Advanced Bash-Scripting Guide:
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# Index

This index / glossary / quick-reference lists many of the important topics covered in the text. Terms are arranged in approximate ASCII sorting order, modified as necessary for enhanced clarity.

Note that commands are indexed in Part 4.

* * *

^ (caret)

- Beginning-of-line, in a Regular Expression

- ^

  ^^

  Uppercase conversion in parameter substitution

~ Tilde

- ~ home directory, corresponds to `$HOME`

- ~/ Current user's home directory

- ~+ Current working directory

- ~- Previous working directory

= Equals sign

- = Variable assignment operator

- = String comparison operator

  == String comparison operator

- =~ Regular Expression match operator

  Example script

< Left angle bracket

- Is-less-than

  String comparison

  Integer comparison within double parentheses

- Redirection

  < `stdin`

  << Here document

  <<< Here string

  <> Opening a file for both reading and writing

> Right angle bracket

- Is-greater-than

  String comparison

  Integer comparison, within double parentheses

- Redirection

  > Redirect `stdout` to a file

  >> Redirect `stdout` to a file, but append

  i>&j Redirect file descriptor `i` to file descriptor `j`

  >&j Redirect `stdout` to file descriptor `j`

  >&2 Redirect `stdout` of a command to `stderr`

  2>&1 Redirect `stderr` to `stdout`

  &> Redirect both `stdout` and `stderr` of a command to a file

  :> `file` Truncate file to zero length

| Pipe, a device for passing the output of a command to another command or to the shell

|| Logical OR test operator

- (dash)

- Prefix to default parameter, in parameter substitution

- Prefix to option flag

- Indicating redirection from `stdin` or `stdout`

- -- (double-dash)

  Prefix to long command options

  C-style variable decrement within double parentheses

; (semicolon)

- As command separator

- \; Escaped semicolon, terminates a find command

- ;; Double-semicolon, terminator in a case option

  Required when ...

  do keyword is on the first line of loop

  terminating curly-bracketed code block

- ;;& ;& Terminators in a case option (version 4+ of Bash).

: Colon

- :> `filename` Truncate file to zero length

- null command, equivalent to the true Bash builtin

- Used in an anonymous here document

- Used in an otherwise empty function

- Used as a function name

! Negation operator, inverts exit status of a test or command

- != not-equal-to String comparison operator

? (question mark)

- Match zero or one characters, in an Extended Regular Expression

- Single-character wild card, in globbing

- In a C-style Trinary operator

// Double forward slash, behavior of cd command toward

. (dot / period)

- . Load a file (into a script), equivalent to source command

- . Match single character, in a Regular Expression

- . Current working directory

  ./ Current working directory

- .. Parent directory

' ... ' (single quotes) strong quoting

" ... " (double quotes) weak quoting

- Double-quoting the backslash (\) character

,

- Comma operator

- ,

  ,,

  Lowercase conversion in parameter substitution

() Parentheses

- ( ... ) Command group; starts a subshell

- ( ... ) Enclose group of Extended Regular Expressions

- >( ... )

  <( ... ) Process substitution

- ... ) Terminates test-condition in case construct

- (( ... )) Double parentheses, in arithmetic expansion

[ Left bracket, test construct

[ ]Brackets

- Array element

- Enclose character set to match in a Regular Expression

- Test construct

[[ ... ]] Double brackets, extended test construct

$ Anchor, in a Regular Expression

$ Prefix to a variable name

$( ... ) Command substitution, setting a variable with output of a command, using parentheses notation

` ... ` Command substitution, using backquotes notation

$[ ... ] Integer expansion (deprecated)

${ ... } Variable manipulation / evaluation

- ${var} Value of a variable

- ${#var} Length of a variable

- ${#@}

  ${#*} Number of positional parameters

- ${parameter?err_msg} Parameter-unset message

- ${parameter-default}

  ${parameter:-default}

  ${parameter=default}

  ${parameter:=default} Set default parameter

- ${parameter+alt_value}

  ${parameter:+alt_value}

  Alternate value of parameter, if set

- ${!var}

  Indirect referencing of a variable, new notation

- ${!#}

  Final positional parameter. (This is an indirect reference to $#.)

