #     done # done # ----------------------------------------------------------------------     let "comparisons -= 1" #  Since "heaviest" element bubbles to bottom,                        #+ we need do one less comparison each pass.  echo echo "$count: ${Countries[@]}"  # Print resultant array at end of each pass. echo let "count += 1"                # Increment pass count.  done                            # End of outer loop                                 # All done.  exit 0

Is it possible to nest arrays within arrays?

#!/bin/bash # "Nested" array.  #  Michael Zick provided this example, #+ with corrections and clarifications by William Park.  AnArray=( $(ls --inode --ignore-backups --almost-all \ 	--directory --full-time --color=none --time=status \ 	--sort=time -l ${PWD} ) )  # Commands and options.  # Spaces are significant . . . and don't quote anything in the above.  SubArray=( ${AnArray[@]:11:1}  ${AnArray[@]:6:5} ) #  This array has six elements: #+     SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]} #      [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} ) # #  Arrays in Bash are (circularly) linked lists #+ of type string (char *). #  So, this isn't actually a nested array, #+ but it's functionally similar.  echo "Current directory and date of last status change:" echo "${SubArray[@]}"  exit 0

Embedded arrays in combination with indirect references create some fascinating possibilities

Example 27-12. Embedded arrays and indirect references

#!/bin/bash # embedded-arrays.sh # Embedded arrays and indirect references.  # This script by Dennis Leeuw. # Used with permission. # Modified by document author.   ARRAY1=(         VAR1_1=value11         VAR1_2=value12         VAR1_3=value13 )  ARRAY2=(         VARIABLE="test"         STRING="VAR1=value1 VAR2=value2 VAR3=value3"         ARRAY21=${ARRAY1[*]} )       # Embed ARRAY1 within this second array.  function print () {         OLD_IFS="$IFS"         IFS=$'\n'       #  To print each array element                         #+ on a separate line.         TEST1="ARRAY2[*]"         local ${!TEST1} # See what happens if you delete this line.         #  Indirect reference. 	#  This makes the components of $TEST1 	#+ accessible to this function.           #  Let's see what we've got so far.         echo         echo "\$TEST1 = $TEST1"       #  Just the name of the variable.         echo; echo         echo "{\$TEST1} = ${!TEST1}"  #  Contents of the variable.                                       #  That's what an indirect                                       #+ reference does.         echo         echo "-------------------------------------------"; echo         echo           # Print variable         echo "Variable VARIABLE: $VARIABLE" 	         # Print a string element         IFS="$OLD_IFS"         TEST2="STRING[*]"         local ${!TEST2}      # Indirect reference (as above).         echo "String element VAR2: $VAR2 from STRING"          # Print an array element         TEST2="ARRAY21[*]"         local ${!TEST2}      # Indirect reference (as above).         echo "Array element VAR1_1: $VAR1_1 from ARRAY21" }  print echo  exit 0  #   As the author of the script notes, #+ "you can easily expand it to create named-hashes in bash." #   (Difficult) exercise for the reader: implement this.

Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a resource-intensive application of this nature should really be written in a compiled language, such as C. It runs excruciatingly slowly as a script.

Example 27-13. The Sieve of Eratosthenes

#!/bin/bash # sieve.sh (ex68.sh)  # Sieve of Eratosthenes # Ancient algorithm for finding prime numbers.  #  This runs a couple of orders of magnitude slower #+ than the equivalent program written in C.  LOWER_LIMIT=1       # Starting with 1. UPPER_LIMIT=1000    # Up to 1000. # (You may set this higher . . . if you have time on your hands.)  PRIME=1 NON_PRIME=0  let SPLIT=UPPER_LIMIT/2 # Optimization: # Need to test numbers only halfway to upper limit. Why?   declare -a Primes # Primes[] is an array.   initialize () { # Initialize the array.  i=$LOWER_LIMIT until [ "$i" -gt "$UPPER_LIMIT" ] do   Primes[i]=$PRIME   let "i += 1" done #  Assume all array members guilty (prime) #+ until proven innocent. }  print_primes () { # Print out the members of the Primes[] array tagged as prime.  i=$LOWER_LIMIT  until [ "$i" -gt "$UPPER_LIMIT" ] do    if [ "${Primes[i]}" -eq "$PRIME" ]   then     printf "%8d" $i     # 8 spaces per number gives nice, even columns.   fi      let "i += 1"    done  }  sift () # Sift out the non-primes. {  let i=$LOWER_LIMIT+1 # Let's start with 2.  until [ "$i" -gt "$UPPER_LIMIT" ] do  if [ "${Primes[i]}" -eq "$PRIME" ] # Don't bother sieving numbers already sieved (tagged as non-prime). then    t=$i    while [ "$t" -le "$UPPER_LIMIT" ]   do     let "t += $i "     Primes[t]=$NON_PRIME     # Tag as non-prime all multiples.   done  fi      let "i += 1" done     }   # ============================================== # main () # Invoke the functions sequentially. initialize sift print_primes # This is what they call structured programming. # ==============================================  echo  exit 0    # -------------------------------------------------------- # # Code below line will not execute, because of 'exit.'  #  This improved version of the Sieve, by Stephane Chazelas, #+ executes somewhat faster.  # Must invoke with command-line argument (limit of primes).  UPPER_LIMIT=$1                  # From command-line. let SPLIT=UPPER_LIMIT/2         # Halfway to max number.  Primes=( '' $(seq $UPPER_LIMIT) )  i=1 until (( ( i += 1 ) > SPLIT ))  # Need check only halfway. do   if [[ -n ${Primes[i]} ]]   then     t=$i     until (( ( t += i ) > UPPER_LIMIT ))     do       Primes[t]=     done   fi   done   echo ${Primes[*]}  exit $?

Example 27-14. The Sieve of Eratosthenes, Optimized

#!/bin/bash # Optimized Sieve of Eratosthenes # Script by Jared Martin, with very minor changes by ABS Guide author. # Used in ABS Guide with permission (thanks!).  # Based on script in Advanced Bash Scripting Guide. # http://tldp.org/LDP/abs/html/arrays.html#PRIMES0 (ex68.sh).  # http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf (reference) # Check results against http://primes.utm.edu/lists/small/1000.txt  # Necessary but not sufficient would be, e.g., #     (($(sieve 7919 | wc -w) == 1000)) && echo "7919 is the 1000th prime"  UPPER_LIMIT=${1:?"Need an upper limit of primes to search."}  Primes=( '' $(seq ${UPPER_LIMIT}) )  typeset -i i t Primes[i=1]='' # 1 is not a prime. until (( ( i += 1 ) > (${UPPER_LIMIT}/i) ))  # Need check only ith-way.   do                                         # Why?     if ((${Primes[t=i*(i-1), i]}))     # Obscure, but instructive, use of arithmetic expansion in subscript.     then       until (( ( t += i ) > ${UPPER_LIMIT} ))         do Primes[t]=; done     fi   done  # echo ${Primes[*]} echo   # Change to original script for pretty-printing (80-col. display). printf "%8d" ${Primes[*]} echo; echo  exit $?

Compare these array-based prime number generators with alternatives that do not use arrays, Example A-15, and Example 16-46.

Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support.

Example 27-15. Emulating a push-down stack

#!/bin/bash # stack.sh: push-down stack simulation  #  Similar to the CPU stack, a push-down stack stores data items #+ sequentially, but releases them in reverse order, last-in first-out.   BP=100            #  Base Pointer of stack array.                   #  Begin at element 100.  SP=$BP            #  Stack Pointer.                   #  Initialize it to "base" (bottom) of stack.  Data=             #  Contents of stack location.                     #  Must use global variable,                   #+ because of limitation on function return range.                     # 100     Base pointer       <-- Base Pointer                   #  99     First data item                   #  98     Second data item                   # ...     More data                   #         Last data item     <-- Stack pointer   declare -a stack   push()            # Push item on stack. { if [ -z "$1" ]    # Nothing to push? then   return fi  let "SP -= 1"     # Bump stack pointer. stack[$SP]=$1  return }  pop()                    # Pop item off stack. { Data=                    # Empty out data item.  if [ "$SP" -eq "$BP" ]   # Stack empty? then   return fi                       #  This also keeps SP from getting past 100,                          #+ i.e., prevents a runaway stack.  Data=${stack[$SP]} let "SP += 1"            # Bump stack pointer. return }  status_report()          # Find out what's happening. { echo "-------------------------------------" echo "REPORT" echo "Stack Pointer = $SP" echo "Just popped \""$Data"\" off the stack." echo "-------------------------------------" echo }   # ======================================================= # Now, for some fun.  echo  # See if you can pop anything off empty stack. pop status_report  echo  push garbage pop status_report     # Garbage in, garbage out.        value1=23;        push $value1 value2=skidoo;    push $value2 value3=LAST;      push $value3  pop               # LAST status_report pop               # skidoo status_report pop               # 23 status_report     # Last-in, first-out!  #  Notice how the stack pointer decrements with each push, #+ and increments with each pop.  echo  exit 0  # =======================================================   # Exercises: # ---------  # 1)  Modify the "push()" function to permit pushing #   + multiple element on the stack with a single function call.  # 2)  Modify the "pop()" function to permit popping #   + multiple element from the stack with a single function call.  # 3)  Add error checking to the critical functions. #     That is, return an error code, depending on #   + successful or unsuccessful completion of the operation, #   + and take appropriate action.  # 4)  Using this script as a starting point, #   + write a stack-based 4-function calculator.

Fancy manipulation of array "subscripts" may require intermediate variables. For projects involving this, again consider using a more powerful programming language, such as Perl or C.

Example 27-16. Complex array application: Exploring a weird mathematical series

#!/bin/bash  # Douglas Hofstadter's notorious "Q-series":  # Q(1) = Q(2) = 1 # Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2  #  This is a "chaotic" integer series with strange #+ and unpredictable behavior. #  The first 20 terms of the series are: #  1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12   #  See Hofstadter's book, _Goedel, Escher, Bach: An Eternal Golden Braid_, #+ p. 137, ff.   LIMIT=100     # Number of terms to calculate. LINEWIDTH=20  # Number of terms printed per line.  Q[1]=1        # First two terms of series are 1. Q[2]=1  echo echo "Q-series [$LIMIT terms]:" echo -n "${Q[1]} "             # Output first two terms. echo -n "${Q[2]} "  for ((n=3; n <= $LIMIT; n++))  # C-like loop expression. do   # Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]]  for n>2 #    Need to break the expression into intermediate terms, #+   since Bash doesn't handle complex array arithmetic very well.    let "n1 = $n - 1"        # n-1   let "n2 = $n - 2"        # n-2      t0=`expr $n - ${Q[n1]}`  # n - Q[n-1]   t1=`expr $n - ${Q[n2]}`  # n - Q[n-2]      T0=${Q[t0]}              # Q[n - Q[n-1]]   T1=${Q[t1]}              # Q[n - Q[n-2]]  Q[n]=`expr $T0 + $T1`      # Q[n - Q[n-1]] + Q[n - Q[n-2]] echo -n "${Q[n]} "  if [ `expr $n % $LINEWIDTH` -eq 0 ]    # Format output. then   #      ^ modulo   echo # Break lines into neat chunks. fi  done  echo  exit 0  #  This is an iterative implementation of the Q-series. #  The more intuitive recursive implementation is left as an exercise. #  Warning: calculating this series recursively takes a VERY long time #+ via a script. C/C++ would be orders of magnitude faster.

Bash supports only one-dimensional arrays, though a little trickery permits simulating multi-dimensional ones.

Example 27-17. Simulating a two-dimensional array, then tilting it

#!/bin/bash # twodim.sh: Simulating a two-dimensional array.  # A one-dimensional array consists of a single row. # A two-dimensional array stores rows sequentially.  Rows=5 Columns=5 # 5 X 5 Array.  declare -a alpha     # char alpha [Rows] [Columns];                      # Unnecessary declaration. Why?  load_alpha () { local rc=0 local index  for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y do     # Use different symbols if you like.   local row=`expr $rc / $Columns`   local column=`expr $rc % $Rows`   let "index = $row * $Rows + $column"   alpha[$index]=$i # alpha[$row][$column]   let "rc += 1" done    #  Simpler would be #+   declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y ) #+ but this somehow lacks the "flavor" of a two-dimensional array. }  print_alpha () { local row=0 local index  echo  while [ "$row" -lt "$Rows" ]   #  Print out in "row major" order: do                             #+ columns vary,                                #+ while row (outer loop) remains the same.   local column=0    echo -n "       "            #  Lines up "square" array with rotated one.      while [ "$column" -lt "$Columns" ]   do     let "index = $row * $Rows + $column"     echo -n "${alpha[index]} "  # alpha[$row][$column]     let "column += 1"   done    let "row += 1"   echo  done    # The simpler equivalent is #     echo ${alpha[*]} | xargs -n $Columns  echo }  filter ()     # Filter out negative array indices. {  echo -n "  "  # Provides the tilt.               # Explain how.  if [[ "$1" -ge 0 &&  "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]] then     let "index = $1 * $Rows + $2"     # Now, print it rotated.     echo -n " ${alpha[index]}"     #           alpha[$row][$column] fi      }       rotate ()  #  Rotate the array 45 degrees -- {          #+ "balance" it on its lower lefthand corner. local row local column  for (( row = Rows; row > -Rows; row-- ))   do       # Step through the array backwards. Why?    for (( column = 0; column < Columns; column++ ))   do      if [ "$row" -ge 0 ]     then       let "t1 = $column - $row"       let "t2 = $column"     else       let "t1 = $column"       let "t2 = $column + $row"     fi        filter $t1 $t2   # Filter out negative array indices.                      # What happens if you don't do this?   done    echo; echo  done   #  Array rotation inspired by examples (pp. 143-146) in #+ "Advanced C Programming on the IBM PC," by Herbert Mayer #+ (see bibliography). #  This just goes to show that much of what can be done in C #+ can also be done in shell scripting.  }   #--------------- Now, let the show begin. ------------# load_alpha     # Load the array. print_alpha    # Print it out.   rotate         # Rotate it 45 degrees counterclockwise. #-----------------------------------------------------#  exit 0  # This is a rather contrived, not to mention inelegant simulation.  # Exercises: # --------- # 1)  Rewrite the array loading and printing functions #     in a more intuitive and less kludgy fashion. # # 2)  Figure out how the array rotation functions work. #     Hint: think about the implications of backwards-indexing an array. # # 3)  Rewrite this script to handle a non-square array, #     such as a 6 X 4 one. #     Try to minimize "distortion" when the array is rotated.

A two-dimensional array is essentially equivalent to a one-dimensional one, but with additional addressing modes for referencing and manipulating the individual elements by row and column position.

For an even more elaborate example of simulating a two-dimensional array, see Example A-10.

For more interesting scripts using arrays, see:

Example 12-3
Example 16-46
Example A-22
Example A-44
Example A-41
Example A-42

Chapter 28. Indirect References

We have seen that referencing a variable, $var, fetches its value. But, what about the value of a value? What about $$var?

The actual notation is \$$var , usually preceded by an eval (and sometimes an echo). This is called an indirect reference.

Example 28-1. Indirect Variable References

#!/bin/bash # ind-ref.sh: Indirect variable referencing. # Accessing the contents of the contents of a variable.  # First, let's fool around a little.  var=23  echo "\$var   = $var"           # $var   = 23 # So far, everything as expected. But ...  echo "\$\$var  = $$var"         # $$var  = 4570var #  Not useful ... #  \$\$ expanded to PID of the script #  -- refer to the entry on the $$ variable -- #+ and "var" is echoed as plain text. #  (Thank you, Jakob Bohm, for pointing this out.)  echo "\\\$\$var = \$$var"       # \$$var = $23 #  As expected. The first $ is escaped and pasted on to #+ the value of var ($var = 23 ). #  Meaningful, but still not useful.   # Now, let's start over and do it the right way.  # ============================================== #   a=letter_of_alphabet   # Variable "a" holds the name of another variable. letter_of_alphabet=z  echo  # Direct reference. echo "a = $a"          # a = letter_of_alphabet  # Indirect reference.   eval a=\$$a # ^^^        Forcing an eval(uation), and ... #        ^   Escaping the first $ ... # ------------------------------------------------------------------------ # The 'eval' forces an update of $a, sets it to the updated value of \$$a. # So, we see why 'eval' so often shows up in indirect reference notation. # ------------------------------------------------------------------------   echo "Now a = $a"    # Now a = z  echo   # Now, let's try changing the second-order reference.  t=table_cell_3 table_cell_3=24 echo "\"table_cell_3\" = $table_cell_3"            # "table_cell_3" = 24 echo -n "dereferenced \"t\" = "; eval echo \$$t    # dereferenced "t" = 24 # In this simple case, the following also works (why?). #         eval t=\$$t; echo "\"t\" = $t"  echo  t=table_cell_3 NEW_VAL=387 table_cell_3=$NEW_VAL echo "Changing value of \"table_cell_3\" to $NEW_VAL." echo "\"table_cell_3\" now $table_cell_3" echo -n "dereferenced \"t\" now "; eval echo \$$t # "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)   echo  # (Thanks, Stephane Chazelas, for clearing up the above behavior.)   #   A more straightforward method is the ${!t} notation, discussed in the #+ "Bash, version 2" section. #   See also ex78.sh.  exit 0

Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers in C, for instance, in table lookup. And, it also has some other very interesting applications. . . .

Nils Radtke shows how to build "dynamic" variable names and evaluate their contents. This can be useful when sourcing configuration files.

#!/bin/bash   # --------------------------------------------- # This could be "sourced" from a separate file. isdnMyProviderRemoteNet=172.16.0.100 isdnYourProviderRemoteNet=10.0.0.10 isdnOnlineService="MyProvider" # ---------------------------------------------         remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)") remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)") remoteNet=$(eval "echo \$isdnMyProviderRemoteNet") remoteNet=$(eval "echo $isdnMyProviderRemoteNet")  echo "$remoteNet"    # 172.16.0.100  # ================================================================  #  And, it gets even better.  #  Consider the following snippet given a variable named getSparc, #+ but no such variable getIa64:  chkMirrorArchs () {    arch="$1";   if [ "$(eval "echo \${$(echo get$(echo -ne $arch |        sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |        sed 's/^.\(.*\)/\1/g')):-false}")" = true ]   then      return 0;   else      return 1;   fi; }  getSparc="true" unset getIa64 chkMirrorArchs sparc echo $?        # 0                # True  chkMirrorArchs Ia64 echo $?        # 1                # False  # Notes: # ----- # Even the to-be-substituted variable name part is built explicitly. # The parameters to the chkMirrorArchs calls are all lower case. # The variable name is composed of two parts: "get" and "Sparc" . . .

Example 28-2. Passing an indirect reference to awk

#!/bin/bash  #  Another version of the "column totaler" script #+ that adds up a specified column (of numbers) in the target file. #  This one uses indirect references.  ARGS=2 E_WRONGARGS=85  if [ $# -ne "$ARGS" ] # Check for proper number of command-line args. then    echo "Usage: `basename $0` filename column-number"    exit $E_WRONGARGS fi  filename=$1         # Name of file to operate on. column_number=$2    # Which column to total up.  #===== Same as original script, up to this point =====#   # A multi-line awk script is invoked by #   awk " #   ... #   ... #   ... #   "   # Begin awk script. # ------------------------------------------------- awk "  { total += \$${column_number} # Indirect reference } END {      print total      }       " "$filename" # Note that awk doesn't need an eval preceding \$$. # ------------------------------------------------- # End awk script.  #  Indirect variable reference avoids the hassles #+ of referencing a shell variable within the embedded awk script. #  Thanks, Stephane Chazelas.   exit $?

This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example 37-2 and Example A-22) makes indirect referencing more intuitive.

Chapter 29. `/dev` and `/proc`

A Linux or UNIX filesystem typically has the /dev and /proc special-purpose directories.

29.1. `/dev`

The /dev directory contains entries for the physical devices that may or may not be present in the hardware. [118] Appropriately enough, these are called device files. As an example, the hard drive partitions containing the mounted filesystem(s) have entries in /dev, as df shows.

                      bash$                                                                    df                                            Filesystem           1k-blocks      Used Available Use%  Mounted on  /dev/hda6               495876    222748    247527  48% /  /dev/hda1                50755      3887     44248   9% /boot  /dev/hda8               367013     13262    334803   4% /home  /dev/hda5              1714416   1123624    503704  70% /usr

Among other things, the /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. [119] This permits mounting an entire filesystem within a single large file. See Example 17-8 and Example 17-7.

A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive partition), /dev/udp (User Datagram Packet port), and /dev/tcp.

For instance:

To manually mount a USB flash drive, append the following line to /etc/fstab. [120]

/dev/sda1    /mnt/flashdrive    auto    noauto,user,noatime    0 0

(See also Example A-23.)

Checking whether a disk is in the CD-burner (soft-linked to /dev/hdc):

head -1 /dev/hdc   #  head: cannot open '/dev/hdc' for reading: No medium found #  (No disc in the drive.)  #  head: error reading '/dev/hdc': Input/output error #  (There is a disk in the drive, but it can't be read; #+  possibly it's an unrecorded CDR blank.)     #  Stream of characters and assorted gibberish #  (There is a pre-recorded disk in the drive, #+ and this is raw output -- a stream of ASCII and binary data.) #  Here we see the wisdom of using 'head' to limit the output #+ to manageable proportions, rather than 'cat' or something similar.   #  Now, it's just a matter of checking/parsing the output and taking #+ appropriate action.

When executing a command on a /dev/tcp/$host/$port pseudo-device file, Bash opens a TCP connection to the associated socket.

The following examples assume an active Internet connection.

Getting the time from nist.gov:

                    bash$                                                              cat </dev/tcp/time.nist.gov/13                                        53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) *

[Mark contributed this example.]

Generalizing the above into a script:

#!/bin/bash # This script must run with root permissions.  URL="time.nist.gov/13"  Time=$(cat </dev/tcp/"$URL") UTC=$(echo "$Time" | awk '{print$3}')   # Third field is UTC (GMT) time. # Exercise: modify this for different time zones.  echo "UTC Time = "$UTC""

Downloading a URL:

                    bash$                                                              exec 5<>/dev/tcp/www.net.cn/80                                        bash$                                                              echo -e "GET / HTTP/1.0\n" >&5                                        bash$                                                              cat <&5

[Thanks, Mark and Mihai Maties.]

Example 29-1. Using /dev/tcp for troubleshooting

#!/bin/bash # dev-tcp.sh: /dev/tcp redirection to check Internet connection.  # Script by Troy Engel. # Used with permission.   TCP_HOST=news-15.net       # A known spam-friendly ISP. TCP_PORT=80                # Port 80 is http.    # Try to connect. (Somewhat similar to a 'ping' . . .)  echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT} MYEXIT=$?  : <<EXPLANATION If bash was compiled with --enable-net-redirections, it has the capability of using a special character device for both TCP and UDP redirections. These redirections are used identically as STDIN/STDOUT/STDERR. The device entries are 30,36 for /dev/tcp:    mknod /dev/tcp c 30 36  >From the bash reference: /dev/tcp/host/port     If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket. EXPLANATION      if [ "X$MYEXIT" = "X0" ]; then   echo "Connection successful. Exit code: $MYEXIT" else   echo "Connection unsuccessful. Exit code: $MYEXIT" fi  exit $MYEXIT

Example 29-2. Playing music

#!/bin/bash # music.sh  # Music without external files  # Author: Antonio Macchi # Used in ABS Guide with permission.   #  /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte), #+ 1 channel (mono)  duration=2000       # If 8000 bytes = 1 second, then 2000 = 1/4 second. volume=$'\xc0'      # Max volume = \xff (or \x00). mute=$'\x80'        # No volume = \x80 (the middle).  function mknote ()  # $1=Note Hz in bytes (e.g. A = 440Hz :: {                   #+ 8000 fps / 440 = 16 :: A = 16 bytes per second)   for t in `seq 0 $duration`   do     test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute   done }  e=`mknote 49` g=`mknote 41` a=`mknote 36` b=`mknote 32` c=`mknote 30` cis=`mknote 29` d=`mknote 27` e2=`mknote 24` n=`mknote 32767` # European notation.  echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \ $n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp # dsp = Digital Signal Processor  exit      # A "bonny" example of an elegant shell script!

29.2. `/proc`

The /proc directory is actually a pseudo-filesystem. The files in /proc mirror currently running system and kernel processes and contain information and statistics about them.

                      bash$                                                                    cat /proc/devices                                            Character devices:    1 mem    2 pty    3 ttyp    4 ttyS    5 cua    7 vcs   10 misc   14 sound   29 fb   36 netlink  128 ptm  136 pts  162 raw  254 pcmcia   Block devices:    1 ramdisk    2 fd    3 ide0    9 md                      bash$                                                                    cat /proc/interrupts                                                                    CPU0           0:      84505          XT-PIC  timer    1:       3375          XT-PIC  keyboard    2:          0          XT-PIC  cascade    5:          1          XT-PIC  soundblaster    8:          1          XT-PIC  rtc   12:       4231          XT-PIC  PS/2 Mouse   14:     109373          XT-PIC  ide0  NMI:          0   ERR:          0                      bash$                                                                    cat /proc/partitions                                            major minor  #blocks  name     rio rmerge rsect ruse wio wmerge wsect wuse running use aveq      3     0    3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030     3     1      52416 hda1 27 395 844 960 4 2 14 180 0 800 1140     3     2          1 hda2 0 0 0 0 0 0 0 0 0 0 0     3     4     165280 hda4 10 0 20 210 0 0 0 0 0 210 210     ...                      bash$                                                                    cat /proc/loadavg                                            0.13 0.42 0.27 2/44 1119                      bash$                                                                    cat /proc/apm                                            1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?                      bash$                                                                    cat /proc/acpi/battery/BAT0/info                                            present:                 yes  design capacity:         43200 mWh  last full capacity:      36640 mWh  battery technology:      rechargeable  design voltage:          10800 mV  design capacity warning: 1832 mWh  design capacity low:     200 mWh  capacity granularity 1:  1 mWh  capacity granularity 2:  1 mWh  model number:            IBM-02K6897  serial number:            1133  battery type:            LION  OEM info:                Panasonic                      bash$                                                                    fgrep Mem /proc/meminfo                                            MemTotal:       515216 kB  MemFree:        266248 kB

Shell scripts may extract data from certain of the files in /proc. [121]

FS=iso                       # ISO filesystem support in kernel?  grep $FS /proc/filesystems   # iso9660

kernel_version=$( awk '{ print $3 }' /proc/version )

CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )  if [ "$CPU" = "Pentium(R)" ] then   run_some_commands   ... else   run_other_commands   ... fi    cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' ) #  Current operating speed (in MHz) of the cpu on your machine. #  On a laptop this may vary, depending on use of battery #+ or AC power.

#!/bin/bash # get-commandline.sh # Get the command-line parameters of a process.  OPTION=cmdline  # Identify PID. pid=$( echo $(pidof "$1") | awk '{ print $1 }' ) # Get only first            ^^^^^^^^^^^^^^^^^^ of multiple instances.  echo echo "Process ID of (first instance of) "$1" = $pid" echo -n "Command-line arguments: " cat /proc/"$pid"/"$OPTION" | xargs -0 echo #   Formats output:        ^^^^^^^^^^^^^^^ #   (Thanks, Han Holl, for the fixup!)  echo; echo   # For example: # sh get-commandline.sh xterm

devfile="/proc/bus/usb/devices" text="Spd" USB1="Spd=12" USB2="Spd=480"   bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}') #                 ^^^^ Stop after first match.  if [ "$bus_speed" = "$USB1" ] then   echo "USB 1.1 port found."   # Do something appropriate for USB 1.1. fi

It is even possible to control certain peripherals with commands sent to the /proc directory.

                                root#                                                                echo on > /proc/acpi/ibm/light

This turns on the Thinklight in certain models of IBM/Lenovo Thinkpads. (May not work on all Linux distros.)

Of course, caution is advised when writing to /proc.

The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The stat and status files keep running statistics on the process, the cmdline file holds the command-line arguments the process was invoked with, and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint.

Example 29-3. Finding the process associated with a PID

#!/bin/bash # pid-identifier.sh: # Gives complete path name to process associated with pid.  ARGNO=1  # Number of arguments the script expects. E_WRONGARGS=65 E_BADPID=66 E_NOSUCHPROCESS=67 E_NOPERMISSION=68 PROCFILE=exe  if [ $# -ne $ARGNO ] then   echo "Usage: `basename $0` PID-number" >&2  # Error message >stderr.   exit $E_WRONGARGS fi    pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 ) # Checks for pid in "ps" listing, field #1. # Then makes sure it is the actual process, not the process invoked by this script. # The last "grep $1" filters out this possibility. # #    pidno=$( ps ax | awk '{ print $1 }' | grep $1 ) #    also works, as Teemu Huovila, points out.  if [ -z "$pidno" ]  #  If, after all the filtering, the result is a zero-length string, then                #+ no running process corresponds to the pid given.   echo "No such process running."   exit $E_NOSUCHPROCESS fi    # Alternatively: #   if ! ps $1 > /dev/null 2>&1 #   then                # no running process corresponds to the pid given. #     echo "No such process running." #     exit $E_NOSUCHPROCESS #    fi  # To simplify the entire process, use "pidof".   if [ ! -r "/proc/$1/$PROCFILE" ]  # Check for read permission. then   echo "Process $1 running, but..."   echo "Can't get read permission on /proc/$1/$PROCFILE."   exit $E_NOPERMISSION  # Ordinary user can't access some files in /proc. fi    # The last two tests may be replaced by: #    if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but                                       # this will test whether it is possible                                       # to send a signal to the process. #    then echo "PID doesn't exist or you're not its owner" >&2 #    exit $E_BADPID #    fi    exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' ) # Or       exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' ) # #  /proc/pid-number/exe is a symbolic link #+ to the complete path name of the invoking process.  if [ -e "$exe_file" ]  #  If /proc/pid-number/exe exists, then                   #+ then the corresponding process exists.   echo "Process #$1 invoked by $exe_file." else   echo "No such process running." fi     #  This elaborate script can *almost* be replaced by #       ps ax | grep $1 | awk '{ print $5 }' #  However, this will not work... #+ because the fifth field of 'ps' is argv[0] of the process, #+ not the executable file path. # # However, either of the following would work. #       find /proc/$1/exe -printf '%l\n' #       lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'  # Additional commentary by Stephane Chazelas.  exit 0

Example 29-4. On-line connect status

#!/bin/bash # connect-stat.sh #  Note that this script may need modification #+ to work with a wireless connection.  PROCNAME=pppd        # ppp daemon PROCFILENAME=status  # Where to look. NOTCONNECTED=85 INTERVAL=2           # Update every 2 seconds.  pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' )  # Finding the process number of 'pppd', the 'ppp daemon'. # Have to filter out the process lines generated by the search itself. # #  However, as Oleg Philon points out, #+ this could have been considerably simplified by using "pidof". #  pidno=$( pidof $PROCNAME ) # #  Moral of the story: #+ When a command sequence gets too complex, look for a shortcut.   if [ -z "$pidno" ]   # If no pid, then process is not running. then   echo "Not connected." # exit $NOTCONNECTED else   echo "Connected."; echo fi  while [ true ]       # Endless loop, script can be improved here. do    if [ ! -e "/proc/$pidno/$PROCFILENAME" ]   # While process running, then "status" file exists.   then     echo "Disconnected." #   exit $NOTCONNECTED   fi  netstat -s | grep "packets received"  # Get some connect statistics. netstat -s | grep "packets delivered"     sleep $INTERVAL   echo; echo  done  exit 0  # As it stands, this script must be terminated with a Control-C.  #    Exercises: #    --------- #    Improve the script so it exits on a "q" keystroke. #    Make the script more user-friendly in other ways. #    Fix the script to work with wireless/DSL connections.

In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the machine.

Chapter 30. Network Programming

The Net's a cross between an elephant and a white elephant sale: it never forgets, and it's always crap.

--Nemo

A Linux system has quite a number of tools for accessing, manipulating, and troubleshooting network connections. We can incorporate some of these tools into scripts -- scripts that expand our knowledge of networking, useful scripts that can facilitate the administration of a network.

Here is a simple CGI script that demonstrates connecting to a remote server.

Example 30-1. Print the server environment

#!/bin/bash # test-cgi.sh # by Michael Zick # Used with permission  # May have to change the location for your site. # (At the ISP's servers, Bash may not be in the usual place.) # Other places: /usr/bin or /usr/local/bin # Might even try it without any path in sha-bang.  # Disable filename globbing. set -f  # Header tells browser what to expect. echo Content-type: text/plain echo  echo CGI/1.0 test script report: echo  echo environment settings: set echo  echo whereis bash? whereis bash echo   echo who are we? echo ${BASH_VERSINFO[*]} echo  echo argc is $#. argv is "$*". echo  # CGI/1.0 expected environment variables.  echo SERVER_SOFTWARE = $SERVER_SOFTWARE echo SERVER_NAME = $SERVER_NAME echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE echo SERVER_PROTOCOL = $SERVER_PROTOCOL echo SERVER_PORT = $SERVER_PORT echo REQUEST_METHOD = $REQUEST_METHOD echo HTTP_ACCEPT = "$HTTP_ACCEPT" echo PATH_INFO = "$PATH_INFO" echo PATH_TRANSLATED = "$PATH_TRANSLATED" echo SCRIPT_NAME = "$SCRIPT_NAME" echo QUERY_STRING = "$QUERY_STRING" echo REMOTE_HOST = $REMOTE_HOST echo REMOTE_ADDR = $REMOTE_ADDR echo REMOTE_USER = $REMOTE_USER echo AUTH_TYPE = $AUTH_TYPE echo CONTENT_TYPE = $CONTENT_TYPE echo CONTENT_LENGTH = $CONTENT_LENGTH  exit 0  # Here document to give short instructions. :<<-'_test_CGI_'  1) Drop this in your http://domain.name/cgi-bin directory. 2) Then, open http://domain.name/cgi-bin/test-cgi.sh.  _test_CGI_

For security purposes, it may be helpful to identify the IP addresses a computer is accessing.

Example 30-2. IP addresses

#!/bin/bash # ip-addresses.sh # List the IP addresses your computer is connected to.  #  Inspired by Greg Bledsoe's ddos.sh script, #  Linux Journal, 09 March 2011. #    URL: #  http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos #  Greg licensed his script under the GPL2, #+ and as a derivative, this script is likewise GPL2.  connection_type=TCP      # Also try UDP. field=2           # Which field of the output we're interested in. no_match=LISTEN   # Filter out records containing this. Why? lsof_args=-ni     # -i lists Internet-associated files.                   # -n preserves numerical IP addresses. 		  # What happens without the -n option? Try it. router="[0-9][0-9][0-9][0-9][0-9]->" #       Delete the router info.  lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |       awk '{print $9}' | cut -d : -f $field | sort | uniq |       sed s/"^$router"//  #  Bledsoe's script assigns the output of a filtered IP list, #  (similar to lines 19-22, above) to a variable. #  He checks for multiple connections to a single IP address, #  then uses: # #    iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset # #  ... within a 60-second delay loop to bounce packets from DDOS attacks.   #  Exercise: #  -------- #  Use the 'iptables' command to extend this script #+ to reject connection attempts from well-known spammer IP domains.

More examples of network programming:

Getting the time from nist.gov
Downloading a URL
A GRE tunnel
Checking if an Internet server is up
Example 16-41
Example A-28
Example A-29
Example 29-1

See also the networking commands in the System and Administrative Commands chapter and the communications commands in the External Filters, Programs and Commands chapter.

Chapter 31. Of Zeros and Nulls

Faultily faultless, icily regular, splendidly null

Dead perfection; no more.

--Alfred Lord Tennyson

/dev/zero ... /dev/null

Uses of /dev/null

Think of /dev/null as a black hole. It is essentially the equivalent of a write-only file. Everything written to it disappears. Attempts to read or output from it result in nothing. All the same, /dev/null can be quite useful from both the command-line and in scripts.

Suppressing stdout.

cat $filename >/dev/null # Contents of the file will not list to stdout.

Suppressing stderr (from Example 16-3).

rm $badname 2>/dev/null #           So error messages [stderr] deep-sixed.

Suppressing output from both stdout and stderr.

cat $filename 2>/dev/null >/dev/null # If "$filename" does not exist, there will be no error message output. # If "$filename" does exist, the contents of the file will not list to stdout. # Therefore, no output at all will result from the above line of code. # #  This can be useful in situations where the return code from a command #+ needs to be tested, but no output is desired. # # cat $filename &>/dev/null #     also works, as Baris Cicek points out.

Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example 2-1 and Example 2-3):

cat /dev/null > /var/log/messages #  : > /var/log/messages   has same effect, but does not spawn a new process.  cat /dev/null > /var/log/wtmp

Automatically emptying the contents of a logfile (especially good for dealing with those nasty "cookies" sent by commercial Web sites):

Example 31-1. Hiding the cookie jar

# Obsolete Netscape browser. # Same principle applies to newer browsers.  if [ -f ~/.netscape/cookies ]  # Remove, if exists. then   rm -f ~/.netscape/cookies fi  ln -s /dev/null ~/.netscape/cookies # All cookies now get sent to a black hole, rather than saved to disk.

Uses of /dev/zero

Like /dev/null, /dev/zero is a pseudo-device file, but it actually produces a stream of nulls (binary zeros, not the ASCII kind). Output written to /dev/zero disappears, and it is fairly difficult to actually read the nulls emitted there, though it can be done with od or a hex editor. The chief use of /dev/zero is creating an initialized dummy file of predetermined length intended as a temporary swap file.

Example 31-2. Setting up a swapfile using /dev/zero

#!/bin/bash # Creating a swap file.  #  A swap file provides a temporary storage cache #+ which helps speed up certain filesystem operations.  ROOT_UID=0         # Root has $UID 0. E_WRONG_USER=85    # Not root?  FILE=/swap BLOCKSIZE=1024 MINBLOCKS=40 SUCCESS=0   # This script must be run as root. if [ "$UID" -ne "$ROOT_UID" ] then   echo; echo "You must be root to run this script."; echo   exit $E_WRONG_USER fi       blocks=${1:-$MINBLOCKS}          #  Set to default of 40 blocks,                                  #+ if nothing specified on command-line. # This is the equivalent of the command block below. # -------------------------------------------------- # if [ -n "$1" ] # then #   blocks=$1 # else #   blocks=$MINBLOCKS # fi # --------------------------------------------------   if [ "$blocks" -lt $MINBLOCKS ] then   blocks=$MINBLOCKS              # Must be at least 40 blocks long. fi     ###################################################################### echo "Creating swap file of size $blocks blocks (KB)." dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks  # Zero out file. mkswap $FILE $blocks             # Designate it a swap file. swapon $FILE                     # Activate swap file. retcode=$?                       # Everything worked? #  Note that if one or more of these commands fails, #+ then it could cause nasty problems. ######################################################################  #  Exercise: #  Rewrite the above block of code so that if it does not execute #+ successfully, then: #    1) an error message is echoed to stderr, #    2) all temporary files are cleaned up, and #    3) the script exits in an orderly fashion with an #+      appropriate error code.  echo "Swap file created and activated."  exit $retcode

Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see Example 17-8) or "securely" deleting a file (see Example 16-61).

Example 31-3. Creating a ramdisk

#!/bin/bash # ramdisk.sh  #  A "ramdisk" is a segment of system RAM memory #+ which acts as if it were a filesystem. #  Its advantage is very fast access (read/write time). #  Disadvantages: volatility, loss of data on reboot or powerdown, #+                less RAM available to system. # #  Of what use is a ramdisk? #  Keeping a large dataset, such as a table or dictionary on ramdisk, #+ speeds up data lookup, since memory access is much faster than disk access.   E_NON_ROOT_USER=70             # Must run as root. ROOTUSER_NAME=root  MOUNTPT=/mnt/ramdisk           # Create with mkdir /mnt/ramdisk. SIZE=2000                      # 2K blocks (change as appropriate) BLOCKSIZE=1024                 # 1K (1024 byte) block size DEVICE=/dev/ram0               # First ram device  username=`id -nu` if [ "$username" != "$ROOTUSER_NAME" ] then   echo "Must be root to run \"`basename $0`\"."   exit $E_NON_ROOT_USER fi  if [ ! -d "$MOUNTPT" ]         #  Test whether mount point already there, then                           #+ so no error if this script is run   mkdir $MOUNTPT               #+ multiple times. fi  ############################################################################## dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE  # Zero out RAM device.                                                       # Why is this necessary? mke2fs $DEVICE                 # Create an ext2 filesystem on it. mount $DEVICE $MOUNTPT         # Mount it. chmod 777 $MOUNTPT             # Enables ordinary user to access ramdisk.                                # However, must be root to unmount it. ############################################################################## # Need to test whether above commands succeed. Could cause problems otherwise. # Exercise: modify this script to make it safer.  echo "\"$MOUNTPT\" now available for use." # The ramdisk is now accessible for storing files, even by an ordinary user.  #  Caution, the ramdisk is volatile, and its contents will disappear #+ on reboot or power loss. #  Copy anything you want saved to a regular directory.  # After reboot, run this script to again set up ramdisk. # Remounting /mnt/ramdisk without the other steps will not work.  #  Suitably modified, this script can by invoked in /etc/rc.d/rc.local, #+ to set up ramdisk automatically at bootup. #  That may be appropriate on, for example, a database server.  exit 0

In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format) UNIX/Linux binaries.

Chapter 32. Debugging

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.

--Brian Kernighan

The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script.

Example 32-1. A buggy script

#!/bin/bash # ex74.sh  # This is a buggy script. # Where, oh where is the error?  a=37  if [$a -gt 27 ] then   echo $a fi    exit $?   # 0! Why?

Output from script:

                    ./ex74.sh: [37: command not found

What's wrong with the above script? Hint: after the if.

Example 32-2. Missing keyword

#!/bin/bash # missing-keyword.sh # What error message will this script generate? And why?  for a in 1 2 3 do   echo "$a" # done     # Required keyword 'done' commented out in line 8.  exit 0     # Will not exit here!  # === #  # From command line, after script terminates:   echo $?    # 2

Output from script:

                    missing-keyword.sh: line 10: syntax error: unexpected end of file

Note that the error message does not necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error.

Error messages may disregard comment lines in a script when reporting the line number of a syntax error.

What if the script executes, but does not work as expected? This is the all too familiar logic error.

Example 32-3. test24: another buggy script

#!/bin/bash  #  This script is supposed to delete all filenames in current directory #+ containing embedded spaces. #  It doesn't work. #  Why not?   badname=`ls | grep ' '`  # Try this: # echo "$badname"  rm "$badname"  exit 0

Try to find out what's wrong with Example 32-3 by uncommenting the echo "$badname" line. Echo statements are useful for seeing whether what you expect is actually what you get.

In this particular case, rm "$badname" will not give the desired results because $badname should not be quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n' . However, there are simpler ways of going about it.

# Correct methods of deleting filenames containing spaces. rm *\ * rm *" "* rm *' '* # Thank you. S.C.

Summarizing the symptoms of a buggy script,

It bombs with a "syntax error" message, or
It runs, but does not work as expected (logic error).
It runs, works as expected, but has nasty side effects (logic bomb).

Tools for debugging non-working scripts include

Inserting echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.

Even better is an echo that echoes only when debug is on.

### debecho (debug-echo), by Stefano Falsetto ### ### Will echo passed parameters only if DEBUG is set to a value. ### debecho () {   if [ ! -z "$DEBUG" ]; then      echo "$1" >&2      #         ^^^ to stderr   fi }  DEBUG=on Whatever=whatnot debecho $Whatever   # whatnot  DEBUG= Whatever=notwhat debecho $Whatever   # (Will not echo.)

Using the tee filter to check processes or data flows at critical points.
Setting option flags -n -v -x

sh -n scriptname checks for syntax errors without actually running the script. This is the equivalent of inserting set -n or set -o noexec into the script. Note that certain types of syntax errors can slip past this check.

sh -v scriptname echoes each command before executing it. This is the equivalent of inserting set -v or set -o verbose in the script.

The -n and -v flags work well together. sh -nv scriptname gives a verbose syntax check.

sh -x scriptname echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting set -x or set -o xtrace in the script.

Inserting set -u or set -o nounset in the script runs it, but gives an unbound variable error message and aborts the script.
```
set -u   # Or   set -o nounset  # Setting a variable to null will not trigger the error/abort. # unset_var=  echo $unset_var   # Unset (and undeclared) variable.  echo "Should not echo!"  # sh t2.sh # t2.sh: line 6: unset_var: unbound variable
```

Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.)

Example 32-4. Testing a condition with an assert

#!/bin/bash # assert.sh  ####################################################################### assert ()                 #  If condition false, {                         #+ exit from script                           #+ with appropriate error message.   E_PARAM_ERR=98   E_ASSERT_FAILED=99     if [ -z "$2" ]          #  Not enough parameters passed   then                    #+ to assert() function.     return $E_PARAM_ERR   #  No damage done.   fi    lineno=$2    if [ ! $1 ]    then     echo "Assertion failed:  \"$1\""     echo "File \"$0\", line $lineno"    # Give name of file and line number.     exit $E_ASSERT_FAILED   # else   #   return   #   and continue executing the script.   fi   } # Insert a similar assert() function into a script you need to debug.     #######################################################################   a=5 b=4 condition="$a -lt $b"     #  Error message and exit from script.                           #  Try setting "condition" to something else                           #+ and see what happens.  assert "$condition" $LINENO # The remainder of the script executes only if the "assert" does not fail.   # Some commands. # Some more commands . . . echo "This statement echoes only if the \"assert\" does not fail." # . . . # More commands . . .  exit $?

Using the $LINENO variable and the caller builtin.
Trapping at exit.

The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [122] It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the first command in the script.

Trapping signals

trap

Specifies an action on receipt of a signal; also useful for debugging.

A simple instance:

trap '' 2 # Ignore interrupt 2 (Control-C), with no action specified.   trap 'echo "Control-C disabled."' 2 # Message when Control-C pressed.

Example 32-5. Trapping at exit

#!/bin/bash # Hunting variables with a trap.  trap 'echo Variable Listing --- a = $a  b = $b' EXIT #  EXIT is the name of the signal generated upon exit from a script. # #  The command specified by the "trap" doesn't execute until #+ the appropriate signal is sent.  echo "This prints before the \"trap\" --" echo "even though the script sees the \"trap\" first." echo  a=39  b=36  exit 0 #  Note that commenting out the 'exit' command makes no difference, #+ since the script exits in any case after running out of commands.

Example 32-6. Cleaning up after Control-C

#!/bin/bash # logon.sh: A quick 'n dirty script to check whether you are on-line yet.  umask 177  # Make sure temp files are not world readable.   TRUE=1 LOGFILE=/var/log/messages #  Note that $LOGFILE must be readable #+ (as root, chmod 644 /var/log/messages). TEMPFILE=temp.$$ #  Create a "unique" temp file name, using process id of the script. #     Using 'mktemp' is an alternative. #     For example: #     TEMPFILE=`mktemp temp.XXXXXX` KEYWORD=address #  At logon, the line "remote IP address xxx.xxx.xxx.xxx" #                      appended to /var/log/messages. ONLINE=22 USER_INTERRUPT=13 CHECK_LINES=100 #  How many lines in log file to check.  trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT #  Cleans up the temp file if script interrupted by control-c.  echo  while [ $TRUE ]  #Endless loop. do   tail -n $CHECK_LINES $LOGFILE> $TEMPFILE   #  Saves last 100 lines of system log file as temp file.   #  Necessary, since newer kernels generate many log messages at log on.   search=`grep $KEYWORD $TEMPFILE`   #  Checks for presence of the "IP address" phrase,   #+ indicating a successful logon.    if [ ! -z "$search" ] #  Quotes necessary because of possible spaces.   then      echo "On-line"      rm -f $TEMPFILE    #  Clean up temp file.      exit $ONLINE   else      echo -n "."        #  The -n option to echo suppresses newline,                         #+ so you get continuous rows of dots.   fi    sleep 1   done     #  Note: if you change the KEYWORD variable to "Exit", #+ this script can be used while on-line #+ to check for an unexpected logoff.  # Exercise: Change the script, per the above note, #           and prettify it.  exit 0   # Nick Drage suggests an alternate method:  while true   do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0   echo -n "."   # Prints dots (.....) until connected.   sleep 2 done  # Problem: Hitting Control-C to terminate this process may be insufficient. #+         (Dots may keep on echoing.) # Exercise: Fix this.    # Stephane Chazelas has yet another alternative:  CHECK_INTERVAL=1  while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD" do echo -n .    sleep $CHECK_INTERVAL done echo "On-line"  # Exercise: Discuss the relative strengths and weaknesses #           of each of these various approaches.

Example 32-7. A Simple Implementation of a Progress Bar

#! /bin/bash # progress-bar2.sh # Author: Graham Ewart (with reformatting by ABS Guide author). # Used in ABS Guide with permission (thanks!).  # Invoke this script with bash. It doesn't work with sh.  interval=1 long_interval=10  {      trap "exit" SIGUSR1      sleep $interval; sleep $interval      while true      do        echo -n '.'     # Use dots.        sleep $interval      done; } &         # Start a progress bar as a background process.  pid=$! trap "echo !; kill -USR1 $pid; wait $pid"  EXIT        # To handle ^C.  echo -n 'Long-running process ' sleep $long_interval echo ' Finished!'  kill -USR1 $pid wait $pid              # Stop the progress bar. trap EXIT  exit $?

The DEBUG argument to trap causes a specified action to execute after every command in a script. This permits tracing variables, for example.

Example 32-8. Tracing a variable

#!/bin/bash  trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG # Echoes the value of $variable after every command.  variable=29; line=$LINENO  echo "  Just initialized \$variable to $variable in line number $line."  let "variable *= 3"; line=$LINENO echo "  Just multiplied \$variable by 3 in line number $line."  exit 0  #  The "trap 'command1 . . . command2 . . .' DEBUG" construct is #+ more appropriate in the context of a complex script, #+ where inserting multiple "echo $variable" statements might be #+ awkward and time-consuming.  # Thanks, Stephane Chazelas for the pointer.   Output of script:  VARIABLE-TRACE> $variable = "" VARIABLE-TRACE> $variable = "29"   Just initialized $variable to 29. VARIABLE-TRACE> $variable = "29" VARIABLE-TRACE> $variable = "87"   Just multiplied $variable by 3. VARIABLE-TRACE> $variable = "87"

Of course, the trap command has other uses aside from debugging, such as disabling certain keystrokes within a script (see Example A-43).

Example 32-9. Running multiple processes (on an SMP box)

#!/bin/bash # parent.sh # Running multiple processes on an SMP box. # Author: Tedman Eng  #  This is the first of two scripts, #+ both of which must be present in the current working directory.     LIMIT=$1         # Total number of process to start NUMPROC=4        # Number of concurrent threads (forks?) PROCID=1         # Starting Process ID echo "My PID is $$"  function start_thread() {         if [ $PROCID -le $LIMIT ] ; then                 ./child.sh $PROCID&                 let "PROCID++"         else            echo "Limit reached."            wait            exit         fi }  while [ "$NUMPROC" -gt 0 ]; do         start_thread;         let "NUMPROC--" done   while true do  trap "start_thread" SIGRTMIN  done  exit 0    # ======== Second script follows ========   #!/bin/bash # child.sh # Running multiple processes on an SMP box. # This script is called by parent.sh. # Author: Tedman Eng  temp=$RANDOM index=$1 shift let "temp %= 5" let "temp += 4" echo "Starting $index  Time:$temp" "$@" sleep ${temp} echo "Ending $index" kill -s SIGRTMIN $PPID  exit 0   # ======================= SCRIPT AUTHOR'S NOTES ======================= # #  It's not completely bug free. #  I ran it with limit = 500 and after the first few hundred iterations, #+ one of the concurrent threads disappeared! #  Not sure if this is collisions from trap signals or something else. #  Once the trap is received, there's a brief moment while executing the #+ trap handler but before the next trap is set.  During this time, it may #+ be possible to miss a trap signal, thus miss spawning a child process.  #  No doubt someone may spot the bug and will be writing  #+ . . . in the future.    # ===================================================================== #    # ----------------------------------------------------------------------#    ################################################################# # The following is the original script written by Vernia Damiano. # Unfortunately, it doesn't work properly. #################################################################  #!/bin/bash  #  Must call script with at least one integer parameter #+ (number of concurrent processes). #  All other parameters are passed through to the processes started.   INDICE=8        # Total number of process to start TEMPO=5         # Maximum sleep time per process E_BADARGS=65    # No arg(s) passed to script.  if [ $# -eq 0 ] # Check for at least one argument passed to script. then   echo "Usage: `basename $0` number_of_processes [passed params]"   exit $E_BADARGS fi  NUMPROC=$1              # Number of concurrent process shift PARAMETRI=( "$@" )      # Parameters of each process  function avvia() {          local temp          local index          temp=$RANDOM          index=$1          shift          let "temp %= $TEMPO"          let "temp += 1"          echo "Starting $index Time:$temp" "$@"          sleep ${temp}          echo "Ending $index"          kill -s SIGRTMIN $$ }  function parti() {          if [ $INDICE -gt 0 ] ; then               avvia $INDICE "${PARAMETRI[@]}" &                 let "INDICE--"          else                 trap : SIGRTMIN          fi }  trap parti SIGRTMIN  while [ "$NUMPROC" -gt 0 ]; do          parti;          let "NUMPROC--" done  wait trap - SIGRTMIN  exit $?  : <<SCRIPT_AUTHOR_COMMENTS I had the need to run a program, with specified options, on a number of different files, using a SMP machine. So I thought [I'd] keep running a specified number of processes and start a new one each time . . . one of these terminates.  The "wait" instruction does not help, since it waits for a given process or *all* process started in background. So I wrote [this] bash script that can do the job, using the "trap" instruction.   --Vernia Damiano SCRIPT_AUTHOR_COMMENTS

trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the remainder of the script. trap SIGNAL restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a script from an undesirable interrupt.

                    trap '' 2  # Signal 2 is Control-C, now disabled. 	command 	command 	command 	trap 2     # Reenables Control-C

Chapter 33. Options

Options are settings that change shell and/or script behavior.

The set command enables options within a script. At the point in the script where you want the options to take effect, use set -o option-name or, in short form, set -option-abbrev. These two forms are equivalent.

                    #!/bin/bash        set -o verbose       # Echoes all commands before executing.

                    #!/bin/bash        set -v       # Exact same effect as above.

To disable an option within a script, use set +o option-name or set +option-abbrev.

                    #!/bin/bash        set -o verbose       # Command echoing on.       command       ...       command        set +o verbose       # Command echoing off.       command       # Not echoed.         set -v       # Command echoing on.       command       ...       command        set +v       # Command echoing off.       command        exit 0

An alternate method of enabling options in a script is to specify them immediately following the #! script header.

                    #!/bin/bash -x       #       # Body of script follows.

It is also possible to enable script options from the command line. Some options that will not work with set are available this way. Among these are -i , force script to run interactive.

bash -v script-name

bash -o verbose script-name

The following is a listing of some useful options. They may be specified in either abbreviated form (preceded by a single dash) or by complete name (preceded by a double dash or by -o).

Table 33-1. Bash options

Abbreviation	Name	Effect
`-B`	brace expansion	Enable brace expansion (default setting = on)
`+B`	brace expansion	Disable brace expansion
`-C`	noclobber	Prevent overwriting of files by redirection (may be overridden by >\|)
`-D`	(none)	List double-quoted strings prefixed by $, but do not execute commands in script
`-a`	allexport	Export all defined variables
`-b`	notify	Notify when jobs running in background terminate (not of much use in a script)
`-c ...`	(none)	Read commands from ...
`checkjobs`		Informs user of any open jobs upon shell exit. Introduced in version 4 of Bash, and still "experimental." Usage: shopt -s checkjobs (Caution: may hang!)
`-e`	errexit	Abort script at first error, when a command exits with non-zero status (except in until or while loops, if-tests, list constructs)
`-f`	noglob	Filename expansion (globbing) disabled
`globstar`	globbing star-match	Enables the ** globbing operator (version 4+ of Bash). Usage: shopt -s globstar
`-i`	interactive	Script runs in interactive mode
`-n`	noexec	Read commands in script, but do not execute them (syntax check)
`-o Option-Name`	(none)	Invoke the Option-Name option
`-o posix`	POSIX	Change the behavior of Bash, or invoked script, to conform to POSIX standard.
`-o pipefail`	pipe failure	Causes a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
`-p`	privileged	Script runs as "suid" (caution!)
`-r`	restricted	Script runs in restricted mode (see Chapter 22).
`-s`	stdin	Read commands from `stdin`
`-t`	(none)	Exit after first command
`-u`	nounset	Attempt to use undefined variable outputs error message, and forces an exit
`-v`	verbose	Print each command to `stdout` before executing it
`-x`	xtrace	Similar to `-v`, but expands commands
`-`	(none)	End of options flag. All other arguments are positional parameters.
`--`	(none)	Unset positional parameters. If arguments given ( `-- arg1 arg2` ), positional parameters set to arguments.

Chapter 34. Gotchas

Turandot: Gli enigmi sono tre, la morte una!

Caleph: No, no! Gli enigmi sono tre, una la vita!

--Puccini

Here are some (non-recommended!) scripting practices that will bring excitement into an otherwise dull life.

Assigning reserved words or characters to variable names.

case=value0       # Causes problems. 23skidoo=value1   # Also problems. # Variable names starting with a digit are reserved by the shell. # Try _23skidoo=value1. Starting variables with an underscore is okay.  # However . . .   using just an underscore will not work. _=25 echo $_           # $_ is a special variable set to last arg of last command. # But . . .       _ is a valid function name!  xyz((!*=value2    # Causes severe problems. # As of version 3 of Bash, periods are not allowed within variable names.

Using a hyphen or other reserved characters in a variable name (or function name).

var-1=23 # Use 'var_1' instead.  function-whatever ()   # Error # Use 'function_whatever ()' instead.    # As of version 3 of Bash, periods are not allowed within function names. function.whatever ()   # Error # Use 'functionWhatever ()' instead.

Using the same name for a variable and a function. This can make a script difficult to understand.

do_something () {   echo "This function does something with \"$1\"." }  do_something=do_something  do_something do_something  # All this is legal, but highly confusing.

Using whitespace inappropriately. In contrast to other programming languages, Bash can be quite finicky about whitespace.

var1 = 23   # 'var1=23' is correct. # On line above, Bash attempts to execute command "var1" # with the arguments "=" and "23". 	 let c = $a - $b   # Instead:   let c=$a-$b   or   let "c = $a - $b"  if [ $a -le 5]    # if [ $a -le 5 ]   is correct. #           ^^      if [ "$a" -le 5 ]   is even better.                   # [[ $a -le 5 ]] also works.

Not terminating with a semicolon the final command in a code block within curly brackets.

{ ls -l; df; echo "Done." } # bash: syntax error: unexpected end of file  { ls -l; df; echo "Done."; } #                        ^     ### Final command needs semicolon.

Assuming uninitialized variables (variables before a value is assigned to them) are "zeroed out". An uninitialized variable has a value of null, not zero.

#!/bin/bash  echo "uninitialized_var = $uninitialized_var" # uninitialized_var =  # However . . . # if $BASH_VERSION ≥ 4.2; then  if [[ ! -v uninitialized_var ]] then   uninitialized_var=0   # Initialize it to zero! fi

Mixing up = and -eq in a test. Remember, = is for comparing literal variables and -eq for integers.

if [ "$a" = 273 ]      # Is $a an integer or string? if [ "$a" -eq 273 ]    # If $a is an integer.  # Sometimes you can interchange -eq and = without adverse consequences. # However . . .   a=273.0   # Not an integer. 	    if [ "$a" = 273 ] then   echo "Comparison works." else     echo "Comparison does not work." fi    # Comparison does not work.  # Same with   a=" 273"  and a="0273".   # Likewise, problems trying to use "-eq" with non-integer values. 	    if [ "$a" -eq 273.0 ] then   echo "a = $a" fi  # Aborts with an error message.   # test.sh: [: 273.0: integer expression expected

Misusing string comparison operators.

Example 34-1. Numerical and string comparison are not equivalent

#!/bin/bash # bad-op.sh: Trying to use a string comparison on integers.  echo number=1  #  The following while-loop has two errors: #+ one blatant, and the other subtle.  while [ "$number" < 5 ]    # Wrong! Should be:  while [ "$number" -lt 5 ] do   echo -n "$number "   let "number += 1" done   #  Attempt to run this bombs with the error message: #+ bad-op.sh: line 10: 5: No such file or directory #  Within single brackets, "<" must be escaped, #+ and even then, it's still wrong for comparing integers.  echo "---------------------"  while [ "$number" \< 5 ]    #  1 2 3 4 do                          #   echo -n "$number "        #  It *seems* to work, but . . .   let "number += 1"         #+ it actually does an ASCII comparison, done                        #+ rather than a numerical one.  echo; echo "---------------------"  # This can cause problems. For example:  lesser=5 greater=105  if [ "$greater" \< "$lesser" ] then   echo "$greater is less than $lesser" fi                          # 105 is less than 5 #  In fact, "105" actually is less than "5" #+ in a string comparison (ASCII sort order).  echo  exit 0

Attempting to use let to set string variables.
```
let "a = hello, you" echo "$a"   # 0
```
Sometimes variables within "test" brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause unexpected behavior. See Example 7-6, Example 20-5, and Example 9-6.
Quoting a variable containing whitespace prevents splitting. Sometimes this produces unintended consequences.
Commands issued from a script may fail to execute because the script owner lacks execute permission for them. If a user cannot invoke a command from the command-line, then putting it into a script will likewise fail. Try changing the attributes of the command in question, perhaps even setting the suid bit (as root, of course).

Attempting to use - as a redirection operator (which it is not) will usually result in an unpleasant surprise.

command1 2> - | command2 # Trying to redirect error output of command1 into a pipe . . . # . . . will not work.	  command1 2>& - | command2  # Also futile.  Thanks, S.C.

Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines may have version 1.XX of Bash as the default installation.

#!/bin/bash  minimum_version=2 # Since Chet Ramey is constantly adding features to Bash, # you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate. E_BAD_VERSION=80  if [ "$BASH_VERSION" \< "$minimum_version" ] then   echo "This script works only with Bash, version $minimum or greater."   echo "Upgrade strongly recommended."   exit $E_BAD_VERSION fi  ...

Using Bash-specific functionality in a Bourne shell script ( #!/bin/sh ) on a non-Linux machine may cause unexpected behavior. A Linux system usually aliases sh to bash, but this does not necessarily hold true for a generic UNIX machine.
Using undocumented features in Bash turns out to be a dangerous practice. In previous releases of this book there were several scripts that depended on the "feature" that, although the maximum value of an exit or return value was 255, that limit did not apply to negative integers. Unfortunately, in version 2.05b and later, that loophole disappeared. See Example 24-9.
In certain contexts, a misleading exit status may be returned. This may occur when setting a local variable within a function or when assigning an arithmetic value to a variable.

The exit status of an arithmetic expression is not equivalent to an error code.

var=1 && ((--var)) && echo $var #        ^^^^^^^^^ Here the and-list terminates with exit status 1. #                     $var doesn't echo! echo $?   # 1

A script with DOS-type newlines ( \r\n ) will fail to execute, since #!/bin/bash\r\n is not recognized, not the same as the expected #!/bin/bash\n . The fix is to convert the script to UNIX-style newlines.

#!/bin/bash  echo "Here"  unix2dos $0    # Script changes itself to DOS format. chmod 755 $0   # Change back to execute permission.                # The 'unix2dos' command removes execute permission.  ./$0           # Script tries to run itself again.                # But it won't work as a DOS file.  echo "There"  exit 0

A shell script headed by #!/bin/sh will not run in full Bash-compatibility mode. Some Bash-specific functions might be disabled. Scripts that need complete access to all the Bash-specific extensions should start with #!/bin/bash .
Putting whitespace in front of the terminating limit string of a here document will cause unexpected behavior in a script.

Putting more than one echo statement in a function whose output is captured.

add2 () {   echo "Whatever ... "   # Delete this line!   let "retval = $1 + $2"     echo $retval     }      num1=12     num2=43     echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"  #   Sum of 12 and 43 = Whatever ...  #   55  #        The "echoes" concatenate.

This will not work.

A script may not export variables back to its parent process, the shell, or to the environment. Just as we learned in biology, a child process can inherit from a parent, but not vice versa.

WHATEVER=/home/bozo export WHATEVER exit 0

                        bash$                                                echo $WHATEVER                                                bash$

Sure enough, back at the command prompt, $WHATEVER remains unset.

Setting and manipulating variables in a subshell, then attempting to use those same variables outside the scope of the subshell will result an unpleasant surprise.

Example 34-2. Subshell Pitfalls

#!/bin/bash # Pitfalls of variables in a subshell.  outer_variable=outer echo echo "outer_variable = $outer_variable" echo  ( # Begin subshell  echo "outer_variable inside subshell = $outer_variable" inner_variable=inner  # Set echo "inner_variable inside subshell = $inner_variable" outer_variable=inner  # Will value change globally? echo "outer_variable inside subshell = $outer_variable"  # Will 'exporting' make a difference? #    export inner_variable #    export outer_variable # Try it and see.  # End subshell )  echo echo "inner_variable outside subshell = $inner_variable"  # Unset. echo "outer_variable outside subshell = $outer_variable"  # Unchanged. echo  exit 0  # What happens if you uncomment lines 19 and 20? # Does it make a difference?

Piping echo output to a read may produce unexpected results. In this scenario, the read acts as if it were running in a subshell. Instead, use the set command (as in Example 15-18).

Example 34-3. Piping the output of echo to a read

#!/bin/bash #  badread.sh: #  Attempting to use 'echo and 'read' #+ to assign variables non-interactively.  #   shopt -s lastpipe  a=aaa b=bbb c=ccc  echo "one two three" | read a b c # Try to reassign a, b, and c.  echo echo "a = $a"  # a = aaa echo "b = $b"  # b = bbb echo "c = $c"  # c = ccc # Reassignment failed.  ### However . . . ##  Uncommenting line 6: #   shopt -s lastpipe ##+ fixes the problem! ### This is a new feature in Bash, version 4.2.  # ------------------------------  # Try the following alternative.  var=`echo "one two three"` set -- $var a=$1; b=$2; c=$3  echo "-------" echo "a = $a"  # a = one echo "b = $b"  # b = two echo "c = $c"  # c = three  # Reassignment succeeded.  # ------------------------------  #  Note also that an echo to a 'read' works within a subshell. #  However, the value of the variable changes *only* within the subshell.  a=aaa          # Starting all over again. b=bbb c=ccc  echo; echo echo "one two three" | ( read a b c; echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" ) # a = one # b = two # c = three echo "-----------------" echo "Outside subshell: " echo "a = $a"  # a = aaa echo "b = $b"  # b = bbb echo "c = $c"  # c = ccc echo  exit 0

In fact, as Anthony Richardson points out, piping to any loop can cause a similar problem.

# Loop piping troubles. #  This example by Anthony Richardson, #+ with addendum by Wilbert Berendsen.   foundone=false find $HOME -type f -atime +30 -size 100k | while true do    read f    echo "$f is over 100KB and has not been accessed in over 30 days"    echo "Consider moving the file to archives."    foundone=true    # ------------------------------------      echo "Subshell level = $BASH_SUBSHELL"    # Subshell level = 1    # Yes, we're inside a subshell.    # ------------------------------------ done     #  foundone will always be false here since it is #+ set to true inside a subshell if [ $foundone = false ] then    echo "No files need archiving." fi  # =====================Now, here is the correct way:=================  foundone=false for f in $(find $HOME -type f -atime +30 -size 100k)  # No pipe here. do    echo "$f is over 100KB and has not been accessed in over 30 days"    echo "Consider moving the file to archives."    foundone=true done     if [ $foundone = false ] then    echo "No files need archiving." fi  # ==================And here is another alternative==================  #  Places the part of the script that reads the variables #+ within a code block, so they share the same subshell. #  Thank you, W.B.  find $HOME -type f -atime +30 -size 100k | {      foundone=false      while read f      do        echo "$f is over 100KB and has not been accessed in over 30 days"        echo "Consider moving the file to archives."        foundone=true      done       if ! $foundone      then        echo "No files need archiving."      fi }

A lookalike problem occurs when trying to write the stdout of a tail -f piped to grep.

tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log #  The "error.log" file will not have anything written to it. #  As Samuli Kaipiainen points out, this results from grep #+ buffering its output. #  The fix is to add the "--line-buffered" parameter to grep.

Using "suid" commands within scripts is risky, as it may compromise system security. [123]
Using shell scripts for CGI programming may be problematic. Shell script variables are not "typesafe," and this can cause undesirable behavior as far as CGI is concerned. Moreover, it is difficult to "cracker-proof" shell scripts.
Bash does not handle the double slash (//) string correctly.
Bash scripts written for Linux or BSD systems may need fixups to run on a commercial UNIX machine. Such scripts often employ the GNU set of commands and filters, which have greater functionality than their generic UNIX counterparts. This is particularly true of such text processing utilites as tr.
Sadly, updates to Bash itself have broken older scripts that used to work perfectly fine. Let us recall how risky it is to use undocumented Bash features.

Danger is near thee --

Beware, beware, beware, beware.

Many brave hearts are asleep in the deep.

So beware --

Beware.

--A.J. Lamb and H.W. Petrie

Chapter 35. Scripting With Style

Get into the habit of writing shell scripts in a structured and systematic manner. Even on-the-fly and "written on the back of an envelope" scripts will benefit if you take a few minutes to plan and organize your thoughts before sitting down and coding.

Herewith are a few stylistic guidelines. This is not (necessarily) intended as an Official Shell Scripting Stylesheet.

35.1. Unofficial Shell Scripting Stylesheet

Comment your code. This makes it easier for others to understand (and appreciate), and easier for you to maintain.

PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" #  It made perfect sense when you wrote it last year, #+ but now it's a complete mystery. #  (From Antek Sawicki's "pw.sh" script.)

Add descriptive headers to your scripts and functions.

#!/bin/bash  #************************************************# #                   xyz.sh                       # #           written by Bozo Bozeman              # #                July 05, 2001                   # #                                                # #           Clean up project files.              # #************************************************#  E_BADDIR=85                       # No such directory. projectdir=/home/bozo/projects    # Directory to clean up.  # --------------------------------------------------------- # # cleanup_pfiles ()                                         # # Removes all files in designated directory.                # # Parameter: $target_directory                              # # Returns: 0 on success, $E_BADDIR if something went wrong. # # --------------------------------------------------------- # cleanup_pfiles () {   if [ ! -d "$1" ]  # Test if target directory exists.   then     echo "$1 is not a directory."     return $E_BADDIR   fi    rm -f "$1"/*   return 0   # Success. }    cleanup_pfiles $projectdir  exit $?

Avoid using "magic numbers," [124] that is, "hard-wired" literal constants. Use meaningful variable names instead. This makes the script easier to understand and permits making changes and updates without breaking the application.

if [ -f /var/log/messages ] then   ... fi #  A year later, you decide to change the script to check /var/log/syslog. #  It is now necessary to manually change the script, instance by instance, #+ and hope nothing breaks.  # A better way: LOGFILE=/var/log/messages  # Only line that needs to be changed. if [ -f "$LOGFILE" ] then   ... fi

Choose descriptive names for variables and functions.

fl=`ls -al $dirname`                 # Cryptic. file_listing=`ls -al $dirname`       # Better.   MAXVAL=10   # All caps used for a script constant. while [ "$index" -le "$MAXVAL" ] ...   E_NOTFOUND=95                        #  Uppercase for an errorcode,                                      #+ and name prefixed with E_. if [ ! -e "$filename" ] then   echo "File $filename not found."   exit $E_NOTFOUND fi     MAIL_DIRECTORY=/var/spool/mail/bozo  #  Uppercase for an environmental export MAIL_DIRECTORY                #+ variable.   GetAnswer ()                         #  Mixed case works well for a {                                    #+ function name, especially   prompt=$1                          #+ when it improves legibility.   echo -n $prompt   read answer   return $answer }    GetAnswer "What is your favorite number? " favorite_number=$? echo $favorite_number   _uservariable=23                     # Permissible, but not recommended. # It's better for user-defined variables not to start with an underscore. # Leave that for system variables.

Use exit codes in a systematic and meaningful way.
```
E_WRONG_ARGS=95 ... ... exit $E_WRONG_ARGS
```
See also Appendix E.

Ender suggests using the exit codes in /usr/include/sysexits.h in shell scripts, though these are primarily intended for C and C++ programming.

Use standardized parameter flags for script invocation. Ender proposes the following set of flags.

-a      All: Return all information (including hidden file info). -b      Brief: Short version, usually for other scripts. -c      Copy, concatenate, etc. -d      Daily: Use information from the whole day, and not merely         information for a specific instance/user. -e      Extended/Elaborate: (often does not include hidden file info). -h      Help: Verbose usage w/descs, aux info, discussion, help.         See also -V. -l      Log output of script. -m      Manual: Launch man-page for base command. -n      Numbers: Numerical data only. -r      Recursive: All files in a directory (and/or all sub-dirs). -s      Setup & File Maintenance: Config files for this script. -u      Usage: List of invocation flags for the script. -v      Verbose: Human readable output, more or less formatted. -V      Version / License / Copy(right|left) / Contribs (email too).

Chapter 36. Miscellany

Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help.

--Tom Duff

36.1. Interactive and non-interactive shells and scripts

An interactive shell reads commands from user input on a tty. Among other things, such a shell reads startup files on activation, displays a prompt, and enables job control by default. The user can interact with the shell.

A shell running a script is always a non-interactive shell. All the same, the script can still access its tty. It is even possible to emulate an interactive shell in a script.

#!/bin/bash MY_PROMPT='$ ' while : do   echo -n "$MY_PROMPT"   read line   eval "$line"   done  exit 0  # This example script, and much of the above explanation supplied by # St�phane Chazelas (thanks again).

Let us consider an interactive script to be one that requires input from the user, usually with read statements (see Example 15-3). "Real life" is actually a bit messier than that. For now, assume an interactive script is bound to a tty, a script that a user has invoked from the console or an xterm.

Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out for automation by non-interactive scripts.

Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never comes. Handle that difficulty by having an expect script or embedded here document feed input to an interactive script running as a background job. In the simplest case, redirect a file to supply input to a read statement (read variable <file). These particular workarounds make possible general purpose scripts that run in either interactive or non-interactive modes.

If a script needs to test whether it is running in an interactive shell, it is simply a matter of finding whether the prompt variable, $PS1 is set. (If the user is being prompted for input, then the script needs to display a prompt.)

if [ -z $PS1 ] # no prompt? ### if [ -v PS1 ]   # On Bash 4.2+ ... then   # non-interactive   ... else   # interactive   ... fi

Alternatively, the script can test for the presence of option "i" in the $- flag.

case $- in *i*)    # interactive shell ;; *)      # non-interactive shell ;; # (Courtesy of "UNIX F.A.Q.," 1993)

However, John Lange describes an alternative method, using the -t test operator.

# Test for a terminal!  fd=0   # stdin  #  As we recall, the -t test option checks whether the stdin, [ -t 0 ], #+ or stdout, [ -t 1 ], in a given script is running in a terminal. if [ -t "$fd" ] then   echo interactive else   echo non-interactive fi   #  But, as John points out: #    if [ -t 0 ] works ... when you're logged in locally #    but fails when you invoke the command remotely via ssh. #    So for a true test you also have to test for a socket.  if [[ -t "$fd" || -p /dev/stdin ]] then   echo interactive else   echo non-interactive fi

Scripts may be forced to run in interactive mode with the -i option or with a #!/bin/bash -i header. Be aware that this can cause erratic script behavior or show error messages even when no error is present.

36.2. Shell Wrappers

A wrapper is a shell script that embeds a system command or utility, that accepts and passes a set of parameters to that command. [125] Wrapping a script around a complex command-line simplifies invoking it. This is expecially useful with sed and awk.

A sed or awk script would normally be invoked from the command-line by a sed -e 'commands' or awk 'commands' . Embedding such a script in a Bash script permits calling it more simply, and makes it reusable. This also enables combining the functionality of sed and awk, for example piping the output of a set of sed commands to awk. As a saved executable file, you can then repeatedly invoke it in its original form or modified, without the inconvenience of retyping it on the command-line.

Example 36-1. shell wrapper

#!/bin/bash  # This simple script removes blank lines from a file. # No argument checking. # # You might wish to add something like: # # E_NOARGS=85 # if [ -z "$1" ] # then #  echo "Usage: `basename $0` target-file" #  exit $E_NOARGS # fi    sed -e /^$/d "$1" # Same as #    sed -e '/^$/d' filename # invoked from the command-line.  #  The '-e' means an "editing" command follows (optional here). #  '^' indicates the beginning of line, '$' the end. #  This matches lines with nothing between the beginning and the end -- #+ blank lines. #  The 'd' is the delete command.  #  Quoting the command-line arg permits #+ whitespace and special characters in the filename.  #  Note that this script doesn't actually change the target file. #  If you need to do that, redirect its output.  exit

Example 36-2. A slightly more complex shell wrapper

#!/bin/bash  #  subst.sh: a script that substitutes one pattern for #+ another in a file, #+ i.e., "sh subst.sh Smith Jones letter.txt". #                     Jones replaces Smith.  ARGS=3         # Script requires 3 arguments. E_BADARGS=85   # Wrong number of arguments passed to script.  if [ $# -ne "$ARGS" ] then   echo "Usage: `basename $0` old-pattern new-pattern filename"   exit $E_BADARGS fi  old_pattern=$1 new_pattern=$2  if [ -f "$3" ] then     file_name=$3 else     echo "File \"$3\" does not exist."     exit $E_BADARGS fi   # ----------------------------------------------- #  Here is where the heavy work gets done. sed -e "s/$old_pattern/$new_pattern/g" $file_name # -----------------------------------------------  #  's' is, of course, the substitute command in sed, #+ and /pattern/ invokes address matching. #  The 'g,' or global flag causes substitution for EVERY #+ occurence of $old_pattern on each line, not just the first. #  Read the 'sed' docs for an in-depth explanation.  exit $?  # Redirect the output of this script to write to a file.

Example 36-3. A generic shell wrapper that writes to a logfile

#!/bin/bash #  logging-wrapper.sh #  Generic shell wrapper that performs an operation #+ and logs it.  DEFAULT_LOGFILE=logfile.txt  # Set the following two variables. OPERATION= #         Can be a complex chain of commands, #+        for example an awk script or a pipe . . .  LOGFILE= if [ -z "$LOGFILE" ] then     # If not set, default to ...   LOGFILE="$DEFAULT_LOGFILE" fi  #         Command-line arguments, if any, for the operation. OPTIONS="$@"   # Log it. echo "`date` + `whoami` + $OPERATION "$@"" >> $LOGFILE # Now, do it. exec $OPERATION "$@"  # It's necessary to do the logging before the operation. # Why?

Example 36-4. A shell wrapper around an awk script

#!/bin/bash # pr-ascii.sh: Prints a table of ASCII characters.  START=33   # Range of printable ASCII characters (decimal). END=127    # Will not work for unprintable characters (> 127).  echo " Decimal   Hex     Character"   # Header. echo " -------   ---     ---------"  for ((i=START; i<=END; i++)) do   echo $i | awk '{printf("  %3d       %2x         %c\n", $1, $1, $1)}' # The Bash printf builtin will not work in this context: #     printf "%c" "$i" done  exit 0   #  Decimal   Hex     Character #  -------   ---     --------- #    33       21         ! #    34       22         " #    35       23         # #    36       24         $ # #    . . . # #   122       7a         z #   123       7b         { #   124       7c         | #   125       7d         }   #  Redirect the output of this script to a file #+ or pipe it to "more":  sh pr-asc.sh | more

Example 36-5. A shell wrapper around another awk script

#!/bin/bash  # Adds up a specified column (of numbers) in the target file. # Floating-point (decimal) numbers okay, because awk can handle them.  ARGS=2 E_WRONGARGS=85  if [ $# -ne "$ARGS" ] # Check for proper number of command-line args. then    echo "Usage: `basename $0` filename column-number"    exit $E_WRONGARGS fi  filename=$1 column_number=$2  #  Passing shell variables to the awk part of the script is a bit tricky. #  One method is to strong-quote the Bash-script variable #+ within the awk script. #     $'$BASH_SCRIPT_VAR' #      ^                ^ #  This is done in the embedded awk script below. #  See the awk documentation for more details.  # A multi-line awk script is here invoked by #   awk ' #   ... #   ... #   ... #   '   # Begin awk script. # ----------------------------- awk '  { total += $'"${column_number}"' } END {      print total }       ' "$filename" # ----------------------------- # End awk script.   #   It may not be safe to pass shell variables to an embedded awk script, #+  so Stephane Chazelas proposes the following alternative: #   --------------------------------------- #   awk -v column_number="$column_number" ' #   { total += $column_number #   } #   END { #       print total #   }' "$filename" #   ---------------------------------------   exit 0

For those scripts needing a single do-it-all tool, a Swiss army knife, there is Perl. Perl combines the capabilities of sed and awk, and throws in a large subset of C, to boot. It is modular and contains support for everything ranging from object-oriented programming up to and including the kitchen sink. Short Perl scripts lend themselves to embedding within shell scripts, and there may be some substance to the claim that Perl can totally replace shell scripting (though the author of the ABS Guide remains skeptical).

Example 36-6. Perl embedded in a Bash script

#!/bin/bash  # Shell commands may precede the Perl script. echo "This precedes the embedded Perl script within \"$0\"." echo "==============================================================="  perl -e 'print "This line prints from an embedded Perl script.\n";' # Like sed, Perl also uses the "-e" option.  echo "===============================================================" echo "However, the script may also contain shell and system commands."  exit 0

It is even possible to combine a Bash script and Perl script within the same file. Depending on how the script is invoked, either the Bash part or the Perl part will execute.

Example 36-7. Bash and Perl scripts combined

#!/bin/bash # bashandperl.sh  echo "Greetings from the Bash part of the script, $0." # More Bash commands may follow here.  exit # End of Bash part of the script.  # =======================================================  #!/usr/bin/perl # This part of the script must be invoked with #    perl -x bashandperl.sh  print "Greetings from the Perl part of the script, $0.\n"; #      Perl doesn't seem to like "echo" ... # More Perl commands may follow here.  # End of Perl part of the script.

                      bash$                                                                    bash bashandperl.sh                                            Greetings from the Bash part of the script.                      bash$                                                                    perl -x bashandperl.sh                                            Greetings from the Perl part of the script.

It is, of course, possible to embed even more exotic scripting languages within shell wrappers. Python, for example ...

Example 36-8. Python embedded in a Bash script

#!/bin/bash # ex56py.sh  # Shell commands may precede the Python script. echo "This precedes the embedded Python script within \"$0.\"" echo "==============================================================="  python -c 'print "This line prints from an embedded Python script.\n";' # Unlike sed and perl, Python uses the "-c" option. python -c 'k = raw_input( "Hit a key to exit to outer script. " )'  echo "===============================================================" echo "However, the script may also contain shell and system commands."  exit 0

Wrapping a script around mplayer and the Google's translation server, you can create something that talks back to you.

Example 36-9. A script that speaks

#!/bin/bash #   Courtesy of: #   http://elinux.org/RPi_Text_to_Speech_(Speech_Synthesis)  #  You must be on-line for this script to work, #+ so you can access the Google translation server. #  Of course, mplayer must be present on your computer.  speak()   {   local IFS=+   # Invoke mplayer, then connect to Google translation server.   /usr/bin/mplayer -ao alsa -really-quiet -noconsolecontrols \  "http://translate.google.com/translate_tts?tl=en&q="$*""   # Google translates, but can also speak.   }  LINES=4  spk=$(tail -$LINES $0) # Tail end of same script! speak "$spk" exit # Browns. Nice talking to you.

One interesting example of a complex shell wrapper is Martin Matusiak's undvd script, which provides an easy-to-use command-line interface to the complex mencoder utility. Another example is Itzchak Rehberg's Ext3Undel, a set of scripts to recover deleted file on an ext3 filesystem.

36.3. Tests and Comparisons: Alternatives

For tests, the [[ ]] construct may be more appropriate than [ ] . Likewise, arithmetic comparisons might benefit from the (( )) construct.

a=8  # All of the comparisons below are equivalent. test "$a" -lt 16 && echo "yes, $a < 16"         # "and list" /bin/test "$a" -lt 16 && echo "yes, $a < 16"  [ "$a" -lt 16 ] && echo "yes, $a < 16"  [[ $a -lt 16 ]] && echo "yes, $a < 16"          # Quoting variables within (( a < 16 )) && echo "yes, $a < 16"             # [[ ]] and (( )) not necessary.  city="New York" # Again, all of the comparisons below are equivalent. test "$city" \< Paris && echo "Yes, Paris is greater than $city"                                   # Greater ASCII order. /bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city"  [ "$city" \< Paris ] && echo "Yes, Paris is greater than $city"  [[ $city < Paris ]] && echo "Yes, Paris is greater than $city"                                   # Need not quote $city.  # Thank you, S.C.

