Bash Pocket Reference

Second Edition

Arnold Robbins

Bash Pocket Reference

by Arnold Robbins

Copyright © 2016 Arnold Robbins. All rights reserved.

Printed in the United States of America.

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- March 2016: Second Edition

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- 2016-02-17: First Release

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978-1-491-94159-1

[M]

The Bash Shell

This pocket reference covers Bash, particularly version 4.4, the primary shell for GNU/Linux and Mac OS X. Bash is available for Solaris and the various BSD systems, and can be easily compiled for just about any other Unix system. It can even be compiled for OpenVMS! The following topics are covered:

- History

- Overview of features

- Invoking the shell

- Command exit status

- Syntax

- Functions

- Variables

- Arithmetic expressions

- Command history

- Programmable completion

- Job control

- Shell options

- Command execution

- Coprocesses

- Restricted shells

- Built-in commands

- Resources

Conventions

Filenames, command names, options, and inline examples are shown in constant width. Input that a user should type in exactly as-is is shown in constant width userinput. Text that should be replaced with real data in examples and syntax descriptions is shown in constant width replaceable. New terms and emphasized words and phrases are shown in italics. Finally, references of the form name(N) refer to the manual page for name in section N of the online manual (accessed via the man command). Shell variable values (including environment variables) are indicated as $VAR.

History

The original Bourne shell distributed with V7 Unix in 1979 became the standard shell for writing shell scripts. The Bourne shell is still found in /bin/sh on many commercial Unix systems. It has not changed that much since its initial release, although it has seen modest enhancements over the years. The most notable new features added were the CDPATH variable and a built-in test command with System III (circa 1980), command hashing and shell functions for System V Release 2 (circa 1984), and the addition of job control features for System V Release 4 (1989).

Because the Berkeley C shell (csh) offered features that were more pleasant for interactive use, such as command history and job control, for a long time the standard practice in the Unix world was to use the Bourne shell for programming and the C shell for daily use. David Korn at Bell Labs was the first developer to enhance the Bourne shell by adding csh-like features to it: history, job control, and additional programmability. Eventually, the Korn shell’s feature set surpassed both that of the Bourne and C shells, while remaining compatible with the former for shell programming. Today, the POSIX standard defines the “standard shell” language and behavior based on the System V Bourne shell, with a selected subset of features from the Korn shell.

The Free Software Foundation, in keeping with its goal to produce a complete Unix work-alike system, developed a clone of the Bourne shell, written from scratch, named “Bash,” the Bourne-Again SHell. Over time, Bash has become a POSIX-compliant version of the shell with many additional features overlapping those of the Korn shell, but Bash is not an exact Korn shell clone. Today, Bash is probably the most widely used Bourne-derived shell.

Overview of Features

The Bash shell provides the following features:

- Input/output redirection

- Wildcard characters for filename abbreviation

- Shell variables and options for customizing the environment

- A built-in command set for writing shell programs

- Shell functions, for modularizing tasks within a shell program

- Job control

- Command-line editing (using the command syntax of either vi or Emacs)

- Access to previous commands (command history) and the ability to edit them

- Integer arithmetic

- Arrays and arithmetic expressions

- Command-name abbreviation (aliasing)

- Upwards compliance with POSIX

- Internationalization facilities

- An arithmetic for loop

Invoking the Shell

The command interpreter for the Bash shell (bash) can be invoked as follows:

bash  [options]  [arguments]

Bash can execute commands from a terminal, from a file (when the first argument is a script), or from standard input (if no arguments remain or if -s is specified). The shell automatically prints prompts if standard input is a terminal, or if -i is given on the command line.

On many systems, /bin/sh is a link to Bash. When invoked as sh, Bash acts more like the traditional Bourne shell: login shells read /etc/profile and ~/.profile, and regular shells read $ENV, if it is set. Full details are available in the bash(1) manpage.

Command-Line Options

Almost all the single-letter command-line options may also be used with the built-in set command (see the entry for set). The options are:

-c str

Read commands from string str.

-D, --dump-strings

Print all $"…" strings in the program.

