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    Advanced Bash-Scripting Guide
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    Computer Technologies  Operating Systems  UNIX  Shells Advanced Bash-Scripting Guide
    Advanced Bash-Scripting Guide

    Advanced Bash-Scripting Guide

    An in-depth exploration of the gentle art of shell scripting

    Mendel Cooper

    Brindle-Phlogiston Associates

    thegrendel@theriver.com

    29 September 2002

    Revision History
    Revision 0.114 June 2000Revised by: mc
    Initial release.
    Revision 0.230 October 2000Revised by: mc
    Bugs fixed, plus much additional material and more example scripts.
    Revision 0.312 February 2001Revised by: mc
    Another major update.
    Revision 0.408 July 2001Revised by: mc
    More bugfixes, much more material, more scripts - a complete revision and expansion of the book.
    Revision 0.503 September 2001Revised by: mc
    Major update. Bugfixes, material added, chapters and sections reorganized.
    Revision 1.014 October 2001Revised by: mc
    Bugfixes, reorganization, material added. Stable release.
    Revision 1.106 January 2002Revised by: mc
    Bugfixes, material and scripts added.
    Revision 1.231 March 2002Revised by: mc
    Bugfixes, material and scripts added.
    Revision 1.302 June 2002Revised by: mc
    'TANGERINE' release: A few bugfixes, much more material and scripts added.
    Revision 1.416 June 2002Revised by: mc
    'MANGO' release: Quite a number of typos fixed, more material and scripts added.
    Revision 1.513 July 2002Revised by: mc
    'PAPAYA' release: A few bugfixes, much more material and scripts added.
    Revision 1.629 September 2002Revised by: mc
    'POMEGRANATE' release: some bugfixes, more material, one more script added.