- ${!varprefix*}

  ${!varprefix@}

  Match names of all previously declared variables beginning with `varprefix`

- ${string:position}

  ${string:position:length} Substring extraction

- ${var#Pattern}

  ${var##Pattern} Substring removal

- ${var%Pattern}

  ${var%%Pattern} Substring removal

- ${string/substring/replacement}

  ${string//substring/replacement}

  ${string/#substring/replacement}

  ${string/%substring/replacement} Substring replacement

$' ... ' String expansion, using escaped characters.

\ Escape the character following

- \< ... \> Angle brackets, escaped, word boundary in a Regular Expression

- \{ N \} "Curly" brackets, escaped, number of character sets to match in an Extended RE

- \; Semicolon, escaped, terminates a find command

- \$$ Indirect reverencing of a variable, old-style notation

- Escaping a newline, to write a multi-line command

&

- &> Redirect both `stdout` and `stderr` of a command to a file

- >&j Redirect `stdout` to file descriptor j

  >&2 Redirect `stdout` of a command to `stderr`

- i>&j Redirect file descriptor i to file descriptor j

  2>&1 Redirect `stderr` to `stdout`

- Closing file descriptors

  n<&- Close input file descriptor n

  0<&-, <&- Close `stdin`

  n>&- Close output file descriptor n

  1>&-, >&- Close `stdout`

- && Logical AND test operator

- Command & Run job in background

# Hashmark, special symbol beginning a script comment

#! Sha-bang, special string starting a shell script

* Asterisk

- Wild card, in globbing

- Any number of characters in a Regular Expression

- ** Exponentiation, arithmetic operator

- ** Extended globbing file-match operator

% Percent sign

- Modulo, division-remainder arithmetic operation

- Substring removal (pattern matching) operator

+ Plus sign

- Character match, in an extended Regular Expression

- Prefix to alternate parameter, in parameter substitution

- ++ C-style variable increment, within double parentheses

* * *

Shell Variables

$_ Last argument to previous command

$- Flags passed to script, using set

$! Process ID of last background job

$? Exit status of a command

$@ All the positional parameters, as separate words

$* All the positional parameters, as a single word

$$ Process ID of the script

$# Number of arguments passed to a function, or to the script itself

$0 Filename of the script

$1 First argument passed to script

$9 Ninth argument passed to script

Table of shell variables

* * * * * *

-a Logical AND compound comparison test

Address database, script example

Advanced Bash Scripting Guide, where to download

Alias

- Removing an alias, using unalias

Anagramming

And list

- To supply default command-line argument

And logical operator &&

Angle brackets, escaped, \< . . . \> word boundary in a Regular Expression

Anonymous here document, using :

Archiving

- rpm

- tar

Arithmetic expansion

- exit status of

- variations of

Arithmetic operators

- combination operators, C-style

  += -= *= /= %=

  | In certain contexts, += can also function as a string concatenation operator.

Arrays

- Associative arrays

  more efficient than conventional arrays

- Bracket notation

- Concatenating, example script

- Copying

- Declaring

  `declare -a array_name`

- Embedded arrays

- Empty arrays, empty elements, example script

- Indirect references

- Initialization

  `array=( element1 element2 ... elementN)`

  Example script

  Using command substitution

- Loading a file into an array

- Multidimensional, simulating

- Nesting and embedding

- Notation and usage

- Number of elements in

  `${#array_name[@]}`

  `${#array_name[*]}`

- Operations

- Passing an array to a function

- As return value from a function

- Special properties, example script

- String operations, example script

- unset deletes array elements

Arrow keys, detecting

ASCII

- Definition

- Scripts for generating ASCII table

awk field-oriented text processing language

- `rand()`, random function

- String manipulation

- Using export to pass a variable to an embedded awk script

* * *

Backlight, setting the brightness

Backquotes, used in command substitution

Base conversion, example script

Bash

- Bad scripting practices

- Basics reviewed, script example

- Command-line options

  Table

- Features that classic Bourne shell lacks

- Internal variables

- Version 2

- Version 3

- Version 4

  Version 4.1

  Version 4.2

.bashrc

`$BASH_SUBSHELL`

Basic commands, external

Batch files, DOS

Batch processing

bc, calculator utility

- In a here document

- Template for calculating a script variable

Bibliography

Bison utility

Bitwise operators

- Example script

Block devices

- testing for

Blocks of code

- Iterating / looping

- Redirection

  Script example: Redirecting output of a a code block

Bootable flash drives, creating

Brace expansion

- Extended, `{a..z}`

- Parameterizing

- With increment and zero-padding (new feature in Bash, version 4)