-i

Create an interactive shell (prompt for input). May not be used with set.

-l, --login

Behave as a login shell.

-O option

Enable shopt option option. Use +O to unset option.

-p

Start up as a privileged user. Do not read $ENV or $BASH_ENV; do not import functions from the environment; and ignore the values of the BASHOPTS, CDPATH, GLOBIGNORE, and SHELLOPTS variables. The normal fixed-name startup files (such as ~/.bash_profile) are read.

-r, --restricted

Create a restricted shell (see the section “Restricted Shells”).

-s

Read commands from standard input. Output from built-in commands goes to file descriptor 1; all other shell output goes to file descriptor 2.

-v, --verbose

Print lines as the shell reads them.

--debugger

If the debugging profile is available at startup, read it and turn on the extdebug option to shopt. For use by the Bash debugger (see http://bashdb.sourceforge.net).

--dump-po-strings

Same as -D, but output in GNU gettext format.

--help

Print a usage message and exit successfully.

--init-file file, --rcfile file

Use file as the startup file instead of ~/.bashrc for interactive shells.

--noediting

Do not use the readline library for input, even in an interactive shell.

--noprofile

Do not read /etc/profile or any of the personal startup files.

--norc

Do not read ~/.bashrc. Enabled automatically when invoked as sh.

--posix

Turn on POSIX mode.

--version

Print a version message and exit.

-, --

End option processing.

See the entry for set for the remaining options.

Arguments

Arguments are assigned to the positional parameters $1, $2, etc. If the first argument is a script, commands are read from it, and the remaining arguments are assigned to $1, $2, and so on. The name of the script is available as $0. The script file itself need not be executable, but it must be readable.

Command Exit Status

When any command exits, it provides a numeric exit status or return value. External commands, such as ls, provide this value to the operating system. Internal commands, such as cd, provide this value directly to the shell.

The shell automatically retrieves the return value when the command exits. An exit status of zero is defined, by convention, to mean true or success. Any other status means false or failure. This is how the shell makes use of commands in its control flow statements such as if, while, and until.

Additionally, the shell makes the return value of the last executed command available in $? where your shell script may access it. Usually you should save it in another variable, since subsequent commands that are part of your script will overwrite it.

Exit values may range from 0 to 255. The shell uses specific numeric values to indicate certain conditions:

Numeric value

Meaning

0

Success

2

Returned by built-in commands to indicate usage errors

126

Command was found but was not executable

127

Command not found

128 + N

Command died due to receiving signal number N

Syntax

This section describes the many symbols peculiar to the shell. The topics are arranged as follows:

- Special files

- Filename metacharacters

- Brace expansion

- Escape sequences

- Quoting

- Command forms

- Redirection forms

Special Files

The shell reads one or more startup files. Some of the files are read only when a shell is a login shell. Bash reads these files, in this order:

- /etc/profile. Executed automatically at login.

- The first file found from this list: ~/.bash_profile, ~/.bash_login, or ~/.profile. Executed automatically at login.

- ~/.bashrc is read by every nonlogin shell. However, if invoked as sh, or with --posix, Bash instead reads $ENV, for POSIX compatibility.

The getpwnam() and getpwuid() C library functions are the sources of home directories for ~name abbreviations. (On personal systems, the user database is stored in /etc/passwd. However, on networked systems, this information may come from NIS, NIS+, LDAP, or some other source, not your workstation password file.)

When an interactive login shell exits, or when non-interactive login shell executes the exit built-in command, Bash reads and executes ~/.bash_logout if that file exists. (A login shell is one where the -l option is set.)

Filename Metacharacters

*

Match any string of zero or more characters.

?

Match any single character.

[abc…]

Match any one of the enclosed characters; a hyphen can specify a range (e.g., a-z, A-Z, 0-9).

[!abc…]

Match any character not enclosed as above.

~

Home directory of the current user.

~name

Home directory of user name.

~+

Current working directory ($PWD).

~-

Previous working directory ($OLDPWD).

With the extglob option on:

?(pattern)

Match zero or one instance of pattern.

