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Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Mendel Cooper

<>

4.1

08 October 2006

Revision History
Revision 3.9	15 May 2006	Revised by: mc
'SPICEBERRY' release: Minor Update.
Revision 4.0	18 Jun 2006	Revised by: mc
'WINTERBERRY' release: Major Update.
Revision 4.1	08 Oct 2006	Revised by: mc
'WAXBERRY' release: Minor Update.

This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little snippets of UNIX® wisdom and lore. It serves as a textbook, a manual for self-study, and 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.

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 is also available. See the for a revision history.

Dedication

For Anita, the source of all the magic

Table of Contents

Part 1. Introduction

1. Why Shell Programming?
2. Starting Off With a Sha-Bang

Part 2. Basics

3. Special Characters
4. Introduction to Variables and Parameters
5. Quoting
6. Exit and Exit Status
7. Tests
8. Operations and Related Topics

Part 3. Beyond the Basics

9. Variables Revisited
10. Loops and Branches
11. Command Substitution
12. Arithmetic Expansion
13. Recess Time

Part 4. Commands

14. Internal Commands and Builtins
15. External Filters, Programs and Commands
16. System and Administrative Commands

Part 5. Advanced Topics

17. Regular Expressions
18. Here Documents
19. I/O Redirection
20. Subshells
21. Restricted Shells
22. Process Substitution
23. Functions
24. Aliases
25. List Constructs
26. Arrays
27. /dev and /proc
28. Of Zeros and Nulls
29. Debugging
30. Options
31. Gotchas
32. Scripting With Style
33. Miscellany
34. Bash, versions 2 and 3

35. Endnotes

35.1. Author's Note
35.2. About the Author
35.3. Where to Go For Help
35.4. Tools Used to Produce This Book
35.5. Credits

Bibliography

A. Contributed Scripts

B. Reference Cards

C. A Sed and Awk Micro-Primer

C.1. Sed
C.2. Awk

D. Exit Codes With Special Meanings

E. A Detailed Introduction to I/O and I/O Redirection

F. Command-Line Options

F.1. Standard Command-Line Options
F.2. Bash Command-Line Options

G. Important Files

H. Important System Directories

I. Localization

J. History Commands

K. A Sample .bashrc File

L. Converting DOS Batch Files to Shell Scripts

M. Exercises

M.1. Analyzing Scripts
M.2. Writing Scripts

N. Revision History

O. Mirror Sites

P. To Do List

Q. Copyright

List of Tables
14-1. Job identifiers
30-1. Bash options
33-1. Numbers representing colors in Escape Sequences
B-1. Special Shell Variables
B-2. TEST Operators: Binary Comparison
B-3. TEST Operators: Files
B-4. Parameter Substitution and Expansion
B-5. String Operations
B-6. Miscellaneous Constructs
C-1. Basic sed operators
C-2. Examples of sed operators
D-1. "Reserved" Exit Codes
L-1. Batch file keywords / variables / operators, and their shell equivalents
L-2. DOS commands and their UNIX equivalents
N-1. Revision History