Brackets, [ ]

- Array element

- Enclose character set to match in a Regular Expression

- Test construct

Brackets, curly, {}, used in

- Code block

- find

- Extended Regular Expressions

- Positional parameters

- xargs

break loop control command

- Parameter (optional)

Builtins in Bash

- Do not fork a subprocess

* * *

case construct

- Command-line parameters, handling

- Globbing, filtering strings with

cat, concatentate file(s)

- Abuse of

- cat scripts

- Less efficient than redirecting `stdin`

- Piping the output of, to a read

- Uses of

Character devices

- testing for

Checksum

Child processes

Colon, : , equivalent to the true Bash builtin

Colorizing scripts

- Cycling through the background colors, example script

- Table of color escape sequences

- Template, colored text on colored background

Comma operator, linking commands or operations

Command-line options

command_not_found_handle () builtin error-handling function (version 4+ of Bash)

Command substitution

- $( ... ), preferred notation

- Backquotes

- Extending the Bash toolset

- Invokes a subshell

- Nesting

- Removes trailing newlines

- Setting variable from loop output

- Word splitting

Comment headers, special purpose

Commenting out blocks of code

- Using an anonymous here document

- Using an if-then construct

Communications and hosts

Compound comparison operators

Compression utilities

- bzip2

- compress

- gzip

- zip

continue loop control command

Control characters

- Control-C, break

- Control-D, terminate / log out / erase

- Control-G, `BEL` (beep)

- Control-H, rubout

- Control-J, newline

- Control-M, carriage return

Coprocesses

cron, scheduling daemon

C-style syntax , for handling variables

Crossword puzzle solver

Cryptography

Curly brackets {}

- in find command

- in an Extended Regular Expression

- in xargs

* * *

Daemons, in UNIX-type OS

date

dc, calculator utility

dd, data duplicator command

- Conversions

- Copying raw data to/from devices

- File deletion, secure

- Keystrokes, capturing

- Options

- Random access on a data stream

- Raspberry Pi, script for preparing a bootable SD card

- Swapfiles, initializing

- Thread on www.linuxquestions.org

Debugging scripts

- Tools

- Trapping at exit

- Trapping signals

Decimal number, Bash interprets numbers as

declare builtin

- options

  case-modification options (version 4+ of Bash)

Default parameters

`/dev` directory

- `/dev/null` pseudo-device file

- `/dev/urandom` pseudo-device file, generating pseudorandom numbers with

- `/dev/zero`, pseudo-device file

Device file

dialog, utility for generating dialog boxes in a script

`$DIRSTACK` directory stack

Disabled commands, in restricted shells

do keyword, begins execution of commands within a loop

done keyword, terminates a loop

DOS batch files, converting to shell scripts

DOS commands, UNIX equivalents of (table)

dot files, "hidden" setup and configuration files

Double brackets [[ ... ]] test construct

- and evaluation of octal/hex constants

Double parentheses (( ... )) arithmetic expansion/evaluation construct

Double quotes " ... " weak quoting

- Double-quoting the backslash (\) character

Double-spacing a text file, using sed

* * *

-e File exists test

echo

- Feeding commands down a pipe

- Setting a variable using command substitution

- `/bin/echo`, external echo command

elif, Contraction of else and if

else

Encrypting files, using openssl

esac, keyword terminating case construct

Environmental variables

-eq , is-equal-to integer comparison test

Eratosthenes, Sieve of, algorithm for generating prime numbers

Escaped characters, special meanings of

- Within $' ... ' string expansion

- Used with Unicode characters

`/etc/fstab` (filesystem mount) file

`/etc/passwd` (user account) file

`$EUID`, Effective user ID

eval, Combine and evaluate expression(s), with variable expansion

- Effects of, Example script

- Forces reevaluation of arguments

- And indirect references

- Risk of using

- Using eval to convert array elements into a command list

- Using eval to select among variables

Evaluation of octal/hex constants within [[ ... ]]

exec command, using in redirection

Exercises

Exit and Exit status

- exit command

- Exit status (exit code, return status of a command)

  Table, Exit codes with special meanings

  Anomalous

  Out of range

  Pipe exit status

  Specified by a function return

  Successful, 0

  `/usr/include/sysexits.h`, system file listing C/C++ standard exit codes

Export, to make available variables to child processes

- Passing a variable to an embedded awk script

expr, Expression evaluator

- Substring extraction

- Substring index (numerical position in string)