*(pattern)

Match zero or more instances of pattern.

+(pattern)

Match one or more instances of pattern.

@(pattern)

Match exactly one instance of pattern.

!(pattern)

Match any strings that don’t match pattern.

The pattern can be a sequence of patterns separated by |, meaning that the match applies to any of the patterns. This extended syntax resembles that available in egrep and awk.

With the globstar option on:

**

Match all files and zero or more subdirectories. When followed by a slash, only directories and subdirectories are matched.

Bash supports the POSIX [[=c=]] notation for matching characters that have the same weight, and [[.c.]] for specifying collating sequences. In addition, character classes, of the form [[:class:]], allow you to match the following classes of characters:

Class

Characters matched

alnum

Alphanumeric characters

alpha

Alphabetic characters

ascii

ASCII characters (not in POSIX)

blank

Space or Tab

cntrl

Control characters

digit

Decimal digits

graph

Nonspace characters

lower

Lowercase characters

print

Printable characters

punct

Punctuation characters

space

Whitespace characters

upper

Uppercase characters

word

[[:word:]] is the same as [[:alnum:]_] (not in POSIX)

xdigit

Hexadecimal digits

Tip

Bash reads scripts one line at a time. It parses each line completely before beginning to execute any of the commands on the line. This has two implications:

- You cannot define an alias and then use it on the same line.

- You should place commands that affect parsing of the script on lines by themselves, before the parts of the script that are affected.

Similar concerns apply to functions; they are parsed all at once, so you cannot turn on the extglob option inside a function body expecting it to affect just that function. Thus, in order to use the extended pattern matching facilities, you should put this command on a line by itself at the beginning of your script:

shopt -s extglob   # enable extended shell patterns

Examples

ls new*

List new and new.1

cat ch?

Match ch9 but not ch10

gvim [D-R]*

Match files beginning with D through R

pr !(*.o|core) | lpr

Print non-object and non-core files

Note

On modern systems, ranges such as [D-R] are not portable; the system’s locale may include more than just the uppercase letters from D to R in the range. However, see the globasciiranges shell option for a way to control this.

Brace Expansion

Bash has long supported brace expansion, based on a similar feature from the C shell. Unlike filename metacharacters, brace expansion is purely textual; the words created by brace expansion do not have to match existing files. There are two forms:

pre{X,Y[,Z…]}post

Expands to preXpost, preYpost, and so on.

pre{start..end[..incr]}post

start and end are either integers or single letters. incr is an integer. The shell expands this construct to the full range between start and end, increasing by incr if supplied.

The prefix and postfix texts are not required for either form. For numeric expansion, start or end or both may be prefixed with one or more leading zeros. The results of expansion are padded with zeros to the maximum of the widths of start and end. Bash ignores leading zeros on incr, always treating it as a decimal value.

Brace expansions may be nested, and the results are not sorted. Brace expansion is performed before other expansions, and the opening and closing braces must not be quoted for Bash to recognize them. Bash skips over command substitutions within brace expansions. To avoid conflict with parameter expansion, ${ cannot start a brace expansion.

Examples

# Expand textually; no sorting
$ echo hi{DDD,BBB,CCC,AAA}there
hiDDDthere hiBBBthere hiCCCthere hiAAAthere

# Expand, then match ch1, ch2, app1, app2
$ ls {ch,app}?

# Expands to mv info info.old
$ mv info{,.old}

# Simple numeric expansion
$ echo 1 to 10 is {1..10}
1 to 10 is 1 2 3 4 5 6 7 8 9 10

# Numeric expansion with increment
$ echo 1 to 10 by 2 is {1..10..2}
1 to 10 by 2 is 1 3 5 7 9

# Numeric expansion with zero padding
$ echo 1 to 10 with zeros is {01..10}
1 to 10 with zeros is 01 02 03 04 05 06 07 08 09 10

Escape Sequences

Bash recognizes and interprets special escape sequences in three different contexts:

- The $'…' quoted string

- Arguments to echo -e and printf %b

- Format strings for printf

The following table lists common escape sequences (those accepted in all contexts) as well as the unique escape sequences for each of the above contexts:

Sequence

Availability

Value

\a

All

ASCII BEL (visual or audible alert)

\b

All

Backspace

\c

echo -e, printf %b

Suppress the terminating newline (like echo -n) and do not print any following characters

\cX

$'…'

Control character X

\e

All

Escape

\E

All

Escape

\f

All

Formfeed

\n

All

Newline

\r

All

Carriage return

\t

All

Tab

\uHHHH

All

Unicode character HHHH

\UHHHHHHHH

All

Unicode character HHHHHHHH

\v

All

Vertical tab

\xHH

All

Hexadecimal value HH

\nnn

$'…', printf

Octal value nnn

\0nnn

echo -e, printf %b

Octal value nnn

\’

$'…'

Single quote

\"

$'…'

Double quote

\?

$'…'

Question mark

\\

All

Backslash

Additionally, the shell interprets a slightly overlapping set of escape sequences in the values of the PS0, PS1, PS2, and PS4 prompt strings. This is discussed in the section “Special Prompt Strings”.

Quoting

Quoting disables a character’s special meaning and allows it to be used literally. The following table displays characters that have special meaning:

Character

Meaning

;

Command separator

&

Background execution

()

Command grouping

|

Pipe

< > &

Redirection symbols

* ? [ ] ~ + - @ !

Filename metacharacters

" ' \

Used in quoting other characters

`

Command substitution

$

Variable substitution (or command or arithmetic substitution)

#

Start a comment that continues to the end of the line

space tab newline

Word separators

These characters can be used for quoting:

"…"

Everything between " and " is taken literally, except for the following characters that keep their special meaning:

$

Variable (or command and arithmetic) substitution will occur.

`

Command substitution will occur.

"

This marks the end of the double quoted string.

'…'

Everything between ' and ' is taken literally, except for another '. You cannot embed another ' within such a quoted string.

\

The character following a \ is taken literally. Use within "…" to escape ", $, and `. Often used to escape itself, spaces, or newlines.

$"…"

Just like "…", except that locale translation is done.

$'…'

Similar to '…', but the quoted text is processed for escape sequences as described in the section “Escape Sequences”.

Examples

$ echo 'Single quotes "protect" double quotes'
Single quotes "protect" double quotes
$ echo "Well, isn’t that \"special\"?"
Well, isn’t that "special"?
$ echo "You have `ls | wc -l` files in `pwd`"
You have      43 files in /home/bob
$ echo "The value of \$x is $x"
The value of $x is 100
$ echo $'A\tB'
A    B

Command Forms

cmd  &

Execute cmd in background.

cmd1 ; cmd2

Command sequence; execute multiple cmds on the same line.

{ cmd1 ; cmd2 ; }

Execute commands as a group in the current shell.

(cmd1 ; cmd2)

Execute commands as a group in a subshell.

cmd1 | cmd2

Pipe; use output from cmd1 as input to cmd2.

cmd1   `cmd2`

Command substitution; use cmd2 output as arguments to cmd1.

cmd1  $(cmd2)

POSIX shell command substitution; nesting is allowed.

cmd  $((expression))

POSIX shell arithmetic substitution. Use the numeric result of expression as a command-line argument to cmd.

cmd1 && cmd2

AND; execute cmd1 and then (if cmd1 succeeds) cmd2. This is a “short circuit” operation: cmd2 is never executed if cmd1 fails.

cmd1 || cmd2

OR; execute either cmd1 or (if cmd1 fails) cmd2. This is a “short circuit” operation; cmd2 is never executed if cmd1 succeeds.

! cmd

NOT; execute cmd, and produce a zero exit status if cmd exits with a nonzero status. Otherwise, produce a nonzero status when cmd exits with a zero status.