List of Examples
2-1. cleanup: A script to clean up the 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 results 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
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
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. Inserting a blank line between paragraphs in a text file
9-11. Converting graphic file formats, with filename change
9-12. Converting streaming audio files to ogg
9-13. Emulating getopt
9-14. Alternate ways of extracting substrings
9-15. Using parameter substitution and error messages
9-16. Parameter substitution and "usage" messages
9-17. Length of a variable
9-18. Pattern matching in parameter substitution
9-19. Renaming file extensions:
9-20. Using pattern matching to parse arbitrary strings
9-21. Matching patterns at prefix or suffix of string
9-22. Using declare to type variables
9-23. Indirect References
9-24. Passing an indirect reference to awk
9-25. Generating random numbers
9-26. Picking a random card from a deck
9-27. Random between values
9-28. Rolling a single die with RANDOM
9-29. Reseeding RANDOM
9-30. Pseudorandom numbers, using awk
9-31. C-type manipulation of variables
10-1. Simple for loops
10-2. for loop with two parameters in each [list] element
10-3. Fileinfo: operating on a file list contained in a variable
10-4. Operating on files with a for loop
10-5. Missing in [list] in a for loop
10-6. Generating the [list] in a for loop with command substitution
10-7. A grep replacement for binary files
10-8. Listing all users on the system
10-9. Checking all the binaries in a directory for authorship
10-10. Listing the symbolic links in a directory
10-11. Symbolic links in a directory, saved to a file
10-12. A C-like for loop
10-13. Using efax in batch mode
10-14. Simple while loop
10-15. Another while loop
10-16. while loop with multiple conditions
10-17. C-like syntax in a while loop
10-18. until loop
10-19. Nested Loop
10-20. Effects of break and continue in a loop
10-21. Breaking out of multiple loop levels
10-22. Continuing at a higher loop level
10-23. Using "continue N" in an actual task
10-24. Using case
10-25. Creating menus using case
10-26. Using command substitution to generate the case variable
10-27. Simple string matching
10-28. Checking for alphabetic input
10-29. Creating menus using select
10-30. Creating menus using select in a function
11-1. Stupid script tricks
11-2. Generating a variable from a loop
11-3. Finding anagrams
14-1. A script that forks off multiple instances of itself
14-2. printf in action
14-3. Variable assignment, using read
14-4. What happens when read has no variable
14-5. Multi-line input to read
14-6. Detecting the arrow keys
14-7. Using read with file redirection
14-8. Problems reading from a pipe
14-9. Changing the current working directory
14-10. Letting "let" do arithmetic.
14-11. Showing the effect of eval
14-12. Forcing a log-off
14-13. A version of "rot13"
14-14. Using eval to force variable substitution in a Perl script
14-15. Using set with positional parameters
14-16. Reversing the positional parameters
14-17. Reassigning the positional parameters
14-18. "Unsetting" a variable
14-19. Using export to pass a variable to an embedded awk script
14-20. Using getopts to read the options/arguments passed to a script
14-21. "Including" a data file
14-22. A (useless) script that sources itself
14-23. Effects of exec
14-24. A script that exec's itself
14-25. Waiting for a process to finish before proceeding
14-26. A script that kills itself
15-1. Using ls to create a table of contents for burning a CDR disk
15-2. Hello or Good-bye
15-3. Badname, eliminate file names in current directory containing bad characters and whitespace.
15-4. Deleting a file by its inode number
15-5. Logfile: Using xargs to monitor system log
15-6. Copying files in current directory to another
15-7. Killing processes by name
15-8. Word frequency analysis using xargs
15-9. Using expr
15-10. Using date
15-11. Word Frequency Analysis
15-12. Which files are scripts?
15-13. Generating 10-digit random numbers
15-14. Using tail to monitor the system log
15-15. Emulating "grep" in a script
15-16. Looking up definitions in Webster's 1913 Dictionary
15-17. Checking words in a list for validity
15-18. toupper: Transforms a file to all uppercase.
15-19. lowercase: Changes all filenames in working directory to lowercase.
15-20. Du: DOS to UNIX text file conversion.
15-21. rot13: rot13, ultra-weak encryption.
15-22. Generating "Crypto-Quote" Puzzles
15-23. Formatted file listing.
15-24. Using column to format a directory listing
15-25. nl: A self-numbering script.
15-26. manview: Viewing formatted manpages
15-27. Using cpio to move a directory tree
15-28. Unpacking an rpm archive
15-29. Stripping comments from C program files
15-30. Exploring /usr/X11R6/bin
15-31. An "improved" strings command
15-32. Using cmp to compare two files within a script.
15-33. basename and dirname
15-34. Checking file integrity
15-35. Uudecoding encoded files
15-36. Finding out where to report a spammer
15-37. Analyzing a spam domain
15-38. Getting a stock quote
15-39. Updating FC4
15-40. Using ssh
15-41. A script that mails itself
15-42. Monthly Payment on a Mortgage
15-43. Base Conversion
15-44. Invoking bc using a "here document"
15-45. Calculating PI
15-46. Converting a decimal number to hexadecimal
15-47. Factoring
15-48. Calculating the hypotenuse of a triangle
15-49. Using seq to generate loop arguments
15-50. Letter Count"
15-51. Using getopt to parse command-line options
15-52. A script that copies itself
15-53. Exercising dd
15-54. Capturing Keystrokes
15-55. Securely deleting a file
15-56. Filename generator
15-57. Converting meters to miles
15-58. Using m4
16-1. Setting a new password
16-2. Setting an erase character
16-3. secret password: Turning off terminal echoing
16-4. Keypress detection
16-5. Checking a remote server for identd
16-6. pidof helps kill a process
16-7. Checking a CD image
16-8. Creating a filesystem in a file
16-9. Adding a new hard drive
16-10. Using umask to hide an output file from prying eyes
16-11. killall, from /etc/rc.d/init.d
18-1. broadcast: Sends message to everyone logged in
18-2. dummyfile: Creates a 2-line dummy file
18-3. Multi-line message using cat
18-4. Multi-line message, with tabs suppressed
18-5. Here document with parameter substitution
18-6. Upload a file pair to "Sunsite" incoming directory
18-7. Parameter substitution turned off
18-8. A script that generates another script
18-9. Here documents and functions
18-10. "Anonymous" Here Document
18-11. Commenting out a block of code
18-12. A self-documenting script
18-13. Prepending a line to a file
18-14. Parsing a mailbox
19-1. Redirecting stdin using exec
19-2. Redirecting stdout using exec
19-3. Redirecting both stdin and stdout in the same script with exec
19-4. Avoiding a subshell
19-5. Redirected while loop
19-6. Alternate form of redirected while loop
19-7. Redirected until loop
19-8. Redirected for loop
19-9. Redirected for loop (both stdin and stdout redirected)
19-10. Redirected if/then test
19-11. Data file "names.data" for above examples
19-12. Logging events
20-1. Variable scope in a subshell
20-2. List User Profiles
20-3. Running parallel processes in subshells
21-1. Running a script in restricted mode
23-1. Simple functions
23-2. Function Taking Parameters
23-3. Functions and command-line args passed to the script
23-4. Passing an indirect reference to a function
23-5. Dereferencing a parameter passed to a function
23-6. Again, dereferencing a parameter passed to a function
23-7. Maximum of two numbers
23-8. Converting numbers to Roman numerals
23-9. Testing large return values in a function
23-10. Comparing two large integers
23-11. Real name from username
23-12. Local variable visibility
23-13. Recursion, using a local variable
23-14. The Towers of Hanoi
24-1. Aliases within a script
24-2. unalias: Setting and unsetting an alias
25-1. Using an "and list" to test for command-line arguments
25-2. Another command-line arg test using an "and list"
25-3. Using "or lists" in combination with an "and list"
26-1. Simple array usage
26-2. Formatting a poem
26-3. Various array operations
26-4. String operations on arrays
26-5. Loading the contents of a script into an array
26-6. Some special properties of arrays
26-7. Of empty arrays and empty elements
26-8. Initializing arrays
26-9. Copying and concatenating arrays
26-10. More on concatenating arrays
26-11. An old friend: The Bubble Sort
26-12. Embedded arrays and indirect references
26-13. Complex array application: Sieve of Eratosthenes
26-14. Emulating a push-down stack
26-15. Complex array application: Exploring a weird mathematical series
26-16. Simulating a two-dimensional array, then tilting it
27-1. Using /dev/tcp for troubleshooting
27-2. Finding the process associated with a PID
27-3. On-line connect status
28-1. Hiding the cookie jar
28-2. Setting up a swapfile using /dev/zero
28-3. Creating a ramdisk
29-1. A buggy script
29-2. Missing keyword
29-3. test24, another buggy script
29-4. Testing a condition with an "assert"
29-5. Trapping at exit
29-6. Cleaning up after Control-C
29-7. Tracing a variable
29-8. Running multiple processes (on an SMP box)
31-1. Numerical and string comparison are not equivalent
31-2. Subshell Pitfalls
31-3. Piping the output of echo to a read
33-1. shell wrapper
33-2. A slightly more complex shell wrapper
33-3. A generic shell wrapper that writes to a logfile
33-4. A shell wrapper around an awk script
33-5. A shell wrapper around another awk script
33-6. Perl embedded in a Bash script
33-7. Bash and Perl scripts combined
33-8. A (useless) script that recursively calls itself
33-9. A (useful) script that recursively calls itself
33-10. Another (useful) script that recursively calls itself
33-11. A "colorized" address database
33-12. Drawing a box
33-13. Echoing colored text
33-14. A "horserace" game
33-15. Return value trickery
33-16. Even more return value trickery
33-17. Passing and returning arrays
33-18. Fun with anagrams
33-19. Widgets invoked from a shell script
34-1. String expansion
34-2. Indirect variable references - the new way
34-3. Simple database application, using indirect variable referencing
34-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards
A-1. mailformat: Formatting an e-mail message
A-2. rn: A simple-minded file rename 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: Calculate number of days between two dates
A-8. Make 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. ftpget: Downloading files via ftp
A-14. password: Generating random 8-character passwords
A-15. fifo: Making daily backups, using named pipes
A-16. Generating prime numbers using the modulo operator
A-17. tree: Displaying a directory tree
A-18. string functions: C-like string functions
A-19. Directory information
A-20. Object-oriented database
A-21. Library of hash functions
A-22. Colorizing text using hash functions
A-23. More on hash functions
A-24. Mounting USB keychain storage devices
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. Basics Reviewed
C-1. Counting Letter Occurrences
K-1. Sample .bashrc file
L-1. VIEWDATA.BAT: DOS Batch File
L-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
P-1. Print the server environment

Part 1. Introduction

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, give additional power and flexibility to scripts. Shell scripts lend themselves exceptionally well to administrative system tasks and other routine repetitive jobs not requiring the bells and whistles of a full-blown tightly structured programming language.