- Substring matching

Extended Regular Expressions

- ? (question mark) Match zero / one characters

- ( ... ) Group of expressions

- \{ N \} "Curly" brackets, escaped, number of character sets to match

- + Character match

* * *

factor, decomposes an integer into its prime factors

- Application: Generating prime numbers

false, returns unsuccessful (1) exit status

Field, a group of characters that comprises an item of data

Files / Archiving

File descriptors

- Closing

  n<&- Close input file descriptor n

  0<&-, <&- Close `stdin`

  n>&- Close output file descriptor n

  1>&-, >&- Close `stdout`

- File handles in C, similarity to

File encryption

find

- {} Curly brackets

- \; Escaped semicolon

Filter

- Using - with file-processing utility as a filter

- Feeding output of a filter back to same filter

Floating point numbers, Bash does not recognize

fold, a filter to wrap lines of text

Forking a child process

for loops

Functions

- Arguments passed referred to by position

- Capturing the return value of a function using echo

- Colon as function name

- Definition must precede first call to function

- Exit status

- Local variables

  and recursion

- Passing an array to a function

- Passing pointers to a function

- Positional parameters

- Recursion

- Redirecting `stdin` of a function

- return

  Multiple return values from a function, example script

  Returning an array from a function

  Return range limits, workarounds

- Shift arguments passed to a function

- Unusual function names

* * *

Games and amusements

- Anagrams

- Anagrams, again

- Bingo Number Generator

- Crossword puzzle solver

- Crypto-Quotes

- Dealing a deck of cards

- Fifteen Puzzle

- Horse race

- Knight's Tour

- "Life" game

- Magic Squares

- Music-playing script

- Nim

- Pachinko

- Perquackey

- Petals Around the Rose

- Podcasting

- Poem

- Speech generation

- Towers of Hanoi

  Graphic version

  Alternate graphic version

getopt, external command for parsing script command-line arguments

- Emulated in a script

getopts, Bash builtin for parsing script command-line arguments

- `$OPTIND` / `$OPTARG`

Global variable

Globbing, filename expansion

- Handling filenames correctly

- Wild cards

- Will not match `dot files`

Golden Ratio (Phi)

-ge , greater-than or equal integer comparison test

-gt , greater-than integer comparison test

groff, text markup and formatting language

Gronsfeld cipher

`$GROUPS`, Groups user belongs to

gzip, compression utility

* * *

Hashing, creating lookup keys in a table

- Example script

head, echo to `stdout` lines at the beginning of a text file

help, gives usage summary of a Bash builtin

Here documents

- Anonymous here documents, using :

  Commenting out blocks of code

  Self-documenting scripts

- bc in a here document

- cat scripts

- Command substitution

- ex scripts

- Function, supplying input to

- Here strings

  Calculating the Golden Ratio

  Prepending text

  As the `stdin` of a loop

  Using read

- Limit string

  ! as a limit string

  Closing limit string may not be indented

  Dash option to limit string, `<<-LimitString`

- Literal text output, for generating program code

- Parameter substitution

  Disabling parameter substitution

- Passing parameters

- Temporary files

- Using vi non-interactively

History commands

`$HOME`, user's home directory

Homework assignment solver

`$HOSTNAME`, system host name

* * *

`$Id` parameter, in rcs (Revision Control System)

if [ condition ]; then ... test construct

- if-grep, if and grep in combination

  Fixup for if-grep test

`$IFS`, Internal field separator variable

- Defaults to whitespace

Integer comparison operators

in, keyword preceding `[list]` in a for loop

Initialization table, `/etc/inittab`

Inline group, i.e., code block

Interactive script, test for

I/O redirection

Indirect referencing of variables

- New notation, introduced in version 2 of Bash ( example script)

iptables, packet filtering and firewall utility

- Usage example

- Example script

Iteration

* * *

Job IDs, table

jot, Emit a sequence of integers. Equivalent to seq.

- Random sequence generation

Just another Bash hacker!

