Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Version 3.1.6

22 January 2005

Mendel Cooper

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 pdf version is also available. See the change log for a revision history.


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
2.1. Invoking the script
2.2. Preliminary Exercises
Part 2. Basics
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
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
Part 3. Beyond the Basics
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. Internal Commands and Builtins
11.1. Job Control Commands
12. External Filters, Programs and Commands
12.1. Basic Commands
12.2. Complex Commands
12.3. Time / Date Commands
12.4. Text Processing Commands
12.5. File and Archiving Commands
12.6. Communications Commands
12.7. Terminal Control Commands
12.8. Math Commands
12.9. Miscellaneous Commands
13. System and Administrative Commands
14. Command Substitution
15. Arithmetic Expansion
16. I/O Redirection
16.1. Using exec
16.2. Redirecting Code Blocks
16.3. Applications
17. Here Documents
17.1. Here Strings
18. Recess Time
Part 4. Advanced Topics
19. Regular Expressions
19.1. A Brief Introduction to Regular Expressions
19.2. Globbing
20. Subshells
21. Restricted Shells
22. Process Substitution
23. Functions
23.1. Complex Functions and Function Complexities
23.2. Local Variables
23.3. Recursion Without Local Variables
24. Aliases
25. List Constructs
26. Arrays
27. Files
28. /dev and /proc
28.1. /dev
28.2. /proc
29. Of Zeros and Nulls
30. Debugging
31. Options
32. Gotchas
33. Scripting With Style
33.1. Unofficial Shell Scripting Stylesheet
34. Miscellany
34.1. Interactive and non-interactive shells and scripts
34.2. Shell Wrappers
34.3. Tests and Comparisons: Alternatives
34.4. Recursion
34.5. "Colorizing" Scripts
34.6. Optimizations
34.7. Assorted Tips
34.8. Security Issues
34.9. Portability Issues
34.10. Shell Scripting Under Windows
35. Bash, versions 2 and 3
35.1. Bash, version2
35.2. Bash, version3
36. Endnotes
36.1. Author's Note
36.2. About the Author
36.3. Where to Go For Help
36.4. Tools Used to Produce This Book
36.4.1. Hardware
36.4.2. Software and Printware
36.5. Credits
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. Standard Command-Line Options
G. Important System Directories
H. Localization
I. History Commands
J. A Sample .bashrc File
K. Converting DOS Batch Files to Shell Scripts
L. Exercises
L.1. Analyzing Scripts
L.2. Writing Scripts
M. Revision History
N. Mirror Sites
O. To Do List
P. Copyright
List of Tables
11-1. Job identifiers
31-1. Bash options
34-1. 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
K-1. Batch file keywords / variables / operators, and their shell equivalents
K-2. DOS commands and their UNIX equivalents
M-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. 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. Inserting a blank line between paragraphs in a text file
9-11. Converting graphic file formats, with filename change
9-12. Alternate ways of extracting substrings
9-13. Using parameter substitution and error messages
9-14. Parameter substitution and "usage" messages
9-15. Length of a variable
9-16. Pattern matching in parameter substitution
9-17. Renaming file extensions:
9-18. Using pattern matching to parse arbitrary strings
9-19. Matching patterns at prefix or suffix of string
9-20. Using declare to type variables
9-21. Indirect References
9-22. Passing an indirect reference to awk
9-23. Generating random numbers
9-24. Picking a random card from a deck
9-25. Random between values
9-26. Rolling a single die with RANDOM
9-27. Reseeding RANDOM
9-28. Pseudorandom numbers, using awk
9-29. 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. 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. Problems reading from a pipe
11-8. Changing the current working directory
11-9. Letting "let" do arithmetic.
11-10. Showing the effect of eval
11-11. Forcing a log-off
11-12. A version of "rot13"
11-13. Using eval to force variable substitution in a Perl script
11-14. Using set with positional parameters
11-15. Reassigning the positional parameters
11-16. "Unsetting" a variable
11-17. Using export to pass a variable to an embedded awk script
11-18. Using getopts to read the options/arguments passed to a script
11-19. "Including" a data file
11-20. A (useless) script that sources itself