Examples

# Format in the background
$ nroff file > file.txt &

# Execute sequentially
$ cd; ls

# All output is redirected
$ (date; who; pwd) > logfile

# Sort file, page output, then print
$ sort file | pr -3 | lpr

# Edit files found by grep
$ gvim `grep -l ifdef *.cpp`

# Specify a list of files to search
$ egrep '(yes|no)' `cat list`

# POSIX version of previous
$ egrep '(yes|no)' $(cat list)

# Faster; not in POSIX
$ egrep '(yes|no)' $(< list)

# Print file if it contains the pattern
# Do so silently, by sending output and
# errors to /dev/null
$ grep XX file > /dev/null 2>&1 && lpr file

# Otherwise, echo an error message
$ grep XX file || echo "XX not found"

Redirection Forms

File descriptor

Name

Common abbreviation

Typical default

0

Standard input

stdin

Keyboard

1

Standard output

stdout

Screen

2

Standard error

stderr

Screen

The usual input source or output destination can be changed, as seen in the following sections.

Simple redirection

cmd > file

Send output of cmd to file (overwrite).

cmd >> file

Send output of cmd to file (append).

cmd < file

Take input for cmd from file.

cmd << text

The contents of the shell script up to a line identical to text become the standard input for cmd (text can be stored in a shell variable). This command form is sometimes called a here document. Input is typed at the keyboard or in the shell program. Commands that typically use this syntax include cat, ex, and sed. (If <<- is used, leading tabs are stripped from the contents of the here document, and the tabs are ignored when comparing input with the end-of-input text marker.) If any part of text is quoted, the input is passed through verbatim. Otherwise, the contents are processed for variable, command, and arithmetic substitutions.

cmd <<< word

Supply text of word, with trailing newline, as input to cmd. (This is known as a here string, from the free version of the rc shell; see the section “Resources”.)

cmd <> file

Open file for reading and writing on the standard input. The contents are not destroyed.[1]

cmd >| file

Send output of cmd to file (overwrite), even if the shell’s noclobber option is set.

Redirection using file descriptors

cmd >&n

Send cmd output to file descriptor n.

cmd m>&n

Same as previous, except that output that would normally go to file descriptor m is sent to file descriptor n instead.

cmd >&-

Close standard output.

cmd <&n

Take input for cmd from file descriptor n.

cmd m<&n

Same as previous, except that input that would normally come from file descriptor m comes from file descriptor n instead.

cmd <&-

Close standard input.

cmd <&n-

Move file descriptor n to standard input by duplicating it and then closing the original.

cmd >&n-

Move file descriptor n to standard output by duplicating it and then closing the original.

Multiple redirection

cmd 2> file

Send standard error to file; standard output remains the same (e.g., the screen).

cmd > file 2>&1

Send both standard output and standard error to file.

cmd >& file

Same as previous.

cmd &> file

Same as previous. Preferred form.

cmd  &>> file

Append both standard output and standard error to file.

cmd > filea 2> fileb

Send standard output to file filea and standard error to file fileb.

cmd  | tee files

Send output of cmd to standard output (usually the terminal) and to files. See tee(1).

cmd 2>&1 | tee files

Send standard output and error output of cmd through a pipe to tee to standard output (usually the terminal) and to files.

cmd  |& tee files

Same as previous.

Bash allows multidigit file descriptor numbers without any special syntax. Most other shells either require a special syntax or do not offer the feature at all.

Note

No space is allowed between file descriptors and a redirection symbol; spacing is optional in the other cases.

Process substitution

cmd  <(command)

Run command with its output connected to a named pipe or an open file in /dev/fd, and place the file’s name in the argument list of cmd. cmd may read the file to see the output of command.

cmd  >(command)

Run command with its input connected to a named pipe or an open file in /dev/fd, and place the file’s name in the argument list of cmd. Output written by cmd to the file is input to command.

Process substitution is available on systems that support either named pipes (FIFOs) or accessing open files via filenames in /dev/fd. (This is true of all modern Unix systems.) It provides a way to create non-linear pipelines.

Process substitution is not available in POSIX mode shells.