* * *

Keywords

- error, if missing

kill, terminate a process by process ID

- Options (`-l`, `-9`)

killall, terminate a process by name

killall script in `/etc/rc.d/init.d`

* * *

lastpipe shell option

-le , less-than or equal integer comparison test

let, setting and carrying out arithmetic operations on variables

- C-style increment and decrement operators

Limit string, in a here document

`$LINENO`, variable indicating the line number where it appears in a script

Link, file (using ln command)

- Invoking script with multiple names, using ln

- symbolic links, ln -s

List constructs

- And list

- Or list

Local variables

- and recursion

Localization

Logical operators (`&&`, `||`, etc.)

Logout file, the `~/.bash_logout` file

Loopback device, mounting a file on a block device

Loops

- break loop control command

- continue loop control command

- C-style loop within double parentheses

  for loop

  while loop

- do (keyword), begins execution of commands within a loop

- done (keyword), terminates a loop

- for loops

  `for` `arg` `in` `[list]`; `do`

  Command substitution to generate `[list]`

  Filename expansion in `[list]`

  Multiple parameters in each `[list]` element

  Omitting `[list]`, defaults to positional parameters

  Parameterizing `[list]`

  Redirection

- in, (keyword) preceding [list] in a for loop

- Nested loops

- Running a loop in the background, script example

- Semicolon required, when do is on first line of loop

  for loop

  while loop

- until loop

  `until [ condition-is-true ]; do`

- while loop

  `while [ condition ]; do`

  Function call inside test brackets

  Multiple conditions

  Omitting test brackets

  Redirection

  while read construct

- Which type of loop to use

Loopback devices

- In `/dev` directory

- Mounting an ISO image

-lt , less-than integer comparison test

* * *

m4, macro processing language

`$MACHTYPE`, Machine type

Magic number, marker at the head of a file indicating the file type

`Makefile`, file containing the list of dependencies used by make command

man, manual page (lookup)

- Man page editor (script)

mapfile builtin, loads an array with a text file

Math commands

Meta-meaning

Morse code training script

Modulo, arithmetic remainder operator

- Application: Generating prime numbers

Mortgage calculations, example script

* * *

-n String not null test

Named pipe, a temporary FIFO buffer

- Example script

nc, netcat, a network toolkit for TCP and UDP ports

-ne, not-equal-to integer comparison test

Negation operator, !, reverses the sense of a test

netstat, Network statistics

Network programming

nl, a filter to number lines of text

Noclobber, `-C` option to Bash to prevent overwriting of files

NOT logical operator, !

null variable assignment, avoiding

* * *

-o Logical OR compound comparison test

Obfuscation

- Colon as function name

- Homework assignment

- Just another Bash hacker!

octal, base-8 numbers

od, octal dump

`$OLDPWD` Previous working directory

openssl encryption utility

Operator

- Definition of

- Precedence

Options, passed to shell or script on command line or by set command

Or list

Or logical operator, ||

* * *

Parameter substitution

- ${parameter+alt_value}

  ${parameter:+alt_value}

  Alternate value of parameter, if set

- ${parameter-default}

  ${parameter:-default}

  ${parameter=default}

  ${parameter:=default}

  Default parameters

- ${!varprefix*}

  ${!varprefix@}

  Parameter name match

- ${parameter?err_msg}

  Parameter-unset message

- ${parameter}

  Value of parameter

- Case modification (version 4+ of Bash).

- Script example

- Table of parameter substitution

Parent / child process problem, a child process cannot export variables to a parent process

Parentheses

- Command group

- Enclose group of Extended Regular Expressions

- Double parentheses, in arithmetic expansion

`$PATH`, the path (location of system binaries)

- Appending directories to `$PATH` using the `+=` operator.

Pathname, a `filename` that incorporates the complete path of a given file.

- Parsing pathnames

Perl, programming language

- Combined in the same file with a Bash script

- Embedded in a Bash script

Perquackey-type anagramming game (Quackey script)

Petals Around the Rose

PID, Process ID, an identification number assigned to a running process.