Table of Contents

1. Why Shell Programming?

2. Starting Off With a Sha-Bang

2.1. Invoking the script
2.2. Preliminary Exercises

Chapter 1. Why 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.

Writing shell scripts is not hard to learn, since the 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 and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules" to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward.

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 shell script is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, or Perl.

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.

When not to use shell scripts

Resource-intensive tasks, especially where speed is a factor (sorting, hashing, etc.)
Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead)
Cross-platform portability required (use C or Java instead)
Complex applications, where structured programming is a necessity (need type-checking of variables, function prototypes, etc.)
Mission-critical applications upon which you are betting the ranch, or the future of the company
Situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism
Project consists of subcomponents with interlocking dependencies
Extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion)
Need native support for multi-dimensional arrays
Need data structures, such as linked lists or trees
Need to generate or manipulate graphics or GUIs
Need direct access to system hardware
Need port or socket I/O
Need to use libraries or interface with legacy code
Proprietary, closed-source applications (shell scripts put the source code right out in the open for all the world to see)

If any of the above applies, consider a more powerful scripting language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a high-level compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step.

We will be using Bash, an acronym 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 all 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, [2] 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 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 was 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), [3] 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 , , or 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, of this book wrote the example scripts that follow.

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 the log files in /var/log

# Cleanup
# Run as root, of course.

cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."

There is nothing unusual here, only a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script becomes a tool, and 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.

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=66       # Can't change directory?
E_NOTROOT=67   # 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 if command line argument 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=65  # Non-numerical argument (bad arg format)
#
#    case "$1" in
#    ""      ) lines=50;;
#    *[!0-9]*) echo "Usage: `basename $0` file-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 # Saves last section of message log file.
mv mesg.temp messages               # Becomes new log directory.


# 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 "Logs cleaned up."

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 refining previously written scripts for increased effectiveness.

The sha-bang ( #!) 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 [4] 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 (line following the sha-bang line), ignoring comments. [5]

#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/usr/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. [6] 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 [7] 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.

#! 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. [8] 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 can 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=65
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 of this. 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, [9] or alternatively bash scriptname. (Not recommended is using sh , 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) [10]

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. [11] 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 system-wide 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

Table of Contents

3. Special Characters

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

5. Quoting

5.1. Quoting Variables
5.2. Escaping

6. Exit and Exit Status

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

8. Operations and Related Topics

8.1. Operators
8.2. Numerical Constants

Chapter 3. Special Characters

Special Characters Found In Scripts and Elsewhere

#

Comments. Lines beginning with a # (with the exception of #!) are comments.

# 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.

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, 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 that "if" and "then" need separation.
                               # Why?
  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

.

"dot" command [period]. Equivalent to source (see Example 14-21). This is a bash builtin.

.

"dot", as a component of a filename. When working with filenames, a 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.

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

.

"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 also Chapter 5.

'

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

,

comma operator. The comma operator 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"

\

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).

:
echo $?   # 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
   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...

Provide a placeholder where a command is expected in a here document. See Example 18-10.

Evaluate string of variables using parameter substitution (as in Example 9-15).

: ${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 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.

bash$ variable=23


bash$ echo variable
variable

bash$ echo $variable
23

The only time a variable appears "naked" -- without the $ prefix -- is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see Example 29-5). 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 alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see Section 9.3, below).

Example 4-1. Variable assignment and substitution

#!/bin/bash

# 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    # Not a variable reference, just the string "hello".

echo $hello
echo ${hello} # Identical to above.

echo "$hello"
echo "${hello}"

echo

hello="A B  C   D"
echo $hello   # A B C D
echo "$hello" # A B  C   D
# As you see, echo $hello   and   echo "$hello"   give different results.
# =======================================
# Quoting a variable preserves whitespace.
# =======================================

echo

echo '$hello'  # $hello
#    ^      ^
#  Variable referencing disabled by single quotes,
#+ which causes the "$" to be interpreted literally.

# Notice the effect of different types of quoting.


hello=    # Setting it to a null value.
echo "\$hello (null value) = $hello"
#  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 older 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"
                                # It still has a null value.
echo

exit 0

An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will usually 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 acts as if it were 0 in an arithmetic operation.
#  This is undocumented (and probably non-portable) behavior.

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.

a=`ls -l`         # Assigns result of 'ls -l' command to 'a'
echo $a           # Unquoted, however, removes tabs and newlines.
echo
echo "$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 actually 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 integer 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: Integer or string?

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

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=""
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.



# Variables in Bash are essentially untyped.

exit 0

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

The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for you.

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 hold information 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

(Thank you, St閜hane 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: $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. [16] 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}
# Or:       lastarg=${!#}
#           (Thanks, Chris Monson.)
# Note that lastarg=${!$#} 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. There must also exist symbolic links to all the alternate names of the script. See Example 15-2.

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-cw
# ln -s /usr/local/bin/wh /usr/local/bin/wh-radb

E_NOARGS=65


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.ripe.net;;
    "wh-ripe") whois $1@whois.ripe.net;;
    "wh-radb") whois $1@whois.radb.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
# Using 'shift' to step through all the positional parameters.

#  Name this script something like shft,
#+ and invoke it with some parameters, for example
#          ./shft a b c def 23 skidoo

until [ -z "$1" ]  # Until all parameters used up...
do
  echo -n "$1 "
  shift
done

echo               # Extra line feed.

exit 0

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

The shift command works in a similar fashion on parameters passed to a function. See Example 33-15.

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, such as the wild card character -- *.)

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. [17]

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

bash$ echo $(ls -l)
total 8 -rw-rw-r-- 1 bozo bozo 130 Aug 21 12:57 t222.sh -rw-rw-r-- 1 bozo bozo 78 Aug 21 12:57 t71.sh


bash$ echo "$(ls -l)"
total 8
 -rw-rw-r--  1 bozo bozo 130 Aug 21 12:57 t222.sh
 -rw-rw-r--  1 bozo bozo  78 Aug 21 12:57 t71.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 -- the variable name [18] -- except $, ` (backquote), and \ (escape). [19] 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. [20] An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

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閜hane 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.

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.

exit 0

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閜hane 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 0xx

Example 5-2. Escaped Characters

#!/bin/bash
# escaped.sh: escaped characters

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 BEEP"
echo $'\n'           # Newline.
echo $'\a'           # Alert (beep).

echo "==============="
echo "QUOTATION MARKS"
# Version 2 and later of Bash permits using the $'\nnn' construct.
# Note that in this case, '\nnn' is an octal value.
echo $'\t \042 \t'   # Quote (") framed by tabs.

# 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 "==============="
echo


# Assigning ASCII characters to a variable.
# ----------------------------------------
quote=$'\042'        # " assigned to a variable.
echo "$quote This is a 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; echo

escape=$'\033'                    # 033 is octal for escape.
echo "\"escape\" echoes as $escape"
#                                   no visible output.

echo; echo

exit 0

See Example 34-1 for another example of the $' ' string expansion construct.

\"

gives the quote its literal meaning

echo "Hello"                  # Hello
echo "\"Hello\", he said."    # "Hello", he said.