Saving file descriptors in variables

Bash allows {variablename} instead of a file descriptor number in redirections. In such a case, the shell uses a file descriptor number greater than 9, and assigns the value to the named shell variable. variablename may name array elements and variables that are special to the shell. For example:

# Save file descriptor number
$ echo foo {foofd}> /tmp/xyzzy
foo
$ echo $foofd
11

This is most often used with redirections with exec, so that you can use the file descriptor later on in a script.

Note

Once you open a file descriptor this way, you are responsible for closing it. Bash will not close it for you.

Special filenames

Bash recognizes several special filenames in redirections. These are interpreted internally by Bash only if they are not present on your system:

/dev/stdin

A duplicate of file descriptor 0.

/dev/stdout

A duplicate of file descriptor 1.

/dev/stderr

A duplicate of file descriptor 2.

/dev/fd/<n>

A duplicate of file descriptor <n>.

/dev/tcp/<host>/<port>

Bash opens a TCP connection to <host>, which is either a hostname or IP address, on port <port> and uses the file descriptor in the redirection.

/dev/udp/<host>/<port>

Bash opens a UDP connection to <host>, which is either a hostname or IP address, on port <port> and uses the file descriptor in the redirection.

Examples

# Copy part1 to book
$ cat part1 > book

# Append part2 and part3
$ cat part2 part3 >> book

# Send report to the big boss
$ mail tim < report

# Here document is sed’s input
$ sed 's/^/XX /g' << END_ARCHIVE
> This is often how a shell archive is "wrapped",
> bundling text for distribution.  You would normally
> run sed from a shell program, not from
> the command line.
> END_ARCHIVE
XX This is often how a shell archive is "wrapped",
XX bundling text for distribution.  You would normally
XX run sed from a shell program, not from
XX the command line.

To redirect standard output to standard error:

$ echo "Usage error: see administrator" 1>&2

The following command sends output (files found) to filelist, and error messages (inaccessible files) to no_access:

$ find / -print > filelist 2> no_access

The following sorts two files and presents the differences between the results using the diff command:

$ diff -u <(sort file1) <(sort file2) | less

Functions

A shell function is a grouping of commands within a shell script. Shell functions let you modularize your program by dividing it up into separate tasks. This way, the code for each task is not repeated every time you need to perform the task. The POSIX shell syntax for defining a function follows the Bourne shell:

name () {
function body’s code comes here
} [redirections]

Functions are invoked in the same way as regular shell built-in commands or external commands. The command-line parameters $1, $2, and so on receive the function’s arguments, temporarily hiding the global values of $1, $2, and so on. $0 remains the name of the full script. For example:

# fatal --- print an error message and die:

fatal () {
# Messages go to standard error.
echo "$0: fatal error:" "$@" >&2
exit 1
}
…
if [ $# = 0 ]    # not enough arguments
then
fatal not enough arguments
fi

A function may use the return command to return an exit status to the calling shell program.

Per the POSIX standard, any redirections given with the function definition are evaluated when the function executes, not when it is defined.

Bash allows you to define functions using a slightly different syntax, as follows:

function name [()] { body } [redirections]

When using the function keyword, the parentheses following the function name are optional.

Functions whose names do not include = or / may be exported into the environment with export -f; see the entry for export.

Functions share traps (see the entry for trap) with the “parent” shell as described in the following table:

Trap type

Shared/not shared

Signal-based traps

Shared until the function redefines the trap

DEBUG

Not shared unless function tracing is enabled (set -T or set -o functrace). If not enabled a DEBUG trap created by a function call remains in place when the function returns

ERR

Not shared unless error tracing is enabled (set -E or set -o errtrace)

EXIT

Shared until the function redefines the trap

RETURN

Not shared unless function tracing is enabled (set -T or set -o functrace)

Functions may have local variables, and they may be recursive. Unlike the Korn shell, the syntax used to define a function is irrelevant.

Function names do not have to be valid shell identifiers (just as external commands are not required to have names that are valid shell identifiers). This does not apply to POSIX mode shells, however. Additionally, POSIX mode shells disallow defining functions with the same name as a POSIX special built-in command. Doing so is an error in interactive shells; it is fatal in noninteractive shells.