Pipe, | , a device for passing the output of a command to another command or to the shell

- Avoiding unnecessary commands in a pipe

- Comments embedded within

- Exit status of a pipe

- Pipefail, set -o pipefail option to indicate exit status within a pipe

- `$PIPESTATUS`, exit status of last executed pipe

- Piping output of a command to a script

- Redirecting `stdin`, rather than using cat in a pipe

Pitfalls

- - (dash) is not redirection operator

- // (double forward slash), behavior of cd command toward

- #!/bin/sh script header disables extended Bash features

- Abuse of cat

- CGI programming, using scripts for

- Closing limit string in a here document, indenting

- DOS-type newlines (\r\n) crash a script

- Double-quoting the backslash (\) character

- eval, risk of using

- Execute permission lacking for commands within a script

- Exit status, anomalous

- Exit status of arithmetic expression not equivalent to an error code

- Export problem, child process to parent process

- Extended Bash features not available

- Failing to quote variables within test brackets

- GNU command set, in cross-platform scripts

- let misuse: attempting to set string variables

- Multiple echo statements in a function whose output is captured

- null variable assignment

- Numerical and string comparison operators not equivalent

  = and -eq not interchangeable

- Omitting terminal semicolon, in a curly-bracketed code block

- Piping

  echo to a loop

  echo to read (however, this problem can be circumvented)

  tail `-f` to grep

- Preserving whitespace within a variable, unintended consequences

- suid commands inside a script

- Undocumented Bash features, danger of

- Updates to Bash breaking older scripts

- Uninitialized variables

- Variable names, inappropriate

- Variables in a subshell, scope limited

- Subshell in while-read loop

- Whitespace, misuse of

Pointers

- and file descriptors

- and functions

- and indirect references

- and variables

Portability issues in shell scripting

- Setting path and umask

- A test suite script (Bash versus classic Bourne shell)

- Using whatis

Positional parameters

- `$@`, as separate words

- `$*`, as a single word

- in functions

POSIX, Portable Operating System Interface / UNIX

- `--posix` option

- 1003.2 standard

- Character classes

`$PPID`, process ID of parent process

Precedence, operator

Prepending lines at head of a file, script example

Prime numbers

- Generating primes using the factor command

- Generating primes using the modulo operator

- Sieve of Eratosthenes, example script

printf, formatted print command

`/proc` directory

- Running processes, files describing

- Writing to files in `/proc`, warning

Process

- Child process

- Parent process

- Process ID (PID)

Process substitution

- To compare contents of directories

- To supply `stdin` of a command

- Template

- while-read loop without a subshell

Programmable completion (tab expansion)

Prompt

- `$PS1`, Main prompt, seen at command line

- `$PS2`, Secondary prompt

Pseudo-code, as problem-solving method

`$PWD`, Current working directory

* * *

Quackey, a Perquackey-type anagramming game (script)

Question mark, ?

- Character match in an Extended Regular Expression

- Single-character wild card, in globbing

- In a C-style Trinary (ternary) operator

Quoting

- Character string

- Variables

  within test brackets

- Whitespace, using quoting to preserve

* * *

Random numbers

- `/dev/urandom`

- `rand()`, random function in awk

- `$RANDOM`, Bash function that returns a pseudorandom integer

- Random sequence generation, using date command

- Random sequence generation, using jot

- Random string, generating

Raspberry Pi (single-board computer)