36.4. Recursion: a script calling itself

Can a script recursively call itself? Indeed.

Example 36-10. A (useless) script that recursively calls itself

#!/bin/bash # recurse.sh  #  Can a script recursively call itself? #  Yes, but is this of any practical use? #  (See the following.)  RANGE=10 MAXVAL=9  i=$RANDOM let "i %= $RANGE"  # Generate a random number between 0 and $RANGE - 1.  if [ "$i" -lt "$MAXVAL" ] then   echo "i = $i"   ./$0             #  Script recursively spawns a new instance of itself. fi                 #  Each child script does the same, until                    #+ a generated $i equals $MAXVAL.  #  Using a "while" loop instead of an "if/then" test causes problems. #  Explain why.  exit 0  # Note: # ---- # This script must have execute permission for it to work properly. # This is the case even if it is invoked by an "sh" command. # Explain why.

Example 36-11. A (useful) script that recursively calls itself

#!/bin/bash # pb.sh: phone book  # Written by Rick Boivie, and used with permission. # Modifications by ABS Guide author.  MINARGS=1     #  Script needs at least one argument. DATAFILE=./phonebook               #  A data file in current working directory               #+ named "phonebook" must exist. PROGNAME=$0 E_NOARGS=70   #  No arguments error.  if [ $# -lt $MINARGS ]; then       echo "Usage: "$PROGNAME" data-to-look-up"       exit $E_NOARGS fi         if [ $# -eq $MINARGS ]; then       grep $1 "$DATAFILE"       # 'grep' prints an error message if $DATAFILE not present. else       ( shift; "$PROGNAME" $* ) | grep $1       # Script recursively calls itself. fi  exit 0        #  Script exits here.               #  Therefore, it's o.k. to put               #+ non-hashmarked comments and data after this point.  # ------------------------------------------------------------------------ Sample "phonebook" datafile:  John Doe        1555 Main St., Baltimore, MD 21228          (410) 222-3333 Mary Moe        9899 Jones Blvd., Warren, NH 03787          (603) 898-3232 Richard Roe     856 E. 7th St., New York, NY 10009          (212) 333-4567 Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678 Zoe Zenobia     4481 N. Baker St., San Francisco, SF 94338  (415) 501-1631 # ------------------------------------------------------------------------  $bash pb.sh Roe Richard Roe     856 E. 7th St., New York, NY 10009          (212) 333-4567 Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678  $bash pb.sh Roe Sam Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678  #  When more than one argument is passed to this script, #+ it prints *only* the line(s) containing all the arguments.

Example 36-12. Another (useful) script that recursively calls itself

#!/bin/bash # usrmnt.sh, written by Anthony Richardson # Used in ABS Guide with permission. # usage: usrmnt.sh # description: mount device, invoking user must be listed in the # MNTUSERS group in the /etc/sudoers file. # ---------------------------------------------------------- # This is a usermount script that reruns itself using sudo. # A user with the proper permissions only has to type # usermount /dev/fd0 /mnt/floppy # instead of # sudo usermount /dev/fd0 /mnt/floppy # I use this same technique for all of my #+ sudo scripts, because I find it convenient. # ---------------------------------------------------------- # If SUDO_COMMAND variable is not set we are not being run through #+ sudo, so rerun ourselves. Pass the user's real and group id . . . if [ -z "$SUDO_COMMAND" ] then mntusr=$(id -u) grpusr=$(id -g) sudo $0 $* exit 0 fi # We will only get here if we are being run by sudo. /bin/mount $* -o uid=$mntusr,gid=$grpusr exit 0 # Additional notes (from the author of this script): # ------------------------------------------------- # 1) Linux allows the "users" option in the /etc/fstab # file so that any user can mount removable media. # But, on a server, I like to allow only a few # individuals access to removable media. # I find using sudo gives me more control. # 2) I also find sudo to be more convenient than # accomplishing this task through groups. # 3) This method gives anyone with proper permissions # root access to the mount command, so be careful # about who you allow access. # You can get finer control over which access can be mounted # by using this same technique in separate mntfloppy, mntcdrom, # and mntsamba scripts.

Too many levels of recursion can exhaust the script's stack space, causing a segfault.

36.5. "Colorizing" Scripts

The ANSI [126] escape sequences set screen attributes, such as bold text, and color of foreground and background. DOS batch files commonly used ANSI escape codes for color output, and so can Bash scripts.

Example 36-13. A "colorized" address database

#!/bin/bash # ex30a.sh: "Colorized" version of ex30.sh. #            Crude address database   clear                                   # Clear the screen.  echo -n "          " echo -e '\E[37;44m'"\033[1mContact List\033[0m"                                         # White on blue background echo; echo echo -e "\033[1mChoose one of the following persons:\033[0m"                                         # Bold tput sgr0                               # Reset attributes. echo "(Enter only the first letter of name.)" echo echo -en '\E[47;34m'"\033[1mE\033[0m"   # Blue tput sgr0                               # Reset colors to "normal." echo "vans, Roland"                     # "[E]vans, Roland" echo -en '\E[47;35m'"\033[1mJ\033[0m"   # Magenta tput sgr0 echo "ambalaya, Mildred" echo -en '\E[47;32m'"\033[1mS\033[0m"   # Green tput sgr0 echo "mith, Julie" echo -en '\E[47;31m'"\033[1mZ\033[0m"   # Red tput sgr0 echo "ane, Morris" echo  read person  case "$person" in # Note variable is quoted.    "E" | "e" )   # Accept upper or lowercase input.   echo   echo "Roland Evans"   echo "4321 Flash Dr."   echo "Hardscrabble, CO 80753"   echo "(303) 734-9874"   echo "(303) 734-9892 fax"   echo "revans@zzy.net"   echo "Business partner & old friend"   ;;    "J" | "j" )   echo   echo "Mildred Jambalaya"   echo "249 E. 7th St., Apt. 19"   echo "New York, NY 10009"   echo "(212) 533-2814"   echo "(212) 533-9972 fax"   echo "milliej@loisaida.com"   echo "Girlfriend"   echo "Birthday: Feb. 11"   ;;  # Add info for Smith & Zane later.            * )    # Default option.	      # Empty input (hitting RETURN) fits here, too.    echo    echo "Not yet in database."   ;;  esac  tput sgr0                               # Reset colors to "normal."  echo  exit 0

Example 36-14. Drawing a box

#!/bin/bash # Draw-box.sh: Drawing a box using ASCII characters.  # Script by Stefano Palmeri, with minor editing by document author. # Minor edits suggested by Jim Angstadt. # Used in the ABS Guide with permission.   ###################################################################### ###  draw_box function doc  ###  #  The "draw_box" function lets the user #+ draw a box in a terminal.        # #  Usage: draw_box ROW COLUMN HEIGHT WIDTH [COLOR]  #  ROW and COLUMN represent the position         #+ of the upper left angle of the box you're going to draw. #  ROW and COLUMN must be greater than 0 #+ and less than current terminal dimension. #  HEIGHT is the number of rows of the box, and must be > 0.  #  HEIGHT + ROW must be <= than current terminal height.  #  WIDTH is the number of columns of the box and must be > 0. #  WIDTH + COLUMN must be <= than current terminal width. # # E.g.: If your terminal dimension is 20x80, #  draw_box 2 3 10 45 is good #  draw_box 2 3 19 45 has bad HEIGHT value (19+2 > 20) #  draw_box 2 3 18 78 has bad WIDTH value (78+3 > 80) # #  COLOR is the color of the box frame. #  This is the 5th argument and is optional. #  0=black 1=red 2=green 3=tan 4=blue 5=purple 6=cyan 7=white. #  If you pass the function bad arguments, #+ it will just exit with code 65, #+ and no messages will be printed on stderr. # #  Clear the terminal before you start to draw a box. #  The clear command is not contained within the function. #  This allows the user to draw multiple boxes, even overlapping ones.  ###  end of draw_box function doc  ###  ######################################################################  draw_box(){  #=============# HORZ="-" VERT="|" CORNER_CHAR="+"  MINARGS=4 E_BADARGS=65 #=============#   if [ $# -lt "$MINARGS" ]; then          # If args are less than 4, exit.     exit $E_BADARGS fi  # Looking for non digit chars in arguments. # Probably it could be done better (exercise for the reader?). if echo $@ | tr -d [:blank:] | tr -d [:digit:] | grep . &> /dev/null; then    exit $E_BADARGS fi  BOX_HEIGHT=`expr $3 - 1`   #  -1 correction needed because angle char "+" BOX_WIDTH=`expr $4 - 1`    #+ is a part of both box height and width. T_ROWS=`tput lines`        #  Define current terminal dimension  T_COLS=`tput cols`         #+ in rows and columns.           if [ $1 -lt 1 ] || [ $1 -gt $T_ROWS ]; then    #  Start checking if arguments    exit $E_BADARGS                             #+ are correct. fi if [ $2 -lt 1 ] || [ $2 -gt $T_COLS ]; then    exit $E_BADARGS fi if [ `expr $1 + $BOX_HEIGHT + 1` -gt $T_ROWS ]; then    exit $E_BADARGS fi if [ `expr $2 + $BOX_WIDTH + 1` -gt $T_COLS ]; then    exit $E_BADARGS fi if [ $3 -lt 1 ] || [ $4 -lt 1 ]; then    exit $E_BADARGS fi                                 # End checking arguments.  plot_char(){                       # Function within a function.    echo -e "\E[${1};${2}H"$3 }  echo -ne "\E[3${5}m"               # Set box frame color, if defined.  # start drawing the box  count=1                                         #  Draw vertical lines using for (( r=$1; count<=$BOX_HEIGHT; r++)); do      #+ plot_char function.   plot_char $r $2 $VERT   let count=count+1 done   count=1 c=`expr $2 + $BOX_WIDTH` for (( r=$1; count<=$BOX_HEIGHT; r++)); do   plot_char $r $c $VERT   let count=count+1 done   count=1                                        #  Draw horizontal lines using for (( c=$2; count<=$BOX_WIDTH; c++)); do      #+ plot_char function.   plot_char $1 $c $HORZ   let count=count+1 done   count=1 r=`expr $1 + $BOX_HEIGHT` for (( c=$2; count<=$BOX_WIDTH; c++)); do   plot_char $r $c $HORZ   let count=count+1 done   plot_char $1 $2 $CORNER_CHAR                   # Draw box angles. plot_char $1 `expr $2 + $BOX_WIDTH` $CORNER_CHAR plot_char `expr $1 + $BOX_HEIGHT` $2 $CORNER_CHAR plot_char `expr $1 + $BOX_HEIGHT` `expr $2 + $BOX_WIDTH` $CORNER_CHAR  echo -ne "\E[0m"             #  Restore old colors.  P_ROWS=`expr $T_ROWS - 1`    #  Put the prompt at bottom of the terminal.  echo -e "\E[${P_ROWS};1H" }         # Now, let's try drawing a box. clear                       # Clear the terminal. R=2      # Row C=3      # Column H=10     # Height W=45     # Width  col=1    # Color (red) draw_box $R $C $H $W $col   # Draw the box.  exit 0  # Exercise: # -------- # Add the option of printing text within the drawn box.

The simplest, and perhaps most useful ANSI escape sequence is bold text, \033[1m ... \033[0m. The \033 represents an escape, the "[1" turns on the bold attribute, while the "[0" switches it off. The "m" terminates each term of the escape sequence.

                      bash$                                                                    echo -e "\033[1mThis is bold text.\033[0m"

A similar escape sequence switches on the underline attribute (on an rxvt and an aterm).

                      bash$                                                                    echo -e "\033[4mThis is underlined text.\033[0m"

With an echo, the -e option enables the escape sequences.

Other escape sequences change the text and/or background color.

                      bash$                                                                    echo -e '\E[34;47mThis prints in blue.'; tput sgr0                                            bash$                                                                    echo -e '\E[33;44m'"yellow text on blue background"; tput sgr0                                            bash$                                                                    echo -e '\E[1;33;44m'"BOLD yellow text on blue background"; tput sgr0

It's usually advisable to set the bold attribute for light-colored foreground text.

The tput sgr0 restores the terminal settings to normal. Omitting this lets all subsequent output from that particular terminal remain blue.

Since tput sgr0 fails to restore terminal settings under certain circumstances, echo -ne \E[0m may be a better choice.

The numbers in the following table work for an rxvt terminal. Results may vary for other terminal emulators.

Table 36-1. Numbers representing colors in Escape Sequences

Color	Foreground	Background
`black`	30	40
`red`	31	41
`green`	32	42
`yellow`	33	43
`blue`	34	44
`magenta`	35	45
`cyan`	36	46
`white`	37	47

Example 36-15. Echoing colored text

#!/bin/bash # color-echo.sh: Echoing text messages in color.  # Modify this script for your own purposes. # It's easier than hand-coding color.  black='\E[30;47m' red='\E[31;47m' green='\E[32;47m' yellow='\E[33;47m' blue='\E[34;47m' magenta='\E[35;47m' cyan='\E[36;47m' white='\E[37;47m'   alias Reset="tput sgr0"      #  Reset text attributes to normal                              #+ without clearing screen.   cecho ()                     # Color-echo.                              # Argument $1 = message                              # Argument $2 = color { local default_msg="No message passed."                              # Doesn't really need to be a local variable.  message=${1:-$default_msg}   # Defaults to default message. color=${2:-$black}           # Defaults to black, if not specified.    echo -e "$color"   echo "$message"   Reset                      # Reset to normal.    return }     # Now, let's try it out. # ---------------------------------------------------- cecho "Feeling blue..." $blue cecho "Magenta looks more like purple." $magenta cecho "Green with envy." $green cecho "Seeing red?" $red cecho "Cyan, more familiarly known as aqua." $cyan cecho "No color passed (defaults to black)."        # Missing $color argument. cecho "\"Empty\" color passed (defaults to black)." ""        # Empty $color argument. cecho        # Missing $message and $color arguments. cecho "" ""        # Empty $message and $color arguments. # ----------------------------------------------------  echo  exit 0  # Exercises: # --------- # 1) Add the "bold" attribute to the 'cecho ()' function. # 2) Add options for colored backgrounds.

Example 36-16. A "horserace" game

#!/bin/bash # horserace.sh: Very simple horserace simulation. # Author: Stefano Palmeri # Used with permission.  ################################################################ #  Goals of the script: #  playing with escape sequences and terminal colors. # #  Exercise: #  Edit the script to make it run less randomly, #+ set up a fake betting shop . . .      #  Um . . . um . . . it's starting to remind me of a movie . . . # #  The script gives each horse a random handicap. #  The odds are calculated upon horse handicap #+ and are expressed in European(?) style. #  E.g., odds=3.75 means that if you bet $1 and win, #+ you receive $3.75. #  #  The script has been tested with a GNU/Linux OS, #+ using xterm and rxvt, and konsole. #  On a machine with an AMD 900 MHz processor, #+ the average race time is 75 seconds.     #  On faster computers the race time would be lower. #  So, if you want more suspense, reset the USLEEP_ARG variable. # #  Script by Stefano Palmeri. ################################################################  E_RUNERR=65  # Check if md5sum and bc are installed.  if ! which bc &> /dev/null; then    echo bc is not installed.      echo "Can\'t run . . . "    exit $E_RUNERR fi if ! which md5sum &> /dev/null; then    echo md5sum is not installed.      echo "Can\'t run . . . "    exit $E_RUNERR fi  #  Set the following variable to slow down script execution. #  It will be passed as the argument for usleep (man usleep)   #+ and is expressed in microseconds (500000 = half a second). USLEEP_ARG=0    #  Clean up the temp directory, restore terminal cursor and  #+ terminal colors -- if script interrupted by Ctl-C. trap 'echo -en "\E[?25h"; echo -en "\E[0m"; stty echo;\ tput cup 20 0; rm -fr  $HORSE_RACE_TMP_DIR'  TERM EXIT #  See the chapter on debugging for an explanation of 'trap.'  # Set a unique (paranoid) name for the temp directory the script needs. HORSE_RACE_TMP_DIR=$HOME/.horserace-`date +%s`-`head -c10 /dev/urandom \ | md5sum | head -c30`  # Create the temp directory and move right in. mkdir $HORSE_RACE_TMP_DIR cd $HORSE_RACE_TMP_DIR   #  This function moves the cursor to line $1 column $2 and then prints $3. #  E.g.: "move_and_echo 5 10 linux" is equivalent to #+ "tput cup 4 9; echo linux", but with one command instead of two. #  Note: "tput cup" defines 0 0 the upper left angle of the terminal, #+ echo defines 1 1 the upper left angle of the terminal. move_and_echo() {           echo -ne "\E[${1};${2}H""$3"  }  # Function to generate a pseudo-random number between 1 and 9.  random_1_9 () {     head -c10 /dev/urandom | md5sum | tr -d [a-z] | tr -d 0 | cut -c1  }  #  Two functions that simulate "movement," when drawing the horses.  draw_horse_one() {                echo -n " "//$MOVE_HORSE// } draw_horse_two(){               echo -n " "\\\\$MOVE_HORSE\\\\  }      # Define current terminal dimension. N_COLS=`tput cols` N_LINES=`tput lines`  # Need at least a 20-LINES X 80-COLUMNS terminal. Check it. if [ $N_COLS -lt 80 ] || [ $N_LINES -lt 20 ]; then    echo "`basename $0` needs a 80-cols X 20-lines terminal."    echo "Your terminal is ${N_COLS}-cols X ${N_LINES}-lines."    exit $E_RUNERR fi   # Start drawing the race field.  # Need a string of 80 chars. See below. BLANK80=`seq -s "" 100 | head -c80`  clear  # Set foreground and background colors to white. echo -ne '\E[37;47m'  # Move the cursor on the upper left angle of the terminal. tput cup 0 0   # Draw six white lines. for n in `seq 5`; do       echo $BLANK80   # Use the 80 chars string to colorize the terminal. done  # Sets foreground color to black.  echo -ne '\E[30m'  move_and_echo 3 1 "START  1"             move_and_echo 3 75 FINISH move_and_echo 1 5 "|" move_and_echo 1 80 "|" move_and_echo 2 5 "|" move_and_echo 2 80 "|" move_and_echo 4 5 "|  2" move_and_echo 4 80 "|" move_and_echo 5 5 "V  3" move_and_echo 5 80 "V"  # Set foreground color to red.  echo -ne '\E[31m'  # Some ASCII art. move_and_echo 1 8 "..@@@..@@@@@...@@@@@.@...@..@@@@..." move_and_echo 2 8 ".@...@...@.......@...@...@.@......." move_and_echo 3 8 ".@@@@@...@.......@...@@@@@.@@@@...." move_and_echo 4 8 ".@...@...@.......@...@...@.@......." move_and_echo 5 8 ".@...@...@.......@...@...@..@@@@..." move_and_echo 1 43 "@@@@...@@@...@@@@..@@@@..@@@@." move_and_echo 2 43 "@...@.@...@.@.....@.....@....." move_and_echo 3 43 "@@@@..@@@@@.@.....@@@@...@@@.." move_and_echo 4 43 "@..@..@...@.@.....@.........@." move_and_echo 5 43 "@...@.@...@..@@@@..@@@@.@@@@.."   # Set foreground and background colors to green. echo -ne '\E[32;42m'  # Draw  eleven green lines. tput cup 5 0 for n in `seq 11`; do       echo $BLANK80 done  # Set foreground color to black.  echo -ne '\E[30m' tput cup 5 0  # Draw the fences.  echo "++++++++++++++++++++++++++++++++++++++\ ++++++++++++++++++++++++++++++++++++++++++"  tput cup 15 0 echo "++++++++++++++++++++++++++++++++++++++\ ++++++++++++++++++++++++++++++++++++++++++"  # Set foreground and background colors to white. echo -ne '\E[37;47m'  # Draw three white lines. for n in `seq 3`; do       echo $BLANK80 done  # Set foreground color to black. echo -ne '\E[30m'  # Create 9 files to stores handicaps. for n in `seq 10 7 68`; do       touch $n done    # Set the first type of "horse" the script will draw. HORSE_TYPE=2  #  Create position-file and odds-file for every "horse". #+ In these files, store the current position of the horse, #+ the type and the odds. for HN in `seq 9`; do       touch horse_${HN}_position       touch odds_${HN}       echo \-1 > horse_${HN}_position       echo $HORSE_TYPE >>  horse_${HN}_position       # Define a random handicap for horse.        HANDICAP=`random_1_9`       # Check if the random_1_9 function returned a good value.       while ! echo $HANDICAP | grep [1-9] &> /dev/null; do                 HANDICAP=`random_1_9`       done       # Define last handicap position for horse.        LHP=`expr $HANDICAP \* 7 + 3`       for FILE in `seq 10 7 $LHP`; do             echo $HN >> $FILE       done                # Calculate odds.       case $HANDICAP in                1) ODDS=`echo $HANDICAP \* 0.25 + 1.25 | bc`                                  echo $ODDS > odds_${HN}               ;;               2 | 3) ODDS=`echo $HANDICAP \* 0.40 + 1.25 | bc`                                        echo $ODDS > odds_${HN}               ;;               4 | 5 | 6) ODDS=`echo $HANDICAP \* 0.55 + 1.25 | bc`                                              echo $ODDS > odds_${HN}               ;;                7 | 8) ODDS=`echo $HANDICAP \* 0.75 + 1.25 | bc`                                        echo $ODDS > odds_${HN}               ;;                9) ODDS=`echo $HANDICAP \* 0.90 + 1.25 | bc`                                   echo $ODDS > odds_${HN}       esac   done   # Print odds. print_odds() { tput cup 6 0 echo -ne '\E[30;42m' for HN in `seq 9`; do       echo "#$HN odds->" `cat odds_${HN}` done }  # Draw the horses at starting line. draw_horses() { tput cup 6 0 echo -ne '\E[30;42m' for HN in `seq 9`; do       echo /\\$HN/\\"                               " done }  print_odds  echo -ne '\E[47m' # Wait for a enter key press to start the race. # The escape sequence '\E[?25l' disables the cursor. tput cup 17 0 echo -e '\E[?25l'Press [enter] key to start the race... read -s  #  Disable normal echoing in the terminal. #  This avoids key presses that might "contaminate" the screen #+ during the race.   stty -echo  # -------------------------------------------------------- # Start the race.  draw_horses echo -ne '\E[37;47m' move_and_echo 18 1 $BLANK80 echo -ne '\E[30m' move_and_echo 18 1 Starting... sleep 1  # Set the column of the finish line. WINNING_POS=74  # Define the time the race started. START_TIME=`date +%s`  # COL variable needed by following "while" construct. COL=0      while [ $COL -lt $WINNING_POS ]; do                               MOVE_HORSE=0                           # Check if the random_1_9 function has returned a good value.           while ! echo $MOVE_HORSE | grep [1-9] &> /dev/null; do                 MOVE_HORSE=`random_1_9`           done                      # Define old type and position of the "randomized horse".           HORSE_TYPE=`cat  horse_${MOVE_HORSE}_position | tail -n 1`           COL=$(expr `cat  horse_${MOVE_HORSE}_position | head -n 1`)                      ADD_POS=1           # Check if the current position is an handicap position.            if seq 10 7 68 | grep -w $COL &> /dev/null; then                 if grep -w $MOVE_HORSE $COL &> /dev/null; then                       ADD_POS=0                       grep -v -w  $MOVE_HORSE $COL > ${COL}_new                       rm -f $COL                       mv -f ${COL}_new $COL                       else ADD_POS=1                 fi            else ADD_POS=1           fi           COL=`expr $COL + $ADD_POS`           echo $COL >  horse_${MOVE_HORSE}_position  # Store new position.                                       # Choose the type of horse to draw.                    case $HORSE_TYPE in                  1) HORSE_TYPE=2; DRAW_HORSE=draw_horse_two                 ;;                 2) HORSE_TYPE=1; DRAW_HORSE=draw_horse_one            esac                  echo $HORSE_TYPE >>  horse_${MOVE_HORSE}_position           # Store current type.                     # Set foreground color to black and background to green.           echo -ne '\E[30;42m'                      # Move the cursor to new horse position.           tput cup `expr $MOVE_HORSE + 5` \ 	  `cat  horse_${MOVE_HORSE}_position | head -n 1`                       # Draw the horse.           $DRAW_HORSE            usleep $USLEEP_ARG                       # When all horses have gone beyond field line 15, reprint odds.            touch fieldline15            if [ $COL = 15 ]; then              echo $MOVE_HORSE >> fieldline15              fi            if [ `wc -l fieldline15 | cut -f1 -d " "` = 9 ]; then                print_odds                : > fieldline15            fi                                 # Define the leading horse.           HIGHEST_POS=`cat *position | sort -n | tail -1`                                # Set background color to white.           echo -ne '\E[47m'           tput cup 17 0           echo -n Current leader: `grep -w $HIGHEST_POS *position | cut -c7`\ 	  "                              "  done    # Define the time the race finished. FINISH_TIME=`date +%s`  # Set background color to green and enable blinking text. echo -ne '\E[30;42m' echo -en '\E[5m'  # Make the winning horse blink. tput cup `expr $MOVE_HORSE + 5` \ `cat  horse_${MOVE_HORSE}_position | head -n 1` $DRAW_HORSE  # Disable blinking text. echo -en '\E[25m'  # Set foreground and background color to white. echo -ne '\E[37;47m' move_and_echo 18 1 $BLANK80  # Set foreground color to black. echo -ne '\E[30m'  # Make winner blink. tput cup 17 0 echo -e "\E[5mWINNER: $MOVE_HORSE\E[25m""  Odds: `cat odds_${MOVE_HORSE}`"\ "  Race time: `expr $FINISH_TIME - $START_TIME` secs"  # Restore cursor and old colors. echo -en "\E[?25h" echo -en "\E[0m"  # Restore echoing. stty echo  # Remove race temp directory. rm -rf $HORSE_RACE_TMP_DIR  tput cup 19 0  exit 0

See also Example A-21, Example A-44, Example A-52, and Example A-40.

There is, however, a major problem with all this. ANSI escape sequences are emphatically non-portable. What works fine on some terminal emulators (or the console) may work differently, or not at all, on others. A "colorized" script that looks stunning on the script author's machine may produce unreadable output on someone else's. This somewhat compromises the usefulness of colorizing scripts, and possibly relegates this technique to the status of a gimmick. Colorized scripts are probably inappropriate in a commercial setting, i.e., your supervisor might disapprove.

Alister's ansi-color utility (based on Moshe Jacobson's color utility considerably simplifies using ANSI escape sequences. It substitutes a clean and logical syntax for the clumsy constructs just discussed.

Henry/teikedvl has likewise created a utility (http://scriptechocolor.sourceforge.net/) to simplify creation of colorized scripts.

36.6. Optimizations

Most shell scripts are quick 'n dirty solutions to non-complex problems. As such, optimizing them for speed is not much of an issue. Consider the case, though, where a script carries out an important task, does it well, but runs too slowly. Rewriting it in a compiled language may not be a palatable option. The simplest fix would be to rewrite the parts of the script that slow it down. Is it possible to apply principles of code optimization even to a lowly shell script?

Check the loops in the script. Time consumed by repetitive operations adds up quickly. If at all possible, remove time-consuming operations from within loops.

Use builtin commands in preference to system commands. Builtins execute faster and usually do not launch a subshell when invoked.

Avoid unnecessary commands, particularly in a pipe.

cat "$file" | grep "$word"  grep "$word" "$file"  #  The above command-lines have an identical effect, #+ but the second runs faster since it launches one fewer subprocess.

The cat command seems especially prone to overuse in scripts.

Certain operators, notably expr, are very inefficient and might be replaced by double parentheses arithmetic expansion. See Example A-59.

Math tests  math via $(( )) real          0m0.294s user          0m0.288s sys           0m0.008s  math via expr: real          1m17.879s   # Much slower! user          0m3.600s sys           0m8.765s  math via let: real          0m0.364s user          0m0.372s sys           0m0.000s

Condition testing constructs in scripts deserve close scrutiny. Substitute case for if-then constructs and combine tests when possible, to minimize script execution time. Again, refer to Example A-59.

Test using "case" construct: real          0m0.329s user          0m0.320s sys           0m0.000s   Test with if [], no quotes: real          0m0.438s user          0m0.432s sys           0m0.008s   Test with if [], quotes: real          0m0.476s user          0m0.452s sys           0m0.024s   Test with if [], using -eq: real          0m0.457s user          0m0.456s sys           0m0.000s

Erik Brandsberg recommends using associative arrays in preference to conventional numeric-indexed arrays in most cases. When overwriting values in a numeric array, there is a significant performance penalty vs. associative arrays. Running a test script confirms this. See Example A-60.

Assignment tests  Assigning a simple variable real          0m0.418s user          0m0.416s sys           0m0.004s  Assigning a numeric index array entry real          0m0.582s user          0m0.564s sys           0m0.016s  Overwriting a numeric index array entry real          0m21.931s user          0m21.913s sys           0m0.016s  Linear reading of numeric index array real          0m0.422s user          0m0.416s sys           0m0.004s  Assigning an associative array entry real          0m1.800s user          0m1.796s sys           0m0.004s  Overwriting an associative array entry real          0m1.798s user          0m1.784s sys           0m0.012s  Linear reading an associative array entry real          0m0.420s user          0m0.420s sys           0m0.000s  Assigning a random number to a simple variable real          0m0.402s user          0m0.388s sys           0m0.016s  Assigning a sparse numeric index array entry randomly into 64k cells real          0m12.678s user          0m12.649s sys           0m0.028s  Reading sparse numeric index array entry real          0m0.087s user          0m0.084s sys           0m0.000s  Assigning a sparse associative array entry randomly into 64k cells real          0m0.698s user          0m0.696s sys           0m0.004s  Reading sparse associative index array entry real          0m0.083s user          0m0.084s sys           0m0.000s

Use the time and times tools to profile computation-intensive commands. Consider rewriting time-critical code sections in C, or even in assembler.

Try to minimize file I/O. Bash is not particularly efficient at handling files, so consider using more appropriate tools for this within the script, such as awk or Perl.

Write your scripts in a modular and coherent form, [127] so they can be reorganized and tightened up as necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but others, such as loop unrolling, are mostly irrelevant. Above all, use common sense.

For an excellent demonstration of how optimization can dramatically reduce the execution time of a script, see Example 16-47.

36.7. Assorted Tips

36.7.1. Ideas for more powerful scripts

You have a problem that you want to solve by writing a Bash script. Unfortunately, you don't know quite where to start. One method is to plunge right in and code those parts of the script that come easily, and write the hard parts as pseudo-code.

#!/bin/bash  ARGCOUNT=1                     # Need name as argument. E_WRONGARGS=65  if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ] #    ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ #  Can't figure out how to code this . . . #+ . . . so write it in pseudo-code.  then   echo "Usage: name-of-script name"   #            ^^^^^^^^^^^^^^     More pseudo-code.   exit $E_WRONGARGS fi   . . .  exit 0   # Later on, substitute working code for the pseudo-code.  # Line 6 becomes: if [ $# -ne "$ARGCOUNT" ]  # Line 12 becomes:   echo "Usage: `basename $0` name"

For an example of using pseudo-code, see the Square Root exercise.

To keep a record of which user scripts have run during a particular session or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times.

# Append (>>) following to end of each script tracked.  whoami>> $SAVE_FILE    # User invoking the script. echo $0>> $SAVE_FILE   # Script name. date>> $SAVE_FILE      # Date and time. echo>> $SAVE_FILE      # Blank line as separator.  #  Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc #+ (something like ~/.scripts-run)

The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning?
```
file=data.txt title="***This is the title line of data text file***"  echo $title | cat - $file >$file.new # "cat -" concatenates stdout to $file. #  End result is #+ to write a new file with $title appended at *beginning*.
```
This is a simplified variant of the Example 19-13 script given earlier. And, of course, sed can also do this.
A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or even a Makefile. It can be invoked as an external shell command in a C program using the system() call, i.e., system("script_name"); .
Setting a variable to the contents of an embedded sed or awk script increases the readability of the surrounding shell wrapper. See Example A-1 and Example 15-20.

Put together files containing your favorite and most useful definitions and functions. As necessary, "include" one or more of these "library files" in scripts with either the dot (.) or source command.

# SCRIPT LIBRARY # ------ -------  # Note: # No "#!" here. # No "live code" either.   # Useful variable definitions  ROOT_UID=0             # Root has $UID 0. E_NOTROOT=101          # Not root user error.  MAXRETVAL=255          # Maximum (positive) return value of a function. SUCCESS=0 FAILURE=-1    # Functions  Usage ()               # "Usage:" message. {   if [ -z "$1" ]       # No arg passed.   then     msg=filename   else     msg=$@   fi    echo "Usage: `basename $0` "$msg"" }     Check_if_root ()       # Check if root running script. {                      # From "ex39.sh" example.   if [ "$UID" -ne "$ROOT_UID" ]   then     echo "Must be root to run this script."     exit $E_NOTROOT   fi }     CreateTempfileName ()  # Creates a "unique" temp filename. {                      # From "ex51.sh" example.   prefix=temp   suffix=`eval date +%s`   Tempfilename=$prefix.$suffix }   isalpha2 ()            # Tests whether *entire string* is alphabetic. {                      # From "isalpha.sh" example.   [ $# -eq 1 ] || return $FAILURE    case $1 in   *[!a-zA-Z]*|"") return $FAILURE;;   *) return $SUCCESS;;   esac                 # Thanks, S.C. }   abs ()                           # Absolute value. {                                # Caution: Max return value = 255.   E_ARGERR=-999999    if [ -z "$1" ]                 # Need arg passed.   then     return $E_ARGERR             # Obvious error value returned.   fi    if [ "$1" -ge 0 ]              # If non-negative,   then                           #     absval=$1                    # stays as-is.   else                           # Otherwise,     let "absval = (( 0 - $1 ))"  # change sign.   fi      return $absval }   tolower ()             #  Converts string(s) passed as argument(s) {                      #+ to lowercase.    if [ -z "$1" ]       #  If no argument(s) passed,   then                 #+ send error message     echo "(null)"      #+ (C-style void-pointer error message)     return             #+ and return from function.   fi      echo "$@" | tr A-Z a-z   # Translate all passed arguments ($@).    return  # Use command substitution to set a variable to function output. # For example: #    oldvar="A seT of miXed-caSe LEtTerS" #    newvar=`tolower "$oldvar"` #    echo "$newvar"    # a set of mixed-case letters # # Exercise: Rewrite this function to change lowercase passed argument(s) #           to uppercase ... toupper()  [easy]. }

Use special-purpose comment headers to increase clarity and legibility in scripts.

## Caution. rm -rf *.zzy   ##  The "-rf" options to "rm" are very dangerous,                ##+ especially with wild cards.  #+ Line continuation. #  This is line 1 #+ of a multi-line comment, #+ and this is the final line.  #* Note.  #o List item.  #> Another point of view. while [ "$var1" != "end" ]    #> while test "$var1" != "end"

Dotan Barak contributes template code for a progress bar in a script.

Example 36-17. A Progress Bar

#!/bin/bash # progress-bar.sh  # Author: Dotan Barak (very minor revisions by ABS Guide author). # Used in ABS Guide with permission (thanks!).   BAR_WIDTH=50 BAR_CHAR_START="[" BAR_CHAR_END="]" BAR_CHAR_EMPTY="." BAR_CHAR_FULL="=" BRACKET_CHARS=2 LIMIT=100  print_progress_bar() {         # Calculate how many characters will be full.         let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"          # Calculate how many characters will be empty.         let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"          # Prepare the bar.         bar_line="${BAR_CHAR_START}"         for ((j=0; j<full_limit; j++)); do                 bar_line="${bar_line}${BAR_CHAR_FULL}"         done          for ((j=0; j<empty_limit; j++)); do                 bar_line="${bar_line}${BAR_CHAR_EMPTY}"         done          bar_line="${bar_line}${BAR_CHAR_END}"          printf "%3d%% %s" $2 ${bar_line} }  # Here is a sample of code that uses it. MAX_PERCENT=100 for ((i=0; i<=MAX_PERCENT; i++)); do         #         usleep 10000         # ... Or run some other commands ...         #         print_progress_bar ${BAR_WIDTH} ${i}         echo -en "\r" done  echo ""  exit

A particularly clever use of if-test constructs is for comment blocks.

#!/bin/bash  COMMENT_BLOCK= #  Try setting the above variable to some value #+ for an unpleasant surprise.  if [ $COMMENT_BLOCK ]; then  Comment block -- ================================= This is a comment line. This is another comment line. This is yet another comment line. =================================  echo "This will not echo."  Comment blocks are error-free! Whee!  fi  echo "No more comments, please."  exit 0

Compare this with using here documents to comment out code blocks.

Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be treated as an integer.

#!/bin/bash  SUCCESS=0 E_BADINPUT=85  test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null # An integer is either equal to 0 or not equal to 0. # 2>/dev/null suppresses error message.  if [ $? -ne "$SUCCESS" ] then   echo "Usage: `basename $0` integer-input"   exit $E_BADINPUT fi  let "sum = $1 + 25"             # Would give error if $1 not integer. echo "Sum = $sum"  # Any variable, not just a command-line parameter, can be tested this way.  exit 0

The 0 - 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to stdout (usually with echo) the "return value," and assign this to a variable. This is actually a variant of command substitution.

Example 36-18. Return value trickery

#!/bin/bash # multiplication.sh  multiply ()                     # Multiplies params passed. {                               # Will accept a variable number of args.    local product=1    until [ -z "$1" ]             # Until uses up arguments passed...   do     let "product *= $1"     shift   done    echo $product                 #  Will not echo to stdout, }                               #+ since this will be assigned to a variable.  mult1=15383; mult2=25211 val1=`multiply $mult1 $mult2` # Assigns stdout (echo) of function to the variable val1. echo "$mult1 X $mult2 = $val1"                   # 387820813  mult1=25; mult2=5; mult3=20 val2=`multiply $mult1 $mult2 $mult3` echo "$mult1 X $mult2 X $mult3 = $val2"          # 2500  mult1=188; mult2=37; mult3=25; mult4=47 val3=`multiply $mult1 $mult2 $mult3 $mult4` echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300  exit 0

The same technique also works for alphanumeric strings. This means that a function can "return" a non-numeric value.

capitalize_ichar ()          #  Capitalizes initial character {                            #+ of argument string(s) passed.    string0="$@"               # Accepts multiple arguments.    firstchar=${string0:0:1}   # First character.   string1=${string0:1}       # Rest of string(s).    FirstChar=`echo "$firstchar" | tr a-z A-Z`                              # Capitalize first character.    echo "$FirstChar$string1"  # Output to stdout.  }    newstring=`capitalize_ichar "every sentence should start with a capital letter."` echo "$newstring"          # Every sentence should start with a capital letter.

It is even possible for a function to "return" multiple values with this method.

Example 36-19. Even more return value trickery

#!/bin/bash # sum-product.sh # A function may "return" more than one value.  sum_and_product ()   # Calculates both sum and product of passed args. {   echo $(( $1 + $2 )) $(( $1 * $2 )) # Echoes to stdout each calculated value, separated by space. }  echo echo "Enter first number " read first  echo echo "Enter second number " read second echo  retval=`sum_and_product $first $second`      # Assigns output of function. sum=`echo "$retval" | awk '{print $1}'`      # Assigns first field. product=`echo "$retval" | awk '{print $2}'`  # Assigns second field.  echo "$first + $second = $sum" echo "$first * $second = $product" echo  exit 0

There can be only one echo statement in the function for this to work. If you alter the previous example:

sum_and_product () {   echo "This is the sum_and_product function." # This messes things up!   echo $(( $1 + $2 )) $(( $1 * $2 )) } ... retval=`sum_and_product $first $second`      # Assigns output of function. # Now, this will not work correctly.

Next in our bag of tricks are techniques for passing an array to a function, then "returning" an array back to the main body of the script.

Passing an array involves loading the space-separated elements of the array into a variable with command substitution. Getting an array back as the "return value" from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the ( ... ) operator to assign it to an array.

Example 36-20. Passing and returning arrays

#!/bin/bash # array-function.sh: Passing an array to a function and ... #                   "returning" an array from a function   Pass_Array () {   local passed_array   # Local variable!   passed_array=( `echo "$1"` )   echo "${passed_array[@]}"   #  List all the elements of the new array   #+ declared and set within the function. }   original_array=( element1 element2 element3 element4 element5 )  echo echo "original_array = ${original_array[@]}" #                      List all elements of original array.   # This is the trick that permits passing an array to a function. # ********************************** argument=`echo ${original_array[@]}` # ********************************** #  Pack a variable #+ with all the space-separated elements of the original array. # # Attempting to just pass the array itself will not work.   # This is the trick that allows grabbing an array as a "return value". # ***************************************** returned_array=( `Pass_Array "$argument"` ) # ***************************************** # Assign 'echoed' output of function to array variable.  echo "returned_array = ${returned_array[@]}"  echo "============================================================="  #  Now, try it again, #+ attempting to access (list) the array from outside the function. Pass_Array "$argument"  # The function itself lists the array, but ... #+ accessing the array from outside the function is forbidden. echo "Passed array (within function) = ${passed_array[@]}" # NULL VALUE since the array is a variable local to the function.  echo  ############################################  # And here is an even more explicit example:  ret_array () {   for element in {11..20}   do     echo "$element "   #  Echo individual elements   done                 #+ of what will be assembled into an array. }  arr=( $(ret_array) )   #  Assemble into array.  echo "Capturing array \"arr\" from function ret_array () ..." echo "Third element of array \"arr\" is ${arr[2]}."   # 13  (zero-indexed) echo -n "Entire array is: " echo ${arr[@]}                # 11 12 13 14 15 16 17 18 19 20  echo  exit 0  #  Nathan Coulter points out that passing arrays with elements containing #+ whitespace breaks this example.

For a more elaborate example of passing arrays to functions, see Example A-10.

Using the double-parentheses construct, it is possible to use C-style syntax for setting and incrementing/decrementing variables and in for and while loops. See Example 11-13 and Example 11-18.
Setting the path and umask at the beginning of a script makes it more portable -- more likely to run on a "foreign" machine whose user may have bollixed up the $PATH and umask.
```
#!/bin/bash PATH=/bin:/usr/bin:/usr/local/bin ; export PATH umask 022   # Files that the script creates will have 755 permission.  # Thanks to Ian D. Allen, for this tip.
```

A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep.

# From "wstrings.sh" example.  wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \ tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

Example 36-21. Fun with anagrams

#!/bin/bash # agram.sh: Playing games with anagrams.  # Find anagrams of... LETTERSET=etaoinshrdlu FILTER='.......'       # How many letters minimum? #       1234567  anagram "$LETTERSET" | # Find all anagrams of the letterset... grep "$FILTER" |       # With at least 7 letters, grep '^is' |           # starting with 'is' grep -v 's$' |         # no plurals grep -v 'ed$'          # no past tense verbs # Possible to add many combinations of conditions and filters.  #  Uses "anagram" utility #+ that is part of the author's "yawl" word list package. #  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz #  http://bash.deta.in/yawl-0.3.2.tar.gz  exit 0                 # End of code.   bash$ sh agram.sh islander isolate isolead isotheral    #  Exercises: #  --------- #  Modify this script to take the LETTERSET as a command-line parameter. #  Parameterize the filters in lines 11 - 13 (as with $FILTER), #+ so that they can be specified by passing arguments to a function.  #  For a slightly different approach to anagramming, #+ see the agram2.sh script.

See also Example 29-4, Example 16-25, and Example A-9.

Use "anonymous here documents" to comment out blocks of code, to save having to individually comment out each line with a #. See Example 19-11.

Running a script on a machine that relies on a command that might not be installed is dangerous. Use whatis to avoid potential problems with this.

CMD=command1                 # First choice. PlanB=command2               # Fallback option.  command_test=$(whatis "$CMD" | grep 'nothing appropriate') #  If 'command1' not found on system , 'whatis' will return #+ "command1: nothing appropriate." # #  A safer alternative is: #     command_test=$(whereis "$CMD" | grep \/) #  But then the sense of the following test would have to be reversed, #+ since the $command_test variable holds content only if #+ the $CMD exists on the system. #     (Thanks, bojster.)   if [[ -z "$command_test" ]]  # Check whether command present. then   $CMD option1 option2       #  Run command1 with options. else                         #  Otherwise,   $PlanB                     #+ run command2.  fi

An if-grep test may not return expected results in an error case, when text is output to stderr, rather that stdout.

if ls -l nonexistent_filename | grep -q 'No such file or directory'   then echo "File \"nonexistent_filename\" does not exist." fi

Redirecting stderr to stdout fixes this.

if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory' #                             ^^^^   then echo "File \"nonexistent_filename\" does not exist." fi  # Thanks, Chris Martin, for pointing this out.

If you absolutely must access a subshell variable outside the subshell, here's a way to do it.

TMPFILE=tmpfile                  # Create a temp file to store the variable.  (   # Inside the subshell ... inner_variable=Inner echo $inner_variable echo $inner_variable >>$TMPFILE  # Append to temp file. )      # Outside the subshell ...  echo; echo "-----"; echo echo $inner_variable             # Null, as expected. echo "-----"; echo  # Now ... read inner_variable <$TMPFILE    # Read back shell variable. rm -f "$TMPFILE"                 # Get rid of temp file. echo "$inner_variable"           # It's an ugly kludge, but it works.

The run-parts command is handy for running a set of command scripts in a particular sequence, especially in combination with cron or at.
For doing multiple revisions on a complex script, use the rcs Revision Control System package.

Among other benefits of this is automatically updated ID header tags. The co command in rcs does a parameter replacement of certain reserved key words, for example, replacing # $Id$ in a script with something like:
```
# $Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $
```

36.7.2. Widgets

It would be nice to be able to invoke X-Windows widgets from a shell script. There happen to exist several packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to be maintained. Fortunately, it is still possible to obtain widtools here.

The widtools (widget tools) package requires the XForms library to be installed. Additionally, the Makefile needs some judicious editing before the package will build on a typical Linux system. Finally, three of the six widgets offered do not work (and, in fact, segfault).

The dialog family of tools offers a method of calling "dialog" widgets from a shell script. The original dialog utility works in a text console, but its successors, gdialog, Xdialog, and kdialog use X-Windows-based widget sets.

Example 36-22. Widgets invoked from a shell script

#!/bin/bash # dialog.sh: Using 'gdialog' widgets.  # Must have 'gdialog' installed on your system to run this script. # Or, you can replace all instance of 'gdialog' below with 'kdialog' ... # Version 1.1 (corrected 04/05/05)  # This script was inspired by the following article. #     "Scripting for X Productivity," by Marco Fioretti, #      LINUX JOURNAL, Issue 113, September 2003, pp. 86-9. # Thank you, all you good people at LJ.   # Input error in dialog box. E_INPUT=85 # Dimensions of display, input widgets. HEIGHT=50 WIDTH=60  # Output file name (constructed out of script name). OUTFILE=$0.output  # Display this script in a text widget. gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH    # Now, we'll try saving input in a file. echo -n "VARIABLE=" > $OUTFILE gdialog --title "User Input" --inputbox "Enter variable, please:" \ $HEIGHT $WIDTH 2>> $OUTFILE   if [ "$?" -eq 0 ] # It's good practice to check exit status. then   echo "Executed \"dialog box\" without errors." else   echo "Error(s) in \"dialog box\" execution."         # Or, clicked on "Cancel", instead of "OK" button.   rm $OUTFILE   exit $E_INPUT fi    # Now, we'll retrieve and display the saved variable. . $OUTFILE   # 'Source' the saved file. echo "The variable input in the \"input box\" was: "$VARIABLE""   rm $OUTFILE  # Clean up by removing the temp file.              # Some applications may need to retain this file.  exit $?  # Exercise: Rewrite this script using the 'zenity' widget set.

The xmessage command is a simple method of popping up a message/query window. For example:

xmessage Fatal error in script! -button exit

The latest entry in the widget sweepstakes is zenity. This utility pops up GTK+ dialog widgets-and-windows, and it works very nicely within a script.

get_info () {   zenity --entry       #  Pops up query window . . .                        #+ and prints user entry to stdout.                         #  Also try the --calendar and --scale options. }  answer=$( get_info )   #  Capture stdout in $answer variable.  echo "User entered: "$answer""

For other methods of scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl (Perl with Gtk extensions), or PyQt (Python with Qt extensions).

36.8. Security Issues

36.8.1. Infected Shell Scripts

A brief warning about script security is indicated. A shell script may contain a worm, trojan, or even a virus. For that reason, never run as root a script (or permit it to be inserted into the system startup scripts in /etc/rc.d) unless you have obtained said script from a trusted source or you have carefully analyzed it to make certain it does nothing harmful.

Various researchers at Bell Labs and other sites, including M. Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the implications of shell script viruses. They conclude that it is all too easy for even a novice, a "script kiddie," to write one. [128]

Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your system from being compromised by a rogue script.

36.8.2. Hiding Shell Script Source

For security purposes, it may be necessary to render a script unreadable. If only there were a utility to create a stripped binary executable from a script. Francisco Rosales' shc -- generic shell script compiler does exactly that.

Unfortunately, according to an article in the October, 2005 Linux Journal, the binary can, in at least some cases, be decrypted to recover the original script source. Still, this could be a useful method of keeping scripts secure from all but the most skilled hackers.

36.8.3. Writing Secure Shell Scripts

Dan Stromberg suggests the following guidelines for writing (relatively) secure shell scripts.

Don't put secret data in environment variables.
Don't pass secret data in an external command's arguments (pass them in via a pipe or redirection instead).
Set your $PATH carefully. Don't just trust whatever path you inherit from the caller if your script is running as root. In fact, whenever you use an environment variable inherited from the caller, think about what could happen if the caller put something misleading in the variable, e.g., if the caller set $HOME to /etc.

36.9. Portability Issues

It is easier to port a shell than a shell script.

--Larry Wall

This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of sh and ksh will find much of value here.

As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX 1003.2 standard. Invoking Bash with the --posix option or inserting a set -o posix at the head of a script causes Bash to conform very closely to this standard. Another alternative is to use a #!/bin/sh sha-bang header in the script, rather than #!/bin/bash. [129] Note that /bin/sh is a link to /bin/bash in Linux and certain other flavors of UNIX, and a script invoked this way disables extended Bash functionality.

Most Bash scripts will run as-is under ksh, and vice-versa, since Chet Ramey has been busily porting ksh features to the latest versions of Bash.

On a commercial UNIX machine, scripts using GNU-specific features of standard commands may not work. This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their proprietary counterparts even on "big-iron" UNIX. Caldera's release of the source to many of the original UNIX utilities has accelerated the trend.

Bash has certain features that the traditional Bourne shell lacks. Among these are:

Certain extended invocation options
Command substitution using $( ) notation
Brace expansion
Certain array operations, and associative arrays
The double brackets extended test construct
The double-parentheses arithmetic-evaluation construct
Certain string manipulation operations
Process substitution
A Regular Expression matching operator
Bash-specific builtins
Coprocesses

See the Bash F.A.Q. for a complete listing.

36.9.1. A Test Suite

Let us illustrate some of the incompatibilities between Bash and the classic Bourne shell. Download and install the "Heirloom Bourne Shell" and run the following script, first using Bash, then the classic sh.

Example 36-23. Test Suite

#!/bin/bash # test-suite.sh # A partial Bash compatibility test suite. # Run this on your version of Bash, or some other shell.  default_option=FAIL         # Tests below will fail unless . . .  echo echo -n "Testing " sleep 1; echo -n ". " sleep 1; echo -n ". " sleep 1; echo ". " echo  # Double brackets String="Double brackets supported?" echo -n "Double brackets test: " if [[ "$String" = "Double brackets supported?" ]] then   echo "PASS" else   echo "FAIL" fi   # Double brackets and regex matching String="Regex matching supported?" echo -n "Regex matching: " if [[ "$String" =~ R.....matching* ]] then   echo "PASS" else   echo "FAIL" fi   # Arrays test_arr=$default_option     # FAIL Array=( If supports arrays will print PASS ) test_arr=${Array[5]} echo "Array test: $test_arr"   # Command Substitution csub_test () {   echo "PASS" }  test_csub=$default_option    # FAIL test_csub=$(csub_test) echo "Command substitution test: $test_csub"  echo  #  Completing this script is an exercise for the reader. #  Add to the above similar tests for double parentheses, #+ brace expansion, process substitution, etc.  exit $?

36.10. Shell Scripting Under Windows

Even users running that other OS can run UNIX-like shell scripts, and therefore benefit from many of the lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates add shell scripting capabilities to Windows.

Another alternative is UWIN, written by David Korn of AT&T, of Korn Shell fame.

In 2006, Microsoft released the Windows Powershell®, which contains limited Bash-like command-line scripting capabilities.

Chapter 37. Bash, versions 2, 3, and 4

37.1. Bash, version 2

The current version of Bash, the one you have running on your machine, is most likely version 2.xx.yy, 3.xx.yy, or 4.xx.yy.

                      bash$                                                                    echo $BASH_VERSION                                            3.2.25(1)-release

The version 2 update of the classic Bash scripting language added array variables, string and parameter expansion, and a better method of indirect variable references, among other features.

Example 37-1. String expansion

#!/bin/bash  # String expansion. # Introduced with version 2 of Bash.  #  Strings of the form $'xxx' #+ have the standard escaped characters interpreted.   echo $'Ringing bell 3 times \a \a \a'      # May only ring once with certain terminals.      # Or ...      # May not ring at all, depending on terminal settings. echo $'Three form feeds \f \f \f' echo $'10 newlines \n\n\n\n\n\n\n\n\n\n' echo $'\102\141\163\150'      #   B   a   s   h      # Octal equivalent of characters.  exit

Example 37-2. Indirect variable references - the new way

#!/bin/bash  # Indirect variable referencing. # This has a few of the attributes of references in C++.   a=letter_of_alphabet letter_of_alphabet=z  echo "a = $a"           # Direct reference.  echo "Now a = ${!a}"    # Indirect reference. #  The ${!variable} notation is more intuitive than the old #+ eval var1=\$$var2  echo  t=table_cell_3 table_cell_3=24 echo "t = ${!t}"                      # t = 24 table_cell_3=387 echo "Value of t changed to ${!t}"    # 387 # No 'eval' necessary.  #  This is useful for referencing members of an array or table, #+ or for simulating a multi-dimensional array. #  An indexing option (analogous to pointer arithmetic) #+ would have been nice. Sigh.  exit 0  # See also, ind-ref.sh example.

Example 37-3. Simple database application, using indirect variable referencing

#!/bin/bash # resistor-inventory.sh # Simple database / table-lookup application.  # ============================================================== # # Data  B1723_value=470                                   # Ohms B1723_powerdissip=.25                             # Watts B1723_colorcode="yellow-violet-brown"             # Color bands B1723_loc=173                                     # Where they are B1723_inventory=78                                # How many  B1724_value=1000 B1724_powerdissip=.25 B1724_colorcode="brown-black-red" B1724_loc=24N B1724_inventory=243  B1725_value=10000 B1725_powerdissip=.125 B1725_colorcode="brown-black-orange" B1725_loc=24N B1725_inventory=89  # ============================================================== #   echo  PS3='Enter catalog number: '  echo  select catalog_number in "B1723" "B1724" "B1725" do   Inv=${catalog_number}_inventory   Val=${catalog_number}_value   Pdissip=${catalog_number}_powerdissip   Loc=${catalog_number}_loc   Ccode=${catalog_number}_colorcode    echo   echo "Catalog number $catalog_number:"   # Now, retrieve value, using indirect referencing.   echo "There are ${!Inv} of  [${!Val} ohm / ${!Pdissip} watt]\   resistors in stock."  #        ^             ^   # As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).   echo "These are located in bin # ${!Loc}."   echo "Their color code is \"${!Ccode}\"."    break done  echo; echo  # Exercises: # --------- # 1) Rewrite this script to read its data from an external file. # 2) Rewrite this script to use arrays, #+   rather than indirect variable referencing. #    Which method is more straightforward and intuitive? #    Which method is easier to code?   # Notes: # ----- #  Shell scripts are inappropriate for anything except the most simple #+ database applications, and even then it involves workarounds and kludges. #  Much better is to use a language with native support for data structures, #+ such as C++ or Java (or even Perl).  exit 0

Example 37-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards

#!/bin/bash # cards.sh  # Deals four random hands from a deck of cards.  UNPICKED=0 PICKED=1  DUPE_CARD=99  LOWER_LIMIT=0 UPPER_LIMIT=51 CARDS_IN_SUIT=13 CARDS=52  declare -a Deck declare -a Suits declare -a Cards #  It would have been easier to implement and more intuitive #+ with a single, 3-dimensional array. #  Perhaps a future version of Bash will support multidimensional arrays.   initialize_Deck () { i=$LOWER_LIMIT until [ "$i" -gt $UPPER_LIMIT ] do   Deck[i]=$UNPICKED   # Set each card of "Deck" as unpicked.   let "i += 1" done echo }  initialize_Suits () { Suits[0]=C #Clubs Suits[1]=D #Diamonds Suits[2]=H #Hearts Suits[3]=S #Spades }  initialize_Cards () { Cards=(2 3 4 5 6 7 8 9 10 J Q K A) # Alternate method of initializing an array. }  pick_a_card () { card_number=$RANDOM let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards. if [ "${Deck[card_number]}" -eq $UNPICKED ] then   Deck[card_number]=$PICKED   return $card_number else     return $DUPE_CARD fi }  parse_card () { number=$1 let "suit_number = number / CARDS_IN_SUIT" suit=${Suits[suit_number]} echo -n "$suit-" let "card_no = number % CARDS_IN_SUIT" Card=${Cards[card_no]} printf %-4s $Card # Print cards in neat columns. }  seed_random ()  # Seed random number generator. {               # What happens if you don't do this? seed=`eval date +%s` let "seed %= 32766" RANDOM=$seed } # Consider other methods of seeding the random number generator.  deal_cards () { echo  cards_picked=0 while [ "$cards_picked" -le $UPPER_LIMIT ] do   pick_a_card   t=$?    if [ "$t" -ne $DUPE_CARD ]   then     parse_card $t      u=$cards_picked+1     # Change back to 1-based indexing, temporarily. Why?     let "u %= $CARDS_IN_SUIT"     if [ "$u" -eq 0 ]   # Nested if/then condition test.     then      echo      echo     fi                  # Each hand set apart with a blank line.      let "cards_picked += 1"   fi   done    echo  return 0 }   # Structured programming: # Entire program logic modularized in functions.  #=============== seed_random initialize_Deck initialize_Suits initialize_Cards deal_cards #===============  exit    # Exercise 1: # Add comments to thoroughly document this script.  # Exercise 2: # Add a routine (function) to print out each hand sorted in suits. # You may add other bells and whistles if you like.  # Exercise 3: # Simplify and streamline the logic of the script.

37.2. Bash, version 3

On July 27, 2004, Chet Ramey released version 3 of Bash. This update fixed quite a number of bugs and added new features.

Some of the more important added features:

A new, more generalized {a..z} brace expansion operator.

#!/bin/bash  for i in {1..10} #  Simpler and more straightforward than #+ for i in $(seq 10) do   echo -n "$i " done  echo  # 1 2 3 4 5 6 7 8 9 10    # Or just . . .  echo {a..z}    #  a b c d e f g h i j k l m n o p q r s t u v w x y z echo {e..m}    #  e f g h i j k l m echo {z..a}    #  z y x w v u t s r q p o n m l k j i h g f e d c b a                #  Works backwards, too. echo {25..30}  #  25 26 27 28 29 30 echo {3..-2}   #  3 2 1 0 -1 -2 echo {X..d}    #  X Y Z [  ] ^ _ ` a b c d                #  Shows (some of) the ASCII characters between Z and a,                #+ but don't rely on this type of behavior because . . . echo {]..a}    #  {]..a}                #  Why?   # You can tack on prefixes and suffixes. echo "Number #"{1..4}, "..."      # Number #1, Number #2, Number #3, Number #4, ...   # You can concatenate brace-expansion sets. echo {1..3}{x..z}" +" "..."      # 1x + 1y + 1z + 2x + 2y + 2z + 3x + 3y + 3z + ...      # Generates an algebraic expression.      # This could be used to find permutations.  # You can nest brace-expansion sets. echo {{a..c},{1..3}}      # a b c 1 2 3      # The "comma operator" splices together strings.  # ########## ######### ############ ########### ######### ############### # Unfortunately, brace expansion does not lend itself to parameterization. var1=1 var2=5 echo {$var1..$var2}   # {1..5}   # Yet, as Emiliano G. points out, using "eval" overcomes this limitation.  start=0 end=10 for index in $(eval echo {$start..$end}) do   echo -n "$index "   # 0 1 2 3 4 5 6 7 8 9 10  done  echo

The ${!array[@]} operator, which expands to all the indices of a given array.

#!/bin/bash  Array=(element-zero element-one element-two element-three)  echo ${Array[0]}   # element-zero                    # First element of array.  echo ${!Array[@]}  # 0 1 2 3                    # All the indices of Array.  for i in ${!Array[@]} do   echo ${Array[i]} # element-zero                    # element-one                    # element-two                    # element-three                    #                    # All the elements in Array. done

The =~ Regular Expression matching operator within a double brackets test expression. (Perl has a similar operator.)

#!/bin/bash  variable="This is a fine mess."  echo "$variable"  # Regex matching with =~ operator within [[ double brackets ]]. if [[ "$variable" =~ T.........fin*es* ]] # NOTE: As of version 3.2 of Bash, expression to match no longer quoted. then   echo "match found"       # match found fi

Or, more usefully:

#!/bin/bash  input=$1   if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]] #                 ^ NOTE: Quoting not necessary, as of version 3.2 of Bash. # NNN-NN-NNNN (where each N is a digit). then   echo "Social Security number."   # Process SSN. else   echo "Not a Social Security number!"   # Or, ask for corrected input. fi

For additional examples of using the =~ operator, see Example A-29, Example 19-14, Example A-35, and Example A-24.

The new set -o pipefail option is useful for debugging pipes. If this option is set, then the exit status of a pipe is the exit status of the last command in the pipe to fail (return a non-zero value), rather than the actual final command in the pipe.

See Example 16-43.

The update to version 3 of Bash breaks a few scripts that worked under earlier versions. Test critical legacy scripts to make sure they still work!

As it happens, a couple of the scripts in the Advanced Bash Scripting Guide had to be fixed up (see Example 9-4, for instance).

37.2.1. Bash, version 3.1

The version 3.1 update of Bash introduces a number of bugfixes and a few minor changes.

The += operator is now permitted in in places where previously only the = assignment operator was recognized.

a=1 echo $a        # 1  a+=5           # Won't work under versions of Bash earlier than 3.1. echo $a        # 15  a+=Hello echo $a        # 15Hello

Here, += functions as a string concatenation operator. Note that its behavior in this particular context is different than within a let construct.

a=1 echo $a        # 1  let a+=5       # Integer arithmetic, rather than string concatenation. echo $a        # 6  let a+=Hello   # Doesn't "add" anything to a. echo $a        # 6

Jeffrey Haemer points out that this concatenation operator can be quite useful. In this instance, we append a directory to the $PATH.

                            bash$                                                                                      echo $PATH                                                        /usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games                            bash$                                                                                      PATH+=:/opt/bin                                                        bash$                                                                                      echo $PATH                                                        /usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games:/opt/bin

37.2.2. Bash, version 3.2

This is pretty much a bugfix update.

In global parameter substitutions, the pattern no longer anchors at the start of the string.
The --wordexp option disables process substitution.

The =~ Regular Expression match operator no longer requires quoting of the pattern within [[ ... ]].

In fact, quoting in this context is not advisable as it may cause regex evaluation to fail. Chet Ramey states in the Bash FAQ that quoting explicitly disables regex evaluation. See also the Ubuntu Bug List and Wikinerds on Bash syntax.

Setting shopt -s compat31 in a script causes reversion to the original behavior.

37.3. Bash, version 4

Chet Ramey announced Version 4 of Bash on the 20th of February, 2009. This release has a number of significant new features, as well as some important bugfixes.

Among the new goodies:

Associative arrays. [130]

Example 37-5. A simple address database

#!/bin/bash4 # fetch_address.sh  declare -A address #       -A option declares associative array.  address[Charles]="414 W. 10th Ave., Baltimore, MD 21236" address[John]="202 E. 3rd St., New York, NY 10009" address[Wilma]="1854 Vermont Ave, Los Angeles, CA 90023"   echo "Charles's address is ${address[Charles]}." # Charles's address is 414 W. 10th Ave., Baltimore, MD 21236. echo "Wilma's address is ${address[Wilma]}." # Wilma's address is 1854 Vermont Ave, Los Angeles, CA 90023. echo "John's address is ${address[John]}." # John's address is 202 E. 3rd St., New York, NY 10009.  echo  echo "${!address[*]}"   # The array indices ... # Charles John Wilma

Example 37-6. A somewhat more elaborate address database

#!/bin/bash4 # fetch_address-2.sh # A more elaborate version of fetch_address.sh.  SUCCESS=0 E_DB=99    # Error code for missing entry.  declare -A address #       -A option declares associative array.   store_address () {   address[$1]="$2"   return $? }   fetch_address () {   if [[ -z "${address[$1]}" ]]   then     echo "$1's address is not in database."     return $E_DB   fi    echo "$1's address is ${address[$1]}."   return $? }   store_address "Lucas Fayne" "414 W. 13th Ave., Baltimore, MD 21236" store_address "Arvid Boyce" "202 E. 3rd St., New York, NY 10009" store_address "Velma Winston" "1854 Vermont Ave, Los Angeles, CA 90023" #  Exercise: #  Rewrite the above store_address calls to read data from a file, #+ then assign field 1 to name, field 2 to address in the array. #  Each line in the file would have a format corresponding to the above. #  Use a while-read loop to read from file, sed or awk to parse the fields.  fetch_address "Lucas Fayne" # Lucas Fayne's address is 414 W. 13th Ave., Baltimore, MD 21236. fetch_address "Velma Winston" # Velma Winston's address is 1854 Vermont Ave, Los Angeles, CA 90023. fetch_address "Arvid Boyce" # Arvid Boyce's address is 202 E. 3rd St., New York, NY 10009. fetch_address "Bozo Bozeman" # Bozo Bozeman's address is not in database.  exit $?   # In this case, exit code = 99, since that is function return.

See Example A-53 for an interesting usage of an associative array.

Elements of the index array may include embedded space characters, or even leading and/or trailing space characters. However, index array elements containing only whitespace are not permitted.

address[   ]="Blank"   # Error!

Enhancements to the case construct: the ;;& and ;& terminators.

Example 37-7. Testing characters

#!/bin/bash4  test_char () {   case "$1" in     [[:print:]] )  echo "$1 is a printable character.";;&       # |     # The ;;& terminator continues to the next pattern test.      |     [[:alnum:]] )  echo "$1 is an alpha/numeric character.";;&  # v     [[:alpha:]] )  echo "$1 is an alphabetic character.";;&     # v     [[:lower:]] )  echo "$1 is a lowercase alphabetic character.";;&     [[:digit:]] )  echo "$1 is an numeric character.";&         # |     # The ;& terminator executes the next statement ...         # |     %%%@@@@@    )  echo "********************************";;    # v #   ^^^^^^^^  ... even with a dummy pattern.   esac }  echo  test_char 3 # 3 is a printable character. # 3 is an alpha/numeric character. # 3 is an numeric character. # ******************************** echo  test_char m # m is a printable character. # m is an alpha/numeric character. # m is an alphabetic character. # m is a lowercase alphabetic character. echo  test_char / # / is a printable character.  echo  # The ;;& terminator can save complex if/then conditions. # The ;& is somewhat less useful.

The new coproc builtin enables two parallel processes to communicate and interact. As Chet Ramey states in the Bash FAQ [131] , ver. 4.01:

There is a new 'coproc' reserved word that specifies a coprocess:
an asynchronous command run with two pipes connected to the creating
shell. Coprocs can be named. The input and output file descriptors
and the PID of the coprocess are available to the calling shell in
variables with coproc-specific names.

George Dimitriu explains,
"... coproc ... is a feature used in Bash process substitution,
which now is made publicly available."
This means it can be explicitly invoked in a script, rather than
just being a behind-the-scenes mechanism used by Bash.

Coprocesses use file descriptors. File descriptors enable processes and pipes to communicate.

#!/bin/bash4 # A coprocess communicates with a while-read loop.   coproc { cat mx_data.txt; sleep 2; } #                         ^^^^^^^ # Try running this without "sleep 2" and see what happens.  while read -u ${COPROC[0]} line    #  ${COPROC[0]} is the do                                 #+ file descriptor of the coprocess.   echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/' done  kill $COPROC_PID                   #  No longer need the coprocess,                                    #+ so kill its PID.

But, be careful!

#!/bin/bash4  echo; echo a=aaa b=bbb c=ccc  coproc echo "one two three" while read -u ${COPROC[0]} a b c;  #  Note that this loop do                                 #+ runs in a subshell.   echo "Inside while-read loop: ";   echo "a = $a"; echo "b = $b"; echo "c = $c"   echo "coproc file descriptor: ${COPROC[0]}" done   # a = one # b = two # c = three # So far, so good, but ...  echo "-----------------" echo "Outside while-read loop: " echo "a = $a"  # a = echo "b = $b"  # b = echo "c = $c"  # c = echo "coproc file descriptor: ${COPROC[0]}" echo #  The coproc is still running, but ... #+ it still doesn't enable the parent process #+ to "inherit" variables from the child process, the while-read loop.  #  Compare this to the "badread.sh" script.

The coprocess is asynchronous, and this might cause a problem. It may terminate before another process has finished communicating with it.

#!/bin/bash4  coproc cpname { for i in {0..10}; do echo "index = $i"; done; } #      ^^^^^^ This is a *named* coprocess. read -u ${cpname[0]} echo $REPLY         #  index = 0 echo ${COPROC[0]}   #+ No output ... the coprocess timed out #  after the first loop iteration.    # However, George Dimitriu has a partial fix.  coproc cpname { for i in {0..10}; do echo "index = $i"; done; sleep 1; echo hi > myo; cat - >> myo; } #       ^^^^^ This is a *named* coprocess.  echo "I am main"$'\04' >&${cpname[1]} myfd=${cpname[0]} echo myfd=$myfd  ### while read -u $myfd ### do ###   echo $REPLY; ### done  echo $cpname_PID  #  Run this with and without the commented-out while-loop, and it is #+ apparent that each process, the executing shell and the coprocess, #+ waits for the other to finish writing in its own write-enabled pipe.

The new mapfile builtin makes it possible to load an array with the contents of a text file without using a loop or command substitution.

#!/bin/bash4  mapfile Arr1 < $0 # Same result as     Arr1=( $(cat $0) ) echo "${Arr1[@]}"  # Copies this entire script out to stdout.  echo "--"; echo  # But, not the same as   read -a   !!! read -a Arr2 < $0 echo "${Arr2[@]}"  # Reads only first line of script into the array.  exit

The read builtin got a minor facelift. The -t timeout option now accepts (decimal) fractional values [132] and the -i option permits preloading the edit buffer. [133] Unfortunately, these enhancements are still a work in progress and not (yet) usable in scripts.

Parameter substitution gets case-modification operators.

#!/bin/bash4  var=veryMixedUpVariable echo ${var}            # veryMixedUpVariable echo ${var^}           # VeryMixedUpVariable #         *              First char --> uppercase. echo ${var^^}          # VERYMIXEDUPVARIABLE #         **             All chars  --> uppercase. echo ${var,}           # veryMixedUpVariable #         *              First char --> lowercase. echo ${var,,}          # verymixedupvariable #         **             All chars  --> lowercase.

The declare builtin now accepts the -l lowercase and -c capitalize options.

#!/bin/bash4  declare -l var1            # Will change to lowercase var1=MixedCaseVARIABLE echo "$var1"               # mixedcasevariable # Same effect as             echo $var1 | tr A-Z a-z  declare -c var2            # Changes only initial char to uppercase. var2=originally_lowercase echo "$var2"               # Originally_lowercase # NOT the same effect as     echo $var2 | tr a-z A-Z

Brace expansion has more options.

Increment/decrement, specified in the final term within braces.

#!/bin/bash4  echo {40..60..2} # 40 42 44 46 48 50 52 54 56 58 60 # All the even numbers, between 40 and 60.  echo {60..40..2} # 60 58 56 54 52 50 48 46 44 42 40 # All the even numbers, between 40 and 60, counting backwards. # In effect, a decrement. echo {60..40..-2} # The same output. The minus sign is not necessary.  # But, what about letters and symbols? echo {X..d} # X Y Z [  ] ^ _ ` a b c d # Does not echo the \ which escapes a space.

Zero-padding, specified in the first term within braces, prefixes each term in the output with the same number of zeroes.

                        bash4$                                                                          echo {010..15}                                                010 011 012 013 014 015                        bash4$                                                                          echo {000..10}                                                000 001 002 003 004 005 006 007 008 009 010

Substring extraction on positional parameters now starts with $0 as the zero-index. (This corrects an inconsistency in the treatment of positional parameters.)

#!/bin/bash # show-params.bash # Requires version 4+ of Bash.  # Invoke this scripts with at least one positional parameter.  E_BADPARAMS=99  if [ -z "$1" ] then   echo "Usage $0 param1 ..."   exit $E_BADPARAMS fi  echo ${@:0}  # bash3 show-params.bash4 one two three # one two three  # bash4 show-params.bash4 one two three # show-params.bash4 one two three  # $0                $1  $2  $3

The new ** globbing operator matches filenames and directories recursively.

#!/bin/bash4 # filelist.bash4  shopt -s globstar  # Must enable globstar, otherwise ** doesn't work.                    # The globstar shell option is new to version 4 of Bash.  echo "Using *"; echo for filename in * do   echo "$filename" done   # Lists only files in current directory ($PWD).  echo; echo "--------------"; echo  echo "Using **" for filename in ** do   echo "$filename" done   # Lists complete file tree, recursively.  exit  Using *  allmyfiles filelist.bash4  --------------  Using **  allmyfiles allmyfiles/file.index.txt allmyfiles/my_music allmyfiles/my_music/me-singing-60s-folksongs.ogg allmyfiles/my_music/me-singing-opera.ogg allmyfiles/my_music/piano-lesson.1.ogg allmyfiles/my_pictures allmyfiles/my_pictures/at-beach-with-Jade.png allmyfiles/my_pictures/picnic-with-Melissa.png filelist.bash4

The new $BASHPID internal variable.

There is a new builtin error-handling function named command_not_found_handle.

#!/bin/bash4  command_not_found_handle () { # Accepts implicit parameters.   echo "The following command is not valid: \""$1\"""   echo "With the following argument(s): \""$2\"" \""$3\"""   # $4, $5 ... } # $1, $2, etc. are not explicitly passed to the function.  bad_command arg1 arg2  # The following command is not valid: "bad_command" # With the following argument(s): "arg1" "arg2"

37.3.1. Bash, version 4.1

Version 4.1 of Bash, released in May, 2010, was primarily a bugfix update.

The printf command now accepts a -v option for setting array indices.
Within double brackets, the > and < string comparison operators now conform to the locale. Since the locale setting may affect the sorting order of string expressions, this has side-effects on comparison tests within [[ ... ]] expressions.

The read builtin now takes a -N option (read -N chars), which causes the read to terminate after chars characters.

Example 37-8. Reading N characters

#!/bin/bash # Requires Bash version -ge 4.1 ...  num_chars=61  read -N $num_chars var < $0   # Read first 61 characters of script! echo "$var" exit  ####### Output of Script #######  #!/bin/bash # Requires Bash version -ge 4.1 ...  num_chars=61

Here documents embedded in $( ... ) command substitution constructs may terminate with a simple ).

Example 37-9. Using a here document to set a variable

#!/bin/bash # here-commsub.sh # Requires Bash version -ge 4.1 ...  multi_line_var=$( cat <<ENDxxx ------------------------------ This is line 1 of the variable This is line 2 of the variable This is line 3 of the variable ------------------------------ ENDxxx)  #  Rather than what Bash 4.0 requires: #+ that the terminating limit string and #+ the terminating close-parenthesis be on separate lines.  # ENDxxx # )   echo "$multi_line_var"  #  Bash still emits a warning, though. #  warning: here-document at line 10 delimited #+ by end-of-file (wanted `ENDxxx')

37.3.2. Bash, version 4.2

Version 4.2 of Bash, released in February, 2011, contains a number of new features and enhancements, in addition to bugfixes.

Bash now supports the the \u and \U Unicode escape.

echo -e '\u2630'   # Horizontal triple bar character. # Equivalent to the more roundabout: echo -e "\xE2\x98\xB0"                    # Recognized by earlier Bash versions.  echo -e '\u220F'   # PI (Greek letter and mathematical symbol) echo -e '\u0416'   # Capital "ZHE" (Cyrillic letter) echo -e '\u2708'   # Airplane (Dingbat font) symbol echo -e '\u2622'   # Radioactivity trefoil  echo -e "The amplifier circuit requires a 100 \u2126 pull-up resistor."   unicode_var='\u2640' echo -e $unicode_var      # Female symbol printf "$unicode_var \n"  # Female symbol, with newline   #  And for something a bit more elaborate . . .  #  We can store Unicode symbols in an associative array, #+ then retrieve them by name. #  Run this in a gnome-terminal or a terminal with a large, bold font #+ for better legibility.  declare -A symbol  # Associative array.  symbol[script_E]='\u2130' symbol[script_F]='\u2131' symbol[script_J]='\u2110' symbol[script_M]='\u2133' symbol[Rx]='\u211E' symbol[TEL]='\u2121' symbol[FAX]='\u213B' symbol[care_of]='\u2105' symbol[account]='\u2100' symbol[trademark]='\u2122'   echo -ne "${symbol[script_E]}   " echo -ne "${symbol[script_F]}   " echo -ne "${symbol[script_J]}   " echo -ne "${symbol[script_M]}   " echo -ne "${symbol[Rx]}   " echo -ne "${symbol[TEL]}   " echo -ne "${symbol[FAX]}   " echo -ne "${symbol[care_of]}   " echo -ne "${symbol[account]}   " echo -ne "${symbol[trademark]}   " echo

The above example uses the $' ... ' string-expansion construct.

When the lastpipe shell option is set, the last command in a pipe doesn't run in a subshell.

Example 37-10. Piping input to a read

#!/bin/bash # lastpipe-option.sh  line=''                   # Null value. echo "\$line = "$line""   # $line =  echo  shopt -s lastpipe         # Error on Bash version -lt 4.2. echo "Exit status of attempting to set \"lastpipe\" option is $?" #     1 if Bash version -lt 4.2, 0 otherwise.  echo  head -1 $0 | read line    # Pipe the first line of the script to read. #            ^^^^^^^^^      Not in a subshell!!!  echo "\$line = "$line"" # Older Bash releases       $line = # Bash version 4.2          $line = #!/bin/bash

This option offers possible "fixups" for these example scripts: Example 34-3 and Example 15-8.

Negative array indices permit counting backwards from the end of an array.

Example 37-11. Negative array indices

#!/bin/bash # neg-array.sh # Requires Bash, version -ge 4.2.  array=( zero one two three four five )   # Six-element array. #         0    1   2    3    4    5 #        -6   -5  -4   -3   -2   -1  # Negative array indices now permitted. echo ${array[-1]}   # five echo ${array[-2]}   # four # ... echo ${array[-6]}   # zero # Negative array indices count backward from the last element+1.  # But, you cannot index past the beginning of the array. echo ${array[-7]}   # array: bad array subscript   # So, what is this new feature good for?  echo "The last element in the array is "${array[-1]}"" # Which is quite a bit more straightforward than: echo "The last element in the array is "${array[${#array[*]}-1]}"" echo  # And ...  index=0 let "neg_element_count = 0 - ${#array[*]}" # Number of elements, converted to a negative number.  while [ $index -gt $neg_element_count ]; do   ((index--)); echo -n "${array[index]} " done  # Lists the elements in the array, backwards.       # We have just simulated the "tac" command on this array.  echo  # See also neg-offset.sh.

Substring extraction uses a negative length parameter to specify an offset from the end of the target string.

Example 37-12. Negative parameter in string-extraction construct

#!/bin/bash # Bash, version -ge 4.2 # Negative length-index in substring extraction. # Important: It changes the interpretation of this construct!  stringZ=abcABC123ABCabc  echo ${stringZ}                              # abcABC123ABCabc #                   Position within string:    0123456789..... echo ${stringZ:2:3}                          #   cAB #  Count 2 chars forward from string beginning, and extract 3 chars. #  ${string:position:length}  #  So far, nothing new, but now ...                                               # abcABC123ABCabc #                   Position within string:    0123....6543210 echo ${stringZ:3:-6}                         #    ABC123 #                ^ #  Index 3 chars forward from beginning and 6 chars backward from end, #+ and extract everything in between. #  ${string:offset-from-front:offset-from-end} #  When the "length" parameter is negative,  #+ it serves as an offset-from-end parameter.  #  See also neg-array.sh.

Chapter 38. Endnotes

38.1. Author's Note

doce ut discas

(Teach, that you yourself may learn.)

How did I come to write a scripting book? It's a strange tale. It seems that a few years back I needed to learn shell scripting -- and what better way to do that than to read a good book on the subject? I was looking to buy a tutorial and reference covering all aspects of the subject. I was looking for a book that would take difficult concepts, turn them inside out, and explain them in excruciating detail, with well-commented examples. [135] In fact, I was looking for this very book, or something very much like it. Unfortunately, it didn't exist, and if I wanted it, I'd have to write it. And so, here we are, folks.

That reminds me of the apocryphal story about a mad professor. Crazy as a loon, the fellow was. At the sight of a book, any book -- at the library, at a bookstore, anywhere -- he would become totally obsessed with the idea that he could have written it, should have written it -- and done a better job of it to boot. He would thereupon rush home and proceed to do just that, write a book with the very same title. When he died some years later, he allegedly had several thousand books to his credit, probably putting even Asimov to shame. The books might not have been any good, who knows, but does that really matter? Here's a fellow who lived his dream, even if he was obsessed by it, driven by it . . . and somehow I can't help admiring the old coot.

38.2. About the Author

Who is this guy anyhow?

The author claims no credentials or special qualifications, [136] other than a compulsion to write. [137]

This book is somewhat of a departure from his other major work, HOW-2 Meet Women: The Shy Man's Guide to Relationships. He has also written the Software-Building HOWTO. Of late, he has been trying his (heavy) hand at fiction: Dave Dawson Over Berlin (First Installment) Dave Dawson Over Berlin (Second Installment) and Dave Dawson Over Berlin (Third Installment) . He also has a few Instructables (here, here, here, here, here, here, and here to his (dis)credit.

A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has emitted a few software truffles, including the cruft one-time pad encryption utility, the mcalc mortgage calculator, the judge Scrabble� adjudicator, the yawl word gaming list package, and the Quacky anagramming gaming package. He got off to a rather shaky start in the computer game -- programming FORTRAN IV on a CDC 3800 (on paper coding pads, with occasional forays on a keypunch machine and a Friden Flexowriter) -- and is not the least bit nostalgic for those days.

Living in an out-of-the-way community with wife and orange tabby, he cherishes human frailty, especially his own. [138]

38.3. Where to Go For Help

The author is no longer supporting or updating this document. He will not answer questions about this book or about general scripting topics.

... sophisticated in mechanism but possibly agile operating under noises being extremely suppressed ...

--CI-300 printer manual

38.4. Tools Used to Produce This Book

38.4.1. Hardware

A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running Red Hat 7.1/7.3. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet.

Update: upgraded to a 770Z Thinkpad (P2-366, 192 meg RAM) running FC3. Anyone feel like donating a later-model laptop to a starving writer <g>?

Update: upgraded to a T61 Thinkpad running Mandriva 2011. No longer starving <g>, but not too proud to accept donations.

38.4.2. Software and Printware

Bram Moolenaar's powerful SGML-aware vim text editor.
OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.
Norman Walsh's DSSSL stylesheets.
DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1-56592-580-7). This is still the standard reference for anyone attempting to write a document in Docbook SGML format.

38.5. Credits

Community participation made this project possible. The author gratefully acknowledges that writing this book would have been unthinkable without help and feedback from all you people out there.

Philippe Martin translated the first version (0.1) of this document into DocBook/SGML. While not on the job at a small French company as a software developer, he enjoys working on GNU/Linux documentation and software, reading literature, playing music, and, for his peace of mind, making merry with friends. You may run across him somewhere in France or in the Basque Country, or you can email him at feloy@free.fr.

Philippe Martin also pointed out that positional parameters past $9 are possible using {bracket} notation. (See Example 4-5).

St�phane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he had, in effect, for a while taken on the role of co-editor for this document. Merci beaucoup!

Paulo Marcel Coelho Aragao offered many corrections, both major and minor, and contributed quite a number of helpful suggestions.

I would like to especially thank Patrick Callahan, Mike Novak, and Pal Domokos for catching bugs, pointing out ambiguities, and for suggesting clarifications and changes in the preliminary version (0.1) of this document. Their lively discussion of shell scripting and general documentation issues inspired me to try to make this document more readable.

I'm grateful to Jim Van Zandt for pointing out errors and omissions in version 0.2 of this document. He also contributed an instructive example script.

Many thanks to Jordi Sanfeliu for giving permission to use his fine tree script (Example A-16), and to Rick Boivie for revising it.