    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. Internal Commands and Builtins
    12. External Filters, Programs and Commands
    13. System and Administrative Commands
    14. Command Substitution
    15. Arithmetic Expansion
    16. I/O Redirection
    17. Here Documents
    18. Recess Time
    Part 4. Advanced Topics
    19. Regular Expressions
    20. Subshells
    21. Restricted Shells
    22. Process Substitution
    23. Functions
    24. Aliases
    25. List Constructs
    26. Arrays
    27. Files
    28. /dev and /proc
    29. Of Zeros and Nulls
    30. Debugging
    31. Options
    32. Gotchas
    33. Scripting With Style
    34. Miscellany
    35. Bash, version 2
    36. Endnotes
    36.1. Author's Note
    36.2. About the Author
    36.3. Tools Used to Produce This Book
    36.4. Credits
    Bibliography
    A. Contributed Scripts
    B. A Sed and Awk Micro-Primer
    B.1. Sed
    B.2. Awk
    C. Exit Codes With Special Meanings
    D. A Detailed Introduction to I/O and I/O Redirection
    E. Localization
    F. History Commands
    G. A Sample .bashrc File
    H. Converting DOS Batch Files to Shell Scripts
    I. Exercises
    I.1. Analyzing Scripts
    I.2. Writing Scripts
    J. Copyright
    List of Tables
    11-1. Job Identifiers
    31-1. bash options
    34-1. Numbers Representing Colors in Escape Sequences
    B-1. Basic sed operators
    B-2. Examples
    C-1. "Reserved" Exit Codes
    H-1. Batch file keywords / variables / operators, and their shell equivalents
    H-2. DOS Commands and Their UNIX Equivalents
    List of Examples
    2-1. cleanup: A script to clean up the log files in /var/log
    2-2. cleanup: An enhanced and generalized version of above script.
    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. arithmetic and string comparisons
    7-5. testing whether a string is null
    7-6. zmost
    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. Converting graphic file formats, with filename change
    9-11. Alternate ways of extracting substrings
    9-12. Using parameter substitution and error messages
    9-13. Parameter substitution and "usage" messages
    9-14. Length of a variable
    9-15. Pattern matching in parameter substitution
    9-16. Renaming file extensions:
    9-17. Using pattern matching to parse arbitrary strings
    9-18. Matching patterns at prefix or suffix of string
    9-19. Using declare to type variables
    9-20. Indirect References
    9-21. Passing an indirect reference to awk
    9-22. Generating random numbers
    9-23. Rolling the die with RANDOM
    9-24. Reseeding RANDOM
    9-25. Pseudorandom numbers, using awk
    9-26. 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 case
    10-24. Creating menus using case
    10-25. Using command substitution to generate the case variable
    10-26. Simple string matching
    10-27. Checking for alphabetic input
    10-28. Creating menus using select
    10-29. Creating menus using select in a function
    11-1. printf in action
    11-2. Variable assignment, using read
    11-3. What happens when read has no variable
    11-4. Multi-line input to read
    11-5. Detecting the arrow keys
    11-6. Using read with file redirection
    11-7. Changing the current working directory
    11-8. Letting let do some arithmetic.
    11-9. Showing the effect of eval
    11-10. Forcing a log-off
    11-11. A version of "rot13"
    11-12. Using set with positional parameters
    11-13. Reassigning the positional parameters
    11-14. "unsetting" a variable
    11-15. Using export to pass a variable to an embedded awk script
    11-16. Using getopts to read the options/arguments passed to a script
    11-17. "Including" a data file
    11-18. A (useless) script that sources itself
    11-19. Effects of exec
    11-20. A script that exec's itself
    11-21. Waiting for a process to finish before proceeding
    11-22. A script that kills itself
    12-1. Using ls to create a table of contents for burning a CDR disk
    12-2. Badname, eliminate file names in current directory containing bad characters and whitespace.
    12-3. Deleting a file by its inode number
    12-4. Logfile using xargs to monitor system log
    12-5. copydir, copying files in current directory to another, using xargs
    12-6. Using expr
    12-7. Using date
    12-8. Word Frequency Analysis
    12-9. Which files are scripts?
    12-10. Generating 10-digit random numbers
    12-11. Using tail to monitor the system log
    12-12. Emulating "grep" in a script
    12-13. Checking words in a list for validity
    12-14. toupper: Transforms a file to all uppercase.
    12-15. lowercase: Changes all filenames in working directory to lowercase.
    12-16. du: DOS to UNIX text file conversion.
    12-17. rot13: rot13, ultra-weak encryption.
    12-18. Generating "Crypto-Quote" Puzzles
    12-19. Formatted file listing.
    12-20. Using column to format a directory listing
    12-21. nl: A self-numbering script.
    12-22. Using cpio to move a directory tree
    12-23. Unpacking an rpm archive
    12-24. stripping comments from C program files
    12-25. Exploring /usr/X11R6/bin
    12-26. An "improved" strings command
    12-27. Using cmp to compare two files within a script.