- Script for preparing a bootable SD card

rcs

read, set value of a variable from `stdin`

- Detecting arrow keys

- Options

- Piping output of cat to read

- "Prepending" text

- Problems piping echo to read

- Redirection from a file to read

- `$REPLY`, default read variable

- Timed input

- while read construct

readline library

Recursion

- Demonstration of

- Factorial

- Fibonacci sequence

- Local variables

- Script calling itself recursively

- Towers of Hanoi

Redirection

- Code blocks

- exec <`filename`,

  to reassign file descriptors

- Introductory-level explanation of I/O redirection

- Open a file for both reading and writing

  `<>filename`

- read input redirected from a file

- `stderr` to `stdout`

  `2>&1`

- `stdin` / `stdout`, using -

- `stdin`of a function

- `stdout` to a file

  `>` ... `>>`

- `stdout` to file descriptor j

  `>&j`

- file descriptor`i` to file descriptor j

  `i>&j`

- `stdout` of a command to `stderr`

  `>&2`

- `stdout` and `stderr` of a command to a file

  `&>`

- tee, redirect to a file output of command(s) partway through a pipe

Reference Cards

- Miscellaneous constructs

- Parameter substitution/expansion

- Special shell variables

- String operations

- Test operators

  Binary comparison

  Files

Regular Expressions

- ^ (caret) Beginning-of-line

- $ (dollar sign) Anchor

- . (dot) Match single character

- * (asterisk) Any number of characters

- [ ] (brackets) Enclose character set to match

- \ (backslash) Escape, interpret following character literally

- \< ... \> (angle brackets, escaped) Word boundary

- Extended REs

  + Character match

  \{ \} Escaped "curly" brackets

  [: :] POSIX character classes

`$REPLY`, Default value associated with read command

Restricted shell, shell (or script) with certain commands disabled

return, command that terminates a function

run-parts

- Running scripts in sequence, without user intervention

* * *

Scope of a variable, definition

Script options, set at command line

Scripting routines, library of useful definitions and functions

Secondary prompt, `$PS2`

Security issues

- nmap, network mapper / port scanner

- sudo

- suid commands inside a script

- Viruses, trojans, and worms in scripts

- Writing secure scripts

sed, pattern-based programming language

- Table, basic operators

- Table, examples of operators

select, construct for menu building

- `in `list`` omitted

Semaphore

Semicolon required, when do keyword is on first line of loop

- When terminating curly-bracketed code block

seq, Emit a sequence of integers. Equivalent to jot.

set, Change value of internal script variables

- set -u, Abort script with error message if attempting to use an undeclared variable.

Shell script, definition of

Shell wrapper, script embedding a command or utility

shift, reassigning positional parameters

`$SHLVL`, shell level, depth to which the shell (or script) is nested

shopt, change shell options

Signal, a message sent to a process

Simulations

- Brownian motion

- Galton board

- Horserace

- Life, game of

- PI, approximating by firing cannonballs

- Pushdown stack

Single quotes (' ... ') strong quoting

Socket, a communication node associated with an I/O port

Sorting

- Bubble sort

- Insertion sort

source, execute a script or, within a script, import a file

- Passing positional parameters

Spam, dealing with

- Example script

- Example script

- Example script

- Example script

Special characters

Stack

- Definition

- Emulating a push-down stack, example script

Standard Deviation, example script

Startup files, Bash

`stdin` and `stdout`

Stopwatch, example script

Strings

- =~ String match operator

- Comparison

- Length

  `${#string}`

- Manipulation

- Manipulation, using awk

- Null string, testing for

- Protecting strings from expansion and/or reinterpretation, script example

  Unprotecting strings, script example

- strchr(), equivalent of

- strlen(), equivalent of

- strings command, find printable strings in a binary or data file

- Substring extraction

  ${string:position}

  ${string:position:length}

  Using expr

- Substring index (numerical position in string)

- Substring matching, using expr

- Substring removal

  ${var#Pattern}

  ${var##Pattern}

  ${var%Pattern}

  ${var%%Pattern}

- Substring replacement

  ${string/substring/replacement}

  ${string//substring/replacement}

  ${string/#substring/replacement}

  ${string/%substring/replacement}

  Script example

- Table of string/substring manipulation and extraction operators

Strong quoting ' ... '

Stylesheet for writing scripts

Subshell

- Command list within parentheses

- Variables, `$BASH_SUBSHELL` and `$SHLVL`

- Variables in a subshell

  scope limited, but ...

  ... can be accessed outside the subshell?

su Substitute user, log on as a different user or as root

suid (set user id) file flag

- suid commands inside a script, not advisable

Symbolic links

Swapfiles

* * *

Tab completion

Table lookup, script example

tail, echo to `stdout` lines at the (tail) end of a text file

tar, archiving utility

tee, redirect to a file output of command(s) partway through a pipe

Terminals

- setserial

- setterm

- stty

- tput

- wall

test command

- Bash builtin

- external command, `/usr/bin/test` (equivalent to `/usr/bin/[`)

Test constructs

Test operators

- -a Logical AND compound comparison

- -e File exists

- -eq is-equal-to (integer comparison)

- -f File is a regular file

- -ge greater-than or equal (integer comparison)

- -gt greater-than (integer comparison)

- -le less-than or equal (integer comparison)

- -lt less-than (integer comparison)

- -n not-zero-length (string comparison)

- -ne not-equal-to (integer comparison)

- -o Logical OR compound comparison

- -u suid flag set, file test

- -z is-zero-length (string comparison)

- = is-equal-to (string comparison)

  == is-equal-to (string comparison)

- < less-than (string comparison)

- < less-than, (integer comparison, within double parentheses)

- <= less-than-or-equal, (integer comparison, within double parentheses)

- > greater-than (string comparison)

- > greater-than, (integer comparison, within double parentheses)

- >= greater-than-or-equal, (integer comparison, within double parentheses)