\$

gives the dollar sign its literal meaning (variable name following \$ will not be referenced)

echo "\$variable01"  # results in $variable01

\\

gives the backslash its literal meaning

echo "\\"  # Results in \

# Whereas . . .

echo "\"   # Invokes secondary prompt from the command line.
           # In a script, gives an error message.

The behavior of \ depends on whether it is itself escaped, quoted, or appearing within command substitution or a here document.

                      #  Simple escaping and quoting
echo \z               #  z
echo \\z              # \z
echo '\z'             # \z
echo '\\z'            # \\z
echo "\z"             # \z
echo "\\z"            # \z

                      #  Command substitution
echo `echo \z`        #  z
echo `echo \\z`       #  z
echo `echo \\\z`      # \z
echo `echo \\\\z`     # \z
echo `echo \\\\\\z`   # \z
echo `echo \\\\\\\z`  # \\z
echo `echo "\z"`      # \z
echo `echo "\\z"`     # \z

                      # Here document
cat <

Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable.

variable=\
echo "$variable"
# Will not work - gives an error message:
# test.sh: : command not found
# A "naked" escape cannot safely be assigned to a variable.
#
#  What actually happens here is that the "\" escapes the newline and
#+ the effect is        variable=echo "$variable"
#+                      invalid variable assignment

variable=\
23skidoo
echo "$variable"        #  23skidoo
                        #  This works, since the second line
                        #+ is a valid variable assignment.

variable=\ 
#        \^    escape followed by space
echo "$variable"        # space

variable=\\
echo "$variable"        # \

variable=\\\
echo "$variable"
# Will not work - gives an error message:
# test.sh: \: command not found
#
#  First escape escapes second one, but the third one is left "naked",
#+ with same result as first instance, above.

variable=\\\\
echo "$variable"        # \\
                        # Second and fourth escapes escaped.
                        # This is o.k.

Escaping a space can prevent word splitting in a command's argument list.

file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"
# List of files as argument(s) to a command.

# Add two files to the list, and list all.
ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list

echo "-------------------------------------------------------------------------"

# What happens if we escape a couple of spaces?
ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
# Error: the first three files concatenated into a single argument to 'ls -l'
#        because the two escaped spaces prevent argument (word) splitting.

The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line.

(cd /source/directory && tar cf - . ) | \
(cd /dest/directory && tar xpvf -)
# Repeating Alan Cox's directory tree copy command,
# but split into two lines for increased legibility.

# As an alternative:
tar cf - -C /source/directory . |
tar xpvf - -C /dest/directory
# See note below.
# (Thanks, St閜hane Chazelas.)

If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line.

echo "foo
bar" 
#foo
#bar

echo

echo 'foo
bar'    # No difference yet.
#foo
#bar

echo

echo foo\
bar     # Newline escaped.
#foobar

echo

echo "foo\
bar"     # Same here, as \ still interpreted as escape within weak quotes.
#foobar

echo

echo 'foo\
bar'     # Escape character \ taken literally because of strong quoting.
#foo\
#bar

# Examples suggested by St閜hane Chazelas.

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 may be used to terminate 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 ). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually may 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 a decimal number 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

. . .

# Will exit with status of last command.
COMMAND_LAST

exit

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

#!/bin/bash

COMMAND_1

. . .

# Will exit with status of last command.
COMMAND_LAST

exit $?

#!/bin/bash

COMMAND1

. . . 

# Will exit with status of last command.
COMMAND_LAST

$? 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." 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 because 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.

$? is especially useful for testing the result of a command in a script (see Example 15-32 and Example 15-17).

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.
#    !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).

# Thanks, St閜hane 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 also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons.
let "1<2" returns 0 (as "1<2" expands to "1") (( 0 && 1 )) returns 1 (as "0 && 1" expands to "0")

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

An if/then construct can contain nested comparisons and tests.

if echo "Next *if* is part of the comparison for the first *if*."

  if [[ $comparison = "integer" ]]
    then (( a < b ))
  else
    [[ $a < $b ]]
  fi

then
  echo '$a is less than $b'
fi

This detailed "if-test" explanation courtesy of St閜hane Chazelas.

Example 7-1. What is truth?

#!/bin/bash

#  Tip:
#  If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.

echo

echo "Testing \"0\""
if [ 0 ]      # zero
then
  echo "0 is true."
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
   # Optional (may be left out if not needed).
   # 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 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

The [[ ]] construct is the more versatile Bash version of [ ]. This is the extended test command, adopted from ksh88.

No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution.

file=/etc/passwd

if [[ -e $file ]]
then
  echo "Password file exists."
fi

Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

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
# 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 ))                                      # 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.

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," [21] 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 (floppy, cdrom, etc.)

-c

file is a character device (keyboard, modem, sound card, etc.)

-p

file is a pipe

-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. [22] 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. [23] 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.

	      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, he can only delete files in it that he owns. 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 
# Used 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. 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 0

Example 28-1, Example 10-7, Example 10-3, Example 28-3, 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" ]

==

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閜hane 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 26-11 for an application of this comparison operator.

-z

string is "null," that is, has zero length

-n

string is not "null."

The -n test absolutely 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. [24]

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


# Lets 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, $string1 stands naked.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi  
# This works fine.
# The [ ] test operator alone detects whether the string is null.
# However it is good practice to quote it ("$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, $string1 stands naked.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi  
# Again, gives correct result.
# Still, it is better to quote it ("$string1"), because . . .


string1="a = b"

if [ $string1 ]       # Again, $string1 stands naked.
then
  echo "String \"string1\" is not null."
else  
  echo "String \"string1\" is null."
fi  
# Not quoting "$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'

NOARGS=65
NOTFOUND=66
NOTGZIP=67

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 $NOARGS
  # Returns 65 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 $NOTFOUND
fi  

if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
  echo "File $1 is not a gzipped file!"
  exit $NOTGZIP
fi  

zcat $1 | more

# Uses the filter 'more.'
# 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 are 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 [ "$exp1" -a "$exp2" ]

Refer to Example 8-3, Example 26-16, 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 above.

if [ condition1 ]
then
  if [ condition2 ]
  then
    do-something  # But only if both "condition1" and "condition2" valid.
  fi  
fi

See Example 34-4 for an example of 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"
echo "z = $z"   # z = 125

%

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

bash$ expr 5 % 3
2

5/3 = 1 with remainder 2

This operator finds use in, among other things, generating numbers within a specific range (see Example 9-25 and Example 9-28) and formatting program output (see Example 26-15 and Example A-6). It can even be used to generate prime numbers, (see Example A-16). Modulo turns up surprisingly often in various 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=65

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 uninitialized variable used in loop,
                                 #+ it results in an error message
                                 #+ on the first pass through loop.

  until [ "$remainder" -eq 0 ]
  do
    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


# Exercise :
# --------
#  Check command-line arguments to make sure they are integers,
#+ and exit the script with an appropriate error message if not.

exit 0

+=

"plus-equal" (increment variable by a constant)

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 Bash are actually signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that takes a variable outside these limits will give an erroneous result.

