Advanced Bash-Scripting Guide

A complete guide to shell scripting, using Bash

19 October 2001

Mendel Cooper

Brindlesoft

thegrendel@theriver.com

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.

The latest update of this document, as an archived "tarball" including both the SGML source and rendered HTML, may be downloaded from the author's home site. See the change log 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
2.1. Invoking the script
2.2. Preliminary Exercises
Part 2. Basics
3. Exit and Exit Status
4. Special Characters
41Special Characters Found In Scripts and Elsewhere
5. Introduction to Variables and Parameters
5.1. Variable Substitution
5.2. Variable Assignment
5.3. Bash Variables Are Untyped
5.4. Special Variable Types
6. Quoting
61Special meanings of certain escaped characters
7. Tests
7.1. Test Constructs
7.2. File test operators
7.3. Comparison operators (binary)
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
111I/O
112Filesystem
113Variables
114Script Behavior
115Commands
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
131Users and Groups
132Terminals
133Information and Statistics
134System Logs
135Job Control
136Process Control and Booting
137Network
138Filesystem
139Backup
1310System Resources
1311Modules
1312Miscellaneous
14. Command Substitution
15. Arithmetic Expansion
151Variations
16. I/O Redirection
161Closing File Descriptors
16.1. Using exec
16.2. Redirecting Code Blocks
16.3. Applications
17. Here Documents
18. Recess Time
Part 4. Advanced Topics
19. Regular Expressions
19.1. A Brief Introduction to Regular Expressions
19.2. Globbing
20. Subshells
201Command List in Parentheses
21. Restricted Shells
211Disabled commands in restricted shells
22. Process Substitution
221Command substitution template
23. Functions
23.1. Complex Functions and Function Complexities
23.2. Local Variables and Recursion
24. Aliases
25. List Constructs
251Chaining together commands
26. Arrays
27. Files
271startup files
272logout file
28. /dev and /proc
28.1. /dev
28.2. /proc
29. Of Zeros and Nulls
291/dev/zero and /dev/null
30. Debugging
301Trapping signals
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. Optimizations
34.5. Assorted Tips
34.6. Oddities
34.7. Portability Issues
35. Bash, version 2
36. Endnotes
36.1. Author's Note
36.2. About the Author
36.3. Tools Used to Produce This Book
36.3.1. Hardware
36.3.2. Software and Printware
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. A Sample .bashrc File
G. Converting DOS Batch Files to Shell Scripts
H. Exercises
H1Easy
H2Medium
H3Difficult
I. Copyright
List of Tables
11-1. Job Identifiers
31-1. bash options
B-1. Basic sed operators
B-2. Examples
C-1. "Reserved" Exit Codes
G-1. Batch file keywords / variables / operators, and their shell equivalents
G-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. exit / exit status
3-2. Negating a condition using !
4-1. Code blocks and I/O redirection
4-2. Saving the results of a code block to a file
4-3. Backup of all files changed in last day
5-1. Variable assignment and substitution
5-2. Plain Variable Assignment
5-3. Variable Assignment, plain and fancy
5-4. Integer or string?
5-5. Positional Parameters
5-6. wh, whois domain name lookup
5-7. Using shift
6-1. Echoing Weird Variables
6-2. Escaped Characters
7-1. What is truth?
7-2. Equivalence of [ ] and test
7-3. Arithmetic Tests using (( ))
7-4. arithmetic and string comparisons
7-5. testing whether a string is null
7-6. zmost
8-1. Using Arithmetic Operations
8-2. Compound Condition Tests Using && and ||
8-3. Representation of numerical constants:
9-1. $IFS and whitespace
9-2. Timed Input
9-3. Once more, timed input
9-4. Am I root?
9-5. arglist: Listing arguments with $* and $@
9-6. Inconsistent $* and $@ behavior
9-7. $* and $@ when $IFS is empty
9-8. underscore variable
9-9. Using param substitution and :
9-10. Length of a variable
9-11. Pattern matching in parameter substitution
9-12. Renaming file extensions:
9-13. Using pattern matching to parse arbitrary strings
9-14. Matching patterns at prefix or suffix of string
9-15. Using declare to type variables
9-16. Indirect References
9-17. Passing an indirect reference to awk
9-18. Generating random numbers
9-19. Rolling the die with RANDOM
9-20. Reseeding RANDOM