Likewise, thanks to Michel Charpentier for permission to use his dc factoring script (Example 16-52).

Kudos to Noah Friedman for permission to use his string function script (Example A-18).

Emmanuel Rouat suggested corrections and additions on command substitution, aliases, and path management. He also contributed a very nice sample .bashrc file (Appendix M).

Heiner Steven kindly gave permission to use his base conversion script, Example 16-48. He also made a number of corrections and many helpful suggestions. Special thanks.

Rick Boivie contributed the delightfully recursive pb.sh script (Example 36-11), revised the tree.sh script (Example A-16), and suggested performance improvements for the monthlypmt.sh script (Example 16-47).

Florian Wisser enlightened me on some of the fine points of testing strings (see Example 7-6), and on other matters.

Oleg Philon sent suggestions concerning cut and pidof.

Michael Zick extended the empty array example to demonstrate some surprising array properties. He also contributed the isspammer scripts (Example 16-41 and Example A-28).

Marc-Jano Knopp sent corrections and clarifications on DOS batch files.

Hyun Jin Cha found several typos in the document in the process of doing a Korean translation. Thanks for pointing these out.

Andreas Abraham sent in a long list of typographical errors and other corrections. Special thanks!

Others contributing scripts, making helpful suggestions, and pointing out errors were Gabor Kiss, Leopold Toetsch, Peter Tillier, Marcus Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell, Jess Thrysoee, Adam Lazur, Bram Moolenaar, Baris Cicek, Greg Keraunen, Keith Matthews, Sandro Magi, Albert Reiner, Dim Segebart, Rory Winston, Lee Bigelow, Wayne Pollock, "jipe," "bojster," "nyal," "Hobbit," "Ender," "Little Monster" (Alexis), "Mark," "Patsie," "vladz," Peggy Russell, Emilio Conti, Ian. D. Allen, Hans-Joerg Diers, Arun Giridhar, Dennis Leeuw, Dan Jacobson, Aurelio Marinho Jargas, Edward Scholtz, Jean Helou, Chris Martin, Lee Maschmeyer, Bruno Haible, Wilbert Berendsen, Sebastien Godard, Bj�n Eriksson, John MacDonald, John Lange, Joshua Tschida, Troy Engel, Manfred Schwarb, Amit Singh, Bill Gradwohl, E. Choroba, David Lombard, Jason Parker, Steve Parker, Bruce W. Clare, William Park, Vernia Damiano, Mihai Maties, Mark Alexander, Jeremy Impson, Ken Fuchs, Jared Martin, Frank Wang, Sylvain Fourmanoit, Matthew Sage, Matthew Walker, Kenny Stauffer, Filip Moritz, Andrzej Stefanski, Daniel Albers, Jeffrey Haemer, Stefano Palmeri, Nils Radtke, Sigurd Solaas, Serghey Rodin, Jeroen Domburg, Alfredo Pironti, Phil Braham, Bruno de Oliveira Schneider, Stefano Falsetto, Chris Morgan, Walter Dnes, Linc Fessenden, Michael Iatrou, Pharis Monalo, Jesse Gough, Fabian Kreutz, Mark Norman, Harald Koenig, Dan Stromberg, Peter Knowles, Francisco Lobo, Mariusz Gniazdowski, Sebastian Arming, Chetankumar Phulpagare, Benno Schulenberg, Tedman Eng, Jochen DeSmet, Juan Nicolas Ruiz, Oliver Beckstein, Achmed Darwish, Dotan Barak, Richard Neill, Albert Siersema, Omair Eshkenazi, Geoff Lee, Graham Ewart, JuanJo Ciarlante, Cliff Bamford, Nathan Coulter, Ramses Rodriguez Martinez, Evgeniy Ivanov, Craig Barnes, George Dimitriu, Kevin LeBlanc, Antonio Macchi, Tomas Pospisek, David Wheeler, Erik Brandsberg, YongYe, Andreas K�hne, P�draig Brady, Joseph Steinhauser, and David Lawyer (himself an author of four HOWTOs).

My gratitude to Chet Ramey and Brian Fox for writing Bash, and building into it elegant and powerful scripting capabilities rivaling those of ksh.

Very special thanks to the hard-working volunteers at the Linux Documentation Project. The LDP hosts a repository of Linux knowledge and lore, and has, to a great extent, enabled the publication of this book.

Thanks and appreciation to IBM, Red Hat, Google, the Free Software Foundation, and all the good people fighting the good fight to keep Open Source software free and open.

Belated thanks to my fourth grade teacher, Miss Spencer, for emotional support and for convincing me that maybe, just maybe I wasn't a total loss.

Thanks most of all to my wife, Anita, for her encouragement, inspiration, and emotional support.

38.6. Disclaimer

(This is a variant of the standard LDP disclaimer.)

No liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors, omissions, and inaccuracies that could cause you to lose data, harm your system, or induce involuntary electrocution, so proceed with appropriate caution. The author takes no responsibility for any damages, incidental or otherwise.

As it happens, it is highly unlikely that either you or your system will suffer ill effects, aside from uncontrollable hiccups. In fact, the raison d'etre of this book is to enable its readers to analyze shell scripts and determine whether they have unanticipated consequences.

Bibliography

Those who do not understand UNIX are condemned to reinvent it, poorly.

--Henry Spencer

Edited by Peter Denning, Computers Under Attack: Intruders, Worms, and Viruses, ACM Press, 1990, 0-201-53067-8.

This compendium contains a couple of articles on shell script viruses.

Ken Burtch, Linux Shell Scripting with Bash, 1st edition, Sams Publishing (Pearson), 2004, 0672326426.

Covers much of the same material as the ABS Guide, though in a different style.

Daniel Goldman, Definitive Guide to Sed, 1st edition, 2013.

This ebook is an excellent introduction to sed. Rather than being a conversion from a printed volume, it was specifically designed and formatted for viewing on an ebook reader. Well-written, informative, and useful as a reference as well as a tutorial. Highly recommended.

Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997, 1-156592-225-5.

Unfolding the full power of shell scripting requires at least a passing familiarity with sed and awk. This is the classic tutorial. It includes an excellent introduction to Regular Expressions. Recommended.

Jeffrey Friedl, Mastering Regular Expressions, O'Reilly and Associates, 2002, 0-596-00289-0.

Still the best all-around reference on Regular Expressions.

Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly and Associates, 2002, 0-596-00343-9.

This excellent manual provides a decent introduction to shell scripting from a sys admin point of view. It includes comprehensive explanations of the startup and initialization scripts in a UNIX system.

Stephen Kochan and Patrick Wood, Unix Shell Programming, Hayden, 1990, 067248448X.

Still considered a standard reference, though somewhat dated, and a bit "wooden" stylistically speaking. [140] In fact, this book was the ABS Guide author's first exposure to UNIX shell scripting, lo these many years ago.

Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox Press, 1996, 1874416680.

Surprisingly good in-depth coverage of various programming languages available for Linux, including a fairly strong chapter on shell scripting.

Herbert Mayer, Advanced C Programming on the IBM PC, Windcrest Books, 1989, 0830693637.

Excellent coverage of algorithms and general programming practices. Highly recommended, but unfortunately out of print.

David Medinets, Unix Shell Programming Tools, McGraw-Hill, 1999, 0070397333.

Pretty good treatment of shell scripting, with examples, and a short intro to Tcl and Perl.

Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998, 1-56592-347-2.

This is a valiant effort at a decent shell primer, but sadly deficient in its coverage of writing scripts and lacking sufficient examples.

Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X.

A very handy pocket reference, despite lacking coverage of Bash-specific features.

Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 3rd edition, O'Reilly and Associates, Random House, 2002, 0-596-00330-7.

Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of being a self-teaching manual. It reproduces much of the Regular Expressions tutorial from the Dougherty and Robbins book, above. The comprehensive coverage of UNIX commands makes this book worthy of a place on your bookshelf.

Clifford Pickover, Computers, Pattern, Chaos, and Beauty, St. Martin's Press, 1990, 0-312-04123-3.

A treasure trove of ideas and recipes for computer-based exploration of mathematical oddities.

George Polya, How To Solve It, Princeton University Press, 1973, 0-691-02356-5.

The classic tutorial on problem-solving methods (algorithms), with special emphasis on how to teach them.

Chet Ramey and Brian Fox, The GNU Bash Reference Manual, Network Theory Ltd, 2003, 0-9541617-7-7.

This manual is the definitive reference for GNU Bash. The authors of this manual, Chet Ramey and Brian Fox, are the original developers of GNU Bash. For each copy sold, the publisher donates $1 to the Free Software Foundation.

Arnold Robbins, Bash Reference Card, SSC, 1998, 1-58731-010-5.

Excellent Bash pocket reference (don't leave home without it, especially if you're a sysadmin). A bargain at $4.95, but unfortunately no longer available for free download.

Arnold Robbins, Effective Awk Programming, Free Software Foundation / O'Reilly and Associates, 2000, 1-882114-26-4.

The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk documentation, and printed copies of the latest version are available from O'Reilly and Associates.

This book served as an inspiration for the author of the ABS Guide.

Bill Rosenblatt, Learning the Korn Shell, O'Reilly and Associates, 1993, 1-56592-054-6.

This well-written book contains some excellent pointers on shell scripting in general.

Paul Sheer, LINUX: Rute User's Tutorial and Exposition, 1st edition, , 2002, 0-13-033351-4.

Very detailed and readable introduction to Linux system administration.

The book is available in print, or on-line.

Ellen Siever and the staff of O'Reilly and Associates, Linux in a Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1-56592-585-8.

The all-around best Linux command reference. It even has a Bash section.

Dave Taylor, Wicked Cool Shell Scripts: 101 Scripts for Linux, Mac OS X, and Unix Systems, 1st edition, No Starch Press, 2004, 1-59327-012-7.

Pretty much what the title promises . . .

The UNIX CD Bookshelf, 3rd edition, O'Reilly and Associates, 2003, 0-596-00392-7.

An array of seven UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $130. Buy this one, even if it means going into debt and not paying the rent.

Update: Seems to have somehow fallen out of print. Ah, well. You can still buy the dead-tree editions of these books.

The O'Reilly books on Perl. (Actually, any O'Reilly books.)

Fioretti, Marco, "Scripting for X Productivity," Linux Journal, Issue 113, September, 2003, pp. 86-9.

Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux Gazette, and his explanation of "The Deep, Dark Secrets of Bash" in issue 56.

Chet Ramey's Bash - The GNU Shell, a two-part series published in issues 3 and 4 of the Linux Journal, July-August 1994.

Examples from the The Bash Scripting Cookbook, by Albing, Vossen, and Newham.

Example shell scripts at zazzybob.

An excellent collection of Bash scripting tips, tricks, and resources at the Bash Hackers Wiki.

Very nice sed, awk, and regular expression tutorials at The UNIX Grymoire.

The GNU sed and gawk manuals. As you recall, gawk is the enhanced GNU version of awk.

Rick Hohensee has written the osimpa i386 assembler entirely as Bash scripts.

dgatwood has a very nice shell script games site, featuring a Tetris� clone and solitaire.

Aurelio Marinho Jargas has written a Regular expression wizard. He has also written an informative book on Regular Expressions, in Portuguese.

William Park has been working on a project to incorporate certain Awk and Python features into Bash. Among these is a gdbm interface. He has released bashdiff on Freshmeat.net. He has an article in the November, 2004 issue of the Linux Gazette on adding string functions to Bash, with a followup article in the December issue, and yet another in the January, 2005 issue.

Peter Knowles has written an elaborate Bash script that generates a book list on the Sony Librie e-book reader. This useful tool facilitates loading non-DRM user content on the Librie (and the newer PRS-xxx-series devices).

Tim Waugh's xmlto is an elaborate Bash script for converting Docbook XML documents to other formats.

Philip Patterson's logforbash logging/debugging script.

AuctionGallery, an application for eBay "power sellers" coded in Bash.

Of historical interest are Colin Needham's original International Movie Database (IMDB) reader polling scripts, which nicely illustrate the use of awk for string parsing. Unfortunately, the URL link is broken.

---

Penguin Pete has quite a number of shell scripting tips and hints on his superb site. Highly recommended.

The excellent Bash Reference Manual, by Chet Ramey and Brian Fox, distributed as part of the bash-2-doc package (available as an rpm). See especially the instructive example scripts in this package.

Assorted comp.os.unix FAQs.

The manpages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, bc, xargs. The texinfo documentation on bash, dd, m4, gawk, and sed.

Appendix A. Contributed Scripts

These scripts, while not fitting into the text of this document, do illustrate some interesting shell programming techniques. Some are useful, too. Have fun analyzing and running them.

Example A-1. mailformat: Formatting an e-mail message

#!/bin/bash # mail-format.sh (ver. 1.1): Format e-mail messages.  # Gets rid of carets, tabs, and also folds excessively long lines.  # ================================================================= #                 Standard Check for Script Argument(s) ARGS=1 E_BADARGS=85 E_NOFILE=86  if [ $# -ne $ARGS ]  # Correct number of arguments passed to script? then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi  if [ -f "$1" ]       # Check if file exists. then     file_name=$1 else     echo "File \"$1\" does not exist."     exit $E_NOFILE fi # -----------------------------------------------------------------  MAXWIDTH=70          # Width to fold excessively long lines to.  # ================================= # A variable can hold a sed script. # It's a useful technique. sedscript='s/^>// s/^  *>// s/^  *// s/		*//' # =================================  #  Delete carets and tabs at beginning of lines, #+ then fold lines to $MAXWIDTH characters. sed "$sedscript" $1 | fold -s --width=$MAXWIDTH                         #  -s option to "fold"                         #+ breaks lines at whitespace, if possible.   #  This script was inspired by an article in a well-known trade journal #+ extolling a 164K MS Windows utility with similar functionality. # #  An nice set of text processing utilities and an efficient #+ scripting language provide an alternative to the bloated executables #+ of a clunky operating system.  exit $?

Example A-2. rn: A simple-minded file renaming utility

This script is a modification of Example 16-22.

#! /bin/bash # rn.sh  # Very simpleminded filename "rename" utility (based on "lowercase.sh"). # #  The "ren" utility, by Vladimir Lanin (lanin@csd2.nyu.edu), #+ does a much better job of this.   ARGS=2 E_BADARGS=85 ONE=1                     # For getting singular/plural right (see below).  if [ $# -ne "$ARGS" ] then   echo "Usage: `basename $0` old-pattern new-pattern"   # As in "rn gif jpg", which renames all gif files in working directory to jpg.   exit $E_BADARGS fi  number=0                  # Keeps track of how many files actually renamed.   for filename in *$1*      #Traverse all matching files in directory. do    if [ -f "$filename" ]  # If finds match...    then      fname=`basename $filename`            # Strip off path.      n=`echo $fname | sed -e "s/$1/$2/"`   # Substitute new for old in filename.      mv $fname $n                          # Rename.      let "number += 1"    fi done     if [ "$number" -eq "$ONE" ]                # For correct grammar. then  echo "$number file renamed." else   echo "$number files renamed." fi   exit $?   # Exercises: # --------- # What types of files will this not work on? # How can this be fixed?

Example A-3. blank-rename: Renames filenames containing blanks

This is an even simpler-minded version of previous script.

#! /bin/bash # blank-rename.sh # # Substitutes underscores for blanks in all the filenames in a directory.  ONE=1                     # For getting singular/plural right (see below). number=0                  # Keeps track of how many files actually renamed. FOUND=0                   # Successful return value.  for filename in *         #Traverse all files in directory. do      echo "$filename" | grep -q " "         #  Check whether filename      if [ $? -eq $FOUND ]                   #+ contains space(s).      then        fname=$filename                      # Yes, this filename needs work.        n=`echo $fname | sed -e "s/ /_/g"`   # Substitute underscore for blank.        mv "$fname" "$n"                     # Do the actual renaming.        let "number += 1"      fi done     if [ "$number" -eq "$ONE" ]                 # For correct grammar. then  echo "$number file renamed." else   echo "$number files renamed." fi   exit 0

Example A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted password

#!/bin/bash  # Example "ex72.sh" modified to use encrypted password.  #  Note that this is still rather insecure, #+ since the decrypted password is sent in the clear. #  Use something like "ssh" if this is a concern.  E_BADARGS=85  if [ -z "$1" ] then   echo "Usage: `basename $0` filename"   exit $E_BADARGS fi    Username=bozo           # Change to suit. pword=/home/bozo/secret/password_encrypted.file # File containing encrypted password.  Filename=`basename $1`  # Strips pathname out of file name.  Server="XXX" Directory="YYY"         # Change above to actual server name & directory.   Password=`cruft <$pword`          # Decrypt password. #  Uses the author's own "cruft" file encryption package, #+ based on the classic "onetime pad" algorithm, #+ and obtainable from: #+ Primary-site:   ftp://ibiblio.org/pub/Linux/utils/file #+                 cruft-0.2.tar.gz [16k]   ftp -n $Server <<End-Of-Session user $Username $Password binary bell cd $Directory put $Filename bye End-Of-Session # -n option to "ftp" disables auto-logon. # Note that "bell" rings 'bell' after each file transfer.  exit 0

Example A-5. copy-cd: Copying a data CD

#!/bin/bash # copy-cd.sh: copying a data CD  CDROM=/dev/cdrom                           # CD ROM device OF=/home/bozo/projects/cdimage.iso         # output file #       /xxxx/xxxxxxxx/                      Change to suit your system. BLOCKSIZE=2048 # SPEED=10                                 # If unspecified, uses max spd. # DEVICE=/dev/cdrom                          older version. DEVICE="1,0,0"  echo; echo "Insert source CD, but do *not* mount it." echo "Press ENTER when ready. " read ready                                 # Wait for input, $ready not used.  echo; echo "Copying the source CD to $OF." echo "This may take a while. Please be patient."  dd if=$CDROM of=$OF bs=$BLOCKSIZE          # Raw device copy.   echo; echo "Remove data CD." echo "Insert blank CDR." echo "Press ENTER when ready. " read ready                                 # Wait for input, $ready not used.  echo "Copying $OF to CDR."  # cdrecord -v -isosize speed=$SPEED dev=$DEVICE $OF   # Old version. wodim -v -isosize dev=$DEVICE $OF # Uses Joerg Schilling's "cdrecord" package (see its docs). # http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html # Newer Linux distros may use "wodim" rather than "cdrecord" ...   echo; echo "Done copying $OF to CDR on device $CDROM."  echo "Do you want to erase the image file (y/n)? "  # Probably a huge file. read answer  case "$answer" in [yY]) rm -f $OF       echo "$OF erased."       ;; *)    echo "$OF not erased.";; esac  echo  # Exercise: # Change the above "case" statement to also accept "yes" and "Yes" as input.  exit 0

Example A-6. Collatz series

#!/bin/bash # collatz.sh  #  The notorious "hailstone" or Collatz series. #  ------------------------------------------- #  1) Get the integer "seed" from the command-line. #  2) NUMBER <-- seed #  3) Print NUMBER. #  4)  If NUMBER is even, divide by 2, or #  5)+ if odd, multiply by 3 and add 1. #  6) NUMBER <-- result  #  7) Loop back to step 3 (for specified number of iterations). # #  The theory is that every such sequence, #+ no matter how large the initial value, #+ eventually settles down to repeating "4,2,1..." cycles, #+ even after fluctuating through a wide range of values. # #  This is an instance of an "iterate," #+ an operation that feeds its output back into its input. #  Sometimes the result is a "chaotic" series.   MAX_ITERATIONS=200 # For large seed numbers (>32000), try increasing MAX_ITERATIONS.  h=${1:-$$}                      #  Seed.                                 #  Use $PID as seed,                                 #+ if not specified as command-line arg.  echo echo "C($h) -*- $MAX_ITERATIONS Iterations" echo  for ((i=1; i<=MAX_ITERATIONS; i++)) do  # echo -n "$h	" #            ^^^  #            tab # printf does it better ... COLWIDTH=%7d printf $COLWIDTH $h    let "remainder = h % 2"   if [ "$remainder" -eq 0 ]   # Even?   then     let "h /= 2"              # Divide by 2.   else     let "h = h*3 + 1"         # Multiply by 3 and add 1.   fi   COLUMNS=10                    # Output 10 values per line. let "line_break = i % $COLUMNS" if [ "$line_break" -eq 0 ] then   echo fi    done  echo  #  For more information on this strange mathematical function, #+ see _Computers, Pattern, Chaos, and Beauty_, by Pickover, p. 185 ff., #+ as listed in the bibliography.  exit 0

Example A-7. days-between: Days between two dates

#!/bin/bash # days-between.sh:    Number of days between two dates. # Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY # # Note: Script modified to account for changes in Bash, v. 2.05b +, #+      that closed the loophole permitting large negative #+      integer return values.  ARGS=2                # Two command-line parameters expected. E_PARAM_ERR=85        # Param error.  REFYR=1600            # Reference year. CENTURY=100 DIY=365 ADJ_DIY=367           # Adjusted for leap year + fraction. MIY=12 DIM=31 LEAPCYCLE=4  MAXRETVAL=255         #  Largest permissible                       #+ positive return value from a function.  diff=                 # Declare global variable for date difference. value=                # Declare global variable for absolute value. day=                  # Declare globals for day, month, year. month= year=   Param_Error ()        # Command-line parameters wrong. {   echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"   echo "       (date must be after 1/3/1600)"   exit $E_PARAM_ERR }     Parse_Date ()                 # Parse date from command-line params. {   month=${1%%/**}   dm=${1%/**}                 # Day and month.   day=${dm#*/}   let "year = `basename $1`"  # Not a filename, but works just the same. }     check_date ()                 # Checks for invalid date(s) passed. {   [ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||   [ "$year" -lt "$REFYR" ] && Param_Error   # Exit script on bad value(s).   # Uses or-list / and-list.   #   # Exercise: Implement more rigorous date checking. }   strip_leading_zero () #  Better to strip possible leading zero(s) {                     #+ from day and/or month   return ${1#0}       #+ since otherwise Bash will interpret them }                     #+ as octal values (POSIX.2, sect 2.9.2.1).   day_index ()          # Gauss' Formula: {                     # Days from March 1, 1600 to date passed as param.                       #           ^^^^^^^^^^^^^   day=$1   month=$2   year=$3    let "month = $month - 2"   if [ "$month" -le 0 ]   then     let "month += 12"     let "year -= 1"   fi      let "year -= $REFYR"   let "indexyr = $year / $CENTURY"     let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \               + $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"   #  For an in-depth explanation of this algorithm, see   #+   http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx     echo $Days  }     calculate_difference ()            # Difference between two day indices. {   let "diff = $1 - $2"             # Global variable. }     abs ()                             #  Absolute value {                                  #  Uses global "value" variable.   if [ "$1" -lt 0 ]                #  If negative   then                             #+ then     let "value = 0 - $1"           #+ change sign,   else                             #+ else     let "value = $1"               #+ leave it alone.   fi }    if [ $# -ne "$ARGS" ]              # Require two command-line params. then   Param_Error fi    Parse_Date $1 check_date $day $month $year       #  See if valid date.  strip_leading_zero $day            #  Remove any leading zeroes day=$?                             #+ on day and/or month. strip_leading_zero $month month=$?  let "date1 = `day_index $day $month $year`"   Parse_Date $2 check_date $day $month $year  strip_leading_zero $day day=$? strip_leading_zero $month month=$?  date2=$(day_index $day $month $year) # Command substitution.   calculate_difference $date1 $date2  abs $diff                            # Make sure it's positive. diff=$value  echo $diff  exit 0  #  Exercise: #  -------- #  If given only one command-line parameter, have the script #+ use today's date as the second.   #  Compare this script with #+ the implementation of Gauss' Formula in a C program at #+    http://buschencrew.hypermart.net/software/datedif

Example A-8. Making a dictionary

#!/bin/bash # makedict.sh  [make dictionary]  # Modification of /usr/sbin/mkdict (/usr/sbin/cracklib-forman) script. # Original script copyright 1993, by Alec Muffett. # #  This modified script included in this document in a manner #+ consistent with the "LICENSE" document of the "Crack" package #+ that the original script is a part of.  #  This script processes text files to produce a sorted list #+ of words found in the files. #  This may be useful for compiling dictionaries #+ and for other lexicographic purposes.   E_BADARGS=85  if [ ! -r "$1" ]                    #  Need at least one then                                #+ valid file argument.   echo "Usage: $0 files-to-process"   exit $E_BADARGS fi     # SORT="sort"                       #  No longer necessary to define                                     #+ options to sort. Changed from                                     #+ original script.  cat $* |                            #  Dump specified files to stdout.         tr A-Z a-z |                #  Convert to lowercase.         tr ' ' '\012' |             #  New: change spaces to newlines. #       tr -cd '\012[a-z][0-9]' |   #  Get rid of everything                                     #+ non-alphanumeric (in orig. script).         tr -c '\012a-z'  '\012' |   #  Rather than deleting non-alpha                                     #+ chars, change them to newlines.         sort |                      #  $SORT options unnecessary now.         uniq |                      #  Remove duplicates.         grep -v '^#' |              #  Delete lines starting with #.         grep -v '^$'                #  Delete blank lines.  exit $?

Example A-9. Soundex conversion

#!/bin/bash # soundex.sh: Calculate "soundex" code for names  # ======================================================= #        Soundex script #              by #         Mendel Cooper #     thegrendel.abs@gmail.com #     reldate: 23 January, 2002 # #   Placed in the Public Domain. # # A slightly different version of this script appeared in #+ Ed Schaefer's July, 2002 "Shell Corner" column #+ in "Unix Review" on-line, #+ http://www.unixreview.com/documents/uni1026336632258/ # =======================================================   ARGCOUNT=1                     # Need name as argument. E_WRONGARGS=90  if [ $# -ne "$ARGCOUNT" ] then   echo "Usage: `basename $0` name"   exit $E_WRONGARGS fi     assign_value ()                #  Assigns numerical value {                              #+ to letters of name.    val1=bfpv                    # 'b,f,p,v' = 1   val2=cgjkqsxz                # 'c,g,j,k,q,s,x,z' = 2   val3=dt                      #  etc.   val4=l   val5=mn   val6=r  # Exceptionally clever use of 'tr' follows. # Try to figure out what is going on here.  value=$( echo "$1" \ | tr -d wh \ | tr $val1 1 | tr $val2 2 | tr $val3 3 \ | tr $val4 4 | tr $val5 5 | tr $val6 6 \ | tr -s 123456 \ | tr -d aeiouy )  # Assign letter values. # Remove duplicate numbers, except when separated by vowels. # Ignore vowels, except as separators, so delete them last. # Ignore 'w' and 'h', even as separators, so delete them first. # # The above command substitution lays more pipe than a plumber <g>.  }     input_name="$1" echo echo "Name = $input_name"   # Change all characters of name input to lowercase. # ------------------------------------------------ name=$( echo $input_name | tr A-Z a-z ) # ------------------------------------------------ # Just in case argument to script is mixed case.   # Prefix of soundex code: first letter of name. # --------------------------------------------   char_pos=0                     # Initialize character position.  prefix0=${name:$char_pos:1} prefix=`echo $prefix0 | tr a-z A-Z`                                # Uppercase 1st letter of soundex.  let "char_pos += 1"            # Bump character position to 2nd letter of name. name1=${name:$char_pos}   # ++++++++++++++++++++++++++ Exception Patch ++++++++++++++++++++++++++++++ #  Now, we run both the input name and the name shifted one char #+ to the right through the value-assigning function. #  If we get the same value out, that means that the first two characters #+ of the name have the same value assigned, and that one should cancel. #  However, we also need to test whether the first letter of the name #+ is a vowel or 'w' or 'h', because otherwise this would bollix things up.  char1=`echo $prefix | tr A-Z a-z`    # First letter of name, lowercased.  assign_value $name s1=$value assign_value $name1 s2=$value assign_value $char1 s3=$value s3=9$s3                              #  If first letter of name is a vowel                                      #+ or 'w' or 'h',                                      #+ then its "value" will be null (unset). 				     #+ Therefore, set it to 9, an otherwise 				     #+ unused value, which can be tested for.   if [[ "$s1" -ne "$s2" || "$s3" -eq 9 ]] then   suffix=$s2 else     suffix=${s2:$char_pos} fi   # ++++++++++++++++++++++ end Exception Patch ++++++++++++++++++++++++++++++   padding=000                    # Use at most 3 zeroes to pad.   soun=$prefix$suffix$padding    # Pad with zeroes.  MAXLEN=4                       # Truncate to maximum of 4 chars. soundex=${soun:0:$MAXLEN}  echo "Soundex = $soundex"  echo  #  The soundex code is a method of indexing and classifying names #+ by grouping together the ones that sound alike. #  The soundex code for a given name is the first letter of the name, #+ followed by a calculated three-number code. #  Similar sounding names should have almost the same soundex codes.  #   Examples: #   Smith and Smythe both have a "S-530" soundex. #   Harrison = H-625 #   Hargison = H-622 #   Harriman = H-655  #  This works out fairly well in practice, but there are numerous anomalies. # # #  The U.S. Census and certain other governmental agencies use soundex, #  as do genealogical researchers. # #  For more information, #+ see the "National Archives and Records Administration home page", #+ http://www.nara.gov/genealogy/soundex/soundex.html    # Exercise: # -------- # Simplify the "Exception Patch" section of this script.  exit 0

Example A-10. Game of Life

#!/bin/bash # life.sh: "Life in the Slow Lane" # Author: Mendel Cooper # License: GPL3  # Version 0.2:   Patched by Daniel Albers #+               to allow non-square grids as input. # Version 0.2.1: Added 2-second delay between generations.  # ##################################################################### # # This is the Bash script version of John Conway's "Game of Life".      # # "Life" is a simple implementation of cellular automata.               # # --------------------------------------------------------------------- # # On a rectangular grid, let each "cell" be either "living" or "dead."  # # Designate a living cell with a dot, and a dead one with a blank space.# #      Begin with an arbitrarily drawn dot-and-blank grid,              # #+     and let this be the starting generation: generation 0.           # # Determine each successive generation by the following rules:          # #   1) Each cell has 8 neighbors, the adjoining cells                   # #+     left, right, top, bottom, and the 4 diagonals.                   # #                                                                       # #                       123                                             # #                       4*5     The * is the cell under consideration.  # #                       678                                             # #                                                                       # # 2) A living cell with either 2 or 3 living neighbors remains alive.   # SURVIVE=2                                                               # # 3) A dead cell with 3 living neighbors comes alive, a "birth."        # BIRTH=3                                                                 # # 4) All other cases result in a dead cell for the next generation.     # # ##################################################################### #   startfile=gen0   # Read the starting generation from the file "gen0" ...                  # Default, if no other file specified when invoking script.                  # if [ -n "$1" ]   # Specify another "generation 0" file. then     startfile="$1" fi    ############################################ #  Abort script if "startfile" not specified #+ and #+ default file "gen0" not present.  E_NOSTARTFILE=86  if [ ! -e "$startfile" ] then   echo "Startfile \""$startfile"\" missing!"   exit $E_NOSTARTFILE fi ############################################   ALIVE1=. DEAD1=_                  # Represent living and dead cells in the start-up file.  #  -----------------------------------------------------# #  This script uses a 10 x 10 grid (may be increased, #+ but a large grid will slow down execution). ROWS=10 COLS=10 #  Change above two variables to match desired grid size. #  -----------------------------------------------------#  GENERATIONS=10          #  How many generations to cycle through.                         #  Adjust this upwards                         #+ if you have time on your hands.  NONE_ALIVE=85           #  Exit status on premature bailout,                         #+ if no cells left alive. DELAY=2                 #  Pause between generations. TRUE=0 FALSE=1 ALIVE=0 DEAD=1  avar=                   # Global; holds current generation. generation=0            # Initialize generation count.  # =================================================================  let "cells = $ROWS * $COLS"   # How many cells.  # Arrays containing "cells." declare -a initial declare -a current  display () {  alive=0                 # How many cells alive at any given time.                         # Initially zero.  declare -a arr arr=( `echo "$1"` )     # Convert passed arg to array.  element_count=${#arr[*]}  local i local rowcheck  for ((i=0; i<$element_count; i++)) do    # Insert newline at end of each row.   let "rowcheck = $i % COLS"   if [ "$rowcheck" -eq 0 ]   then     echo                # Newline.     echo -n "      "    # Indent.   fi      cell=${arr[i]}    if [ "$cell" = . ]   then     let "alive += 1"   fi      echo -n "$cell" | sed -e 's/_/ /g'   # Print out array, changing underscores to spaces. done    return  }  IsValid ()                            # Test if cell coordinate valid. {    if [ -z "$1"  -o -z "$2" ]          # Mandatory arguments missing?   then     return $FALSE   fi  local row local lower_limit=0                   # Disallow negative coordinate. local upper_limit local left local right  let "upper_limit = $ROWS * $COLS - 1" # Total number of cells.   if [ "$1" -lt "$lower_limit" -o "$1" -gt "$upper_limit" ] then   return $FALSE                       # Out of array bounds. fi    row=$2 let "left = $row * $COLS"             # Left limit. let "right = $left + $COLS - 1"       # Right limit.  if [ "$1" -lt "$left" -o "$1" -gt "$right" ] then   return $FALSE                       # Beyond row boundary. fi    return $TRUE                          # Valid coordinate.  }     IsAlive ()              #  Test whether cell is alive.                         #  Takes array, cell number, and {                       #+ state of cell as arguments.   GetCount "$1" $2      #  Get alive cell count in neighborhood.   local nhbd=$?    if [ "$nhbd" -eq "$BIRTH" ]  # Alive in any case.   then     return $ALIVE   fi    if [ "$3" = "." -a "$nhbd" -eq "$SURVIVE" ]   then                  # Alive only if previously alive.     return $ALIVE   fi      return $DEAD          # Defaults to dead.  }     GetCount ()             # Count live cells in passed cell's neighborhood.                         # Two arguments needed: 			# $1) variable holding array 			# $2) cell number {   local cell_number=$2   local array   local top   local center   local bottom   local r   local row   local i   local t_top   local t_cen   local t_bot   local count=0   local ROW_NHBD=3    array=( `echo "$1"` )    let "top = $cell_number - $COLS - 1"    # Set up cell neighborhood.   let "center = $cell_number - 1"   let "bottom = $cell_number + $COLS - 1"   let "r = $cell_number / $COLS"    for ((i=0; i<$ROW_NHBD; i++))           # Traverse from left to right.    do     let "t_top = $top + $i"     let "t_cen = $center + $i"     let "t_bot = $bottom + $i"       let "row = $r"                        # Count center row.     IsValid $t_cen $row                   # Valid cell position?     if [ $? -eq "$TRUE" ]     then       if [ ${array[$t_cen]} = "$ALIVE1" ] # Is it alive?       then                                # If yes, then ...         let "count += 1"                  # Increment count.       fi	     fi        let "row = $r - 1"                    # Count top row.               IsValid $t_top $row     if [ $? -eq "$TRUE" ]     then       if [ ${array[$t_top]} = "$ALIVE1" ] # Redundancy here.       then                                # Can it be optimized?         let "count += 1"       fi	     fi        let "row = $r + 1"                    # Count bottom row.     IsValid $t_bot $row     if [ $? -eq "$TRUE" ]     then       if [ ${array[$t_bot]} = "$ALIVE1" ]        then         let "count += 1"       fi	     fi      done       if [ ${array[$cell_number]} = "$ALIVE1" ]   then     let "count -= 1"        #  Make sure value of tested cell itself   fi                        #+ is not counted.     return $count    }  next_gen ()               # Update generation array. {  local array local i=0  array=( `echo "$1"` )     # Convert passed arg to array.  while [ "$i" -lt "$cells" ] do   IsAlive "$1" $i ${array[$i]}   # Is the cell alive?   if [ $? -eq "$ALIVE" ]   then                           #  If alive, then     array[$i]=.                  #+ represent the cell as a period.   else       array[$i]="_"                #  Otherwise underscore    fi                            #+ (will later be converted to space).   let "i += 1"  done      #    let "generation += 1"       # Increment generation count. ###  Why was the above line commented out?   # Set variable to pass as parameter to "display" function. avar=`echo ${array[@]}`   # Convert array back to string variable. display "$avar"           # Display it. echo; echo echo "Generation $generation  -  $alive alive"  if [ "$alive" -eq 0 ] then   echo   echo "Premature exit: no more cells alive!"   exit $NONE_ALIVE        #  No point in continuing fi                        #+ if no live cells.  }   # =========================================================  # main () # {  # Load initial array with contents of startup file. initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\ # Delete lines containing '#' comment character.            sed -e 's/\./\. /g' -e 's/_/_ /g'` ) # Remove linefeeds and insert space between elements.  clear          # Clear screen.  echo #         Title setterm -reverse on echo "=======================" setterm -reverse off echo "    $GENERATIONS generations" echo "           of" echo "\"Life in the Slow Lane\"" setterm -reverse on echo "=======================" setterm -reverse off  sleep $DELAY   # Display "splash screen" for 2 seconds.   # -------- Display first generation. -------- Gen0=`echo ${initial[@]}` display "$Gen0"           # Display only. echo; echo echo "Generation $generation  -  $alive alive" sleep $DELAY # -------------------------------------------   let "generation += 1"     # Bump generation count. echo  # ------- Display second generation. ------- Cur=`echo ${initial[@]}` next_gen "$Cur"          # Update & display. sleep $DELAY # ------------------------------------------  let "generation += 1"     # Increment generation count.  # ------ Main loop for displaying subsequent generations ------ while [ "$generation" -le "$GENERATIONS" ] do   Cur="$avar"   next_gen "$Cur"   let "generation += 1"   sleep $DELAY done # ==============================================================  echo # }  exit 0   # CEOF:EOF    # The grid in this script has a "boundary problem." # The the top, bottom, and sides border on a void of dead cells. # Exercise: Change the script to have the grid wrap around, # +         so that the left and right sides will "touch,"       # +         as will the top and bottom. # # Exercise: Create a new "gen0" file to seed this script. #           Use a 12 x 16 grid, instead of the original 10 x 10 one. #           Make the necessary changes to the script, #+          so it will run with the altered file. # # Exercise: Modify this script so that it can determine the grid size #+          from the "gen0" file, and set any variables necessary #+          for the script to run. #           This would make unnecessary any changes to variables #+          in the script for an altered grid size. # # Exercise: Optimize this script. #           It has redundant code.

Example A-11. Data file for Game of Life

# gen0 # # This is an example "generation 0" start-up file for "life.sh". # -------------------------------------------------------------- #  The "gen0" file is a 10 x 10 grid using a period (.) for live cells, #+ and an underscore (_) for dead ones. We cannot simply use spaces #+ for dead cells in this file because of a peculiarity in Bash arrays. #  [Exercise for the reader: explain this.] # # Lines beginning with a '#' are comments, and the script ignores them. __.__..___ __.._.____ ____.___.. _._______. ____._____ ..__...___ ____._____ ___...____ __.._..___ _..___..__

+++

The following script is by Mark Moraes of the University of Toronto. See the file Moraes-COPYRIGHT for permissions and restrictions. This file is included in the combined HTML/source tarball of the ABS Guide.

Example A-12. behead: Removing mail and news message headers

#! /bin/sh #  Strips off the header from a mail/News message i.e. till the first #+ empty line. #  Author: Mark Moraes, University of Toronto  # ==> These comments added by author of this document.  if [ $# -eq 0 ]; then # ==> If no command-line args present, then works on file redirected to stdin. 	sed -e '1,/^$/d' -e '/^[ 	]*$/d' 	# --> Delete empty lines and all lines until  	# --> first one beginning with white space. else # ==> If command-line args present, then work on files named. 	for i do 		sed -e '1,/^$/d' -e '/^[ 	]*$/d' $i 		# --> Ditto, as above. 	done fi  exit  # ==> Exercise: Add error checking and other options. # ==> # ==> Note that the small sed script repeats, except for the arg passed. # ==> Does it make sense to embed it in a function? Why or why not?   /*  * Copyright University of Toronto 1988, 1989.  * Written by Mark Moraes  *  * Permission is granted to anyone to use this software for any purpose on  * any computer system, and to alter it and redistribute it freely, subject  * to the following restrictions:  *  * 1. The author and the University of Toronto are not responsible   *    for the consequences of use of this software, no matter how awful,   *    even if they arise from flaws in it.  *  * 2. The origin of this software must not be misrepresented, either by  *    explicit claim or by omission.  Since few users ever read sources,  *    credits must appear in the documentation.  *  * 3. Altered versions must be plainly marked as such, and must not be  *    misrepresented as being the original software.  Since few users  *    ever read sources, credits must appear in the documentation.  *  * 4. This notice may not be removed or altered.  */

Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution operators discussed in Section 10.2.

Example A-13. password: Generating random 8-character passwords

#!/bin/bash # # #  Random password generator for Bash 2.x + #+ by Antek Sawicki <tenox@tenox.tc>, #+ who generously gave usage permission to the ABS Guide author. # # ==> Comments added by document author ==>   MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz" # ==> Password will consist of alphanumeric characters. LENGTH="8" # ==> May change 'LENGTH' for longer password.   while [ "${n:=1}" -le "$LENGTH" ] # ==> Recall that := is "default substitution" operator. # ==> So, if 'n' has not been initialized, set it to 1. do 	PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}" 	# ==> Very clever, but tricky.  	# ==> Starting from the innermost nesting... 	# ==> ${#MATRIX} returns length of array MATRIX.  	# ==> $RANDOM%${#MATRIX} returns random number between 1 	# ==> and [length of MATRIX] - 1.  	# ==> ${MATRIX:$(($RANDOM%${#MATRIX})):1} 	# ==> returns expansion of MATRIX at random position, by length 1.  	# ==> See {var:pos:len} parameter substitution in Chapter 9. 	# ==> and the associated examples.  	# ==> PASS=... simply pastes this result onto previous PASS (concatenation).  	# ==> To visualize this more clearly, uncomment the following line 	#                 echo "$PASS" 	# ==> to see PASS being built up, 	# ==> one character at a time, each iteration of the loop.  	let n+=1 	# ==> Increment 'n' for next pass. done  echo "$PASS"      # ==> Or, redirect to a file, as desired.  exit 0

James R. Van Zandt contributed this script which uses named pipes and, in his words, "really exercises quoting and escaping."

Example A-14. fifo: Making daily backups, using named pipes

#!/bin/bash # ==> Script by James R. Van Zandt, and used here with his permission.  # ==> Comments added by author of this document.       HERE=`uname -n`    # ==> hostname   THERE=bilbo   echo "starting remote backup to $THERE at `date +%r`"   # ==> `date +%r` returns time in 12-hour format, i.e. "08:08:34 PM".      # make sure /pipe really is a pipe and not a plain file   rm -rf /pipe   mkfifo /pipe       # ==> Create a "named pipe", named "/pipe" ...      # ==> 'su xyz' runs commands as user "xyz".   # ==> 'ssh' invokes secure shell (remote login client).   su xyz -c "ssh $THERE \"cat > /home/xyz/backup/${HERE}-daily.tar.gz\" < /pipe"&   cd /   tar -czf - bin boot dev etc home info lib man root sbin share usr var > /pipe   # ==> Uses named pipe, /pipe, to communicate between processes:   # ==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe.    # ==> The end result is this backs up the main directories, from / on down.    # ==>  What are the advantages of a "named pipe" in this situation,   # ==>+ as opposed to an "anonymous pipe", with |?   # ==>  Will an anonymous pipe even work here?    # ==>  Is it necessary to delete the pipe before exiting the script?   # ==>  How could that be done?     exit 0

St�phane Chazelas used the following script to demonstrate generating prime numbers without arrays.

Example A-15. Generating prime numbers using the modulo operator

#!/bin/bash # primes.sh: Generate prime numbers, without using arrays. # Script contributed by Stephane Chazelas.  #  This does *not* use the classic "Sieve of Eratosthenes" algorithm, #+ but instead the more intuitive method of testing each candidate number #+ for factors (divisors), using the "%" modulo operator.   LIMIT=1000                    # Primes, 2 ... 1000.  Primes() {  (( n = $1 + 1 ))             # Bump to next integer.  shift                        # Next parameter in list. #  echo "_n=$n i=$i_"    if (( n == LIMIT ))  then echo $*  return  fi   for i; do                    # "i" set to "@", previous values of $n. #   echo "-n=$n i=$i-"    (( i * i > n )) && break   # Optimization.    (( n % i )) && continue    # Sift out non-primes using modulo operator.    Primes $n $@               # Recursion inside loop.    return    done     Primes $n $@ $n            #  Recursion outside loop.                               #  Successively accumulate 			      #+ positional parameters.                               #  "$@" is the accumulating list of primes. }  Primes 1  exit $?  # Pipe output of the script to 'fmt' for prettier printing.  #  Uncomment lines 16 and 24 to help figure out what is going on.  #  Compare the speed of this algorithm for generating primes #+ with the Sieve of Eratosthenes (ex68.sh).   #  Exercise: Rewrite this script without recursion.

Rick Boivie's revision of Jordi Sanfeliu's tree script.

Example A-16. tree: Displaying a directory tree

#!/bin/bash # tree.sh  #  Written by Rick Boivie. #  Used with permission. #  This is a revised and simplified version of a script #+ by Jordi Sanfeliu (the original author), and patched by Ian Kjos. #  This script replaces the earlier version used in #+ previous releases of the Advanced Bash Scripting Guide. #  Copyright (c) 2002, by Jordi Sanfeliu, Rick Boivie, and Ian Kjos.  # ==> Comments added by the author of this document.   search () { for dir in `echo *` #  ==> `echo *` lists all the files in current working directory, #+ ==> without line breaks. #  ==> Similar effect to for dir in * #  ==> but "dir in `echo *`" will not handle filenames with blanks. do   if [ -d "$dir" ] ; then # ==> If it is a directory (-d)...   zz=0                    # ==> Temp variable, keeping track of                           #     directory level.   while [ $zz != $1 ]     # Keep track of inner nested loop.     do       echo -n "| "        # ==> Display vertical connector symbol,                           # ==> with 2 spaces & no line feed                           #     in order to indent.       zz=`expr $zz + 1`   # ==> Increment zz.     done      if [ -L "$dir" ] ; then # ==> If directory is a symbolic link...       echo "+---$dir" `ls -l $dir | sed 's/^.*'$dir' //'`       # ==> Display horiz. connector and list directory name, but...       # ==> delete date/time part of long listing.     else       echo "+---$dir"       # ==> Display horizontal connector symbol...       # ==> and print directory name.       numdirs=`expr $numdirs + 1` # ==> Increment directory count.       if cd "$dir" ; then         # ==> If can move to subdirectory...         search `expr $1 + 1`      # with recursion ;-)         # ==> Function calls itself.         cd ..       fi     fi   fi done }  if [ $# != 0 ] ; then   cd $1   # Move to indicated directory.   #else   # stay in current directory fi  echo "Initial directory = `pwd`" numdirs=0  search 0 echo "Total directories = $numdirs"  exit 0

Patsie's version of a directory tree script.

Example A-17. tree2: Alternate directory tree script

#!/bin/bash # tree2.sh  # Lightly modified/reformatted by ABS Guide author. # Included in ABS Guide with permission of script author (thanks!).  ## Recursive file/dirsize checking script, by Patsie ## ## This script builds a list of files/directories and their size (du -akx) ## and processes this list to a human readable tree shape ## The 'du -akx' is only as good as the permissions the owner has. ## So preferably run as root* to get the best results, or use only on ## directories for which you have read permissions. Anything you can't ## read is not in the list.  #* ABS Guide author advises caution when running scripts as root!   ##########  THIS IS CONFIGURABLE  ##########  TOP=5                   # Top 5 biggest (sub)directories. MAXRECURS=5             # Max 5 subdirectories/recursions deep. E_BL=80                 # Blank line already returned. E_DIR=81                # Directory not specified.   ##########  DON'T CHANGE ANYTHING BELOW THIS LINE  ##########  PID=$$                            # Our own process ID. SELF=`basename $0`                # Our own program name. TMP="/tmp/${SELF}.${PID}.tmp"     # Temporary 'du' result.  # Convert number to dotted thousand. function dot { echo "            $*" |                sed -e :a -e 's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta' |                tail -c 12; }  # Usage: tree <recursion> <indent prefix> <min size> <directory> function tree {   recurs="$1"           # How deep nested are we?   prefix="$2"           # What do we display before file/dirname?   minsize="$3"          # What is the minumum file/dirsize?   dirname="$4"          # Which directory are we checking?  # Get ($TOP) biggest subdirs/subfiles from TMP file.   LIST=`egrep "[[:space:]]${dirname}/[^/]*$" "$TMP" |         awk '{if($1>'$minsize') print;}' | sort -nr | head -$TOP`   [ -z "$LIST" ] && return        # Empty list, then go back.    cnt=0   num=`echo "$LIST" | wc -l`      # How many entries in the list.    ## Main loop   echo "$LIST" | while read size name; do     ((cnt+=1))		          # Count entry number.     bname=`basename "$name"`      # We only need a basename of the entry.     [ -d "$name" ] && bname="$bname/"                                   # If it's a directory, append a slash.     echo "`dot $size`$prefix +-$bname"                                   # Display the result.     #  Call ourself recursively if it's a directory     #+ and we're not nested too deep ($MAXRECURS).     #  The recursion goes up: $((recurs+1))     #  The prefix gets a space if it's the last entry,     #+ or a pipe if there are more entries.     #  The minimum file/dirsize becomes     #+ a tenth of his parent: $((size/10)).     # Last argument is the full directory name to check.     if [ -d "$name" -a $recurs -lt $MAXRECURS ]; then       [ $cnt -lt $num ] \         || (tree $((recurs+1)) "$prefix  " $((size/10)) "$name") \         && (tree $((recurs+1)) "$prefix |" $((size/10)) "$name")     fi   done    [ $? -eq 0 ] && echo "           $prefix"   # Every time we jump back add a 'blank' line.   return $E_BL   # We return 80 to tell we added a blank line already. }  ###                ### ###  main program  ### ###                ###  rootdir="$@" [ -d "$rootdir" ] ||   { echo "$SELF: Usage: $SELF <directory>" >&2; exit $E_DIR; }   # We should be called with a directory name.  echo "Building inventory list, please wait ..."      # Show "please wait" message. du -akx "$rootdir" 1>"$TMP" 2>/dev/null      # Build a temporary list of all files/dirs and their size. size=`tail -1 "$TMP" | awk '{print $1}'`      # What is our rootdirectory's size? echo "`dot $size` $rootdir"      # Display rootdirectory's entry. tree 0 "" 0 "$rootdir"      # Display the tree below our rootdirectory.  rm "$TMP" 2>/dev/null      # Clean up TMP file.  exit $?

Noah Friedman permitted use of his string function script. It essentially reproduces some of the C-library string manipulation functions.

Example A-18. string functions: C-style string functions

#!/bin/bash  # string.bash --- bash emulation of string(3) library routines # Author: Noah Friedman <friedman@prep.ai.mit.edu> # ==>     Used with his kind permission in this document. # Created: 1992-07-01 # Last modified: 1993-09-29 # Public domain  # Conversion to bash v2 syntax done by Chet Ramey  # Commentary: # Code:  #:docstring strcat: # Usage: strcat s1 s2 # # Strcat appends the value of variable s2 to variable s1.  # # Example: #    a="foo" #    b="bar" #    strcat a b #    echo $a #    => foobar # #:end docstring:  ###;;;autoload   ==> Autoloading of function commented out. function strcat () {     local s1_val s2_val      s1_val=${!1}                        # indirect variable expansion     s2_val=${!2}     eval "$1"=\'"${s1_val}${s2_val}"\'     # ==> eval $1='${s1_val}${s2_val}' avoids problems,     # ==> if one of the variables contains a single quote. }  #:docstring strncat: # Usage: strncat s1 s2 $n #  # Line strcat, but strncat appends a maximum of n characters from the value # of variable s2.  It copies fewer if the value of variabl s2 is shorter # than n characters.  Echoes result on stdout. # # Example: #    a=foo #    b=barbaz #    strncat a b 3 #    echo $a #    => foobar # #:end docstring:  ###;;;autoload function strncat () {     local s1="$1"     local s2="$2"     local -i n="$3"     local s1_val s2_val      s1_val=${!s1}                       # ==> indirect variable expansion     s2_val=${!s2}      if [ ${#s2_val} -gt ${n} ]; then        s2_val=${s2_val:0:$n}            # ==> substring extraction     fi      eval "$s1"=\'"${s1_val}${s2_val}"\'     # ==> eval $1='${s1_val}${s2_val}' avoids problems,     # ==> if one of the variables contains a single quote. }  #:docstring strcmp: # Usage: strcmp $s1 $s2 # # Strcmp compares its arguments and returns an integer less than, equal to, # or greater than zero, depending on whether string s1 is lexicographically # less than, equal to, or greater than string s2. #:end docstring:  ###;;;autoload function strcmp () {     [ "$1" = "$2" ] && return 0      [ "${1}" '<' "${2}" ] > /dev/null && return -1      return 1 }  #:docstring strncmp: # Usage: strncmp $s1 $s2 $n #  # Like strcmp, but makes the comparison by examining a maximum of n # characters (n less than or equal to zero yields equality). #:end docstring:  ###;;;autoload function strncmp () {     if [ -z "${3}" -o "${3}" -le "0" ]; then        return 0     fi         if [ ${3} -ge ${#1} -a ${3} -ge ${#2} ]; then        strcmp "$1" "$2"        return $?     else        s1=${1:0:$3}        s2=${2:0:$3}        strcmp $s1 $s2        return $?     fi }  #:docstring strlen: # Usage: strlen s # # Strlen returns the number of characters in string literal s. #:end docstring:  ###;;;autoload function strlen () {     eval echo "\${#${1}}"     # ==> Returns the length of the value of the variable     # ==> whose name is passed as an argument. }  #:docstring strspn: # Usage: strspn $s1 $s2 #  # Strspn returns the length of the maximum initial segment of string s1, # which consists entirely of characters from string s2. #:end docstring:  ###;;;autoload function strspn () {     # Unsetting IFS allows whitespace to be handled as normal chars.      local IFS=     local result="${1%%[!${2}]*}"       echo ${#result} }  #:docstring strcspn: # Usage: strcspn $s1 $s2 # # Strcspn returns the length of the maximum initial segment of string s1, # which consists entirely of characters not from string s2. #:end docstring:  ###;;;autoload function strcspn () {     # Unsetting IFS allows whitspace to be handled as normal chars.      local IFS=     local result="${1%%[${2}]*}"       echo ${#result} }  #:docstring strstr: # Usage: strstr s1 s2 #  # Strstr echoes a substring starting at the first occurrence of string s2 in # string s1, or nothing if s2 does not occur in the string.  If s2 points to # a string of zero length, strstr echoes s1. #:end docstring:  ###;;;autoload function strstr () {     # if s2 points to a string of zero length, strstr echoes s1     [ ${#2} -eq 0 ] && { echo "$1" ; return 0; }      # strstr echoes nothing if s2 does not occur in s1     case "$1" in     *$2*) ;;     *) return 1;;     esac      # use the pattern matching code to strip off the match and everything     # following it     first=${1/$2*/}      # then strip off the first unmatched portion of the string     echo "${1##$first}" }  #:docstring strtok: # Usage: strtok s1 s2 # # Strtok considers the string s1 to consist of a sequence of zero or more # text tokens separated by spans of one or more characters from the # separator string s2.  The first call (with a non-empty string s1 # specified) echoes a string consisting of the first token on stdout. The # function keeps track of its position in the string s1 between separate # calls, so that subsequent calls made with the first argument an empty # string will work through the string immediately following that token.  In # this way subsequent calls will work through the string s1 until no tokens # remain.  The separator string s2 may be different from call to call. # When no token remains in s1, an empty value is echoed on stdout. #:end docstring:  ###;;;autoload function strtok () {  : }  #:docstring strtrunc: # Usage: strtrunc $n $s1 {$s2} {$...} # # Used by many functions like strncmp to truncate arguments for comparison. # Echoes the first n characters of each string s1 s2 ... on stdout.  #:end docstring:  ###;;;autoload function strtrunc () {     n=$1 ; shift     for z; do         echo "${z:0:$n}"     done }  # provide string  # string.bash ends here   # ========================================================================== # # ==> Everything below here added by the document author.  # ==> Suggested use of this script is to delete everything below here, # ==> and "source" this file into your own scripts.  # strcat string0=one string1=two echo echo "Testing \"strcat\" function:" echo "Original \"string0\" = $string0" echo "\"string1\" = $string1" strcat string0 string1 echo "New \"string0\" = $string0" echo  # strlen echo echo "Testing \"strlen\" function:" str=123456789 echo "\"str\" = $str" echo -n "Length of \"str\" = " strlen str echo    # Exercise: # -------- # Add code to test all the other string functions above.   exit 0

Michael Zick's complex array example uses the md5sum check sum command to encode directory information.

Example A-19. Directory information

#! /bin/bash # directory-info.sh # Parses and lists directory information.  # NOTE: Change lines 273 and 353 per "README" file.  # Michael Zick is the author of this script. # Used here with his permission.  # Controls # If overridden by command arguments, they must be in the order: #   Arg1: "Descriptor Directory" #   Arg2: "Exclude Paths" #   Arg3: "Exclude Directories" # # Environment Settings override Defaults. # Command arguments override Environment Settings.  # Default location for content addressed file descriptors. MD5UCFS=${1:-${MD5UCFS:-'/tmpfs/ucfs'}}  # Directory paths never to list or enter declare -a \   EXCLUDE_PATHS=${2:-${EXCLUDE_PATHS:-'(/proc /dev /devfs /tmpfs)'}}  # Directories never to list or enter declare -a \   EXCLUDE_DIRS=${3:-${EXCLUDE_DIRS:-'(ucfs lost+found tmp wtmp)'}}  # Files never to list or enter declare -a \   EXCLUDE_FILES=${3:-${EXCLUDE_FILES:-'(core "Name with Spaces")'}}   # Here document used as a comment block. : <<LSfieldsDoc # # # # # List Filesystem Directory Information # # # # # # #	ListDirectory "FileGlob" "Field-Array-Name" # or #	ListDirectory -of "FileGlob" "Field-Array-Filename" #	'-of' meaning 'output to filename' # # # # #  String format description based on: ls (GNU fileutils) version 4.0.36  Produces a line (or more) formatted: inode permissions hard-links owner group ... 32736 -rw-------    1 mszick   mszick  size    day month date hh:mm:ss year path 2756608 Sun Apr 20 08:53:06 2003 /home/mszick/core  Unless it is formatted: inode permissions hard-links owner group ... 266705 crw-rw----    1    root  uucp  major minor day month date hh:mm:ss year path 4,  68 Sun Apr 20 09:27:33 2003 /dev/ttyS4 NOTE: that pesky comma after the major number  NOTE: the 'path' may be multiple fields: /home/mszick/core /proc/982/fd/0 -> /dev/null /proc/982/fd/1 -> /home/mszick/.xsession-errors /proc/982/fd/13 -> /tmp/tmpfZVVOCs (deleted) /proc/982/fd/7 -> /tmp/kde-mszick/ksycoca /proc/982/fd/8 -> socket:[11586] /proc/982/fd/9 -> pipe:[11588]  If that isn't enough to keep your parser guessing, either or both of the path components may be relative: ../Built-Shared -> Built-Static ../linux-2.4.20.tar.bz2 -> ../../../SRCS/linux-2.4.20.tar.bz2  The first character of the 11 (10?) character permissions field: 's' Socket 'd' Directory 'b' Block device 'c' Character device 'l' Symbolic link NOTE: Hard links not marked - test for identical inode numbers on identical filesystems. All information about hard linked files are shared, except for the names and the name's location in the directory system. NOTE: A "Hard link" is known as a "File Alias" on some systems. '-' An undistingushed file  Followed by three groups of letters for: User, Group, Others Character 1: '-' Not readable; 'r' Readable Character 2: '-' Not writable; 'w' Writable Character 3, User and Group: Combined execute and special '-' Not Executable, Not Special 'x' Executable, Not Special 's' Executable, Special 'S' Not Executable, Special Character 3, Others: Combined execute and sticky (tacky?) '-' Not Executable, Not Tacky 'x' Executable, Not Tacky 't' Executable, Tacky 'T' Not Executable, Tacky  Followed by an access indicator Haven't tested this one, it may be the eleventh character or it may generate another field ' ' No alternate access '+' Alternate access LSfieldsDoc   ListDirectory() { 	local -a T 	local -i of=0		# Default return in variable #	OLD_IFS=$IFS		# Using BASH default ' \t\n'  	case "$#" in 	3)	case "$1" in 		-of)	of=1 ; shift ;; 		 * )	return 1 ;; 		esac ;; 	2)	: ;;		# Poor man's "continue" 	*)	return 1 ;; 	esac  	# NOTE: the (ls) command is NOT quoted (") 	T=( $(ls --inode --ignore-backups --almost-all --directory \ 	--full-time --color=none --time=status --sort=none \ 	--format=long $1) )  	case $of in 	# Assign T back to the array whose name was passed as $2 		0) eval $2=\( \"\$\{T\[@\]\}\" \) ;; 	# Write T into filename passed as $2 		1) echo "${T[@]}" > "$2" ;; 	esac 	return 0    }  # # # # # Is that string a legal number? # # # # # # #	IsNumber "Var" # # # # # There has to be a better way, sigh...  IsNumber() { 	local -i int 	if [ $# -eq 0 ] 	then 		return 1 	else 		(let int=$1)  2>/dev/null 		return $?	# Exit status of the let thread 	fi }  # # # # # Index Filesystem Directory Information # # # # # # #	IndexList "Field-Array-Name" "Index-Array-Name" # or #	IndexList -if Field-Array-Filename Index-Array-Name #	IndexList -of Field-Array-Name Index-Array-Filename #	IndexList -if -of Field-Array-Filename Index-Array-Filename # # # # #  : <<IndexListDoc Walk an array of directory fields produced by ListDirectory  Having suppressed the line breaks in an otherwise line oriented report, build an index to the array element which starts each line.  Each line gets two index entries, the first element of each line (inode) and the element that holds the pathname of the file.  The first index entry pair (Line-Number==0) are informational: Index-Array-Name[0] : Number of "Lines" indexed Index-Array-Name[1] : "Current Line" pointer into Index-Array-Name  The following index pairs (if any) hold element indexes into the Field-Array-Name per: Index-Array-Name[Line-Number * 2] : The "inode" field element. NOTE: This distance may be either +11 or +12 elements. Index-Array-Name[(Line-Number * 2) + 1] : The "pathname" element. NOTE: This distance may be a variable number of elements. Next line index pair for Line-Number+1. IndexListDoc    IndexList() { 	local -a LIST			# Local of listname passed 	local -a -i INDEX=( 0 0 )	# Local of index to return 	local -i Lidx Lcnt 	local -i if=0 of=0		# Default to variable names  	case "$#" in			# Simplistic option testing 		0) return 1 ;; 		1) return 1 ;; 		2) : ;;			# Poor man's continue 		3) case "$1" in 			-if) if=1 ;; 			-of) of=1 ;; 			 * ) return 1 ;; 		   esac ; shift ;; 		4) if=1 ; of=1 ; shift ; shift ;; 		*) return 1 	esac  	# Make local copy of list 	case "$if" in 		0) eval LIST=\( \"\$\{$1\[@\]\}\" \) ;; 		1) LIST=( $(cat $1) ) ;; 	esac  	# Grok (grope?) the array 	Lcnt=${#LIST[@]} 	Lidx=0 	until (( Lidx >= Lcnt )) 	do 	if IsNumber ${LIST[$Lidx]} 	then 		local -i inode name 		local ft 		inode=Lidx 		local m=${LIST[$Lidx+2]}	# Hard Links field 		ft=${LIST[$Lidx+1]:0:1} 	# Fast-Stat 		case $ft in 		b)	((Lidx+=12)) ;;		# Block device 		c)	((Lidx+=12)) ;;		# Character device 		*)	((Lidx+=11)) ;;		# Anything else 		esac 		name=Lidx 		case $ft in 		-)	((Lidx+=1)) ;;		# The easy one 		b)	((Lidx+=1)) ;;		# Block device 		c)	((Lidx+=1)) ;;		# Character device 		d)	((Lidx+=1)) ;;		# The other easy one 		l)	((Lidx+=3)) ;;		# At LEAST two more fields #  A little more elegance here would handle pipes, #+ sockets, deleted files - later. 		*)	until IsNumber ${LIST[$Lidx]} || ((Lidx >= Lcnt)) 			do 				((Lidx+=1)) 			done 			;;			# Not required 		esac 		INDEX[${#INDEX[*]}]=$inode 		INDEX[${#INDEX[*]}]=$name 		INDEX[0]=${INDEX[0]}+1		# One more "line" found # echo "Line: ${INDEX[0]} Type: $ft Links: $m Inode: \ # ${LIST[$inode]} Name: ${LIST[$name]}"  	else 		((Lidx+=1)) 	fi 	done 	case "$of" in 		0) eval $2=\( \"\$\{INDEX\[@\]\}\" \) ;; 		1) echo "${INDEX[@]}" > "$2" ;; 	esac 	return 0				# What could go wrong? }  # # # # # Content Identify File # # # # # # #	DigestFile Input-Array-Name Digest-Array-Name # or #	DigestFile -if Input-FileName Digest-Array-Name # # # # #  # Here document used as a comment block. : <<DigestFilesDoc  The key (no pun intended) to a Unified Content File System (UCFS) is to distinguish the files in the system based on their content. Distinguishing files by their name is just so 20th Century.  The content is distinguished by computing a checksum of that content. This version uses the md5sum program to generate a 128 bit checksum representative of the file's contents. There is a chance that two files having different content might generate the same checksum using md5sum (or any checksum).  Should that become a problem, then the use of md5sum can be replace by a cyrptographic signature.  But until then...  The md5sum program is documented as outputting three fields (and it does), but when read it appears as two fields (array elements).  This is caused by the lack of whitespace between the second and third field. So this function gropes the md5sum output and returns: 	[0]	32 character checksum in hexidecimal (UCFS filename) 	[1]	Single character: ' ' text file, '*' binary file 	[2]	Filesystem (20th Century Style) name 	Note: That name may be the character '-' indicating STDIN read.  DigestFilesDoc    DigestFile() { 	local if=0		# Default, variable name 	local -a T1 T2  	case "$#" in 	3)	case "$1" in 		-if)	if=1 ; shift ;; 		 * )	return 1 ;; 		esac ;; 	2)	: ;;		# Poor man's "continue" 	*)	return 1 ;; 	esac  	case $if in 	0) eval T1=\( \"\$\{$1\[@\]\}\" \) 	   T2=( $(echo ${T1[@]} | md5sum -) ) 	   ;; 	1) T2=( $(md5sum $1) ) 	   ;; 	esac  	case ${#T2[@]} in 	0) return 1 ;; 	1) return 1 ;; 	2) case ${T2[1]:0:1} in		# SanScrit-2.0.5 	   \*) T2[${#T2[@]}]=${T2[1]:1} 	       T2[1]=\* 	       ;; 	    *) T2[${#T2[@]}]=${T2[1]} 	       T2[1]=" " 	       ;; 	   esac 	   ;; 	3) : ;; # Assume it worked 	*) return 1 ;; 	esac  	local -i len=${#T2[0]} 	if [ $len -ne 32 ] ; then return 1 ; fi 	eval $2=\( \"\$\{T2\[@\]\}\" \) }  # # # # # Locate File # # # # # # #	LocateFile [-l] FileName Location-Array-Name # or #	LocateFile [-l] -of FileName Location-Array-FileName # # # # #  # A file location is Filesystem-id and inode-number  # Here document used as a comment block. : <<StatFieldsDoc 	Based on stat, version 2.2 	stat -t and stat -lt fields 	[0]	name 	[1]	Total size 		File - number of bytes 		Symbolic link - string length of pathname 	[2]	Number of (512 byte) blocks allocated 	[3]	File type and Access rights (hex) 	[4]	User ID of owner 	[5]	Group ID of owner 	[6]	Device number 	[7]	Inode number 	[8]	Number of hard links 	[9]	Device type (if inode device) Major 	[10]	Device type (if inode device) Minor 	[11]	Time of last access 		May be disabled in 'mount' with noatime 		atime of files changed by exec, read, pipe, utime, mknod (mmap?) 		atime of directories changed by addition/deletion of files 	[12]	Time of last modification 		mtime of files changed by write, truncate, utime, mknod 		mtime of directories changed by addtition/deletion of files 	[13]	Time of last change 		ctime reflects time of changed inode information (owner, group 		permissions, link count -*-*- Per: 	Return code: 0 	Size of array: 14 	Contents of array 	Element 0: /home/mszick 	Element 1: 4096 	Element 2: 8 	Element 3: 41e8 	Element 4: 500 	Element 5: 500 	Element 6: 303 	Element 7: 32385 	Element 8: 22 	Element 9: 0 	Element 10: 0 	Element 11: 1051221030 	Element 12: 1051214068 	Element 13: 1051214068  	For a link in the form of linkname -> realname 	stat -t  linkname returns the linkname (link) information 	stat -lt linkname returns the realname information  	stat -tf and stat -ltf fields 	[0]	name 	[1]	ID-0?		# Maybe someday, but Linux stat structure 	[2]	ID-0?		# does not have either LABEL nor UUID 				# fields, currently information must come 				# from file-system specific utilities 	These will be munged into: 	[1]	UUID if possible 	[2]	Volume Label if possible 	Note: 'mount -l' does return the label and could return the UUID  	[3]	Maximum length of filenames 	[4]	Filesystem type 	[5]	Total blocks in the filesystem 	[6]	Free blocks 	[7]	Free blocks for non-root user(s) 	[8]	Block size of the filesystem 	[9]	Total inodes 	[10]	Free inodes  -*-*- Per: 	Return code: 0 	Size of array: 11 	Contents of array 	Element 0: /home/mszick 	Element 1: 0 	Element 2: 0 	Element 3: 255 	Element 4: ef53 	Element 5: 2581445 	Element 6: 2277180 	Element 7: 2146050 	Element 8: 4096 	Element 9: 1311552 	Element 10: 1276425  StatFieldsDoc   #	LocateFile [-l] FileName Location-Array-Name #	LocateFile [-l] -of FileName Location-Array-FileName  LocateFile() { 	local -a LOC LOC1 LOC2 	local lk="" of=0  	case "$#" in 	0) return 1 ;; 	1) return 1 ;; 	2) : ;; 	*) while (( "$#" > 2 )) 	   do 	      case "$1" in 	       -l) lk=-1 ;; 	      -of) of=1 ;; 	        *) return 1 ;; 	      esac 	   shift            done ;; 	esac  # More Sanscrit-2.0.5       # LOC1=( $(stat -t $lk $1) )       # LOC2=( $(stat -tf $lk $1) )       # Uncomment above two lines if system has "stat" command installed. 	LOC=( ${LOC1[@]:0:1} ${LOC1[@]:3:11} 	      ${LOC2[@]:1:2} ${LOC2[@]:4:1} )  	case "$of" in 		0) eval $2=\( \"\$\{LOC\[@\]\}\" \) ;; 		1) echo "${LOC[@]}" > "$2" ;; 	esac 	return 0 # Which yields (if you are lucky, and have "stat" installed) # -*-*- Location Discriptor -*-*- #	Return code: 0 #	Size of array: 15 #	Contents of array #	Element 0: /home/mszick		20th Century name #	Element 1: 41e8			Type and Permissions #	Element 2: 500			User #	Element 3: 500			Group #	Element 4: 303			Device #	Element 5: 32385		inode #	Element 6: 22			Link count #	Element 7: 0			Device Major #	Element 8: 0			Device Minor #	Element 9: 1051224608		Last Access #	Element 10: 1051214068		Last Modify #	Element 11: 1051214068		Last Status #	Element 12: 0			UUID (to be) #	Element 13: 0			Volume Label (to be) #	Element 14: ef53		Filesystem type }    # And then there was some test code  ListArray() # ListArray Name { 	local -a Ta  	eval Ta=\( \"\$\{$1\[@\]\}\" \) 	echo 	echo "-*-*- List of Array -*-*-" 	echo "Size of array $1: ${#Ta[*]}" 	echo "Contents of array $1:" 	for (( i=0 ; i<${#Ta[*]} ; i++ )) 	do 	    echo -e "\tElement $i: ${Ta[$i]}" 	done 	return 0 }  declare -a CUR_DIR # For small arrays ListDirectory "${PWD}" CUR_DIR ListArray CUR_DIR  declare -a DIR_DIG DigestFile CUR_DIR DIR_DIG echo "The new \"name\" (checksum) for ${CUR_DIR[9]} is ${DIR_DIG[0]}"  declare -a DIR_ENT # BIG_DIR # For really big arrays - use a temporary file in ramdisk # BIG-DIR # ListDirectory -of "${CUR_DIR[11]}/*" "/tmpfs/junk2" ListDirectory "${CUR_DIR[11]}/*" DIR_ENT  declare -a DIR_IDX # BIG-DIR # IndexList -if "/tmpfs/junk2" DIR_IDX IndexList DIR_ENT DIR_IDX  declare -a IDX_DIG # BIG-DIR # DIR_ENT=( $(cat /tmpfs/junk2) ) # BIG-DIR # DigestFile -if /tmpfs/junk2 IDX_DIG DigestFile DIR_ENT IDX_DIG # Small (should) be able to parallize IndexList & DigestFile # Large (should) be able to parallize IndexList & DigestFile & the assignment echo "The \"name\" (checksum) for the contents of ${PWD} is ${IDX_DIG[0]}"  declare -a FILE_LOC LocateFile ${PWD} FILE_LOC ListArray FILE_LOC  exit 0

St�phane Chazelas demonstrates object-oriented programming in a Bash script.

Mariusz Gniazdowski contributed a hash library for use in scripts.

Example A-20. Library of hash functions

# Hash: # Hash function library # Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com> # Date: 2005-04-07  # Functions making emulating hashes in Bash a little less painful.   #    Limitations: #  * Only global variables are supported. #  * Each hash instance generates one global variable per value. #  * Variable names collisions are possible #+   if you define variable like __hash__hashname_key #  * Keys must use chars that can be part of a Bash variable name #+   (no dashes, periods, etc.). #  * The hash is created as a variable: #    ... hashname_keyname #    So if somone will create hashes like: #      myhash_ + mykey = myhash__mykey #      myhash + _mykey = myhash__mykey #    Then there will be a collision. #    (This should not pose a major problem.)   Hash_config_varname_prefix=__hash__   # Emulates:  hash[key]=value # # Params: # 1 - hash # 2 - key # 3 - value function hash_set { 	eval "${Hash_config_varname_prefix}${1}_${2}=\"${3}\"" }   # Emulates:  value=hash[key] # # Params: # 1 - hash # 2 - key # 3 - value (name of global variable to set) function hash_get_into { 	eval "$3=\"\$${Hash_config_varname_prefix}${1}_${2}\"" }   # Emulates:  echo hash[key] # # Params: # 1 - hash # 2 - key # 3 - echo params (like -n, for example) function hash_echo { 	eval "echo $3 \"\$${Hash_config_varname_prefix}${1}_${2}\"" }   # Emulates:  hash1[key1]=hash2[key2] # # Params: # 1 - hash1 # 2 - key1 # 3 - hash2 # 4 - key2 function hash_copy { eval "${Hash_config_varname_prefix}${1}_${2}\ =\"\$${Hash_config_varname_prefix}${3}_${4}\"" }   # Emulates:  hash[keyN-1]=hash[key2]=...hash[key1] # # Copies first key to rest of keys. # # Params: # 1 - hash1 # 2 - key1 # 3 - key2 # . . . # N - keyN function hash_dup {   local hashName="$1" keyName="$2"   shift 2   until [ ${#} -le 0 ]; do     eval "${Hash_config_varname_prefix}${hashName}_${1}\ =\"\$${Hash_config_varname_prefix}${hashName}_${keyName}\""   shift;   done; }   # Emulates:  unset hash[key] # # Params: # 1 - hash # 2 - key function hash_unset { 	eval "unset ${Hash_config_varname_prefix}${1}_${2}" }   # Emulates something similar to:  ref=&hash[key] # # The reference is name of the variable in which value is held. # # Params: # 1 - hash # 2 - key # 3 - ref - Name of global variable to set. function hash_get_ref_into { 	eval "$3=\"${Hash_config_varname_prefix}${1}_${2}\"" }   # Emulates something similar to:  echo &hash[key] # # That reference is name of variable in which value is held. # # Params: # 1 - hash # 2 - key # 3 - echo params (like -n for example) function hash_echo_ref { 	eval "echo $3 \"${Hash_config_varname_prefix}${1}_${2}\"" }    # Emulates something similar to:  $$hash[key](param1, param2, ...) # # Params: # 1 - hash # 2 - key # 3,4, ... - Function parameters function hash_call {   local hash key   hash=$1   key=$2   shift 2   eval "eval \"\$${Hash_config_varname_prefix}${hash}_${key} \\\"\\\$@\\\"\"" }   # Emulates something similar to:  isset(hash[key]) or hash[key]==NULL # # Params: # 1 - hash # 2 - key # Returns: # 0 - there is such key # 1 - there is no such key function hash_is_set {   eval "if [[ \"\${${Hash_config_varname_prefix}${1}_${2}-a}\" = \"a\" &&    \"\${${Hash_config_varname_prefix}${1}_${2}-b}\" = \"b\" ]]     then return 1; else return 0; fi" }   # Emulates something similar to: #   foreach($hash as $key => $value) { fun($key,$value); } # # It is possible to write different variations of this function. # Here we use a function call to make it as "generic" as possible. # # Params: # 1 - hash # 2 - function name function hash_foreach {   local keyname oldIFS="$IFS"   IFS=' '   for i in $(eval "echo \${!${Hash_config_varname_prefix}${1}_*}"); do     keyname=$(eval "echo \${i##${Hash_config_varname_prefix}${1}_}")     eval "$2 $keyname \"\$$i\""   done IFS="$oldIFS" }  #  NOTE: In lines 103 and 116, ampersand changed. #  But, it doesn't matter, because these are comment lines anyhow.

Here is an example script using the foregoing hash library.

Example A-21. Colorizing text using hash functions

#!/bin/bash # hash-example.sh: Colorizing text. # Author: Mariusz Gniazdowski <mariusz.gn-at-gmail.com>  . Hash.lib      # Load the library of functions.  hash_set colors red          "\033[0;31m" hash_set colors blue         "\033[0;34m" hash_set colors light_blue   "\033[1;34m" hash_set colors light_red    "\033[1;31m" hash_set colors cyan         "\033[0;36m" hash_set colors light_green  "\033[1;32m" hash_set colors light_gray   "\033[0;37m" hash_set colors green        "\033[0;32m" hash_set colors yellow       "\033[1;33m" hash_set colors light_purple "\033[1;35m" hash_set colors purple       "\033[0;35m" hash_set colors reset_color  "\033[0;00m"   # $1 - keyname # $2 - value try_colors() { 	echo -en "$2" 	echo "This line is $1." } hash_foreach colors try_colors hash_echo colors reset_color -en  echo -e '\nLet us overwrite some colors with yellow.\n' # It's hard to read yellow text on some terminals. hash_dup colors yellow   red light_green blue green light_gray cyan hash_foreach colors try_colors hash_echo colors reset_color -en  echo -e '\nLet us delete them and try colors once more . . .\n'  for i in red light_green blue green light_gray cyan; do 	hash_unset colors $i done hash_foreach colors try_colors hash_echo colors reset_color -en  hash_set other txt "Other examples . . ." hash_echo other txt hash_get_into other txt text echo $text  hash_set other my_fun try_colors hash_call other my_fun   purple "`hash_echo colors purple`" hash_echo colors reset_color -en  echo; echo "Back to normal?"; echo  exit $?  #  On some terminals, the "light" colors print in bold, #  and end up looking darker than the normal ones. #  Why is this?

An example illustrating the mechanics of hashing, but from a different point of view.

Example A-22. More on hash functions

#!/bin/bash # $Id: ha.sh,v 1.2 2005/04/21 23:24:26 oliver Exp $ # Copyright 2005 Oliver Beckstein # Released under the GNU Public License # Author of script granted permission for inclusion in ABS Guide. # (Thank you!)  #---------------------------------------------------------------- # pseudo hash based on indirect parameter expansion # API: access through functions: #  # create the hash: #   #      newhash Lovers # # add entries (note single quotes for spaces) #     #      addhash Lovers Tristan Isolde #      addhash Lovers 'Romeo Montague' 'Juliet Capulet' # # access value by key # #      gethash Lovers Tristan   ---->  Isolde # # show all keys # #      keyshash Lovers         ----> 'Tristan'  'Romeo Montague' # # # Convention: instead of perls' foo{bar} = boing' syntax, # use #       '_foo_bar=boing' (two underscores, no spaces) # # 1) store key   in _NAME_keys[] # 2) store value in _NAME_values[] using the same integer index # The integer index for the last entry is _NAME_ptr # # NOTE: No error or sanity checks, just bare bones.   function _inihash () {     # private function     # call at the beginning of each procedure     # defines: _keys _values _ptr     #     # Usage: _inihash NAME     local name=$1     _keys=_${name}_keys     _values=_${name}_values     _ptr=_${name}_ptr }  function newhash () {     # Usage: newhash NAME     #        NAME should not contain spaces or dots.     #        Actually: it must be a legal name for a Bash variable.     # We rely on Bash automatically recognising arrays.     local name=$1      local _keys _values _ptr     _inihash ${name}     eval ${_ptr}=0 }   function addhash () {     # Usage: addhash NAME KEY 'VALUE with spaces'     #        arguments with spaces need to be quoted with single quotes ''     local name=$1 k="$2" v="$3"      local _keys _values _ptr     _inihash ${name}      #echo "DEBUG(addhash): ${_ptr}=${!_ptr}"      eval let ${_ptr}=${_ptr}+1     eval "$_keys[${!_ptr}]=\"${k}\""     eval "$_values[${!_ptr}]=\"${v}\"" }  function gethash () {     #  Usage: gethash NAME KEY     #         Returns boing     #         ERR=0 if entry found, 1 otherwise     #  That's not a proper hash --     #+ we simply linearly search through the keys.     local name=$1 key="$2"      local _keys _values _ptr      local k v i found h     _inihash ${name}          # _ptr holds the highest index in the hash     found=0      for i in $(seq 1 ${!_ptr}); do 	h="\${${_keys}[${i}]}"  #  Safer to do it in two steps, 	eval k=${h}             #+ especially when quoting for spaces. 	if [ "${k}" = "${key}" ]; then found=1; break; fi     done;      [ ${found} = 0 ] && return 1;     # else: i is the index that matches the key     h="\${${_values}[${i}]}"     eval echo "${h}"     return 0;	 }  function keyshash () {     # Usage: keyshash NAME     # Returns list of all keys defined for hash name.     local name=$1 key="$2"      local _keys _values _ptr      local k i h     _inihash ${name}          # _ptr holds the highest index in the hash     for i in $(seq 1 ${!_ptr}); do 	h="\${${_keys}[${i}]}"   #  Safer to do it in two steps, 	eval k=${h}              #+ especially when quoting for spaces. 	echo -n "'${k}' "     done; }   # -----------------------------------------------------------------------  # Now, let's test it. # (Per comments at the beginning of the script.) newhash Lovers addhash Lovers Tristan Isolde addhash Lovers 'Romeo Montague' 'Juliet Capulet'  # Output results. echo gethash Lovers Tristan      # Isolde echo keyshash Lovers             # 'Tristan' 'Romeo Montague' echo; echo   exit 0  # Exercise: # --------  # Add error checks to the functions.

Now for a script that installs and mounts those cute USB keychain solid-state "hard drives."