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)

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 run fast enough to permit its use on the fly.

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 negate

!

bitwise NOT

^

bitwise XOR

^=

"bitwise XOR-equal"

logical operators

&&

and (logical)

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 within [ ... ] construct.

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

||

or (logical)

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 within [ ... ] 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), but only the last operation is returned.

let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1"               # t1 = 11

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 10-12.

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
# 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 0       # Thanks, Rich Bartell and Stephane Chazelas, for clarification.

Part 3. Beyond the Basics

Table of Contents

9. Variables Revisited

9.1. Internal Variables
9.2. Manipulating Strings
9.3. Parameter Substitution
9.4. Typing variables: declare or typeset
9.5. Indirect References to Variables
9.6. $RANDOM: generate random integer
9.7. The Double Parentheses Construct

10. Loops and Branches

10.1. Loops
10.2. Nested Loops
10.3. Loop Control
10.4. Testing and Branching

11. Command Substitution

12. Arithmetic Expansion

13. Recess Time

Chapter 9. Variables Revisited

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 20-1 for usage.

$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
3.00.14(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.

$DIRSTACK

the top value in the directory stack (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.

$GLOBIGNORE

A list of filename patterns to be excluded from matching in globbing.

$GROUPS

groups current user belongs to

This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd.

root# echo $GROUPS
0


root# echo ${GROUPS[1]}
1


root# echo ${GROUPS[5]}
6

$HOME

home directory of the user, usually /home/username (see Example 9-15)

$HOSTNAME

The hostname command assigns the system name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 9-15.

$HOSTTYPE

host type

Like $MACHTYPE, identifies the system hardware.

bash$ echo $HOSTTYPE
i686

$IFS

internal field separator

This variable determines how Bash recognizes fields, or word boundaries when it interprets character strings.

$IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS. See Example 5-1.

bash$ echo $IFS | cat -vte
$
(Show tabs and display "$" at end-of-line.)



bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
(Read commands from string and assign any arguments to pos params.)

$IFS does not handle whitespace the same as it does other characters.

Example 9-1. $IFS and whitespace

#!/bin/bash
# $IFS treats whitespace differently than other characters.

output_args_one_per_line()
{
  for arg
  do echo "[$arg]"
  done
}

echo; echo "IFS=\" \""
echo "-------"

IFS=" "
var=" a  b c   "
output_args_one_per_line $var  # output_args_one_per_line `echo " a  b c   "`
#
# [a]
# [b]
# [c]


echo; echo "IFS=:"
echo "-----"

IFS=:
var=":a::b:c:::"               # Same as above, but substitute ":" for " ".
output_args_one_per_line $var
#
# []
# [a]
# []
# [b]
# [c]
# []
# []
# []

# The same thing happens with the "FS" field separator in awk.

# Thank you, Stephane Chazelas.

echo

exit 0

(Thanks, S. C., for clarification and examples.)

See also Example 15-37, Example 10-7, and Example 18-14 for instructive examples of using $IFS.

$IGNOREEOF

ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.

$LC_COLLATE

Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in filename globbing.

As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, ls [A-M]* would match both File1.txt and file1.txt. To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc.

$LC_CTYPE

This internal variable controls character interpretation in globbing and pattern matching.

$LINENO

This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes.

# *** BEGIN DEBUG BLOCK ***
last_cmd_arg=$_  # Save it.

echo "At line number $LINENO, variable \"v1\" = $v1"
echo "Last command argument processed = $last_cmd_arg"
# *** END DEBUG BLOCK ***

$MACHTYPE

machine type

Identifies the system hardware.

bash$ echo $MACHTYPE
i686

$OLDPWD

old working directory ("OLD-print-working-directory", previous directory you were in)

$OSTYPE

operating system type

bash$ echo $OSTYPE
linux

$PATH

path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc.

When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, $PATH, a list of directories, separated by colons. Normally, the system stores the $PATH definition in /etc/profile and/or ~/.bashrc (see Appendix G).

bash$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original $PATH (a child process, such as a script, may not change the environment of the parent process, the shell).

The current "working directory", ./, is usually omitted from the $PATH as a security measure.

$PIPESTATUS

Array variable holding exit status(es) of last executed foreground pipe. Interestingly enough, this does not necessarily give the same result as the exit status of the last executed command.

bash$ echo $PIPESTATUS
0

bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $PIPESTATUS
141

bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127

The members of the $PIPESTATUS array hold the exit status of each respective command executed in a pipe. $PIPESTATUS[0] holds the exit status of the first command in the pipe, $PIPESTATUS[1] the exit status of the second command, and so on.

The $PIPESTATUS variable may contain an erroneous 0 value in a login shell (in releases prior to 3.0 of Bash).

tcsh% bash

bash$ who | grep nobody | sort
bash$ echo ${PIPESTATUS[*]}
0

The above lines contained in a script would produce the expected 0 1 0 output.

Thank you, Wayne Pollock for pointing this out and supplying the above example.

The $PIPESTATUS variable gives unexpected results in some contexts.

bash$ echo $BASH_VERSION
3.00.14(1)-release

bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
 0       0       0

bash$ echo ${PIPESTATUS[@]}
141 127 0

Chet Ramey attributes the above output to the behavior of ls. If ls writes to a pipe whose output is not read, then SIGPIPE kills it, and its exit status is 141. Otherwise its exit status is 0, as expected. This likewise is the case for tr.

$PIPESTATUS is a "volatile" variable. It needs to be captured immediately after the pipe in question, before any other command intervenes.

bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
 0       0       0

bash$ echo ${PIPESTATUS[@]}
0 127 0

bash$ echo ${PIPESTATUS[@]}
0

The pipefail option may be useful in cases where $PIPESTATUS does not give the desired information.

$PPID

The $PPID of a process is the process ID (pid) of its parent process. [25]

Compare this with the pidof command.

$PROMPT_COMMAND

A variable holding a command to be executed just before the primary prompt, $PS1 is to be displayed.

$PS1

This is the main prompt, seen at the command line.

$PS2

The secondary prompt, seen when additional input is expected. It displays as ">".

$PS3

The tertiary prompt, displayed in a select loop (see Example 10-29).

$PS4

The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x option. It displays as "+".

$PWD

working directory (directory you are in at the time)

This is the analog to the pwd builtin command.

#!/bin/bash

E_WRONG_DIRECTORY=73

clear # Clear screen.

TargetDirectory=/home/bozo/projects/GreatAmericanNovel

cd $TargetDirectory
echo "Deleting stale files in $TargetDirectory."

if [ "$PWD" != "$TargetDirectory" ]
then    # Keep from wiping out wrong directory by accident.
  echo "Wrong directory!"
  echo "In $PWD, rather than $TargetDirectory!"
  echo "Bailing out!"
  exit $E_WRONG_DIRECTORY
fi  

rm -rf *
rm .[A-Za-z0-9]*    # Delete dotfiles.
# rm -f .[^.]* ..?*   to remove filenames beginning with multiple dots.
# (shopt -s dotglob; rm -f *)   will also work.
# Thanks, S.C. for pointing this out.

