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Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Mendel Cooper


23 October 2005

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.


For Anita, the source of all the magic

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. Emulating getopt
9-13. Alternate ways of extracting substrings
9-14. Using parameter substitution and error messages
9-15. Parameter substitution and "usage" messages
9-16. Length of a variable
9-17. Pattern matching in parameter substitution
9-18. Renaming file extensions:
9-19. Using pattern matching to parse arbitrary strings
9-20. Matching patterns at prefix or suffix of string
9-21. Using declare to type variables
9-22. Indirect References
9-23. Passing an indirect reference to awk
9-24. Generating random numbers
9-25. Picking a random card from a deck
9-26. Random between values
9-27. Rolling a single die with RANDOM
9-28. Reseeding RANDOM
9-29. Pseudorandom numbers, using awk
9-30. 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. A script that forks off multiple instances of itself
11-2. printf in action
11-3. Variable assignment, using read
11-4. What happens when read has no variable
11-5. Multi-line input to read
11-6. Detecting the arrow keys
11-7. Using read with file redirection
11-8. Problems reading from a pipe
11-9. Changing the current working directory
11-10. Letting "let" do arithmetic.
11-11. Showing the effect of eval
11-12. Forcing a log-off
11-13. A version of "rot13"
11-14. Using eval to force variable substitution in a Perl script
11-15. Using set with positional parameters
11-16. Reassigning the positional parameters
11-17. "Unsetting" a variable
11-18. Using export to pass a variable to an embedded awk script
11-19. Using getopts to read the options/arguments passed to a script
11-20. "Including" a data file
11-21. A (useless) script that sources itself
11-22. Effects of exec
11-23. A script that exec's itself
11-24. Waiting for a process to finish before proceeding
11-25. 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. Looking up definitions in Webster's 1913 Dictionary
12-17. Checking words in a list for validity
12-18. toupper: Transforms a file to all uppercase.
12-19. lowercase: Changes all filenames in working directory to lowercase.
12-20. Du: DOS to UNIX text file conversion.
12-21. rot13: rot13, ultra-weak encryption.
12-22. Generating "Crypto-Quote" Puzzles
12-23. Formatted file listing.
12-24. Using column to format a directory listing
12-25. nl: A self-numbering script.
12-26. manview: Viewing formatted manpages
12-27. Using cpio to move a directory tree
12-28. Unpacking an rpm archive
12-29. Stripping comments from C program files
12-30. Exploring /usr/X11R6/bin
12-31. An "improved" strings command
12-32. Using cmp to compare two files within a script.
12-33. basename and dirname
12-34. Checking file integrity
12-35. Uudecoding encoded files
12-36. Finding out where to report a spammer
12-37. Analyzing a spam domain
12-38. Getting a stock quote
12-39. Updating FC4
12-40. Using ssh
12-41. A script that mails itself
12-42. Monthly Payment on a Mortgage
12-43. Base Conversion
12-44. Invoking bc using a "here document"
12-45. Calculating PI
12-46. Converting a decimal number to hexadecimal
12-47. Factoring
12-48. Calculating the hypotenuse of a triangle
12-49. Using seq to generate loop arguments
12-50. Letter Count"
12-51. Using getopt to parse command-line options
12-52. A script that copies itself
12-53. Exercising dd
12-54. Capturing Keystrokes
12-55. Securely deleting a file
12-56. Filename generator
12-57. Converting meters to miles
12-58. 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. Checking a remote server for identd
13-6. pidof helps kill a process
13-7. Checking a CD image
13-8. Creating a filesystem in a file
13-9. Adding a new hard drive
13-10. Using umask to hide an output file from prying eyes
13-11. killall, from /etc/rc.d/init.d
14-1. Stupid script tricks
14-2. Generating a variable from a loop
14-3. Finding anagrams
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. broadcast: Sends message to everyone logged in
17-2. dummyfile: Creates a 2-line dummy file
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
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. Mounting USB keychain storage devices
A-24. Preserving weblogs
A-25. Protecting literal strings
A-26. Unprotecting literal strings
A-27. Spammer Identification
A-28. Spammer Hunt
A-29. Making wget easier to use
A-30. A "podcasting" script
A-31. Basics Reviewed
A-32. An expanded cd command
C-1. Counting Letter Occurrences
K-1. Sample .bashrc file
L-2. Shell Script Conversion of VIEWDATA.BAT
P-1. Print the server environment

  Published under the terms of the GNU General Public License Design by Interspire