Example A-23. Mounting USB keychain storage devices

#!/bin/bash # ==> usb.sh # ==> Script for mounting and installing pen/keychain USB storage devices. # ==> Runs as root at system startup (see below). # ==> # ==> Newer Linux distros (2004 or later) autodetect # ==> and install USB pen drives, and therefore don't need this script. # ==> But, it's still instructive.   #  This code is free software covered by GNU GPL license version 2 or above. #  Please refer to http://www.gnu.org/ for the full license text. # #  Some code lifted from usb-mount by Michael Hamilton's usb-mount (LGPL) #+ see http://users.actrix.co.nz/michael/usbmount.html # #  INSTALL #  ------- #  Put this in /etc/hotplug/usb/diskonkey. #  Then look in /etc/hotplug/usb.distmap, and copy all usb-storage entries #+ into /etc/hotplug/usb.usermap, substituting "usb-storage" for "diskonkey". #  Otherwise this code is only run during the kernel module invocation/removal #+ (at least in my tests), which defeats the purpose. # #  TODO #  ---- #  Handle more than one diskonkey device at one time (e.g. /dev/diskonkey1 #+ and /mnt/diskonkey1), etc. The biggest problem here is the handling in #+ devlabel, which I haven't yet tried. # #  AUTHOR and SUPPORT #  ------------------ #  Konstantin Riabitsev, <icon linux duke edu>. #  Send any problem reports to my email address at the moment. # # ==> Comments added by ABS Guide author.    SYMLINKDEV=/dev/diskonkey MOUNTPOINT=/mnt/diskonkey DEVLABEL=/sbin/devlabel DEVLABELCONFIG=/etc/sysconfig/devlabel IAM=$0  ## # Functions lifted near-verbatim from usb-mount code. # function allAttachedScsiUsb {   find /proc/scsi/ -path '/proc/scsi/usb-storage*' -type f |   xargs grep -l 'Attached: Yes' } function scsiDevFromScsiUsb {   echo $1 | awk -F"[-/]" '{ n=$(NF-1);   print "/dev/sd" substr("abcdefghijklmnopqrstuvwxyz", n+1, 1) }' }  if [ "${ACTION}" = "add" ] && [ -f "${DEVICE}" ]; then     ##     # lifted from usbcam code.     #     if [ -f /var/run/console.lock ]; then         CONSOLEOWNER=`cat /var/run/console.lock`     elif [ -f /var/lock/console.lock ]; then         CONSOLEOWNER=`cat /var/lock/console.lock`     else         CONSOLEOWNER=     fi     for procEntry in $(allAttachedScsiUsb); do         scsiDev=$(scsiDevFromScsiUsb $procEntry)         #  Some bug with usb-storage?         #  Partitions are not in /proc/partitions until they are accessed         #+ somehow.         /sbin/fdisk -l $scsiDev >/dev/null         ##         #  Most devices have partitioning info, so the data would be on         #+ /dev/sd?1. However, some stupider ones don't have any partitioning         #+ and use the entire device for data storage. This tries to         #+ guess semi-intelligently if we have a /dev/sd?1 and if not, then         #+ it uses the entire device and hopes for the better.         #         if grep -q `basename $scsiDev`1 /proc/partitions; then             part="$scsiDev""1"         else             part=$scsiDev         fi         ##         #  Change ownership of the partition to the console user so they can         #+ mount it.         #         if [ ! -z "$CONSOLEOWNER" ]; then             chown $CONSOLEOWNER:disk $part         fi         ##         # This checks if we already have this UUID defined with devlabel.         # If not, it then adds the device to the list.         #         prodid=`$DEVLABEL printid -d $part`         if ! grep -q $prodid $DEVLABELCONFIG; then             # cross our fingers and hope it works             $DEVLABEL add -d $part -s $SYMLINKDEV 2>/dev/null         fi         ##         # Check if the mount point exists and create if it doesn't.         #         if [ ! -e $MOUNTPOINT ]; then             mkdir -p $MOUNTPOINT         fi         ##         # Take care of /etc/fstab so mounting is easy.         #         if ! grep -q "^$SYMLINKDEV" /etc/fstab; then             # Add an fstab entry             echo -e \                 "$SYMLINKDEV\t\t$MOUNTPOINT\t\tauto\tnoauto,owner,kudzu 0 0" \                 >> /etc/fstab         fi     done     if [ ! -z "$REMOVER" ]; then         ##         # Make sure this script is triggered on device removal.         #         mkdir -p `dirname $REMOVER`         ln -s $IAM $REMOVER     fi elif [ "${ACTION}" = "remove" ]; then     ##     # If the device is mounted, unmount it cleanly.     #     if grep -q "$MOUNTPOINT" /etc/mtab; then         # unmount cleanly         umount -l $MOUNTPOINT     fi     ##     # Remove it from /etc/fstab if it's there.     #     if grep -q "^$SYMLINKDEV" /etc/fstab; then         grep -v "^$SYMLINKDEV" /etc/fstab > /etc/.fstab.new         mv -f /etc/.fstab.new /etc/fstab     fi fi  exit 0

Converting a text file to HTML format.

Example A-24. Converting to HTML

#!/bin/bash # tohtml.sh [v. 0.2.01, reldate: 04/13/12, a teeny bit less buggy]  # Convert a text file to HTML format. # Author: Mendel Cooper # License: GPL3 # Usage: sh tohtml.sh < textfile > htmlfile # Script can easily be modified to accept source and target filenames.  #    Assumptions: # 1) Paragraphs in (target) text file are separated by a blank line. # 2) Jpeg images (*.jpg) are located in "images" subdirectory. #    In the target file, the image names are enclosed in square brackets, #    for example, [image01.jpg]. # 3) Emphasized (italic) phrases begin with a space+underscore #+   or the first character on the line is an underscore, #+   and end with an underscore+space or underscore+end-of-line.   # Settings FNTSIZE=2        # Small-medium font size IMGDIR="images"  # Image directory # Headers HDR01='<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">' HDR02='<!-- Converted to HTML by ***tohtml.sh*** script -->' HDR03='<!-- script author: M. Leo Cooper <thegrendel.abs@gmail.com> -->' HDR10='<html>' HDR11='<head>' HDR11a='</head>' HDR12a='<title>' HDR12b='</title>' HDR121='<META NAME="GENERATOR" CONTENT="tohtml.sh script">' HDR13='<body bgcolor="#dddddd">'   # Change background color to suit. HDR14a='<font size=' HDR14b='>' # Footers FTR10='</body>' FTR11='</html>' # Tags BOLD="<b>" CENTER="<center>" END_CENTER="</center>" LF="<br>"   write_headers ()   {   echo "$HDR01"   echo   echo "$HDR02"   echo "$HDR03"   echo   echo   echo "$HDR10"   echo "$HDR11"   echo "$HDR121"   echo "$HDR11a"   echo "$HDR13"   echo   echo -n "$HDR14a"   echo -n "$FNTSIZE"   echo "$HDR14b"   echo   echo "$BOLD"        # Everything in bold (more easily readable).   }   process_text ()   {   while read line     # Read one line at a time.   do     {     if [ ! "$line" ]  # Blank line?     then              # Then new paragraph must follow.       echo       echo "$LF"      # Insert two <br> tags.       echo "$LF"       echo       continue        # Skip the underscore test.     else              # Otherwise . . .        if [[ "$line" =~ \[*jpg\] ]]    # Is a graphic?       then                            # Strip away brackets.         temp=$( echo "$line" | sed -e 's/\[//' -e 's/\]//' )         line=""$CENTER" <img src="\"$IMGDIR"/$temp\"> "$END_CENTER" "                                       # Add image tag.                                       # And, center it.       fi      fi       echo "$line" | grep -q _     if [ "$?" -eq 0 ]    # If line contains underscore ...     then       # ===================================================       # Convert underscored phrase to italics.       temp=$( echo "$line" |               sed -e 's/ _/ <i>/' -e 's/_/<\/i> /' |               sed -e 's/^_/<i>/'  -e 's/_/<\/i>/' )       #  Process only underscores prefixed by space,       #+ or at beginning or end of line.       #  Do not convert underscores embedded within a word!       line="$temp"       # Slows script execution. Can be optimized?       # ===================================================     fi       #   echo     echo "$line" #   echo #   Don't want extra blank lines in generated text!     } # End while   done   }   # End process_text ()   write_footers ()  # Termination tags.   {   echo "$FTR10"   echo "$FTR11"   }   # main () { # ========= write_headers process_text write_footers # ========= #         }  exit $?  #  Exercises: #  --------- #  1) Fixup: Check for closing underscore before a comma or period. #  2) Add a test for the presence of a closing underscore #+    in phrases to be italicized.

Here is something to warm the hearts of webmasters and mistresses: a script that saves weblogs.

Example A-25. Preserving weblogs

#!/bin/bash # archiveweblogs.sh v1.0  # Troy Engel <tengel@fluid.com> # Slightly modified by document author. # Used with permission. # #  This script will preserve the normally rotated and #+ thrown away weblogs from a default RedHat/Apache installation. #  It will save the files with a date/time stamp in the filename, #+ bzipped, to a given directory. # #  Run this from crontab nightly at an off hour, #+ as bzip2 can suck up some serious CPU on huge logs: #  0 2 * * * /opt/sbin/archiveweblogs.sh   PROBLEM=66  # Set this to your backup dir. BKP_DIR=/opt/backups/weblogs  # Default Apache/RedHat stuff LOG_DAYS="4 3 2 1" LOG_DIR=/var/log/httpd LOG_FILES="access_log error_log"  # Default RedHat program locations LS=/bin/ls MV=/bin/mv ID=/usr/bin/id CUT=/bin/cut COL=/usr/bin/column BZ2=/usr/bin/bzip2  # Are we root? USER=`$ID -u` if [ "X$USER" != "X0" ]; then   echo "PANIC: Only root can run this script!"   exit $PROBLEM fi  # Backup dir exists/writable? if [ ! -x $BKP_DIR ]; then   echo "PANIC: $BKP_DIR doesn't exist or isn't writable!"   exit $PROBLEM fi  # Move, rename and bzip2 the logs for logday in $LOG_DAYS; do   for logfile in $LOG_FILES; do     MYFILE="$LOG_DIR/$logfile.$logday"     if [ -w $MYFILE ]; then       DTS=`$LS -lgo --time-style=+%Y%m%d $MYFILE | $COL -t | $CUT -d ' ' -f7`       $MV $MYFILE $BKP_DIR/$logfile.$DTS       $BZ2 $BKP_DIR/$logfile.$DTS     else       # Only spew an error if the file exits (ergo non-writable).       if [ -f $MYFILE ]; then         echo "ERROR: $MYFILE not writable. Skipping."       fi     fi   done done  exit 0

How to keep the shell from expanding and reinterpreting text strings.

Example A-26. Protecting literal strings

#! /bin/bash # protect_literal.sh  # set -vx  :<<-'_Protect_Literal_String_Doc'      Copyright (c) Michael S. Zick, 2003; All Rights Reserved     License: Unrestricted reuse in any form, for any purpose.     Warranty: None     Revision: $ID$      Documentation redirected to the Bash no-operation.     Bash will '/dev/null' this block when the script is first read.     (Uncomment the above set command to see this action.)      Remove the first (Sha-Bang) line when sourcing this as a library     procedure.  Also comment out the example use code in the two     places where shown.       Usage:         _protect_literal_str 'Whatever string meets your ${fancy}'         Just echos the argument to standard out, hard quotes         restored.          $(_protect_literal_str 'Whatever string meets your ${fancy}')         as the right-hand-side of an assignment statement.      Does:         As the right-hand-side of an assignment, preserves the         hard quotes protecting the contents of the literal during         assignment.      Notes:         The strange names (_*) are used to avoid trampling on         the user's chosen names when this is sourced as a         library.  _Protect_Literal_String_Doc  # The 'for illustration' function form  _protect_literal_str() {  # Pick an un-used, non-printing character as local IFS. # Not required, but shows that we are ignoring it.     local IFS=$'\x1B'               # \ESC character  # Enclose the All-Elements-Of in hard quotes during assignment.     local tmp=$'\x27'$@$'\x27' #    local tmp=$'\''$@$'\''         # Even uglier.      local len=${#tmp}               # Info only.     echo $tmp is $len long.         # Output AND information. }  # This is the short-named version. _pls() {     local IFS=$'x1B'                # \ESC character (not required)     echo $'\x27'$@$'\x27'           # Hard quoted parameter glob }  # :<<-'_Protect_Literal_String_Test' # # # Remove the above "# " to disable this code. # # #  # See how that looks when printed. echo echo "- - Test One - -" _protect_literal_str 'Hello $user' _protect_literal_str 'Hello "${username}"' echo  # Which yields: # - - Test One - - # 'Hello $user' is 13 long. # 'Hello "${username}"' is 21 long.  #  Looks as expected, but why all of the trouble? #  The difference is hidden inside the Bash internal order #+ of operations. #  Which shows when you use it on the RHS of an assignment.  # Declare an array for test values. declare -a arrayZ  # Assign elements with various types of quotes and escapes. arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )  # Now list that array and see what is there. echo "- - Test Two - -" for (( i=0 ; i<${#arrayZ[*]} ; i++ )) do     echo  Element $i: ${arrayZ[$i]} is: ${#arrayZ[$i]} long. done echo  # Which yields: # - - Test Two - - # Element 0: zero is: 4 long.           # Our marker element # Element 1: 'Hello ${Me}' is: 13 long. # Our "$(_pls '...' )" # Element 2: Hello ${You} is: 12 long.  # Quotes are missing # Element 3: \'Pass: \' is: 10 long.    # ${pw} expanded to nothing  # Now make an assignment with that result. declare -a array2=( ${arrayZ[@]} )  # And print what happened. echo "- - Test Three - -" for (( i=0 ; i<${#array2[*]} ; i++ )) do     echo  Element $i: ${array2[$i]} is: ${#array2[$i]} long. done echo  # Which yields: # - - Test Three - - # Element 0: zero is: 4 long.           # Our marker element. # Element 1: Hello ${Me} is: 11 long.   # Intended result. # Element 2: Hello is: 5 long.          # ${You} expanded to nothing. # Element 3: 'Pass: is: 6 long.         # Split on the whitespace. # Element 4: ' is: 1 long.              # The end quote is here now.  #  Our Element 1 has had its leading and trailing hard quotes stripped. #  Although not shown, leading and trailing whitespace is also stripped. #  Now that the string contents are set, Bash will always, internally, #+ hard quote the contents as required during its operations.  #  Why? #  Considering our "$(_pls 'Hello ${Me}')" construction: #  " ... " -> Expansion required, strip the quotes. #  $( ... ) -> Replace with the result of..., strip this. #  _pls ' ... ' -> called with literal arguments, strip the quotes. #  The result returned includes hard quotes; BUT the above processing #+ has already been done, so they become part of the value assigned. # #  Similarly, during further usage of the string variable, the ${Me} #+ is part of the contents (result) and survives any operations #  (Until explicitly told to evaluate the string).  #  Hint: See what happens when the hard quotes ($'\x27') are replaced #+ with soft quotes ($'\x22') in the above procedures. #  Interesting also is to remove the addition of any quoting.  # _Protect_Literal_String_Test # # # Remove the above "# " to disable this code. # # #  exit 0

But, what if you want the shell to expand and reinterpret strings?

Example A-27. Unprotecting literal strings

#! /bin/bash # unprotect_literal.sh  # set -vx  :<<-'_UnProtect_Literal_String_Doc'      Copyright (c) Michael S. Zick, 2003; All Rights Reserved     License: Unrestricted reuse in any form, for any purpose.     Warranty: None     Revision: $ID$      Documentation redirected to the Bash no-operation. Bash will     '/dev/null' this block when the script is first read.     (Uncomment the above set command to see this action.)      Remove the first (Sha-Bang) line when sourcing this as a library     procedure.  Also comment out the example use code in the two     places where shown.       Usage:         Complement of the "$(_pls 'Literal String')" function.         (See the protect_literal.sh example.)          StringVar=$(_upls ProtectedSringVariable)      Does:         When used on the right-hand-side of an assignment statement;         makes the substitions embedded in the protected string.      Notes:         The strange names (_*) are used to avoid trampling on         the user's chosen names when this is sourced as a         library.   _UnProtect_Literal_String_Doc  _upls() {     local IFS=$'x1B'                # \ESC character (not required)     eval echo $@                    # Substitution on the glob. }  # :<<-'_UnProtect_Literal_String_Test' # # # Remove the above "# " to disable this code. # # #   _pls() {     local IFS=$'x1B'                # \ESC character (not required)     echo $'\x27'$@$'\x27'           # Hard quoted parameter glob }  # Declare an array for test values. declare -a arrayZ  # Assign elements with various types of quotes and escapes. arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )  # Now make an assignment with that result. declare -a array2=( ${arrayZ[@]} )  # Which yielded: # - - Test Three - - # Element 0: zero is: 4 long            # Our marker element. # Element 1: Hello ${Me} is: 11 long    # Intended result. # Element 2: Hello is: 5 long           # ${You} expanded to nothing. # Element 3: 'Pass: is: 6 long          # Split on the whitespace. # Element 4: ' is: 1 long               # The end quote is here now.  # set -vx  #  Initialize 'Me' to something for the embedded ${Me} substitution. #  This needs to be done ONLY just prior to evaluating the #+ protected string. #  (This is why it was protected to begin with.)  Me="to the array guy."  # Set a string variable destination to the result. newVar=$(_upls ${array2[1]})  # Show what the contents are. echo $newVar  # Do we really need a function to do this? newerVar=$(eval echo ${array2[1]}) echo $newerVar  #  I guess not, but the _upls function gives us a place to hang #+ the documentation on. #  This helps when we forget what a # construction like: #+ $(eval echo ... ) means.  # What if Me isn't set when the protected string is evaluated? unset Me newestVar=$(_upls ${array2[1]}) echo $newestVar  # Just gone, no hints, no runs, no errors.  #  Why in the world? #  Setting the contents of a string variable containing character #+ sequences that have a meaning in Bash is a general problem in #+ script programming. # #  This problem is now solved in eight lines of code #+ (and four pages of description).  #  Where is all this going? #  Dynamic content Web pages as an array of Bash strings. #  Content set per request by a Bash 'eval' command #+ on the stored page template. #  Not intended to replace PHP, just an interesting thing to do. ### #  Don't have a webserver application? #  No problem, check the example directory of the Bash source; #+ there is a Bash script for that also.  # _UnProtect_Literal_String_Test # # # Remove the above "# " to disable this code. # # #  exit 0

This interesting script helps hunt down spammers.

Example A-28. Spammer Identification

#!/bin/bash  # $Id: is_spammer.bash,v 1.12.2.11 2004/10/01 21:42:33 mszick Exp $ # Above line is RCS info.  # The latest version of this script is available from http://www.morethan.org. # # Spammer-identification # by Michael S. Zick # Used in the ABS Guide with permission.    ####################################################### # Documentation # See also "Quickstart" at end of script. #######################################################  :<<-'__is_spammer_Doc_'      Copyright (c) Michael S. Zick, 2004     License: Unrestricted reuse in any form, for any purpose.     Warranty: None -{Its a script; the user is on their own.}-  Impatient?     Application code: goto "# # # Hunt the Spammer' program code # # #"     Example output: ":<<-'_is_spammer_outputs_'"     How to use: Enter script name without arguments.                 Or goto "Quickstart" at end of script.  Provides     Given a domain name or IP(v4) address as input:      Does an exhaustive set of queries to find the associated     network resources (short of recursing into TLDs).      Checks the IP(v4) addresses found against Blacklist     nameservers.      If found to be a blacklisted IP(v4) address,     reports the blacklist text records.     (Usually hyper-links to the specific report.)  Requires     A working Internet connection.     (Exercise: Add check and/or abort if not on-line when running script.)     Bash with arrays (2.05b+).      The external program 'dig' --     a utility program provided with the 'bind' set of programs.     Specifically, the version which is part of Bind series 9.x     See: http://www.isc.org      All usages of 'dig' are limited to wrapper functions,     which may be rewritten as required.     See: dig_wrappers.bash for details.          ("Additional documentation" -- below)  Usage     Script requires a single argument, which may be:     1) A domain name;     2) An IP(v4) address;     3) A filename, with one name or address per line.      Script accepts an optional second argument, which may be:     1) A Blacklist server name;     2) A filename, with one Blacklist server name per line.      If the second argument is not provided, the script uses     a built-in set of (free) Blacklist servers.      See also, the Quickstart at the end of this script (after 'exit').  Return Codes     0 - All OK     1 - Script failure     2 - Something is Blacklisted  Optional environment variables     SPAMMER_TRACE         If set to a writable file,         script will log an execution flow trace.      SPAMMER_DATA         If set to a writable file, script will dump its         discovered data in the form of GraphViz file.         See: http://www.research.att.com/sw/tools/graphviz      SPAMMER_LIMIT         Limits the depth of resource tracing.          Default is 2 levels.          A setting of 0 (zero) means 'unlimited' . . .           Caution: script might recurse the whole Internet!          A limit of 1 or 2 is most useful when processing         a file of domain names and addresses.         A higher limit can be useful when hunting spam gangs.   Additional documentation     Download the archived set of scripts     explaining and illustrating the function contained within this script.     http://bash.deta.in/mszick_clf.tar.bz2   Study notes     This script uses a large number of functions.     Nearly all general functions have their own example script.     Each of the example scripts have tutorial level comments.  Scripting project     Add support for IP(v6) addresses.     IP(v6) addresses are recognized but not processed.  Advanced project     Add the reverse lookup detail to the discovered information.      Report the delegation chain and abuse contacts.      Modify the GraphViz file output to include the     newly discovered information.  __is_spammer_Doc_  #######################################################     #### Special IFS settings used for string parsing. ####  # Whitespace == :Space:Tab:Line Feed:Carriage Return: WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'  # No Whitespace == Line Feed:Carriage Return NO_WSP=$'\x0A'$'\x0D'  # Field separator for dotted decimal IP addresses ADR_IFS=${NO_WSP}'.'  # Array to dotted string conversions DOT_IFS='.'${WSP_IFS}  # # # Pending operations stack machine # # # # This set of functions described in func_stack.bash. # (See "Additional documentation" above.) # # #  # Global stack of pending operations. declare -f -a _pending_ # Global sentinel for stack runners declare -i _p_ctrl_ # Global holder for currently executing function declare -f _pend_current_  # # # Debug version only - remove for regular use # # # # # The function stored in _pend_hook_ is called # immediately before each pending function is # evaluated.  Stack clean, _pend_current_ set. # # This thingy demonstrated in pend_hook.bash. declare -f _pend_hook_ # # #  # The do nothing function pend_dummy() { : ; }  # Clear and initialize the function stack. pend_init() {     unset _pending_[@]     pend_func pend_stop_mark     _pend_hook_='pend_dummy'  # Debug only. }  # Discard the top function on the stack. pend_pop() {     if [ ${#_pending_[@]} -gt 0 ]     then         local -i _top_         _top_=${#_pending_[@]}-1         unset _pending_[$_top_]     fi }  # pend_func function_name [$(printf '%q\n' arguments)] pend_func() {     local IFS=${NO_WSP}     set -f     _pending_[${#_pending_[@]}]=$@     set +f }  # The function which stops the release: pend_stop_mark() {     _p_ctrl_=0 }  pend_mark() {     pend_func pend_stop_mark }  # Execute functions until 'pend_stop_mark' . . . pend_release() {     local -i _top_             # Declare _top_ as integer.     _p_ctrl_=${#_pending_[@]}     while [ ${_p_ctrl_} -gt 0 ]     do        _top_=${#_pending_[@]}-1        _pend_current_=${_pending_[$_top_]}        unset _pending_[$_top_]        $_pend_hook_            # Debug only.        eval $_pend_current_     done }  # Drop functions until 'pend_stop_mark' . . . pend_drop() {     local -i _top_     local _pd_ctrl_=${#_pending_[@]}     while [ ${_pd_ctrl_} -gt 0 ]     do        _top_=$_pd_ctrl_-1        if [ "${_pending_[$_top_]}" == 'pend_stop_mark' ]        then            unset _pending_[$_top_]            break        else            unset _pending_[$_top_]            _pd_ctrl_=$_top_        fi     done     if [ ${#_pending_[@]} -eq 0 ]     then         pend_func pend_stop_mark     fi }  #### Array editors ####  # This function described in edit_exact.bash. # (See "Additional documentation," above.) # edit_exact <excludes_array_name> <target_array_name> edit_exact() {     [ $# -eq 2 ] ||     [ $# -eq 3 ] || return 1     local -a _ee_Excludes     local -a _ee_Target     local _ee_x     local _ee_t     local IFS=${NO_WSP}     set -f     eval _ee_Excludes=\( \$\{$1\[@\]\} \)     eval _ee_Target=\( \$\{$2\[@\]\} \)     local _ee_len=${#_ee_Target[@]}     # Original length.     local _ee_cnt=${#_ee_Excludes[@]}   # Exclude list length.     [ ${_ee_len} -ne 0 ] || return 0    # Can't edit zero length.     [ ${_ee_cnt} -ne 0 ] || return 0    # Can't edit zero length.     for (( x = 0; x < ${_ee_cnt} ; x++ ))     do         _ee_x=${_ee_Excludes[$x]}         for (( n = 0 ; n < ${_ee_len} ; n++ ))         do             _ee_t=${_ee_Target[$n]}             if [ x"${_ee_t}" == x"${_ee_x}" ]             then                 unset _ee_Target[$n]     # Discard match.                 [ $# -eq 2 ] && break    # If 2 arguments, then done.             fi         done     done     eval $2=\( \$\{_ee_Target\[@\]\} \)     set +f     return 0 }  # This function described in edit_by_glob.bash. # edit_by_glob <excludes_array_name> <target_array_name> edit_by_glob() {     [ $# -eq 2 ] ||     [ $# -eq 3 ] || return 1     local -a _ebg_Excludes     local -a _ebg_Target     local _ebg_x     local _ebg_t     local IFS=${NO_WSP}     set -f     eval _ebg_Excludes=\( \$\{$1\[@\]\} \)     eval _ebg_Target=\( \$\{$2\[@\]\} \)     local _ebg_len=${#_ebg_Target[@]}     local _ebg_cnt=${#_ebg_Excludes[@]}     [ ${_ebg_len} -ne 0 ] || return 0     [ ${_ebg_cnt} -ne 0 ] || return 0     for (( x = 0; x < ${_ebg_cnt} ; x++ ))     do         _ebg_x=${_ebg_Excludes[$x]}         for (( n = 0 ; n < ${_ebg_len} ; n++ ))         do             [ $# -eq 3 ] && _ebg_x=${_ebg_x}'*'  #  Do prefix edit             if [ ${_ebg_Target[$n]:=} ]          #+ if defined & set.             then                 _ebg_t=${_ebg_Target[$n]/#${_ebg_x}/}                 [ ${#_ebg_t} -eq 0 ] && unset _ebg_Target[$n]             fi         done     done     eval $2=\( \$\{_ebg_Target\[@\]\} \)     set +f     return 0 }  # This function described in unique_lines.bash. # unique_lines <in_name> <out_name> unique_lines() {     [ $# -eq 2 ] || return 1     local -a _ul_in     local -a _ul_out     local -i _ul_cnt     local -i _ul_pos     local _ul_tmp     local IFS=${NO_WSP}     set -f     eval _ul_in=\( \$\{$1\[@\]\} \)     _ul_cnt=${#_ul_in[@]}     for (( _ul_pos = 0 ; _ul_pos < ${_ul_cnt} ; _ul_pos++ ))     do         if [ ${_ul_in[${_ul_pos}]:=} ]      # If defined & not empty         then             _ul_tmp=${_ul_in[${_ul_pos}]}             _ul_out[${#_ul_out[@]}]=${_ul_tmp}             for (( zap = _ul_pos ; zap < ${_ul_cnt} ; zap++ ))             do                 [ ${_ul_in[${zap}]:=} ] &&                 [ 'x'${_ul_in[${zap}]} == 'x'${_ul_tmp} ] &&                     unset _ul_in[${zap}]             done         fi     done     eval $2=\( \$\{_ul_out\[@\]\} \)     set +f     return 0 }  # This function described in char_convert.bash. # to_lower <string> to_lower() {     [ $# -eq 1 ] || return 1     local _tl_out     _tl_out=${1//A/a}     _tl_out=${_tl_out//B/b}     _tl_out=${_tl_out//C/c}     _tl_out=${_tl_out//D/d}     _tl_out=${_tl_out//E/e}     _tl_out=${_tl_out//F/f}     _tl_out=${_tl_out//G/g}     _tl_out=${_tl_out//H/h}     _tl_out=${_tl_out//I/i}     _tl_out=${_tl_out//J/j}     _tl_out=${_tl_out//K/k}     _tl_out=${_tl_out//L/l}     _tl_out=${_tl_out//M/m}     _tl_out=${_tl_out//N/n}     _tl_out=${_tl_out//O/o}     _tl_out=${_tl_out//P/p}     _tl_out=${_tl_out//Q/q}     _tl_out=${_tl_out//R/r}     _tl_out=${_tl_out//S/s}     _tl_out=${_tl_out//T/t}     _tl_out=${_tl_out//U/u}     _tl_out=${_tl_out//V/v}     _tl_out=${_tl_out//W/w}     _tl_out=${_tl_out//X/x}     _tl_out=${_tl_out//Y/y}     _tl_out=${_tl_out//Z/z}     echo ${_tl_out}     return 0 }  #### Application helper functions ####  # Not everybody uses dots as separators (APNIC, for example). # This function described in to_dot.bash # to_dot <string> to_dot() {     [ $# -eq 1 ] || return 1     echo ${1//[#|@|%]/.}     return 0 }  # This function described in is_number.bash. # is_number <input> is_number() {     [ "$#" -eq 1 ]    || return 1  # is blank?     [ x"$1" == 'x0' ] && return 0  # is zero?     local -i tst     let tst=$1 2>/dev/null         # else is numeric!     return $? }  # This function described in is_address.bash. # is_address <input> is_address() {     [ $# -eq 1 ] || return 1    # Blank ==> false     local -a _ia_input     local IFS=${ADR_IFS}     _ia_input=( $1 )     if  [ ${#_ia_input[@]} -eq 4 ]  &&         is_number ${_ia_input[0]}   &&         is_number ${_ia_input[1]}   &&         is_number ${_ia_input[2]}   &&         is_number ${_ia_input[3]}   &&         [ ${_ia_input[0]} -lt 256 ] &&         [ ${_ia_input[1]} -lt 256 ] &&         [ ${_ia_input[2]} -lt 256 ] &&         [ ${_ia_input[3]} -lt 256 ]     then         return 0     else         return 1     fi }  #  This function described in split_ip.bash. #  split_ip <IP_address> #+ <array_name_norm> [<array_name_rev>] split_ip() {     [ $# -eq 3 ] ||              #  Either three     [ $# -eq 2 ] || return 1     #+ or two arguments     local -a _si_input     local IFS=${ADR_IFS}     _si_input=( $1 )     IFS=${WSP_IFS}     eval $2=\(\ \$\{_si_input\[@\]\}\ \)     if [ $# -eq 3 ]     then         # Build query order array.         local -a _dns_ip         _dns_ip[0]=${_si_input[3]}         _dns_ip[1]=${_si_input[2]}         _dns_ip[2]=${_si_input[1]}         _dns_ip[3]=${_si_input[0]}         eval $3=\(\ \$\{_dns_ip\[@\]\}\ \)     fi     return 0 }  # This function described in dot_array.bash. # dot_array <array_name> dot_array() {     [ $# -eq 1 ] || return 1     # Single argument required.     local -a _da_input     eval _da_input=\(\ \$\{$1\[@\]\}\ \)     local IFS=${DOT_IFS}     local _da_output=${_da_input[@]}     IFS=${WSP_IFS}     echo ${_da_output}     return 0 }  # This function described in file_to_array.bash # file_to_array <file_name> <line_array_name> file_to_array() {     [ $# -eq 2 ] || return 1  # Two arguments required.     local IFS=${NO_WSP}     local -a _fta_tmp_     _fta_tmp_=( $(cat $1) )     eval $2=\( \$\{_fta_tmp_\[@\]\} \)     return 0 }  #  Columnized print of an array of multi-field strings. #  col_print <array_name> <min_space> < #+ tab_stop [tab_stops]> col_print() {     [ $# -gt 2 ] || return 0     local -a _cp_inp     local -a _cp_spc     local -a _cp_line     local _cp_min     local _cp_mcnt     local _cp_pos     local _cp_cnt     local _cp_tab     local -i _cp     local -i _cpf     local _cp_fld     # WARNING: FOLLOWING LINE NOT BLANK -- IT IS QUOTED SPACES.     local _cp_max='                                                            '     set -f     local IFS=${NO_WSP}     eval _cp_inp=\(\ \$\{$1\[@\]\}\ \)     [ ${#_cp_inp[@]} -gt 0 ] || return 0 # Empty is easy.     _cp_mcnt=$2     _cp_min=${_cp_max:1:${_cp_mcnt}}     shift     shift     _cp_cnt=$#     for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))     do         _cp_spc[${#_cp_spc[@]}]="${_cp_max:2:$1}" #"         shift     done     _cp_cnt=${#_cp_inp[@]}     for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))     do         _cp_pos=1         IFS=${NO_WSP}$'\x20'         _cp_line=( ${_cp_inp[${_cp}]} )         IFS=${NO_WSP}         for (( _cpf = 0 ; _cpf < ${#_cp_line[@]} ; _cpf++ ))         do             _cp_tab=${_cp_spc[${_cpf}]:${_cp_pos}}             if [ ${#_cp_tab} -lt ${_cp_mcnt} ]             then                 _cp_tab="${_cp_min}"             fi             echo -n "${_cp_tab}"             (( _cp_pos = ${_cp_pos} + ${#_cp_tab} ))             _cp_fld="${_cp_line[${_cpf}]}"             echo -n ${_cp_fld}             (( _cp_pos = ${_cp_pos} + ${#_cp_fld} ))         done         echo     done     set +f     return 0 }  # # # # 'Hunt the Spammer' data flow # # # #  # Application return code declare -i _hs_RC  # Original input, from which IP addresses are removed # After which, domain names to check declare -a uc_name  # Original input IP addresses are moved here # After which, IP addresses to check declare -a uc_address  # Names against which address expansion run # Ready for name detail lookup declare -a chk_name  # Addresses against which name expansion run # Ready for address detail lookup declare -a chk_address  #  Recursion is depth-first-by-name. #  The expand_input_address maintains this list #+ to prohibit looking up addresses twice during #+ domain name recursion. declare -a been_there_addr been_there_addr=( '127.0.0.1' ) # Whitelist localhost  # Names which we have checked (or given up on) declare -a known_name  # Addresses which we have checked (or given up on) declare -a known_address  #  List of zero or more Blacklist servers to check. #  Each 'known_address' will be checked against each server, #+ with negative replies and failures suppressed. declare -a list_server  # Indirection limit - set to zero == no limit indirect=${SPAMMER_LIMIT:=2}  # # # # 'Hunt the Spammer' information output data # # # #  # Any domain name may have multiple IP addresses. # Any IP address may have multiple domain names. # Therefore, track unique address-name pairs. declare -a known_pair declare -a reverse_pair  #  In addition to the data flow variables; known_address #+ known_name and list_server, the following are output to the #+ external graphics interface file.  # Authority chain, parent -> SOA fields. declare -a auth_chain  # Reference chain, parent name -> child name declare -a ref_chain  # DNS chain - domain name -> address declare -a name_address  # Name and service pairs - domain name -> service declare -a name_srvc  # Name and resource pairs - domain name -> Resource Record declare -a name_resource  # Parent and Child pairs - parent name -> child name # This MAY NOT be the same as the ref_chain followed! declare -a parent_child  # Address and Blacklist hit pairs - address->server declare -a address_hits  # Dump interface file data declare -f _dot_dump _dot_dump=pend_dummy   # Initially a no-op  #  Data dump is enabled by setting the environment variable SPAMMER_DATA #+ to the name of a writable file. declare _dot_file  # Helper function for the dump-to-dot-file function # dump_to_dot <array_name> <prefix> dump_to_dot() {     local -a _dda_tmp     local -i _dda_cnt     local _dda_form='    '${2}'%04u %s\n'     local IFS=${NO_WSP}     eval _dda_tmp=\(\ \$\{$1\[@\]\}\ \)     _dda_cnt=${#_dda_tmp[@]}     if [ ${_dda_cnt} -gt 0 ]     then         for (( _dda = 0 ; _dda < _dda_cnt ; _dda++ ))         do             printf "${_dda_form}" \                    "${_dda}" "${_dda_tmp[${_dda}]}" >>${_dot_file}         done     fi }  # Which will also set _dot_dump to this function . . . dump_dot() {     local -i _dd_cnt     echo '# Data vintage: '$(date -R) >${_dot_file}     echo '# ABS Guide: is_spammer.bash; v2, 2004-msz' >>${_dot_file}     echo >>${_dot_file}     echo 'digraph G {' >>${_dot_file}      if [ ${#known_name[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known domain name nodes' >>${_dot_file}         _dd_cnt=${#known_name[@]}         for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))         do             printf '    N%04u [label="%s"] ;\n' \                    "${_dd}" "${known_name[${_dd}]}" >>${_dot_file}         done     fi      if [ ${#known_address[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known address nodes' >>${_dot_file}         _dd_cnt=${#known_address[@]}         for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))         do             printf '    A%04u [label="%s"] ;\n' \                    "${_dd}" "${known_address[${_dd}]}" >>${_dot_file}         done     fi      echo                                   >>${_dot_file}     echo '/*'                              >>${_dot_file}     echo ' * Known relationships :: User conversion to'  >>${_dot_file}     echo ' * graphic form by hand or program required.'  >>${_dot_file}     echo ' *'                              >>${_dot_file}      if [ ${#auth_chain[@]} -gt 0 ]     then       echo >>${_dot_file}       echo '# Authority ref. edges followed & field source.' >>${_dot_file}         dump_to_dot auth_chain AC     fi      if [ ${#ref_chain[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Name ref. edges followed and field source.' >>${_dot_file}         dump_to_dot ref_chain RC     fi      if [ ${#name_address[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known name->address edges' >>${_dot_file}         dump_to_dot name_address NA     fi      if [ ${#name_srvc[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known name->service edges' >>${_dot_file}         dump_to_dot name_srvc NS     fi      if [ ${#name_resource[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known name->resource edges' >>${_dot_file}         dump_to_dot name_resource NR     fi      if [ ${#parent_child[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known parent->child edges' >>${_dot_file}         dump_to_dot parent_child PC     fi      if [ ${#list_server[@]} -gt 0 ]     then         echo >>${_dot_file}         echo '# Known Blacklist nodes' >>${_dot_file}         _dd_cnt=${#list_server[@]}         for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))         do             printf '    LS%04u [label="%s"] ;\n' \                    "${_dd}" "${list_server[${_dd}]}" >>${_dot_file}         done     fi      unique_lines address_hits address_hits     if [ ${#address_hits[@]} -gt 0 ]     then       echo >>${_dot_file}       echo '# Known address->Blacklist_hit edges' >>${_dot_file}       echo '# CAUTION: dig warnings can trigger false hits.' >>${_dot_file}        dump_to_dot address_hits AH     fi     echo          >>${_dot_file}     echo ' *'     >>${_dot_file}     echo ' * That is a lot of relationships. Happy graphing.' >>${_dot_file}     echo ' */'    >>${_dot_file}     echo '}'      >>${_dot_file}     return 0 }  # # # # 'Hunt the Spammer' execution flow # # # #  #  Execution trace is enabled by setting the #+ environment variable SPAMMER_TRACE to the name of a writable file. declare -a _trace_log declare _log_file  # Function to fill the trace log trace_logger() {     _trace_log[${#_trace_log[@]}]=${_pend_current_} }  # Dump trace log to file function variable. declare -f _log_dump _log_dump=pend_dummy   # Initially a no-op.  # Dump the trace log to a file. dump_log() {     local -i _dl_cnt     _dl_cnt=${#_trace_log[@]}     for (( _dl = 0 ; _dl < _dl_cnt ; _dl++ ))     do         echo ${_trace_log[${_dl}]} >> ${_log_file}     done     _dl_cnt=${#_pending_[@]}     if [ ${_dl_cnt} -gt 0 ]     then         _dl_cnt=${_dl_cnt}-1         echo '# # # Operations stack not empty # # #' >> ${_log_file}         for (( _dl = ${_dl_cnt} ; _dl >= 0 ; _dl-- ))         do             echo ${_pending_[${_dl}]} >> ${_log_file}         done     fi }  # # # Utility program 'dig' wrappers # # # # #  These wrappers are derived from the #+ examples shown in dig_wrappers.bash. # #  The major difference is these return #+ their results as a list in an array. # #  See dig_wrappers.bash for details and #+ use that script to develop any changes. # # # #  # Short form answer: 'dig' parses answer.  # Forward lookup :: Name -> Address # short_fwd <domain_name> <array_name> short_fwd() {     local -a _sf_reply     local -i _sf_rc     local -i _sf_cnt     IFS=${NO_WSP} echo -n '.' # echo 'sfwd: '${1}   _sf_reply=( $(dig +short ${1} -c in -t a 2>/dev/null) )   _sf_rc=$?   if [ ${_sf_rc} -ne 0 ]   then     _trace_log[${#_trace_log[@]}]='## Lookup error '${_sf_rc}' on '${1}' ##' # [ ${_sf_rc} -ne 9 ] && pend_drop         return ${_sf_rc}     else         # Some versions of 'dig' return warnings on stdout.         _sf_cnt=${#_sf_reply[@]}         for (( _sf = 0 ; _sf < ${_sf_cnt} ; _sf++ ))         do             [ 'x'${_sf_reply[${_sf}]:0:2} == 'x;;' ] &&                 unset _sf_reply[${_sf}]         done         eval $2=\( \$\{_sf_reply\[@\]\} \)     fi     return 0 }  # Reverse lookup :: Address -> Name # short_rev <ip_address> <array_name> short_rev() {     local -a _sr_reply     local -i _sr_rc     local -i _sr_cnt     IFS=${NO_WSP} echo -n '.' # echo 'srev: '${1}   _sr_reply=( $(dig +short -x ${1} 2>/dev/null) )   _sr_rc=$?   if [ ${_sr_rc} -ne 0 ]   then     _trace_log[${#_trace_log[@]}]='## Lookup error '${_sr_rc}' on '${1}' ##' # [ ${_sr_rc} -ne 9 ] && pend_drop         return ${_sr_rc}     else         # Some versions of 'dig' return warnings on stdout.         _sr_cnt=${#_sr_reply[@]}         for (( _sr = 0 ; _sr < ${_sr_cnt} ; _sr++ ))         do             [ 'x'${_sr_reply[${_sr}]:0:2} == 'x;;' ] &&                 unset _sr_reply[${_sr}]         done         eval $2=\( \$\{_sr_reply\[@\]\} \)     fi     return 0 }  # Special format lookup used to query blacklist servers. # short_text <ip_address> <array_name> short_text() {     local -a _st_reply     local -i _st_rc     local -i _st_cnt     IFS=${NO_WSP} # echo 'stxt: '${1}   _st_reply=( $(dig +short ${1} -c in -t txt 2>/dev/null) )   _st_rc=$?   if [ ${_st_rc} -ne 0 ]   then     _trace_log[${#_trace_log[@]}]='##Text lookup error '${_st_rc}' on '${1}'##' # [ ${_st_rc} -ne 9 ] && pend_drop         return ${_st_rc}     else         # Some versions of 'dig' return warnings on stdout.         _st_cnt=${#_st_reply[@]}         for (( _st = 0 ; _st < ${#_st_cnt} ; _st++ ))         do             [ 'x'${_st_reply[${_st}]:0:2} == 'x;;' ] &&                 unset _st_reply[${_st}]         done         eval $2=\( \$\{_st_reply\[@\]\} \)     fi     return 0 }  # The long forms, a.k.a., the parse it yourself versions  # RFC 2782   Service lookups # dig +noall +nofail +answer _ldap._tcp.openldap.org -t srv # _<service>._<protocol>.<domain_name> # _ldap._tcp.openldap.org. 3600   IN     SRV    0 0 389 ldap.openldap.org. # domain TTL Class SRV Priority Weight Port Target  # Forward lookup :: Name -> poor man's zone transfer # long_fwd <domain_name> <array_name> long_fwd() {     local -a _lf_reply     local -i _lf_rc     local -i _lf_cnt     IFS=${NO_WSP} echo -n ':' # echo 'lfwd: '${1}   _lf_reply=( $(      dig +noall +nofail +answer +authority +additional \          ${1} -t soa ${1} -t mx ${1} -t any 2>/dev/null) )   _lf_rc=$?   if [ ${_lf_rc} -ne 0 ]   then     _trace_log[${#_trace_log[@]}]='# Zone lookup err '${_lf_rc}' on '${1}' #' # [ ${_lf_rc} -ne 9 ] && pend_drop         return ${_lf_rc}     else         # Some versions of 'dig' return warnings on stdout.         _lf_cnt=${#_lf_reply[@]}         for (( _lf = 0 ; _lf < ${_lf_cnt} ; _lf++ ))         do             [ 'x'${_lf_reply[${_lf}]:0:2} == 'x;;' ] &&                 unset _lf_reply[${_lf}]         done         eval $2=\( \$\{_lf_reply\[@\]\} \)     fi     return 0 } #  The reverse lookup domain name corresponding to the IPv6 address: #      4321:0:1:2:3:4:567:89ab #  would be (nibble, I.E: Hexdigit) reversed: #  b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA.  # Reverse lookup :: Address -> poor man's delegation chain # long_rev <rev_ip_address> <array_name> long_rev() {     local -a _lr_reply     local -i _lr_rc     local -i _lr_cnt     local _lr_dns     _lr_dns=${1}'.in-addr.arpa.'     IFS=${NO_WSP} echo -n ':' # echo 'lrev: '${1}   _lr_reply=( $(        dig +noall +nofail +answer +authority +additional \            ${_lr_dns} -t soa ${_lr_dns} -t any 2>/dev/null) )   _lr_rc=$?   if [ ${_lr_rc} -ne 0 ]   then     _trace_log[${#_trace_log[@]}]='# Deleg lkp error '${_lr_rc}' on '${1}' #' # [ ${_lr_rc} -ne 9 ] && pend_drop         return ${_lr_rc}     else         # Some versions of 'dig' return warnings on stdout.         _lr_cnt=${#_lr_reply[@]}         for (( _lr = 0 ; _lr < ${_lr_cnt} ; _lr++ ))         do             [ 'x'${_lr_reply[${_lr}]:0:2} == 'x;;' ] &&                 unset _lr_reply[${_lr}]         done         eval $2=\( \$\{_lr_reply\[@\]\} \)     fi     return 0 }  # # # Application specific functions # # #  # Mung a possible name; suppresses root and TLDs. # name_fixup <string> name_fixup(){     local -a _nf_tmp     local -i _nf_end     local _nf_str     local IFS     _nf_str=$(to_lower ${1})     _nf_str=$(to_dot ${_nf_str})     _nf_end=${#_nf_str}-1     [ ${_nf_str:${_nf_end}} != '.' ] &&         _nf_str=${_nf_str}'.'     IFS=${ADR_IFS}     _nf_tmp=( ${_nf_str} )     IFS=${WSP_IFS}     _nf_end=${#_nf_tmp[@]}     case ${_nf_end} in     0) # No dots, only dots.         echo         return 1     ;;     1) # Only a TLD.         echo         return 1     ;;     2) # Maybe okay.        echo ${_nf_str}        return 0        # Needs a lookup table?        if [ ${#_nf_tmp[1]} -eq 2 ]        then # Country coded TLD.            echo            return 1        else            echo ${_nf_str}            return 0        fi     ;;     esac     echo ${_nf_str}     return 0 }  # Grope and mung original input(s). split_input() {     [ ${#uc_name[@]} -gt 0 ] || return 0     local -i _si_cnt     local -i _si_len     local _si_str     unique_lines uc_name uc_name     _si_cnt=${#uc_name[@]}     for (( _si = 0 ; _si < _si_cnt ; _si++ ))     do         _si_str=${uc_name[$_si]}         if is_address ${_si_str}         then             uc_address[${#uc_address[@]}]=${_si_str}             unset uc_name[$_si]         else             if ! uc_name[$_si]=$(name_fixup ${_si_str})             then                 unset ucname[$_si]             fi         fi     done   uc_name=( ${uc_name[@]} )   _si_cnt=${#uc_name[@]}   _trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd name input(s).#'   _si_cnt=${#uc_address[@]}   _trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd addr input(s).#'     return 0 }  # # # Discovery functions -- recursively interlocked by external data # # # # # # The leading 'if list is empty; return 0' in each is required. # # #  # Recursion limiter # limit_chk() <next_level> limit_chk() {     local -i _lc_lmt     # Check indirection limit.     if [ ${indirect} -eq 0 ] || [ $# -eq 0 ]     then         # The 'do-forever' choice         echo 1                 # Any value will do.         return 0               # OK to continue.     else         # Limiting is in effect.         if [ ${indirect} -lt ${1} ]         then             echo ${1}          # Whatever.             return 1           # Stop here.         else             _lc_lmt=${1}+1     # Bump the given limit.             echo ${_lc_lmt}    # Echo it.             return 0           # OK to continue.         fi     fi }  # For each name in uc_name: #     Move name to chk_name. #     Add addresses to uc_address. #     Pend expand_input_address. #     Repeat until nothing new found. # expand_input_name <indirection_limit> expand_input_name() {     [ ${#uc_name[@]} -gt 0 ] || return 0     local -a _ein_addr     local -a _ein_new     local -i _ucn_cnt     local -i _ein_cnt     local _ein_tst     _ucn_cnt=${#uc_name[@]}      if  ! _ein_cnt=$(limit_chk ${1})     then         return 0     fi      for (( _ein = 0 ; _ein < _ucn_cnt ; _ein++ ))     do         if short_fwd ${uc_name[${_ein}]} _ein_new         then           for (( _ein_cnt = 0 ; _ein_cnt < ${#_ein_new[@]}; _ein_cnt++ ))           do               _ein_tst=${_ein_new[${_ein_cnt}]}               if is_address ${_ein_tst}               then                   _ein_addr[${#_ein_addr[@]}]=${_ein_tst}               fi     done         fi     done     unique_lines _ein_addr _ein_addr     # Scrub duplicates.     edit_exact chk_address _ein_addr     # Scrub pending detail.     edit_exact known_address _ein_addr   # Scrub already detailed.  if [ ${#_ein_addr[@]} -gt 0 ]        # Anything new?  then    uc_address=( ${uc_address[@]} ${_ein_addr[@]} )    pend_func expand_input_address ${1}    _trace_log[${#_trace_log[@]}]='#Add '${#_ein_addr[@]}' unchkd addr inp.#'     fi     edit_exact chk_name uc_name          # Scrub pending detail.     edit_exact known_name uc_name        # Scrub already detailed.     if [ ${#uc_name[@]} -gt 0 ]     then         chk_name=( ${chk_name[@]} ${uc_name[@]}  )         pend_func detail_each_name ${1}     fi     unset uc_name[@]     return 0 }  # For each address in uc_address: #     Move address to chk_address. #     Add names to uc_name. #     Pend expand_input_name. #     Repeat until nothing new found. # expand_input_address <indirection_limit> expand_input_address() {     [ ${#uc_address[@]} -gt 0 ] || return 0     local -a _eia_addr     local -a _eia_name     local -a _eia_new     local -i _uca_cnt     local -i _eia_cnt     local _eia_tst     unique_lines uc_address _eia_addr     unset uc_address[@]     edit_exact been_there_addr _eia_addr     _uca_cnt=${#_eia_addr[@]}     [ ${_uca_cnt} -gt 0 ] &&         been_there_addr=( ${been_there_addr[@]} ${_eia_addr[@]} )      for (( _eia = 0 ; _eia < _uca_cnt ; _eia++ ))      do        if short_rev ${_eia_addr[${_eia}]} _eia_new        then          for (( _eia_cnt = 0 ; _eia_cnt < ${#_eia_new[@]} ; _eia_cnt++ ))          do            _eia_tst=${_eia_new[${_eia_cnt}]}            if _eia_tst=$(name_fixup ${_eia_tst})            then              _eia_name[${#_eia_name[@]}]=${_eia_tst}        fi      done            fi     done     unique_lines _eia_name _eia_name     # Scrub duplicates.     edit_exact chk_name _eia_name        # Scrub pending detail.     edit_exact known_name _eia_name      # Scrub already detailed.  if [ ${#_eia_name[@]} -gt 0 ]        # Anything new?  then    uc_name=( ${uc_name[@]} ${_eia_name[@]} )    pend_func expand_input_name ${1}    _trace_log[${#_trace_log[@]}]='#Add '${#_eia_name[@]}' unchkd name inp.#'     fi     edit_exact chk_address _eia_addr     # Scrub pending detail.     edit_exact known_address _eia_addr   # Scrub already detailed.     if [ ${#_eia_addr[@]} -gt 0 ]        # Anything new?     then         chk_address=( ${chk_address[@]} ${_eia_addr[@]} )         pend_func detail_each_address ${1}     fi     return 0 }  # The parse-it-yourself zone reply. # The input is the chk_name list. # detail_each_name <indirection_limit> detail_each_name() {     [ ${#chk_name[@]} -gt 0 ] || return 0     local -a _den_chk       # Names to check     local -a _den_name      # Names found here     local -a _den_address   # Addresses found here     local -a _den_pair      # Pairs found here     local -a _den_rev       # Reverse pairs found here     local -a _den_tmp       # Line being parsed     local -a _den_auth      # SOA contact being parsed     local -a _den_new       # The zone reply     local -a _den_pc        # Parent-Child gets big fast     local -a _den_ref       # So does reference chain     local -a _den_nr        # Name-Resource can be big     local -a _den_na        # Name-Address     local -a _den_ns        # Name-Service     local -a _den_achn      # Chain of Authority     local -i _den_cnt       # Count of names to detail     local -i _den_lmt       # Indirection limit     local _den_who          # Named being processed     local _den_rec          # Record type being processed     local _den_cont         # Contact domain     local _den_str          # Fixed up name string     local _den_str2         # Fixed up reverse     local IFS=${WSP_IFS}      # Local, unique copy of names to check     unique_lines chk_name _den_chk     unset chk_name[@]       # Done with globals.      # Less any names already known     edit_exact known_name _den_chk     _den_cnt=${#_den_chk[@]}      # If anything left, add to known_name.     [ ${_den_cnt} -gt 0 ] &&         known_name=( ${known_name[@]} ${_den_chk[@]} )      # for the list of (previously) unknown names . . .     for (( _den = 0 ; _den < _den_cnt ; _den++ ))     do         _den_who=${_den_chk[${_den}]}         if long_fwd ${_den_who} _den_new         then             unique_lines _den_new _den_new             if [ ${#_den_new[@]} -eq 0 ]             then                 _den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}             fi              # Parse each line in the reply.             for (( _line = 0 ; _line < ${#_den_new[@]} ; _line++ ))             do                 IFS=${NO_WSP}$'\x09'$'\x20'                 _den_tmp=( ${_den_new[${_line}]} )                 IFS=${WSP_IFS}               # If usable record and not a warning message . . .               if [ ${#_den_tmp[@]} -gt 4 ] && [ 'x'${_den_tmp[0]} != 'x;;' ]               then                     _den_rec=${_den_tmp[3]}                     _den_nr[${#_den_nr[@]}]=${_den_who}' '${_den_rec}                     # Begin at RFC1033 (+++)                     case ${_den_rec} in  #<name> [<ttl>]  [<class>] SOA <origin> <person>                     SOA) # Start Of Authority     if _den_str=$(name_fixup ${_den_tmp[0]})     then       _den_name[${#_den_name[@]}]=${_den_str}       _den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str}' SOA'       # SOA origin -- domain name of master zone record       if _den_str2=$(name_fixup ${_den_tmp[4]})       then         _den_name[${#_den_name[@]}]=${_den_str2}         _den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str2}' SOA.O'       fi       # Responsible party e-mail address (possibly bogus).       # Possibility of first.last@domain.name ignored.       set -f       if _den_str2=$(name_fixup ${_den_tmp[5]})       then         IFS=${ADR_IFS}         _den_auth=( ${_den_str2} )         IFS=${WSP_IFS}         if [ ${#_den_auth[@]} -gt 2 ]         then           _den_cont=${_den_auth[1]}           for (( _auth = 2 ; _auth < ${#_den_auth[@]} ; _auth++ ))           do             _den_cont=${_den_cont}'.'${_den_auth[${_auth}]}           done           _den_name[${#_den_name[@]}]=${_den_cont}'.'           _den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_cont}'. SOA.C'                                 fi         fi         set +f                         fi                     ;;         A) # IP(v4) Address Record       if _den_str=$(name_fixup ${_den_tmp[0]})       then         _den_name[${#_den_name[@]}]=${_den_str}         _den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}         _den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}         _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' A'       else         _den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'         _den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}         _den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain A'       fi       _den_address[${#_den_address[@]}]=${_den_tmp[4]}       _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}              ;;               NS) # Name Server Record              # Domain name being serviced (may be other than current)                if _den_str=$(name_fixup ${_den_tmp[0]})                  then                    _den_name[${#_den_name[@]}]=${_den_str}                    _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' NS'               # Domain name of service provider              if _den_str2=$(name_fixup ${_den_tmp[4]})              then                _den_name[${#_den_name[@]}]=${_den_str2}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' NSH'                _den_ns[${#_den_ns[@]}]=${_den_str2}' NS'                _den_pc[${#_den_pc[@]}]=${_den_str}' '${_den_str2}               fi                fi                     ;;               MX) # Mail Server Record                  # Domain name being serviced (wildcards not handled here)              if _den_str=$(name_fixup ${_den_tmp[0]})              then                _den_name[${#_den_name[@]}]=${_den_str}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MX'              fi              # Domain name of service provider              if _den_str=$(name_fixup ${_den_tmp[5]})              then                _den_name[${#_den_name[@]}]=${_den_str}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MXH'                _den_ns[${#_den_ns[@]}]=${_den_str}' MX'                _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}              fi                     ;;               PTR) # Reverse address record                   # Special name              if _den_str=$(name_fixup ${_den_tmp[0]})              then                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' PTR'                # Host name (not a CNAME)                if _den_str2=$(name_fixup ${_den_tmp[4]})                then                  _den_rev[${#_den_rev[@]}]=${_den_str}' '${_den_str2}                  _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' PTRH'                  _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}                fi              fi                     ;;               AAAA) # IP(v6) Address Record              if _den_str=$(name_fixup ${_den_tmp[0]})              then                _den_name[${#_den_name[@]}]=${_den_str}                _den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}                _den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' AAAA'                else                  _den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'                  _den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}                  _den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain'                fi                # No processing for IPv6 addresses                _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}                     ;;               CNAME) # Alias name record                     # Nickname              if _den_str=$(name_fixup ${_den_tmp[0]})              then                _den_name[${#_den_name[@]}]=${_den_str}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CNAME'                _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}              fi                     # Hostname              if _den_str=$(name_fixup ${_den_tmp[4]})              then                _den_name[${#_den_name[@]}]=${_den_str}                _den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CHOST'                _den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}              fi                     ;; #            TXT) #            ;;                     esac                 fi             done         else # Lookup error == 'A' record 'unknown address'             _den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}         fi     done      # Control dot array growth.     unique_lines _den_achn _den_achn      # Works best, all the same.     edit_exact auth_chain _den_achn       # Works best, unique items.     if [ ${#_den_achn[@]} -gt 0 ]     then         IFS=${NO_WSP}         auth_chain=( ${auth_chain[@]} ${_den_achn[@]} )         IFS=${WSP_IFS}     fi      unique_lines _den_ref _den_ref      # Works best, all the same.     edit_exact ref_chain _den_ref       # Works best, unique items.     if [ ${#_den_ref[@]} -gt 0 ]     then         IFS=${NO_WSP}         ref_chain=( ${ref_chain[@]} ${_den_ref[@]} )         IFS=${WSP_IFS}     fi      unique_lines _den_na _den_na     edit_exact name_address _den_na     if [ ${#_den_na[@]} -gt 0 ]     then         IFS=${NO_WSP}         name_address=( ${name_address[@]} ${_den_na[@]} )         IFS=${WSP_IFS}     fi      unique_lines _den_ns _den_ns     edit_exact name_srvc _den_ns     if [ ${#_den_ns[@]} -gt 0 ]     then         IFS=${NO_WSP}         name_srvc=( ${name_srvc[@]} ${_den_ns[@]} )         IFS=${WSP_IFS}     fi      unique_lines _den_nr _den_nr     edit_exact name_resource _den_nr     if [ ${#_den_nr[@]} -gt 0 ]     then         IFS=${NO_WSP}         name_resource=( ${name_resource[@]} ${_den_nr[@]} )         IFS=${WSP_IFS}     fi      unique_lines _den_pc _den_pc     edit_exact parent_child _den_pc     if [ ${#_den_pc[@]} -gt 0 ]     then         IFS=${NO_WSP}         parent_child=( ${parent_child[@]} ${_den_pc[@]} )         IFS=${WSP_IFS}     fi      # Update list known_pair (Address and Name).     unique_lines _den_pair _den_pair     edit_exact known_pair _den_pair     if [ ${#_den_pair[@]} -gt 0 ]  # Anything new?     then         IFS=${NO_WSP}         known_pair=( ${known_pair[@]} ${_den_pair[@]} )         IFS=${WSP_IFS}     fi      # Update list of reverse pairs.     unique_lines _den_rev _den_rev     edit_exact reverse_pair _den_rev     if [ ${#_den_rev[@]} -gt 0 ]   # Anything new?     then         IFS=${NO_WSP}         reverse_pair=( ${reverse_pair[@]} ${_den_rev[@]} )         IFS=${WSP_IFS}     fi      # Check indirection limit -- give up if reached.     if ! _den_lmt=$(limit_chk ${1})     then         return 0     fi  # Execution engine is LIFO. Order of pend operations is important. # Did we define any new addresses? unique_lines _den_address _den_address    # Scrub duplicates. edit_exact known_address _den_address     # Scrub already processed. edit_exact un_address _den_address        # Scrub already waiting. if [ ${#_den_address[@]} -gt 0 ]          # Anything new? then   uc_address=( ${uc_address[@]} ${_den_address[@]} )   pend_func expand_input_address ${_den_lmt}   _trace_log[${#_trace_log[@]}]='# Add '${#_den_address[@]}' unchkd addr. #'     fi  # Did we find any new names? unique_lines _den_name _den_name          # Scrub duplicates. edit_exact known_name _den_name           # Scrub already processed. edit_exact uc_name _den_name              # Scrub already waiting. if [ ${#_den_name[@]} -gt 0 ]             # Anything new? then   uc_name=( ${uc_name[@]} ${_den_name[@]} )   pend_func expand_input_name ${_den_lmt}   _trace_log[${#_trace_log[@]}]='#Added '${#_den_name[@]}' unchkd name#'     fi     return 0 }  # The parse-it-yourself delegation reply # Input is the chk_address list. # detail_each_address <indirection_limit> detail_each_address() {     [ ${#chk_address[@]} -gt 0 ] || return 0     unique_lines chk_address chk_address     edit_exact known_address chk_address     if [ ${#chk_address[@]} -gt 0 ]     then         known_address=( ${known_address[@]} ${chk_address[@]} )         unset chk_address[@]     fi     return 0 }  # # # Application specific output functions # # #  # Pretty print the known pairs. report_pairs() {     echo     echo 'Known network pairs.'     col_print known_pair 2 5 30      if [ ${#auth_chain[@]} -gt 0 ]     then         echo         echo 'Known chain of authority.'         col_print auth_chain 2 5 30 55     fi      if [ ${#reverse_pair[@]} -gt 0 ]     then         echo         echo 'Known reverse pairs.'         col_print reverse_pair 2 5 55     fi     return 0 }  # Check an address against the list of blacklist servers. # A good place to capture for GraphViz: address->status(server(reports)) # check_lists <ip_address> check_lists() {     [ $# -eq 1 ] || return 1     local -a _cl_fwd_addr     local -a _cl_rev_addr     local -a _cl_reply     local -i _cl_rc     local -i _ls_cnt     local _cl_dns_addr     local _cl_lkup      split_ip ${1} _cl_fwd_addr _cl_rev_addr     _cl_dns_addr=$(dot_array _cl_rev_addr)'.'     _ls_cnt=${#list_server[@]}     echo '    Checking address '${1}     for (( _cl = 0 ; _cl < _ls_cnt ; _cl++ ))     do       _cl_lkup=${_cl_dns_addr}${list_server[${_cl}]}       if short_text ${_cl_lkup} _cl_reply       then         if [ ${#_cl_reply[@]} -gt 0 ]         then           echo '        Records from '${list_server[${_cl}]}           address_hits[${#address_hits[@]}]=${1}' '${list_server[${_cl}]}           _hs_RC=2           for (( _clr = 0 ; _clr < ${#_cl_reply[@]} ; _clr++ ))           do             echo '            '${_cl_reply[${_clr}]}           done         fi       fi     done     return 0 }  # # # The usual application glue # # #  # Who did it? credits() {    echo    echo 'Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz' }  # How to use it? # (See also, "Quickstart" at end of script.) usage() {     cat <<-'_usage_statement_'     The script is_spammer.bash requires either one or two arguments.      arg 1) May be one of:         a) A domain name         b) An IPv4 address         c) The name of a file with any mix of names            and addresses, one per line.      arg 2) May be one of:         a) A Blacklist server domain name         b) The name of a file with Blacklist server            domain names, one per line.         c) If not present, a default list of (free)            Blacklist servers is used.         d) If a filename of an empty, readable, file            is given,            Blacklist server lookup is disabled.      All script output is written to stdout.      Return codes: 0 -> All OK, 1 -> Script failure,                   2 -> Something is Blacklisted.      Requires the external program 'dig' from the 'bind-9'     set of DNS programs.  See: http://www.isc.org      The domain name lookup depth limit defaults to 2 levels.     Set the environment variable SPAMMER_LIMIT to change.     SPAMMER_LIMIT=0 means 'unlimited'      Limit may also be set on the command-line.     If arg#1 is an integer, the limit is set to that value     and then the above argument rules are applied.      Setting the environment variable 'SPAMMER_DATA' to a filename     will cause the script to write a GraphViz graphic file.      For the development version;     Setting the environment variable 'SPAMMER_TRACE' to a filename     will cause the execution engine to log a function call trace.  _usage_statement_ }  # The default list of Blacklist servers: # Many choices, see: http://www.spews.org/lists.html  declare -a default_servers # See: http://www.spamhaus.org (Conservative, well maintained) default_servers[0]='sbl-xbl.spamhaus.org' # See: http://ordb.org (Open mail relays) default_servers[1]='relays.ordb.org' # See: http://www.spamcop.net/ (You can report spammers here) default_servers[2]='bl.spamcop.net' # See: http://www.spews.org (An 'early detect' system) default_servers[3]='l2.spews.dnsbl.sorbs.net' # See: http://www.dnsbl.us.sorbs.net/using.shtml default_servers[4]='dnsbl.sorbs.net' # See: http://dsbl.org/usage (Various mail relay lists) default_servers[5]='list.dsbl.org' default_servers[6]='multihop.dsbl.org' default_servers[7]='unconfirmed.dsbl.org'  # User input argument #1 setup_input() {     if [ -e ${1} ] && [ -r ${1} ]  # Name of readable file     then         file_to_array ${1} uc_name         echo 'Using filename >'${1}'< as input.'     else         if is_address ${1}          # IP address?         then             uc_address=( ${1} )             echo 'Starting with address >'${1}'<'         else                       # Must be a name.             uc_name=( ${1} )             echo 'Starting with domain name >'${1}'<'         fi     fi     return 0 }  # User input argument #2 setup_servers() {     if [ -e ${1} ] && [ -r ${1} ]  # Name of a readable file     then         file_to_array ${1} list_server         echo 'Using filename >'${1}'< as blacklist server list.'     else         list_server=( ${1} )         echo 'Using blacklist server >'${1}'<'     fi     return 0 }  # User environment variable SPAMMER_TRACE live_log_die() {     if [ ${SPAMMER_TRACE:=} ]    # Wants trace log?     then         if [ ! -e ${SPAMMER_TRACE} ]         then             if ! touch ${SPAMMER_TRACE} 2>/dev/null             then                 pend_func echo $(printf '%q\n' \                 'Unable to create log file >'${SPAMMER_TRACE}'<')                 pend_release                 exit 1             fi             _log_file=${SPAMMER_TRACE}             _pend_hook_=trace_logger             _log_dump=dump_log         else             if [ ! -w ${SPAMMER_TRACE} ]             then                 pend_func echo $(printf '%q\n' \                 'Unable to write log file >'${SPAMMER_TRACE}'<')                 pend_release                 exit 1             fi             _log_file=${SPAMMER_TRACE}             echo '' > ${_log_file}             _pend_hook_=trace_logger             _log_dump=dump_log         fi     fi     return 0 }  # User environment variable SPAMMER_DATA data_capture() {     if [ ${SPAMMER_DATA:=} ]    # Wants a data dump?     then         if [ ! -e ${SPAMMER_DATA} ]         then             if ! touch ${SPAMMER_DATA} 2>/dev/null             then                 pend_func echo $(printf '%q]n' \                 'Unable to create data output file >'${SPAMMER_DATA}'<')                 pend_release                 exit 1             fi             _dot_file=${SPAMMER_DATA}             _dot_dump=dump_dot         else             if [ ! -w ${SPAMMER_DATA} ]             then                 pend_func echo $(printf '%q\n' \                 'Unable to write data output file >'${SPAMMER_DATA}'<')                 pend_release                 exit 1             fi             _dot_file=${SPAMMER_DATA}             _dot_dump=dump_dot         fi     fi     return 0 }  # Grope user specified arguments. do_user_args() {     if [ $# -gt 0 ] && is_number $1     then         indirect=$1         shift     fi      case $# in                     # Did user treat us well?         1)             if ! setup_input $1    # Needs error checking.             then                 pend_release                 $_log_dump                 exit 1             fi             list_server=( ${default_servers[@]} )             _list_cnt=${#list_server[@]}             echo 'Using default blacklist server list.'             echo 'Search depth limit: '${indirect}             ;;         2)             if ! setup_input $1    # Needs error checking.             then                 pend_release                 $_log_dump                 exit 1             fi             if ! setup_servers $2  # Needs error checking.             then                 pend_release                 $_log_dump                 exit 1             fi             echo 'Search depth limit: '${indirect}             ;;         *)             pend_func usage             pend_release             $_log_dump             exit 1             ;;     esac     return 0 }  # A general purpose debug tool. # list_array <array_name> list_array() {     [ $# -eq 1 ] || return 1  # One argument required.      local -a _la_lines     set -f     local IFS=${NO_WSP}     eval _la_lines=\(\ \$\{$1\[@\]\}\ \)     echo     echo "Element count "${#_la_lines[@]}" array "${1}     local _ln_cnt=${#_la_lines[@]}      for (( _i = 0; _i < ${_ln_cnt}; _i++ ))     do         echo 'Element '$_i' >'${_la_lines[$_i]}'<'     done     set +f     return 0 }  # # # 'Hunt the Spammer' program code # # # pend_init                               # Ready stack engine. pend_func credits                       # Last thing to print.  # # # Deal with user # # # live_log_die                            # Setup debug trace log. data_capture                            # Setup data capture file. echo do_user_args $@  # # # Haven't exited yet - There is some hope # # # # Discovery group - Execution engine is LIFO - pend # in reverse order of execution. _hs_RC=0                                # Hunt the Spammer return code pend_mark     pend_func report_pairs              # Report name-address pairs.      # The two detail_* are mutually recursive functions.     # They also pend expand_* functions as required.     # These two (the last of ???) exit the recursion.     pend_func detail_each_address       # Get all resources of addresses.     pend_func detail_each_name          # Get all resources of names.      #  The two expand_* are mutually recursive functions,     #+ which pend additional detail_* functions as required.     pend_func expand_input_address 1    # Expand input names by address.     pend_func expand_input_name 1       # #xpand input addresses by name.      # Start with a unique set of names and addresses.     pend_func unique_lines uc_address uc_address     pend_func unique_lines uc_name uc_name      # Separate mixed input of names and addresses.     pend_func split_input pend_release  # # # Pairs reported -- Unique list of IP addresses found echo _ip_cnt=${#known_address[@]} if [ ${#list_server[@]} -eq 0 ] then     echo 'Blacklist server list empty, none checked.' else     if [ ${_ip_cnt} -eq 0 ]     then         echo 'Known address list empty, none checked.'     else         _ip_cnt=${_ip_cnt}-1   # Start at top.         echo 'Checking Blacklist servers.'         for (( _ip = _ip_cnt ; _ip >= 0 ; _ip-- ))         do           pend_func check_lists $( printf '%q\n' ${known_address[$_ip]} )         done     fi fi pend_release $_dot_dump                   # Graphics file dump $_log_dump                   # Execution trace echo   ############################## # Example output from script # ############################## :<<-'_is_spammer_outputs_'  ./is_spammer.bash 0 web4.alojamentos7.com  Starting with domain name >web4.alojamentos7.com< Using default blacklist server list. Search depth limit: 0 .:....::::...:::...:::.......::..::...:::.......:: Known network pairs.     66.98.208.97             web4.alojamentos7.com.     66.98.208.97             ns1.alojamentos7.com.     69.56.202.147            ns2.alojamentos.ws.     66.98.208.97             alojamentos7.com.     66.98.208.97             web.alojamentos7.com.     69.56.202.146            ns1.alojamentos.ws.     69.56.202.146            alojamentos.ws.     66.235.180.113           ns1.alojamentos.org.     66.235.181.192           ns2.alojamentos.org.     66.235.180.113           alojamentos.org.     66.235.180.113           web6.alojamentos.org.     216.234.234.30           ns1.theplanet.com.     12.96.160.115            ns2.theplanet.com.     216.185.111.52           mail1.theplanet.com.     69.56.141.4              spooling.theplanet.com.     216.185.111.40           theplanet.com.     216.185.111.40           www.theplanet.com.     216.185.111.52           mail.theplanet.com.  Checking Blacklist servers.   Checking address 66.98.208.97       Records from dnsbl.sorbs.net   "Spam Received See: http://www.dnsbl.sorbs.net/lookup.shtml?66.98.208.97"     Checking address 69.56.202.147     Checking address 69.56.202.146     Checking address 66.235.180.113     Checking address 66.235.181.192     Checking address 216.185.111.40     Checking address 216.234.234.30     Checking address 12.96.160.115     Checking address 216.185.111.52     Checking address 69.56.141.4  Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz  _is_spammer_outputs_  exit ${_hs_RC}  #################################################### #  The script ignores everything from here on down # #+ because of the 'exit' command, just above.      # ####################################################    Quickstart ==========   Prerequisites    Bash version 2.05b or 3.00 (bash --version)   A version of Bash which supports arrays. Array    support is included by default Bash configurations.    'dig,' version 9.x.x (dig $HOSTNAME, see first line of output)   A version of dig which supports the +short options.    See: dig_wrappers.bash for details.    Optional Prerequisites    'named,' a local DNS caching program. Any flavor will do.   Do twice: dig $HOSTNAME    Check near bottom of output for: SERVER: 127.0.0.1#53   That means you have one running.    Optional Graphics Support    'date,' a standard *nix thing. (date -R)    dot Program to convert graphic description file to a    diagram. (dot -V)   A part of the Graph-Viz set of programs.   See: [http://www.research.att.com/sw/tools/graphviz||GraphViz]    'dotty,' a visual editor for graphic description files.   Also a part of the Graph-Viz set of programs.      Quick Start  In the same directory as the is_spammer.bash script;  Do: ./is_spammer.bash   Usage Details  1. Blacklist server choices.    (a) To use default, built-in list: Do nothing.    (b) To use your own list:       i. Create a file with a single Blacklist server         domain name per line.      ii. Provide that filename as the last argument to          the script.    (c) To use a single Blacklist server: Last argument        to the script.    (d) To disable Blacklist lookups:      i. Create an empty file (touch spammer.nul)        Your choice of filename.      ii. Provide the filename of that empty file as the          last argument to the script.  2. Search depth limit.    (a) To use the default value of 2: Do nothing.    (b) To set a different limit:        A limit of 0 means: no limit.      i. export SPAMMER_LIMIT=1        or whatever limit you want.      ii. OR provide the desired limit as the first         argument to the script.  3. Optional execution trace log.    (a) To use the default setting of no log output: Do nothing.    (b) To write an execution trace log:       export SPAMMER_TRACE=spammer.log       or whatever filename you want.  4. Optional graphic description file.    (a) To use the default setting of no graphic file: Do nothing.    (b) To write a Graph-Viz graphic description file:       export SPAMMER_DATA=spammer.dot       or whatever filename you want.  5. Where to start the search.    (a) Starting with a single domain name:      i. Without a command-line search limit: First         argument to script.      ii. With a command-line search limit: Second          argument to script.    (b) Starting with a single IP address:      i. Without a command-line search limit: First         argument to script.      ii. With a command-line search limit: Second          argument to script.    (c) Starting with (mixed) multiple name(s) and/or address(es):       Create a file with one name or address per line.       Your choice of filename.      i. Without a command-line search limit: Filename as         first argument to script.      ii. With a command-line search limit: Filename as          second argument to script.  6. What to do with the display output.    (a) To view display output on screen: Do nothing.    (b) To save display output to a file: Redirect stdout to a filename.    (c) To discard display output: Redirect stdout to /dev/null.  7. Temporary end of decision making.     press RETURN     wait (optionally, watch the dots and colons).  8. Optionally check the return code.    (a) Return code 0: All OK    (b) Return code 1: Script setup failure    (c) Return code 2: Something was blacklisted.  9. Where is my graph (diagram)?  The script does not directly produce a graph (diagram).  It only produces a graphic description file. You can  process the graphic descriptor file that was output  with the 'dot' program.  Until you edit that descriptor file, to describe the  relationships you want shown, all that you will get is  a bunch of labeled name and address nodes.  All of the script's discovered relationships are within  a comment block in the graphic descriptor file, each  with a descriptive heading.  The editing required to draw a line between a pair of  nodes from the information in the descriptor file may  be done with a text editor.   Given these lines somewhere in the descriptor file:  # Known domain name nodes  N0000 [label="guardproof.info."] ;  N0002 [label="third.guardproof.info."] ;    # Known address nodes  A0000 [label="61.141.32.197"] ;    /*  # Known name->address edges  NA0000 third.guardproof.info. 61.141.32.197    # Known parent->child edges  PC0000 guardproof.info. third.guardproof.info.   */  Turn that into the following lines by substituting node  identifiers into the relationships:  # Known domain name nodes  N0000 [label="guardproof.info."] ;  N0002 [label="third.guardproof.info."] ;    # Known address nodes  A0000 [label="61.141.32.197"] ;    # PC0000 guardproof.info. third.guardproof.info.  N0000->N0002 ;    # NA0000 third.guardproof.info. 61.141.32.197  N0002->A0000 ;    /*  # Known name->address edges  NA0000 third.guardproof.info. 61.141.32.197    # Known parent->child edges  PC0000 guardproof.info. third.guardproof.info.   */  Process that with the 'dot' program, and you have your  first network diagram.  In addition to the conventional graphic edges, the  descriptor file includes similar format pair-data that  describes services, zone records (sub-graphs?),  blacklisted addresses, and other things which might be  interesting to include in your graph. This additional  information could be displayed as different node  shapes, colors, line sizes, etc.  The descriptor file can also be read and edited by a  Bash script (of course). You should be able to find  most of the functions required within the  "is_spammer.bash" script.  # End Quickstart.    Additional Note ========== ====  Michael Zick points out that there is a "makeviz.bash" interactive Web site at rediris.es. Can't give the full URL, since this is not a publically accessible site.

Another anti-spam script.

Example A-29. Spammer Hunt

#!/bin/bash # whx.sh: "whois" spammer lookup # Author: Walter Dnes # Slight revisions (first section) by ABS Guide author. # Used in ABS Guide with permission.  # Needs version 3.x or greater of Bash to run (because of =~ operator). # Commented by script author and ABS Guide author.    E_BADARGS=85        # Missing command-line arg. E_NOHOST=86         # Host not found. E_TIMEOUT=87        # Host lookup timed out. E_UNDEF=88          # Some other (undefined) error.  HOSTWAIT=10         # Specify up to 10 seconds for host query reply.                     # The actual wait may be a bit longer. OUTFILE=whois.txt   # Output file. PORT=4321   if [ -z "$1" ]      # Check for (required) command-line arg. then   echo "Usage: $0 domain name or IP address"   exit $E_BADARGS fi   if [[ "$1" =~ [a-zA-Z][a-zA-Z]$ ]]  #  Ends in two alpha chars? then                                  #  It's a domain name &&                                       #+ must do host lookup.   IPADDR=$(host -W $HOSTWAIT $1 | awk '{print $4}')                                       #  Doing host lookup                                       #+ to get IP address. 				      #  Extract final field. else   IPADDR="$1"                         #  Command-line arg was IP address. fi  echo; echo "IP Address is: "$IPADDR""; echo  if [ -e "$OUTFILE" ] then   rm -f "$OUTFILE"   echo "Stale output file \"$OUTFILE\" removed."; echo fi   #  Sanity checks. #  (This section needs more work.) #  =============================== if [ -z "$IPADDR" ] # No response. then   echo "Host not found!"   exit $E_NOHOST    # Bail out. fi  if [[ "$IPADDR" =~ ^[;;] ]] #  ;; Connection timed out; no servers could be reached. then   echo "Host lookup timed out!"   exit $E_TIMEOUT   # Bail out. fi  if [[ "$IPADDR" =~ [(NXDOMAIN)]$ ]] #  Host xxxxxxxxx.xxx not found: 3(NXDOMAIN) then   echo "Host not found!"   exit $E_NOHOST    # Bail out. fi  if [[ "$IPADDR" =~ [(SERVFAIL)]$ ]] #  Host xxxxxxxxx.xxx not found: 2(SERVFAIL) then   echo "Host not found!"   exit $E_NOHOST    # Bail out. fi     # ======================== Main body of script ========================  AFRINICquery() { #  Define the function that queries AFRINIC. Echo a notification to the #+ screen, and then run the actual query, redirecting output to $OUTFILE.    echo "Searching for $IPADDR in whois.afrinic.net"   whois -h whois.afrinic.net "$IPADDR" > $OUTFILE  #  Check for presence of reference to an rwhois. #  Warn about non-functional rwhois.infosat.net server #+ and attempt rwhois query.   if grep -e "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE"   then     echo " " >> $OUTFILE     echo "***" >> $OUTFILE     echo "***" >> $OUTFILE     echo "Warning: rwhois.infosat.net was not working \       as of 2005/02/02" >> $OUTFILE     echo "         when this script was written." >> $OUTFILE     echo "***" >> $OUTFILE     echo "***" >> $OUTFILE     echo " " >> $OUTFILE     RWHOIS=`grep "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\     sed "s/\(^.*\)\(rwhois\..*\)\(:4.*\)/\2/"`     whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE   fi }  APNICquery() {   echo "Searching for $IPADDR in whois.apnic.net"   whois -h whois.apnic.net "$IPADDR" > $OUTFILE  #  Just  about  every  country has its own internet registrar. #  I don't normally bother consulting them, because the regional registry #+ usually supplies sufficient information. #  There are a few exceptions, where the regional registry simply #+ refers to the national registry for direct data. #  These are Japan and South Korea in APNIC, and Brasil in LACNIC. #  The following if statement checks $OUTFILE (whois.txt) for the presence #+ of "KR" (South Korea) or "JP" (Japan) in the country field. #  If either is found, the query is re-run against the appropriate #+ national registry.    if grep -E "^country:[ ]+KR$" "$OUTFILE"   then     echo "Searching for $IPADDR in whois.krnic.net"     whois -h whois.krnic.net "$IPADDR" >> $OUTFILE   elif grep -E "^country:[ ]+JP$" "$OUTFILE"   then     echo "Searching for $IPADDR in whois.nic.ad.jp"     whois -h whois.nic.ad.jp "$IPADDR"/e >> $OUTFILE   fi }  ARINquery() {   echo "Searching for $IPADDR in whois.arin.net"   whois -h whois.arin.net "$IPADDR" > $OUTFILE  #  Several large internet providers listed by ARIN have their own #+ internal whois service, referred to as "rwhois". #  A large block of IP addresses is listed with the provider #+ under the ARIN registry. #  To get the IP addresses of 2nd-level ISPs or other large customers, #+ one has to refer to the rwhois server on port 4321. #  I originally started with a bunch of "if" statements checking for #+ the larger providers. #  This approach is unwieldy, and there's always another rwhois server #+ that I didn't know about. #  A more elegant approach is to check $OUTFILE for a reference #+ to a whois server, parse that server name out of the comment section, #+ and re-run the query against the appropriate rwhois server. #  The parsing looks a bit ugly, with a long continued line inside #+ backticks. #  But it only has to be done once, and will work as new servers are added. #@   ABS Guide author comment: it isn't all that ugly, and is, in fact, #@+  an instructive use of Regular Expressions.    if grep -E "^Comment: .*rwhois.[^ ]+" "$OUTFILE"   then     RWHOIS=`grep -e "^Comment:.*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\     sed "s/^\(.*\)\(rwhois\.[^ ]\+\)\(.*$\)/\2/"`     echo "Searching for $IPADDR in ${RWHOIS}"     whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE   fi }  LACNICquery() {   echo "Searching for $IPADDR in whois.lacnic.net"   whois -h whois.lacnic.net "$IPADDR" > $OUTFILE  #  The  following if statement checks $OUTFILE (whois.txt) for #+ the presence of "BR" (Brasil) in the country field. #  If it is found, the query is re-run against whois.registro.br.    if grep -E "^country:[ ]+BR$" "$OUTFILE"   then     echo "Searching for $IPADDR in whois.registro.br"     whois -h whois.registro.br "$IPADDR" >> $OUTFILE   fi }  RIPEquery() {   echo "Searching for $IPADDR in whois.ripe.net"   whois -h whois.ripe.net "$IPADDR" > $OUTFILE }  #  Initialize a few variables. #  * slash8 is the most significant octet #  * slash16 consists of the two most significant octets #  * octet2 is the second most significant octet     slash8=`echo $IPADDR | cut -d. -f 1`   if [ -z "$slash8" ]  # Yet another sanity check.   then     echo "Undefined error!"     exit $E_UNDEF   fi slash16=`echo $IPADDR | cut -d. -f 1-2` #                             ^ Period specified as 'cut" delimiter.   if [ -z "$slash16" ]   then     echo "Undefined error!"     exit $E_UNDEF   fi octet2=`echo $slash16 | cut -d. -f 2`   if [ -z "$octet2" ]   then     echo "Undefined error!"     exit $E_UNDEF   fi   #  Check for various odds and ends of reserved space. #  There is no point in querying for those addresses.  if [ $slash8 == 0 ]; then   echo $IPADDR is '"This Network"' space\; Not querying elif [ $slash8 == 10 ]; then   echo $IPADDR is RFC1918 space\; Not querying elif [ $slash8 == 14 ]; then   echo $IPADDR is '"Public Data Network"' space\; Not querying elif [ $slash8 == 127 ]; then   echo $IPADDR is loopback space\; Not querying elif [ $slash16 == 169.254 ]; then   echo $IPADDR is link-local space\; Not querying elif [ $slash8 == 172 ] && [ $octet2 -ge 16 ] && [ $octet2 -le 31 ];then   echo $IPADDR is RFC1918 space\; Not querying elif [ $slash16 == 192.168 ]; then   echo $IPADDR is RFC1918 space\; Not querying elif [ $slash8 -ge 224 ]; then   echo $IPADDR is either Multicast or reserved space\; Not querying elif [ $slash8 -ge 200 ] && [ $slash8 -le 201 ]; then LACNICquery "$IPADDR" elif [ $slash8 -ge 202 ] && [ $slash8 -le 203 ]; then APNICquery "$IPADDR" elif [ $slash8 -ge 210 ] && [ $slash8 -le 211 ]; then APNICquery "$IPADDR" elif [ $slash8 -ge 218 ] && [ $slash8 -le 223 ]; then APNICquery "$IPADDR"  #  If we got this far without making a decision, query ARIN. #  If a reference is found in $OUTFILE to APNIC, AFRINIC, LACNIC, or RIPE, #+ query the appropriate whois server.  else   ARINquery "$IPADDR"   if grep "whois.afrinic.net" "$OUTFILE"; then     AFRINICquery "$IPADDR"   elif grep -E "^OrgID:[ ]+RIPE$" "$OUTFILE"; then     RIPEquery "$IPADDR"   elif grep -E "^OrgID:[ ]+APNIC$" "$OUTFILE"; then     APNICquery "$IPADDR"   elif grep -E "^OrgID:[ ]+LACNIC$" "$OUTFILE"; then     LACNICquery "$IPADDR"   fi fi  #@  --------------------------------------------------------------- #   Try also: #   wget http://logi.cc/nw/whois.php3?ACTION=doQuery&DOMAIN=$IPADDR #@  ---------------------------------------------------------------  #  We've  now  finished  the querying. #  Echo a copy of the final result to the screen.  cat $OUTFILE # Or "less $OUTFILE" . . .   exit 0  #@  ABS Guide author comments: #@  Nothing fancy here, but still a very useful tool for hunting spammers. #@  Sure, the script can be cleaned up some, and it's still a bit buggy, #@+ (exercise for reader), but all the same, it's a nice piece of coding #@+ by Walter Dnes. #@  Thank you!