Bash uses a dynamic scoping model, whereby variables declared with local are visible inside that function and in other functions that it calls. This is different from many other Bourne-style shells.

Tip

Be careful not to use exit from within a function unless you really wish to terminate the entire program.

Variables

This section describes the following:

- Variable assignment

- Variable substitution

- Indirect variables (namerefs)

- Built-in shell variables

- Other shell variables

- Arrays

- Special prompt strings

Variable Assignment

Variable names consist of any number of letters, digits, or underscores. Upper- and lowercase letters are distinct, and names may not start with a digit. Variables are assigned values using the = operator. There must not be any whitespace between the variable name and the value. You can make multiple assignments on the same line by separating each one with whitespace:

firstname=Arnold lastname=Robbins numkids=4 numpets=1

By convention, names for variables used or set by the shell have all uppercase letters; however, you can use uppercase names in your scripts if you use a name that isn’t special to the shell.

By default, the shell treats variable values as strings, even if the value of the string is all digits. However, when a value is assigned to an integer variable (created via declare -i), Bash evaluates the righthand side of the assignment as an expression (see the section “Arithmetic Expressions”). For example:

$ i=5+3 ; echo $i
5+3
$ declare -i jj ; jj=5+3 ; echo $jj
8

The += operator allows you to add or append the righthand side of the assignment to an existing value. Integer variables treat the righthand side as an expression, which is evaluated and added to the value. Arrays add the new elements to the array (see the section “Arrays”). For example:

$ name=Arnold              String variable
$ name+=" Robbins" ; echo $name
Arnold Robbins
$ declare -i jj ; jj=3+5   Integer variable
$ echo $jj
8
$ jj+=2+4 ; echo $jj
14
$ pets=(blacky rusty)      Array variable
$ echo ${pets[*]}
blacky rusty
$ pets+=(raincloud sophie)
$ echo ${pets[*]}
blacky rusty raincloud sophie

Variable Substitution

No spaces should be used in the following expressions. The colon (:) is optional; if it’s included, var must be nonnull as well as set. The variable substitution forms honor the value of the shell nocasematch option.

In noninteractive shells with set -u enabled, using an unset variable with the #, ##, %, %%, //, /#, /%, ^, ^^, ,, and ,, substitutions causes the shell to exit.

Consider single-quoted text in a variable substitution, such as ${var:=a'special-text'b}. Here, special-text is recognized as being quoted. However, in POSIX mode, when a variable substitution occurs inside double quotes, such single quotes do not define a new, nested quoting context. There are exceptions: single quotes do provide quoting when used with the #, ##, %, %%, //, /#, /%, ^, ^^, ,, and ,, substitutions.

var=value …

Set each variable var to a value.

${var}

Use value of var; braces are optional if var is separated from the following text. They are required for array variables.

${var:-value}

Use var if set; otherwise, use value.

${var:=value}

Use var if set; otherwise, use value and assign value to var.

${var:?value}

Use var if set; otherwise, print value and exit (if not interactive). If value isn’t supplied, print the phrase parameter null or not set to stderr.

${var:+value}

Use value if var is set; otherwise, use nothing.

${#var}

Use the length of var.

${#*}, ${#@}

Use the number of positional parameters.

${var#pattern}

Use value of var after removing text matching pattern from the left. Remove the shortest matching piece.

${var##pattern}

Same as #pattern, but remove the longest matching piece.

${var%pattern}

Use value of var after removing text matching pattern from the right. Remove the shortest matching piece.

${var%%pattern}

Same as %pattern, but remove the longest matching piece.

${var^pattern}

Convert the case of var to uppercase. The pattern is evaluated as for filename matching. If the first letter of var’s value matches the pattern, it is converted to uppercase. var can be * or @, in which case the positional parameters are modified. var can also be an array subscripted by * or @, in which case the substitution is applied to all the elements of the array.

${var^^pattern}

Same as ^pattern, but apply the match to every letter in the string.

${var,pattern}

Same as ^pattern, but convert matching characters to lowercase. Applies only to the first character in the string.