# Filenames may contain all characters in the 0 - 255 range, except "/".
# Deleting files beginning with weird characters is left as an exercise.

# Various other operations here, as necessary.

echo
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo


exit 0

$REPLY

The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself.

#!/bin/bash
# reply.sh

# REPLY is the default value for a 'read' command.

echo
echo -n "What is your favorite vegetable? "
read

echo "Your favorite vegetable is $REPLY."
#  REPLY holds the value of last "read" if and only if
#+ no variable supplied.

echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of \$REPLY is still $REPLY."
#  $REPLY is still set to its previous value because
#+ the variable $fruit absorbed the new "read" value.

echo

exit 0

$SECONDS

The number of seconds the script has been running.

#!/bin/bash

TIME_LIMIT=10
INTERVAL=1

echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo

while [ "$SECONDS" -le "$TIME_LIMIT" ]
do
  if [ "$SECONDS" -eq 1 ]
  then
    units=second
  else  
    units=seconds
  fi

  echo "This script has been running $SECONDS $units."
  #  On a slow or overburdened machine, the script may skip a count
  #+ every once in a while.
  sleep $INTERVAL
done

echo -e "\a"  # Beep!

exit 0

$SHELLOPTS

the list of enabled shell options, a readonly variable

bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs

$SHLVL

Shell level, how deeply Bash is nested. If, at the command line, $SHLVL is 1, then in a script it will increment to 2.

This variable is not affected by subshells. Use $BASH_SUBSHELL when you need an indication of subshell nesting.

$TMOUT

If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after $time seconds. This will cause a logout.

As of version 2.05b of Bash, it is now possible to use $TMOUT in a script in combination with read.

# Works in scripts for Bash, versions 2.05b and later.

TMOUT=3    # Prompt times out at three seconds.

echo "What is your favorite song?"
echo "Quickly now, you only have $TMOUT seconds to answer!"
read song

if [ -z "$song" ]
then
  song="(no answer)"
  # Default response.
fi

echo "Your favorite song is $song."

There are other, more complex, ways of implementing timed input in a script. One alternative is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see Example 29-5) the interrupt generated by the timing loop (whew!).

Example 9-2. Timed Input

#!/bin/bash
# timed-input.sh

# TMOUT=3    Also works, as of newer versions of Bash.


TIMELIMIT=3  # Three seconds in this instance. May be set to different value.

PrintAnswer()
{
  if [ "$answer" = TIMEOUT ]
  then
    echo $answer
  else       # Don't want to mix up the two instances. 
    echo "Your favorite veggie is $answer"
    kill $!  # Kills no longer needed TimerOn function running in background.
             # $! is PID of last job running in background.
  fi

}  



TimerOn()
{
  sleep $TIMELIMIT && kill -s 14 $$ &
  # Waits 3 seconds, then sends sigalarm to script.
}  

Int14Vector()
{
  answer="TIMEOUT"
  PrintAnswer
  exit 14
}  

trap Int14Vector 14   # Timer interrupt (14) subverted for our purposes.

echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer


#  Admittedly, this is a kludgy implementation of timed input,
#+ however the "-t" option to "read" simplifies this task.
#  See "t-out.sh", below.

#  If you need something really elegant...
#+ consider writing the application in C or C++,
#+ using appropriate library functions, such as 'alarm' and 'setitimer'.

exit 0

An alternative is using stty.

Example 9-3. Once more, timed input

#!/bin/bash
# timeout.sh

#  Written by Stephane Chazelas,
#+ and modified by the document author.

INTERVAL=5                # timeout interval

timedout_read() {
  timeout=$1
  varname=$2
  old_tty_settings=`stty -g`
  stty -icanon min 0 time ${timeout}0
  eval read $varname      # or just  read $varname
  stty "$old_tty_settings"
  # See man page for "stty".
}

echo; echo -n "What's your name? Quick! "
timedout_read $INTERVAL your_name

#  This may not work on every terminal type.
#  The maximum timeout depends on the terminal.
#+ (it is often 25.5 seconds).

echo

if [ ! -z "$your_name" ]  # If name input before timeout...
then
  echo "Your name is $your_name."
else
  echo "Timed out."
fi

echo

# The behavior of this script differs somewhat from "timed-input.sh".
# At each keystroke, the counter resets.

exit 0

Perhaps the simplest method is using the -t option to read.

Example 9-4. Timed read

#!/bin/bash
# t-out.sh
# Inspired by a suggestion from "syngin seven" (thanks).


TIMELIMIT=4         # 4 seconds

read -t $TIMELIMIT variable <&1
#                           ^^^
#  In this instance, "<&1" is needed for Bash 1.x and 2.x,
#  but unnecessary for Bash 3.x.

echo

if [ -z "$variable" ]  # Is null?
then
  echo "Timed out, variable still unset."
else  
  echo "variable = $variable"
fi  

exit 0

$UID

user ID number

current user's user identification number, as recorded in /etc/passwd

This is the current user's real id, even if she has temporarily assumed another identity through su. $UID is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin.

Example 9-5. Am I root?

#!/bin/bash
# am-i-root.sh:   Am I root or not?

ROOT_UID=0   # Root has $UID 0.

if [ "$UID" -eq "$ROOT_UID" ]  # Will the real "root" please stand up?
then
  echo "You are root."
else
  echo "You are just an ordinary user (but mom loves you just the same)."
fi

exit 0


# ============================================================= #
# Code below will not execute, because the script already exited.

# An alternate method of getting to the root of matters:

ROOTUSER_NAME=root

username=`id -nu`              # Or...   username=`whoami`
if [ "$username" = "$ROOTUSER_NAME" ]
then
  echo "Rooty, toot, toot. You are root."
else
  echo "You are just a regular fella."
fi

9.2. 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

expr "$string" : '.*'

stringZ=abcABC123ABCabc

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

Example 9-10. Inserting a blank line between paragraphs in a text file

#!/bin/bash
# paragraph-space.sh

# Inserts a blank line between paragraphs of a single-spaced text file.
# Usage: $0

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
#       |------|

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
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, [27] 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.

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}

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

${string##substring}

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

stringZ=abcABC123ABCabc
#       |----|
#       |----------|

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'.

${string%substring}

Strips 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}

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

stringZ=abcABC123ABCabc
#                    ||
#        |------------|

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 9-11. 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.

Example 9-12. 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
#      Appropriate codecs may need to be installed for this script to work.
# Uses the "ogg" library and "oggenc":
#      http://www.xiph.org/


OFILEPREF=${1%%ra}      # Strip off the "ra" suffix.
OFILESUFF=wav           # Suffix for wav file.
OUTFILE="$OFILEPREF""$OFILESUFF"
E_NOARGS=65

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 audio file, the *.ra 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.

A simple emulation of getopt using substring extraction constructs.

Example 9-13. 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

---

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.

${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'.

${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'.