"Little Monster's" front end to wget.

Example A-30. Making wget easier to use

#!/bin/bash # wgetter2.bash  # Author: Little Monster [monster@monstruum.co.uk] # ==> Used in ABS Guide with permission of script author. # ==> This script still needs debugging and fixups (exercise for reader). # ==> It could also use some additional editing in the comments.   #  This is wgetter2 -- #+ a Bash script to make wget a bit more friendly, and save typing.  #  Carefully crafted by Little Monster. #  More or less complete on 02/02/2005. #  If you think this script can be improved, #+ email me at: monster@monstruum.co.uk # ==> and cc: to the author of the ABS Guide, please. #  This script is licenced under the GPL. #  You are free to copy, alter and re-use it, #+ but please don't try to claim you wrote it. #  Log your changes here instead.  # ======================================================================= # changelog:  # 07/02/2005.  Fixups by Little Monster. # 02/02/2005.  Minor additions by Little Monster. #              (See after # +++++++++++ ) # 29/01/2005.  Minor stylistic edits and cleanups by author of ABS Guide. #              Added exit error codes. # 22/11/2004.  Finished initial version of second version of wgetter: #              wgetter2 is born. # 01/12/2004.  Changed 'runn' function so it can be run 2 ways -- #              either ask for a file name or have one input on the CL. # 01/12/2004.  Made sensible handling of no URL's given. # 01/12/2004.  Made loop of main options, so you don't #              have to keep calling wgetter 2 all the time. #              Runs as a session instead. # 01/12/2004.  Added looping to 'runn' function. #              Simplified and improved. # 01/12/2004.  Added state to recursion setting. #              Enables re-use of previous value. # 05/12/2004.  Modified the file detection routine in the 'runn' function #              so it's not fooled by empty values, and is cleaner. # 01/02/2004.  Added cookie finding routine from later version (which  #              isn't ready yet), so as not to have hard-coded paths. # =======================================================================  # Error codes for abnormal exit. E_USAGE=67        # Usage message, then quit. E_NO_OPTS=68      # No command-line args entered. E_NO_URLS=69      # No URLs passed to script. E_NO_SAVEFILE=70  # No save filename passed to script. E_USER_EXIT=71    # User decides to quit.   #  Basic default wget command we want to use. #  This is the place to change it, if required. #  NB: if using a proxy, set http_proxy = yourproxy in .wgetrc. #  Otherwise delete --proxy=on, below. # ==================================================================== CommandA="wget -nc -c -t 5 --progress=bar --random-wait --proxy=on -r" # ====================================================================    # -------------------------------------------------------------------- # Set some other variables and explain them.  pattern=" -A .jpg,.JPG,.jpeg,.JPEG,.gif,.GIF,.htm,.html,.shtml,.php"                     # wget's option to only get certain types of file.                     # comment out if not using today=`date +%F`    # Used for a filename. home=$HOME          # Set HOME to an internal variable.                     # In case some other path is used, change it here. depthDefault=3      # Set a sensible default recursion. Depth=$depthDefault # Otherwise user feedback doesn't tie in properly. RefA=""             # Set blank referring page. Flag=""             #  Default to not saving anything,                     #+ or whatever else might be wanted in future. lister=""           # Used for passing a list of urls directly to wget. Woptions=""         # Used for passing wget some options for itself. inFile=""           # Used for the run function. newFile=""          # Used for the run function. savePath="$home/w-save" Config="$home/.wgetter2rc"                     #  This is where some variables can be stored,                      #+ if permanently changed from within the script. Cookie_List="$home/.cookielist"                     # So we know where the cookies are kept . . . cFlag=""            # Part of the cookie file selection routine.  # Define the options available. Easy to change letters here if needed. # These are the optional options; you don't just wait to be asked.  save=s   # Save command instead of executing it. cook=c   # Change cookie file for this session. help=h   # Usage guide. list=l   # Pass wget the -i option and URL list. runn=r   # Run saved commands as an argument to the option. inpu=i   # Run saved commands interactively. wopt=w   # Allow to enter options to pass directly to wget. # --------------------------------------------------------------------   if [ -z "$1" ]; then   # Make sure we get something for wget to eat.    echo "You must at least enter a URL or option!"    echo "-$help for usage."    exit $E_NO_OPTS fi    # +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ # added added added added added added added added added added added added  if [ ! -e "$Config" ]; then   # See if configuration file exists.    echo "Creating configuration file, $Config"    echo "# This is the configuration file for wgetter2" > "$Config"    echo "# Your customised settings will be saved in this file" >> "$Config" else    source $Config             # Import variables we set outside the script. fi  if [ ! -e "$Cookie_List" ]; then    # Set up a list of cookie files, if there isn't one.    echo "Hunting for cookies . . ."    find -name cookies.txt >> $Cookie_List # Create the list of cookie files. fi #  Isolate this in its own 'if' statement,    #+ in case we got interrupted while searching.  if [ -z "$cFlag" ]; then # If we haven't already done this . . .    echo                  # Make a nice space after the command prompt.    echo "Looks like you haven't set up your source of cookies yet."    n=0                   #  Make sure the counter                          #+ doesn't contain random values.    while read; do       Cookies[$n]=$REPLY # Put the cookie files we found into an array.       echo "$n) ${Cookies[$n]}"  # Create a menu.       n=$(( n + 1 ))     # Increment the counter.    done < $Cookie_List   # Feed the read statement.    echo "Enter the number of the cookie file you want to use."    echo "If you won't be using cookies, just press RETURN."    echo    echo "I won't be asking this again. Edit $Config"    echo "If you decide to change at a later date"    echo "or use the -${cook} option for per session changes."    read    if [ ! -z $REPLY ]; then   # User didn't just press return.       Cookie=" --load-cookies ${Cookies[$REPLY]}"       # Set the variable here as well as in the config file.        echo "Cookie=\" --load-cookies ${Cookies[$REPLY]}\"" >> $Config    fi    echo "cFlag=1" >> $Config  # So we know not to ask again. fi  # end added section end added section end added section end added section # +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++    # Another variable. # This one may or may not be subject to variation. # A bit like the small print. CookiesON=$Cookie # echo "cookie file is $CookiesON" # For debugging. # echo "home is ${home}"           # For debugging.                                    # Got caught with this one!   wopts() { echo "Enter options to pass to wget." echo "It is assumed you know what you're doing." echo echo "You can pass their arguments here too." # That is to say, everything passed here is passed to wget.  read Wopts # Read in the options to be passed to wget.  Woptions=" $Wopts" #         ^  Why the leading space? # Assign to another variable. # Just for fun, or something . . .  echo "passing options ${Wopts} to wget" # Mainly for debugging. # Is cute.  return }   save_func() { echo "Settings will be saved." if [ ! -d $savePath ]; then  #  See if directory exists.    mkdir $savePath           #  Create the directory to save things in                              #+ if it isn't already there. fi  Flag=S # Tell the final bit of code what to do. # Set a flag since stuff is done in main.  return }   usage() # Tell them how it works. {     echo "Welcome to wgetter.  This is a front end to wget."     echo "It will always run wget with these options:"     echo "$CommandA"     echo "and the pattern to match: $pattern \ (which you can change at the top of this script)."     echo "It will also ask you for recursion depth, \ and if you want to use a referring page."     echo "Wgetter accepts the following options:"     echo ""     echo "-$help : Display this help."     echo "-$save : Save the command to a file $savePath/wget-($today) \ instead of running it."     echo "-$runn : Run saved wget commands instead of starting a new one -"     echo "Enter filename as argument to this option."     echo "-$inpu : Run saved wget commands interactively --"     echo "The script will ask you for the filename."     echo "-$cook : Change the cookies file for this session."     echo "-$list : Tell wget to use URL's from a list instead of \ from the command-line."     echo "-$wopt : Pass any other options direct to wget."     echo ""     echo "See the wget man page for additional options \ you can pass to wget."     echo ""      exit $E_USAGE  # End here. Don't process anything else. }    list_func() #  Gives the user the option to use the -i option to wget,             #+ and a list of URLs. { while [ 1 ]; do    echo "Enter the name of the file containing URL's (press q to change your mind)."    read urlfile    if [ ! -e "$urlfile" ] && [ "$urlfile" != q ]; then        # Look for a file, or the quit option.        echo "That file does not exist!"    elif [ "$urlfile" = q ]; then # Check quit option.        echo "Not using a url list."        return    else       echo "using $urlfile."       echo "If you gave url's on the command-line, I'll use those first."                             # Report wget standard behaviour to the user.       lister=" -i $urlfile" # This is what we want to pass to wget.       return    fi done }   cookie_func() # Give the user the option to use a different cookie file. { while [ 1 ]; do    echo "Change the cookies file. Press return if you don't want to change  it."    read Cookies    # NB: this is not the same as Cookie, earlier.    # There is an 's' on the end.    # Bit like chocolate chips.    if [ -z "$Cookies" ]; then                 # Escape clause for wusses.       return    elif [ ! -e "$Cookies" ]; then       echo "File does not exist.  Try again." # Keep em going . . .    else        CookiesON=" --load-cookies $Cookies"   # File is good -- use it!        return    fi done }    run_func() { if [ -z "$OPTARG" ]; then # Test to see if we used the in-line option or the query one.    if [ ! -d "$savePath" ]; then      # If directory doesn't exist . . .       echo "$savePath does not appear to exist."       echo "Please supply path and filename of saved wget commands:"       read newFile          until [ -f "$newFile" ]; do  # Keep going till we get something.             echo "Sorry, that file does not exist.  Please try again."             # Try really hard to get something.             read newFile          done   # ----------------------------------------------------------------------- #       if [ -z ( grep wget ${newfile} ) ]; then         # Assume they haven't got the right file and bail out. #       echo "Sorry, that file does not contain wget commands.  Aborting." #       exit #       fi # # This is bogus code. # It doesn't actually work. # If anyone wants to fix it, feel free! # -----------------------------------------------------------------------         filePath="${newFile}"    else    echo "Save path is $savePath"      echo "Please enter name of the file which you want to use."      echo "You have a choice of:"      ls $savePath                                    # Give them a choice.      read inFile        until [ -f "$savePath/$inFile" ]; do         #  Keep going till                                                     #+ we get something.           if [ ! -f "${savePath}/${inFile}" ]; then # If file doesn't exist.              echo "Sorry, that file does not exist.  Please choose from:"              ls $savePath                           # If a mistake is made.              read inFile           fi          done       filePath="${savePath}/${inFile}"  # Make one variable . . .    fi else filePath="${savePath}/${OPTARG}"   # Which can be many things . . . fi  if [ ! -f "$filePath" ]; then           # If a bogus file got through.    echo "You did not specify a suitable file."    echo "Run this script with the -${save} option first."    echo "Aborting."    exit $E_NO_SAVEFILE fi echo "Using: $filePath" while read; do     eval $REPLY     echo "Completed: $REPLY" done < $filePath  # Feed the actual file we are using into a 'while' loop.  exit }    # Fish out any options we are using for the script. # This is based on the demo in "Learning The Bash Shell" (O'Reilly). while getopts ":$save$cook$help$list$runn:$inpu$wopt" opt do   case $opt in      $save) save_func;;   #  Save some wgetter sessions for later.      $cook) cookie_func;; #  Change cookie file.      $help) usage;;       #  Get help.      $list) list_func;;   #  Allow wget to use a list of URLs.      $runn) run_func;;    #  Useful if you are calling wgetter from,                           #+ for example, a cron script.      $inpu) run_func;;    #  When you don't know what your files are named.      $wopt) wopts;;       #  Pass options directly to wget.         \?) echo "Not a valid option."             echo "Use -${wopt} to pass options directly to wget,"             echo "or -${help} for help";;      # Catch anything else.   esac done shift $((OPTIND - 1))     # Do funky magic stuff with $#.   if [ -z "$1" ] && [ -z "$lister" ]; then                           #  We should be left with at least one URL                           #+ on the command-line, unless a list is  			  #+ being used -- catch empty CL's.    echo "No URL's given! You must enter them on the same line as wgetter2."    echo "E.g.,  wgetter2 http://somesite http://anothersite."    echo "Use $help option for more information."    exit $E_NO_URLS        # Bail out, with appropriate error code. fi  URLS=" $@" # Use this so that URL list can be changed if we stay in the option loop.  while [ 1 ]; do    # This is where we ask for the most used options.    # (Mostly unchanged from version 1 of wgetter)    if [ -z $curDepth ]; then       Current=""    else Current=" Current value is $curDepth"    fi        echo "How deep should I go? \ (integer: Default is $depthDefault.$Current)"        read Depth   # Recursion -- how far should we go?        inputB=""    # Reset this to blank on each pass of the loop.        echo "Enter the name of the referring page (default is none)."        read inputB  # Need this for some sites.         echo "Do you want to have the output logged to the terminal"        echo "(y/n, default is yes)?"        read noHide  # Otherwise wget will just log it to a file.         case $noHide in    # Now you see me, now you don't.           y|Y ) hide="";;           n|N ) hide=" -b";;             * ) hide="";;        esac         if [ -z ${Depth} ]; then        #  User accepted either default or current depth,        #+ in which case Depth is now empty.           if [ -z ${curDepth} ]; then           #  See if a depth was set on a previous iteration.              Depth="$depthDefault"              #  Set the default recursion depth if nothing              #+ else to use.           else Depth="$curDepth" #  Otherwise, set the one we used before.           fi        fi    Recurse=" -l $Depth"          # Set how deep we want to go.    curDepth=$Depth               # Remember setting for next time.         if [ ! -z $inputB ]; then           RefA=" --referer=$inputB"   # Option to use referring page.        fi     WGETTER="${CommandA}${pattern}${hide}${RefA}${Recurse}\ ${CookiesON}${lister}${Woptions}${URLS}"    #  Just string the whole lot together . . .    #  NB: no embedded spaces.    #  They are in the individual elements so that if any are empty,    #+ we don't get an extra space.     if [ -z "${CookiesON}" ] && [ "$cFlag" = "1" ] ; then        echo "Warning -- can't find cookie file"        #  This should be changed,        #+ in case the user has opted to not use cookies.    fi     if [ "$Flag" = "S" ]; then       echo "$WGETTER" >> $savePath/wget-${today}       #  Create a unique filename for today, or append to it if it exists.       echo "$inputB" >> $savePath/site-list-${today}       #  Make a list, so it's easy to refer back to,       #+ since the whole command is a bit confusing to look at.       echo "Command saved to the file $savePath/wget-${today}"            # Tell the user.       echo "Referring page URL saved to the file$ \ savePath/site-list-${today}"            # Tell the user.       Saver=" with save option"       # Stick this somewhere, so it appears in the loop if set.    else        echo "*****************"        echo "*****Getting*****"        echo "*****************"        echo ""        echo "$WGETTER"        echo ""        echo "*****************"        eval "$WGETTER"    fi         echo ""        echo "Starting over$Saver."        echo "If you want to stop, press q."        echo "Otherwise, enter some URL's:"        # Let them go again. Tell about save option being set.         read        case $REPLY in        # Need to change this to a 'trap' clause.           q|Q ) exit $E_USER_EXIT;;  # Exercise for the reader?             * ) URLS=" $REPLY";;        esac         echo "" done   exit 0

Example A-31. A podcasting script

#!/bin/bash  #  bashpodder.sh: #  By Linc 10/1/2004 #  Find the latest script at #+ http://linc.homeunix.org:8080/scripts/bashpodder #  Last revision 12/14/2004 - Many Contributors! #  If you use this and have made improvements or have comments #+ drop me an email at linc dot fessenden at gmail dot com #  I'd appreciate it!  # ==>  ABS Guide extra comments.  # ==>  Author of this script has kindly granted permission # ==>+ for inclusion in ABS Guide.   # ==> ################################################################ #  # ==> What is "podcasting"?  # ==> It's broadcasting "radio shows" over the Internet. # ==> These shows can be played on iPods and other music file players.  # ==> This script makes it possible. # ==> See documentation at the script author's site, above.  # ==> ################################################################   # Make script crontab friendly: cd $(dirname $0) # ==> Change to directory where this script lives.  # datadir is the directory you want podcasts saved to: datadir=$(date +%Y-%m-%d) # ==> Will create a date-labeled directory, named: YYYY-MM-DD  # Check for and create datadir if necessary: if test ! -d $datadir         then         mkdir $datadir fi  # Delete any temp file: rm -f temp.log  #  Read the bp.conf file and wget any url not already #+ in the podcast.log file: while read podcast   do # ==> Main action follows.   file=$(wget -q $podcast -O - | tr '\r' '\n' | tr \' \" | \ sed -n 's/.*url="\([^"]*\)".*/\1/p')   for url in $file                 do                 echo $url >> temp.log                 if ! grep "$url" podcast.log > /dev/null                         then                         wget -q -P $datadir "$url"                 fi                 done     done < bp.conf  # Move dynamically created log file to permanent log file: cat podcast.log >> temp.log sort temp.log | uniq > podcast.log rm temp.log # Create an m3u playlist: ls $datadir | grep -v m3u > $datadir/podcast.m3u   exit 0  ################################################# For a different scripting approach to Podcasting, see Phil Salkie's article,  "Internet Radio to Podcast with Shell Tools" in the September, 2005 issue of LINUX JOURNAL, http://www.linuxjournal.com/article/8171 #################################################

Example A-32. Nightly backup to a firewire HD

#!/bin/bash # nightly-backup.sh # http://www.richardneill.org/source.php#nightly-backup-rsync # Copyright (c) 2005 Richard Neill <backup@richardneill.org>. # This is Free Software licensed under the GNU GPL. # ==> Included in ABS Guide with script author's kind permission. # ==> (Thanks!)  #  This does a backup from the host computer to a locally connected #+ firewire HDD using rsync and ssh. #  (Script should work with USB-connected device (see lines 40-43). #  It then rotates the backups. #  Run it via cron every night at 5am. #  This only backs up the home directory. #  If ownerships (other than the user's) should be preserved, #+ then run the rsync process as root (and re-instate the -o). #  We save every day for 7 days, then every week for 4 weeks, #+ then every month for 3 months.  #  See: http://www.mikerubel.org/computers/rsync_snapshots/ #+ for more explanation of the theory. #  Save as: $HOME/bin/nightly-backup_firewire-hdd.sh  #  Known bugs: #  ---------- #  i)  Ideally, we want to exclude ~/.tmp and the browser caches.  #  ii) If the user is sitting at the computer at 5am, #+     and files are modified while the rsync is occurring, #+     then the BACKUP_JUSTINCASE branch gets triggered. #      To some extent, this is a  #+     feature, but it also causes a "disk-space leak".      ##### BEGIN CONFIGURATION SECTION ############################################ LOCAL_USER=rjn                # User whose home directory should be backed up. MOUNT_POINT=/backup           # Mountpoint of backup drive.                               # NO trailing slash!                               # This must be unique (eg using a udev symlink) # MOUNT_POINT=/media/disk     # For USB-connected device. SOURCE_DIR=/home/$LOCAL_USER  # NO trailing slash - it DOES matter to rsync. BACKUP_DEST_DIR=$MOUNT_POINT/backup/`hostname -s`.${LOCAL_USER}.nightly_backup DRY_RUN=false                 #If true, invoke rsync with -n, to do a dry run.                               # Comment out or set to false for normal use. VERBOSE=false                 # If true, make rsync verbose.                               # Comment out or set to false otherwise. COMPRESS=false                # If true, compress.                               # Good for internet, bad on LAN.                               # Comment out or set to false otherwise.  ### Exit Codes ### E_VARS_NOT_SET=64 E_COMMANDLINE=65 E_MOUNT_FAIL=70 E_NOSOURCEDIR=71 E_UNMOUNTED=72 E_BACKUP=73 ##### END CONFIGURATION SECTION ##############################################   # Check that all the important variables have been set: if [ -z "$LOCAL_USER" ] ||    [ -z "$SOURCE_DIR" ] ||    [ -z "$MOUNT_POINT" ]  ||    [ -z "$BACKUP_DEST_DIR" ] then    echo 'One of the variables is not set! Edit the file: $0. BACKUP FAILED.'    exit $E_VARS_NOT_SET fi  if [ "$#" != 0 ]  # If command-line param(s) . . . then              # Here document(ation).   cat <<-ENDOFTEXT     Automatic Nightly backup run from cron.     Read the source for more details: $0     The backup directory is $BACKUP_DEST_DIR .     It will be created if necessary; initialisation is no longer required.      WARNING: Contents of $BACKUP_DEST_DIR are rotated.     Directories named 'backup.\$i' will eventually be DELETED.     We keep backups from every day for 7 days (1-8),     then every week for 4 weeks (9-12),     then every month for 3 months (13-15).      You may wish to add this to your crontab using 'crontab -e'     #  Back up files: $SOURCE_DIR to $BACKUP_DEST_DIR     #+ every night at 3:15 am          15 03 * * * /home/$LOCAL_USER/bin/nightly-backup_firewire-hdd.sh      Don't forget to verify the backups are working,     especially if you don't read cron's mail!" 	ENDOFTEXT    exit $E_COMMANDLINE fi   # Parse the options. # ==================  if [ "$DRY_RUN" == "true" ]; then   DRY_RUN="-n"   echo "WARNING:"   echo "THIS IS A 'DRY RUN'!"   echo "No data will actually be transferred!" else   DRY_RUN="" fi  if [ "$VERBOSE" == "true" ]; then   VERBOSE="-v" else   VERBOSE="" fi  if [ "$COMPRESS" == "true" ]; then   COMPRESS="-z" else   COMPRESS="" fi   #  Every week (actually of 8 days) and every month, #+ extra backups are preserved. DAY_OF_MONTH=`date +%d`            # Day of month (01..31). if [ $DAY_OF_MONTH = 01 ]; then    # First of month.   MONTHSTART=true elif [ $DAY_OF_MONTH = 08 \     -o $DAY_OF_MONTH = 16 \     -o $DAY_OF_MONTH = 24 ]; then     # Day 8,16,24  (use 8, not 7 to better handle 31-day months)       WEEKSTART=true fi    #  Check that the HDD is mounted. #  At least, check that *something* is mounted here! #  We can use something unique to the device, rather than just guessing #+ the scsi-id by having an appropriate udev rule in #+ /etc/udev/rules.d/10-rules.local #+ and by putting a relevant entry in /etc/fstab. #  Eg: this udev rule: # BUS="scsi", KERNEL="sd*", SYSFS{vendor}="WDC WD16", # SYSFS{model}="00JB-00GVA0     ", NAME="%k", SYMLINK="lacie_1394d%n"  if mount | grep $MOUNT_POINT >/dev/null; then   echo "Mount point $MOUNT_POINT is indeed mounted. OK" else   echo -n "Attempting to mount $MOUNT_POINT..."	            # If it isn't mounted, try to mount it.   sudo mount $MOUNT_POINT 2>/dev/null    if mount | grep $MOUNT_POINT >/dev/null; then     UNMOUNT_LATER=TRUE     echo "OK"     #  Note: Ensure that this is also unmounted     #+ if we exit prematurely with failure.   else     echo "FAILED"     echo -e "Nothing is mounted at $MOUNT_POINT. BACKUP FAILED!"     exit $E_MOUNT_FAIL   fi fi   # Check that source dir exists and is readable. if [ ! -r  $SOURCE_DIR ] ; then   echo "$SOURCE_DIR does not exist, or cannot be read. BACKUP FAILED."   exit $E_NOSOURCEDIR fi   # Check that the backup directory structure is as it should be. # If not, create it. # Create the subdirectories. # Note that backup.0 will be created as needed by rsync.  for ((i=1;i<=15;i++)); do   if [ ! -d $BACKUP_DEST_DIR/backup.$i ]; then     if /bin/mkdir -p $BACKUP_DEST_DIR/backup.$i ; then     #  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  No [ ] test brackets. Why?       echo "Warning: directory $BACKUP_DEST_DIR/backup.$i is missing,"       echo "or was not initialised. (Re-)creating it."     else       echo "ERROR: directory $BACKUP_DEST_DIR/backup.$i"       echo "is missing and could not be created."     if  [ "$UNMOUNT_LATER" == "TRUE" ]; then         # Before we exit, unmount the mount point if necessary.         cd 	sudo umount $MOUNT_POINT && 	echo "Unmounted $MOUNT_POINT again. Giving up."     fi       exit $E_UNMOUNTED   fi fi done   #  Set the permission to 700 for security #+ on an otherwise permissive multi-user system. if ! /bin/chmod 700 $BACKUP_DEST_DIR ; then   echo "ERROR: Could not set permissions on $BACKUP_DEST_DIR to 700."    if  [ "$UNMOUNT_LATER" == "TRUE" ]; then   # Before we exit, unmount the mount point if necessary.      cd ; sudo umount $MOUNT_POINT \      && echo "Unmounted $MOUNT_POINT again. Giving up."   fi    exit $E_UNMOUNTED fi  # Create the symlink: current -> backup.1 if required. # A failure here is not critical. cd $BACKUP_DEST_DIR if [ ! -h current ] ; then   if ! /bin/ln -s backup.1 current ; then     echo "WARNING: could not create symlink current -> backup.1"   fi fi   # Now, do the rsync. echo "Now doing backup with rsync..." echo "Source dir: $SOURCE_DIR" echo -e "Backup destination dir: $BACKUP_DEST_DIR\n"   /usr/bin/rsync $DRY_RUN $VERBOSE -a -S --delete --modify-window=60 \ --link-dest=../backup.1 $SOURCE_DIR $BACKUP_DEST_DIR/backup.0/  #  Only warn, rather than exit if the rsync failed, #+ since it may only be a minor problem. #  E.g., if one file is not readable, rsync will fail. #  This shouldn't prevent the rotation. #  Not using, e.g., `date +%a`  since these directories #+ are just full of links and don't consume *that much* space.  if [ $? != 0 ]; then   BACKUP_JUSTINCASE=backup.`date +%F_%T`.justincase   echo "WARNING: the rsync process did not entirely succeed."   echo "Something might be wrong."   echo "Saving an extra copy at: $BACKUP_JUSTINCASE"   echo "WARNING: if this occurs regularly, a LOT of space will be consumed,"   echo "even though these are just hard-links!" fi  # Save a readme in the backup parent directory. # Save another one in the recent subdirectory. echo "Backup of $SOURCE_DIR on `hostname` was last run on \ `date`" > $BACKUP_DEST_DIR/README.txt echo "This backup of $SOURCE_DIR on `hostname` was created on \ `date`" > $BACKUP_DEST_DIR/backup.0/README.txt  # If we are not in a dry run, rotate the backups. [ -z "$DRY_RUN" ] &&    #  Check how full the backup disk is.   #  Warn if 90%. if 98% or more, we'll probably fail, so give up.   #  (Note: df can output to more than one line.)   #  We test this here, rather than before   #+ so that rsync may possibly have a chance.   DISK_FULL_PERCENT=`/bin/df $BACKUP_DEST_DIR |   tr "\n" ' ' | awk '{print $12}' | grep -oE [0-9]+ `   echo "Disk space check on backup partition \   $MOUNT_POINT $DISK_FULL_PERCENT% full."   if [ $DISK_FULL_PERCENT -gt 90 ]; then     echo "Warning: Disk is greater than 90% full."   fi   if [ $DISK_FULL_PERCENT -gt 98 ]; then     echo "Error: Disk is full! Giving up."       if  [ "$UNMOUNT_LATER" == "TRUE" ]; then         # Before we exit, unmount the mount point if necessary.         cd; sudo umount $MOUNT_POINT &&         echo "Unmounted $MOUNT_POINT again. Giving up."       fi     exit $E_UNMOUNTED   fi    # Create an extra backup.  # If this copy fails, give up.  if [ -n "$BACKUP_JUSTINCASE" ]; then    if ! /bin/cp -al $BACKUP_DEST_DIR/backup.0 \       $BACKUP_DEST_DIR/$BACKUP_JUSTINCASE    then      echo "ERROR: Failed to create extra copy \      $BACKUP_DEST_DIR/$BACKUP_JUSTINCASE"      if  [ "$UNMOUNT_LATER" == "TRUE" ]; then        # Before we exit, unmount the mount point if necessary.        cd ;sudo umount $MOUNT_POINT &&        echo "Unmounted $MOUNT_POINT again. Giving up."      fi      exit $E_UNMOUNTED    fi  fi    # At start of month, rotate the oldest 8.  if [ "$MONTHSTART" == "true" ]; then    echo -e "\nStart of month. \    Removing oldest backup: $BACKUP_DEST_DIR/backup.15"  &&    /bin/rm -rf  $BACKUP_DEST_DIR/backup.15  &&    echo "Rotating monthly,weekly backups: \    $BACKUP_DEST_DIR/backup.[8-14] -> $BACKUP_DEST_DIR/backup.[9-15]"  &&      /bin/mv $BACKUP_DEST_DIR/backup.14 $BACKUP_DEST_DIR/backup.15  &&      /bin/mv $BACKUP_DEST_DIR/backup.13 $BACKUP_DEST_DIR/backup.14  &&      /bin/mv $BACKUP_DEST_DIR/backup.12 $BACKUP_DEST_DIR/backup.13  &&      /bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12  &&      /bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11  &&      /bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10  &&      /bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9   # At start of week, rotate the second-oldest 4.  elif [ "$WEEKSTART" == "true" ]; then    echo -e "\nStart of week. \    Removing oldest weekly backup: $BACKUP_DEST_DIR/backup.12"  &&    /bin/rm -rf  $BACKUP_DEST_DIR/backup.12  &&     echo "Rotating weekly backups: \    $BACKUP_DEST_DIR/backup.[8-11] -> $BACKUP_DEST_DIR/backup.[9-12]"  &&      /bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12  &&      /bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11  &&      /bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10  &&      /bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9   else    echo -e "\nRemoving oldest daily backup: $BACKUP_DEST_DIR/backup.8"  &&      /bin/rm -rf  $BACKUP_DEST_DIR/backup.8   fi  &&   # Every day, rotate the newest 8.  echo "Rotating daily backups: \  $BACKUP_DEST_DIR/backup.[1-7] -> $BACKUP_DEST_DIR/backup.[2-8]"  &&      /bin/mv $BACKUP_DEST_DIR/backup.7 $BACKUP_DEST_DIR/backup.8  &&      /bin/mv $BACKUP_DEST_DIR/backup.6 $BACKUP_DEST_DIR/backup.7  &&      /bin/mv $BACKUP_DEST_DIR/backup.5 $BACKUP_DEST_DIR/backup.6  &&      /bin/mv $BACKUP_DEST_DIR/backup.4 $BACKUP_DEST_DIR/backup.5  &&      /bin/mv $BACKUP_DEST_DIR/backup.3 $BACKUP_DEST_DIR/backup.4  &&      /bin/mv $BACKUP_DEST_DIR/backup.2 $BACKUP_DEST_DIR/backup.3  &&      /bin/mv $BACKUP_DEST_DIR/backup.1 $BACKUP_DEST_DIR/backup.2  &&      /bin/mv $BACKUP_DEST_DIR/backup.0 $BACKUP_DEST_DIR/backup.1  &&   SUCCESS=true   if  [ "$UNMOUNT_LATER" == "TRUE" ]; then   # Unmount the mount point if it wasn't mounted to begin with.   cd ; sudo umount $MOUNT_POINT && echo "Unmounted $MOUNT_POINT again." fi   if [ "$SUCCESS" == "true" ]; then   echo 'SUCCESS!'   exit 0 fi  # Should have already exited if backup worked. echo 'BACKUP FAILED! Is this just a dry run? Is the disk full?) ' exit $E_BACKUP

Example A-33. An expanded cd command

########################################################################### # #       cdll #       by Phil Braham # #       ############################################ #       Latest version of this script available from #       http://freshmeat.net/projects/cd/ #       ############################################ # #       .cd_new # #       An enhancement of the Unix cd command # #       There are unlimited stack entries and special entries. The stack #       entries keep the last cd_maxhistory #       directories that have been used. The special entries can be #       assigned to commonly used directories. # #       The special entries may be pre-assigned by setting the environment #       variables CDSn or by using the -u or -U command. # #       The following is a suggestion for the .profile file: # #               . cdll              #  Set up the cd command #       alias cd='cd_new'           #  Replace the cd command #               cd -U               #  Upload pre-assigned entries for #                                   #+ the stack and special entries #               cd -D               #  Set non-default mode #               alias @="cd_new @"  #  Allow @ to be used to get history # #       For help type: # #               cd -h or #               cd -H # # ########################################################################### # #       Version 1.2.1 # #       Written by Phil Braham - Realtime Software Pty Ltd #       (realtime@mpx.com.au) #       Please send any suggestions or enhancements to the author (also at #       phil@braham.net) # ############################################################################  cd_hm () {         ${PRINTF} "%s" "cd [dir] [0-9] [@[s|h] [-g [<dir>]] [-d] \ [-D] [-r<n>] [dir|0-9] [-R<n>] [<dir>|0-9]    [-s<n>] [-S<n>] [-u] [-U] [-f] [-F] [-h] [-H] [-v]     <dir> Go to directory     0-n         Go to previous directory (0 is previous, 1 is last but 1 etc)                 n is up to max history (default is 50)     @           List history and special entries     @h          List history entries     @s          List special entries     -g [<dir>]  Go to literal name (bypass special names)                 This is to allow access to dirs called '0','1','-h' etc     -d          Change default action - verbose. (See note)     -D          Change default action - silent. (See note)     -s<n> Go to the special entry <n>*     -S<n> Go to the special entry <n>                 and replace it with the current dir*     -r<n> [<dir>] Go to directory <dir>                               and then put it on special entry <n>*     -R<n> [<dir>] Go to directory <dir>                               and put current dir on special entry <n>*     -a<n>       Alternative suggested directory. See note below.     -f [<file>] File entries to <file>.     -u [<file>] Update entries from <file>.                 If no filename supplied then default file                 (${CDPath}${2:-"$CDFile"}) is used                 -F and -U are silent versions     -v          Print version number     -h          Help     -H          Detailed help      *The special entries (0 - 9) are held until log off, replaced by another      entry or updated with the -u command      Alternative suggested directories:     If a directory is not found then CD will suggest any     possibilities. These are directories starting with the same letters     and if any are found they are listed prefixed with -a<n>     where <n> is a number.     It's possible to go to the directory by entering cd -a<n>     on the command line.          The directory for -r<n> or -R<n> may be a number.     For example:         $ cd -r3 4  Go to history entry 4 and put it on special entry 3         $ cd -R3 4  Put current dir on the special entry 3                     and go to history entry 4         $ cd -s3    Go to special entry 3          Note that commands R,r,S and s may be used without a number     and refer to 0:         $ cd -s     Go to special entry 0         $ cd -S     Go to special entry 0 and make special                     entry 0 current dir         $ cd -r 1   Go to history entry 1 and put it on special entry 0         $ cd -r     Go to history entry 0 and put it on special entry 0     "         if ${TEST} "$CD_MODE" = "PREV"         then                 ${PRINTF} "$cd_mnset"         else                 ${PRINTF} "$cd_mset"         fi }  cd_Hm () {         cd_hm         ${PRINTF} "%s" "         The previous directories (0-$cd_maxhistory) are stored in the         environment variables CD[0] - CD[$cd_maxhistory]         Similarly the special directories S0 - $cd_maxspecial are in         the environment variable CDS[0] - CDS[$cd_maxspecial]         and may be accessed from the command line          The default pathname for the -f and -u commands is $CDPath         The default filename for the -f and -u commands is $CDFile          Set the following environment variables:             CDL_PROMPTLEN  - Set to the length of prompt you require.                 Prompt string is set to the right characters of the                 current directory.                 If not set then prompt is left unchanged             CDL_PROMPT_PRE - Set to the string to prefix the prompt.                 Default is:                     non-root:  \"\\[\\e[01;34m\\]\"  (sets colour to blue).                     root:      \"\\[\\e[01;31m\\]\"  (sets colour to red).             CDL_PROMPT_POST    - Set to the string to suffix the prompt.                 Default is:                     non-root:  \"\\[\\e[00m\\]$\"                                 (resets colour and displays $).                     root:      \"\\[\\e[00m\\]#\"                                 (resets colour and displays #).             CDPath - Set the default path for the -f & -u options.                      Default is home directory             CDFile - Set the default filename for the -f & -u options.                      Default is cdfile          "     cd_version  }  cd_version () {  printf "Version: ${VERSION_MAJOR}.${VERSION_MINOR} Date: ${VERSION_DATE}\n" }  # # Truncate right. # # params: #   p1 - string #   p2 - length to truncate to # # returns string in tcd # cd_right_trunc () {     local tlen=${2}     local plen=${#1}     local str="${1}"     local diff     local filler="<--"     if ${TEST} ${plen} -le ${tlen}     then         tcd="${str}"     else         let diff=${plen}-${tlen}         elen=3         if ${TEST} ${diff} -le 2         then             let elen=${diff}         fi         tlen=-${tlen}         let tlen=${tlen}+${elen}         tcd=${filler:0:elen}${str:tlen}     fi }  # # Three versions of do history: #    cd_dohistory  - packs history and specials side by side #    cd_dohistoryH - Shows only hstory #    cd_dohistoryS - Shows only specials # cd_dohistory () {     cd_getrc         ${PRINTF} "History:\n"     local -i count=${cd_histcount}     while ${TEST} ${count} -ge 0     do         cd_right_trunc "${CD[count]}" ${cd_lchar}             ${PRINTF} "%2d %-${cd_lchar}.${cd_lchar}s " ${count} "${tcd}"          cd_right_trunc "${CDS[count]}" ${cd_rchar}             ${PRINTF} "S%d %-${cd_rchar}.${cd_rchar}s\n" ${count} "${tcd}"         count=${count}-1     done }  cd_dohistoryH () {     cd_getrc         ${PRINTF} "History:\n"         local -i count=${cd_maxhistory}         while ${TEST} ${count} -ge 0         do           ${PRINTF} "${count} %-${cd_flchar}.${cd_flchar}s\n" ${CD[$count]}           count=${count}-1         done }  cd_dohistoryS () {     cd_getrc         ${PRINTF} "Specials:\n"         local -i count=${cd_maxspecial}         while ${TEST} ${count} -ge 0         do           ${PRINTF} "S${count} %-${cd_flchar}.${cd_flchar}s\n" ${CDS[$count]}           count=${count}-1         done }  cd_getrc () {     cd_flchar=$(stty -a | awk -F \;     '/rows/ { print $2 $3 }' | awk -F \  '{ print $4 }')     if ${TEST} ${cd_flchar} -ne 0     then         cd_lchar=${cd_flchar}/2-5         cd_rchar=${cd_flchar}/2-5             cd_flchar=${cd_flchar}-5     else             cd_flchar=${FLCHAR:=75} 	    # cd_flchar is used for for the @s & @h history             cd_lchar=${LCHAR:=35}             cd_rchar=${RCHAR:=35}     fi }  cd_doselection () {         local -i nm=0         cd_doflag="TRUE"         if ${TEST} "${CD_MODE}" = "PREV"         then                 if ${TEST} -z "$cd_npwd"                 then                         cd_npwd=0                 fi         fi         tm=$(echo "${cd_npwd}" | cut -b 1)     if ${TEST} "${tm}" = "-"     then         pm=$(echo "${cd_npwd}" | cut -b 2)         nm=$(echo "${cd_npwd}" | cut -d $pm -f2)         case "${pm}" in              a) cd_npwd=${cd_sugg[$nm]} ;;              s) cd_npwd="${CDS[$nm]}" ;;              S) cd_npwd="${CDS[$nm]}" ; CDS[$nm]=`pwd` ;;              r) cd_npwd="$2" ; cd_specDir=$nm ; cd_doselection "$1" "$2";;              R) cd_npwd="$2" ; CDS[$nm]=`pwd` ; cd_doselection "$1" "$2";;         esac     fi      if ${TEST} "${cd_npwd}" != "." -a "${cd_npwd}" \ != ".." -a "${cd_npwd}" -le ${cd_maxhistory} >>/dev/null 2>&1     then       cd_npwd=${CD[$cd_npwd]}      else        case "$cd_npwd" in                 @)  cd_dohistory ; cd_doflag="FALSE" ;;                @h) cd_dohistoryH ; cd_doflag="FALSE" ;;                @s) cd_dohistoryS ; cd_doflag="FALSE" ;;                -h) cd_hm ; cd_doflag="FALSE" ;;                -H) cd_Hm ; cd_doflag="FALSE" ;;                -f) cd_fsave "SHOW" $2 ; cd_doflag="FALSE" ;;                -u) cd_upload "SHOW" $2 ; cd_doflag="FALSE" ;;                -F) cd_fsave "NOSHOW" $2 ; cd_doflag="FALSE" ;;                -U) cd_upload "NOSHOW" $2 ; cd_doflag="FALSE" ;;                -g) cd_npwd="$2" ;;                -d) cd_chdefm 1; cd_doflag="FALSE" ;;                -D) cd_chdefm 0; cd_doflag="FALSE" ;;                -r) cd_npwd="$2" ; cd_specDir=0 ; cd_doselection "$1" "$2";;                -R) cd_npwd="$2" ; CDS[0]=`pwd` ; cd_doselection "$1" "$2";;                -s) cd_npwd="${CDS[0]}" ;;                -S) cd_npwd="${CDS[0]}"  ; CDS[0]=`pwd` ;;                -v) cd_version ; cd_doflag="FALSE";;        esac     fi }  cd_chdefm () {         if ${TEST} "${CD_MODE}" = "PREV"         then                 CD_MODE=""                 if ${TEST} $1 -eq 1                 then                         ${PRINTF} "${cd_mset}"                 fi         else                 CD_MODE="PREV"                 if ${TEST} $1 -eq 1                 then                         ${PRINTF} "${cd_mnset}"                 fi         fi }  cd_fsave () {         local sfile=${CDPath}${2:-"$CDFile"}         if ${TEST} "$1" = "SHOW"         then                 ${PRINTF} "Saved to %s\n" $sfile         fi         ${RM} -f ${sfile}         local -i count=0         while ${TEST} ${count} -le ${cd_maxhistory}         do                 echo "CD[$count]=\"${CD[$count]}\"" >> ${sfile}                 count=${count}+1         done         count=0         while ${TEST} ${count} -le ${cd_maxspecial}         do                 echo "CDS[$count]=\"${CDS[$count]}\"" >> ${sfile}                 count=${count}+1         done }  cd_upload () {         local sfile=${CDPath}${2:-"$CDFile"}         if ${TEST} "${1}" = "SHOW"         then                 ${PRINTF} "Loading from %s\n" ${sfile}         fi         . ${sfile} }  cd_new () {     local -i count     local -i choose=0          cd_npwd="${1}"         cd_specDir=-1         cd_doselection "${1}" "${2}"          if ${TEST} ${cd_doflag} = "TRUE"         then                 if ${TEST} "${CD[0]}" != "`pwd`"                 then                         count=$cd_maxhistory                         while ${TEST} $count -gt 0                         do                                 CD[$count]=${CD[$count-1]}                                 count=${count}-1                         done                         CD[0]=`pwd`                 fi                 command cd "${cd_npwd}" 2>/dev/null         if ${TEST} $? -eq 1         then             ${PRINTF} "Unknown dir: %s\n" "${cd_npwd}"             local -i ftflag=0             for i in "${cd_npwd}"*             do                 if ${TEST} -d "${i}"                 then                     if ${TEST} ${ftflag} -eq 0                     then                         ${PRINTF} "Suggest:\n"                         ftflag=1                 fi                     ${PRINTF} "\t-a${choose} %s\n" "$i"                                         cd_sugg[$choose]="${i}"                     choose=${choose}+1         fi             done         fi         fi          if ${TEST} ${cd_specDir} -ne -1         then                 CDS[${cd_specDir}]=`pwd`         fi          if ${TEST} ! -z "${CDL_PROMPTLEN}"         then         cd_right_trunc "${PWD}" ${CDL_PROMPTLEN}             cd_rp=${CDL_PROMPT_PRE}${tcd}${CDL_PROMPT_POST}                 export PS1="$(echo -ne ${cd_rp})"         fi } ######################################################################### #                                                                       # #                        Initialisation here                            # #                                                                       # ######################################################################### # VERSION_MAJOR="1" VERSION_MINOR="2.1" VERSION_DATE="24-MAY-2003" # alias cd=cd_new # # Set up commands RM=/bin/rm TEST=test PRINTF=printf              # Use builtin printf  ######################################################################### #                                                                       # # Change this to modify the default pre- and post prompt strings.       # # These only come into effect if CDL_PROMPTLEN is set.                  # #                                                                       # ######################################################################### if ${TEST} ${EUID} -eq 0 then #   CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="$HOSTNAME@"}     CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;31m\\]"}  # Root is in red     CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]#"} else     CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;34m\\]"}  # Users in blue     CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]$"} fi ######################################################################### # # cd_maxhistory defines the max number of history entries allowed. typeset -i cd_maxhistory=50  ######################################################################### # # cd_maxspecial defines the number of special entries. typeset -i cd_maxspecial=9 # # ######################################################################### # #  cd_histcount defines the number of entries displayed in #+ the history command. typeset -i cd_histcount=9 # ######################################################################### export CDPath=${HOME}/ #  Change these to use a different                                      # #+ default path and filename                                            # export CDFile=${CDFILE:=cdfile}           # for the -u and -f commands  # # #########################################################################                                                                         # typeset -i cd_lchar cd_rchar cd_flchar                         #  This is the number of chars to allow for the # cd_flchar=${FLCHAR:=75} #+ cd_flchar is used for for the @s & @h history#  typeset -ax CD CDS # cd_mset="\n\tDefault mode is now set - entering cd with no parameters \ has the default action\n\tUse cd -d or -D for cd to go to \ previous directory with no parameters\n" cd_mnset="\n\tNon-default mode is now set - entering cd with no \ parameters is the same as entering cd 0\n\tUse cd -d or \ -D to change default cd action\n"  # ==================================================================== #    : <<DOCUMENTATION  Written by Phil Braham. Realtime Software Pty Ltd. Released under GNU license. Free to use. Please pass any modifications or comments to the author Phil Braham:  realtime@mpx.com.au =======================================================================  cdll is a replacement for cd and incorporates similar functionality to the bash pushd and popd commands but is independent of them.  This version of cdll has been tested on Linux using Bash. It will work on most Linux versions but will probably not work on other shells without modification.  Introduction ============  cdll allows easy moving about between directories. When changing to a new directory the current one is automatically put onto a stack. By default 50 entries are kept, but this is configurable. Special directories can be kept for easy access - by default up to 10, but this is configurable. The most recent stack entries and the special entries can be easily viewed.  The directory stack and special entries can be saved to, and loaded from, a file. This allows them to be set up on login, saved before logging out or changed when moving project to project.  In addition, cdll provides a flexible command prompt facility that allows, for example, a directory name in colour that is truncated from the left if it gets too long.   Setting up cdll ===============  Copy cdll to either your local home directory or a central directory such as /usr/bin (this will require root access).  Copy the file cdfile to your home directory. It will require read and write access. This a default file that contains a directory stack and special entries.  To replace the cd command you must add commands to your login script. The login script is one or more of:      /etc/profile     ~/.bash_profile     ~/.bash_login     ~/.profile     ~/.bashrc     /etc/bash.bashrc.local      To setup your login, ~/.bashrc is recommended, for global (and root) setup add the commands to /etc/bash.bashrc.local      To set up on login, add the command:     . <dir>/cdll For example if cdll is in your local home directory:     . ~/cdll If in /usr/bin then:     . /usr/bin/cdll  If you want to use this instead of the buitin cd command then add:     alias cd='cd_new' We would also recommend the following commands:     alias @='cd_new @'     cd -U     cd -D  If you want to use cdll's prompt facilty then add the following:     CDL_PROMPTLEN=nn Where nn is a number described below. Initially 99 would be suitable number.  Thus the script looks something like this:      ######################################################################     # CD Setup     ######################################################################     CDL_PROMPTLEN=21        # Allow a prompt length of up to 21 characters     . /usr/bin/cdll         # Initialise cdll     alias cd='cd_new'       # Replace the built in cd command     alias @='cd_new @'      # Allow @ at the prompt to display history     cd -U                   # Upload directories     cd -D                   # Set default action to non-posix     ######################################################################  The full meaning of these commands will become clear later.  There are a couple of caveats. If another program changes the directory without calling cdll, then the directory won't be put on the stack and also if the prompt facility is used then this will not be updated. Two programs that can do this are pushd and popd. To update the prompt and stack simply enter:      cd .      Note that if the previous entry on the stack is the current directory then the stack is not updated.  Usage =====   cd [dir] [0-9] [@[s|h] [-g <dir>] [-d] [-D] [-r<n>]    [dir|0-9] [-R<n>] [<dir>|0-9] [-s<n>] [-S<n>]    [-u] [-U] [-f] [-F] [-h] [-H] [-v]      <dir>       Go to directory     0-n         Goto previous directory (0 is previous,                 1 is last but 1, etc.)                 n is up to max history (default is 50)     @           List history and special entries (Usually available as $ @)     @h          List history entries     @s          List special entries     -g [<dir>]  Go to literal name (bypass special names)                 This is to allow access to dirs called '0','1','-h' etc     -d          Change default action - verbose. (See note)     -D          Change default action - silent. (See note)     -s<n>       Go to the special entry <n>     -S<n>       Go to the special entry <n>                       and replace it with the current dir     -r<n> [<dir>] Go to directory <dir>                               and then put it on special entry <n>     -R<n> [<dir>] Go to directory <dir>                               and put current dir on special entry <n>     -a<n>       Alternative suggested directory. See note below.     -f [<file>] File entries to <file>.     -u [<file>] Update entries from <file>.                 If no filename supplied then default file (~/cdfile) is used                 -F and -U are silent versions     -v          Print version number     -h          Help     -H          Detailed help    Examples ========  These examples assume non-default mode is set (that is, cd with no parameters will go to the most recent stack directory), that aliases have been set up for cd and @ as described above and that cd's prompt facility is active and the prompt length is 21 characters.      /home/phil$ @     # List the entries with the @     History:     # Output of the @ command     .....     # Skipped these entries for brevity     1 /home/phil/ummdev               S1 /home/phil/perl     # Most recent two history entries     0 /home/phil/perl/eg              S0 /home/phil/umm/ummdev     # and two special entries are shown          /home/phil$ cd /home/phil/utils/Cdll     # Now change directories     /home/phil/utils/Cdll$ @     # Prompt reflects the directory.     History:     # New history     .....        1 /home/phil/perl/eg              S1 /home/phil/perl     # History entry 0 has moved to 1     0 /home/phil                      S0 /home/phil/umm/ummdev     # and the most recent has entered         To go to a history entry:      /home/phil/utils/Cdll$ cd 1     # Go to history entry 1.     /home/phil/perl/eg$     # Current directory is now what was 1      To go to a special entry:      /home/phil/perl/eg$ cd -s1     # Go to special entry 1     /home/phil/umm/ummdev$     # Current directory is S1  To go to a directory called, for example, 1:      /home/phil$ cd -g 1     # -g ignores the special meaning of 1     /home/phil/1$      To put current directory on the special list as S1:     cd -r1 .        #  OR     cd -R1 .        #  These have the same effect if the directory is                     #+ . (the current directory)  To go to a directory and add it as a special     The directory for -r<n> or -R<n> may be a number.   For example:         $ cd -r3 4  Go to history entry 4 and put it on special entry 3         $ cd -R3 4  Put current dir on the special entry 3 and go to                     history entry 4         $ cd -s3    Go to special entry 3      Note that commands R,r,S and s may be used without a number and     refer to 0:         $ cd -s     Go to special entry 0         $ cd -S     Go to special entry 0 and make special entry 0                     current dir         $ cd -r 1   Go to history entry 1 and put it on special entry 0         $ cd -r     Go to history entry 0 and put it on special entry 0       Alternative suggested directories:      If a directory is not found, then CD will suggest any     possibilities. These are directories starting with the same letters     and if any are found they are listed prefixed with -a<n>     where <n> is a number. It's possible to go to the directory     by entering cd -a<n> on the command line.          Use cd -d or -D to change default cd action. cd -H will show         current action.          The history entries (0-n) are stored in the environment variables         CD[0] - CD[n]         Similarly the special directories S0 - 9 are in the environment         variable CDS[0] - CDS[9]         and may be accessed from the command line, for example:                      ls -l ${CDS[3]}             cat ${CD[8]}/file.txt          The default pathname for the -f and -u commands is ~         The default filename for the -f and -u commands is cdfile   Configuration =============      The following environment variables can be set:              CDL_PROMPTLEN  - Set to the length of prompt you require.             Prompt string is set to the right characters of the current             directory. If not set, then prompt is left unchanged. Note             that this is the number of characters that the directory is             shortened to, not the total characters in the prompt.              CDL_PROMPT_PRE - Set to the string to prefix the prompt.                 Default is:                     non-root:  "\\[\\e[01;34m\\]"  (sets colour to blue).                     root:      "\\[\\e[01;31m\\]"  (sets colour to red).              CDL_PROMPT_POST    - Set to the string to suffix the prompt.                 Default is:                     non-root:  "\\[\\e[00m\\]$"                                (resets colour and displays $).                     root:      "\\[\\e[00m\\]#"                                (resets colour and displays #).          Note:             CDL_PROMPT_PRE & _POST only t          CDPath - Set the default path for the -f & -u options.                  Default is home directory         CDFile - Set the default filename for the -f & -u options.                  Default is cdfile       There are three variables defined in the file cdll which control the     number of entries stored or displayed. They are in the section labeled     'Initialisation here' towards the end of the file.          cd_maxhistory       - The number of history entries stored.                               Default is 50.         cd_maxspecial       - The number of special entries allowed.                               Default is 9.         cd_histcount        - The number of history and special entries                               displayed. Default is 9.      Note that cd_maxspecial should be >= cd_histcount to avoid displaying     special entries that can't be set.   Version: 1.2.1 Date: 24-MAY-2003  DOCUMENTATION

Example A-34. A soundcard setup script

#!/bin/bash # soundcard-on.sh  #  Script author: Mkarcher #  http://www.thinkwiki.org/wiki  ... #  /Script_for_configuring_the_CS4239_sound_chip_in_PnP_mode #  ABS Guide author made minor changes and added comments. #  Couldn't contact script author to ask for permission to use, but ... #+ the script was released under the FDL, #+ so its use here should be both legal and ethical.  #  Sound-via-pnp-script for Thinkpad 600E #+ and possibly other computers with onboard CS4239/CS4610 #+ that do not work with the PCI driver #+ and are not recognized by the PnP code of snd-cs4236. #  Also for some 770-series Thinkpads, such as the 770x. #  Run as root user, of course. # #  These are old and very obsolete laptop computers, #+ but this particular script is very instructive, #+ as it shows how to set up and hack device files.    #  Search for sound card pnp device:  for dev in /sys/bus/pnp/devices/* do   grep CSC0100 $dev/id > /dev/null && WSSDEV=$dev   grep CSC0110 $dev/id > /dev/null && CTLDEV=$dev done # On 770x: # WSSDEV = /sys/bus/pnp/devices/00:07 # CTLDEV = /sys/bus/pnp/devices/00:06 # These are symbolic links to /sys/devices/pnp0/ ...   #  Activate devices: #  Thinkpad boots with devices disabled unless "fast boot" is turned off #+ (in BIOS).  echo activate > $WSSDEV/resources echo activate > $CTLDEV/resources   # Parse resource settings.  { read # Discard "state = active" (see below).   read bla port1   read bla port2   read bla port3   read bla irq   read bla dma1   read bla dma2  # The "bla's" are labels in the first field: "io," "state," etc.  # These are discarded.   #  Hack: with PnPBIOS: ports are: port1: WSS, port2:  #+ OPL, port3: sb (unneeded)  #       with ACPI-PnP:ports are: port1: OPL, port2: sb, port3: WSS  #  (ACPI bios seems to be wrong here, the PnP-card-code in snd-cs4236.c  #+  uses the PnPBIOS port order)  #  Detect port order using the fixed OPL port as reference.   if [ ${port2%%-*} = 0x388 ]  #            ^^^^  Strip out everything following hyphen in port address.  #                  So, if port1 is 0x530-0x537  #+                 we're left with 0x530 -- the start address of the port.  then    # PnPBIOS: usual order    port=${port1%%-*}    oplport=${port2%%-*}  else    # ACPI: mixed-up order    port=${port3%%-*}    oplport=${port1%%-*}  fi  } < $WSSDEV/resources # To see what's going on here: # --------------------------- #   cat /sys/devices/pnp0/00:07/resources # #   state = active #   io 0x530-0x537 #   io 0x388-0x38b #   io 0x220-0x233 #   irq 5 #   dma 1 #   dma 0 #   ^^^   "bla" labels in first field (discarded).    { read # Discard first line, as above.   read bla port1   cport=${port1%%-*}   #            ^^^^   # Just want _start_ address of port. } < $CTLDEV/resources   # Load the module:  modprobe --ignore-install snd-cs4236 port=$port cport=$cport\ fm_port=$oplport irq=$irq dma1=$dma1 dma2=$dma2 isapnp=0 index=0 # See the modprobe manpage.  exit $?

Example A-35. Locating split paragraphs in a text file

#!/bin/bash # find-splitpara.sh #  Finds split paragraphs in a text file, #+ and tags the line numbers.   ARGCOUNT=1       # Expect one arg. OFF=0            # Flag states. ON=1 E_WRONGARGS=85  file="$1"        # Target filename. lineno=1         # Line number. Start at 1. Flag=$OFF        # Blank line flag.  if [ $# -ne "$ARGCOUNT" ] then   echo "Usage: `basename $0` FILENAME"   exit $E_WRONGARGS fi    file_read ()     # Scan file for pattern, then print line. { while read line do    if [[ "$line" =~ ^[a-z] && $Flag -eq $ON ]]      then  # Line begins with lowercase character, following blank line.      echo -n "$lineno::   "      echo "$line"   fi     if [[ "$line" =~ ^$ ]]      then       #  If blank line,      Flag=$ON   #+ set flag.   else      Flag=$OFF   fi    ((lineno++))  done } < $file  # Redirect file into function's stdin.  file_read   exit $?   # ---------------------------------------------------------------- This is line one of an example paragraph, bla, bla, bla. This is line two, and line three should follow on next line, but  there is a blank line separating the two parts of the paragraph. # ----------------------------------------------------------------  Running this script on a file containing the above paragraph yields:  4::   there is a blank line separating the two parts of the paragraph.   There will be additional output for all the other split paragraphs in the target file.

Example A-36. Insertion sort

#!/bin/bash # insertion-sort.bash: Insertion sort implementation in Bash #                      Heavy use of Bash array features: #+                     (string) slicing, merging, etc # URL: http://www.lugmen.org.ar/~jjo/jjotip/insertion-sort.bash.d #+          /insertion-sort.bash.sh # # Author: JuanJo Ciarlante <jjo@irrigacion.gov.ar> # Lightly reformatted by ABS Guide author. # License: GPLv2 # Used in ABS Guide with author's permission (thanks!). # # Test with:   ./insertion-sort.bash -t # Or:          bash insertion-sort.bash -t # The following *doesn't* work: #              sh insertion-sort.bash -t #  Why not? Hint: which Bash-specific features are disabled #+ when running a script by 'sh script.sh'? # : ${DEBUG:=0}  # Debug, override with:  DEBUG=1 ./scriptname . . . # Parameter substitution -- set DEBUG to 0 if not previously set.  # Global array: "list" typeset -a list # Load whitespace-separated numbers from stdin. if [ "$1" = "-t" ]; then DEBUG=1         read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) # Random list. #                    ^ ^  process substition else         read -a list fi numelem=${#list[*]}  #  Shows the list, marking the element whose index is $1 #+ by surrounding it with the two chars passed as $2. #  Whole line prefixed with $3. showlist()   {   echo "$3"${list[@]:0:$1} ${2:0:1}${list[$1]}${2:1:1} ${list[@]:$1+1};   }  # Loop _pivot_ -- from second element to end of list. for(( i=1; i<numelem; i++ )) do         ((DEBUG))&&showlist i "[]" " "         # From current _pivot_, back to first element.         for(( j=i; j; j-- )) do                 # Search for the 1st elem. less than current "pivot" . . .                 [[ "${list[j-1]}" -le "${list[i]}" ]] && break         done 	(( i==j )) && continue ## No insertion was needed for this element. 	# . . . Move list[i] (pivot) to the left of list[j]:         list=(${list[@]:0:j} ${list[i]} ${list[j]}\ 	#         {0,j-1}        {i}       {j}               ${list[@]:j+1:i-(j+1)} ${list[@]:i+1}) 	#         {j+1,i-1}              {i+1,last} 	((DEBUG))&&showlist j "<>" "*" done   echo echo  "------" echo $'Result:\n'${list[@]}  exit $?

Example A-37. Standard Deviation

#!/bin/bash # sd.sh: Standard Deviation  #  The Standard Deviation indicates how consistent a set of data is. #  It shows to what extent the individual data points deviate from the #+ arithmetic mean, i.e., how much they "bounce around" (or cluster). #  It is essentially the average deviation-distance of the #+ data points from the mean.  # =========================================================== # #    To calculate the Standard Deviation: # # 1  Find the arithmetic mean (average) of all the data points. # 2  Subtract each data point from the arithmetic mean, #    and square that difference. # 3  Add all of the individual difference-squares in # 2. # 4  Divide the sum in # 3 by the number of data points. #    This is known as the "variance." # 5  The square root of # 4 gives the Standard Deviation. # =========================================================== #  count=0         # Number of data points; global. SC=9            # Scale to be used by bc. Nine decimal places. E_DATAFILE=90   # Data file error.  # ----------------- Set data file --------------------- if [ ! -z "$1" ]  # Specify filename as cmd-line arg? then   datafile="$1" #  ASCII text file, else            #+ one (numerical) data point per line!   datafile=sample.dat fi              #  See example data file, below.  if [ ! -e "$datafile" ] then   echo "\""$datafile"\" does not exist!"   exit $E_DATAFILE fi # -----------------------------------------------------   arith_mean () {   local rt=0         # Running total.   local am=0         # Arithmetic mean.   local ct=0         # Number of data points.    while read value   # Read one data point at a time.   do     rt=$(echo "scale=$SC; $rt + $value" | bc)     (( ct++ ))   done    am=$(echo "scale=$SC; $rt / $ct" | bc)    echo $am; return $ct   # This function "returns" TWO values!   #  Caution: This little trick will not work if $ct > 255!   #  To handle a larger number of data points,   #+ simply comment out the "return $ct" above. } <"$datafile"   # Feed in data file.  sd () {   mean1=$1  # Arithmetic mean (passed to function).   n=$2      # How many data points.   sum2=0    # Sum of squared differences ("variance").   avg2=0    # Average of $sum2.   sdev=0    # Standard Deviation.    while read value   # Read one line at a time.   do     diff=$(echo "scale=$SC; $mean1 - $value" | bc)     # Difference between arith. mean and data point.     dif2=$(echo "scale=$SC; $diff * $diff" | bc) # Squared.     sum2=$(echo "scale=$SC; $sum2 + $dif2" | bc) # Sum of squares.   done      avg2=$(echo "scale=$SC; $sum2 / $n" | bc)  # Avg. of sum of squares.     sdev=$(echo "scale=$SC; sqrt($avg2)" | bc) # Square root =     echo $sdev                                 # Standard Deviation.  } <"$datafile"   # Rewinds data file.   # ======================================================= # mean=$(arith_mean); count=$?   # Two returns from function! std_dev=$(sd $mean $count)  echo echo "Number of data points in \""$datafile"\" = $count" echo "Arithmetic mean (average) = $mean" echo "Standard Deviation = $std_dev" echo # ======================================================= #  exit  #  This script could stand some drastic streamlining, #+ but not at the cost of reduced legibility, please.   # ++++++++++++++++++++++++++++++++++++++++ # # A sample data file (sample1.dat):  # 18.35 # 19.0 # 18.88 # 18.91 # 18.64   # $ sh sd.sh sample1.dat  # Number of data points in "sample1.dat" = 5 # Arithmetic mean (average) = 18.756000000 # Standard Deviation = .235338054 # ++++++++++++++++++++++++++++++++++++++++ #

Example A-38. A pad file generator for shareware authors

#!/bin/bash # pad.sh  ####################################################### #               PAD (xml) file creator #+ Written by Mendel Cooper <thegrendel.abs@gmail.com>. #+ Released to the Public Domain. # #  Generates a "PAD" descriptor file for shareware #+ packages, according to the specifications #+ of the ASP. #  http://www.asp-shareware.org/pad #######################################################   # Accepts (optional) save filename as a command-line argument. if [ -n "$1" ] then   savefile=$1 else   savefile=save_file.xml               # Default save_file name. fi     # ===== PAD file headers ===== HDR1="<?xml version=\"1.0\" encoding=\"Windows-1252\" ?>" HDR2="<XML_DIZ_INFO>" HDR3="<MASTER_PAD_VERSION_INFO>" HDR4="\t<MASTER_PAD_VERSION>1.15</MASTER_PAD_VERSION>" HDR5="\t<MASTER_PAD_INFO>Portable Application Description, or PAD for short, is a data set that is used by shareware authors to disseminate information to anyone interested in their software products. To find out more go to http://www.asp-shareware.org/pad</MASTER_PAD_INFO>" HDR6="</MASTER_PAD_VERSION_INFO>" # ============================   fill_in () {   if [ -z "$2" ]   then     echo -n "$1? "     # Get user input.   else     echo -n "$1 $2? "  # Additional query?   fi      read var             # May paste to fill in field.                        # This shows how flexible "read" can be.    if [ -z "$var" ]   then     echo -e "\t\t<$1 />" >>$savefile    # Indent with 2 tabs.     return   else     echo -e "\t\t<$1>$var</$1>" >>$savefile     return ${#var}     # Return length of input string.   fi }      check_field_length ()  # Check length of program description fields. {   # $1 = maximum field length   # $2 = actual field length   if [ "$2" -gt "$1" ]   then     echo "Warning: Maximum field length of $1 characters exceeded!"   fi }    clear                  # Clear screen. echo "PAD File Creator" echo "--- ---- -------" echo  # Write File Headers to file. echo $HDR1 >$savefile echo $HDR2 >>$savefile echo $HDR3 >>$savefile echo -e $HDR4 >>$savefile echo -e $HDR5 >>$savefile echo $HDR6 >>$savefile   # Company_Info echo "COMPANY INFO" CO_HDR="Company_Info" echo "<$CO_HDR>" >>$savefile  fill_in Company_Name fill_in Address_1 fill_in Address_2 fill_in City_Town  fill_in State_Province fill_in Zip_Postal_Code fill_in Country  # If applicable: # fill_in ASP_Member "[Y/N]" # fill_in ASP_Member_Number # fill_in ESC_Member "[Y/N]"  fill_in Company_WebSite_URL  clear   # Clear screen between sections.     # Contact_Info echo "CONTACT INFO" CONTACT_HDR="Contact_Info" echo "<$CONTACT_HDR>" >>$savefile fill_in Author_First_Name fill_in Author_Last_Name fill_in Author_Email fill_in Contact_First_Name fill_in Contact_Last_Name fill_in Contact_Email echo -e "\t</$CONTACT_HDR>" >>$savefile    # END Contact_Info  clear     # Support_Info echo "SUPPORT INFO" SUPPORT_HDR="Support_Info" echo "<$SUPPORT_HDR>" >>$savefile fill_in Sales_Email fill_in Support_Email fill_in General_Email fill_in Sales_Phone fill_in Support_Phone fill_in General_Phone fill_in Fax_Phone echo -e "\t</$SUPPORT_HDR>" >>$savefile    # END Support_Info  echo "</$CO_HDR>" >>$savefile # END Company_Info  clear  # Program_Info  echo "PROGRAM INFO" PROGRAM_HDR="Program_Info" echo "<$PROGRAM_HDR>" >>$savefile fill_in Program_Name fill_in Program_Version fill_in Program_Release_Month fill_in Program_Release_Day fill_in Program_Release_Year fill_in Program_Cost_Dollars fill_in Program_Cost_Other fill_in Program_Type "[Shareware/Freeware/GPL]" fill_in Program_Release_Status "[Beta, Major Upgrade, etc.]" fill_in Program_Install_Support fill_in Program_OS_Support "[Win9x/Win2k/Linux/etc.]" fill_in Program_Language "[English/Spanish/etc.]"  echo; echo    # File_Info  echo "FILE INFO" FILEINFO_HDR="File_Info" echo "<$FILEINFO_HDR>" >>$savefile fill_in Filename_Versioned fill_in Filename_Previous fill_in Filename_Generic fill_in Filename_Long fill_in File_Size_Bytes fill_in File_Size_K fill_in File_Size_MB echo -e "\t</$FILEINFO_HDR>" >>$savefile   # END File_Info   clear    # Expire_Info  echo "EXPIRE INFO" EXPIRE_HDR="Expire_Info" echo "<$EXPIRE_HDR>" >>$savefile fill_in Has_Expire_Info "Y/N" fill_in Expire_Count fill_in Expire_Based_On fill_in Expire_Other_Info fill_in Expire_Month fill_in Expire_Day fill_in Expire_Year echo -e "\t</$EXPIRE_HDR>" >>$savefile   # END Expire_Info   clear    # More Program_Info echo "ADDITIONAL PROGRAM INFO" fill_in Program_Change_Info fill_in Program_Specific_Category fill_in Program_Categories fill_in Includes_JAVA_VM "[Y/N]" fill_in Includes_VB_Runtime "[Y/N]" fill_in Includes_DirectX "[Y/N]"   # END More Program_Info  echo "</$PROGRAM_HDR>" >>$savefile # END Program_Info   clear  # Program Description echo "PROGRAM DESCRIPTIONS" PROGDESC_HDR="Program_Descriptions" echo "<$PROGDESC_HDR>" >>$savefile  LANG="English" echo "<$LANG>" >>$savefile  fill_in Keywords "[comma + space separated]" echo echo "45, 80, 250, 450, 2000 word program descriptions" echo "(may cut and paste into field)" #  It would be highly appropriate to compose the following #+ "Char_Desc" fields with a text editor, #+ then cut-and-paste the text into the answer fields. echo echo "              |---------------45 characters---------------|" fill_in Char_Desc_45 check_field_length 45 "$?" echo fill_in Char_Desc_80 check_field_length 80 "$?"  fill_in Char_Desc_250 check_field_length 250 "$?"  fill_in Char_Desc_450 fill_in Char_Desc_2000  echo "</$LANG>" >>$savefile echo "</$PROGDESC_HDR>" >>$savefile # END Program Description  clear echo "Done."; echo; echo echo "Save file is:  \""$savefile"\""  exit 0

Example A-39. A man page editor

#!/bin/bash # maned.sh # A rudimentary man page editor  # Version: 0.1 (Alpha, probably buggy) # Author: Mendel Cooper <thegrendel.abs@gmail.com> # Reldate: 16 June 2008 # License: GPL3   savefile=      # Global, used in multiple functions. E_NOINPUT=90   # User input missing (error). May or may not be critical.  # =========== Markup Tags ============ # TopHeader=".TH" NameHeader=".SH NAME" SyntaxHeader=".SH SYNTAX" SynopsisHeader=".SH SYNOPSIS" InstallationHeader=".SH INSTALLATION" DescHeader=".SH DESCRIPTION" OptHeader=".SH OPTIONS" FilesHeader=".SH FILES" EnvHeader=".SH ENVIRONMENT" AuthHeader=".SH AUTHOR" BugsHeader=".SH BUGS" SeeAlsoHeader=".SH SEE ALSO" BOLD=".B" # Add more tags, as needed. # See groff docs for markup meanings. # ==================================== #  start () { clear                  # Clear screen. echo "ManEd" echo "-----" echo echo "Simple man page creator" echo "Author: Mendel Cooper" echo "License: GPL3" echo; echo; echo }  progname () {   echo -n "Program name? "   read name    echo -n "Manpage section? [Hit RETURN for default (\"1\") ]  "   read section   if [ -z "$section" ]   then     section=1   # Most man pages are in section 1.   fi    if [ -n "$name" ]   then     savefile=""$name"."$section""       #  Filename suffix = section.     echo -n "$1 " >>$savefile     name1=$(echo "$name" | tr a-z A-Z)  #  Change to uppercase,                                         #+ per man page convention.     echo -n "$name1" >>$savefile   else     echo "Error! No input."             # Mandatory input.     exit $E_NOINPUT                     # Critical!     #  Exercise: The script-abort if no filename input is a bit clumsy.     #            Rewrite this section so a default filename is used     #+           if no input.   fi    echo -n "  \"$section\"">>$savefile   # Append, always append.    echo -n "Version? "   read ver   echo -n " \"Version $ver \"">>$savefile   echo >>$savefile    echo -n "Short description [0 - 5 words]? "   read sdesc   echo "$NameHeader">>$savefile   echo ""$BOLD" "$name"">>$savefile   echo "\- "$sdesc"">>$savefile  }  fill_in () { # This function more or less copied from "pad.sh" script.   echo -n "$2? "       # Get user input.   read var             # May paste (a single line only!) to fill in field.    if [ -n "$var" ]   then     echo "$1 " >>$savefile     echo -n "$var" >>$savefile   else                 # Don't append empty field to file.     return $E_NOINPUT  # Not critical here.   fi    echo >>$savefile  }       end () { clear echo -n "Would you like to view the saved man page (y/n)? " read ans if [ "$ans" = "n" -o "$ans" = "N" ]; then exit; fi exec less "$savefile"  #  Exit script and hand off control to "less" ...                        #+ ... which formats for viewing man page source. }   # ---------------------------------------- # start progname "$TopHeader" fill_in "$SynopsisHeader" "Synopsis" fill_in "$DescHeader" "Long description" # May paste in *single line* of text. fill_in "$OptHeader" "Options" fill_in "$FilesHeader" "Files" fill_in "$AuthHeader" "Author" fill_in "$BugsHeader" "Bugs" fill_in "$SeeAlsoHeader" "See also" # fill_in "$OtherHeader" ... as necessary. end    # ... exit not needed. # ---------------------------------------- #  #  Note that the generated man page will usually #+ require manual fine-tuning with a text editor. #  However, it's a distinct improvement upon #+ writing man source from scratch #+ or even editing a blank man page template.  #  The main deficiency of the script is that it permits #+ pasting only a single text line into the input fields. #  This may be a long, cobbled-together line, which groff #  will automatically wrap and hyphenate. #  However, if you want multiple (newline-separated) paragraphs, #+ these must be inserted by manual text editing on the #+ script-generated man page. #  Exercise (difficult): Fix this!  #  This script is not nearly as elaborate as the #+ full-featured "manedit" package #+ http://freshmeat.net/projects/manedit/ #+ but it's much easier to use.

Example A-40. Petals Around the Rose

#!/bin/bash -i # petals.sh  ######################################################################### # Petals Around the Rose                                                # #                                                                       # # Version 0.1 Created by Serghey Rodin                                  # # Version 0.2 Modded by ABS Guide Author                                # #                                                                       # # License: GPL3                                                         # # Used in ABS Guide with permission.                                    # # ##################################################################### #  hits=0      # Correct guesses. WIN=6       # Mastered the game. ALMOST=5    # One short of mastery. EXIT=exit   # Give up early?  RANDOM=$$   # Seeds the random number generator from PID of script.   # Bones (ASCII graphics for dice) bone1[1]="|         |" bone1[2]="|       o |" bone1[3]="|       o |" bone1[4]="| o     o |" bone1[5]="| o     o |" bone1[6]="| o     o |" bone2[1]="|    o    |" bone2[2]="|         |" bone2[3]="|    o    |" bone2[4]="|         |" bone2[5]="|    o    |" bone2[6]="| o     o |" bone3[1]="|         |" bone3[2]="| o       |" bone3[3]="| o       |" bone3[4]="| o     o |" bone3[5]="| o     o |" bone3[6]="| o     o |" bone="+---------+"    # Functions  instructions () {    clear   echo -n "Do you need instructions? (y/n) "; read ans   if [ "$ans" = "y" -o "$ans" = "Y" ]; then     clear     echo -e '\E[34;47m'  # Blue type.  #  "cat document"     cat <<INSTRUCTIONSZZZ The name of the game is Petals Around the Rose, and that name is significant. Five dice will roll and you must guess the "answer" for each roll. It will be zero or an even number. After your guess, you will be told the answer for the roll, but . . . that's ALL the information you will get.  Six consecutive correct guesses admits you to the Fellowship of the Rose. INSTRUCTIONSZZZ      echo -e "\033[0m"    # Turn off blue.     else clear   fi  }   fortune () {   RANGE=7   FLOOR=0   number=0   while [ "$number" -le $FLOOR ]   do     number=$RANDOM     let "number %= $RANGE"   # 1 - 6.   done    return $number }    throw () { # Calculate each individual die.   fortune; B1=$?   fortune; B2=$?   fortune; B3=$?   fortune; B4=$?   fortune; B5=$?    calc () { # Function embedded within a function!     case "$1" in        3   ) rose=2;;        5   ) rose=4;;        *   ) rose=0;;     esac    # Simplified algorithm.             # Doesn't really get to the heart of the matter.     return $rose   }    answer=0   calc "$B1"; answer=$(expr $answer + $(echo $?))   calc "$B2"; answer=$(expr $answer + $(echo $?))   calc "$B3"; answer=$(expr $answer + $(echo $?))   calc "$B4"; answer=$(expr $answer + $(echo $?))   calc "$B5"; answer=$(expr $answer + $(echo $?)) }    game () { # Generate graphic display of dice throw.   throw     echo -e "\033[1m"    # Bold.   echo -e "\n"   echo -e "$bone\t$bone\t$bone\t$bone\t$bone"   echo -e \  "${bone1[$B1]}\t${bone1[$B2]}\t${bone1[$B3]}\t${bone1[$B4]}\t${bone1[$B5]}"   echo -e \  "${bone2[$B1]}\t${bone2[$B2]}\t${bone2[$B3]}\t${bone2[$B4]}\t${bone2[$B5]}"   echo -e \  "${bone3[$B1]}\t${bone3[$B2]}\t${bone3[$B3]}\t${bone3[$B4]}\t${bone3[$B5]}"   echo -e "$bone\t$bone\t$bone\t$bone\t$bone"   echo -e "\n\n\t\t"     echo -e "\033[0m"    # Turn off bold.   echo -n "There are how many petals around the rose? " }    # ============================================================== #  instructions  while [ "$petal" != "$EXIT" ]    # Main loop. do   game   read petal   echo "$petal" | grep [0-9] >/dev/null  # Filter response for digit.                                          # Otherwise just roll dice again.   if [ "$?" -eq 0 ]   # If-loop #1.   then     if [ "$petal" == "$answer" ]; then    # If-loop #2.     	echo -e "\nCorrect. There are $petal petals around the rose.\n"         (( hits++ ))          if [ "$hits" -eq "$WIN" ]; then   # If-loop #3.           echo -e '\E[31;47m'  # Red type.           echo -e "\033[1m"    # Bold.           echo "You have unraveled the mystery of the Rose Petals!"           echo "Welcome to the Fellowship of the Rose!!!"           echo "(You are herewith sworn to secrecy.)"; echo           echo -e "\033[0m"    # Turn off red & bold.           break                # Exit!         else echo "You have $hits correct so far."; echo          if [ "$hits" -eq "$ALMOST" ]; then           echo "Just one more gets you to the heart of the mystery!"; echo         fi        fi                                  # Close if-loop #3.      else       echo -e "\nWrong. There are $answer petals around the rose.\n"       hits=0   # Reset number of correct guesses.     fi                                    # Close if-loop #2.      echo -n "Hit ENTER for the next roll, or type \"exit\" to end. "     read     if [ "$REPLY" = "$EXIT" ]; then exit     fi    fi                  # Close if-loop #1.    clear done                  # End of main (while) loop.  ###  exit $?  # Resources: # --------- # 1) http://en.wikipedia.org/wiki/Petals_Around_the_Rose #    (Wikipedia entry.) # 2) http://www.borrett.id.au/computing/petals-bg.htm #    (How Bill Gates coped with the Petals Around the Rose challenge.)