9-21. 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. Checking all the binaries in a directory for authorship
10-9. Listing the symbolic links in a directory
10-10. Symbolic links in a directory, saved to a file
10-11. A C-like for loop
10-12. Using efax in batch mode
10-13. Simple while loop
10-14. Another while loop
10-15. while loop with multiple conditions
10-16. C-like syntax in a while loop
10-17. until loop
10-18. Nested Loop
10-19. Effects of break and continue in a loop
10-20. Breaking out of multiple loop levels
10-21. Continuing at a higher loop level
10-22. Using case
10-23. Creating menus using case
10-24. Using command substitution to generate the case variable
10-25. Simple string matching
10-26. Checking for alphabetic input
10-27. Creating menus using select
10-28. Creating menus using select in a function
11-1. printf in action
11-2. Variable assignment, using read
11-3. Multi-line input to read
11-4. Using read with file redirection
11-5. Changing the current working directory
11-6. Letting let do some arithmetic.
11-7. Showing the effect of eval
11-8. Forcing a log-off
11-9. A version of "rot13"
11-10. Using set with positional parameters
11-11. "unsetting" a variable
11-12. Using export to pass a variable to an embedded awk script
11-13. Using getopts to read the options/arguments passed to a script
11-14. "Including" a data file
11-15. Effects of exec
11-16. Waiting for a process to finish before proceeding
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. Using cmp to compare two files within a script.
12-9. Generating 10-digit random numbers
12-10. Using tail to monitor the system log
12-11. Emulating "grep" in a script
12-12. Checking words in a list for validity
12-13. toupper: Transforms a file to all uppercase.
12-14. lowercase: Changes all filenames in working directory to lowercase.
12-15. du: DOS to UNIX text file conversion.
12-16. rot13: rot13, ultra-weak encryption.
12-17. Generating "Crypto-Quote" Puzzles
12-18. Formatted file listing.
12-19. Using column to format a directory listing
12-20. nl: A self-numbering script.
12-21. Using cpio to move a directory tree
12-22. Unpacking an rpm archive
12-23. stripping comments from C program files
12-24. Exploring /usr/X11R6/bin
12-25. basename and dirname
12-26. uudecoding encoded files
12-27. Monthly Payment on a Mortgage
12-28. Base Conversion
12-29. Using seq to generate loop arguments
12-30. Capturing Keystrokes
12-31. Securely deleting a file
12-32. 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
16-1. Redirecting stdin using exec
16-2. Redirected while loop
16-3. Alternate form of redirected while loop
16-4. Redirected until loop
16-5. Redirected for loop
16-6. Redirected for loop (both stdin and stdout redirected)
16-7. Redirected if/then test
16-8. 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. "Anonymous" Here Document
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. Some special properties of arrays
26-3. Of empty arrays and empty elements
26-4. An old friend: The Bubble Sort
26-5. Complex array application: Sieve of Erastosthenes
26-6. Complex array application: Exploring a weird mathematical series
26-7. 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. test24, another buggy script
30-3. Testing a condition with an "assert"
30-4. Trapping at exit
30-5. Cleaning up after Control-C
30-6. Tracing a variable
32-1. Subshell Pitfalls
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 script that recursively calls itself
35-1. String expansion
35-2. Indirect variable references - the new way
35-3. 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. encryptedpw: Uploading to an ftp site, using a locally encrypted password
A-5. copy-cd: Copying a data CD
A-6. days-between: Calculate number of days between two dates
A-7. behead: Removing mail and news message headers
A-8. ftpget: Downloading files via ftp
A-9. password: Generating random 8-character passwords
A-10. fifo: Making daily backups, using named pipes
A-11. Generating prime numbers using the modulo operator
A-12. tree: Displaying a directory tree
A-13. string functions: C-like string functions
A-14. Object-oriented database
F-1. Sample .bashrc file
G-1. VIEWDATA.BAT: DOS Batch File
G-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
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  Introduction