- || Logical OR

- && Logical AND

- ! Negation operator, inverts exit status of a test

  != not-equal-to (string comparison)

- Tables of test operators

  Binary comparison

  File

Text and text file processing

Time / Date

Timed input

- Using read -t

- Using stty

- Using timing loop

- Using `$TMOUT`

Tips and hints for Bash scripts

- Array, as return value from a function

  Associative array more efficient than a numerically-indexed array

- Capturing the return value of a function, using echo

- CGI programming, using scripts for

- Comment blocks

  Using anonymous here documents

  Using if-then constructs

- Comment headers, special purpose

- C-style syntax , for manipulating variables

- Double-spacing a text file

- Filenames prefixed with a dash, removing

- Filter, feeding output back to same filter

- Function return value workarounds

- if-grep test fixup

- Library of useful definitions and functions

- null variable assignment, avoiding

- Passing an array to a function

- `$PATH`, appending to, using the `+=` operator.

- Prepending lines at head of a file

- Progress bar template

- Pseudo-code

- rcs

- Redirecting a test to `/dev/null` to suppress output

- Running scripts in sequence without user intervention, using run-parts

- Script as embedded command

- Script portability

  Setting path and umask

  Using whatis

- Setting script variable to a block of embedded sed or awk code

- Speeding up script execution by disabling unicode

- Subshell variable, accessing outside the subshell

- Testing a variable to see if it contains only digits

- Testing whether a command exists, using type

- Tracking script usage

- while-read loop without a subshell

- Widgets, invoking from a script

`$TMOUT`, Timeout interval

Token, a symbol that may expand to a keyword or command

tput, terminal-control command

tr, character translation filter

- DOS to Unix text file conversion

- Options

- Soundex, example script

- Variants

Trap, specifying an action upon receipt of a signal

Trinary (ternary) operator, C-style, `var>10?88:99`

- in double-parentheses construct

- in let construct

true, returns successful (0) exit status

typeset builtin

- options

* * *

`$UID`, User ID number

unalias, to remove an alias

uname, output system information

Unicode, encoding standard for representing letters and symbols

- Disabling unicode to optimize script

Uninitialized variables

uniq, filter to remove duplicate lines from a sorted file

unset, delete a shell variable

until loop

until [ condition-is-true ]; do

* * *

Variables

- Array operations on

- Assignment

  Script example

  Script example

  Script example

- Bash internal variables

- Block of sed or awk code, setting a variable to

- C-style increment/decrement/trinary operations

- Change value of internal script variables using set

- declare, to modify the properties of variables

- Deleting a shell variable using unset

- Environmental

- Expansion / Substring replacement operators

- Indirect referencing

  `eval variable1=\$$variable2`

  Newer notation

  `${!variable}`

- Integer

- Integer / string (variables are untyped)

- Length

  `${#var}`

- Lvalue

- Manipulating and expanding

- Name and value of a variable, distinguishing between

- Null string, testing for

- Null variable assignment, avoiding

- Quoting

  within test brackets

  to preserve whitespace

- rvalue

- Setting to null value

- In subshell not visible to parent shell

- Testing a variable if it contains only digits

- Typing, restricting the properties of a variable

- Undeclared, error message

- Uninitialized

- Unquoted variable, splitting

- Unsetting

- Untyped

* * *

wait, suspend script execution

- To remedy script hang

Weak quoting " ... "

while loop

while [ condition ]; do

- C-style syntax

- Calling a function within test brackets

- Multiple conditions

- Omitting test brackets

- while read construct

  Avoiding a subshell

Whitespace, spaces, tabs, and newline characters

- `$IFS` defaults to

- Inappropriate use of

- Preceding closing limit string in a here document, error

- Preceding script comments

- Quoting, to preserve whitespace within strings or variables

- [:space:], POSIX character class

who, information about logged on users

- w

- whoami

- logname

Widgets

Wild card characters

- Asterisk *

- In `[list]` constructs

- Question mark ?

- Will not match `dot files`

Word splitting

- Definition

- Resulting from command substitution

Wrapper, shell

* * *

xargs, Filter for grouping arguments

- Curly brackets

- Limiting arguments passed

- Options

- Processes arguments one at a time

- Whitespace, handling

* * *

yes

- Emulation

* * *

-z String is null

Zombie, a process that has terminated, but not yet been killed by its parent

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