Example A-41. Quacky: a Perquackey-type word game

#!/bin/bash # qky.sh  ############################################################## # QUACKEY: a somewhat simplified version of Perquackey [TM]. # #                                                            # # Author: Mendel Cooper  <thegrendel.abs@gmail.com>          # # version 0.1.02      03 May, 2008                           # # License: GPL3                                              # ##############################################################  WLIST=/usr/share/dict/word.lst #                     ^^^^^^^^  Word list file found here. #  ASCII word list, one word per line, UNIX format. #  A suggested list is the script author's "yawl" word list package. #  http://bash.deta.in/yawl-0.3.2.tar.gz #    or #  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz  NONCONS=0     # Word not constructable from letter set. CONS=1        # Constructable. SUCCESS=0 NG=1 FAILURE='' NULL=0        # Zero out value of letter (if found). MINWLEN=3     # Minimum word length. MAXCAT=5      # Maximum number of words in a given category. PENALTY=200   # General-purpose penalty for unacceptable words. total= E_DUP=70      # Duplicate word error.  TIMEOUT=10    # Time for word input.  NVLET=10      # 10 letters for non-vulnerable. VULET=13      # 13 letters for vulnerable (not yet implemented!).  declare -a Words declare -a Status declare -a Score=( 0 0 0 0 0 0 0 0 0 0 0 )   letters=( a n s r t m l k p r b c i d s i d z e w u e t f e y e r e f e g t g h h i t r s c i t i d i j a t a o l a m n a n o v n w o s e l n o s p a q e e r a b r s a o d s t g t i t l u e u v n e o x y m r k ) #  Letter distribution table shamelessly borrowed from "Wordy" game, #+ ca. 1992, written by a certain fine fellow named Mendel Cooper.  declare -a LS  numelements=${#letters[@]} randseed="$1"  instructions () {   clear   echo "Welcome to QUACKEY, the anagramming word construction game."; echo   echo -n "Do you need instructions? (y/n) "; read ans     if [ "$ans" = "y" -o "$ans" = "Y" ]; then      clear      echo -e '\E[31;47m'  # Red foreground. '\E[34;47m' for blue.      cat <<INSTRUCTION1  QUACKEY is a variant of Perquackey [TM]. The rules are the same, but the scoring is simplified and plurals of previously played words are allowed. "Vulnerable" play is not yet implemented, but it is otherwise feature-complete.  As the game begins, the player gets 10 letters. The object is to construct valid dictionary words of at least 3-letter length from the letterset. Each word-length category -- 3-letter, 4-letter, 5-letter, ... -- fills up with the fifth word entered, and no further words in that category are accepted.  The penalty for too-short (two-letter), duplicate, unconstructable, and invalid (not in dictionary) words is -200. The same penalty applies to attempts to enter a word in a filled-up category.  INSTRUCTION1    echo -n "Hit ENTER for next page of instructions. "; read az1       cat <<INSTRUCTION2  The scoring mostly corresponds to classic Perquackey: The first 3-letter word scores    60, plus   10 for each additional one. The first 4-letter word scores   120, plus   20 for each additional one. The first 5-letter word scores   200, plus   50 for each additional one. The first 6-letter word scores   300, plus  100 for each additional one. The first 7-letter word scores   500, plus  150 for each additional one. The first 8-letter word scores   750, plus  250 for each additional one. The first 9-letter word scores  1000, plus  500 for each additional one. The first 10-letter word scores 2000, plus 2000 for each additional one.  Category completion bonuses are: 3-letter words   100 4-letter words   200 5-letter words   400 6-letter words   800 7-letter words  2000 8-letter words 10000 This is a simplification of the absurdly baroque Perquackey bonus scoring system.  INSTRUCTION2    echo -n "Hit ENTER for final page of instructions. "; read az1       cat <<INSTRUCTION3   Hitting just ENTER for a word entry ends the game.  Individual word entry is timed to a maximum of 10 seconds. *** Timing out on an entry ends the game. *** Aside from that, the game is untimed.  -------------------------------------------------- Game statistics are automatically saved to a file. --------------------------------------------------  For competitive ("duplicate") play, a previous letterset may be duplicated by repeating the script's random seed, command-line parameter \$1. For example, "qky 7633" specifies the letterset  c a d i f r h u s k ... INSTRUCTION3    echo; echo -n "Hit ENTER to begin game. "; read az1         echo -e "\033[0m"    # Turn off red.      else clear   fi    clear  }    seed_random () {                         #  Seed random number generator.   if [ -n "$randseed" ]   #  Can specify random seed.   then                    #+ for play in competitive mode. #   RANDOM="$randseed"     echo "RANDOM seed set to "$randseed""   else     randseed="$$"         # Or get random seed from process ID.     echo "RANDOM seed not specified, set to Process ID of script ($$)."   fi    RANDOM="$randseed"    echo }   get_letset () {   element=0   echo -n "Letterset:"    for lset in $(seq $NVLET)   do  # Pick random letters to fill out letterset.     LS[element]="${letters[$((RANDOM%numelements))]}"     ((element++))   done    echo   echo "${LS[@]}"  }   add_word () {   wrd="$1"   local idx=0    Status[0]=""   Status[3]=""   Status[4]=""    while [ "${Words[idx]}" != '' ]   do     if [ "${Words[idx]}" = "$wrd" ]     then       Status[3]="Duplicate-word-PENALTY"       let "Score[0]= 0 - $PENALTY"       let "Score[1]-=$PENALTY"       return $E_DUP     fi      ((idx++))   done    Words[idx]="$wrd"   get_score  }  get_score() {   local wlen=0   local score=0   local bonus=0   local first_word=0   local add_word=0   local numwords=0    wlen=${#wrd}   numwords=${Score[wlen]}   Score[2]=0   Status[4]=""   # Initialize "bonus" to 0.    case "$wlen" in     3) first_word=60        add_word=10;;     4) first_word=120        add_word=20;;     5) first_word=200        add_word=50;;     6) first_word=300        add_word=100;;     7) first_word=500        add_word=150;;     8) first_word=750        add_word=250;;     9) first_word=1000        add_word=500;;    10) first_word=2000        add_word=2000;;   # This category modified from original rules!       esac    ((Score[wlen]++))   if [ ${Score[wlen]} -eq $MAXCAT ]   then   # Category completion bonus scoring simplified!     case $wlen in       3 ) bonus=100;;       4 ) bonus=200;;       5 ) bonus=400;;       6 ) bonus=800;;       7 ) bonus=2000;;       8 ) bonus=10000;;     esac  # Needn't worry about 9's and 10's.     Status[4]="Category-$wlen-completion***BONUS***"     Score[2]=$bonus   else     Status[4]=""   # Erase it.   fi       let "score =  $first_word +   $add_word * $numwords"     if [ "$numwords" -eq 0 ]     then       Score[0]=$score     else       Score[0]=$add_word     fi   #  All this to distinguish last-word score          #+ from total running score.   let "Score[1] += ${Score[0]}"   let "Score[1] += ${Score[2]}"  }    get_word () {   local wrd=''   read -t $TIMEOUT wrd   # Timed read.   echo $wrd }  is_constructable () { # This is the most complex and difficult-to-write function.   local -a local_LS=( "${LS[@]}" )  # Local copy of letter set.   local is_found=0   local idx=0   local pos   local strlen   local local_word=( "$1" )   strlen=${#local_word}    while [ "$idx" -lt "$strlen" ]   do     is_found=$(expr index "${local_LS[*]}" "${local_word:idx:1}")     if [ "$is_found" -eq "$NONCONS" ] # Not constructable!     then       echo "$FAILURE"; return     else       ((pos = ($is_found - 1) / 2))   # Compensate for spaces betw. letters!       local_LS[pos]=$NULL             # Zero out used letters.       ((idx++))                       # Bump index.     fi   done    echo "$SUCCESS"   return }  is_valid () { # Surprisingly easy to check if word in dictionary ...   fgrep -qw "$1" "$WLIST"   # ... courtesy of 'grep' ...   echo $? }  check_word () {   if [ -z "$1" ]   then     return   fi    Status[1]=""   Status[2]=""   Status[3]=""   Status[4]=""    iscons=$(is_constructable "$1")   if [ "$iscons" ]   then     Status[1]="constructable"      v=$(is_valid "$1")     if [ "$v" -eq "$SUCCESS" ]     then       Status[2]="valid"        strlen=${#1}        if [ ${Score[strlen]} -eq "$MAXCAT" ]   # Category full!       then         Status[3]="Category-$strlen-overflow-PENALTY"         return $NG       fi        case "$strlen" in         1 | 2 )         Status[3]="Two-letter-word-PENALTY"         return $NG;;         * )  	Status[3]="" 	return $SUCCESS;;       esac     else       Status[3]="Not-valid-PENALTY"       return $NG     fi   else     Status[3]="Not-constructable-PENALTY"        return $NG   fi    ### FIXME: Streamline the above code block.  }   display_words () {   local idx=0   local wlen0    clear   echo "Letterset:   ${LS[@]}"   echo "Threes:    Fours:    Fives:     Sixes:    Sevens:    Eights:"   echo "------------------------------------------------------------"         while [ "${Words[idx]}" != '' ]   do    wlen0=${#Words[idx]}    case "$wlen0" in      3) ;;      4) echo -n "           " ;;      5) echo -n "                     " ;;      6) echo -n "                                " ;;      7) echo -n "                                          " ;;      8) echo -n "                                                     " ;;    esac    echo "${Words[idx]}"    ((idx++))   done    ### FIXME: The word display is pretty crude. }   play () {   word="Start game"   # Dummy word, to start ...    while [ "$word" ]   #  If player just hits return (null word),   do                  #+ then game ends.     echo "$word: "${Status[@]}""     echo -n "Last score: [${Score[0]}]   TOTAL score: [${Score[1]}]:     Next word: "     total=${Score[1]}     word=$(get_word)     check_word "$word"      if [ "$?" -eq "$SUCCESS" ]     then       add_word "$word"     else       let "Score[0]= 0 - $PENALTY"       let "Score[1]-=$PENALTY"     fi    display_words   done   # Exit game.    ### FIXME: The play () function calls too many other functions.   ### This verges on "spaghetti code" !!! }  end_of_game () { # Save and display stats.    #######################Autosave##########################   savefile=qky.save.$$   #                 ^^ PID of script   echo `date` >> $savefile   echo "Letterset # $randseed  (random seed) ">> $savefile   echo -n "Letterset: " >> $savefile   echo "${LS[@]}" >> $savefile   echo "---------" >> $savefile   echo "Words constructed:" >> $savefile   echo "${Words[@]}" >> $savefile   echo >> $savefile   echo "Score: $total" >> $savefile    echo "Statistics for this round saved in \""$savefile"\""   #########################################################    echo "Score for this round: $total"   echo "Words:  ${Words[@]}" }  # ---------# instructions seed_random get_letset play end_of_game # ---------#  exit $?  # TODO: # # 1) Clean up code! # 2) Prettify the display_words () function (maybe with widgets?). # 3) Improve the time-out ... maybe change to untimed entry, #+   but with a time limit for the overall round.    # 4) An on-screen countdown timer would be nice. # 5) Implement "vulnerable" mode of play for compatibility with classic #+   version of the game. # 6) Improve save-to-file capability (and maybe make it optional). # 7) Fix bugs!!!  # For more info, reference: # http://bash.deta.in/qky.README.html

Example A-42. Nim

#!/bin/bash # nim.sh: Game of Nim  # Author: Mendel Cooper # Reldate: 15 July 2008 # License: GPL3  ROWS=5     # Five rows of pegs (or matchsticks). WON=91     # Exit codes to keep track of wins/losses. LOST=92    # Possibly useful if running in batch mode.   QUIT=99 peg_msg=   # Peg/Pegs? Rows=( 0 5 4 3 2 1 )   # Array holding play info. # ${Rows[0]} holds total number of pegs, updated after each turn. # Other array elements hold number of pegs in corresponding row.  instructions () {   clear   tput bold   echo "Welcome to the game of Nim."; echo   echo -n "Do you need instructions? (y/n) "; read ans     if [ "$ans" = "y" -o "$ans" = "Y" ]; then      clear      echo -e '\E[33;41m'  # Yellow fg., over red bg.; bold.      cat <<INSTRUCTIONS  Nim is a game with roots in the distant past. This particular variant starts with five rows of pegs.  1:    | | | | |  2:     | | | |  3:      | | |  4:       | |  5:        |   The number at the left identifies the row.  The human player moves first, and alternates turns with the bot. A turn consists of removing at least one peg from a single row. It is permissable to remove ALL the pegs from a row. For example, in row 2, above, the player can remove 1, 2, 3, or 4 pegs. The player who removes the last peg loses.  The strategy consists of trying to be the one who removes the next-to-last peg(s), leaving the loser with the final peg.  To exit the game early, hit ENTER during your turn. INSTRUCTIONS  echo; echo -n "Hit ENTER to begin game. "; read azx        echo -e "\033[0m"    # Restore display.       else tput sgr0; clear   fi  clear  }   tally_up () {   let "Rows[0] = ${Rows[1]} + ${Rows[2]} + ${Rows[3]} + ${Rows[4]} + \   ${Rows[5]}"    # Add up how many pegs remaining. }   display () {   index=1   # Start with top row.   echo    while [ "$index" -le "$ROWS" ]   do     p=${Rows[index]}     echo -n "$index:   "          # Show row number.    # ------------------------------------------------   # Two concurrent inner loops.        indent=$index       while [ "$indent" -gt 0 ]       do         echo -n " "               # Staggered rows.         ((indent--))              # Spacing between pegs.       done      while [ "$p" -gt 0 ]     do       echo -n "| "       ((p--))     done   # -----------------------------------------------    echo   ((index++))   done      tally_up    rp=${Rows[0]}    if [ "$rp" -eq 1 ]   then     peg_msg=peg     final_msg="Game over."   else             # Game not yet over . . .     peg_msg=pegs     final_msg=""   # . . . So "final message" is blank.   fi    echo "      $rp $peg_msg remaining."   echo "      "$final_msg""     echo }  player_move () {    echo "Your move:"    echo -n "Which row? "   while read idx   do                   # Validity check, etc.      if [ -z "$idx" ]   # Hitting return quits.     then         echo "Premature exit."; echo         tput sgr0      # Restore display.         exit $QUIT     fi      if [ "$idx" -gt "$ROWS" -o "$idx" -lt 1 ]   # Bounds check.     then       echo "Invalid row number!"       echo -n "Which row? "     else       break     fi     # TODO:     # Add check for non-numeric input.     # Also, script crashes on input outside of range of long double.     # Fix this.    done    echo -n "Remove how many? "   while read num   do                   # Validity check.    if [ -z "$num" ]   then     echo "Premature exit."; echo     tput sgr0          # Restore display.     exit $QUIT   fi      if [ "$num" -gt ${Rows[idx]} -o "$num" -lt 1 ]     then       echo "Cannot remove $num!"       echo -n "Remove how many? "     else       break     fi   done   # TODO:   # Add check for non-numeric input.   # Also, script crashes on input outside of range of long double.   # Fix this.    let "Rows[idx] -= $num"    display   tally_up    if [ ${Rows[0]} -eq 1 ]   then    echo "      Human wins!"    echo "      Congratulations!"    tput sgr0   # Restore display.    echo    exit $WON   fi    if [ ${Rows[0]} -eq 0 ]   then          # Snatching defeat from the jaws of victory . . .     echo "      Fool!"     echo "      You just removed the last peg!"     echo "      Bot wins!"     tput sgr0   # Restore display.     echo     exit $LOST   fi }   bot_move () {    row_b=0   while [[ $row_b -eq 0 || ${Rows[row_b]} -eq 0 ]]   do     row_b=$RANDOM          # Choose random row.     let "row_b %= $ROWS"   done     num_b=0   r0=${Rows[row_b]}    if [ "$r0" -eq 1 ]   then     num_b=1   else     let "num_b = $r0 - 1"          #  Leave only a single peg in the row.   fi     #  Not a very strong strategy,          #+ but probably a bit better than totally random.    let "Rows[row_b] -= $num_b"   echo -n "Bot:  "   echo "Removing from row $row_b ... "    if [ "$num_b" -eq 1 ]   then     peg_msg=peg   else     peg_msg=pegs   fi    echo "      $num_b $peg_msg."    display   tally_up    if [ ${Rows[0]} -eq 1 ]   then    echo "      Bot wins!"    tput sgr0   # Restore display.    exit $WON   fi  }   # ================================================== # instructions     # If human player needs them . . . tput bold        # Bold characters for easier viewing. display          # Show game board.  while [ true ]   # Main loop. do               # Alternate human and bot turns.   player_move   bot_move done # ================================================== #  # Exercise: # -------- # Improve the bot's strategy. # There is, in fact, a Nim strategy that can force a win. # See the Wikipedia article on Nim:  http://en.wikipedia.org/wiki/Nim # Recode the bot to use this strategy (rather difficult).  #  Curiosities: #  ----------- #  Nim played a prominent role in Alain Resnais' 1961 New Wave film, #+ Last Year at Marienbad. # #  In 1978, Leo Christopherson wrote an animated version of Nim, #+ Android Nim, for the TRS-80 Model I.

Example A-43. A command-line stopwatch

#!/bin/sh # sw.sh # A command-line Stopwatch  # Author: P�draig Brady #    http://www.pixelbeat.org/scripts/sw #    (Minor reformatting by ABS Guide author.) #    Used in ABS Guide with script author's permission. # Notes: #    This script starts a few processes per lap, in addition to #    the shell loop processing, so the assumption is made that #    this takes an insignificant amount of time compared to #    the response time of humans (~.1s) (or the keyboard #    interrupt rate (~.05s)). #    '?' for splits must be entered twice if characters #    (erroneously) entered before it (on the same line). #    '?' since not generating a signal may be slightly delayed #    on heavily loaded systems. #    Lap timings on ubuntu may be slightly delayed due to: #    https://bugs.launchpad.net/bugs/62511 # Changes: #    V1.0, 23 Aug 2005, Initial release #    V1.1, 26 Jul 2007, Allow both splits and laps from single invocation. #                       Only start timer after a key is pressed. #                       Indicate lap number #                       Cache programs at startup so there is less error #                       due to startup delays. #    V1.2, 01 Aug 2007, Work around `date` commands that don't have #                       nanoseconds. #                       Use stty to change interrupt keys to space for #                       laps etc. #                       Ignore other input as it causes problems. #    V1.3, 01 Aug 2007, Testing release. #    V1.4, 02 Aug 2007, Various tweaks to get working under ubuntu #                       and Mac OS X. #    V1.5, 27 Jun 2008, set LANG=C as got vague bug report about it.  export LANG=C  ulimit -c 0   # No coredumps from SIGQUIT. trap '' TSTP  # Ignore Ctrl-Z just in case. save_tty=`stty -g` && trap "stty $save_tty" EXIT  # Restore tty on exit. stty quit ' ' # Space for laps rather than Ctrl-\. stty eof  '?' # ? for splits rather than Ctrl-D. stty -echo    # Don't echo input.  cache_progs() {     stty > /dev/null     date > /dev/null     grep . < /dev/null     (echo "import time" | python) 2> /dev/null     bc < /dev/null     sed '' < /dev/null     printf '1' > /dev/null     /usr/bin/time false 2> /dev/null     cat < /dev/null } cache_progs   # To minimise startup delay.  date +%s.%N | grep -qF 'N' && use_python=1 # If `date` lacks nanoseconds. now() {     if [ "$use_python" ]; then         echo "import time; print time.time()" 2>/dev/null | python     else         printf "%.2f" `date +%s.%N`     fi }  fmt_seconds() {     seconds=$1     mins=`echo $seconds/60 | bc`     if [ "$mins" != "0" ]; then         seconds=`echo "$seconds - ($mins*60)" | bc`         echo "$mins:$seconds"     else         echo "$seconds"     fi }  total() {     end=`now`     total=`echo "$end - $start" | bc`     fmt_seconds $total }  stop() {     [ "$lapped" ] && lap "$laptime" "display"     total     exit }  lap() {     laptime=`echo "$1" | sed -n 's/.*real[^0-9.]*\(.*\)/\1/p'`     [ ! "$laptime" -o "$laptime" = "0.00" ] && return     # Signals too frequent.     laptotal=`echo $laptime+0$laptotal | bc`     if [ "$2" = "display" ]; then         lapcount=`echo 0$lapcount+1 | bc`         laptime=`fmt_seconds $laptotal`         echo $laptime "($lapcount)"         lapped="true"         laptotal="0"     fi }  echo -n "Space for lap | ? for split | Ctrl-C to stop | Space to start...">&2  while true; do     trap true INT QUIT  # Set signal handlers.     laptime=`/usr/bin/time -p 2>&1 cat >/dev/null`     ret=$?     trap '' INT QUIT    # Ignore signals within this script.     if [ $ret -eq 1 -o $ret -eq 2 -o $ret -eq 130 ]; then # SIGINT = stop         [ ! "$start" ] && { echo >&2; exit; }         stop     elif [ $ret -eq 3 -o $ret -eq 131 ]; then             # SIGQUIT = lap         if [ ! "$start" ]; then             start=`now` || exit 1             echo >&2             continue         fi         lap "$laptime" "display"     else                # eof = split         [ ! "$start" ] && continue         total         lap "$laptime"  # Update laptotal.     fi done  exit $?

Example A-44. An all-purpose shell scripting homework assignment solution

#!/bin/bash #  homework.sh: All-purpose homework assignment solution. #  Author: M. Leo Cooper #  If you substitute your own name as author, then it is plagiarism, #+ possibly a lesser sin than cheating on your homework! #  License: Public Domain  #  This script may be turned in to your instructor #+ in fulfillment of ALL shell scripting homework assignments. #  It's sparsely commented, but you, the student, can easily remedy that. #  The script author repudiates all responsibility!  DLA=1 P1=2 P2=4 P3=7 PP1=0 PP2=8 MAXL=9 E_LZY=99  declare -a L L[0]="3 4 0 17 29 8 13 18 19 17 20 2 19 14 17 28" L[1]="8 29 12 14 18 19 29 4 12 15 7 0 19 8 2 0 11 11 24 29 17 4 6 17 4 19" L[2]="29 19 7 0 19 29 8 29 7 0 21 4 29 13 4 6 11 4 2 19 4 3" L[3]="19 14 29 2 14 12 15 11 4 19 4 29 19 7 8 18 29" L[4]="18 2 7 14 14 11 22 14 17 10 29 0 18 18 8 6 13 12 4 13 19 26" L[5]="15 11 4 0 18 4 29 0 2 2 4 15 19 29 12 24 29 7 20 12 1 11 4 29" L[6]="4 23 2 20 18 4 29 14 5 29 4 6 17 4 6 8 14 20 18 29" L[7]="11 0 25 8 13 4 18 18 27" L[8]="0 13 3 29 6 17 0 3 4 29 12 4 29 0 2 2 14 17 3 8 13 6 11 24 26" L[9]="19 7 0 13 10 29 24 14 20 26"  declare -a \ alph=( A B C D E F G H I J K L M N O P Q R S T U V W X Y Z . , : ' ' )   pt_lt () {   echo -n "${alph[$1]}"   echo -n -e "\a"   sleep $DLA }  b_r () {  echo -e '\E[31;48m\033[1m' }  cr () {  echo -e "\a"  sleep $DLA }  restore () {   echo -e '\033[0m'            # Bold off.   tput sgr0                    # Normal. }   p_l () {   for ltr in $1   do     pt_lt "$ltr"   done }  # ---------------------- b_r  for i in $(seq 0 $MAXL) do   p_l "${L[i]}"   if [[ "$i" -eq "$P1" || "$i" -eq "$P2" || "$i" -eq "$P3" ]]   then     cr   elif [[ "$i" -eq "$PP1" || "$i" -eq "$PP2" ]]   then     cr; cr   fi done  restore # ----------------------  echo  exit $E_LZY  #  A typical example of an obfuscated script that is difficult #+ to understand, and frustrating to maintain. #  In your career as a sysadmin, you'll run into these critters #+ all too often.

Example A-45. The Knight's Tour

#!/bin/bash # ktour.sh  # author: mendel cooper # reldate: 12 Jan 2009 # license: public domain # (Not much sense GPLing something that's pretty much in the common #+ domain anyhow.)  ################################################################### #             The Knight's Tour, a classic problem.               # #             =====================================               # #  The knight must move onto every square of the chess board,     # #  but cannot revisit any square he has already visited.          # #                                                                 # #  And just why is Sir Knight unwelcome for a return visit?       # #  Could it be that he has a habit of partying into the wee hours # #+ of the morning?                                                # #  Possibly he leaves pizza crusts in the bed, empty beer bottles # #+ all over the floor, and clogs the plumbing. . . .              # #                                                                 # #  -------------------------------------------------------------  # #                                                                 # #  Usage: ktour.sh [start-square] [stupid]                        # #                                                                 # #  Note that start-square can be a square number                  # #+ in the range 0 - 63 ... or                                     # #  a square designator in conventional chess notation,            # #  such as a1, f5, h3, etc.                                       # #                                                                 # #  If start-square-number not supplied,                           # #+ then starts on a random square somewhere on the board.         # #                                                                 # # "stupid" as second parameter sets the stupid strategy.          # #                                                                 # #  Examples:                                                      # #  ktour.sh 23          starts on square #23 (h3)                 # #  ktour.sh g6 stupid   starts on square #46,                     # #                       using "stupid" (non-Warnsdorff) strategy. # ###################################################################  DEBUG=      # Set this to echo debugging info to stdout. SUCCESS=0 FAIL=99 BADMOVE=-999 FAILURE=1 LINELEN=21  # How many moves to display per line. # ---------------------------------------- # # Board array params ROWS=8   # 8 x 8 board. COLS=8 let "SQUARES = $ROWS * $COLS" let "MAX = $SQUARES - 1" MIN=0 # 64 squares on board, indexed from 0 to 63.  VISITED=1 UNVISITED=-1 UNVSYM="##" # ---------------------------------------- # # Global variables. startpos=    # Starting position (square #, 0 - 63). currpos=     # Current position. movenum=     # Move number. CRITPOS=37   # Have to patch for f5 starting position!  declare -i board # Use a one-dimensional array to simulate a two-dimensional one. # This can make life difficult and result in ugly kludges; see below. declare -i moves  # Offsets from current knight position.   initialize_board () {   local idx    for idx in {0..63}   do     board[$idx]=$UNVISITED   done }    print_board () {   local idx    echo "    _____________________________________"   for row in {7..0}               #  Reverse order of rows ...   do                              #+ so it prints in chessboard order.     let "rownum = $row + 1"       #  Start numbering rows at 1.     echo -n "$rownum  |"          #  Mark board edge with border and     for column in {0..7}          #+ "algebraic notation."     do       let "idx = $ROWS*$row + $column"       if [ ${board[idx]} -eq $UNVISITED ]       then         echo -n "$UNVSYM   "      ##       else                        # Mark square with move number.         printf "%02d " "${board[idx]}"; echo -n "  "       fi     done     echo -e -n "\b\b\b|"  # \b is a backspace.     echo                  # -e enables echoing escaped chars.   done    echo "    -------------------------------------"   echo "     a    b    c    d    e    f    g    h" }    failure() { # Whine, then bail out.   echo   print_board   echo   echo    "   Waah!!! Ran out of squares to move to!"   echo -n "   Knight's Tour attempt ended"   echo    " on $(to_algebraic $currpos) [square #$currpos]"   echo    "   after just $movenum moves!"   echo   exit $FAIL }    xlat_coords ()   #  Translate x/y coordinates to board position {                #+ (board-array element #).   #  For user input of starting board position as x/y coords.   #  This function not used in initial release of ktour.sh.   #  May be used in an updated version, for compatibility with   #+ standard implementation of the Knight's Tour in C, Python, etc.   if [ -z "$1" -o -z "$2" ]   then     return $FAIL   fi    local xc=$1   local yc=$2    let "board_index = $xc * $ROWS + yc"    if [ $board_index -lt $MIN -o $board_index -gt $MAX ]   then     return $FAIL    # Strayed off the board!   else     return $board_index   fi }    to_algebraic ()   #  Translate board position (board-array element #) {                 #+ to standard algebraic notation used by chess players.   if [ -z "$1" ]   then     return $FAIL   fi    local element_no=$1   # Numerical board position.   local col_arr=( a b c d e f g h )   local row_arr=( 1 2 3 4 5 6 7 8 )    let "row_no = $element_no / $ROWS"   let "col_no = $element_no % $ROWS"   t1=${col_arr[col_no]}; t2=${row_arr[row_no]}   local apos=$t1$t2   # Concatenate.   echo $apos }    from_algebraic ()   #  Translate standard algebraic chess notation {                   #+ to numerical board position (board-array element #).                     #  Or recognize numerical input & return it unchanged.   if [ -z "$1" ]   then     return $FAIL   fi   # If no command-line arg, then will default to random start pos.    local ix   local ix_count=0   local b_index     # Board index [0-63]   local alpos="$1"    arow=${alpos:0:1} # position = 0, length = 1   acol=${alpos:1:1}    if [[ $arow =~ [[:digit:]] ]]   #  Numerical input?   then       #  POSIX char class     if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!       then return $FAIL     else if [ $alpos -gt $MAX ]   # Off board?       then return $FAIL     else return $alpos            #  Return digit(s) unchanged . . .       fi                          #+ if within range.     fi   fi    if [[ $acol -eq $MIN || $acol -gt $ROWS ]]   then        # Outside of range 1 - 8?     return $FAIL   fi    for ix in a b c d e f g h   do  # Convert column letter to column number.    if [ "$arow" = "$ix" ]    then      break    fi   ((ix_count++))    # Find index count.   done    ((acol--))        # Decrementing converts to zero-based array.   let "b_index = $ix_count + $acol * $ROWS"    if [ $b_index -gt $MAX ]   # Off board?   then     return $FAIL   fi        return $b_index  }   generate_moves ()   #  Calculate all valid knight moves, {                   #+ relative to current position ($1),                     #+ and store in ${moves} array.   local kt_hop=1    #  One square  :: short leg of knight move.   local kt_skip=2   #  Two squares :: long leg  of knight move.   local valmov=0    #  Valid moves.   local row_pos; let "row_pos = $1 % $COLS"     let "move1 = -$kt_skip + $ROWS"           # 2 sideways to-the-left,  1 up     if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]   # An ugly, ugly kludge!     then                                           # Can't move off board.       move1=$BADMOVE                               # Not even temporarily.     else       ((valmov++))     fi   let "move2 = -$kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-left,  2 up     if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]    # Kludge continued ...     then       move2=$BADMOVE     else       ((valmov++))     fi   let "move3 =  $kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-right, 2 up     if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]     then       move3=$BADMOVE     else       ((valmov++))     fi   let "move4 =  $kt_skip + $ROWS"           # 2 sideways to-the-right, 1 up     if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]     then       move4=$BADMOVE     else       ((valmov++))     fi   let "move5 =  $kt_skip - $ROWS"           # 2 sideways to-the-right, 1 dn     if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]     then       move5=$BADMOVE     else       ((valmov++))     fi   let "move6 =  $kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-right, 2 dn     if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]     then       move6=$BADMOVE     else       ((valmov++))     fi   let "move7 = -$kt_hop - $kt_skip * $ROWS" # 1 sideways to-the-left,  2 dn     if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]     then       move7=$BADMOVE     else       ((valmov++))     fi   let "move8 = -$kt_skip - $ROWS"           # 2 sideways to-the-left,  1 dn     if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]     then       move8=$BADMOVE     else       ((valmov++))     fi   # There must be a better way to do this.    local m=( $valmov $move1 $move2 $move3 $move4 $move5 $move6 $move7 $move8 )   # ${moves[0]} = number of valid moves.   # ${moves[1]} ... ${moves[8]} = possible moves.   echo "${m[*]}"    # Elements of array to stdout for capture in a var.  }    is_on_board ()  # Is position actually on the board? {   if [[ "$1" -lt "$MIN" || "$1" -gt "$MAX" ]]   then     return $FAILURE   else     return $SUCCESS   fi }    do_move ()      # Move the knight! {   local valid_moves=0   local aapos   currposl="$1"   lmin=$ROWS   iex=0   squarel=   mpm=   mov=   declare -a p_moves    ########################## DECIDE-MOVE #############################   if [ $startpos -ne $CRITPOS ]   then   # CRITPOS = square #37     decide_move   else                     # Needs a special patch for startpos=37 !!!     decide_move_patched    # Why this particular move and no other ???   fi   ####################################################################    (( ++movenum ))          # Increment move count.   let "square = $currposl + ${moves[iex]}"    ##################    DEBUG    ###############   if [ "$DEBUG" ]     then debug   # Echo debugging information.   fi   ##############################################    if [[ "$square" -gt $MAX || "$square" -lt $MIN ||         ${board[square]} -ne $UNVISITED ]]   then     (( --movenum ))              #  Decrement move count,     echo "RAN OUT OF SQUARES!!!" #+ since previous one was invalid.     return $FAIL   fi    board[square]=$movenum   currpos=$square       # Update current position.   ((valid_moves++));    # moves[0]=$valid_moves   aapos=$(to_algebraic $square)   echo -n "$aapos "   test $(( $Moves % $LINELEN )) -eq 0 && echo   # Print LINELEN=21 moves per line. A valid tour shows 3 complete lines.   return $valid_moves   # Found a square to move to! }    do_move_stupid()   #  Dingbat algorithm, {                  #+ courtesy of script author, *not* Warnsdorff.   local valid_moves=0   local movloc   local squareloc   local aapos   local cposloc="$1"    for movloc in {1..8}   do       # Move to first-found unvisited square.     let "squareloc = $cposloc + ${moves[movloc]}"     is_on_board $squareloc     if [ $? -eq $SUCCESS ] && [ ${board[squareloc]} -eq $UNVISITED ]     then   # Add conditions to above if-test to improve algorithm.       (( ++movenum ))       board[squareloc]=$movenum       currpos=$squareloc     # Update current position.       ((valid_moves++));     # moves[0]=$valid_moves       aapos=$(to_algebraic $squareloc)       echo -n "$aapos "       test $(( $Moves % $LINELEN )) -eq 0 && echo   # Print 21 moves/line.       return $valid_moves    # Found a square to move to!     fi   done    return $FAIL   #  If no square found in all 8 loop iterations,   #+ then Knight's Tour attempt ends in failure.    #  Dingbat algorithm will typically fail after about 30 - 40 moves,   #+ but executes _much_ faster than Warnsdorff's in do_move() function. }    decide_move ()         #  Which move will we make? {                      #  But, fails on startpos=37 !!!   for mov in {1..8}   do     let "squarel = $currposl + ${moves[mov]}"     is_on_board $squarel     if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]     then   #  Find accessible square with least possible future moves.            #  This is Warnsdorff's algorithm.            #  What happens is that the knight wanders toward the outer edge            #+ of the board, then pretty much spirals inward.            #  Given two or more possible moves with same value of            #+ least-possible-future-moves, this implementation chooses            #+ the _first_ of those moves.            #  This means that there is not necessarily a unique solution            #+ for any given starting position.        possible_moves $squarel       mpm=$?       p_moves[mov]=$mpm              if [ $mpm -lt $lmin ]  # If less than previous minimum ...       then #     ^^         lmin=$mpm            # Update minimum.         iex=$mov             # Save index.       fi      fi   done }    decide_move_patched ()         #  Decide which move to make, {  #        ^^^^^^^            #+ but only if startpos=37 !!!   for mov in {1..8}   do     let "squarel = $currposl + ${moves[mov]}"     is_on_board $squarel     if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]     then       possible_moves $squarel       mpm=$?       p_moves[mov]=$mpm              if [ $mpm -le $lmin ]  # If less-than-or equal to prev. minimum!       then #     ^^         lmin=$mpm         iex=$mov       fi      fi   done                       # There has to be a better way to do this. }    possible_moves ()            #  Calculate number of possible moves, {                            #+ given the current position.    if [ -z "$1" ]   then     return $FAIL   fi    local curr_pos=$1   local valid_movl=0   local icx=0   local movl   local sq   declare -a movesloc    movesloc=( $(generate_moves $curr_pos) )    for movl in {1..8}   do     let "sq = $curr_pos + ${movesloc[movl]}"     is_on_board $sq     if [ $? -eq $SUCCESS ] && [ ${board[sq]} -eq $UNVISITED ]     then       ((valid_movl++));     fi   done    return $valid_movl         # Found a square to move to! }   strategy () {   echo    if [ -n "$STUPID" ]   then     for Moves in {1..63}     do       cposl=$1       moves=( $(generate_moves $currpos) )       do_move_stupid "$currpos"       if [ $? -eq $FAIL ]       then         failure       fi       done   fi    #  Don't need an "else" clause here,   #+ because Stupid Strategy will always fail and exit!   for Moves in {1..63}   do     cposl=$1     moves=( $(generate_moves $currpos) )     do_move "$currpos"     if [ $? -eq $FAIL ]     then       failure     fi    done         #  Could have condensed above two do-loops into a single one,   echo  #+ but this would have slowed execution.    print_board   echo   echo "Knight's Tour ends on $(to_algebraic $currpos) [square #$currpos]."   return $SUCCESS }  debug () {       # Enable this by setting DEBUG=1 near beginning of script.   local n    echo "================================="   echo "  At move number  $movenum:"   echo " *** possible moves = $mpm ***" # echo "### square = $square ###"   echo "lmin = $lmin"   echo "${moves[@]}"    for n in {1..8}   do     echo -n "($n):${p_moves[n]} "   done    echo   echo "iex = $iex :: moves[iex] = ${moves[iex]}"   echo "square = $square"   echo "================================="   echo } # Gives pretty complete status after ea. move.    # =============================================================== # # int main () { from_algebraic "$1" startpos=$? if [ "$startpos" -eq "$FAIL" ]          # Okay even if no $1. then   #         ^^^^^^^^^^^              Okay even if input -lt 0.   echo "No starting square specified (or illegal input)."   let "startpos = $RANDOM % $SQUARES"   # 0 - 63 permissable range. fi   if [ "$2" = "stupid" ] then   STUPID=1   echo -n "     ### Stupid Strategy ###" else   STUPID=''   echo -n "  *** Warnsdorff's Algorithm ***" fi   initialize_board  movenum=0 board[startpos]=$movenum   # Mark each board square with move number. currpos=$startpos algpos=$(to_algebraic $startpos)  echo; echo "Starting from $algpos [square #$startpos] ..."; echo echo -n "Moves:"  strategy "$currpos"  echo  exit 0   # return 0;  # }      # End of main() pseudo-function. # =============================================================== #   # Exercises: # --------- # # 1) Extend this example to a 10 x 10 board or larger. # 2) Improve the "stupid strategy" by modifying the #    do_move_stupid function. #    Hint: Prevent straying into corner squares in early moves #          (the exact opposite of Warnsdorff's algorithm!). # 3) This script could stand considerable improvement and #    streamlining, especially in the poorly-written #    generate_moves() function #    and in the DECIDE-MOVE patch in the do_move() function. #    Must figure out why standard algorithm fails for startpos=37 ... #+   but _not_ on any other, including symmetrical startpos=26. #    Possibly, when calculating possible moves, counts the move back #+   to the originating square. If so, it might be a relatively easy fix.

Example A-46. Magic Squares

#!/bin/bash # msquare.sh # Magic Square generator (odd-order squares only!)  # Author: mendel cooper # reldate: 19 Jan. 2009 # License: Public Domain # A C-program by the very talented Kwon Young Shin inspired this script. #     http://user.chollian.net/~brainstm/MagicSquare.htm  # Definition: A "magic square" is a two-dimensional array #             of integers in which all the rows, columns, #             and *long* diagonals add up to the same number. #             Being "square," the array has the same number #             of rows and columns. That number is the "order." # An example of a magic square of order 3 is: #   8  1  6    #   3  5  7    #   4  9  2    # All the rows, columns, and the two long diagonals add up to 15.   # Globals EVEN=2 MAXSIZE=31   # 31 rows x 31 cols. E_usage=90   # Invocation error. dimension= declare -i square  usage_message () {   echo "Usage: $0 order"   echo "   ... where \"order\" (square size) is an ODD integer"   echo "       in the range 3 - 31."   #  Actually works for squares up to order 159,   #+ but large squares will not display pretty-printed in a term window.   #  Try increasing MAXSIZE, above.   exit $E_usage }   calculate ()       # Here's where the actual work gets done. {   local row col index dimadj j k cell_val=1   dimension=$1    let "dimadj = $dimension * 3"; let "dimadj /= 2"   # x 1.5, then truncate.    for ((j=0; j < dimension; j++))   do     for ((k=0; k < dimension; k++))     do  # Calculate indices, then convert to 1-dim. array index.         # Bash doesn't support multidimensional arrays. Pity.       let "col = $k - $j + $dimadj"; let "col %= $dimension"       let "row = $j * 2 - $k + $dimension"; let "row %= $dimension"       let "index = $row*($dimension) + $col"       square[$index]=cell_val; ((cell_val++))     done   done }     # Plain math, visualization not required.   print_square ()               # Output square, one row at a time. {   local row col idx d1   let "d1 = $dimension - 1"   # Adjust for zero-indexed array.     for row in $(seq 0 $d1)   do      for col in $(seq 0 $d1)     do       let "idx = $row * $dimension + $col"       printf "%3d " "${square[idx]}"; echo -n "  "     done   # Displays up to 13th order neatly in 80-column term window.      echo   # Newline after each row.   done }   ################################################# if [[ -z "$1" ]] || [[ "$1" -gt $MAXSIZE ]] then   usage_message fi  let "test_even = $1 % $EVEN" if [ $test_even -eq 0 ] then           # Can't handle even-order squares.   usage_message fi  calculate $1 print_square   # echo "${square[@]}"   # DEBUG  exit $? #################################################   # Exercises: # --------- # 1) Add a function to calculate the sum of each row, column, #    and *long* diagonal. The sums must match. #    This is the "magic constant" of that particular order square. # 2) Have the print_square function auto-calculate how much space #    to allot between square elements for optimized display. #    This might require parameterizing the "printf" line. # 3) Add appropriate functions for generating magic squares #    with an *even* number of rows/columns. #    This is non-trivial(!). #    See the URL for Kwon Young Shin, above, for help.

Example A-47. Fifteen Puzzle

#!/bin/bash # fifteen.sh  # Classic "Fifteen Puzzle" # Author: Antonio Macchi # Lightly edited and commented by ABS Guide author. # Used in ABS Guide with permission. (Thanks!)  #  The invention of the Fifteen Puzzle is attributed to either #+ Sam Loyd or Noyes Palmer Chapman. #  The puzzle was wildly popular in the late 19th-century.  #  Object: Rearrange the numbers so they read in order, #+ from 1 - 15:   ________________ #                |  1   2   3   4 | #                |  5   6   7   8 | #                |  9  10  11  12 | #                | 13  14  15     | #                 ----------------   ####################### # Constants           #   SQUARES=16          #   FAIL=70             #   E_PREMATURE_EXIT=80 # #######################   ######## # Data # ########  Puzzle=( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 " " )   ############# # Functions # #############  function swap {   local tmp    tmp=${Puzzle[$1]}   Puzzle[$1]=${Puzzle[$2]}   Puzzle[$2]=$tmp }   function Jumble { # Scramble the pieces at beginning of round.   local i pos1 pos2    for i in {1..100}   do     pos1=$(( $RANDOM % $SQUARES))     pos2=$(( $RANDOM % $SQUARES ))     swap $pos1 $pos2   done }   function PrintPuzzle {   local i1 i2 puzpos   puzpos=0    clear   echo "Enter  quit  to exit."; echo   # Better that than Ctl-C.    echo ",----.----.----.----."   # Top border.   for i1 in {1..4}   do     for i2 in {1..4}      do       printf "| %2s " "${Puzzle[$puzpos]}"       (( puzpos++ ))     done     echo "|"                     # Right-side border.     test $i1 = 4 || echo "+----+----+----+----+"   done   echo "'----'----'----'----'"   # Bottom border. }   function GetNum { # Test for valid input.   local puznum garbage    while true   do  	  echo "Moves: $moves" # Also counts invalid moves.     read -p "Number to move: " puznum garbage       if [ "$puznum" = "quit" ]; then echo; exit $E_PREMATURE_EXIT; fi     test -z "$puznum" -o -n "${puznum//[0-9]/}" && continue     test $puznum -gt 0 -a $puznum -lt $SQUARES && break   done   return $puznum }   function GetPosFromNum { # $1 = puzzle-number   local puzpos    for puzpos in {0..15}   do     test "${Puzzle[$puzpos]}" = "$1" && break   done   return $puzpos }   function Move { # $1=Puzzle-pos   test $1 -gt 3 && test "${Puzzle[$(( $1 - 4 ))]}" = " "\        && swap $1 $(( $1 - 4 )) && return 0   test $(( $1%4 )) -ne 3 && test "${Puzzle[$(( $1 + 1 ))]}" = " "\        && swap $1 $(( $1 + 1 )) && return 0   test $1 -lt 12 && test "${Puzzle[$(( $1 + 4 ))]}" = " "\        && swap $1 $(( $1 + 4 )) && return 0   test $(( $1%4 )) -ne 0 && test "${Puzzle[$(( $1 - 1 ))]}" = " " &&\        swap $1 $(( $1 - 1 )) && return 0   return 1 }   function Solved {   local pos    for pos in {0..14}   do     test "${Puzzle[$pos]}" = $(( $pos + 1 )) || return $FAIL     # Check whether number in each square = square number.   done   return 0   # Successful solution. }   ################### MAIN () #######################{ moves=0 Jumble  while true   # Loop continuously until puzzle solved. do   echo; echo   PrintPuzzle   echo   while true   do     GetNum     puznum=$?     GetPosFromNum $puznum     puzpos=$?     ((moves++))     Move $puzpos && break   done   Solved && break done  echo;echo PrintPuzzle echo; echo "BRAVO!"; echo  exit 0 ###################################################}  #  Exercise: #  -------- #  Rewrite the script to display the letters A - O, #+ rather than the numbers 1 - 15.

Example A-48. The Towers of Hanoi, graphic version

#! /bin/bash # The Towers Of Hanoi # Original script (hanoi.bash) copyright (C) 2000 Amit Singh. # All Rights Reserved. # http://hanoi.kernelthread.com  #  hanoi2.bash #  Version 2.00: modded for ASCII-graphic display. #  Version 2.01: fixed no command-line param bug. #  Uses code contributed by Antonio Macchi, #+ with heavy editing by ABS Guide author. #  This variant falls under the original copyright, see above. #  Used in ABS Guide with Amit Singh's permission (thanks!).   ###   Variables && sanity check   ###  E_NOPARAM=86 E_BADPARAM=87            # Illegal no. of disks passed to script. E_NOEXIT=88  DISKS=${1:-$E_NOPARAM}   # Must specify how many disks. Moves=0  MWIDTH=7 MARGIN=2 # Arbitrary "magic" constants; work okay for relatively small # of disks. # BASEWIDTH=51   # Original code. let "basewidth = $MWIDTH * $DISKS + $MARGIN"       # "Base" beneath rods. # Above "algorithm" could likely stand improvement.  ###   Display variables   ### let "disks1 = $DISKS - 1" let "spaces1 = $DISKS"  let "spaces2 = 2 * $DISKS"   let "lastmove_t = $DISKS - 1"                      # Final move?   declare -a Rod1 Rod2 Rod3  ###   #########################   ###   function repeat  {  # $1=char $2=number of repetitions   local n           # Repeat-print a character.      for (( n=0; n<$2; n++ )); do     echo -n "$1"   done }  function FromRod  {   local rod summit weight sequence    while true; do     rod=$1     test ${rod/[^123]/} || continue      sequence=$(echo $(seq 0 $disks1 | tac))     for summit in $sequence; do       eval weight=\${Rod${rod}[$summit]}       test $weight -ne 0 &&            { echo "$rod $summit $weight"; return; }     done   done }   function ToRod  { # $1=previous (FromRod) weight   local rod firstfree weight sequence      while true; do     rod=$2     test ${rod/[^123]} || continue      sequence=$(echo $(seq 0 $disks1 | tac))     for firstfree in $sequence; do       eval weight=\${Rod${rod}[$firstfree]}       test $weight -gt 0 && { (( firstfree++ )); break; }     done     test $weight -gt $1 -o $firstfree = 0 &&          { echo "$rod $firstfree"; return; }   done }   function PrintRods  {   local disk rod empty fill sp sequence     repeat " " $spaces1   echo -n "|"   repeat " " $spaces2   echo -n "|"   repeat " " $spaces2   echo "|"    sequence=$(echo $(seq 0 $disks1 | tac))   for disk in $sequence; do     for rod in {1..3}; do       eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))       eval fill=\${Rod${rod}[$disk]}       repeat " " $empty       test $fill -gt 0 && repeat "*" $fill || echo -n "|"       repeat " " $empty     done     echo   done   repeat "=" $basewidth   # Print "base" beneath rods.   echo }   display () {   echo   PrintRods    # Get rod-number, summit and weight   first=( `FromRod $1` )   eval Rod${first[0]}[${first[1]}]=0    # Get rod-number and first-free position   second=( `ToRod ${first[2]} $2` )   eval Rod${second[0]}[${second[1]}]=${first[2]}   echo; echo; echo if [ "${Rod3[lastmove_t]}" = 1 ] then   # Last move? If yes, then display final position.     echo "+  Final Position: $Moves moves"; echo     PrintRods   fi }   # From here down, almost the same as original (hanoi.bash) script.  dohanoi() {   # Recursive function.     case $1 in     0)         ;;     *)         dohanoi "$(($1-1))" $2 $4 $3 	if [ "$Moves" -ne 0 ]         then 	  echo "+  Position after move $Moves"         fi         ((Moves++))         echo -n "   Next move will be:  "         echo $2 "-->" $3           display $2 $3         dohanoi "$(($1-1))" $4 $3 $2         ;;     esac }   setup_arrays () {   local dim n elem    let "dim1 = $1 - 1"   elem=$dim1    for n in $(seq 0 $dim1)   do    let "Rod1[$elem] = 2 * $n + 1"    Rod2[$n]=0    Rod3[$n]=0    ((elem--))   done }   ###   Main   ###  setup_arrays $DISKS echo; echo "+  Start Position"  case $# in     1) case $(($1>0)) in     # Must have at least one disk.        1)            disks=$1            dohanoi $1 1 3 2 #          Total moves = 2^n - 1, where n = number of disks. 	   echo            exit 0;            ;;        *)            echo "$0: Illegal value for number of disks";            exit $E_BADPARAM;            ;;        esac     ;;     *)        clear        echo "usage: $0 N"        echo "       Where \"N\" is the number of disks."        exit $E_NOPARAM;        ;; esac  exit $E_NOEXIT   # Shouldn't exit here.  # Note: # Redirect script output to a file, otherwise it scrolls off display.

Example A-49. The Towers of Hanoi, alternate graphic version

#! /bin/bash # The Towers Of Hanoi # Original script (hanoi.bash) copyright (C) 2000 Amit Singh. # All Rights Reserved. # http://hanoi.kernelthread.com  #  hanoi2.bash #  Version 2: modded for ASCII-graphic display. #  Uses code contributed by Antonio Macchi, #+ with heavy editing by ABS Guide author. #  This variant also falls under the original copyright, see above. #  Used in ABS Guide with Amit Singh's permission (thanks!).   #   Variables   # E_NOPARAM=86 E_BADPARAM=87   # Illegal no. of disks passed to script. E_NOEXIT=88 DELAY=2         # Interval, in seconds, between moves. Change, if desired. DISKS=$1 Moves=0  MWIDTH=7 MARGIN=2 # Arbitrary "magic" constants, work okay for relatively small # of disks. # BASEWIDTH=51   # Original code. let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods. # Above "algorithm" could likely stand improvement.  # Display variables. let "disks1 = $DISKS - 1" let "spaces1 = $DISKS"  let "spaces2 = 2 * $DISKS"   let "lastmove_t = $DISKS - 1"                # Final move?   declare -a Rod1 Rod2 Rod3  #################   function repeat  {  # $1=char $2=number of repetitions   local n           # Repeat-print a character.      for (( n=0; n<$2; n++ )); do     echo -n "$1"   done }  function FromRod  {   local rod summit weight sequence    while true; do     rod=$1     test ${rod/[^123]/} || continue      sequence=$(echo $(seq 0 $disks1 | tac))     for summit in $sequence; do       eval weight=\${Rod${rod}[$summit]}       test $weight -ne 0 &&            { echo "$rod $summit $weight"; return; }     done   done }   function ToRod  { # $1=previous (FromRod) weight   local rod firstfree weight sequence      while true; do     rod=$2     test ${rod/[^123]} || continue      sequence=$(echo $(seq 0 $disks1 | tac))     for firstfree in $sequence; do       eval weight=\${Rod${rod}[$firstfree]}       test $weight -gt 0 && { (( firstfree++ )); break; }     done     test $weight -gt $1 -o $firstfree = 0 &&          { echo "$rod $firstfree"; return; }   done }   function PrintRods  {   local disk rod empty fill sp sequence    tput cup 5 0    repeat " " $spaces1   echo -n "|"   repeat " " $spaces2   echo -n "|"   repeat " " $spaces2   echo "|"    sequence=$(echo $(seq 0 $disks1 | tac))   for disk in $sequence; do     for rod in {1..3}; do       eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))       eval fill=\${Rod${rod}[$disk]}       repeat " " $empty       test $fill -gt 0 && repeat "*" $fill || echo -n "|"       repeat " " $empty     done     echo   done   repeat "=" $basewidth   # Print "base" beneath rods.   echo }   display () {   echo   PrintRods    # Get rod-number, summit and weight   first=( `FromRod $1` )   eval Rod${first[0]}[${first[1]}]=0    # Get rod-number and first-free position   second=( `ToRod ${first[2]} $2` )   eval Rod${second[0]}[${second[1]}]=${first[2]}     if [ "${Rod3[lastmove_t]}" = 1 ]   then   # Last move? If yes, then display final position.     tput cup 0 0     echo; echo "+  Final Position: $Moves moves"     PrintRods   fi    sleep $DELAY }  # From here down, almost the same as original (hanoi.bash) script.  dohanoi() {   # Recursive function.     case $1 in     0)         ;;     *)         dohanoi "$(($1-1))" $2 $4 $3 	if [ "$Moves" -ne 0 ]         then 	  tput cup 0 0 	  echo; echo "+  Position after move $Moves"         fi         ((Moves++))         echo -n "   Next move will be:  "         echo $2 "-->" $3         display $2 $3         dohanoi "$(($1-1))" $4 $3 $2         ;;     esac }  setup_arrays () {   local dim n elem    let "dim1 = $1 - 1"   elem=$dim1    for n in $(seq 0 $dim1)   do    let "Rod1[$elem] = 2 * $n + 1"    Rod2[$n]=0    Rod3[$n]=0    ((elem--))   done }   ###   Main   ###  trap "tput cnorm" 0 tput civis clear  setup_arrays $DISKS  tput cup 0 0 echo; echo "+  Start Position"  case $# in     1) case $(($1>0)) in     # Must have at least one disk.        1)            disks=$1            dohanoi $1 1 3 2 #          Total moves = 2^n - 1, where n = # of disks. 	   echo            exit 0;            ;;        *)            echo "$0: Illegal value for number of disks";            exit $E_BADPARAM;            ;;        esac     ;;     *)        echo "usage: $0 N"        echo "       Where \"N\" is the number of disks."        exit $E_NOPARAM;        ;; esac  exit $E_NOEXIT   # Shouldn't exit here.  #  Exercise: #  -------- #  There is a minor bug in the script that causes the display of #+ the next-to-last move to be skipped. #+ Fix this.

Example A-50. An alternate version of the getopt-simple.sh script

#!/bin/bash # UseGetOpt.sh  # Author: Peggy Russell <prusselltechgroup@gmail.com>  UseGetOpt () {   declare inputOptions   declare -r E_OPTERR=85   declare -r ScriptName=${0##*/}   declare -r ShortOpts="adf:hlt"   declare -r LongOpts="aoption,debug,file:,help,log,test"  DoSomething () {     echo "The function name is '${FUNCNAME}'"     #  Recall that $FUNCNAME is an internal variable     #+ holding the name of the function it is in.   }    inputOptions=$(getopt -o "${ShortOpts}" --long \               "${LongOpts}" --name "${ScriptName}" -- "${@}")    if [[ ($? -ne 0) || ($# -eq 0) ]]; then     echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"     exit $E_OPTERR   fi    eval set -- "${inputOptions}"    # Only for educational purposes. Can be removed.   #-----------------------------------------------   echo "++ Test: Number of arguments: [$#]"   echo '++ Test: Looping through "$@"'   for a in "$@"; do     echo "  ++ [$a]"   done   #-----------------------------------------------    while true; do     case "${1}" in       --aoption | -a)  # Argument found.         echo "Option [$1]"         ;;        --debug | -d)    # Enable informational messages.         echo "Option [$1] Debugging enabled"         ;;        --file | -f)     #  Check for optional argument.         case "$2" in   #+ Double colon is optional argument.           "")          #  Not there.               echo "Option [$1] Use default"               shift               ;;            *) # Got it              echo "Option [$1] Using input [$2]"              shift              ;;          esac         DoSomething         ;;        --log | -l) # Enable Logging.         echo "Option [$1] Logging enabled"         ;;        --test | -t) # Enable testing.         echo "Option [$1] Testing enabled"         ;;        --help | -h)         echo "Option [$1] Display help"         break         ;;        --)   # Done! $# is argument number for "--", $@ is "--"         echo "Option [$1] Dash Dash"         break         ;;         *)         echo "Major internal error!"         exit 8         ;;      esac     echo "Number of arguments: [$#]"     shift   done    shift   # Only for educational purposes. Can be removed.   #----------------------------------------------------------------------   echo "++ Test: Number of arguments after \"--\" is [$#] They are: [$@]"   echo '++ Test: Looping through "$@"'   for a in "$@"; do     echo "  ++ [$a]"   done   #----------------------------------------------------------------------    }  ################################### M A I N ######################## #  If you remove "function UseGetOpt () {" and corresponding "}", #+ you can uncomment the "exit 0" line below, and invoke this script #+ with the various options from the command-line. #------------------------------------------------------------------- # exit 0  echo "Test 1" UseGetOpt -f myfile one "two three" four  echo;echo "Test 2" UseGetOpt -h  echo;echo "Test 3 - Short Options" UseGetOpt -adltf myfile  anotherfile  echo;echo "Test 4 - Long Options" UseGetOpt --aoption --debug --log --test --file myfile anotherfile  exit

Example A-51. The version of the UseGetOpt.sh example used in the Tab Expansion appendix

#!/bin/bash  #  UseGetOpt-2.sh #  Modified version of the script for illustrating tab-expansion #+ of command-line options. #  See the "Introduction to Tab Expansion" appendix.  #  Possible options: -a -d -f -l -t -h #+                   --aoption, --debug --file --log --test -- help --  #  Author of original script: Peggy Russell <prusselltechgroup@gmail.com>   # UseGetOpt () {   declare inputOptions   declare -r E_OPTERR=85   declare -r ScriptName=${0##*/}   declare -r ShortOpts="adf:hlt"   declare -r LongOpts="aoption,debug,file:,help,log,test"  DoSomething () {     echo "The function name is '${FUNCNAME}'"   }    inputOptions=$(getopt -o "${ShortOpts}" --long \               "${LongOpts}" --name "${ScriptName}" -- "${@}")    if [[ ($? -ne 0) || ($# -eq 0) ]]; then     echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"     exit $E_OPTERR   fi    eval set -- "${inputOptions}"     while true; do     case "${1}" in       --aoption | -a)  # Argument found.         echo "Option [$1]"         ;;        --debug | -d)    # Enable informational messages.         echo "Option [$1] Debugging enabled"         ;;        --file | -f)     #  Check for optional argument.         case "$2" in   #+ Double colon is optional argument.           "")          #  Not there.               echo "Option [$1] Use default"               shift               ;;            *) # Got it              echo "Option [$1] Using input [$2]"              shift              ;;          esac         DoSomething         ;;        --log | -l) # Enable Logging.         echo "Option [$1] Logging enabled"         ;;        --test | -t) # Enable testing.         echo "Option [$1] Testing enabled"         ;;        --help | -h)         echo "Option [$1] Display help"         break         ;;        --)   # Done! $# is argument number for "--", $@ is "--"         echo "Option [$1] Dash Dash"         break         ;;         *)         echo "Major internal error!"         exit 8         ;;      esac     echo "Number of arguments: [$#]"     shift   done    shift    #  }  exit

Example A-52. Cycling through all the possible color backgrounds

#!/bin/bash  # show-all-colors.sh # Displays all 256 possible background colors, using ANSI escape sequences. # Author: Chetankumar Phulpagare # Used in ABS Guide with permission.  T1=8 T2=6 T3=36 offset=0  for num1 in {0..7} do {    for num2 in {0,1}        do {           shownum=`echo "$offset + $T1 * ${num2} + $num1" | bc`           echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"           }        done    echo    } done  offset=16 for num1 in {0..5} do {    for num2 in {0..5}        do {           for num3 in {0..5}               do {                  shownum=`echo "$offset + $T2 * ${num3} \                  + $num2 + $T3 * ${num1}" | bc`                  echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"                  }                done           echo           }        done } done  offset=232 for num1 in {0..23} do {    shownum=`expr $offset + $num1`    echo -en "\E[0;48;5;${shownum}m ${shownum}\E[0m" } done  echo

Example A-53. Morse Code Practice

#!/bin/bash # sam.sh, v. .01a # Still Another Morse (code training script) # With profuse apologies to Sam (F.B.) Morse. # Author: Mendel Cooper # License: GPL3 # Reldate: 05/25/11  # Morse code training script. # Converts arguments to audible dots and dashes. # Note: lowercase input only at this time.    # Get the wav files from the source tarball: # http://bash.deta.in/abs-guide-latest.tar.bz2 DOT='soundfiles/dot.wav' DASH='soundfiles/dash.wav' # Maybe move soundfiles to /usr/local/sounds?  LETTERSPACE=300000  # Microseconds. WORDSPACE=980000 # Nice and slow, for beginners. Maybe 5 wpm?  EXIT_MSG="May the Morse be with you!" E_NOARGS=75         # No command-line args?    declare -A morse    # Associative array! # ======================================= # morse[a]="dot; dash" morse[b]="dash; dot; dot; dot" morse[c]="dash; dot; dash; dot" morse[d]="dash; dot; dot" morse[e]="dot" morse[f]="dot; dot; dash; dot" morse[g]="dash; dash; dot" morse[h]="dot; dot; dot; dot" morse[i]="dot; dot;" morse[j]="dot; dash; dash; dash" morse[k]="dash; dot; dash" morse[l]="dot; dash; dot; dot" morse[m]="dash; dash" morse[n]="dash; dot" morse[o]="dash; dash; dash" morse[p]="dot; dash; dash; dot" morse[q]="dash; dash; dot; dash" morse[r]="dot; dash; dot" morse[s]="dot; dot; dot" morse[t]="dash" morse[u]="dot; dot; dash" morse[v]="dot; dot; dot; dash" morse[w]="dot; dash; dash" morse[x]="dash; dot; dot; dash" morse[y]="dash; dot; dash; dash" morse[z]="dash; dash; dot; dot" morse[0]="dash; dash; dash; dash; dash" morse[1]="dot; dash; dash; dash; dash" morse[2]="dot; dot; dash; dash; dash" morse[3]="dot; dot; dot; dash; dash" morse[4]="dot; dot; dot; dot; dash" morse[5]="dot; dot; dot; dot; dot" morse[6]="dash; dot; dot; dot; dot" morse[7]="dash; dash; dot; dot; dot" morse[8]="dash; dash; dash; dot; dot" morse[9]="dash; dash; dash; dash; dot" # The following must be escaped or quoted. morse[?]="dot; dot; dash; dash; dot; dot" morse[.]="dot; dash; dot; dash; dot; dash" morse[,]="dash; dash; dot; dot; dash; dash" morse[/]="dash; dot; dot; dash; dot" morse[\@]="dot; dash; dash; dot; dash; dot" # ======================================= #  play_letter () {   eval ${morse[$1]}   # Play dots, dashes from appropriate sound files.   # Why is 'eval' necessary here?   usleep $LETTERSPACE # Pause in between letters. }  extract_letters () {                     # Slice string apart, letter by letter.   local pos=0         # Starting at left end of string.   local len=1         # One letter at a time.   strlen=${#1}    while [ $pos -lt $strlen ]   do     letter=${1:pos:len}     #      ^^^^^^^^^^^^    See Chapter 10.1.     play_letter $letter     echo -n "*"       #    Mark letter just played.     ((pos++))   done }  ######### Play the sounds ############ dot()  { aplay "$DOT" 2&>/dev/null;  } dash() { aplay "$DASH" 2&>/dev/null; } ######################################  no_args () {     declare -a usage     usage=( $0 word1 word2 ... )      echo "Usage:"; echo     echo ${usage[*]}     for index in 0 1 2 3     do       extract_letters ${usage[index]}            usleep $WORDSPACE       echo -n " "     # Print space between words.     done #   echo "Usage: $0 word1 word2 ... "     echo; echo }   # int main() # {  clear                 # Clear the terminal screen. echo "            SAM" echo "Still Another Morse code trainer" echo "    Author: Mendel Cooper" echo; echo;  if [ -z "$1" ] then   no_args   echo; echo; echo "$EXIT_MSG"; echo   exit $E_NOARGS fi  echo; echo "$*"       # Print text that will be played.  until [ -z "$1" ] do   extract_letters $1   shift           # On to next word.   usleep $WORDSPACE   echo -n " "     # Print space between words. done  echo; echo; echo "$EXIT_MSG"; echo  exit 0 # }  #  Exercises: #  --------- #  1) Have the script accept either lowercase or uppercase words #+    as arguments. Hint: Use 'tr' . . . #  2) Have the script optionally accept input from a text file.

Example A-54. Base64 encoding/decoding

#!/bin/bash # base64.sh: Bash implementation of Base64 encoding and decoding. # # Copyright (c) 2011 vladz <vladz@devzero.fr> # Used in ABSG with permission (thanks!). # #  Encode or decode original Base64 (and also Base64url) #+ from STDIN to STDOUT. # #    Usage: # #    Encode #    $ ./base64.sh < binary-file > binary-file.base64 #    Decode #    $ ./base64.sh -d < binary-file.base64 > binary-file # # Reference: # #    [1]  RFC4648 - "The Base16, Base32, and Base64 Data Encodings" #         http://tools.ietf.org/html/rfc4648#section-5   # The base64_charset[] array contains entire base64 charset, # and additionally the character "=" ... base64_charset=( {A..Z} {a..z} {0..9} + / = )                 # Nice illustration of brace expansion.  #  Uncomment the ### line below to use base64url encoding instead of #+ original base64. ### base64_charset=( {A..Z} {a..z} {0..9} - _ = )  #  Output text width when encoding #+ (64 characters, just like openssl output). text_width=64  function display_base64_char { #  Convert a 6-bit number (between 0 and 63) into its corresponding values #+ in Base64, then display the result with the specified text width.   printf "${base64_charset[$1]}"; (( width++ ))   (( width % text_width == 0 )) && printf "\n" }  function encode_base64 { # Encode three 8-bit hexadecimal codes into four 6-bit numbers.   #    We need two local int array variables:   #    c8[]: to store the codes of the 8-bit characters to encode   #    c6[]: to store the corresponding encoded values on 6-bit   declare -a -i c8 c6    #  Convert hexadecimal to decimal.   c8=( $(printf "ibase=16; ${1:0:2}\n${1:2:2}\n${1:4:2}\n" | bc) )    #  Let's play with bitwise operators   #+ (3x8-bit into 4x6-bits conversion).   (( c6[0] = c8[0] >> 2 ))   (( c6[1] = ((c8[0] &  3) << 4) | (c8[1] >> 4) ))    # The following operations depend on the c8 element number.   case ${#c8[*]} in      3) (( c6[2] = ((c8[1] & 15) << 2) | (c8[2] >> 6) ))        (( c6[3] = c8[2] & 63 )) ;;     2) (( c6[2] = (c8[1] & 15) << 2 ))        (( c6[3] = 64 )) ;;     1) (( c6[2] = c6[3] = 64 )) ;;   esac    for char in ${c6[@]}; do     display_base64_char ${char}   done }  function decode_base64 { # Decode four base64 characters into three hexadecimal ASCII characters.   #  c8[]: to store the codes of the 8-bit characters   #  c6[]: to store the corresponding Base64 values on 6-bit   declare -a -i c8 c6    # Find decimal value corresponding to the current base64 character.   for current_char in ${1:0:1} ${1:1:1} ${1:2:1} ${1:3:1}; do      [ "${current_char}" = "=" ] && break       position=0      while [ "${current_char}" != "${base64_charset[${position}]}" ]; do         (( position++ ))      done       c6=( ${c6[*]} ${position} )   done    #  Let's play with bitwise operators   #+ (4x8-bit into 3x6-bits conversion).   (( c8[0] = (c6[0] << 2) | (c6[1] >> 4) ))    # The next operations depends on the c6 elements number.   case ${#c6[*]} in     3) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))        (( c8[2] = (c6[2] & 3) << 6 )); unset c8[2] ;;     4) (( c8[1] = ( (c6[1] & 15) << 4) | (c6[2] >> 2) ))        (( c8[2] = ( (c6[2] &  3) << 6) |  c6[3] )) ;;   esac    for char in ${c8[*]}; do      printf "\x$(printf "%x" ${char})"   done }   # main ()  if [ "$1" = "-d" ]; then   # decode    # Reformat STDIN in pseudo 4x6-bit groups.   content=$(cat - | tr -d "\n" | sed -r "s/(.{4})/\1 /g")    for chars in ${content}; do decode_base64 ${chars}; done  else   # Make a hexdump of stdin and reformat in 3-byte groups.   content=$(cat - | xxd -ps -u | sed -r "s/(\w{6})/\1 /g" |             tr -d "\n")    for chars in ${content}; do encode_base64 ${chars}; done    echo  fi

Example A-55. Inserting text in a file using sed

#!/bin/bash #  Prepends a string at a specified line #+ in files with names ending in "sample" #+ in the current working directory. #  000000000000000000000000000000000000 #  This script overwrites files! #  Be careful running it in a directory #+ where you have important files!!! #  000000000000000000000000000000000000  #  Create a couple of files to operate on ... #  01sample #  02sample #  ... etc. #  These files must not be empty, else the prepend will not work.  lineno=1            # Append at line 1 (prepend). filespec="*sample"  # Filename pattern to operate on.  string=$(whoami)    # Will set your username as string to insert.                     # It could just as easily be any other string.  for file in $filespec # Specify which files to alter. do #        ^^^^^^^^^  sed -i ""$lineno"i "$string"" $file #    ^^ -i option edits files in-place. #                 ^ Insert (i) command.  echo ""$file" altered!" done  echo "Warning: files possibly clobbered!"  exit 0  # Exercise: # Add error checking to this script. # It needs it badly.

Example A-56. The Gronsfeld Cipher

#!/bin/bash # gronsfeld.bash  # License: GPL3 # Reldate 06/23/11  #  This is an implementation of the Gronsfeld Cipher. #  It's essentially a stripped-down variant of the  #+ polyalphabetic Vigen�re Tableau, but with only 10 alphabets. #  The classic Gronsfeld has a numeric sequence as the key word, #+ but here we substitute a letter string, for ease of use. #  Allegedly, this cipher was invented by the eponymous Count Gronsfeld #+ in the 17th Century. It was at one time considered to be unbreakable. #  Note that this is ###not### a secure cipher by modern standards.  #  Global Variables  # Enc_suffix="29379"   #  Encrypted text output with this 5-digit suffix.                       #  This functions as a decryption flag,                      #+ and when used to generate passwords adds security. Default_key="gronsfeldk"                      #  The script uses this if key not entered below                      #  (at "Keychain").                      #  Change the above two values frequently                      #+ for added security.  GROUPLEN=5           #  Output in groups of 5 letters, per tradition. alpha1=( abcdefghijklmnopqrstuvwxyz ) alpha2=( {A..Z} )    #  Output in all caps, per tradition.                      #  Use   alpha2=( {a..z} )   for password generator. wraplen=26           #  Wrap around if past end of alphabet. dflag=               #  Decrypt flag (set if $Enc_suffix present). E_NOARGS=76          #  Missing command-line args? DEBUG=77             #  Debugging flag. declare -a offsets   #  This array holds the numeric shift values for                      #+ encryption/decryption.  ########Keychain######### key=  ### Put key here!!!       # 10 characters! #########################    # Function : () { # Encrypt or decrypt, depending on whether $dflag is set.   # Why ": ()" as a function name? Just to prove that it can be done.    local idx keydx mlen off1 shft   local plaintext="$1"   local mlen=${#plaintext}  for (( idx=0; idx<$mlen; idx++ )) do   let "keydx = $idx % $keylen"   shft=${offsets[keydx]}    if [ -n "$dflag" ]   then                  # Decrypt!     let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) - $shft"     # Shift backward to decrypt.   else                  # Encrypt!     let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) + $shft"     # Shift forward to encrypt.     test $(( $idx % $GROUPLEN)) = 0 && echo -n " "  # Groups of 5 letters.     #  Comment out above line for output as a string without whitespace,     #+ for example, if using the script as a password generator.   fi    ((off1--))   # Normalize. Why is this necessary?        if [ $off1 -lt 0 ]       then     # Catch negative indices.         let "off1 += $wraplen"       fi    ((off1 %= $wraplen))   # Wrap around if past end of alphabet.    echo -n "${alpha2[off1]}"  done    if [ -z "$dflag" ]   then     echo " $Enc_suffix" #   echo "$Enc_suffix"  # For password generator.   else     echo   fi } # End encrypt/decrypt function.    # int main () {  # Check for command-line args. if [ -z "$1" ] then    echo "Usage: $0 TEXT TO ENCODE/DECODE"    exit $E_NOARGS fi  if [ ${!#} == "$Enc_suffix" ] #    ^^^^^ Final command-line arg. then   dflag=ON   echo -n "+"           # Flag decrypted text with a "+" for easy ID. fi  if [ -z "$key" ] then   key="$Default_key"    # "gronsfeldk" per above. fi  keylen=${#key}  for (( idx=0; idx<$keylen; idx++ )) do  # Calculate shift values for encryption/decryption.   offsets[idx]=$(expr index "${alpha1[*]}" ${key:idx:1})   # Normalize.   ((offsets[idx]--))  #  Necessary because "expr index" starts at 1,                       #+ whereas array count starts at 0.   # Generate array of numerical offsets corresponding to the key.   # There are simpler ways to accomplish this. done  args=$(echo "$*" | sed -e 's/ //g' | tr A-Z a-z | sed -e 's/[0-9]//g') # Remove whitespace and digits from command-line args. # Can modify to also remove punctuation characters, if desired.           # Debug:          # echo "$args"; exit $DEBUG  : "$args"               # Call the function named ":". # : is a null operator, except . . . when it's a function name!  exit $?    # } End-of-script   #   **************************************************************   # #   This script can function as a  password generator, #+  with several minor mods, see above. #   That would allow an easy-to-remember password, even the word #+ "password" itself, which encrypts to vrgfotvo29379 #+  a fairly secure password not susceptible to a dictionary attack. #   Or, you could use your own name (surely that's easy to remember!). #   For example, Bozo Bozeman encrypts to hfnbttdppkt29379. #   **************************************************************   #

Example A-57. Bingo Number Generator

#!/bin/bash # bingo.sh # Bingo number generator # Reldate 20Aug12, License: Public Domain  ####################################################################### # This script generates bingo numbers. # Hitting a key generates a new number. # Hitting 'q' terminates the script. # In a given run of the script, there will be no duplicate numbers. # When the script terminates, it prints a log of the numbers generated. #######################################################################  MIN=1       # Lowest allowable bingo number. MAX=75      # Highest allowable bingo number. COLS=15     # Numbers in each column (B I N G O). SINGLE_DIGIT_MAX=9  declare -a Numbers Prefix=(B I N G O)  initialize_Numbers () {  # Zero them out to start.    # They'll be incremented if chosen.    local index=0    until [ "$index" -gt $MAX ]    do      Numbers[index]=0      ((index++))    done     Numbers[0]=1   # Flag zero, so it won't be selected. }   generate_number () {    local number     while [ 1 ]    do      let "number = $(expr $RANDOM % $MAX)"      if [ ${Numbers[number]} -eq 0 ]    # Number not yet called.      then        let "Numbers[number]+=1"         # Flag it in the array.        break                            # And terminate loop.      fi   # Else if already called, loop and generate another number.    done    # Exercise: Rewrite this more elegantly as an until-loop.     return $number }   print_numbers_called () {   # Print out the called number log in neat columns.     # echo ${Numbers[@]}  local pre2=0                #  Prefix a zero, so columns will align                             #+ on single-digit numbers.  echo "Number Stats"  for (( index=1; index<=MAX; index++)) do   count=${Numbers[index]}   let "t = $index - 1"      # Normalize, since array begins with index 0.   let "column = $(expr $t / $COLS)"   pre=${Prefix[column]} # echo -n "${Prefix[column]} "  if [ $(expr $t % $COLS) -eq 0 ] then   echo   # Newline at end of row. fi    if [ "$index" -gt $SINGLE_DIGIT_MAX ]  # Check for single-digit number.   then     echo -n "$pre$index#$count "   else    # Prefix a zero.     echo -n "$pre$pre2$index#$count "   fi  done }    # main () { RANDOM=$$   # Seed random number generator.  initialize_Numbers   # Zero out the number tracking array.  clear echo "Bingo Number Caller"; echo  while [[ "$key" != "q" ]]   # Main loop. do   read -s -n1 -p "Hit a key for the next number [q to exit] " key   # Usually 'q' exits, but not always.   # Can always hit Ctl-C if q fails.   echo    generate_number; new_number=$?    let "column = $(expr $new_number / $COLS)"   echo -n "${Prefix[column]} "   # B-I-N-G-O    echo $new_number done  echo; echo  # Game over ... print_numbers_called echo; echo "[#0 = not called . . . #1 = called]"  echo  exit 0 # }   # Certainly, this script could stand some improvement. #See also the author's Instructable: #www.instructables.com/id/Binguino-An-Arduino-based-Bingo-Number-Generato/

To end this section, a review of the basics . . . and more.