9.2.1. 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 9-14. Alternate ways of extracting 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.

exit 0

9.2.2. Further Discussion

For more on string manipulation in scripts, refer to Section 9.3 and the relevant section of the expr command listing. For script examples, see:

9.3. 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.

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".
#
#  Commands follow.

See also Example 3-4, Example 28-2, 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, [28] as above.

echo ${username=`whoami`}
# Variable "username" is now set to `whoami`.

${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.

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

Example 9-15. 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."}
#  If 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 9-16. 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 15-9). 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 9-17. 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}

Remove from $var the shortest/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}

Remove from $var the shortest/longest part of $Pattern that matches the back end of $var.

Version 2 of Bash added additional options.

Example 9-18. 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 9-19. 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-14 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 9-20. 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 9-21. 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 all previously declared variables beginning with varprefix.

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

# Bash, version 2.04, adds this feature.

9.4. Typing variables: declare or typeset

The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing 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

(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.

-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

declare -a indices

The variable indices will be treated as an array.

-f functions

declare -f

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

declare -f function_name

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

-x export

declare -x var3

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

-x var=$value

declare -x var3=373

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

Example 9-22. 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.

9.5. Indirect References to Variables

Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation.

Example 9-23. Indirect References

#!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.

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
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.)


# Another method is the ${!t} notation, discussed in "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 9-24. 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=65

if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
then
   echo "Usage: `basename $0` filename column-number"
   exit $E_WRONGARGS
fi

filename=$1
column_number=$2

#===== 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"
# ------------------------------------------------
# End awk script.

#  Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
#  Thanks, Stephane Chazelas.


exit 0

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 34-2 and Example A-23) makes indirect referencing more intuitive.

9.6. $RANDOM: generate random integer

$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom [29] integer in the range 0 - 32767. It should not be used to generate an encryption key.

Example 9-25. 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 9-26. 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

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 9-27. 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.
# 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    "Expects up to 3 passed parameters, but all are completely optional."
      echo    "min is the minimum value"
      echo    "max is the maximum value"
      echo    "multiple specifies that the answer must be 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    "Successful completion returns 0, unsuccessful completion returns"
      echo    "function syntax and 1."
      echo    "The answer is returned in the global variable randomBetweenAnswer"
      echo    "Negative values for any passed parameter are 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.

displacement=$((0-minimum))
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
   answer[i+displacement]=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+displacement]=$((answer[randomBetweenAnswer+displacement]+1))
done



# Let's check the results

for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
   [ ${answer[i+displacement]} -eq 0 ] && echo "We never got an answer of $i." || echo "${i} occurred ${answer[i+displacement]} 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 9-28. 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         # Throw count.

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) let "ones += 1";;   # Since die has no "zero", this corresponds to 1.
  1) let "twos += 1";;   # And this to 2, etc.
  2) let "threes += 1";;
  3) let "fours += 1";;
  4) let "fives += 1";;
  5) let "sixes += 1";;
esac
}

echo


while [ "$throw" -lt "$MAXTHROWS" ]
do
  let "die1 = RANDOM % $PIPS"
  update_count $die1
  let "throw += 1"
done  

print_result

exit 0

#  The scores should distribute fairly evenly, assuming RANDOM is fairly 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. [30] (This mirrors the behavior of the random() function in C.)

Example 9-29. Reseeding RANDOM

#!/bin/bash
# seeding-random.sh: Seeding the RANDOM variable.

MAXCOUNT=25       # How many numbers to generate.

random_numbers ()
{
count=0
while [ "$count" -lt "$MAXCOUNT" ]
do
  number=$RANDOM
  echo -n "$number "
  let "count += 1"
done  
}

echo; echo

RANDOM=1          # Setting RANDOM seeds the random number generator.
random_numbers

echo; echo

RANDOM=1          # Same seed for RANDOM...
random_numbers    # ...reproduces the exact same number series.
                  #
                  # When is it useful to duplicate a "random" number series?

echo; echo

RANDOM=2          # Trying again, but with a different 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 $2 }')
#  Pseudo-random output fetched
#+ from /dev/urandom (system pseudo-random device-file),
#+ then converted to line of printable (octal) numbers by "od",
#+ finally "awk" retrieves just one number for SEED.
RANDOM=$SEED
random_numbers

echo; echo

exit 0

The /dev/urandom 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 and Example 15-13), or using dd (see Example 15-55), or even piping to md5sum (see Example 33-14).

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

Example 9-30. Pseudorandom numbers, using awk

#!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 - 1.
# 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 fail to do this?)

# 2) Using an integer multiplier as a scaling factor, generate random numbers 
#+   in the range between 10 and 100.

# 3) Same as exercise #2, above, but generate random integers this time.

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

9.7. 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-type manipulation of variables in Bash.

Example 9-31. C-type manipulation of variables

#!/bin/bash
# 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"

(( a++ ))     # Post-increment 'a', C-style.
echo "a (after a++) = $a"

(( a-- ))     # Post-decrement 'a', C-style.
echo "a (after a--) = $a"


(( ++a ))     # Pre-increment 'a', C-style.
echo "a (after ++a) = $a"

(( --a ))     # Pre-decrement 'a', C-style.
echo "a (after --a) = $a"

echo

########################################################
#  Note that, as in C, pre- and post-decrement operators
#+ have slightly 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."
echo "t = $t "        # Yes!

echo


# -----------------
# Easter Egg alert!
# -----------------
#  Chet Ramey apparently 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 now out.

# See also "for" and "while" loops using the ((...)) construct.

# These work only with Bash, version 2.04 or later.

exit 0

Chapter 10. Loops and Branches

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

10.1. Loops

A loop is a block of code that iterates [31] 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
燾ommand(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 10-1. 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

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.
do
  echo $planet
done

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 14-15) to force parsing of each [list] element and assignment of each component to the positional parameters.

Example 10-2. 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 10-3. 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 $9 "         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

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

Example 10-4. Operating on files with a for loop

#!/bin/bash
# list-glob.sh: Generating [list] in a for-loop, using "globbing"

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-16. See also Example 14-16.

Example 10-5. 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 15-49, Example 10-10 and Example 15-43.

Example 10-6. 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 10-7. 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 10-8. 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               ^^^^^^^^^^^^^^^^^
do
  echo "USER #$n = $name"
  let "n += 1"
done  


# USER #1 = root
# USER #2 = bin
# USER #3 = daemon
# ...
# USER #30 = bozo

exit 0

#  Exercise:
#  --------
#  How is it that an ordinary user (or a script run by same)
#+ can read /etc/passwd?
#  Isn't this a security hole? Why or why not?

A final example of the [list] resulting from command substitution.

Example 10-9. Checking all the binaries in a directory for authorship

#!/bin/bash
# findstring.sh:
# Find a particular string in 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 0

#  Exercise (easy):
#  ---------------
#  Convert this script to take command-line parameters
#+ for $directory and $fstring.

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

Example 10-10. 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.
#  Even this will only pick up the first field of each argument.