    12-28. basename and dirname
    12-29. Checking file integrity
    12-30. uudecoding encoded files
    12-31. A script that mails itself
    12-32. Monthly Payment on a Mortgage
    12-33. Base Conversion
    12-34. Another way to invoke bc
    12-35. Calculating PI
    12-36. Converting a decimal number to hexadecimal
    12-37. Factoring
    12-38. Calculating the hypotenuse of a triangle
    12-39. Using seq to generate loop arguments
    12-40. Using getopt to parse command-line options
    12-41. Capturing Keystrokes
    12-42. Securely deleting a file
    12-43. Filename generator
    12-44. Converting meters to miles
    12-45. Using m4
    13-1. setting an erase character
    13-2. secret password: Turning off terminal echoing
    13-3. Keypress detection
    13-4. pidof helps kill a process
    13-5. Checking a CD image
    13-6. Creating a filesystem in a file
    13-7. Adding a new hard drive
    13-8. killall, from /etc/rc.d/init.d
    14-1. Stupid script tricks
    14-2. Generating a variable from a loop
    16-1. Redirecting stdin using exec
    16-2. Redirecting stdout using exec
    16-3. Redirecting both stdin and stdout in the same script with exec
    16-4. Redirected while loop
    16-5. Alternate form of redirected while loop
    16-6. Redirected until loop
    16-7. Redirected for loop
    16-8. Redirected for loop (both stdin and stdout redirected)
    16-9. Redirected if/then test
    16-10. Data file "names.data" for above examples
    16-11. Logging events
    17-1. dummyfile: Creates a 2-line dummy file
    17-2. broadcast: Sends message to everyone logged in
    17-3. Multi-line message using cat
    17-4. Multi-line message, with tabs suppressed
    17-5. Here document with parameter substitution
    17-6. Parameter substitution turned off
    17-7. upload: Uploads a file pair to "Sunsite" incoming directory
    17-8. Here documents and functions
    17-9. "Anonymous" Here Document
    17-10. Commenting out a block of code
    17-11. A self-documenting script
    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 function
    23-2. Function Taking Parameters
    23-3. Maximum of two numbers
    23-4. Converting numbers to Roman numerals
    23-5. Testing large return values in a function
    23-6. Comparing two large integers
    23-7. Real name from username
    23-8. Local variable visibility
    23-9. Recursion, using a local variable
    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. Some special properties of arrays
    26-4. Of empty arrays and empty elements
    26-5. An old friend: The Bubble Sort
    26-6. Complex array application: Sieve of Eratosthenes
    26-7. Emulating a push-down stack
    26-8. Complex array application: Exploring a weird mathematical series
    26-9. Simulating a two-dimensional array, then tilting it
    28-1. Finding the process associated with a PID
    28-2. On-line connect status
    29-1. Hiding the cookie jar
    29-2. Setting up a swapfile using /dev/zero
    29-3. Creating a ramdisk
    30-1. A buggy script
    30-2. Missing keyword
    30-3. test24, another buggy script
    30-4. Testing a condition with an "assert"
    30-5. Trapping at exit
    30-6. Cleaning up after Control-C
    30-7. Tracing a variable
    32-1. Subshell Pitfalls
    32-2. Piping the output of echo to a read
    34-1. shell wrapper
    34-2. A slightly more complex shell wrapper
    34-3. A shell wrapper around an awk script
    34-4. Perl embedded in a Bash script
    34-5. Bash and Perl scripts combined
    34-6. A (useless) script that recursively calls itself
    34-7. A (useful) script that recursively calls itself
    34-8. A "colorized" address database
    34-9. Echoing colored text
    34-10. Return value trickery
    34-11. Even more return value trickery
    34-12. Passing and returning arrays
    34-13. Fun with anagrams
    35-1. String expansion
    35-2. Indirect variable references - the new way
    35-3. Simple database application, using indirect variable referencing
    35-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards
    A-1. manview: Viewing formatted manpages
    A-2. mailformat: Formatting an e-mail message
    A-3. rn: A simple-minded file rename utility
    A-4. blank-rename: renames filenames containing blanks
    A-5. encryptedpw: Uploading to an ftp site, using a locally encrypted password
    A-6. copy-cd: Copying a data CD
    A-7. Collatz series
    A-8. days-between: Calculate number of days between two dates
    A-9. Make a "dictionary"
    A-10. Soundex conversion
    A-11. "Game of Life"
    A-12. Data file for "Game of Life"
    A-13. behead: Removing mail and news message headers
    A-14. ftpget: Downloading files via ftp
    A-15. password: Generating random 8-character passwords
    A-16. fifo: Making daily backups, using named pipes
    A-17. Generating prime numbers using the modulo operator
    A-18. tree: Displaying a directory tree
    A-19. string functions: C-like string functions
    A-20. Object-oriented database
    G-1. Sample .bashrc file
    H-1. VIEWDATA.BAT: DOS Batch File
    H-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
      Next
      Introduction
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