Example A-58. Basics Reviewed

#!/bin/bash # basics-reviewed.bash  # File extension == *.bash == specific to Bash  #   Copyright (c) Michael S. Zick, 2003; All rights reserved. #   License: Use in any form, for any purpose. #   Revision: $ID$ # #              Edited for layout by M.C. #   (author of the "Advanced Bash Scripting Guide") #   Fixes and updates (04/08) by Cliff Bamford.   #  This script tested under Bash versions 2.04, 2.05a and 2.05b. #  It may not work with earlier versions. #  This demonstration script generates one --intentional-- #+ "command not found" error message. See line 436.  #  The current Bash maintainer, Chet Ramey, has fixed the items noted #+ for later versions of Bash.            ###-------------------------------------------###         ###  Pipe the output of this script to 'more' ###         ###+ else it will scroll off the page.        ###         ###                                           ###         ###  You may also redirect its output         ###         ###+ to a file for examination.               ###           ###-------------------------------------------###    #  Most of the following points are described at length in #+ the text of the foregoing "Advanced Bash Scripting Guide." #  This demonstration script is mostly just a reorganized presentation. #      -- msz  # Variables are not typed unless otherwise specified.  #  Variables are named. Names must contain a non-digit. #  File descriptor names (as in, for example: 2>&1) #+ contain ONLY digits.  # Parameters and Bash array elements are numbered. # (Parameters are very similar to Bash arrays.)  # A variable name may be undefined (null reference). unset VarNull  # A variable name may be defined but empty (null contents). VarEmpty=''                         # Two, adjacent, single quotes.  # A variable name may be defined and non-empty. VarSomething='Literal'  # A variable may contain: #   * A whole number as a signed 32-bit (or larger) integer #   * A string # A variable may also be an array.  #  A string may contain embedded blanks and may be treated #+ as if it where a function name with optional arguments.  #  The names of variables and the names of functions #+ are in different namespaces.   #  A variable may be defined as a Bash array either explicitly or #+ implicitly by the syntax of the assignment statement. #  Explicit: declare -a ArrayVar    # The echo command is a builtin. echo $VarSomething  # The printf command is a builtin. # Translate %s as: String-Format printf %s $VarSomething         # No linebreak specified, none output. echo                            # Default, only linebreak output.     # The Bash parser word breaks on whitespace. # Whitespace, or the lack of it is significant. # (This holds true in general; there are, of course, exceptions.)     # Translate the DOLLAR_SIGN character as: Content-Of.  # Extended-Syntax way of writing Content-Of: echo ${VarSomething}  #  The ${ ... } Extended-Syntax allows more than just the variable #+ name to be specified. #  In general, $VarSomething can always be written as: ${VarSomething}.  # Call this script with arguments to see the following in action.    #  Outside of double-quotes, the special characters @ and * #+ specify identical behavior. #  May be pronounced as: All-Elements-Of.  #  Without specification of a name, they refer to the #+ pre-defined parameter Bash-Array.    # Glob-Pattern references echo $*                         # All parameters to script or function echo ${*}                       # Same  # Bash disables filename expansion for Glob-Patterns. # Only character matching is active.   # All-Elements-Of references echo $@                         # Same as above echo ${@}                       # Same as above     #  Within double-quotes, the behavior of Glob-Pattern references #+ depends on the setting of IFS (Input Field Separator). #  Within double-quotes, All-Elements-Of references behave the same.   #  Specifying only the name of a variable holding a string refers #+ to all elements (characters) of a string.   #  To specify an element (character) of a string, #+ the Extended-Syntax reference notation (see below) MAY be used.     #  Specifying only the name of a Bash array references #+ the subscript zero element, #+ NOT the FIRST DEFINED nor the FIRST WITH CONTENTS element.  #  Additional qualification is needed to reference other elements, #+ which means that the reference MUST be written in Extended-Syntax. #  The general form is: ${name[subscript]}.  #  The string forms may also be used: ${name:subscript} #+ for Bash-Arrays when referencing the subscript zero element.   # Bash-Arrays are implemented internally as linked lists, #+ not as a fixed area of storage as in some programming languages.   #   Characteristics of Bash arrays (Bash-Arrays): #   --------------------------------------------  #   If not otherwise specified, Bash-Array subscripts begin with #+  subscript number zero. Literally: [0] #   This is called zero-based indexing. ### #   If not otherwise specified, Bash-Arrays are subscript packed #+  (sequential subscripts without subscript gaps). ### #   Negative subscripts are not allowed. ### #   Elements of a Bash-Array need not all be of the same type. ### #   Elements of a Bash-Array may be undefined (null reference). #       That is, a Bash-Array may be "subscript sparse." ### #   Elements of a Bash-Array may be defined and empty (null contents). ### #   Elements of a Bash-Array may contain: #     * A whole number as a signed 32-bit (or larger) integer #     * A string #     * A string formated so that it appears to be a function name #     + with optional arguments ### #   Defined elements of a Bash-Array may be undefined (unset). #       That is, a subscript packed Bash-Array may be changed #   +   into a subscript sparse Bash-Array. ### #   Elements may be added to a Bash-Array by defining an element #+  not previously defined. ### # For these reasons, I have been calling them "Bash-Arrays". # I'll return to the generic term "array" from now on. #     -- msz   echo "========================================================="  #  Lines 202 - 334 supplied by Cliff Bamford. (Thanks!) #  Demo --- Interaction with Arrays, quoting, IFS, echo, * and @   ---   #+ all affect how things work  ArrayVar[0]='zero'                    # 0 normal ArrayVar[1]=one                       # 1 unquoted literal ArrayVar[2]='two'                     # 2 normal ArrayVar[3]='three'                   # 3 normal ArrayVar[4]='I am four'               # 4 normal with spaces ArrayVar[5]='five'                    # 5 normal unset ArrayVar[6]                     # 6 undefined ArrayValue[7]='seven'                 # 7 normal ArrayValue[8]=''                      # 8 defined but empty ArrayValue[9]='nine'                  # 9 normal   echo '--- Here is the array we are using for this test' echo echo "ArrayVar[0]='zero'             # 0 normal" echo "ArrayVar[1]=one                # 1 unquoted literal" echo "ArrayVar[2]='two'              # 2 normal" echo "ArrayVar[3]='three'            # 3 normal" echo "ArrayVar[4]='I am four'        # 4 normal with spaces" echo "ArrayVar[5]='five'             # 5 normal" echo "unset ArrayVar[6]              # 6 undefined" echo "ArrayValue[7]='seven'          # 7 normal" echo "ArrayValue[8]=''               # 8 defined but empty" echo "ArrayValue[9]='nine'           # 9 normal" echo   echo echo '---Case0: No double-quotes, Default IFS of space,tab,newline ---' IFS=$'\x20'$'\x09'$'\x0A'            # In exactly this order. echo 'Here is: printf %q {${ArrayVar[*]}' printf %q ${ArrayVar[*]} echo echo 'Here is: printf %q {${ArrayVar[@]}' printf %q ${ArrayVar[@]} echo echo 'Here is: echo ${ArrayVar[*]}' echo  ${ArrayVar[@]} echo 'Here is: echo {${ArrayVar[@]}' echo ${ArrayVar[@]}  echo echo '---Case1: Within double-quotes - Default IFS of space-tab-  newline ---' IFS=$'\x20'$'\x09'$'\x0A'	    #  These three bytes, echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---Case2: Within double-quotes - IFS is q' IFS='q' echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---Case3: Within double-quotes - IFS is ^' IFS='^' echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---Case4: Within double-quotes - IFS is ^ followed by   space,tab,newline' IFS=$'^'$'\x20'$'\x09'$'\x0A'       # ^ + space tab newline echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---Case6: Within double-quotes - IFS set and empty ' IFS='' echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---Case7: Within double-quotes - IFS is unset' unset IFS echo 'Here is: printf %q "{${ArrayVar[*]}"' printf %q "${ArrayVar[*]}" echo echo 'Here is: printf %q "{${ArrayVar[@]}"' printf %q "${ArrayVar[@]}" echo echo 'Here is: echo "${ArrayVar[*]}"' echo  "${ArrayVar[@]}" echo 'Here is: echo "{${ArrayVar[@]}"' echo "${ArrayVar[@]}"  echo echo '---End of Cases---' echo "========================================================="; echo    # Put IFS back to the default. # Default is exactly these three bytes. IFS=$'\x20'$'\x09'$'\x0A'           # In exactly this order.  # Interpretation of the above outputs: #   A Glob-Pattern is I/O; the setting of IFS matters. ### #   An All-Elements-Of does not consider IFS settings. ### #   Note the different output using the echo command and the #+  quoted format operator of the printf command.   #  Recall: #   Parameters are similar to arrays and have the similar behaviors. ### #  The above examples demonstrate the possible variations. #  To retain the shape of a sparse array, additional script #+ programming is required. ### #  The source code of Bash has a routine to output the #+ [subscript]=value   array assignment format. #  As of version 2.05b, that routine is not used, #+ but that might change in future releases.    # The length of a string, measured in non-null elements (characters): echo echo '- - Non-quoted references - -' echo 'Non-Null character count: '${#VarSomething}' characters.'  # test='Lit'$'\x00''eral'           # $'\x00' is a null character. # echo ${#test}                     # See that?    #  The length of an array, measured in defined elements, #+ including null content elements. echo echo 'Defined content count: '${#ArrayVar[@]}' elements.' # That is NOT the maximum subscript (4). # That is NOT the range of the subscripts (1 . . 4 inclusive). # It IS the length of the linked list. ### #  Both the maximum subscript and the range of the subscripts may #+ be found with additional script programming.  # The length of a string, measured in non-null elements (characters): echo echo '- - Quoted, Glob-Pattern references - -' echo 'Non-Null character count: '"${#VarSomething}"' characters.'  #  The length of an array, measured in defined elements, #+ including null-content elements. echo echo 'Defined element count: '"${#ArrayVar[*]}"' elements.'  #  Interpretation: Substitution does not effect the ${# ... } operation. #  Suggestion: #  Always use the All-Elements-Of character #+ if that is what is intended (independence from IFS).    #  Define a simple function. #  I include an underscore in the name #+ to make it distinctive in the examples below. ### #  Bash separates variable names and function names #+ in different namespaces. #  The Mark-One eyeball isn't that advanced. ### _simple() {     echo -n 'SimpleFunc'$@          #  Newlines are swallowed in }                                   #+ result returned in any case.   # The ( ... ) notation invokes a command or function. # The $( ... ) notation is pronounced: Result-Of.   # Invoke the function _simple echo echo '- - Output of function _simple - -' _simple                             # Try passing arguments. echo # or (_simple)                           # Try passing arguments. echo  echo '- Is there a variable of that name? -' echo $_simple not defined           # No variable by that name.  # Invoke the result of function _simple (Error msg intended)  ### $(_simple)                          # Gives an error message: #                          line 436: SimpleFunc: command not found #                          ---------------------------------------  echo ###  #  The first word of the result of function _simple #+ is neither a valid Bash command nor the name of a defined function. ### # This demonstrates that the output of _simple is subject to evaluation. ### # Interpretation: #   A function can be used to generate in-line Bash commands.   # A simple function where the first word of result IS a bash command: ### _print() {     echo -n 'printf %q '$@ }  echo '- - Outputs of function _print - -' _print parm1 parm2                  # An Output NOT A Command. echo  $(_print parm1 parm2)               #  Executes: printf %q parm1 parm2                                     #  See above IFS examples for the                                     #+ various possibilities. echo  $(_print $VarSomething)             # The predictable result. echo    # Function variables # ------------------  echo echo '- - Function variables - -' # A variable may represent a signed integer, a string or an array. # A string may be used like a function name with optional arguments.  # set -vx                           #  Enable if desired declare -f funcVar                  #+ in namespace of functions  funcVar=_print                      # Contains name of function. $funcVar parm1                      # Same as _print at this point. echo  funcVar=$(_print )                  # Contains result of function. $funcVar                            # No input, No output. $funcVar $VarSomething              # The predictable result. echo  funcVar=$(_print $VarSomething)     #  $VarSomething replaced HERE. $funcVar                            #  The expansion is part of the echo                                #+ variable contents.  funcVar="$(_print $VarSomething)"   #  $VarSomething replaced HERE. $funcVar                            #  The expansion is part of the echo                                #+ variable contents.  #  The difference between the unquoted and the double-quoted versions #+ above can be seen in the "protect_literal.sh" example. #  The first case above is processed as two, unquoted, Bash-Words. #  The second case above is processed as one, quoted, Bash-Word.     # Delayed replacement # -------------------  echo echo '- - Delayed replacement - -' funcVar="$(_print '$VarSomething')" # No replacement, single Bash-Word. eval $funcVar                       # $VarSomething replaced HERE. echo  VarSomething='NewThing' eval $funcVar                       # $VarSomething replaced HERE. echo  # Restore the original setting trashed above. VarSomething=Literal  #  There are a pair of functions demonstrated in the #+ "protect_literal.sh" and "unprotect_literal.sh" examples. #  These are general purpose functions for delayed replacement literals #+ containing variables.      # REVIEW: # ------  #  A string can be considered a Classic-Array of elements (characters). #  A string operation applies to all elements (characters) of the string #+ (in concept, anyway). ### #  The notation: ${array_name[@]} represents all elements of the #+ Bash-Array: array_name. ### #  The Extended-Syntax string operations can be applied to all #+ elements of an array. ### #  This may be thought of as a For-Each operation on a vector of strings. ### #  Parameters are similar to an array. #  The initialization of a parameter array for a script #+ and a parameter array for a function only differ #+ in the initialization of ${0}, which never changes its setting. ### #  Subscript zero of the script's parameter array contains #+ the name of the script. ### #  Subscript zero of a function's parameter array DOES NOT contain #+ the name of the function. #  The name of the current function is accessed by the $FUNCNAME variable. ### #  A quick, review list follows (quick, not short).  echo echo '- - Test (but not change) - -' echo '- null reference -' echo -n ${VarNull-'NotSet'}' '          # NotSet echo ${VarNull}                         # NewLine only echo -n ${VarNull:-'NotSet'}' '         # NotSet echo ${VarNull}                         # Newline only  echo '- null contents -' echo -n ${VarEmpty-'Empty'}' '          # Only the space echo ${VarEmpty}                        # Newline only echo -n ${VarEmpty:-'Empty'}' '         # Empty echo ${VarEmpty}                        # Newline only  echo '- contents -' echo ${VarSomething-'Content'}          # Literal echo ${VarSomething:-'Content'}         # Literal  echo '- Sparse Array -' echo ${ArrayVar[@]-'not set'}  # ASCII-Art time # State     Y==yes, N==no #           -       :- # Unset     Y       Y       ${# ... } == 0 # Empty     N       Y       ${# ... } == 0 # Contents  N       N       ${# ... } > 0  #  Either the first and/or the second part of the tests #+ may be a command or a function invocation string. echo echo '- - Test 1 for undefined - -' declare -i t _decT() {     t=$t-1 }  # Null reference, set: t == -1 t=${#VarNull}                           # Results in zero. ${VarNull- _decT }                      # Function executes, t now -1. echo $t  # Null contents, set: t == 0 t=${#VarEmpty}                          # Results in zero. ${VarEmpty- _decT }                     # _decT function NOT executed. echo $t  # Contents, set: t == number of non-null characters VarSomething='_simple'                  # Set to valid function name. t=${#VarSomething}                      # non-zero length ${VarSomething- _decT }                 # Function _simple executed. echo $t                                 # Note the Append-To action.  # Exercise: clean up that example. unset t unset _decT VarSomething=Literal  echo echo '- - Test and Change - -' echo '- Assignment if null reference -' echo -n ${VarNull='NotSet'}' '          # NotSet NotSet echo ${VarNull} unset VarNull  echo '- Assignment if null reference -' echo -n ${VarNull:='NotSet'}' '         # NotSet NotSet echo ${VarNull} unset VarNull  echo '- No assignment if null contents -' echo -n ${VarEmpty='Empty'}' '          # Space only echo ${VarEmpty} VarEmpty=''  echo '- Assignment if null contents -' echo -n ${VarEmpty:='Empty'}' '         # Empty Empty echo ${VarEmpty} VarEmpty=''  echo '- No change if already has contents -' echo ${VarSomething='Content'}          # Literal echo ${VarSomething:='Content'}         # Literal   # "Subscript sparse" Bash-Arrays ### #  Bash-Arrays are subscript packed, beginning with #+ subscript zero unless otherwise specified. ### #  The initialization of ArrayVar was one way #+ to "otherwise specify".  Here is the other way: ### echo declare -a ArraySparse ArraySparse=( [1]=one [2]='' [4]='four' ) # [0]=null reference, [2]=null content, [3]=null reference  echo '- - Array-Sparse List - -' # Within double-quotes, default IFS, Glob-Pattern  IFS=$'\x20'$'\x09'$'\x0A' printf %q "${ArraySparse[*]}" echo  #  Note that the output does not distinguish between "null content" #+ and "null reference". #  Both print as escaped whitespace. ### #  Note also that the output does NOT contain escaped whitespace #+ for the "null reference(s)" prior to the first defined element. ### # This behavior of 2.04, 2.05a and 2.05b has been reported #+ and may change in a future version of Bash.  #  To output a sparse array and maintain the [subscript]=value #+ relationship without change requires a bit of programming. #  One possible code fragment: ### # local l=${#ArraySparse[@]}        # Count of defined elements # local f=0                         # Count of found subscripts # local i=0                         # Subscript to test (                                   # Anonymous in-line function     for (( l=${#ArraySparse[@]}, f = 0, i = 0 ; f < l ; i++ ))     do         # 'if defined then...'         ${ArraySparse[$i]+ eval echo '\ ['$i']='${ArraySparse[$i]} ; (( f++ )) }     done )  # The reader coming upon the above code fragment cold #+ might want to review "command lists" and "multiple commands on a line" #+ in the text of the foregoing "Advanced Bash Scripting Guide." ### #  Note: #  The "read -a array_name" version of the "read" command #+ begins filling array_name at subscript zero. #  ArraySparse does not define a value at subscript zero. ### #  The user needing to read/write a sparse array to either #+ external storage or a communications socket must invent #+ a read/write code pair suitable for their purpose. ### # Exercise: clean it up.  unset ArraySparse  echo echo '- - Conditional alternate (But not change)- -' echo '- No alternate if null reference -' echo -n ${VarNull+'NotSet'}' ' echo ${VarNull} unset VarNull  echo '- No alternate if null reference -' echo -n ${VarNull:+'NotSet'}' ' echo ${VarNull} unset VarNull  echo '- Alternate if null contents -' echo -n ${VarEmpty+'Empty'}' '              # Empty echo ${VarEmpty} VarEmpty=''  echo '- No alternate if null contents -' echo -n ${VarEmpty:+'Empty'}' '             # Space only echo ${VarEmpty} VarEmpty=''  echo '- Alternate if already has contents -'  # Alternate literal echo -n ${VarSomething+'Content'}' '        # Content Literal echo ${VarSomething}  # Invoke function echo -n ${VarSomething:+ $(_simple) }' '    # SimpleFunc Literal echo ${VarSomething} echo  echo '- - Sparse Array - -' echo ${ArrayVar[@]+'Empty'}                 # An array of 'Empty'(ies) echo  echo '- - Test 2 for undefined - -'  declare -i t _incT() {     t=$t+1 }  #  Note: #  This is the same test used in the sparse array #+ listing code fragment.  # Null reference, set: t == -1 t=${#VarNull}-1                     # Results in minus-one. ${VarNull+ _incT }                  # Does not execute. echo $t' Null reference'  # Null contents, set: t == 0 t=${#VarEmpty}-1                    # Results in minus-one. ${VarEmpty+ _incT }                 # Executes. echo $t'  Null content'  # Contents, set: t == (number of non-null characters) t=${#VarSomething}-1                # non-null length minus-one ${VarSomething+ _incT }             # Executes. echo $t'  Contents'  # Exercise: clean up that example. unset t unset _incT  # ${name?err_msg} ${name:?err_msg} #  These follow the same rules but always exit afterwards #+ if an action is specified following the question mark. #  The action following the question mark may be a literal #+ or a function result. ### #  ${name?} ${name:?} are test-only, the return can be tested.     # Element operations # ------------------  echo echo '- - Trailing sub-element selection - -'  #  Strings, Arrays and Positional parameters  #  Call this script with multiple arguments #+ to see the parameter selections.  echo '- All -' echo ${VarSomething:0}              # all non-null characters echo ${ArrayVar[@]:0}               # all elements with content echo ${@:0}                         # all parameters with content;                                     # ignoring parameter[0]  echo echo '- All after -' echo ${VarSomething:1}              # all non-null after character[0] echo ${ArrayVar[@]:1}               # all after element[0] with content echo ${@:2}                         # all after param[1] with content  echo echo '- Range after -' echo ${VarSomething:4:3}            # ral                                     # Three characters after                                     # character[3]  echo '- Sparse array gotch -' echo ${ArrayVar[@]:1:2}     #  four - The only element with content.                             #  Two elements after (if that many exist).                             #  the FIRST WITH CONTENTS                             #+ (the FIRST WITH  CONTENTS is being                             #+ considered as if it                             #+ were subscript zero). #  Executed as if Bash considers ONLY array elements with CONTENT #  printf %q "${ArrayVar[@]:0:3}"    # Try this one  #  In versions 2.04, 2.05a and 2.05b, #+ Bash does not handle sparse arrays as expected using this notation. # #  The current Bash maintainer, Chet Ramey, has corrected this.   echo '- Non-sparse array -' echo ${@:2:2}               # Two parameters following parameter[1]  # New victims for string vector examples: stringZ=abcABC123ABCabc arrayZ=( abcabc ABCABC 123123 ABCABC abcabc ) sparseZ=( [1]='abcabc' [3]='ABCABC' [4]='' [5]='123123' )  echo echo ' - - Victim string - -'$stringZ'- - ' echo ' - - Victim array - -'${arrayZ[@]}'- - ' echo ' - - Sparse array - -'${sparseZ[@]}'- - ' echo ' - [0]==null ref, [2]==null ref, [4]==null content - ' echo ' - [1]=abcabc [3]=ABCABC [5]=123123 - ' echo ' - non-null-reference count: '${#sparseZ[@]}' elements'  echo echo '- - Prefix sub-element removal - -' echo '- - Glob-Pattern match must include the first character. - -' echo '- - Glob-Pattern may be a literal or a function result. - -' echo   # Function returning a simple, Literal, Glob-Pattern _abc() {     echo -n 'abc' }  echo '- Shortest prefix -' echo ${stringZ#123}                 # Unchanged (not a prefix). echo ${stringZ#$(_abc)}             # ABC123ABCabc echo ${arrayZ[@]#abc}               # Applied to each element.  # echo ${sparseZ[@]#abc}            # Version-2.05b core dumps. # Has since been fixed by Chet Ramey.  # The -it would be nice- First-Subscript-Of # echo ${#sparseZ[@]#*}             # This is NOT valid Bash.  echo echo '- Longest prefix -' echo ${stringZ##1*3}                # Unchanged (not a prefix) echo ${stringZ##a*C}                # abc echo ${arrayZ[@]##a*c}              # ABCABC 123123 ABCABC  # echo ${sparseZ[@]##a*c}           # Version-2.05b core dumps. # Has since been fixed by Chet Ramey.  echo echo '- - Suffix sub-element removal - -' echo '- - Glob-Pattern match must include the last character. - -' echo '- - Glob-Pattern may be a literal or a function result. - -' echo echo '- Shortest suffix -' echo ${stringZ%1*3}                 # Unchanged (not a suffix). echo ${stringZ%$(_abc)}             # abcABC123ABC echo ${arrayZ[@]%abc}               # Applied to each element.  # echo ${sparseZ[@]%abc}            # Version-2.05b core dumps. # Has since been fixed by Chet Ramey.  # The -it would be nice- Last-Subscript-Of # echo ${#sparseZ[@]%*}             # This is NOT valid Bash.  echo echo '- Longest suffix -' echo ${stringZ%%1*3}                # Unchanged (not a suffix) echo ${stringZ%%b*c}                # a echo ${arrayZ[@]%%b*c}              # a ABCABC 123123 ABCABC a  # echo ${sparseZ[@]%%b*c}           # Version-2.05b core dumps. # Has since been fixed by Chet Ramey.  echo echo '- - Sub-element replacement - -' echo '- - Sub-element at any location in string. - -' echo '- - First specification is a Glob-Pattern - -' echo '- - Glob-Pattern may be a literal or Glob-Pattern function result. - -' echo '- - Second specification may be a literal or function result. - -' echo '- - Second specification may be unspecified. Pronounce that' echo '    as: Replace-With-Nothing (Delete) - -' echo    # Function returning a simple, Literal, Glob-Pattern _123() {     echo -n '123' }  echo '- Replace first occurrence -' echo ${stringZ/$(_123)/999}         # Changed (123 is a component). echo ${stringZ/ABC/xyz}             # xyzABC123ABCabc echo ${arrayZ[@]/ABC/xyz}           # Applied to each element. echo ${sparseZ[@]/ABC/xyz}          # Works as expected.  echo echo '- Delete first occurrence -' echo ${stringZ/$(_123)/} echo ${stringZ/ABC/} echo ${arrayZ[@]/ABC/} echo ${sparseZ[@]/ABC/}  #  The replacement need not be a literal, #+ since the result of a function invocation is allowed. #  This is general to all forms of replacement. echo echo '- Replace first occurrence with Result-Of -' echo ${stringZ/$(_123)/$(_simple)}  # Works as expected. echo ${arrayZ[@]/ca/$(_simple)}     # Applied to each element. echo ${sparseZ[@]/ca/$(_simple)}    # Works as expected.  echo echo '- Replace all occurrences -' echo ${stringZ//[b2]/X}             # X-out b's and 2's echo ${stringZ//abc/xyz}            # xyzABC123ABCxyz echo ${arrayZ[@]//abc/xyz}          # Applied to each element. echo ${sparseZ[@]//abc/xyz}         # Works as expected.  echo echo '- Delete all occurrences -' echo ${stringZ//[b2]/} echo ${stringZ//abc/} echo ${arrayZ[@]//abc/} echo ${sparseZ[@]//abc/}  echo echo '- - Prefix sub-element replacement - -' echo '- - Match must include the first character. - -' echo  echo '- Replace prefix occurrences -' echo ${stringZ/#[b2]/X}             # Unchanged (neither is a prefix). echo ${stringZ/#$(_abc)/XYZ}        # XYZABC123ABCabc echo ${arrayZ[@]/#abc/XYZ}          # Applied to each element. echo ${sparseZ[@]/#abc/XYZ}         # Works as expected.  echo echo '- Delete prefix occurrences -' echo ${stringZ/#[b2]/} echo ${stringZ/#$(_abc)/} echo ${arrayZ[@]/#abc/} echo ${sparseZ[@]/#abc/}  echo echo '- - Suffix sub-element replacement - -' echo '- - Match must include the last character. - -' echo  echo '- Replace suffix occurrences -' echo ${stringZ/%[b2]/X}             # Unchanged (neither is a suffix). echo ${stringZ/%$(_abc)/XYZ}        # abcABC123ABCXYZ echo ${arrayZ[@]/%abc/XYZ}          # Applied to each element. echo ${sparseZ[@]/%abc/XYZ}         # Works as expected.  echo echo '- Delete suffix occurrences -' echo ${stringZ/%[b2]/} echo ${stringZ/%$(_abc)/} echo ${arrayZ[@]/%abc/} echo ${sparseZ[@]/%abc/}  echo echo '- - Special cases of null Glob-Pattern - -' echo  echo '- Prefix all -' # null substring pattern means 'prefix' echo ${stringZ/#/NEW}               # NEWabcABC123ABCabc echo ${arrayZ[@]/#/NEW}             # Applied to each element. echo ${sparseZ[@]/#/NEW}            # Applied to null-content also.                                     # That seems reasonable.  echo echo '- Suffix all -' # null substring pattern means 'suffix' echo ${stringZ/%/NEW}               # abcABC123ABCabcNEW echo ${arrayZ[@]/%/NEW}             # Applied to each element. echo ${sparseZ[@]/%/NEW}            # Applied to null-content also.                                     # That seems reasonable.  echo echo '- - Special case For-Each Glob-Pattern - -' echo '- - - - This is a nice-to-have dream - - - -' echo  _GenFunc() {     echo -n ${0}                    # Illustration only.     # Actually, that would be an arbitrary computation. }  # All occurrences, matching the AnyThing pattern. # Currently //*/ does not match null-content nor null-reference. # /#/ and /%/ does match null-content but not null-reference. echo ${sparseZ[@]//*/$(_GenFunc)}   #  A possible syntax would be to make #+ the parameter notation used within this construct mean: #   ${1} - The full element #   ${2} - The prefix, if any, to the matched sub-element #   ${3} - The matched sub-element #   ${4} - The suffix, if any, to the matched sub-element # # echo ${sparseZ[@]//*/$(_GenFunc ${3})}   # Same as ${1} here. # Perhaps it will be implemented in a future version of Bash.   exit 0

Example A-59. Testing execution times of various commands

#!/bin/bash #  test-execution-time.sh #  Example by Erik Brandsberg, for testing execution time #+ of certain operations. #  Referenced in the "Optimizations" section of "Miscellany" chapter.  count=50000 echo "Math tests" echo "Math via \$(( ))" time for (( i=0; i< $count; i++)) do   result=$(( $i%2 )) done  echo "Math via *expr*:" time for (( i=0; i< $count; i++)) do   result=`expr "$i%2"` done  echo "Math via *let*:" time for (( i=0; i< $count; i++)) do   let result=$i%2 done  echo echo "Conditional testing tests"  echo "Test via case:" time for (( i=0; i< $count; i++)) do   case $(( $i%2 )) in     0) : ;;     1) : ;;   esac done  echo "Test with if [], no quotes:" time for (( i=0; i< $count; i++)) do   if [ $(( $i%2 )) = 0 ]; then      :   else      :   fi done  echo "Test with if [], quotes:" time for (( i=0; i< $count; i++)) do   if [ "$(( $i%2 ))" = "0" ]; then      :   else      :   fi done  echo "Test with if [], using -eq:" time for (( i=0; i< $count; i++)) do   if [ $(( $i%2 )) -eq 0 ]; then      :   else      :   fi done  exit $?

Example A-60. Associative arrays vs. conventional arrays (execution times)

#!/bin/bash #  assoc-arr-test.sh #  Benchmark test script to compare execution times of #  numeric-indexed array vs. associative array. #     Thank you, Erik Brandsberg.  count=100000       # May take a while for some of the tests below. declare simple     # Can change to 20000, if desired. declare -a array1 declare -A array2 declare -a array3 declare -A array4  echo "===Assignment tests===" echo  echo "Assigning a simple variable:" # References $i twice to equalize lookup times. time for (( i=0; i< $count; i++)); do         simple=$i$i done  echo "---"  echo "Assigning a numeric index array entry:" time for (( i=0; i< $count; i++)); do         array1[$i]=$i done  echo "---"  echo "Overwriting a numeric index array entry:" time for (( i=0; i< $count; i++)); do         array1[$i]=$i done  echo "---"  echo "Linear reading of numeric index array:" time for (( i=0; i< $count; i++)); do         simple=array1[$i] done  echo "---"  echo "Assigning an associative array entry:" time for (( i=0; i< $count; i++)); do         array2[$i]=$i done  echo "---"  echo "Overwriting an associative array entry:" time for (( i=0; i< $count; i++)); do         array2[$i]=$i done  echo "---"  echo "Linear reading an associative array entry:" time for (( i=0; i< $count; i++)); do         simple=array2[$i] done  echo "---"  echo "Assigning a random number to a simple variable:" time for (( i=0; i< $count; i++)); do         simple=$RANDOM done  echo "---"  echo "Assign a sparse numeric index array entry randomly into 64k cells:" time for (( i=0; i< $count; i++)); do         array3[$RANDOM]=$i done  echo "---"  echo "Reading sparse numeric index array entry:" time for value in "${array3[@]}"i; do         simple=$value done  echo "---"  echo "Assigning a sparse associative array entry randomly into 64k cells:" time for (( i=0; i< $count; i++)); do         array4[$RANDOM]=$i done  echo "---"  echo "Reading sparse associative index array entry:" time for value in "${array4[@]}"; do         simple=$value done  exit $?

Appendix B. Reference Cards

The following reference cards provide a useful summary of certain scripting concepts. The foregoing text treats these matters in more depth, as well as giving usage examples.

Table B-1. Special Shell Variables

Variable	Meaning
`$0`	Filename of script
`$1`	Positional parameter #1
`$2 - $9`	Positional parameters #2 - #9
`${10}`	Positional parameter #10
`$#`	Number of positional parameters
`"$*"`	All the positional parameters (as a single word) *
`"$@"`	All the positional parameters (as separate strings)
`${#*}`	Number of positional parameters
`${#@}`	Number of positional parameters
`$?`	Return value
`$$`	Process ID (PID) of script
`$-`	Flags passed to script (using set)
`$_`	Last argument of previous command
`$!`	Process ID (PID) of last job run in background

* Must be quoted, otherwise it defaults to $@.

Table B-2. TEST Operators: Binary Comparison

Operator	Meaning	Operator	Meaning

Arithmetic Comparison		String Comparison
`-eq`	Equal to	`=`	Equal to
		`==`	Equal to
`-ne`	Not equal to	`!=`	Not equal to
`-lt`	Less than	`\<`	Less than (ASCII) *
`-le`	Less than or equal to
`-gt`	Greater than	`\>`	Greater than (ASCII) *
`-ge`	Greater than or equal to
		`-z`	String is empty
		`-n`	String is not empty

Arithmetic Comparison	within double parentheses (( ... ))
`>`	Greater than
`>=`	Greater than or equal to
`<`	Less than
`<=`	Less than or equal to

* If within a double-bracket [[ ... ]] test construct, then no escape \ is needed.

Table B-3. TEST Operators: Files

Operator	Tests Whether	Operator	Tests Whether
`-e`	File exists	`-s`	File is not zero size
`-f`	File is a regular file
`-d`	File is a directory	`-r`	File has read permission
`-h`	File is a symbolic link	`-w`	File has write permission
`-L`	File is a symbolic link	`-x`	File has execute permission
`-b`	File is a block device
`-c`	File is a character device	`-g`	sgid flag set
`-p`	File is a pipe	`-u`	suid flag set
`-S`	File is a socket	`-k`	"sticky bit" set
`-t`	File is associated with a terminal

`-N`	File modified since it was last read	`F1 -nt F2`	File F1 is newer than F2 *
`-O`	You own the file	`F1 -ot F2`	File F1 is older than F2 *
`-G`	Group id of file same as yours	`F1 -ef F2`	Files F1 and F2 are hard links to the same file *

`!`	NOT (inverts sense of above tests)

* Binary operator (requires two operands).

Table B-4. Parameter Substitution and Expansion

Expression	Meaning
`${var}`	Value of `var` (same as `$var` )

`${var-$DEFAULT}`	If `var` not set, evaluate expression as `$DEFAULT` *
`${var:-$DEFAULT}`	If `var` not set or is empty, evaluate expression as `$DEFAULT` *

`${var=$DEFAULT}`	If `var` not set, evaluate expression as `$DEFAULT` *
`${var:=$DEFAULT}`	If `var` not set or is empty, evaluate expression as `$DEFAULT` *

`${var+$OTHER}`	If `var` set, evaluate expression as `$OTHER` , otherwise as null string
`${var:+$OTHER}`	If `var` set, evaluate expression as `$OTHER` , otherwise as null string

`${var?$ERR_MSG}`	If `var` not set, print `$ERR_MSG` and abort script with an exit status of 1.*
`${var:?$ERR_MSG}`	If `var` not set, print `$ERR_MSG` and abort script with an exit status of 1.*

`${!varprefix*}`	Matches all previously declared variables beginning with `varprefix`
`${!varprefix@}`	Matches all previously declared variables beginning with `varprefix`

* If var is set, evaluate the expression as $var with no side-effects.

# Note that some of the above behavior of operators has changed from earlier versions of Bash.

Table B-5. String Operations

Expression	Meaning
`${#string}`	Length of `$string`

`${string:position}`	Extract substring from `$string` at `$position`
`${string:position:length}`	Extract `$length` characters substring from `$string` at `$position` [zero-indexed, first character is at position 0]

`${string#substring}`	Strip shortest match of `$substring` from front of `$string`
`${string##substring}`	Strip longest match of `$substring` from front of `$string`
`${string%substring}`	Strip shortest match of `$substring` from back of `$string`
`${string%%substring}`	Strip longest match of `$substring` from back of `$string`

`${string/substring/replacement}`	Replace first match of `$substring` with `$replacement`
`${string//substring/replacement}`	Replace all matches of `$substring` with `$replacement`
`${string/#substring/replacement}`	If `$substring` matches front end of `$string` , substitute `$replacement` for `$substring`
`${string/%substring/replacement}`	If `$substring` matches back end of `$string` , substitute `$replacement` for `$substring`


`expr match "$string" '$substring'`	Length of matching `$substring` * at beginning of `$string`
`expr "$string" : '$substring'`	Length of matching `$substring` * at beginning of `$string`
`expr index "$string" $substring`	Numerical position in `$string` of first character in `$substring` * that matches [0 if no match, first character counts as position 1]
`expr substr $string $position $length`	Extract `$length` characters from `$string` starting at `$position` [0 if no match, first character counts as position 1]
`expr match "$string" '$$substring$'`	Extract `$substring` *, searching from beginning of `$string`
`expr "$string" : '$$substring$'`	Extract `$substring` * , searching from beginning of `$string`
`expr match "$string" '.*$$substring$'`	Extract `$substring` *, searching from end of `$string`
`expr "$string" : '.*$$substring$'`	Extract `$substring` *, searching from end of `$string`

* Where $substring is a Regular Expression.

Table B-6. Miscellaneous Constructs

Expression	Interpretation

Brackets
`if [ CONDITION ]`	Test construct
`if [[ CONDITION ]]`	Extended test construct
`Array[1]=element1`	Array initialization
`[a-z]`	Range of characters within a Regular Expression

Curly Brackets
`${variable}`	Parameter substitution
`${!variable}`	Indirect variable reference
`{ command1; command2; . . . commandN; }`	Block of code
`{string1,string2,string3,...}`	Brace expansion
`{a..z}`	Extended brace expansion
`{}`	Text replacement, after find and xargs


Parentheses
`( command1; command2 )`	Command group executed within a subshell
`Array=(element1 element2 element3)`	Array initialization
`result=$(COMMAND)`	Command substitution, new style
`>(COMMAND)`	Process substitution
`<(COMMAND)`	Process substitution

Double Parentheses
`(( var = 78 ))`	Integer arithmetic
`var=$(( 20 + 5 ))`	Integer arithmetic, with variable assignment
`(( var++ ))`	C-style variable increment
`(( var-- ))`	C-style variable decrement
`(( var0 = var1<98?9:21 ))`	C-style ternary operation

Quoting
`"$variable"`	"Weak" quoting
`'string'`	'Strong' quoting

Back Quotes
result=`COMMAND`	Command substitution, classic style

Appendix C. A Sed and Awk Micro-Primer

This is a very brief introduction to the sed and awk text processing utilities. We will deal with only a few basic commands here, but that will suffice for understanding simple sed and awk constructs within shell scripts.

sed: a non-interactive text file editor

awk: a field-oriented pattern processing language with a C-style syntax

For all their differences, the two utilities share a similar invocation syntax, use regular expressions , read input by default from stdin, and output to stdout. These are well-behaved UNIX tools, and they work together well. The output from one can be piped to the other, and their combined capabilities give shell scripts some of the power of Perl.

One important difference between the utilities is that while shell scripts can easily pass arguments to sed, it is more cumbersome for awk (see Example 36-5 and Example 28-2).

C.1. Sed

Sed is a non-interactive [141] stream editor. It receives text input, whether from stdin or from a file, performs certain operations on specified lines of the input, one line at a time, then outputs the result to stdout or to a file. Within a shell script, sed is usually one of several tool components in a pipe.

Sed determines which lines of its input that it will operate on from the address range passed to it. [142] Specify this address range either by line number or by a pattern to match. For example, 3d signals sed to delete line 3 of the input, and /Windows/d tells sed that you want every line of the input containing a match to "Windows" deleted.

Of all the operations in the sed toolkit, we will focus primarily on the three most commonly used ones. These are printing (to stdout), deletion, and substitution.

Table C-1. Basic sed operators

Operator	Name	Effect
`[address-range]/p`	print	Print [specified address range]
`[address-range]/d`	delete	Delete [specified address range]
`s/pattern1/pattern2/`	substitute	Substitute pattern2 for first instance of pattern1 in a line
`[address-range]/s/pattern1/pattern2/`	substitute	Substitute pattern2 for first instance of pattern1 in a line, over `address-range`
`[address-range]/y/pattern1/pattern2/`	transform	replace any character in pattern1 with the corresponding character in pattern2, over `address-range` (equivalent of tr)
`[address] i pattern Filename`	insert	Insert pattern at address indicated in file Filename. Usually used with `-i` `in-place` option.
`g`	global	Operate on every pattern match within each matched line of input

Unless the g (global) operator is appended to a substitute command, the substitution operates only on the first instance of a pattern match within each line.

From the command-line and in a shell script, a sed operation may require quoting and certain options.

sed -e '/^$/d' $filename # The -e option causes the next string to be interpreted as an editing instruction. #  (If passing only a single instruction to sed, the "-e" is optional.) #  The "strong" quotes ('') protect the RE characters in the instruction #+ from reinterpretation as special characters by the body of the script. # (This reserves RE expansion of the instruction for sed.) # # Operates on the text contained in file $filename.

In certain cases, a sed editing command will not work with single quotes.

filename=file1.txt pattern=BEGIN    sed "/^$pattern/d" "$filename"  # Works as specified. # sed '/^$pattern/d' "$filename"    has unexpected results. #        In this instance, with strong quoting (' ... '), #+      "$pattern" will not expand to "BEGIN".

Sed uses the -e option to specify that the following string is an instruction or set of instructions. If there is only a single instruction contained in the string, then this may be omitted.

sed -n '/xzy/p' $filename # The -n option tells sed to print only those lines matching the pattern. # Otherwise all input lines would print. # The -e option not necessary here since there is only a single editing instruction.

Table C-2. Examples of sed operators

Notation	Effect
`8d`	Delete 8th line of input.
`/^$/d`	Delete all blank lines.
`1,/^$/d`	Delete from beginning of input up to, and including first blank line.
`/Jones/p`	Print only lines containing "Jones" (with -n option).
`s/Windows/Linux/`	Substitute "Linux" for first instance of "Windows" found in each input line.
`s/BSOD/stability/g`	Substitute "stability" for every instance of "BSOD" found in each input line.
`s/ *$//`	Delete all spaces at the end of every line.
`s/00*/0/g`	Compress all consecutive sequences of zeroes into a single zero.
`echo "Working on it." \| sed -e '1i How far are you along?'`	Prints "How far are you along?" as first line, "Working on it" as second.
`5i 'Linux is great.' file.txt`	Inserts 'Linux is great.' at line 5 of the file file.txt.
`/GUI/d`	Delete all lines containing "GUI".
`s/GUI//g`	Delete all instances of "GUI", leaving the remainder of each line intact.

Substituting a zero-length string for another is equivalent to deleting that string within a line of input. This leaves the remainder of the line intact. Applying s/GUI// to the line

                                          The most important parts of any application are its GUI and sound effects

results in

                    The most important parts of any application are its  and sound effects

A backslash forces the sed replacement command to continue on to the next line. This has the effect of using the newline at the end of the first line as the replacement string. This substitution replaces line-beginning spaces with a newline. The net result is to replace paragraph indents with a blank line between paragraphs.

An address range followed by one or more operations may require open and closed curly brackets, with appropriate newlines.

/[0-9A-Za-z]/,/^$/{ /^$/d }

This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text file, but retaining the blank line(s) between paragraphs.

The usual delimiter that sed uses is /. However, sed allows other delimiters, such as %. This is useful when / is part of a replacement string, as in a file pathname. See Example 11-10 and Example 16-32.

A quick way to double-space a text file is sed G filename .

For illustrative examples of sed within shell scripts, see:

Example 36-1
Example 36-2
Example 16-3
Example A-2
Example 16-17
Example 16-27
Example A-12
Example A-16
Example A-17
Example 16-32
Example 11-10
Example 16-48
Example A-1
Example 16-14
Example 16-12
Example A-10
Example 19-12
Example 16-19
Example A-29
Example A-31
Example A-24
Example A-43
Example A-55

For a more extensive treatment of sed, refer to the pertinent references in the Bibliography.

C.2. Awk

Awk [143] is a full-featured text processing language with a syntax reminiscent of C. While it possesses an extensive set of operators and capabilities, we will cover only a few of these here - the ones most useful in shell scripts.

Awk breaks each line of input passed to it into fields. By default, a field is a string of consecutive characters delimited by whitespace, though there are options for changing this. Awk parses and operates on each separate field. This makes it ideal for handling structured text files -- especially tables -- data organized into consistent chunks, such as rows and columns.

Strong quoting and curly brackets enclose blocks of awk code within a shell script.

# $1 is field #1, $2 is field #2, etc.  echo one two | awk '{print $1}' # one  echo one two | awk '{print $2}' # two  # But what is field #0 ($0)? echo one two | awk '{print $0}' # one two # All the fields!   awk '{print $3}' $filename # Prints field #3 of file $filename to stdout.  awk '{print $1 $5 $6}' $filename # Prints fields #1, #5, and #6 of file $filename.  awk '{print $0}' $filename # Prints the entire file! # Same effect as:   cat $filename . . . or . . . sed '' $filename

We have just seen the awk print command in action. The only other feature of awk we need to deal with here is variables. Awk handles variables similarly to shell scripts, though a bit more flexibly.

{ total += ${column_number} }

This adds the value of column_number to the running total of total >. Finally, to print "total", there is an END command block, executed after the script has processed all its input.

Corresponding to the END, there is a BEGIN, for a code block to be performed before awk starts processing its input.

The following example illustrates how awk can add text-parsing tools to a shell script.

Example C-1. Counting Letter Occurrences

#! /bin/sh # letter-count2.sh: Counting letter occurrences in a text file. # # Script by nyal [nyal@voila.fr]. # Used in ABS Guide with permission. # Recommented and reformatted by ABS Guide author. # Version 1.1: Modified to work with gawk 3.1.3. #              (Will still work with earlier versions.)   INIT_TAB_AWK="" # Parameter to initialize awk script. count_case=0 FILE_PARSE=$1  E_PARAMERR=85  usage() {     echo "Usage: letter-count.sh file letters" 2>&1     # For example:   ./letter-count2.sh filename.txt a b c     exit $E_PARAMERR  # Too few arguments passed to script. }  if [ ! -f "$1" ] ; then     echo "$1: No such file." 2>&1     usage                 # Print usage message and exit. fi   if [ -z "$2" ] ; then     echo "$2: No letters specified." 2>&1     usage fi   shift                      # Letters specified. for letter in `echo $@`    # For each one . . .   do   INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \   \"$letter\"; final_tab[${count_case}] = 0; "    # Pass as parameter to awk script below.   count_case=`expr $count_case + 1` done  # DEBUG: # echo $INIT_TAB_AWK;  cat $FILE_PARSE | # Pipe the target file to the following awk script.  # --------------------------------------------------------------------- # Earlier version of script: # awk -v tab_search=0 -v final_tab=0 -v tab=0 -v \ # nb_letter=0 -v chara=0 -v chara2=0 \  awk \ "BEGIN { $INIT_TAB_AWK } \ { split(\$0, tab, \"\"); \ for (chara in tab) \ { for (chara2 in tab_search) \ { if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \ END { for (chara in final_tab) \ { print tab_search[chara] \" => \" final_tab[chara] } }" # --------------------------------------------------------------------- #  Nothing all that complicated, just . . . #+ for-loops, if-tests, and a couple of specialized functions.  exit $?  # Compare this script to letter-count.sh.

For simpler examples of awk within shell scripts, see:

Example 15-14
Example 20-8
Example 16-32
Example 36-5
Example 28-2
Example 15-20
Example 29-3
Example 29-4
Example 11-3
Example 16-61
Example 9-16
Example 16-4
Example 10-6
Example 36-19
Example 11-9
Example 36-4
Example 16-53
Example T-3

That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the Bibliography.

Appendix D. Parsing and Managing Pathnames

Emmanual Rouat contributed the following example of parsing and transforming filenames and, in particular, pathnames. It draws heavily on the functionality of sed.

#!/usr/bin/env bash #----------------------------------------------------------- # Management of PATH, LD_LIBRARY_PATH, MANPATH variables... # By Emmanuel Rouat <no-email> # (Inspired by the bash documentation 'pathfuncs' and on # discussions found on stackoverflow: # http://stackoverflow.com/questions/370047/ # http://stackoverflow.com/questions/273909/#346860 ) # Last modified: Sat Sep 22 12:01:55 CEST 2012 # # The following functions handle spaces correctly. # These functions belong in .bash_profile rather than in # .bashrc, I guess. # # The modular aspect of these functions should make it easy # to expand them to handle path substitutions instead # of path removal etc.... # # See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/ # (item 43) for an explanation of the 'duplicate-entries' removal # (it's a nice trick!) #-----------------------------------------------------------  # Show $@ (usually PATH) as list. function p_show() { local p="$@" && for p; do [[ ${!p} ]] && echo -e ${!p//:/\\n}; done }  # Filter out empty lines, multiple/trailing slashes, and duplicate entries. function p_filter() { awk '/^[ \t]*$/ {next} {sub(/\/+$/, "");gsub(/\/+/, "/")}!x[$0]++' ;}  # Rebuild list of items into ':' separated word (PATH-like). function p_build() { paste -sd: ;}  # Clean $1 (typically PATH) and rebuild it function p_clean() { local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}  # Remove $1 from $2 (found on stackoverflow, with modifications). function p_rm() { local d=$(echo $1 | p_filter) p=${2} &&   eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}  #  Same as previous, but filters on a pattern (dangerous... #+ don't use 'bin' or '/' as pattern!). function p_rmpat() { local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |   p_filter | grep -v "${d}" | p_build)' ;}  # Delete $1 from $2 and append it cleanly. function p_append() { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&   eval ${p}='$(p_show ${p} d | p_build)' ;}  # Delete $1 from $2 and prepend it cleanly. function p_prepend() { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&   eval ${p}='$(p_show d ${p} | p_build)' ;}  # Some tests: echo MYPATH="/bin:/usr/bin/:/bin://bin/" p_append "/project//my project/bin" MYPATH echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'" echo "(result should be: /bin:/usr/bin:/project/my project/bin)" echo $MYPATH  echo MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin" p_prepend "/project//my project/bin" MYOTHERPATH echo "Prepend '/project//my project/bin' \ to '/bin:/usr/bin/:/bin:/project//my project/bin/'" echo "(result should be: /project/my project/bin:/bin:/usr/bin)" echo $MYOTHERPATH  echo p_prepend "/project//my project/bin" FOOPATH  # FOOPATH doesn't exist. echo "Prepend '/project//my project/bin' to an unset variable" echo "(result should be: /project/my project/bin)" echo $FOOPATH  echo BARPATH="/a:/b/://b c://a:/my local pub" p_clean BARPATH echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'" echo "(result should be: /a:/b:/b c:/my local pub)" echo $BARPATH

***

David Wheeler kindly permitted me to use his instructive examples.

Doing it correctly: A quick summary by David Wheeler http://www.dwheeler.com/essays/filenames-in-shell.html  So, how can you process filenames correctly in shell? Here's a quick summary about how to do it correctly, for the impatient who "just want the answer". In short: Double-quote to use "$variable" instead of $variable, set IFS to just newline and tab, prefix all globs/filenames so they cannot begin with "-" when expanded, and use one of a few templates that work correctly. Here are some of those templates that work correctly:    IFS="$(printf '\n\t')"  # Remove SPACE, so filenames with spaces work well.   #  Correct glob use:  #+ always use "for" loop, prefix glob, check for existence:  for file in ./* ; do          # Use "./*" ... NEVER bare "*" ...    if [ -e "$file" ] ; then    # Make sure it isn't an empty match.      COMMAND ... "$file" ...    fi  done     # Correct glob use, but requires nonstandard bash extension.  shopt -s nullglob  #  Bash extension,                     #+ so that empty glob matches will work.  for file in ./* ; do        # Use "./*", NEVER bare "*"    COMMAND ... "$file" ...  done     #  These handle all filenames correctly;  #+ can be unwieldy if COMMAND is large:  find ... -exec COMMAND... {} \;  find ... -exec COMMAND... {} \+ # If multiple files are okay for COMMAND.     #  This skips filenames with control characters  #+ (including tab and newline).  IFS="$(printf '\n\t')"  controlchars="$(printf '*[\001-\037\177]*')"  for file in $(find . ! -name "$controlchars"') ; do    COMMAND "$file" ...  done     #  Okay if filenames can't contain tabs or newlines --  #+ beware the assumption.  IFS="$(printf '\n\t')"  for file in $(find .) ; do    COMMAND "$file" ...  done     # Requires nonstandard but common extensions in find and xargs:  find . -print0 | xargs -0 COMMAND   # Requires nonstandard extensions to find and to shell (bash works).  # variables might not stay set once the loop ends:  find . -print0 | while IFS="" read -r -d "" file ; do ...    COMMAND "$file" # Use quoted "$file", not $file, everywhere.  done     #  Requires nonstandard extensions to find and to shell (bash works).  #  Underlying system must include named pipes (FIFOs)  #+ or the /dev/fd mechanism.  #  In this version, variables *do* stay set after the loop ends,  #  and you can read from stdin.  #+ (Change the 4 to another number if fd 4 is needed.)   while IFS="" read -r -d "" file <&4 ; do    COMMAND "$file"   # Use quoted "$file" -- not $file, everywhere.  done 4< <(find . -print0)    #  Named pipe version.  #  Requires nonstandard extensions to find and to shell's read (bash ok).  #  Underlying system must include named pipes (FIFOs).  #  Again, in this version, variables *do* stay set after the loop ends,  #  and you can read from stdin.  # (Change the 4 to something else if fd 4 needed).   mkfifo mypipe   find . -print0 > mypipe &  while IFS="" read -r -d "" file <&4 ; do    COMMAND "$file" # Use quoted "$file", not $file, everywhere.  done 4< mypipe

Appendix E. Exit Codes With Special Meanings

Table E-1. Reserved Exit Codes

Exit Code Number	Meaning	Example	Comments
`1`	Catchall for general errors	let "var1 = 1/0"	Miscellaneous errors, such as "divide by zero" and other impermissible operations
`2`	Misuse of shell builtins (according to Bash documentation)	empty_function() {}	Missing keyword or command, or permission problem (and diff return code on a failed binary file comparison).
`126`	Command invoked cannot execute	/dev/null	Permission problem or command is not an executable
`127`	"command not found"	illegal_command	Possible problem with `$PATH` or a typo
`128`	Invalid argument to exit	exit 3.14159	exit takes only integer args in the range 0 - 255 (see first footnote)
`128+n`	Fatal error signal "n"	kill -9 `$PPID` of script	`$?` returns 137 (128 + 9)
`130`	Script terminated by Control-C	Ctl-C	Control-C is fatal error signal 2, (130 = 128 + 2, see above)
`255*`	Exit status out of range	exit -1	exit takes only integer args in the range 0 - 255

According to the above table, exit codes 1 - 2, 126 - 165, and 255 [144] have special meanings, and should therefore be avoided for user-specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error code a "command not found" or a user-defined one?). However, many scripts use an exit 1 as a general bailout-upon-error. Since exit code 1 signifies so many possible errors, it is not particularly useful in debugging.

There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success), to conform with the C/C++ standard. This would allot 50 valid codes, and make troubleshooting scripts more straightforward. [145] All user-defined exit codes in the accompanying examples to this document conform to this standard, except where overriding circumstances exist, as in Example 9-2.

Issuing a $? from the command-line after a shell script exits gives results consistent with the table above only from the Bash or sh prompt. Running the C-shell or tcsh may give different values in some cases.

Appendix F. A Detailed Introduction to I/O and I/O Redirection

written by St�phane Chazelas, and revised by the document author

A command expects the first three file descriptors to be available. The first, fd 0 (standard input, stdin), is for reading. The other two (fd 1, stdout and fd 2, stderr) are for writing.

There is a stdin, stdout, and a stderr associated with each command. ls 2>&1 means temporarily connecting the stderr of the ls command to the same "resource" as the shell's stdout.

By convention, a command reads its input from fd 0 (stdin), prints normal output to fd 1 (stdout), and error ouput to fd 2 (stderr). If one of those three fd's is not open, you may encounter problems:

                bash$                                                  cat /etc/passwd >&-                                cat: standard output: Bad file descriptor

For example, when xterm runs, it first initializes itself. Before running the user's shell, xterm opens the terminal device (/dev/pts/<n> or something similar) three times.

At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits them in turn, except when you redirect the command. Redirection means reassigning one of the file descriptors to another file (or a pipe, or anything permissible). File descriptors may be reassigned locally (for a command, a command group, a subshell, a while or if or case or for loop...), or globally, for the remainder of the shell (using exec).

ls > /dev/null means running ls with its fd 1 connected to /dev/null.

                  bash$                                                        lsof -a -p $$ -d0,1,2                                    COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME  bash    363 bozo        0u   CHR  136,1         3 /dev/pts/1  bash    363 bozo        1u   CHR  136,1         3 /dev/pts/1  bash    363 bozo        2u   CHR  136,1         3 /dev/pts/1                  bash$                                                        exec 2> /dev/null                                    bash$                                                        lsof -a -p $$ -d0,1,2                                    COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME  bash    371 bozo        0u   CHR  136,1         3 /dev/pts/1  bash    371 bozo        1u   CHR  136,1         3 /dev/pts/1  bash    371 bozo        2w   CHR    1,3       120 /dev/null                  bash$                                                        bash -c 'lsof -a -p $$ -d0,1,2' | cat                                    COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME  lsof    379 root    0u   CHR  136,1         3 /dev/pts/1  lsof    379 root    1w  FIFO    0,0      7118 pipe  lsof    379 root    2u   CHR  136,1         3 /dev/pts/1                  bash$                                                        echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)"                                    COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME  lsof    426 root    0u   CHR  136,1         3 /dev/pts/1  lsof    426 root    1w  FIFO    0,0      7520 pipe  lsof    426 root    2w  FIFO    0,0      7520 pipe

This works for different types of redirection.

Exercise: Analyze the following script.

#! /usr/bin/env bash  mkfifo /tmp/fifo1 /tmp/fifo2 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)  exec 3>&1 (  (   (    while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \    | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2     echo 1st, to stdout    sleep 1    echo 2nd, to stderr >&2    sleep 1    echo 3rd, to fd 3 >&3    sleep 1    echo 4th, to fd 4 >&4    sleep 1    echo 5th, to fd 5 >&5    sleep 1    echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5    sleep 1    echo 7th, to fd 6 >&6    sleep 1    echo 8th, to fd 7 >&7    sleep 1    echo 9th, to fd 8 >&8    ) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&-  ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&- ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&-  rm -f /tmp/fifo1 /tmp/fifo2   # For each command and subshell, figure out which fd points to what. # Good luck!  exit 0

Appendix G. Command-Line Options

Many executables, whether binaries or script files, accept options to modify their run-time behavior. For example: from the command-line, typing command -o would invoke command, with option o.

G.1. Standard Command-Line Options

Over time, there has evolved a loose standard for the meanings of command-line option flags. The GNU utilities conform more closely to this "standard" than older UNIX utilities.

Traditionally, UNIX command-line options consist of a dash, followed by one or more lowercase letters. The GNU utilities added a double-dash, followed by a complete word or compound word.

The two most widely-accepted options are:

-h

--help

Help: Give usage message and exit.
-v

--version

Version: Show program version and exit.

Other common options are:

-a

--all

All: show all information or operate on all arguments.
-l

--list

List: list files or arguments without taking other action.
-o

Output filename
-q

--quiet

Quiet: suppress stdout.
-r

-R

--recursive

Recursive: Operate recursively (down directory tree).
-v

--verbose

Verbose: output additional information to stdout or stderr.
-z

--compress

Compress: apply compression (usually gzip).

However:

In tar and gawk:

-f

--file

File: filename follows.
In cp, mv, rm:

-f

--force

Force: force overwrite of target file(s).

Many UNIX and Linux utilities deviate from this "standard," so it is dangerous to assume that a given option will behave in a standard way. Always check the man page for the command in question when in doubt.

A complete table of recommended options for the GNU utilities is available at the GNU standards page.

G.2. Bash Command-Line Options

Bash itself has a number of command-line options. Here are some of the more useful ones.

-c

Read commands from the following string and assign any arguments to the positional parameters.

                        bash$                                                                          bash -c 'set a b c d; IFS="+-;"; echo "$*"'                                                a+b+c+d

-r

--restricted

Runs the shell, or a script, in restricted mode.
--posix

Forces Bash to conform to POSIX mode.
--version

Display Bash version information and exit.
--

End of options. Anything further on the command line is an argument, not an option.

Appendix H. Important Files

startup files

: These files contain the aliases and environmental variables made available to Bash running as a user shell and to all Bash scripts invoked after system initialization.
/etc/profile: Systemwide defaults, mostly setting the environment (all Bourne-type shells, not just Bash [146])
/etc/bashrc: systemwide functions and aliases for Bash
$HOME/.bash_profile: user-specific Bash environmental default settings, found in each user's home directory (the local counterpart to /etc/profile)
$HOME/.bashrc: user-specific Bash init file, found in each user's home directory (the local counterpart to /etc/bashrc). Only interactive shells and user scripts read this file. See Appendix M for a sample .bashrc file.

logout file

$HOME/.bash_logout: user-specific instruction file, found in each user's home directory. Upon exit from a login (Bash) shell, the commands in this file execute.

data files

/etc/passwd: A listing of all the user accounts on the system, their identities, their home directories, the groups they belong to, and their default shell. Note that the user passwords are not stored in this file, [147] but in /etc/shadow in encrypted form.

system configuration files

/etc/sysconfig/hwconf

Listing and description of attached hardware devices. This information is in text form and can be extracted and parsed.

                        bash$                                                                          grep -A 5 AUDIO /etc/sysconfig/hwconf                                                class: AUDIO  bus: PCI  detached: 0  driver: snd-intel8x0  desc: "Intel Corporation 82801CA/CAM AC'97 Audio Controller"  vendorId: 8086

This file is present on Red Hat and Fedora Core installations, but may be missing from other distros.

Appendix I. Important System Directories

Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories.

/bin

Binaries (executables). Basic system programs and utilities (such as bash).
/usr/bin [148]

More system binaries.
/usr/local/bin

Miscellaneous binaries local to the particular machine.
/sbin

System binaries. Basic system administrative programs and utilities (such as fsck).
/usr/sbin

More system administrative programs and utilities.
/etc

Et cetera. Systemwide configuration scripts.

Of particular interest are the /etc/fstab (filesystem table), /etc/mtab (mounted filesystem table), and the /etc/inittab files.
/etc/rc.d

Boot scripts, on Red Hat and derivative distributions of Linux.
/usr/share/doc

Documentation for installed packages.
/usr/man

The systemwide manpages.
/dev

Device directory. Entries (but not mount points) for physical and virtual devices. See Chapter 29.
/proc

Process directory. Contains information and statistics about running processes and kernel parameters. See Chapter 29.
/sys

Systemwide device directory. Contains information and statistics about device and device names. This is newly added to Linux with the 2.6.X kernels.
/mnt

Mount. Directory for mounting hard drive partitions, such as /mnt/dos, and physical devices. In newer Linux distros, the /media directory has taken over as the preferred mount point for I/O devices.
/media

In newer Linux distros, the preferred mount point for I/O devices, such as CD/DVD drives or USB flash drives.
/var

Variable (changeable) system files. This is a catchall "scratchpad" directory for data generated while a Linux/UNIX machine is running.
/var/log

Systemwide log files.
/var/spool/mail

User mail spool.
/lib

Systemwide library files.
/usr/lib

More systemwide library files.
/tmp

System temporary files.

/boot

System boot directory. The kernel, module links, system map, and boot manager reside here.

Altering files in this directory may result in an unbootable system.

Appendix J. An Introduction to Programmable Completion

The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete the command sequence. [149] If multiple completions are possible, then [Tab] lists them all. Let's see how it works.

                  bash$                                                        xtra[Tab]                                    xtraceroute       xtrapin           xtrapproto  xtraceroute.real  xtrapinfo         xtrapreset  xtrapchar         xtrapout          xtrapstats                  bash$                                                        xtrac[Tab]                                    xtraceroute       xtraceroute.real                  bash$                                                        xtraceroute.r[Tab]                                    xtraceroute.real

Tab completion also works for variables and path names.

                  bash$                                                        echo $BASH[Tab]                                    $BASH                 $BASH_COMPLETION      $BASH_SUBSHELL  $BASH_ARGC            $BASH_COMPLETION_DIR  $BASH_VERSINFO  $BASH_ARGV            $BASH_LINENO          $BASH_VERSION  $BASH_COMMAND         $BASH_SOURCE                  bash$                                                        echo /usr/local/[Tab]                                    bin/     etc/     include/ libexec/ sbin/    src/       doc/     games/   lib/     man/     share/

The Bash complete and compgen builtins make it possible for tab completion to recognize partial parameters and options to commands. In a very simple case, we can use complete from the command-line to specify a short list of acceptable parameters.

                bash$                                                  touch sample_command                                bash$                                                  touch file1.txt file2.txt file2.doc file30.txt file4.zzz                                bash$                                                  chmod +x sample_command                                bash$                                                  complete -f -X '!*.txt' sample_command                                bash$                                                  ./sample[Tab][Tab]                                sample_command                file1.txt   file2.txt   file30.txt

The -f option to complete specifies filenames, and -X the filter pattern.

For anything more complex, we could write a script that specifies a list of acceptable command-line parameters. The compgen builtin expands a list of arguments to generate completion matches.

Let us take a modified version of the UseGetOpt.sh script as an example command. This script accepts a number of command-line parameters, preceded by either a single or double dash. And here is the corresponding completion script, by convention given a filename corresponding to its associated command.

Example J-1. Completion script for UseGetOpt.sh

# file: UseGetOpt-2 # UseGetOpt-2.sh parameter-completion  _UseGetOpt-2 ()   #  By convention, the function name {                 #+ starts with an underscore.   local cur   # Pointer to current completion word.   # By convention, it's named "cur" but this isn't strictly necessary.    COMPREPLY=()   # Array variable storing the possible completions.   cur=${COMP_WORDS[COMP_CWORD]}    case "$cur" in     -*)     COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \                                --file --log --test --help --' -- $cur ) );; #   Generate the completion matches and load them into $COMPREPLY array. #   xx) May add more cases here. #   yy) #   zz)   esac    return 0 }  complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh #        ^^ ^^^^^^^^^^^^  Invokes the function _UseGetOpt-2.

Now, let's try it.

                bash$                                                  source UseGetOpt-2                                bash$                                                  ./UseGetOpt-2.sh -[Tab]                                --         --aoption  --debug    --file     --help     --log     --test  -a         -d         -f         -h         -l         -t                bash$                                                  ./UseGetOpt-2.sh --[Tab]                                --         --aoption  --debug    --file     --help     --log     --test

We begin by sourcing the "completion script." This sets the command-line parameters. [150]

In the first instance, hitting [Tab] after a single dash, the output is all the possible parameters preceded by one or more dashes. Hitting [Tab] after two dashes gives the possible parameters preceded by two or more dashes.

Now, just what is the point of having to jump through flaming hoops to enable command-line tab completion? It saves keystrokes. [151]

Resources:

Bash programmable completion project

Mitch Frazier's Linux Journal article, More on Using the Bash Complete Command

Steve's excellent two-part article, "An Introduction to Bash Completion": Part 1 and Part 2

Appendix K. Localization

Localization is an undocumented Bash feature.

A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get output from the same script in English.

To create a localized script, use the following template to write all messages to the user (error messages, prompts, etc.).

#!/bin/bash # localized.sh #  Script by St�phane Chazelas, #+ modified by Bruno Haible, bugfixed by Alfredo Pironti.  . gettext.sh  E_CDERROR=65  error() {   printf "$@" >&2   exit $E_CDERROR }  cd $var || error "`eval_gettext \"Can\'t cd to \\\$var.\"`" #  The triple backslashes (escapes) in front of $var needed #+ "because eval_gettext expects a string #+ where the variable values have not yet been substituted." #    -- per Bruno Haible read -p "`gettext \"Enter the value: \"`" var #  ...   #  ------------------------------------------------------------------ #  Alfredo Pironti comments:  #  This script has been modified to not use the $"..." syntax in #+ favor of the "`gettext \"...\"`" syntax. #  This is ok, but with the new localized.sh program, the commands #+ "bash -D filename" and "bash --dump-po-string filename" #+ will produce no output #+ (because those command are only searching for the $"..." strings)! #  The ONLY way to extract strings from the new file is to use the # 'xgettext' program. However, the xgettext program is buggy.  # Note that 'xgettext' has another bug. # # The shell fragment: #    gettext -s "I like Bash" # will be correctly extracted, but . . . #    xgettext -s "I like Bash" # . . . fails! #  'xgettext' will extract "-s" because #+ the command only extracts the #+ very first argument after the 'gettext' word.   #  Escape characters: # #  To localize a sentence like #     echo -e "Hello\tworld!" #+ you must use #     echo -e "`gettext \"Hello\\tworld\"`" #  The "double escape character" before the `t' is needed because #+ 'gettext' will search for a string like: 'Hello\tworld' #  This is because gettext will read one literal `\') #+ and will output a string like "Bonjour\tmonde", #+ so the 'echo' command will display the message correctly. # #  You may not use #     echo "`gettext -e \"Hello\tworld\"`" #+ due to the xgettext bug explained above.    # Let's localize the following shell fragment: #     echo "-h display help and exit" # # First, one could do this: #     echo "`gettext \"-h display help and exit\"`" #  This way 'xgettext' will work ok, #+ but the 'gettext' program will read "-h" as an option! # # One solution could be #     echo "`gettext -- \"-h display help and exit\"`" #  This way 'gettext' will work, #+ but 'xgettext' will extract "--", as referred to above. # # The workaround you may use to get this string localized is #     echo -e "`gettext \"\\0-h display help and exit\"`" #  We have added a \0 (NULL) at the beginning of the sentence. #  This way 'gettext' works correctly, as does 'xgettext.' #  Moreover, the NULL character won't change the behavior #+ of the 'echo' command. #  ------------------------------------------------------------------

                  bash$                                                        bash -D localized.sh                                    "Can't cd to %s."  "Enter the value: "

This lists all the localized text. (The -D option lists double-quoted strings prefixed by a $, without executing the script.)

                  bash$                                                        bash --dump-po-strings localized.sh                                    #: a:6  msgid "Can't cd to %s."  msgstr ""  #: a:7  msgid "Enter the value: "  msgstr ""

The --dump-po-strings option to Bash resembles the -D option, but uses gettext "po" format.

Bruno Haible points out:

Starting with gettext-0.12.2, xgettext -o - localized.sh is recommended instead of bash --dump-po-strings localized.sh, because xgettext . . .

1. understands the gettext and eval_gettext commands (whereas bash --dump-po-strings understands only its deprecated $"..." syntax)

2. can extract comments placed by the programmer, intended to be read by the translator.

This shell code is then not specific to Bash any more; it works the same way with Bash 1.x and other /bin/sh implementations.

Now, build a language.po file for each language that the script will be translated into, specifying the msgstr . Alfredo Pironti gives the following example:

fr.po:

#: a:6 msgid "Can't cd to $var." msgstr "Impossible de se positionner dans le repertoire $var." #: a:7 msgid "Enter the value: " msgstr "Entrez la valeur : "  #  The string are dumped with the variable names, not with the %s syntax, #+ similar to C programs. #+ This is a very cool feature if the programmer uses #+ variable names that make sense!

Then, run msgfmt.

msgfmt -o localized.sh.mo fr.po

Place the resulting localized.sh.mo file in the /usr/local/share/locale/fr/LC_MESSAGES directory, and at the beginning of the script, insert the lines:

TEXTDOMAINDIR=/usr/local/share/locale TEXTDOMAIN=localized.sh

If a user on a French system runs the script, she will get French messages.

With older versions of Bash or other shells, localization requires gettext, using the -s option. In this case, the script becomes:

#!/bin/bash # localized.sh  E_CDERROR=65  error() {   local format=$1   shift   printf "$(gettext -s "$format")" "$@" >&2   exit $E_CDERROR } cd $var || error "Can't cd to %s." "$var" read -p "$(gettext -s "Enter the value: ")" var # ...

The TEXTDOMAIN and TEXTDOMAINDIR variables need to be set and exported to the environment. This should be done within the script itself.

---

This appendix written by St�phane Chazelas, with modifications suggested by Alfredo Pironti, and by Bruno Haible, maintainer of GNU gettext.

Appendix L. History Commands

The Bash shell provides command-line tools for editing and manipulating a user's command history. This is primarily a convenience, a means of saving keystrokes.

Bash history commands:

history
fc

                  bash$                                                        history                                                        1  mount /mnt/cdrom     2  cd /mnt/cdrom     3  ls      ...

Internal variables associated with Bash history commands:

$HISTCMD
$HISTCONTROL
$HISTIGNORE
$HISTFILE
$HISTFILESIZE
$HISTSIZE
$HISTTIMEFORMAT (Bash, ver. 3.0 or later)
!!
!$
!#
!N
!-N
!STRING
!?STRING?
^STRING^string^

Unfortunately, the Bash history tools find no use in scripting.

#!/bin/bash # history.sh # A (vain) attempt to use the 'history' command in a script.  history                      # No output.  var=$(history); echo "$var"  # $var is empty.  #  History commands are, by default, disabled within a script. #  However, as dhw points out, #+ set -o history #+ enables the history mechanism.  set -o history var=$(history); echo "$var"   # 1  var=$(history)

                  bash$                                                        ./history.sh                                    (no output)

The Advancing in the Bash Shell site gives a good introduction to the use of history commands in Bash.

Appendix M. Sample `.bashrc` and `.bash_profile` Files

The ~/.bashrc file determines the behavior of interactive shells. A good look at this file can lead to a better understanding of Bash.

Emmanuel Rouat contributed the following very elaborate .bashrc file, written for a Linux system. He welcomes reader feedback on it.

Study the file carefully, and feel free to reuse code snippets and functions from it in your own .bashrc file or even in your scripts.

Example M-1. Sample .bashrc file

# =============================================================== # # # PERSONAL $HOME/.bashrc FILE for bash-3.0 (or later) # By Emmanuel Rouat [no-email] # # Last modified: Tue Nov 20 22:04:47 CET 2012  #  This file is normally read by interactive shells only. #+ Here is the place to define your aliases, functions and #+ other interactive features like your prompt. # #  The majority of the code here assumes you are on a GNU #+ system (most likely a Linux box) and is often based on code #+ found on Usenet or Internet. # #  See for instance: #  http://tldp.org/LDP/abs/html/index.html #  http://www.caliban.org/bash #  http://www.shelldorado.com/scripts/categories.html #  http://www.dotfiles.org # #  The choice of colors was done for a shell with a dark background #+ (white on black), and this is usually also suited for pure text-mode #+ consoles (no X server available). If you use a white background, #+ you'll have to do some other choices for readability. # #  This bashrc file is a bit overcrowded. #  Remember, it is just just an example. #  Tailor it to your needs. # # =============================================================== #  # --> Comments added by HOWTO author.  # If not running interactively, don't do anything [ -z "$PS1" ] && return   #------------------------------------------------------------- # Source global definitions (if any) #-------------------------------------------------------------   if [ -f /etc/bashrc ]; then       . /etc/bashrc   # --> Read /etc/bashrc, if present. fi   #-------------------------------------------------------------- #  Automatic setting of $DISPLAY (if not set already). #  This works for me - your mileage may vary. . . . #  The problem is that different types of terminals give #+ different answers to 'who am i' (rxvt in particular can be #+ troublesome) - however this code seems to work in a majority #+ of cases. #--------------------------------------------------------------  function get_xserver () {     case $TERM in         xterm )             XSERVER=$(who am i | awk '{print $NF}' | tr -d ')''(' )             # Ane-Pieter Wieringa suggests the following alternative:             #  I_AM=$(who am i)             #  SERVER=${I_AM#*(}             #  SERVER=${SERVER%*)}             XSERVER=${XSERVER%%:*}             ;;             aterm | rxvt)             # Find some code that works here. ...             ;;     esac }  if [ -z ${DISPLAY:=""} ]; then     get_xserver     if [[ -z ${XSERVER}  || ${XSERVER} == $(hostname) ||        ${XSERVER} == "unix" ]]; then           DISPLAY=":0.0"          # Display on local host.     else        DISPLAY=${XSERVER}:0.0     # Display on remote host.     fi fi  export DISPLAY  #------------------------------------------------------------- # Some settings #-------------------------------------------------------------  #set -o nounset     # These  two options are useful for debugging. #set -o xtrace alias debug="set -o nounset; set -o xtrace"  ulimit -S -c 0      # Don't want coredumps. set -o notify set -o noclobber set -o ignoreeof   # Enable options: shopt -s cdspell shopt -s cdable_vars shopt -s checkhash shopt -s checkwinsize shopt -s sourcepath shopt -s no_empty_cmd_completion shopt -s cmdhist shopt -s histappend histreedit histverify shopt -s extglob       # Necessary for programmable completion.  # Disable options: shopt -u mailwarn unset MAILCHECK        # Don't want my shell to warn me of incoming mail.   #------------------------------------------------------------- # Greeting, motd etc. ... #-------------------------------------------------------------  # Color definitions (taken from Color Bash Prompt HowTo). # Some colors might look different of some terminals. # For example, I see 'Bold Red' as 'orange' on my screen, # hence the 'Green' 'BRed' 'Red' sequence I often use in my prompt.   # Normal Colors Black='\e[0;30m'        # Black Red='\e[0;31m'          # Red Green='\e[0;32m'        # Green Yellow='\e[0;33m'       # Yellow Blue='\e[0;34m'         # Blue Purple='\e[0;35m'       # Purple Cyan='\e[0;36m'         # Cyan White='\e[0;37m'        # White  # Bold BBlack='\e[1;30m'       # Black BRed='\e[1;31m'         # Red BGreen='\e[1;32m'       # Green BYellow='\e[1;33m'      # Yellow BBlue='\e[1;34m'        # Blue BPurple='\e[1;35m'      # Purple BCyan='\e[1;36m'        # Cyan BWhite='\e[1;37m'       # White  # Background On_Black='\e[40m'       # Black On_Red='\e[41m'         # Red On_Green='\e[42m'       # Green On_Yellow='\e[43m'      # Yellow On_Blue='\e[44m'        # Blue On_Purple='\e[45m'      # Purple On_Cyan='\e[46m'        # Cyan On_White='\e[47m'       # White  NC="\e[m"               # Color Reset   ALERT=${BWhite}${On_Red} # Bold White on red background    echo -e "${BCyan}This is BASH ${BRed}${BASH_VERSION%.*}${BCyan}\ - DISPLAY on ${BRed}$DISPLAY${NC}\n" date if [ -x /usr/games/fortune ]; then     /usr/games/fortune -s     # Makes our day a bit more fun.... :-) fi  function _exit()              # Function to run upon exit of shell. {     echo -e "${BRed}Hasta la vista, baby${NC}" } trap _exit EXIT  #------------------------------------------------------------- # Shell Prompt - for many examples, see: #       http://www.debian-administration.org/articles/205 #       http://www.askapache.com/linux/bash-power-prompt.html #       http://tldp.org/HOWTO/Bash-Prompt-HOWTO #       https://github.com/nojhan/liquidprompt #------------------------------------------------------------- # Current Format: [TIME USER@HOST PWD] > # TIME: #    Green     == machine load is low #    Orange    == machine load is medium #    Red       == machine load is high #    ALERT     == machine load is very high # USER: #    Cyan      == normal user #    Orange    == SU to user #    Red       == root # HOST: #    Cyan      == local session #    Green     == secured remote connection (via ssh) #    Red       == unsecured remote connection # PWD: #    Green     == more than 10% free disk space #    Orange    == less than 10% free disk space #    ALERT     == less than 5% free disk space #    Red       == current user does not have write privileges #    Cyan      == current filesystem is size zero (like /proc) # >: #    White     == no background or suspended jobs in this shell #    Cyan      == at least one background job in this shell #    Orange    == at least one suspended job in this shell # #    Command is added to the history file each time you hit enter, #    so it's available to all shells (using 'history -a').   # Test connection type: if [ -n "${SSH_CONNECTION}" ]; then     CNX=${Green}        # Connected on remote machine, via ssh (good). elif [[ "${DISPLAY%%:0*}" != "" ]]; then     CNX=${ALERT}        # Connected on remote machine, not via ssh (bad). else     CNX=${BCyan}        # Connected on local machine. fi  # Test user type: if [[ ${USER} == "root" ]]; then     SU=${Red}           # User is root. elif [[ ${USER} != $(logname) ]]; then     SU=${BRed}          # User is not login user. else     SU=${BCyan}         # User is normal (well ... most of us are). fi    NCPU=$(grep -c 'processor' /proc/cpuinfo)    # Number of CPUs SLOAD=$(( 100*${NCPU} ))        # Small load MLOAD=$(( 200*${NCPU} ))        # Medium load XLOAD=$(( 400*${NCPU} ))        # Xlarge load  # Returns system load as percentage, i.e., '40' rather than '0.40)'. function load() {     local SYSLOAD=$(cut -d " " -f1 /proc/loadavg | tr -d '.')     # System load of the current host.     echo $((10#$SYSLOAD))       # Convert to decimal. }  # Returns a color indicating system load. function load_color() {     local SYSLOAD=$(load)     if [ ${SYSLOAD} -gt ${XLOAD} ]; then         echo -en ${ALERT}     elif [ ${SYSLOAD} -gt ${MLOAD} ]; then         echo -en ${Red}     elif [ ${SYSLOAD} -gt ${SLOAD} ]; then         echo -en ${BRed}     else         echo -en ${Green}     fi }  # Returns a color according to free disk space in $PWD. function disk_color() {     if [ ! -w "${PWD}" ] ; then         echo -en ${Red}         # No 'write' privilege in the current directory.     elif [ -s "${PWD}" ] ; then         local used=$(command df -P "$PWD" |                    awk 'END {print $5} {sub(/%/,"")}')         if [ ${used} -gt 95 ]; then             echo -en ${ALERT}           # Disk almost full (>95%).         elif [ ${used} -gt 90 ]; then             echo -en ${BRed}            # Free disk space almost gone.         else             echo -en ${Green}           # Free disk space is ok.         fi     else         echo -en ${Cyan}         # Current directory is size '0' (like /proc, /sys etc).     fi }  # Returns a color according to running/suspended jobs. function job_color() {     if [ $(jobs -s | wc -l) -gt "0" ]; then         echo -en ${BRed}     elif [ $(jobs -r | wc -l) -gt "0" ] ; then         echo -en ${BCyan}     fi }  # Adds some text in the terminal frame (if applicable).   # Now we construct the prompt. PROMPT_COMMAND="history -a" case ${TERM} in   *term | rxvt | linux)         PS1="\[\$(load_color)\][\A\[${NC}\] "         # Time of day (with load info):         PS1="\[\$(load_color)\][\A\[${NC}\] "         # User@Host (with connection type info):         PS1=${PS1}"\[${SU}\]\u\[${NC}\]@\[${CNX}\]\h\[${NC}\] "         # PWD (with 'disk space' info):         PS1=${PS1}"\[\$(disk_color)\]\W]\[${NC}\] "         # Prompt (with 'job' info):         PS1=${PS1}"\[\$(job_color)\]>\[${NC}\] "         # Set title of current xterm:         PS1=${PS1}"\[\e]0;[\u@\h] \w\a\]"         ;;     *)         PS1="(\A \u@\h \W) > " # --> PS1="(\A \u@\h \w) > "                                # --> Shows full pathname of current dir.         ;; esac    export TIMEFORMAT=$'\nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n' export HISTIGNORE="&:bg:fg:ll:h" export HISTTIMEFORMAT="$(echo -e ${BCyan})[%d/%m %H:%M:%S]$(echo -e ${NC}) " export HISTCONTROL=ignoredups export HOSTFILE=$HOME/.hosts    # Put a list of remote hosts in ~/.hosts   #============================================================ # #  ALIASES AND FUNCTIONS # #  Arguably, some functions defined here are quite big. #  If you want to make this file smaller, these functions can #+ be converted into scripts and removed from here. # #============================================================  #------------------- # Personnal Aliases #-------------------  alias rm='rm -i' alias cp='cp -i' alias mv='mv -i' # -> Prevents accidentally clobbering files. alias mkdir='mkdir -p'  alias h='history' alias j='jobs -l' alias which='type -a' alias ..='cd ..'  # Pretty-print of some PATH variables: alias path='echo -e ${PATH//:/\\n}' alias libpath='echo -e ${LD_LIBRARY_PATH//:/\\n}'   alias du='du -kh'    # Makes a more readable output. alias df='df -kTh'  #------------------------------------------------------------- # The 'ls' family (this assumes you use a recent GNU ls). #------------------------------------------------------------- # Add colors for filetype and  human-readable sizes by default on 'ls': alias ls='ls -h --color' alias lx='ls -lXB'         #  Sort by extension. alias lk='ls -lSr'         #  Sort by size, biggest last. alias lt='ls -ltr'         #  Sort by date, most recent last. alias lc='ls -ltcr'        #  Sort by/show change time,most recent last. alias lu='ls -ltur'        #  Sort by/show access time,most recent last.  # The ubiquitous 'll': directories first, with alphanumeric sorting: alias ll="ls -lv --group-directories-first" alias lm='ll |more'        #  Pipe through 'more' alias lr='ll -R'           #  Recursive ls. alias la='ll -A'           #  Show hidden files. alias tree='tree -Csuh'    #  Nice alternative to 'recursive ls' ...   #------------------------------------------------------------- # Tailoring 'less' #-------------------------------------------------------------  alias more='less' export PAGER=less export LESSCHARSET='latin1' export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&-'                 # Use this if lesspipe.sh exists. export LESS='-i -N -w  -z-4 -g -e -M -X -F -R -P%t?f%f \ :stdin .?pb%pb\%:?lbLine %lb:?bbByte %bb:-...'  # LESS man page colors (makes Man pages more readable). export LESS_TERMCAP_mb=$'\E[01;31m' export LESS_TERMCAP_md=$'\E[01;31m' export LESS_TERMCAP_me=$'\E[0m' export LESS_TERMCAP_se=$'\E[0m' export LESS_TERMCAP_so=$'\E[01;44;33m' export LESS_TERMCAP_ue=$'\E[0m' export LESS_TERMCAP_us=$'\E[01;32m'   #------------------------------------------------------------- # Spelling typos - highly personnal and keyboard-dependent :-) #-------------------------------------------------------------  alias xs='cd' alias vf='cd' alias moer='more' alias moew='more' alias kk='ll'   #------------------------------------------------------------- # A few fun ones #-------------------------------------------------------------  # Adds some text in the terminal frame (if applicable).  function xtitle() {     case "$TERM" in     *term* | rxvt)         echo -en  "\e]0;$*\a" ;;     *)  ;;     esac }   # Aliases that use xtitle alias top='xtitle Processes on $HOST && top' alias make='xtitle Making $(basename $PWD) ; make'  # .. and functions function man() {     for i ; do         xtitle The $(basename $1|tr -d .[:digit:]) manual         command man -a "$i"     done }   #------------------------------------------------------------- # Make the following commands run in background automatically: #-------------------------------------------------------------  function te()  # wrapper around xemacs/gnuserv {     if [ "$(gnuclient -batch -eval t 2>&-)" == "t" ]; then        gnuclient -q "$@";     else        ( xemacs "$@" &);     fi }  function soffice() { command soffice "$@" & } function firefox() { command firefox "$@" & } function xpdf() { command xpdf "$@" & }   #------------------------------------------------------------- # File & strings related functions: #-------------------------------------------------------------   # Find a file with a pattern in name: function ff() { find . -type f -iname '*'"$*"'*' -ls ; }  # Find a file with pattern $1 in name and Execute $2 on it: function fe() { find . -type f -iname '*'"${1:-}"'*' \ -exec ${2:-file} {} \;  ; }  #  Find a pattern in a set of files and highlight them: #+ (needs a recent version of egrep). function fstr() {     OPTIND=1     local mycase=""     local usage="fstr: find string in files. Usage: fstr [-i] \"pattern\" [\"filename pattern\"] "     while getopts :it opt     do         case "$opt" in            i) mycase="-i " ;;            *) echo "$usage"; return ;;         esac     done     shift $(( $OPTIND - 1 ))     if [ "$#" -lt 1 ]; then         echo "$usage"         return;     fi     find . -type f -name "${2:-*}" -print0 | \ xargs -0 egrep --color=always -sn ${case} "$1" 2>&- | more  }   function swap() { # Swap 2 filenames around, if they exist (from Uzi's bashrc).     local TMPFILE=tmp.$$      [ $# -ne 2 ] && echo "swap: 2 arguments needed" && return 1     [ ! -e $1 ] && echo "swap: $1 does not exist" && return 1     [ ! -e $2 ] && echo "swap: $2 does not exist" && return 1      mv "$1" $TMPFILE     mv "$2" "$1"     mv $TMPFILE "$2" }  function extract()      # Handy Extract Program {     if [ -f $1 ] ; then         case $1 in             *.tar.bz2)   tar xvjf $1     ;;             *.tar.gz)    tar xvzf $1     ;;             *.bz2)       bunzip2 $1      ;;             *.rar)       unrar x $1      ;;             *.gz)        gunzip $1       ;;             *.tar)       tar xvf $1      ;;             *.tbz2)      tar xvjf $1     ;;             *.tgz)       tar xvzf $1     ;;             *.zip)       unzip $1        ;;             *.Z)         uncompress $1   ;;             *.7z)        7z x $1         ;;             *)           echo "'$1' cannot be extracted via >extract<" ;;         esac     else         echo "'$1' is not a valid file!"     fi }   # Creates an archive (*.tar.gz) from given directory. function maketar() { tar cvzf "${1%%/}.tar.gz"  "${1%%/}/"; }  # Create a ZIP archive of a file or folder. function makezip() { zip -r "${1%%/}.zip" "$1" ; }  # Make your directories and files access rights sane. function sanitize() { chmod -R u=rwX,g=rX,o= "$@" ;}  #------------------------------------------------------------- # Process/system related functions: #-------------------------------------------------------------   function my_ps() { ps $@ -u $USER -o pid,%cpu,%mem,bsdtime,command ; } function pp() { my_ps f | awk '!/awk/ && $0~var' var=${1:-".*"} ; }   function killps()   # kill by process name {     local pid pname sig="-TERM"   # default signal     if [ "$#" -lt 1 ] || [ "$#" -gt 2 ]; then         echo "Usage: killps [-SIGNAL] pattern"         return;     fi     if [ $# = 2 ]; then sig=$1 ; fi     for pid in $(my_ps| awk '!/awk/ && $0~pat { print $1 }' pat=${!#} )     do         pname=$(my_ps | awk '$1~var { print $5 }' var=$pid )         if ask "Kill process $pid <$pname> with signal $sig?"             then kill $sig $pid         fi     done }  function mydf()         # Pretty-print of 'df' output. {                       # Inspired by 'dfc' utility.     for fs ; do          if [ ! -d $fs ]         then           echo -e $fs" :No such file or directory" ; continue         fi          local info=( $(command df -P $fs | awk 'END{ print $2,$3,$5 }') )         local free=( $(command df -Pkh $fs | awk 'END{ print $4 }') )         local nbstars=$(( 20 * ${info[1]} / ${info[0]} ))         local out="["         for ((j=0;j<20;j++)); do             if [ ${j} -lt ${nbstars} ]; then                out=$out"*"             else                out=$out"-"             fi         done         out=${info[2]}" "$out"] ("$free" free on "$fs")"         echo -e $out     done }   function my_ip() # Get IP adress on ethernet. {     MY_IP=$(/sbin/ifconfig eth0 | awk '/inet/ { print $2 } ' |       sed -e s/addr://)     echo ${MY_IP:-"Not connected"} }  function ii()   # Get current host related info. {     echo -e "\nYou are logged on ${BRed}$HOST"     echo -e "\n${BRed}Additionnal information:$NC " ; uname -a     echo -e "\n${BRed}Users logged on:$NC " ; w -hs |              cut -d " " -f1 | sort | uniq     echo -e "\n${BRed}Current date :$NC " ; date     echo -e "\n${BRed}Machine stats :$NC " ; uptime     echo -e "\n${BRed}Memory stats :$NC " ; free     echo -e "\n${BRed}Diskspace :$NC " ; mydf / $HOME     echo -e "\n${BRed}Local IP Address :$NC" ; my_ip     echo -e "\n${BRed}Open connections :$NC "; netstat -pan --inet;     echo }  #------------------------------------------------------------- # Misc utilities: #-------------------------------------------------------------  function repeat()       # Repeat n times command. {     local i max     max=$1; shift;     for ((i=1; i <= max ; i++)); do  # --> C-like syntax         eval "$@";     done }   function ask()          # See 'killps' for example of use. {     echo -n "$@" '[y/n] ' ; read ans     case "$ans" in         y*|Y*) return 0 ;;         *) return 1 ;;     esac }  function corename()   # Get name of app that created a corefile. {     for file ; do         echo -n $file : ; gdb --core=$file --batch | head -1     done }    #========================================================================= # #  PROGRAMMABLE COMPLETION SECTION #  Most are taken from the bash 2.05 documentation and from Ian McDonald's # 'Bash completion' package (http://www.caliban.org/bash/#completion) #  You will in fact need bash more recent then 3.0 for some features. # #  Note that most linux distributions now provide many completions # 'out of the box' - however, you might need to make your own one day, #  so I kept those here as examples. #=========================================================================  if [ "${BASH_VERSION%.*}" \< "3.0" ]; then     echo "You will need to upgrade to version 3.0 for full \           programmable completion features"     return fi  shopt -s extglob        # Necessary.  complete -A hostname   rsh rcp telnet rlogin ftp ping disk complete -A export     printenv complete -A variable   export local readonly unset complete -A enabled    builtin complete -A alias      alias unalias complete -A function   function complete -A user       su mail finger  complete -A helptopic  help     # Currently same as builtins. complete -A shopt      shopt complete -A stopped -P '%' bg complete -A job -P '%'     fg jobs disown  complete -A directory  mkdir rmdir complete -A directory   -o default cd  # Compression complete -f -o default -X '*.+(zip|ZIP)'  zip complete -f -o default -X '!*.+(zip|ZIP)' unzip complete -f -o default -X '*.+(z|Z)'      compress complete -f -o default -X '!*.+(z|Z)'     uncompress complete -f -o default -X '*.+(gz|GZ)'    gzip complete -f -o default -X '!*.+(gz|GZ)'   gunzip complete -f -o default -X '*.+(bz2|BZ2)'  bzip2 complete -f -o default -X '!*.+(bz2|BZ2)' bunzip2 complete -f -o default -X '!*.+(zip|ZIP|z|Z|gz|GZ|bz2|BZ2)' extract   # Documents - Postscript,pdf,dvi..... complete -f -o default -X '!*.+(ps|PS)'  gs ghostview ps2pdf ps2ascii complete -f -o default -X \ '!*.+(dvi|DVI)' dvips dvipdf xdvi dviselect dvitype complete -f -o default -X '!*.+(pdf|PDF)' acroread pdf2ps complete -f -o default -X '!*.@(@(?(e)ps|?(E)PS|pdf|PDF)?\ (.gz|.GZ|.bz2|.BZ2|.Z))' gv ggv complete -f -o default -X '!*.texi*' makeinfo texi2dvi texi2html texi2pdf complete -f -o default -X '!*.tex' tex latex slitex complete -f -o default -X '!*.lyx' lyx complete -f -o default -X '!*.+(htm*|HTM*)' lynx html2ps complete -f -o default -X \ '!*.+(doc|DOC|xls|XLS|ppt|PPT|sx?|SX?|csv|CSV|od?|OD?|ott|OTT)' soffice  # Multimedia complete -f -o default -X \ '!*.+(gif|GIF|jp*g|JP*G|bmp|BMP|xpm|XPM|png|PNG)' xv gimp ee gqview complete -f -o default -X '!*.+(mp3|MP3)' mpg123 mpg321 complete -f -o default -X '!*.+(ogg|OGG)' ogg123 complete -f -o default -X \ '!*.@(mp[23]|MP[23]|ogg|OGG|wav|WAV|pls|\ m3u|xm|mod|s[3t]m|it|mtm|ult|flac)' xmms complete -f -o default -X '!*.@(mp?(e)g|MP?(E)G|wma|avi|AVI|\ asf|vob|VOB|bin|dat|vcd|ps|pes|fli|viv|rm|ram|yuv|mov|MOV|qt|\ QT|wmv|mp3|MP3|ogg|OGG|ogm|OGM|mp4|MP4|wav|WAV|asx|ASX)' xine    complete -f -o default -X '!*.pl'  perl perl5   #  This is a 'universal' completion function - it works when commands have #+ a so-called 'long options' mode , ie: 'ls --all' instead of 'ls -a' #  Needs the '-o' option of grep #+ (try the commented-out version if not available).  #  First, remove '=' from completion word separators #+ (this will allow completions like 'ls --color=auto' to work correctly).  COMP_WORDBREAKS=${COMP_WORDBREAKS/=/}   _get_longopts() {   #$1 --help | sed  -e '/--/!d' -e 's/.*--\([^[:space:].,]*\).*/--\1/'| \   #grep ^"$2" |sort -u ;     $1 --help | grep -o -e "--[^[:space:].,]*" | grep -e "$2" |sort -u }  _longopts() {     local cur     cur=${COMP_WORDS[COMP_CWORD]}      case "${cur:-*}" in        -*)      ;;         *)      return ;;     esac      case "$1" in        \~*)     eval cmd="$1" ;;          *)     cmd="$1" ;;     esac     COMPREPLY=( $(_get_longopts ${1} ${cur} ) ) } complete  -o default -F _longopts configure bash complete  -o default -F _longopts wget id info a2ps ls recode  _tar() {     local cur ext regex tar untar      COMPREPLY=()     cur=${COMP_WORDS[COMP_CWORD]}      # If we want an option, return the possible long options.     case "$cur" in         -*)     COMPREPLY=( $(_get_longopts $1 $cur ) ); return 0;;     esac      if [ $COMP_CWORD -eq 1 ]; then         COMPREPLY=( $( compgen -W 'c t x u r d A' -- $cur ) )         return 0     fi      case "${COMP_WORDS[1]}" in         ?(-)c*f)             COMPREPLY=( $( compgen -f $cur ) )             return 0             ;;         +([^Izjy])f)             ext='tar'             regex=$ext             ;;         *z*f)             ext='tar.gz'             regex='t\(ar\.\)\(gz\|Z\)'             ;;         *[Ijy]*f)             ext='t?(ar.)bz?(2)'             regex='t\(ar\.\)bz2\?'             ;;         *)             COMPREPLY=( $( compgen -f $cur ) )             return 0             ;;      esac      if [[ "$COMP_LINE" == tar*.$ext' '* ]]; then         # Complete on files in tar file.         #         # Get name of tar file from command line.         tar=$( echo "$COMP_LINE" | \                         sed -e 's|^.* \([^ ]*'$regex'\) .*$|\1|' )         # Devise how to untar and list it.         untar=t${COMP_WORDS[1]//[^Izjyf]/}          COMPREPLY=( $( compgen -W "$( echo $( tar $untar $tar \                                 2>/dev/null ) )" -- "$cur" ) )         return 0      else         # File completion on relevant files.         COMPREPLY=( $( compgen -G $cur\*.$ext ) )      fi      return 0  }  complete -F _tar -o default tar  _make() {     local mdef makef makef_dir="." makef_inc gcmd cur prev i;     COMPREPLY=();     cur=${COMP_WORDS[COMP_CWORD]};     prev=${COMP_WORDS[COMP_CWORD-1]};     case "$prev" in         -*f)             COMPREPLY=($(compgen -f $cur ));             return 0             ;;     esac;     case "$cur" in         -*)             COMPREPLY=($(_get_longopts $1 $cur ));             return 0             ;;     esac;      # ... make reads     #          GNUmakefile,     #     then makefile     #     then Makefile ...     if [ -f ${makef_dir}/GNUmakefile ]; then         makef=${makef_dir}/GNUmakefile     elif [ -f ${makef_dir}/makefile ]; then         makef=${makef_dir}/makefile     elif [ -f ${makef_dir}/Makefile ]; then         makef=${makef_dir}/Makefile     else        makef=${makef_dir}/*.mk         # Local convention.     fi       #  Before we scan for targets, see if a Makefile name was     #+ specified with -f.     for (( i=0; i < ${#COMP_WORDS[@]}; i++ )); do         if [[ ${COMP_WORDS[i]} == -f ]]; then             # eval for tilde expansion             eval makef=${COMP_WORDS[i+1]}             break         fi     done     [ ! -f $makef ] && return 0      # Deal with included Makefiles.     makef_inc=$( grep -E '^-?include' $makef |                  sed -e "s,^.* ,"$makef_dir"/," )     for file in $makef_inc; do         [ -f $file ] && makef="$makef $file"     done       #  If we have a partial word to complete, restrict completions     #+ to matches of that word.     if [ -n "$cur" ]; then gcmd='grep "^$cur"' ; else gcmd=cat ; fi      COMPREPLY=( $( awk -F':' '/^[a-zA-Z0-9][^$#\/\t=]*:([^=]|$)/ \                                {split($1,A,/ /);for(i in A)print A[i]}' \                                 $makef 2>/dev/null | eval $gcmd  ))  }  complete -F _make -X '+($*|*.[cho])' make gmake pmake     _killall() {     local cur prev     COMPREPLY=()     cur=${COMP_WORDS[COMP_CWORD]}      #  Get a list of processes     #+ (the first sed evaluation     #+ takes care of swapped out processes, the second     #+ takes care of getting the basename of the process).     COMPREPLY=( $( ps -u $USER -o comm  | \         sed -e '1,1d' -e 's#[]\[]##g' -e 's#^.*/##'| \         awk '{if ($0 ~ /^'$cur'/) print $0}' ))      return 0 }  complete -F _killall killall killps    # Local Variables: # mode:shell-script # sh-shell:bash # End:

And, here is a snippet from Andrzej Szelachowski's instructive .bash_profile file.