11-21. Effects of exec
11-22. A script that exec's itself
11-23. Waiting for a process to finish before proceeding
11-24. A script that kills itself
12-1. Using ls to create a table of contents for burning a CDR disk
12-2. Hello or Good-bye
12-3. Badname, eliminate file names in current directory containing bad characters and whitespace.
12-4. Deleting a file by its inode number
12-5. Logfile: Using xargs to monitor system log
12-6. Copying files in current directory to another
12-7. Killing processes by name
12-8. Word frequency analysis using xargs
12-9. Using expr
12-10. Using date
12-11. Word Frequency Analysis
12-12. Which files are scripts?
12-13. Generating 10-digit random numbers
12-14. Using tail to monitor the system log
12-15. Emulating "grep" in a script
12-16. Checking words in a list for validity
12-17. toupper: Transforms a file to all uppercase.
12-18. lowercase: Changes all filenames in working directory to lowercase.
12-19. Du: DOS to UNIX text file conversion.
12-20. rot13: rot13, ultra-weak encryption.
12-21. Generating "Crypto-Quote" Puzzles
12-22. Formatted file listing.
12-23. Using column to format a directory listing
12-24. nl: A self-numbering script.
12-25. manview: Viewing formatted manpages
12-26. Using cpio to move a directory tree
12-27. Unpacking an rpm archive
12-28. Stripping comments from C program files
12-29. Exploring /usr/X11R6/bin
12-30. An "improved" strings command
12-31. Using cmp to compare two files within a script.
12-32. basename and dirname
12-33. Checking file integrity
12-34. Uudecoding encoded files
12-35. Checking a spam domain
12-36. Getting a stock quote
12-37. Using ssh
12-38. A script that mails itself
12-39. Monthly Payment on a Mortgage
12-40. Base Conversion
12-41. Invoking bc using a "here document"
12-42. Calculating PI
12-43. Converting a decimal number to hexadecimal
12-44. Factoring
12-45. Calculating the hypotenuse of a triangle
12-46. Using seq to generate loop arguments
12-47. Using getopt to parse command-line options
12-48. A script that copies itself
12-49. Exercising dd
12-50. Capturing Keystrokes
12-51. Securely deleting a file
12-52. Filename generator
12-53. Converting meters to miles
12-54. Using m4
13-1. Setting a new password
13-2. Setting an erase character
13-3. secret password: Turning off terminal echoing
13-4. Keypress detection
13-5. pidof helps kill a process
13-6. Checking a CD image
13-7. Creating a filesystem in a file
13-8. Adding a new hard drive
13-9. 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. Avoiding a subshell
16-5. Redirected while loop
16-6. Alternate form of redirected while loop
16-7. Redirected until loop
16-8. Redirected for loop
16-9. Redirected for loop (both stdin and stdout redirected)
16-10. Redirected if/then test
16-11. Data file "" for above examples
16-12. 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. Upload a file pair to "Sunsite" incoming directory
17-7. Parameter substitution turned off
17-8. A script that generates another script
17-9. Here documents and functions
17-10. "Anonymous" Here Document
17-11. Commenting out a block of code
17-12. A self-documenting script
17-13. Prepending a line to a file
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
28-1. Using /dev/tcp for troubleshooting
28-2. Finding the process associated with a PID
28-3. 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
30-8. Running multiple processes (on an SMP box)
32-1. Numerical and string comparison are not equivalent
32-2. Subshell Pitfalls
32-3. 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. Another (useful) script that recursively calls itself
34-9. A "colorized" address database
34-10. Drawing a box
34-11. Echoing colored text
34-12. Return value trickery
34-13. Even more return value trickery
34-14. Passing and returning arrays
34-15. Fun with anagrams
34-16. Widgets invoked from a shell script
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. 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. Mounting USB keychain storage devices
A-22. Preserving weblogs
A-23. Protecting literal strings
A-24. Unprotecting literal strings
A-25. Spammer Identification
A-26. Basics Reviewed
A-27. An expanded cd command
C-1. Counting Letter Occurrences
J-1. Sample .bashrc file
K-2. Shell Script Conversion of VIEWDATA.BAT
O-1. Print the server environment