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

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

Example 10-11. 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.

exit 0

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

Example 10-12. A C-like for loop

#!/bin/bash
# Two 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

# +==========================================+

# Now, let's do the same, using C-like syntax.

LIMIT=10

for ((a=1; a <= LIMIT ; a++))  # Double parentheses, and "LIMIT" with no "$".
do
  echo -n "$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++))  # The comma chains together operations.
do
  echo -n "$a-$b "
done

echo; echo

exit 0

See also Example 26-15, Example 26-16, and Example A-6.

---

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

Example 10-13. Using efax in batch mode

#!/bin/bash
# Faxing (must have 'efax' package installed).

EXPECTED_ARGS=2
E_BADARGS=65

if [ $# -ne $EXPECTED_ARGS ]
# Check for proper no. 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 /dev/ttyS3 -o1 -t "T$1" $fil   # Finally, do the work.


#  As S.C. points out, the for-loop can be eliminated with
#     efax -d /dev/ttyS3 -o1 -t "T$1" $2.0*
#+ but it's not quite as instructive [grin].

exit 0

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
燾ommand(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 certain specialized while loops, as, for example, a getopts construct, deviate somewhat from the standard template given here.

Example 10-14. 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 10-15. 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 10-16. 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 last 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-like syntax by using the double parentheses construct (see also Example 9-31).

Example 10-17. C-like syntax in a while loop

#!/bin/bash
# wh-loopc.sh: Count to 10 in a "while" loop.

LIMIT=10
a=1

while [ "$a" -le $LIMIT ]
do
  echo -n "$a "
  let "a+=1"
done           # No surprises, so far.

echo; echo

# +=================================================================+

# Now, 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, and no "$" preceding variables.
do
  echo -n "$a "
  ((a += 1))   # let "a+=1"
  # Yes, indeed.
  # Double parentheses permit incrementing a variable with C-like syntax.
done

echo

# Now, C programmers can feel right at home in Bash.

exit 0

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
燾ommand(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 10-18. 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  

exit 0

10.2. 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 10-19. 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 26-11 for an illustration of nested while loops, and Example 26-13 to see a while loop nested inside an until loop.

10.3. Loop Control

Commands Affecting Loop Behavior

break, continue

The break and continue loop control commands [32] 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 (repetition) of the loop, skipping all the remaining commands in that particular loop cycle.

Example 10-20. 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 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 10-21. 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 10-22. 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 ]
    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 never echo.
  done  
  # --------------------------------------------------------------------

done

echo; echo

# Exercise:
# Come up with a meaningful use for "continue N" in a script.

exit 0

Example 10-23. 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

#  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.

10.4. 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" )
?TT CLASS="REPLACEABLE" >command...
?;

?$condition2" )
?TT CLASS="REPLACEABLE" >command...
?;

esac

Quoting the variables is not mandatory, since word splitting does not take place.
Each test line ends with a right paren ).
Each condition block ends with a double semicolon ;;.
The entire case block terminates with an esac (case spelled backwards).

Example 10-24. 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 10-25. 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 Floppy 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##*/} "; 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 an 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 10-26. 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 10-27. Simple string matching

#!/bin/bash
# match-string.sh: simple string matching

match_string ()
{
  MATCH=0
  NOMATCH=90
  PARAMS=2     # Function requires 2 arguments.
  BAD_PARAMS=91

  [ $# -eq $PARAMS ] || return $BAD_PARAMS

  case "$1" in
  "$2") return $MATCH;;
  *   ) return $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 10-28. Checking for alphabetic input

#!/bin/bash
# isalpha.sh: Using a "case" structure to filter a string.

SUCCESS=0
FAILURE=-1

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
?TT CLASS="REPLACEABLE" >command...
燽reak
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 that this may be changed.

Example 10-29. Creating menus using select

#!/bin/bash

PS3='Choose your favorite vegetable: ' # Sets the prompt string.

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 0

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

Compare this to the behavior of a

for variable [in list]

construct with the in list omitted.

Example 10-30. 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 tomatoes spinach
#         $1    $2   $3      $4       $5       $6
#         passed to choice_of() function

exit 0

Chapter 11. Command Substitution

Command substitution reassigns the output of a command [33] or even multiple commands; it literally plugs the command output into another context. [34]

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.

Thanks, S.C.

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=`

#  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 11-1. Stupid script tricks

#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.


dangerous_variable=`cat /boot/vmlinuz`   # The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (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 11-2. 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
#+ within a variable declaration.

exit 0

The $(COMMAND) 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. [35]

word_count=$( wc -w $(ls -l | awk '{print $9}') )

Or, for something a bit more elaborate . . .

Example 11-3. 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://personal.riverusers.com/~thegrendel/yawl-0.3.2.tar.gz

E_NOARGS=66
E_BADARG=67
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 example of anagram finding.

exit $?

Examples of command substitution in shell scripts:

Chapter 12. 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:

Chapter 13. 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

Mastering the commands on your Linux machine is an indispensable prelude to writing effective shell scripts.

This section covers the following commands:

. (See also source)
ac
adduser
agetty
agrep
ar
arch
at
autoload
awk (See also Using awk for math operations)
badblocks
banner
basename
batch
bc
bg
bind
bison
builtin
bzgrep
bzip2
cal
caller
cat
cd
chattr
chfn
chgrp
chkconfig
chmod
chown
chroot
cksum
clear
clock
cmp
col
colrm
column
comm
command
compress
cp
cpio
cron
crypt
csplit
cu
cut
date
dc
dd
debugfs
declare
depmod
df
dialog
diff
diff3
diffstat
dig
dirname
dirs
disown
dmesg
doexec
dos2unix
du
dump
dumpe2fs
e2fsck
echo
egrep
enable
enscript
env
eqn
eval
exec
exit (Related topic: exit status)
expand
export
expr
factor
false
fdformat
fdisk
fg
fgrep
file
find
finger
flex
flock
fmt
fold
free
fsck
ftp
fuser
getopt
getopts
gettext
getty
gnome-mount
grep
groff
groupmod
groups (Related topic: the $GROUPS variable)
gs
gzip
halt
hash
hdparm
head
help
hexdump
host
hostid
hostname (Related topic: the $HOSTNAME variable)
hwclock
iconv
id (Related topic: the $UID variable)
ifconfig
info
infocmp
init
insmod
install
ipcalc
iwconfig
jobs
join
jot
kill
killall
last
lastcomm
lastlog
ldd
less
let
lex
ln
locate
lockfile
logger
logname
logout
logrotate
look
losetup
lp
ls
lsdev
lsmod
lsof
lspci
lsusb
ltrace
lynx
m4
mail
mailto
make
MAKEDEV
man
mcookie
md5sum
mesg
mimencode
mkbootdisk
mkdir
mke2fs
mkfifo
mknod
mkswap
mktemp
mmencode
modinfo
modprobe
more
mount
msgfmt
mv
nc
netconfig
netstat
newgrp
nice
nl
nm
nmap
nohup
nslookup
objdump
od
passwd
paste
patch (Related topic: diff)
pathchk
pgrep
pidof
ping
pkill
popd
pr
printenv
管理员在2009年8月13日编辑了该文章文章。

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