Example M-2. .bash_profile file

# From Andrzej Szelachowski's ~/.bash_profile:   #  Note that a variable may require special treatment #+ if it will be exported.  DARKGRAY='\e[1;30m' LIGHTRED='\e[1;31m' GREEN='\e[32m' YELLOW='\e[1;33m' LIGHTBLUE='\e[1;34m' NC='\e[m'  PCT="\`if [[ \$EUID -eq 0 ]]; then T='$LIGHTRED' ; else T='$LIGHTBLUE'; fi;  echo \$T \`"  #  For "literal" command substitution to be assigned to a variable, #+ use escapes and double quotes: #+       PCT="\` ... \`" . . . #  Otherwise, the value of PCT variable is assigned only once, #+ when the variable is exported/read from .bash_profile, #+ and it will not change afterwards even if the user ID changes.   PS1="\n$GREEN[\w] \n$DARKGRAY($PCT\t$DARKGRAY)-($PCT\u$DARKGRAY)-($PCT\! $DARKGRAY)$YELLOW-> $NC"  #  Escape a variables whose value changes: #        if [[ \$EUID -eq 0 ]], #  Otherwise the value of the EUID variable will be assigned only once, #+ as above.  #  When a variable is assigned, it should be called escaped: #+       echo \$T, #  Otherwise the value of the T variable is taken from the moment the PCT  #+ variable is exported/read from .bash_profile. #  So, in this example it would be null.  #  When a variable's value contains a semicolon it should be strong quoted: #        T='$LIGHTRED', #  Otherwise, the semicolon will be interpreted as a command separator.   #  Variables PCT and PS1 can be merged into a new PS1 variable:  PS1="\`if [[ \$EUID -eq 0 ]]; then PCT='$LIGHTRED'; else PCT='$LIGHTBLUE'; fi;  echo '\n$GREEN[\w] \n$DARKGRAY('\$PCT'\t$DARKGRAY)-\ ('\$PCT'\u$DARKGRAY)-('\$PCT'\!$DARKGRAY)$YELLOW-> $NC'\`"  # The trick is to use strong quoting for parts of old PS1 variable.

Appendix N. Converting DOS Batch Files to Shell Scripts

Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file language allowed writing some fairly powerful scripts and applications, though they often required extensive kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent shell scripting ones.

Table N-1. Batch file keywords / variables / operators, and their shell equivalents

Batch File Operator	Shell Script Equivalent	Meaning
`%`	$	command-line parameter prefix
`/`	-	command option flag
`\`	/	directory path separator
`==`	=	(equal-to) string comparison test
`!==!`	!=	(not equal-to) string comparison test
`\|`	\|	pipe
`@`	set `+v`	do not echo current command
`*`	*	filename "wild card"
`>`	>	file redirection (overwrite)
`>>`	>>	file redirection (append)
`<`	<	redirect `stdin`
`%VAR%`	$VAR	environmental variable
`REM`	#	comment
`NOT`	!	negate following test
`NUL`	`/dev/null`	"black hole" for burying command output
`ECHO`	echo	echo (many more option in Bash)
`ECHO.`	echo	echo blank line
`ECHO OFF`	set `+v`	do not echo command(s) following
`FOR %%VAR IN (LIST) DO`	for var in [list]; do	"for" loop
`:LABEL`	none (unnecessary)	label
`GOTO`	none (use a function)	jump to another location in the script
`PAUSE`	sleep	pause or wait an interval
`CHOICE`	case or select	menu choice
`IF`	if	if-test
`IF EXIST FILENAME`	if [ -e filename ]	test if file exists
`IF !%N==!`	if [ -z "$N" ]	if replaceable parameter "N" not present
`CALL`	source or . (dot operator)	"include" another script
`COMMAND /C`	source or . (dot operator)	"include" another script (same as CALL)
`SET`	export	set an environmental variable
`SHIFT`	shift	left shift command-line argument list
`SGN`	-lt or -gt	sign (of integer)
`ERRORLEVEL`	$?	exit status
`CON`	`stdin`	"console" (`stdin`)
`PRN`	`/dev/lp0`	(generic) printer device
`LPT1`	`/dev/lp0`	first printer device
`COM1`	`/dev/ttyS0`	first serial port

Batch files usually contain DOS commands. These must be translated into their UNIX equivalents in order to convert a batch file into a shell script.

Table N-2. DOS commands and their UNIX equivalents

DOS Command	UNIX Equivalent	Effect
`ASSIGN`	ln	link file or directory
`ATTRIB`	chmod	change file permissions
`CD`	cd	change directory
`CHDIR`	cd	change directory
`CLS`	clear	clear screen
`COMP`	diff, comm, cmp	file compare
`COPY`	cp	file copy
`Ctl-C`	Ctl-C	break (signal)
`Ctl-Z`	Ctl-D	EOF (end-of-file)
`DEL`	rm	delete file(s)
`DELTREE`	rm -rf	delete directory recursively
`DIR`	ls -l	directory listing
`ERASE`	rm	delete file(s)
`EXIT`	exit	exit current process
`FC`	comm, cmp	file compare
`FIND`	grep	find strings in files
`MD`	mkdir	make directory
`MKDIR`	mkdir	make directory
`MORE`	more	text file paging filter
`MOVE`	mv	move
`PATH`	$PATH	path to executables
`REN`	mv	rename (move)
`RENAME`	mv	rename (move)
`RD`	rmdir	remove directory
`RMDIR`	rmdir	remove directory
`SORT`	sort	sort file
`TIME`	date	display system time
`TYPE`	cat	output file to `stdout`
`XCOPY`	cp	(extended) file copy

Virtually all UNIX and shell operators and commands have many more options and enhancements than their DOS and batch file counterparts. Many DOS batch files rely on auxiliary utilities, such as ask.com, a crippled counterpart to read.

DOS supports only a very limited and incompatible subset of filename wild-card expansion, recognizing just the * and ? characters.

Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better than the original.

Example N-1. VIEWDATA.BAT: DOS Batch File

REM VIEWDATA  REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS" REM                           BY PAUL SOMERSON   @ECHO OFF  IF !%1==! GOTO VIEWDATA REM  IF NO COMMAND-LINE ARG... FIND "%1" C:\BOZO\BOOKLIST.TXT GOTO EXIT0 REM  PRINT LINE WITH STRING MATCH, THEN EXIT.  :VIEWDATA TYPE C:\BOZO\BOOKLIST.TXT | MORE REM  SHOW ENTIRE FILE, 1 PAGE AT A TIME.  :EXIT0

The script conversion is somewhat of an improvement. [152]

Example N-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT

#!/bin/bash # viewdata.sh # Conversion of VIEWDATA.BAT to shell script.  DATAFILE=/home/bozo/datafiles/book-collection.data ARGNO=1  # @ECHO OFF                 Command unnecessary here.  if [ $# -lt "$ARGNO" ]    # IF !%1==! GOTO VIEWDATA then   less $DATAFILE          # TYPE C:\MYDIR\BOOKLIST.TXT | MORE else   grep "$1" $DATAFILE     # FIND "%1" C:\MYDIR\BOOKLIST.TXT fi    exit 0                    # :EXIT0  #  GOTOs, labels, smoke-and-mirrors, and flimflam unnecessary. #  The converted script is short, sweet, and clean, #+ which is more than can be said for the original.

Ted Davis' Shell Scripts on the PC site had a set of comprehensive tutorials on the old-fashioned art of batch file programming. Unfortunately the page has vanished without a trace.

Appendix O. Exercises

The exercises that follow test and extend your knowledge of scripting. Think of them as a challenge, as an entertaining way to take you further along the stony path toward UNIX wizardry.

On a dingy side street in a run-down section of Hoboken, New Jersey,
there sits a nondescript squat two-story brick building with an inscription
incised on a marble plate in its wall:

Bash Scripting Hall of Fame

Inside, among various dusty uninteresting exhibits is a corroding,
cobweb-festooned brass plaque inscribed with a short, very short
list of those few persons who have successfully mastered the material
in theAdvanced Bash Scripting Guide, as evidenced by their performance
on the following Exercise sections.

(Alas, the author of theABS Guide is not represented among the exhibits.
This is possibly due to malicious rumors about lack of credentials and
deficient scripting skills.)

O.1. Analyzing Scripts

Examine the following script. Run it, then explain what it does. Annotate the script and rewrite it in a more compact and elegant manner.

#!/bin/bash  MAX=10000     for((nr=1; nr<$MAX; nr++))   do      let "t1 = nr % 5"     if [ "$t1" -ne 3 ]     then       continue     fi      let "t2 = nr % 7"     if [ "$t2" -ne 4 ]     then       continue     fi      let "t3 = nr % 9"     if [ "$t3" -ne 5 ]     then       continue     fi    break   # What happens when you comment out this line? Why?    done    echo "Number = $nr"   exit 0

---

Explain what the following script does. It is really just a parameterized command-line pipe.

#!/bin/bash  DIRNAME=/usr/bin FILETYPE="shell script" LOGFILE=logfile  file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l  exit 0

---

Examine and explain the following script. For hints, you might refer to the listings for find and stat.

#!/bin/bash  # Author:  Nathan Coulter # This code is released to the public domain. # The author gave permission to use this code snippet in the ABS Guide.  find -maxdepth 1 -type f -printf '%f\000'  | {    while read -d $'\000'; do       mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'       )-$REPLY"    done }  # Warning: Test-drive this script in a "scratch" directory. # It will somehow affect all the files there.

---

A reader sent in the following code snippet.

while read LINE do   echo $LINE done < `tail -f /var/log/messages`

He wished to write a script tracking changes to the system log file, /var/log/messages. Unfortunately, the above code block hangs and does nothing useful. Why? Fix this so it does work. (Hint: rather than redirecting the stdin of the loop, try a pipe.)

---

Analyze the following "one-liner" (here split into two lines for clarity) contributed by Rory Winston:

export SUM=0; for f in $(find src -name "*.java"); do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM

Hint: First, break the script up into bite-sized sections. Then, carefully examine its use of double-parentheses arithmetic, the export command, the find command, the wc command, and awk.

---

Analyze Example A-10, and reorganize it in a simplified and more logical style. See how many of the variables can be eliminated, and try to optimize the script to speed up its execution time.

Alter the script so that it accepts any ordinary ASCII text file as input for its initial "generation". The script will read the first $ROW*$COL characters, and set the occurrences of vowels as "living" cells. Hint: be sure to translate the spaces in the input file to underscore characters.

O.2. Writing Scripts

Write a script to carry out each of the following tasks.

EASY

Self-reproducing Script

Write a script that backs itself up, that is, copies itself to a file named backup.sh.

Hint: Use the cat command and the appropriate positional parameter.

Home Directory Listing

Perform a recursive directory listing on the user's home directory and save the information to a file. Compress the file, have the script prompt the user to insert a USB flash drive, then press ENTER. Finally, save the file to the flash drive after making certain the flash drive has properly mounted by parsing the output of df. Note that the flash drive must be unmounted before it is removed.

Converting for loops to while and until loops

Convert the for loops in Example 11-1 to while loops. Hint: store the data in an array and step through the array elements.

Having already done the "heavy lifting," now convert the loops in the example to until loops.

Changing the line spacing of a text file

Write a script that reads each line of a target file, then writes the line back to stdout, but with an extra blank line following. This has the effect of double-spacing the file.

Include all necessary code to check whether the script gets the necessary command-line argument (a filename), and whether the specified file exists.

When the script runs correctly, modify it to triple-space the target file.

Finally, write a script to remove all blank lines from the target file, single-spacing it.

Backwards Listing

Write a script that echoes itself to stdout, but backwards.

Automatically Decompressing Files

Given a list of filenames as input, this script queries each target file (parsing the output of the file command) for the type of compression used on it. Then the script automatically invokes the appropriate decompression command (gunzip, bunzip2, unzip, uncompress, or whatever). If a target file is not compressed, the script emits a warning message, but takes no other action on that particular file.

Unique System ID

Generate a "unique" 6-digit hexadecimal identifier for your computer. Do not use the flawed hostid command. Hint: md5sum /etc/passwd , then select the first 6 digits of output.

Backup

Archive as a "tarball" (*.tar.gz file) all the files in your home directory tree (/home/your-name) that have been modified in the last 24 hours. Hint: use find.

Optional: you may use this as the basis of a backup script.

Checking whether a process is still running

Given a process ID (PID) as an argument, this script will check, at user-specified intervals, whether the given process is still running. You may use the ps and sleep commands.

Primes

Print (to stdout) all prime numbers between 60000 and 63000. The output should be nicely formatted in columns (hint: use printf).

Lottery Numbers

One type of lottery involves picking five different numbers, in the range of 1 - 50. Write a script that generates five pseudorandom numbers in this range, with no duplicates. The script will give the option of echoing the numbers to stdout or saving them to a file, along with the date and time the particular number set was generated. (If your script consistently generates winning lottery numbers, then you can retire on the proceeds and leave shell scripting to those of us who have to work for a living.)

INTERMEDIATE

Integer or String

Write a script function that determines if an argument passed to it is an integer or a string. The function will return TRUE (0) if passed an integer, and FALSE (1) if passed a string.

Hint: What does the following expression return when $1 is not an integer?

expr $1 + 0

ASCII to Integer

The atoi function in C converts a string character to an integer. Write a shell script function that performs the same operation. Likewise, write a shell script function that does the inverse, mirroring the C itoa function which converts an integer into an ASCII character.

Managing Disk Space

List, one at a time, all files larger than 100K in the /home/username directory tree. Give the user the option to delete or compress the file, then proceed to show the next one. Write to a logfile the names of all deleted files and the deletion times.

Banner

Simulate the functionality of the deprecated banner command in a script.

Removing Inactive Accounts

Inactive accounts on a network server waste disk space and may become a security risk. Write an administrative script (to be invoked by root or the cron daemon) that checks for and deletes user accounts that have not been accessed within the last 90 days.

Enforcing Disk Quotas

Write a script for a multi-user system that checks users' disk usage. If a user surpasses a preset limit (500 MB, for example) in her /home/username directory, then the script automatically sends her a "pigout" warning e-mail.

The script will use the du and mail commands. As an option, it will allow setting and enforcing quotas using the quota and setquota commands.

Logged in User Information

For all logged in users, show their real names and the time and date of their last login.

Hint: use who, lastlog, and parse /etc/passwd.

Safe Delete

Implement, as a script, a "safe" delete command, sdel.sh. Filenames passed as command-line arguments to this script are not deleted, but instead gzipped if not already compressed (use file to check), then moved to a ~/TRASH directory. Upon invocation, the script checks the ~/TRASH directory for files older than 48 hours and permanently deletes them. (An better alternative might be to have a second script handle this, periodically invoked by the cron daemon.)

Extra credit: Write the script so it can handle files and directories recursively. This would give it the capability of "safely deleting" entire directory structures.

Making Change

What is the most efficient way to make change for $1.68, using only coins in common circulations (up to 25c)? It's 6 quarters, 1 dime, a nickel, and three cents.

Given any arbitrary command-line input in dollars and cents ($*.??), calculate the change, using the minimum number of coins. If your home country is not the United States, you may use your local currency units instead. The script will need to parse the command-line input, then change it to multiples of the smallest monetary unit (cents or whatever). Hint: look at Example 24-8.

Quadratic Equations

Solve a quadratic equation of the form Ax^2 + Bx + C = 0 . Have a script take as arguments the coefficients, A , B , and C , and return the solutions to five decimal places.

Hint: pipe the coefficients to bc, using the well-known formula, x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A .

Table of Logarithms

Using the bc and printf commands, print out a nicely-formatted table of eight-place natural logarithms in the interval between 0.00 and 100.00, in steps of .01.

Hint: bc requires the -l option to load the math library.

Unicode Table

Using Example T-1 as a template, write a script that prints to a file a complete Unicode table.

Hint: Use the -e option to echo: echo -e '\uXXXX', where XXXX is the Unicode numerical character designation. This requires version 4.2 or later of Bash.

Sum of Matching Numbers

Find the sum of all five-digit numbers (in the range 10000 - 99999) containing exactly two out of the following set of digits: { 4, 5, 6 }. These may repeat within the same number, and if so, they count once for each occurrence.

Some examples of matching numbers are 42057, 74638, and 89515.

Lucky Numbers

A lucky number is one whose individual digits add up to 7, in successive additions. For example, 62431 is a lucky number (6 + 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the lucky numbers between 1000 and 10000.

Craps

Borrowing the ASCII graphics from Example A-40, write a script that plays the well-known gambling game of craps. The script will accept bets from one or more players, roll the dice, and keep track of wins and losses, as well as of each player's bankroll.

Tic-tac-toe

Write a script that plays the child's game of tic-tac-toe against a human player. The script will let the human choose whether to take the first move. The script will follow an optimal strategy, and therefore never lose. To simplify matters, you may use ASCII graphics:

                          o | x |    ----------      | x |    ----------      | o |          Your move, human (row, column)?

Alphabetizing a String

Alphabetize (in ASCII order) an arbitrary string read from the command-line.

Parsing

Parse /etc/passwd, and output its contents in nice, easy-to-read tabular form.

Logging Logins

Parse /var/log/messages to produce a nicely formatted file of user logins and login times. The script may need to run as root. (Hint: Search for the string "LOGIN.")

Pretty-Printing a Data File

Certain database and spreadsheet packages use save-files with the fields separated by commas, commonly referred to as comma-separated values or CSVs. Other applications often need to parse these files.

Given a data file with comma-separated fields, of the form:

Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989 Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612 ...

Reformat the data and print it out to stdout in labeled, evenly-spaced columns.

Justification

Given ASCII text input either from stdin or a file, adjust the word spacing to right-justify each line to a user-specified line-width, then send the output to stdout.

Mailing List

Using the mail command, write a script that manages a simple mailing list. The script automatically e-mails the monthly company newsletter, read from a specified text file, and sends it to all the addresses on the mailing list, which the script reads from another specified file.

Generating Passwords

Generate pseudorandom 8-character passwords, using characters in the ranges [0-9], [A-Z], [a-z]. Each password must contain at least two digits.

Monitoring a User

You suspect that one particular user on the network has been abusing her privileges and possibly attempting to hack the system. Write a script to automatically monitor and log her activities when she's signed on. The log file will save entries for the previous week, and delete those entries more than seven days old.

You may use last, lastlog, and lastcomm to aid your surveillance of the suspected fiend.

Checking for Broken Links

Using lynx with the -traversal option, write a script that checks a Web site for broken links.

DIFFICULT

Testing Passwords

Write a script to check and validate passwords. The object is to flag "weak" or easily guessed password candidates.

A trial password will be input to the script as a command-line parameter. To be considered acceptable, a password must meet the following minimum qualifications:

Minimum length of 8 characters
Must contain at least one numeric character
Must contain at least one of the following non-alphabetic characters: @, #, $, %, &, *, +, -, =

Optional:

Do a dictionary check on every sequence of at least four consecutive alphabetic characters in the password under test. This will eliminate passwords containing embedded "words" found in a standard dictionary.
Enable the script to check all the passwords on your system. These do not reside in /etc/passwd.

This exercise tests mastery of Regular Expressions.

Cross Reference

Write a script that generates a cross-reference (concordance) on a target file. The output will be a listing of all word occurrences in the target file, along with the line numbers in which each word occurs. Traditionally, linked list constructs would be used in such applications. Therefore, you should investigate arrays in the course of this exercise. Example 16-12 is probably not a good place to start.

Square Root

Write a script to calculate square roots of numbers using Newton's Method.

The algorithm for this, expressed as a snippet of Bash pseudo-code is:

#  (Isaac) Newton's Method for speedy extraction #+ of square roots.  guess = $argument #  $argument is the number to find the square root of. #  $guess is each successive calculated "guess" -- or trial solution -- #+ of the square root. #  Our first "guess" at a square root is the argument itself.  oldguess = 0 # $oldguess is the previous $guess.  tolerance = .000001 # To how close a tolerance we wish to calculate.  loopcnt = 0 # Let's keep track of how many times through the loop. # Some arguments will require more loop iterations than others.   while [ ABS( $guess $oldguess ) -gt $tolerance ] #       ^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.  #      "ABS" is a (floating point) function to find the absolute value #+      of the difference between the two terms. #             So, as long as difference between current and previous #+            trial solution (guess) exceeds the tolerance, keep looping.  do    oldguess = $guess  # Update $oldguess to previous $guess.  #  =======================================================    guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0 #        = 1/2 ( ($oldguess **2 + $argument) / $oldguess ) #  equivalent to: #        = 1/2 ( $oldguess + $argument / $oldguess ) #  that is, "averaging out" the trial solution and #+ the proportion of argument deviation #+ (in effect, splitting the error in half). #  This converges on an accurate solution #+ with surprisingly few loop iterations . . . #+ for arguments > $tolerance, of course. #  =======================================================     (( loopcnt++ ))     # Update loop counter. done

It's a simple enough recipe, and seems at first glance easy enough to convert into a working Bash script. The problem, though, is that Bash has no native support for floating point numbers. So, the script writer needs to use bc or possibly awk to convert the numbers and do the calculations. It could get rather messy . . .

Logging File Accesses

Log all accesses to the files in /etc during the course of a single day. This information should include the filename, user name, and access time. If any alterations to the files take place, that will be flagged. Write this data as tabular (tab-separated) formatted records in a logfile.

Monitoring Processes

Write a script to continually monitor all running processes and to keep track of how many child processes each parent spawns. If a process spawns more than five children, then the script sends an e-mail to the system administrator (or root) with all relevant information, including the time, PID of the parent, PIDs of the children, etc. The script appends a report to a log file every ten minutes.

Strip Comments

Strip all comments from a shell script whose name is specified on the command-line. Note that the initial #! line must not be stripped out.

Strip HTML Tags

Strip all the HTML tags from a specified HTML file, then reformat it into lines between 60 and 75 characters in length. Reset paragraph and block spacing, as appropriate, and convert HTML tables to their approximate text equivalent.

XML Conversion

Convert an XML file to both HTML and text format.

Optional: A script that converts Docbook/SGML to XML.

Chasing Spammers

Write a script that analyzes a spam e-mail by doing DNS lookups on the IP addresses in the headers to identify the relay hosts as well as the originating ISP. The script will forward the unaltered spam message to the responsible ISPs. Of course, it will be necessary to filter out your own ISP's IP address, so you don't end up complaining about yourself.

As necessary, use the appropriate network analysis commands.

For some ideas, see Example 16-41 and Example A-28.

Optional: Write a script that searches through a list of e-mail messages and deletes the spam according to specified filters.

Creating man pages

Write a script that automates the process of creating man pages.

Given a text file which contains information to be formatted into a man page, the script will read the file, then invoke the appropriate groff commands to output the corresponding man page to stdout. The text file contains blocks of information under the standard man page headings, i.e., NAME, SYNOPSIS, DESCRIPTION, etc.

Example A-39 is an instructive first step.

Hex Dump

Do a hex(adecimal) dump on a binary file specified as an argument to the script. The output should be in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4-byte hex number, and the final field the ASCII equivalent of the previous 8 fields.

The obvious followup to this is to extend the hex dump script into a disassembler. Using a lookup table, or some other clever gimmick, convert the hex values into 80x86 op codes.

Emulating a Shift Register

Using Example 27-15 as an inspiration, write a script that emulates a 64-bit shift register as an array. Implement functions to load the register, shift left, shift right, and rotate it. Finally, write a function that interprets the register contents as eight 8-bit ASCII characters.

Calculating Determinants

Write a script that calculates determinants [153] by recursively expanding the minors. Use a 4 x 4 determinant as a test case.

Hidden Words

Write a "word-find" puzzle generator, a script that hides 10 input words in a 10 x 10 array of random letters. The words may be hidden across, down, or diagonally.

Optional: Write a script that solves word-find puzzles. To keep this from becoming too difficult, the solution script will find only horizontal and vertical words. (Hint: Treat each row and column as a string, and search for substrings.)

Anagramming

Anagram 4-letter input. For example, the anagrams of word are: do or rod row word. You may use /usr/share/dict/linux.words as the reference list.

Word Ladders

A "word ladder" is a sequence of words, with each successive word in the sequence differing from the previous one by a single letter.

For example, to "ladder" from mark to vase:

mark --> park --> part --> past --> vast --> vase          ^           ^       ^      ^           ^

Write a script that solves word ladder puzzles. Given a starting and an ending word, the script will list all intermediate steps in the "ladder." Note that all words in the sequence must be legitimate dictionary words.

Fog Index

The "fog index" of a passage of text estimates its reading difficulty, as a number corresponding roughly to a school grade level. For example, a passage with a fog index of 12 should be comprehensible to anyone with 12 years of schooling.

The Gunning version of the fog index uses the following algorithm.

Choose a section of the text at least 100 words in length.
Count the number of sentences (a portion of a sentence truncated by the boundary of the text section counts as one).
Find the average number of words per sentence.

AVE_WDS_SEN = TOTAL_WORDS / SENTENCES
Count the number of "difficult" words in the segment -- those containing at least 3 syllables. Divide this quantity by total words to get the proportion of difficult words.

PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS
The Gunning fog index is the sum of the above two quantities, multiplied by 0.4, then rounded to the nearest integer.

G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )

Step 4 is by far the most difficult portion of the exercise. There exist various algorithms for estimating the syllable count of a word. A rule-of-thumb formula might consider the number of letters in a word and the vowel-consonant mix.

A strict interpretation of the Gunning fog index does not count compound words and proper nouns as "difficult" words, but this would enormously complicate the script.

Calculating PI using Buffon's Needle

The Eighteenth Century French mathematician de Buffon came up with a novel experiment. Repeatedly drop a needle of length n onto a wooden floor composed of long and narrow parallel boards. The cracks separating the equal-width floorboards are a fixed distance d apart. Keep track of the total drops and the number of times the needle intersects a crack on the floor. The ratio of these two quantities turns out to be a fractional multiple of PI.

In the spirit of Example 16-50, write a script that runs a Monte Carlo simulation of Buffon's Needle. To simplify matters, set the needle length equal to the distance between the cracks, n = d .

Hint: there are actually two critical variables: the distance from the center of the needle to the nearest crack, and the inclination angle of the needle to that crack. You may use bc to handle the calculations.

Playfair Cipher

Implement the Playfair (Wheatstone) Cipher in a script.

The Playfair Cipher encrypts text by substitution of digrams (2-letter groupings). It is traditional to use a 5 x 5 letter scrambled-alphabet key square for the encryption and decryption.

                          C O D E S    A B F G H    I K L M N    P Q R T U    V W X Y Z  Each letter of the alphabet appears once, except "I" also represents "J". The arbitrarily chosen key word, "CODES" comes first, then all the rest of the alphabet, in order from left to right, skipping letters already used.  To encrypt, separate the plaintext message into digrams (2-letter groups). If a group has two identical letters, delete the second, and form a new group. If there is a single letter left over at the end, insert a "null" character, typically an "X."  THIS IS A TOP SECRET MESSAGE  TH IS IS AT OP SE CR ET ME SA GE    For each digram, there are three possibilities. -----------------------------------------------  1) Both letters will be on the same row of the key square:    For each letter, substitute the one immediately to the right, in that    row. If necessary, wrap around left to the beginning of the row.  or  2) Both letters will be in the same column of the key square:    For each letter, substitute the one immediately below it, in that    row. If necessary, wrap around to the top of the column.  or  3) Both letters will form the corners of a rectangle within the key square:    For each letter, substitute the one on the other corner the rectangle    which lies on the same row.   The "TH" digram falls under case #3. G H M N T U           (Rectangle with "T" and "H" at corners)  T --> U H --> G   The "SE" digram falls under case #1. C O D E S     (Row containing "S" and "E")  S --> C  (wraps around left to beginning of row) E --> S  =========================================================================  To decrypt encrypted text, reverse the above procedure under cases #1 and #2 (move in opposite direction for substitution). Under case #3, just take the remaining two corners of the rectangle.   Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a fairly detailed description of the Playfair Cipher and its solution methods.

This script will have three main sections

Generating the key square, based on a user-input keyword.
Encrypting a plaintext message.
Decrypting encrypted text.

The script will make extensive use of arrays and functions. You may use Example A-56 as an inspiration.

Please do not send the author your solutions to these exercises. There are more appropriate ways to impress him with your cleverness, such as submitting bugfixes and suggestions for improving the book.

Appendix P. Revision History

                This document first appeared as a 60-page HOWTO in the late spring       of 2000. Since then, it has gone through quite a number of updates       and revisions. This book could not have been written without the       assistance of the Linux community, and especially of the volunteers       of the Linux Documentation Project.

Here is the e-mail to the LDP requesting permission to submit version 0.1.

From thegrendel@theriver.com Sat Jun 10 09:05:33 2000 -0700 Date: Sat, 10 Jun 2000 09:05:28 -0700 (MST) From: "M. Leo Cooper" <thegrendel@theriver.com> X-Sender: thegrendel@localhost To: ldp-discuss@lists.linuxdoc.org Subject: Permission to submit HOWTO Dear HOWTO Coordinator, I am working on and would like to submit to the LDP a HOWTO on the subject of "Bash Scripting" (shell scripting, using 'bash'). As it happens, I have been writing this document, off and on, for about the last eight months or so, and I could produce a first draft in ASCII text format in a matter of just a few more days. I began writing this out of frustration at being unable to find a decent book on shell scripting. I managed to locate some pretty good articles on various aspects of scripting, but nothing like a complete, beginning-to-end tutorial. Well, in keeping with my philosophy, if all else fails, do it yourself. As it stands, this proposed "Bash-Scripting HOWTO" would serve as a combination tutorial and reference, with the heavier emphasis on the tutorial. It assumes Linux experience, but only a very basic level of programming skills. Interspersed with the text are 79 illustrative example scripts of varying complexity, all liberally commented. There are even exercises for the reader. At this stage, I'm up to 18,000+ words (124k), and that's over 50 pages of text (whew!). I haven't mentioned that I've previously authored an LDP HOWTO, the "Software-Building HOWTO", which I wrote in Linuxdoc/SGML. I don't know if I could handle Docbook/SGML, and I'm glad you have volunteers to do the conversion. You people seem to have gotten on a more organized basis these last few months. Working with Greg Hankins and Tim Bynum was nice, but a professional team is even nicer. Anyhow, please advise. Mendel Cooper thegrendel@theriver.com

Table P-1. Revision History

Release	Date	Comments
0.1	14 Jun 2000	Initial release.
`0.2`	30 Oct 2000	Bugs fixed, plus much additional material and more example scripts.
`0.3`	12 Feb 2001	Major update.
`0.4`	08 Jul 2001	Complete revision and expansion of the book.
`0.5`	03 Sep 2001	Major update: Bugfixes, material added, sections reorganized.
`1.0`	14 Oct 2001	Stable release: Bugfixes, reorganization, material added.
`1.1`	06 Jan 2002	Bugfixes, material and scripts added.
`1.2`	31 Mar 2002	Bugfixes, material and scripts added.
`1.3`	02 Jun 2002	TANGERINE release: A few bugfixes, much more material and scripts added.
`1.4`	16 Jun 2002	MANGO release: A number of typos fixed, more material and scripts.
`1.5`	13 Jul 2002	PAPAYA release: A few bugfixes, much more material and scripts added.
`1.6`	29 Sep 2002	POMEGRANATE release: Bugfixes, more material, one more script.
`1.7`	05 Jan 2003	COCONUT release: A couple of bugfixes, more material, one more script.
`1.8`	10 May 2003	BREADFRUIT release: A number of bugfixes, more scripts and material.
`1.9`	21 Jun 2003	PERSIMMON release: Bugfixes, and more material.
`2.0`	24 Aug 2003	GOOSEBERRY release: Major update.
`2.1`	14 Sep 2003	HUCKLEBERRY release: Bugfixes, and more material.
`2.2`	31 Oct 2003	CRANBERRY release: Major update.
`2.3`	03 Jan 2004	STRAWBERRY release: Bugfixes and more material.
`2.4`	25 Jan 2004	MUSKMELON release: Bugfixes.
`2.5`	15 Feb 2004	STARFRUIT release: Bugfixes and more material.
`2.6`	15 Mar 2004	SALAL release: Minor update.
`2.7`	18 Apr 2004	MULBERRY release: Minor update.
`2.8`	11 Jul 2004	ELDERBERRY release: Minor update.
`3.0`	03 Oct 2004	LOGANBERRY release: Major update.
`3.1`	14 Nov 2004	BAYBERRY release: Bugfix update.
`3.2`	06 Feb 2005	BLUEBERRY release: Minor update.
`3.3`	20 Mar 2005	RASPBERRY release: Bugfixes, much material added.
`3.4`	08 May 2005	TEABERRY release: Bugfixes, stylistic revisions.
`3.5`	05 Jun 2005	BOXBERRY release: Bugfixes, some material added.
`3.6`	28 Aug 2005	POKEBERRY release: Bugfixes, some material added.
`3.7`	23 Oct 2005	WHORTLEBERRY release: Bugfixes, some material added.
`3.8`	26 Feb 2006	BLAEBERRY release: Bugfixes, some material added.
`3.9`	15 May 2006	SPICEBERRY release: Bugfixes, some material added.
`4.0`	18 Jun 2006	WINTERBERRY release: Major reorganization.
`4.1`	08 Oct 2006	WAXBERRY release: Minor update.
`4.2`	10 Dec 2006	SPARKLEBERRY release: Important update.
`4.3`	29 Apr 2007	INKBERRY release: Bugfixes, material added.
`5.0`	24 Jun 2007	SERVICEBERRY release: Major update.
`5.1`	10 Nov 2007	LINGONBERRY release: Minor update.
`5.2`	16 Mar 2008	SILVERBERRY release: Important update.
`5.3`	11 May 2008	GOLDENBERRY release: Minor update.
`5.4`	21 Jul 2008	ANGLEBERRY release: Major update.
`5.5`	23 Nov 2008	FARKLEBERRY release: Minor update.
`5.6`	26 Jan 2009	WORCESTERBERRY release: Minor update.
`6.0`	23 Mar 2009	THIMBLEBERRY release: Major update.
`6.1`	30 Sep 2009	BUFFALOBERRY release: Minor update.
`6.2`	17 Mar 2010	ROWANBERRY release: Minor update.
`6.3`	30 Apr 2011	SWOZZLEBERRY release: Major update.
`6.4`	30 Aug 2011	VORTEXBERRY release: Minor update.
`6.5`	05 Apr 2012	TUNGSTENBERRY release: Minor update.
`6.6`	27 Nov 2012	YTTERBIUMBERRY release: Minor update.
`10`	10 Mar 2014	YTTERBIUMBERRY release: License change.

Appendix Q. Download and Mirror Sites

The latest update of this document, as an archived, bzip2-ed "tarball" including both the SGML source and rendered HTML, may be downloaded from the author's home site). A pdf version is also available (mirror site). There is likewise an epub version, courtesy of Craig Barnes and Michael Satke. The change log gives a detailed revision history. The ABS Guide even has its own freshmeat.net/freecode page to keep track of major updates, user comments, and popularity ratings for the project.

The legacy hosting site for this document is the Linux Documentation Project, which maintains many other Guides and HOWTOs as well.

Many thanks to Ronny Bangsund for donating server space to host this project.

Appendix R. To Do List

A comprehensive survey of incompatibilities between Bash and the classic Bourne shell.
Same as above, but for the Korn shell (ksh).

Appendix S. Copyright

The Advanced Bash Scripting Guide is herewith granted to the PUBLIC DOMAIN. This has the following implications and consequences.

A.  All previous releases of the Advanced Bash Scripting Guide     are as well granted to the Public Domain.  A1. All printed editions, whether authorized by the author or not,     are as well granted to the Public Domain. This legally overrides     any stated intention or wishes of the publishers. Any statement     of copyright is void and invalid.     THERE ARE NO EXCEPTIONS TO THIS.  A2. Any release of the Advanced Bash Scripting Guide, whether in     electronic or print form is granted to the Public Domain by the     express directive of the author and previous copyright holder, Mendel     Cooper. No other person(s) or entities have ever held a valid copyright.  B.  As a Public Domain document, unlimited copying and distribution rights     are granted. There can be NO restrictions. If anyone has published or will     in the future publish an original or modified version of this document,     then only additional original material may be copyrighted. The core     work will remain in the Public Domain.

By law, distributors and publishers (including on-line publishers) are prohibited from imposing any conditions, strictures, or provisions on this document, any previous versions, or any derivative versions. The author asserts that he has not entered into any contractual obligations that would alter the foregoing declarations.

Essentially, you may freely distribute this book or any derivative thereof in electronic or printed form. If you have previously purchased or are in possession of a printed copy of a current or previous edition, you have the LEGAL RIGHT to copy and/or redistribute it, regardless of any copyright notice. Any copyright notice is void.

Additionally, the author wishes to state his intention that:

If you copy or distribute this book, kindly DO NOT use the materials within, or any portion thereof, in a patent or copyright lawsuit against the Open Source community, its developers, its distributors, or against any of its associated software or documentation including, but not limited to, the Linux kernel, Open Office, Samba, and Wine. Kindly DO NOT use any of the materials within this book in testimony or depositions as a plaintiff's "expert witness" in any lawsuit against the Open Source community, any of its developers, its distributors, or any of its associated software or documentation.

A Public Domain license essentially does not restrict ANY legitimate distribution or use of this book. The author especially encourages its (royalty-free!) use for classroom and instructional purposes.

To date, limited print rights (Lulu edition) have been granted to one individual and to no one else. Neither that individual nor Lulu holds or ever has held a valid copyright.

It has come to the attention of the author that unauthorized electronic and print editions of this book are being sold commercially on itunes®, amazon.com and elsewhere. These are illegal and pirated editions produced without the author's permission, and readers of this book are strongly urged not to purchase them. In fact, these pirated editions are now legal, but necessarily fall into the Public Domain, and any copyright notices contained within them are invalid and void.

The author produced this book in a manner consistent with the spirit of the LDP Manifesto.

Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French, German, Italian, Russian, Czech, Chinese, Indonesian, Dutch, Romanian, Bulgarian, and Turkish translations are also available or in progress. If you wish to translate this document into another language, please feel free to do so, subject to the terms stated above. The author wishes to be notified of such efforts.

Appendix T. ASCII Table

Traditionally, a book of this sort has an ASCII Table appendix. This book does not. Instead, here are several short scripts, each of which generates a complete ASCII table.

Example T-1. A script that generates an ASCII table

#!/bin/bash # ascii.sh # ver. 0.2, reldate 26 Aug 2008 # Patched by ABS Guide author.  # Original script by Sebastian Arming. # Used with permission (thanks!).  exec >ASCII.txt         #  Save stdout to file,                         #+ as in the example scripts                         #+ reassign-stdout.sh and upperconv.sh.  MAXNUM=256 COLUMNS=5 OCT=8 OCTSQU=64 LITTLESPACE=-3 BIGSPACE=-5  i=1 # Decimal counter o=1 # Octal counter  while [ "$i" -lt "$MAXNUM" ]; do  # We don't have to count past 400 octal.         paddi="    $i"         echo -n "${paddi: $BIGSPACE}  "       # Column spacing.         paddo="00$o" #       echo -ne "\\${paddo: $LITTLESPACE}"   # Original.         echo -ne "\\0${paddo: $LITTLESPACE}"  # Fixup. #                   ^         echo -n "     "         if (( i % $COLUMNS == 0)); then       # New line.            echo         fi         ((i++, o++))         # The octal notation for 8 is 10, and 64 decimal is 100 octal.         (( i % $OCT == 0))    && ((o+=2))         (( i % $OCTSQU == 0)) && ((o+=20)) done  exit $?  # Compare this script with the "pr-asc.sh" example. # This one handles "unprintable" characters.  # Exercise: # Rewrite this script to use decimal numbers, rather than octal.

Example T-2. Another ASCII table script

#!/bin/bash # Script author: Joseph Steinhauser # Lightly edited by ABS Guide author, but not commented. # Used in ABS Guide with permission.  #------------------------------------------------------------------------- #-- File:  ascii.sh    Print ASCII chart, base 10/8/16         (JETS-2012) #------------------------------------------------------------------------- #-- Usage: ascii [oct|dec|hex|help|8|10|16] #-- #-- This script prints out a summary of ASCII char codes from Zero to 127. #-- Numeric values may be printed in Base10, Octal, or Hex. #-- #-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default. #-- For more detail, man ascii . . . #-------------------------------------------------------------------------  [ -n "$BASH_VERSION" ] && shopt -s extglob  case "$1" in    oct|[Oo]?([Cc][Tt])|8)       Obase=Octal;  Numy=3o;;    hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex;    Numy=2X;;    help|?(-)[h?])        sed -n '2,/^[ ]*$/p' $0;exit;;    code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p'   $0;exit;;    *) Obase=Decimal esac # CODE is actually shorter than the chart!  printf "\t\t## $Obase ASCII Chart ##\n\n"; FM1="|%0${Numy:-3d}"; LD=-1  AB="nul soh stx etx eot enq ack bel bs tab nl vt np cr so si dle" AD="dc1 dc2 dc3 dc4 nak syn etb can em sub esc fs gs rs us sp"  for TOK in $AB $AD; do ABR[$((LD+=1))]=$TOK; done; ABR[127]=del  IDX=0 while [ $IDX -le 127 ] && CHR="${ABR[$IDX]}"    do ((${#CHR}))&& FM2='%-3s'|| FM2=`printf '\\\\%o  ' $IDX`       printf "$FM1 $FM2" "$IDX" $CHR; (( (IDX+=1)%8))||echo '|'    done  exit $?

Example T-3. A third ASCII table script, using awk

#!/bin/bash # ASCII table script, using awk. # Author: Joseph Steinhauser # Used in ABS Guide with permission.   #------------------------------------------------------------------------- #-- File:  ascii     Print ASCII chart, base 10/8/16         (JETS-2010) #------------------------------------------------------------------------- #-- Usage: ascii [oct|dec|hex|help|8|10|16] #-- #-- This script prints a summary of ASCII char codes from Zero to 127. #-- Numeric values may be printed in Base10, Octal, or Hex (Base16). #-- #-- Format Based on: /usr/share/lib/pub/ascii with base-10 as default. #-- For more detail, man ascii #-------------------------------------------------------------------------  [ -n "$BASH_VERSION" ] && shopt -s extglob  case "$1" in    oct|[Oo]?([Cc][Tt])|8)       Obase=Octal;  Numy=3o;;    hex|[Hh]?([Ee][Xx])|16|[Xx]) Obase=Hex;    Numy=2X;;    help|?(-)[h?])        sed -n '2,/^[ ]*$/p' $0;exit;;    code|[Cc][Oo][Dd][Ee])sed -n '/case/,$p'   $0;exit;;    *) Obase=Decimal esac export Obase   # CODE is actually shorter than the chart!  awk 'BEGIN{print "\n\t\t## "ENVIRON["Obase"]" ASCII Chart ##\n"            ab="soh,stx,etx,eot,enq,ack,bel,bs,tab,nl,vt,np,cr,so,si,dle,"            ad="dc1,dc2,dc3,dc4,nak,syn,etb,can,em,sub,esc,fs,gs,rs,us,sp"            split(ab ad,abr,",");abr[0]="nul";abr[127]="del";            fm1="|%0'"${Numy:- 4d}"' %-3s"            for(idx=0;idx<128;idx++){fmt=fm1 (++colz%8?"":"|\n")            printf(fmt,idx,(idx in abr)?abr[idx]:sprintf("%c",idx))} }'  exit $?

Index

This index / glossary / quick-reference lists many of the important topics covered in the text. Terms are arranged in approximate ASCII sorting order, modified as necessary for enhanced clarity.

Note that commands are indexed in Part 4.

* * *

^ (caret)

Beginning-of-line, in a Regular Expression
^

^^

Uppercase conversion in parameter substitution

~ Tilde

~ home directory, corresponds to $HOME
~/ Current user's home directory
~+ Current working directory
~- Previous working directory

= Equals sign

= Variable assignment operator
= String comparison operator

== String comparison operator
=~ Regular Expression match operator

Example script

< Left angle bracket

Is-less-than

String comparison

Integer comparison within double parentheses
Redirection

< stdin

<< Here document

<<< Here string

<> Opening a file for both reading and writing

> Right angle bracket

Is-greater-than

String comparison

Integer comparison, within double parentheses
Redirection

> Redirect stdout to a file

>> Redirect stdout to a file, but append

i>&j Redirect file descriptor i to file descriptor j

>&j Redirect stdout to file descriptor j

>&2 Redirect stdout of a command to stderr

2>&1 Redirect stderr to stdout

&> Redirect both stdout and stderr of a command to a file

:> file Truncate file to zero length

| Pipe, a device for passing the output of a command to another command or to the shell

|| Logical OR test operator

- (dash)

Prefix to default parameter, in parameter substitution
Prefix to option flag
Indicating redirection from stdin or stdout
-- (double-dash)

Prefix to long command options

C-style variable decrement within double parentheses

; (semicolon)

As command separator
\; Escaped semicolon, terminates a find command
;; Double-semicolon, terminator in a case option

Required when ...

do keyword is on the first line of loop

terminating curly-bracketed code block
;;& ;& Terminators in a case option (version 4+ of Bash).

: Colon

:> filename Truncate file to zero length
null command, equivalent to the true Bash builtin
Used in an anonymous here document
Used in an otherwise empty function
Used as a function name

! Negation operator, inverts exit status of a test or command

!= not-equal-to String comparison operator

? (question mark)

Match zero or one characters, in an Extended Regular Expression
Single-character wild card, in globbing
In a C-style Trinary operator

// Double forward slash, behavior of cd command toward

. (dot / period)

. Load a file (into a script), equivalent to source command
. Match single character, in a Regular Expression
. Current working directory

./ Current working directory
.. Parent directory

' ... ' (single quotes) strong quoting

" ... " (double quotes) weak quoting

Double-quoting the backslash (\) character

Comma operator
,

,,

Lowercase conversion in parameter substitution

() Parentheses

( ... ) Command group; starts a subshell
( ... ) Enclose group of Extended Regular Expressions
>( ... )

<( ... ) Process substitution
... ) Terminates test-condition in case construct
(( ... )) Double parentheses, in arithmetic expansion

[ Left bracket, test construct

[ ]Brackets

Array element
Enclose character set to match in a Regular Expression
Test construct

[[ ... ]] Double brackets, extended test construct

$ Anchor, in a Regular Expression

$ Prefix to a variable name

$( ... ) Command substitution, setting a variable with output of a command, using parentheses notation

` ... ` Command substitution, using backquotes notation

$[ ... ] Integer expansion (deprecated)

${ ... } Variable manipulation / evaluation

${var} Value of a variable
${#var} Length of a variable
${#@}

${#*} Number of positional parameters
${parameter?err_msg} Parameter-unset message
${parameter-default}

${parameter:-default}

${parameter=default}

${parameter:=default} Set default parameter
${parameter+alt_value}

${parameter:+alt_value}

Alternate value of parameter, if set
${!var}

Indirect referencing of a variable, new notation
${!#}

Final positional parameter. (This is an indirect reference to $#.)
${!varprefix*}

${!varprefix@}

Match names of all previously declared variables beginning with varprefix
${string:position}

${string:position:length} Substring extraction
${var#Pattern}

${var##Pattern} Substring removal
${var%Pattern}

${var%%Pattern} Substring removal
${string/substring/replacement}

${string//substring/replacement}

${string/#substring/replacement}

${string/%substring/replacement} Substring replacement

$' ... ' String expansion, using escaped characters.

\ Escape the character following

\< ... \> Angle brackets, escaped, word boundary in a Regular Expression
\{ N \} "Curly" brackets, escaped, number of character sets to match in an Extended RE
\; Semicolon, escaped, terminates a find command
\$$ Indirect reverencing of a variable, old-style notation
Escaping a newline, to write a multi-line command

&> Redirect both stdout and stderr of a command to a file
>&j Redirect stdout to file descriptor j

>&2 Redirect stdout of a command to stderr
i>&j Redirect file descriptor i to file descriptor j

2>&1 Redirect stderr to stdout
Closing file descriptors

n<&- Close input file descriptor n

0<&-, <&- Close stdin

n>&- Close output file descriptor n

1>&-, >&- Close stdout
&& Logical AND test operator
Command & Run job in background

# Hashmark, special symbol beginning a script comment

#! Sha-bang, special string starting a shell script

* Asterisk

Wild card, in globbing
Any number of characters in a Regular Expression
** Exponentiation, arithmetic operator
** Extended globbing file-match operator

% Percent sign

Modulo, division-remainder arithmetic operation
Substring removal (pattern matching) operator

+ Plus sign

Character match, in an extended Regular Expression
Prefix to alternate parameter, in parameter substitution
++ C-style variable increment, within double parentheses

* * *

Shell Variables

$_ Last argument to previous command

$- Flags passed to script, using set

$! Process ID of last background job

$? Exit status of a command

$@ All the positional parameters, as separate words

$* All the positional parameters, as a single word

$$ Process ID of the script

$# Number of arguments passed to a function, or to the script itself

$0 Filename of the script

$1 First argument passed to script

$9 Ninth argument passed to script

Table of shell variables

* * * * * *

-a Logical AND compound comparison test

Address database, script example

Advanced Bash Scripting Guide, where to download

Alias

Removing an alias, using unalias

Anagramming

And list

To supply default command-line argument

And logical operator &&

Angle brackets, escaped, \< . . . \> word boundary in a Regular Expression

Anonymous here document, using :

Archiving

Arithmetic expansion

exit status of
variations of

Arithmetic operators

combination operators, C-style

+= -= *= /= %=

In certain contexts, += can also function as a string concatenation operator.

Arrays

Associative arrays

more efficient than conventional arrays
Bracket notation
Concatenating, example script
Copying
Declaring

declare -a array_name
Embedded arrays
Empty arrays, empty elements, example script
Indirect references
Initialization

array=( element1 element2 ... elementN)

Example script

Using command substitution
Loading a file into an array
Multidimensional, simulating
Nesting and embedding
Notation and usage
Number of elements in

${#array_name[@]}

${#array_name[*]}
Operations
Passing an array to a function
As return value from a function
Special properties, example script
String operations, example script
unset deletes array elements

Arrow keys, detecting

ASCII

Definition
Scripts for generating ASCII table

awk field-oriented text processing language

rand(), random function
String manipulation
Using export to pass a variable to an embedded awk script

* * *

Backlight, setting the brightness

Backquotes, used in command substitution

Base conversion, example script

Bash

Bad scripting practices
Basics reviewed, script example
Command-line options

Table
Features that classic Bourne shell lacks
Internal variables
Version 2
Version 3
Version 4

Version 4.1

Version 4.2

.bashrc

$BASH_SUBSHELL

Basic commands, external

Batch files, DOS

Batch processing

bc, calculator utility

In a here document
Template for calculating a script variable

Bibliography

Bison utility

Bitwise operators

Example script

Block devices

testing for

Blocks of code

Iterating / looping
Redirection

Script example: Redirecting output of a a code block

Bootable flash drives, creating

Brace expansion

Extended, {a..z}
Parameterizing
With increment and zero-padding (new feature in Bash, version 4)

Brackets, [ ]

Array element
Enclose character set to match in a Regular Expression
Test construct

Brackets, curly, {}, used in

Code block
find
Extended Regular Expressions
Positional parameters
xargs

break loop control command

Parameter (optional)

Builtins in Bash

Do not fork a subprocess

* * *

case construct

Command-line parameters, handling
Globbing, filtering strings with

cat, concatentate file(s)

Abuse of
cat scripts
Less efficient than redirecting stdin
Piping the output of, to a read
Uses of

Character devices

testing for

Checksum

Child processes

Colon, : , equivalent to the true Bash builtin

Colorizing scripts

Cycling through the background colors, example script
Table of color escape sequences
Template, colored text on colored background

Comma operator, linking commands or operations

Command-line options

command_not_found_handle () builtin error-handling function (version 4+ of Bash)

Command substitution

$( ... ), preferred notation
Backquotes
Extending the Bash toolset
Invokes a subshell
Nesting
Removes trailing newlines
Setting variable from loop output
Word splitting

Comment headers, special purpose

Commenting out blocks of code

Using an anonymous here document
Using an if-then construct

Communications and hosts

Compound comparison operators

Compression utilities

bzip2
compress
gzip
zip

continue loop control command

Control characters

Control-C, break
Control-D, terminate / log out / erase
Control-G, BEL (beep)
Control-H, rubout
Control-J, newline
Control-M, carriage return

Coprocesses

cron, scheduling daemon

C-style syntax , for handling variables

Crossword puzzle solver

Cryptography

Curly brackets {}

in find command
in an Extended Regular Expression
in xargs

* * *

Daemons, in UNIX-type OS

date

dc, calculator utility

dd, data duplicator command

Conversions
Copying raw data to/from devices
File deletion, secure
Keystrokes, capturing
Options
Random access on a data stream
Raspberry Pi, script for preparing a bootable SD card
Swapfiles, initializing
Thread on www.linuxquestions.org

Debugging scripts

Tools
Trapping at exit
Trapping signals

Decimal number, Bash interprets numbers as

declare builtin

options

case-modification options (version 4+ of Bash)

Default parameters

/dev directory

/dev/null pseudo-device file
/dev/urandom pseudo-device file, generating pseudorandom numbers with
/dev/zero, pseudo-device file

Device file

dialog, utility for generating dialog boxes in a script

$DIRSTACK directory stack

Disabled commands, in restricted shells

do keyword, begins execution of commands within a loop

done keyword, terminates a loop

DOS batch files, converting to shell scripts

DOS commands, UNIX equivalents of (table)

dot files, "hidden" setup and configuration files

Double brackets [[ ... ]] test construct

and evaluation of octal/hex constants

Double parentheses (( ... )) arithmetic expansion/evaluation construct

Double quotes " ... " weak quoting

Double-quoting the backslash (\) character

Double-spacing a text file, using sed

* * *

-e File exists test

echo

Feeding commands down a pipe
Setting a variable using command substitution
/bin/echo, external echo command

elif, Contraction of else and if

else

Encrypting files, using openssl

esac, keyword terminating case construct

Environmental variables

-eq , is-equal-to integer comparison test

Eratosthenes, Sieve of, algorithm for generating prime numbers

Escaped characters, special meanings of

Within $' ... ' string expansion
Used with Unicode characters

/etc/fstab (filesystem mount) file

/etc/passwd (user account) file

$EUID, Effective user ID

eval, Combine and evaluate expression(s), with variable expansion

Effects of, Example script
Forces reevaluation of arguments
And indirect references
Risk of using
Using eval to convert array elements into a command list
Using eval to select among variables

Evaluation of octal/hex constants within [[ ... ]]

exec command, using in redirection

Exercises

Exit and Exit status

exit command
Exit status (exit code, return status of a command)

Table, Exit codes with special meanings

Anomalous

Out of range

Pipe exit status

Specified by a function return

Successful, 0

/usr/include/sysexits.h, system file listing C/C++ standard exit codes

Export, to make available variables to child processes

Passing a variable to an embedded awk script

expr, Expression evaluator

Substring extraction
Substring index (numerical position in string)
Substring matching

Extended Regular Expressions

? (question mark) Match zero / one characters
( ... ) Group of expressions
\{ N \} "Curly" brackets, escaped, number of character sets to match
+ Character match

* * *

factor, decomposes an integer into its prime factors

Application: Generating prime numbers

false, returns unsuccessful (1) exit status

Field, a group of characters that comprises an item of data

Files / Archiving

File descriptors

Closing

n<&- Close input file descriptor n

0<&-, <&- Close stdin

n>&- Close output file descriptor n

1>&-, >&- Close stdout
File handles in C, similarity to

File encryption

find

{} Curly brackets
\; Escaped semicolon

Filter

Using - with file-processing utility as a filter
Feeding output of a filter back to same filter

Floating point numbers, Bash does not recognize

fold, a filter to wrap lines of text

Forking a child process

for loops

Functions

Arguments passed referred to by position
Capturing the return value of a function using echo
Colon as function name
Definition must precede first call to function
Exit status
Local variables

and recursion
Passing an array to a function
Passing pointers to a function
Positional parameters
Recursion
Redirecting stdin of a function
return

Multiple return values from a function, example script

Returning an array from a function

Return range limits, workarounds
Shift arguments passed to a function
Unusual function names

* * *

Games and amusements

Anagrams
Anagrams, again
Bingo Number Generator
Crossword puzzle solver
Crypto-Quotes
Dealing a deck of cards
Fifteen Puzzle
Horse race
Knight's Tour
"Life" game
Magic Squares
Music-playing script
Nim
Pachinko
Perquackey
Petals Around the Rose
Podcasting
Poem
Speech generation
Towers of Hanoi

Graphic version

Alternate graphic version

getopt, external command for parsing script command-line arguments

Emulated in a script

getopts, Bash builtin for parsing script command-line arguments

$OPTIND / $OPTARG

Global variable

Globbing, filename expansion

Handling filenames correctly
Wild cards
Will not match dot files

Golden Ratio (Phi)

-ge , greater-than or equal integer comparison test

-gt , greater-than integer comparison test

groff, text markup and formatting language

Gronsfeld cipher

$GROUPS, Groups user belongs to

gzip, compression utility

* * *

Hashing, creating lookup keys in a table

Example script

head, echo to stdout lines at the beginning of a text file

help, gives usage summary of a Bash builtin

Here documents

Anonymous here documents, using :

Commenting out blocks of code

Self-documenting scripts
bc in a here document
cat scripts
Command substitution
ex scripts
Function, supplying input to
Here strings

Calculating the Golden Ratio

Prepending text

As the stdin of a loop

Using read
Limit string

! as a limit string

Closing limit string may not be indented

Dash option to limit string, <<-LimitString
Literal text output, for generating program code
Parameter substitution

Disabling parameter substitution
Passing parameters
Temporary files
Using vi non-interactively

History commands

$HOME, user's home directory

Homework assignment solver

$HOSTNAME, system host name

* * *

$Id parameter, in rcs (Revision Control System)

if [ condition ]; then ... test construct

if-grep, if and grep in combination

Fixup for if-grep test

$IFS, Internal field separator variable

Defaults to whitespace

Integer comparison operators

in, keyword preceding [list] in a for loop

Initialization table, /etc/inittab

Inline group, i.e., code block

Interactive script, test for

I/O redirection

Indirect referencing of variables

New notation, introduced in version 2 of Bash ( example script)

iptables, packet filtering and firewall utility

Usage example
Example script

Iteration

* * *

Job IDs, table

jot, Emit a sequence of integers. Equivalent to seq.

Random sequence generation

Just another Bash hacker!

* * *

Keywords

error, if missing

kill, terminate a process by process ID

Options (-l, -9)

killall, terminate a process by name

killall script in /etc/rc.d/init.d

* * *

lastpipe shell option

-le , less-than or equal integer comparison test

let, setting and carrying out arithmetic operations on variables

C-style increment and decrement operators

Limit string, in a here document

$LINENO, variable indicating the line number where it appears in a script

Link, file (using ln command)

Invoking script with multiple names, using ln
symbolic links, ln -s

List constructs

And list
Or list

Local variables

and recursion

Localization

Logical operators (&&, ||, etc.)

Logout file, the ~/.bash_logout file

Loopback device, mounting a file on a block device

Loops

break loop control command
continue loop control command
C-style loop within double parentheses

for loop

while loop
do (keyword), begins execution of commands within a loop
done (keyword), terminates a loop
for loops

for arg in [list]; do

Command substitution to generate [list]

Filename expansion in [list]

Multiple parameters in each [list] element

Omitting [list], defaults to positional parameters

Parameterizing [list]

Redirection
in, (keyword) preceding [list] in a for loop
Nested loops
Running a loop in the background, script example
Semicolon required, when do is on first line of loop

for loop

while loop
until loop

until [ condition-is-true ]; do
while loop

while [ condition ]; do

Function call inside test brackets

Multiple conditions

Omitting test brackets

Redirection

while read construct
Which type of loop to use

Loopback devices

In /dev directory
Mounting an ISO image

-lt , less-than integer comparison test

* * *

m4, macro processing language

$MACHTYPE, Machine type

Magic number, marker at the head of a file indicating the file type

Makefile, file containing the list of dependencies used by make command

man, manual page (lookup)

Man page editor (script)

mapfile builtin, loads an array with a text file

Math commands

Meta-meaning

Morse code training script

Modulo, arithmetic remainder operator

Application: Generating prime numbers

Mortgage calculations, example script

* * *

-n String not null test

Named pipe, a temporary FIFO buffer

Example script

nc, netcat, a network toolkit for TCP and UDP ports

-ne, not-equal-to integer comparison test

Negation operator, !, reverses the sense of a test

netstat, Network statistics

Network programming

nl, a filter to number lines of text

Noclobber, -C option to Bash to prevent overwriting of files

NOT logical operator, !

null variable assignment, avoiding

* * *

-o Logical OR compound comparison test

Obfuscation

Colon as function name
Homework assignment
Just another Bash hacker!

octal, base-8 numbers

od, octal dump

$OLDPWD Previous working directory

openssl encryption utility

Operator

Definition of
Precedence

Options, passed to shell or script on command line or by set command

Or list

Or logical operator, ||

* * *

Parameter substitution

${parameter+alt_value}

${parameter:+alt_value}

Alternate value of parameter, if set
${parameter-default}

${parameter:-default}

${parameter=default}

${parameter:=default}

Default parameters
${!varprefix*}

${!varprefix@}

Parameter name match
${parameter?err_msg}

Parameter-unset message
${parameter}

Value of parameter
Case modification (version 4+ of Bash).
Script example
Table of parameter substitution

Parent / child process problem, a child process cannot export variables to a parent process

Parentheses

Command group
Enclose group of Extended Regular Expressions
Double parentheses, in arithmetic expansion

$PATH, the path (location of system binaries)

Appending directories to $PATH using the += operator.

Pathname, a filename that incorporates the complete path of a given file.

Parsing pathnames

Perl, programming language

Combined in the same file with a Bash script
Embedded in a Bash script

Perquackey-type anagramming game (Quackey script)

Petals Around the Rose

PID, Process ID, an identification number assigned to a running process.

Pipe, | , a device for passing the output of a command to another command or to the shell

Avoiding unnecessary commands in a pipe
Comments embedded within
Exit status of a pipe
Pipefail, set -o pipefail option to indicate exit status within a pipe
$PIPESTATUS, exit status of last executed pipe
Piping output of a command to a script
Redirecting stdin, rather than using cat in a pipe

Pitfalls

- (dash) is not redirection operator
// (double forward slash), behavior of cd command toward
#!/bin/sh script header disables extended Bash features
Abuse of cat
CGI programming, using scripts for
Closing limit string in a here document, indenting
DOS-type newlines (\r\n) crash a script
Double-quoting the backslash (\) character
eval, risk of using
Execute permission lacking for commands within a script
Exit status, anomalous
Exit status of arithmetic expression not equivalent to an error code
Export problem, child process to parent process
Extended Bash features not available
Failing to quote variables within test brackets
GNU command set, in cross-platform scripts
let misuse: attempting to set string variables
Multiple echo statements in a function whose output is captured
null variable assignment
Numerical and string comparison operators not equivalent

= and -eq not interchangeable
Omitting terminal semicolon, in a curly-bracketed code block
Piping

echo to a loop

echo to read (however, this problem can be circumvented)

tail -f to grep
Preserving whitespace within a variable, unintended consequences
suid commands inside a script
Undocumented Bash features, danger of
Updates to Bash breaking older scripts
Uninitialized variables
Variable names, inappropriate
Variables in a subshell, scope limited
Subshell in while-read loop
Whitespace, misuse of

Pointers

and file descriptors
and functions
and indirect references
and variables

Portability issues in shell scripting

Setting path and umask
A test suite script (Bash versus classic Bourne shell)
Using whatis

Positional parameters

$@, as separate words
$*, as a single word
in functions

POSIX, Portable Operating System Interface / UNIX

--posix option
1003.2 standard
Character classes

$PPID, process ID of parent process

Precedence, operator

Prepending lines at head of a file, script example

Prime numbers

Generating primes using the factor command
Generating primes using the modulo operator
Sieve of Eratosthenes, example script

printf, formatted print command

/proc directory

Running processes, files describing
Writing to files in /proc, warning

Process

Child process
Parent process
Process ID (PID)

Process substitution

To compare contents of directories
To supply stdin of a command
Template
while-read loop without a subshell

Programmable completion (tab expansion)

Prompt

$PS1, Main prompt, seen at command line
$PS2, Secondary prompt

Pseudo-code, as problem-solving method

$PWD, Current working directory

* * *

Quackey, a Perquackey-type anagramming game (script)

Question mark, ?

Character match in an Extended Regular Expression
Single-character wild card, in globbing
In a C-style Trinary (ternary) operator

Quoting

Character string
Variables

within test brackets
Whitespace, using quoting to preserve

* * *

Random numbers

/dev/urandom
rand(), random function in awk
$RANDOM, Bash function that returns a pseudorandom integer
Random sequence generation, using date command
Random sequence generation, using jot
Random string, generating

Raspberry Pi (single-board computer)

Script for preparing a bootable SD card

rcs

read, set value of a variable from stdin

Detecting arrow keys
Options
Piping output of cat to read
"Prepending" text
Problems piping echo to read
Redirection from a file to read
$REPLY, default read variable
Timed input
while read construct

readline library

Recursion

Demonstration of
Factorial
Fibonacci sequence
Local variables
Script calling itself recursively
Towers of Hanoi

Redirection

Code blocks
exec <filename,

to reassign file descriptors
Introductory-level explanation of I/O redirection
Open a file for both reading and writing

<>filename
read input redirected from a file
stderr to stdout

2>&1
stdin / stdout, using -
stdinof a function
stdout to a file

> ... >>
stdout to file descriptor j

>&j
file descriptori to file descriptor j

i>&j
stdout of a command to stderr

>&2
stdout and stderr of a command to a file

&>
tee, redirect to a file output of command(s) partway through a pipe

Reference Cards

Miscellaneous constructs
Parameter substitution/expansion
Special shell variables
String operations
Test operators

Binary comparison

Files

Regular Expressions

^ (caret) Beginning-of-line
$ (dollar sign) Anchor
. (dot) Match single character
* (asterisk) Any number of characters
[ ] (brackets) Enclose character set to match
\ (backslash) Escape, interpret following character literally
\< ... \> (angle brackets, escaped) Word boundary
Extended REs

+ Character match

\{ \} Escaped "curly" brackets

[: :] POSIX character classes

$REPLY, Default value associated with read command

Restricted shell, shell (or script) with certain commands disabled

return, command that terminates a function

run-parts

Running scripts in sequence, without user intervention

* * *

Scope of a variable, definition

Script options, set at command line

Scripting routines, library of useful definitions and functions

Secondary prompt, $PS2

Security issues

nmap, network mapper / port scanner
sudo
suid commands inside a script
Viruses, trojans, and worms in scripts
Writing secure scripts

sed, pattern-based programming language

Table, basic operators
Table, examples of operators

select, construct for menu building

in list omitted

Semaphore

Semicolon required, when do keyword is on first line of loop

When terminating curly-bracketed code block

seq, Emit a sequence of integers. Equivalent to jot.

set, Change value of internal script variables

set -u, Abort script with error message if attempting to use an undeclared variable.

Shell script, definition of

Shell wrapper, script embedding a command or utility

shift, reassigning positional parameters

$SHLVL, shell level, depth to which the shell (or script) is nested

shopt, change shell options

Signal, a message sent to a process

Simulations

Brownian motion
Galton board
Horserace
Life, game of
PI, approximating by firing cannonballs
Pushdown stack

Single quotes (' ... ') strong quoting

Socket, a communication node associated with an I/O port

Sorting

Bubble sort
Insertion sort

source, execute a script or, within a script, import a file

Passing positional parameters

Spam, dealing with

Example script
Example script
Example script
Example script

Special characters

Stack

Definition
Emulating a push-down stack, example script

Standard Deviation, example script

Startup files, Bash

stdin and stdout

Stopwatch, example script

Strings

=~ String match operator
Comparison
Length

${#string}
Manipulation
Manipulation, using awk
Null string, testing for
Protecting strings from expansion and/or reinterpretation, script example

Unprotecting strings, script example
strchr(), equivalent of
strlen(), equivalent of
strings command, find printable strings in a binary or data file
Substring extraction

${string:position}

${string:position:length}

Using expr
Substring index (numerical position in string)
Substring matching, using expr
Substring removal

${var#Pattern}

${var##Pattern}

${var%Pattern}

${var%%Pattern}
Substring replacement

${string/substring/replacement}

${string//substring/replacement}

${string/#substring/replacement}

${string/%substring/replacement}

Script example
Table of string/substring manipulation and extraction operators

Strong quoting ' ... '

Stylesheet for writing scripts

Subshell

Command list within parentheses
Variables, $BASH_SUBSHELL and $SHLVL
Variables in a subshell

scope limited, but ...

... can be accessed outside the subshell?

su Substitute user, log on as a different user or as root

suid (set user id) file flag

suid commands inside a script, not advisable

Symbolic links

Swapfiles

* * *

Tab completion

Table lookup, script example

tail, echo to stdout lines at the (tail) end of a text file

tar, archiving utility

tee, redirect to a file output of command(s) partway through a pipe

Terminals

setserial
setterm
stty
tput
wall

test command

Bash builtin
external command, /usr/bin/test (equivalent to /usr/bin/[)

Test constructs

Test operators

-a Logical AND compound comparison
-e File exists
-eq is-equal-to (integer comparison)
-f File is a regular file
-ge greater-than or equal (integer comparison)
-gt greater-than (integer comparison)
-le less-than or equal (integer comparison)
-lt less-than (integer comparison)
-n not-zero-length (string comparison)
-ne not-equal-to (integer comparison)
-o Logical OR compound comparison
-u suid flag set, file test
-z is-zero-length (string comparison)
= is-equal-to (string comparison)

== is-equal-to (string comparison)
< less-than (string comparison)
< less-than, (integer comparison, within double parentheses)
<= less-than-or-equal, (integer comparison, within double parentheses)
> greater-than (string comparison)
> greater-than, (integer comparison, within double parentheses)
>= greater-than-or-equal, (integer comparison, within double parentheses)
|| Logical OR
&& Logical AND
! Negation operator, inverts exit status of a test

!= not-equal-to (string comparison)
Tables of test operators

Binary comparison

File

Text and text file processing

Time / Date

Timed input

Using read -t
Using stty
Using timing loop
Using $TMOUT

Tips and hints for Bash scripts

Array, as return value from a function

Associative array more efficient than a numerically-indexed array
Capturing the return value of a function, using echo
CGI programming, using scripts for
Comment blocks

Using anonymous here documents

Using if-then constructs
Comment headers, special purpose
C-style syntax , for manipulating variables
Double-spacing a text file
Filenames prefixed with a dash, removing
Filter, feeding output back to same filter
Function return value workarounds
if-grep test fixup
Library of useful definitions and functions
null variable assignment, avoiding
Passing an array to a function
$PATH, appending to, using the += operator.
Prepending lines at head of a file
Progress bar template
Pseudo-code
rcs
Redirecting a test to /dev/null to suppress output
Running scripts in sequence without user intervention, using run-parts
Script as embedded command
Script portability

Setting path and umask

Using whatis
Setting script variable to a block of embedded sed or awk code
Speeding up script execution by disabling unicode
Subshell variable, accessing outside the subshell
Testing a variable to see if it contains only digits
Testing whether a command exists, using type
Tracking script usage
while-read loop without a subshell
Widgets, invoking from a script

$TMOUT, Timeout interval

Token, a symbol that may expand to a keyword or command

tput, terminal-control command

tr, character translation filter

DOS to Unix text file conversion
Options
Soundex, example script
Variants

Trap, specifying an action upon receipt of a signal

Trinary (ternary) operator, C-style, var>10?88:99

in double-parentheses construct
in let construct

true, returns successful (0) exit status

typeset builtin

options

* * *

$UID, User ID number

unalias, to remove an alias

uname, output system information

Unicode, encoding standard for representing letters and symbols

Disabling unicode to optimize script

Uninitialized variables

uniq, filter to remove duplicate lines from a sorted file

unset, delete a shell variable

until loop

until [ condition-is-true ]; do

* * *

Variables

Array operations on
Assignment

Script example

Script example

Script example
Bash internal variables
Block of sed or awk code, setting a variable to
C-style increment/decrement/trinary operations
Change value of internal script variables using set
declare, to modify the properties of variables
Deleting a shell variable using unset
Environmental
Expansion / Substring replacement operators
Indirect referencing

eval variable1=\$$variable2

Newer notation

${!variable}
Integer
Integer / string (variables are untyped)
Length

${#var}
Lvalue
Manipulating and expanding
Name and value of a variable, distinguishing between
Null string, testing for
Null variable assignment, avoiding
Quoting

within test brackets

to preserve whitespace
rvalue
Setting to null value
In subshell not visible to parent shell
Testing a variable if it contains only digits
Typing, restricting the properties of a variable
Undeclared, error message
Uninitialized
Unquoted variable, splitting
Unsetting
Untyped

* * *

wait, suspend script execution

To remedy script hang

Weak quoting " ... "

while loop

while [ condition ]; do

C-style syntax
Calling a function within test brackets
Multiple conditions
Omitting test brackets
while read construct

Avoiding a subshell

Whitespace, spaces, tabs, and newline characters

$IFS defaults to
Inappropriate use of
Preceding closing limit string in a here document, error
Preceding script comments
Quoting, to preserve whitespace within strings or variables
[:space:], POSIX character class

who, information about logged on users

w
whoami
logname

Widgets

Wild card characters

Asterisk *
In [list] constructs
Question mark ?
Will not match dot files

Word splitting

Definition
Resulting from command substitution

Wrapper, shell

* * *

xargs, Filter for grouping arguments

Curly brackets
Limiting arguments passed
Options
Processes arguments one at a time
Whitespace, handling

* * *

yes

Emulation

* * *

-z String is null

Zombie, a process that has terminated, but not yet been killed by its parent

Write a Script That Connects to 100 Hosts,

Source: https://tldp.org/LDP/abs/html/abs-guide.html

Write a Script That Connects to 100 Hosts,

Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Dedication

Part 1. Introduction

Chapter 1. Shell Programming!

Chapter 2. Starting Off With a Sha-Bang

2.1. Invoking the script

2.2. Preliminary Exercises

Part 2. Basics

Chapter 3. Special Characters

Chapter 4. Introduction to Variables and Parameters

4.1. Variable Substitution

4.2. Variable Assignment

4.3. Bash Variables Are Untyped

4.4. Special Variable Types

Chapter 5. Quoting

5.1. Quoting Variables

5.2. Escaping

Chapter 6. Exit and Exit Status

Chapter 7. Tests

7.1. Test Constructs

7.2. File test operators

7.3. Other Comparison Operators

7.4. Nested if/then Condition Tests

7.5. Testing Your Knowledge of Tests

Chapter 8. Operations and Related Topics

8.1. Operators

8.2. Numerical Constants

8.3. The Double-Parentheses Construct

8.4. Operator Precedence

Part 3. Beyond the Basics

Chapter 9. Another Look at Variables

9.1. Internal Variables

9.2. Typing variables: declare or typeset

9.2.1. Another use for declare

9.3. $RANDOM: generate random integer

Chapter 10. Manipulating Variables

10.1. Manipulating Strings

10.1.1. Manipulating strings using awk

10.1.2. Further Reference

10.2. Parameter Substitution

Chapter 11. Loops and Branches

11.1. Loops

11.2. Nested Loops

11.3. Loop Control

11.4. Testing and Branching

Chapter 12. Command Substitution

Chapter 13. Arithmetic Expansion

Chapter 14. Recess Time

Part 4. Commands

Chapter 15. Internal Commands and Builtins

15.1. Job Control Commands

Chapter 16. External Filters, Programs and Commands

16.1. Basic Commands

16.2. Complex Commands

16.3. Time / Date Commands

16.4. Text Processing Commands

16.5. File and Archiving Commands

16.6. Communications Commands

16.7. Terminal Control Commands

16.8. Math Commands

16.9. Miscellaneous Commands

Chapter 17. System and Administrative Commands

17.1. Analyzing a System Script

Part 5. Advanced Topics

Chapter 18. Regular Expressions

18.1. A Brief Introduction to Regular Expressions

18.2. Globbing

Chapter 19. Here Documents

19.1. Here Strings

Chapter 20. I/O Redirection

20.1. Using exec

20.2. Redirecting Code Blocks

20.3. Applications

Chapter 21. Subshells

Chapter 22. Restricted Shells

Chapter 23. Process Substitution

Chapter 24. Functions

24.1. Complex Functions and Function Complexities

7.4. Nested `if/then` Condition Tests

Chapter 29. `/dev` and `/proc`

29.1. `/dev`

29.2. `/proc`

Appendix M. Sample `.bashrc` and `.bash_profile` Files