${var,,pattern}

Same as ,pattern, but apply the match to every letter in the string.

${var@a}

Use the flag values (as for declare) representing var’s attributes. var may be an array subscripted with @ or *, in which case the transform applies to all the elements.

${var@A}

A string in the form of a command or assignment statement that if evaluated recreates var and its attributes. var may be an array subscripted with @ or *, in which case the transform applies to all the elements.

${var@E}

The value of var with $'…' escape sequences evaluated (see the section “Escape Sequences”). var may be an array subscripted with @ or *, in which case the transform applies to all the elements.

${var@P}

The value of var with prompt string escape sequences evaluated (see the section “Special Prompt Strings”). var may be an array subscripted with @ or *, in which case the transform applies to all the elements.

${var@Q}

The value of var quoted in a way that allows entering the values as input. var may be an array subscripted with @ or *, in which case the transform applies to all the elements.

${!prefix*}, ${!prefix@}

List of variables whose names begin with prefix.

${var:pos}, ${var:pos:len}

Starting at position pos (0-based) in variable var, extract len characters, or extract rest of string if no len. pos and len may be arithmetic expressions. A negative len counts from the end of the string.

When var is * or @, the expansion is performed upon the positional parameters. If pos is zero, then $0 is included in the resulting list. Similarly, var can be an array indexed by * or @.

${var/pat/repl}

Use value of var, with first match of pat replaced with repl.

${var/pat}

Use value of var, with first match of pat deleted.

${var//pat/repl}

Use value of var, with every match of pat replaced with repl.

${var/#pat/repl}

Use value of var, with match of pat replaced with repl. Match must occur at the beginning of the value.

${var/%pat/repl}

Use value of var, with match of pat replaced with repl. Match must occur at the end of the value.

${!var}

Use value of var as name of variable whose value should be used (indirect reference).

Examples

$ u=up d=down blank=         Assign values to three variables
(last is null)
$ echo ${u}root              Braces are needed here
uproot
$ echo ${u-$d}               Display value of u or d; u is set,
up                           so print it
$ echo ${tmp-`date`}         If tmp not set, execute date
Tue Feb  2 22:52:57 EST 2016
$ echo ${blank="no data"}    blank is set, so it is printed
(blank line)
$ echo ${blank:="no data"}   blank is set but null, print string
no data
$ echo $blank                blank now has a new value
no data

# Take the current directory name and remove the
# longest character string ending with /, which
# removes the leading pathname and leaves the tail
$ tail=${PWD##*/}

# Use a famous word
$ word=supercalifragilisticexpialidocious

# Modify the case of the first character
$ echo ${word^[r-t]}
Supercalifragilisticexpialidocious

# Modify the case of all matching characters
$ echo ${word^^[r-t]}
SupeRcalifRagiliSTicexpialidociouS

Indirect Variables (namerefs)

Indirect variables, or namerefs, are variables that name a second variable. All actions (references, assignments, and attribute changes) applied to the nameref are done to the variable named by the nameref’s value. Namerefs are created using declare -n, removed using unset -n, and tested for with test -R. For example:

$ greeting="hello, world"        Regular variable assignment
$ declare -n message=greeting    Declare the nameref
$ echo $message                  Access through it
hello, world                     Value is $greeting
$ message="bye now"              Assign through the nameref
$ echo $greeting                 Demonstrate the change
bye now

Bash also provides a special syntax that lets one variable indirectly reference another, but assignments with this syntax are not possible:

$ text=greeting      Regular variable assignment
$ echo ${!text}      Use the alias
bye now

When a nameref is used as the control variable in a for loop, the loop terms are treated as variable names and the nameref refers to each in turn:

$ declare -n nr            Set up nameref
$ i=1                      A simple counter
$ for nr in v1 v2 v3       Start a loop
> do
>     nr=$((i++))          Each variable gets a unique value
> done
$ echo $v1 $v2 $v3         Show results
1 2 3

Converting an existing variable into a nameref disables the -c, -i, -l and -u attributes (see the entry for declare).

Built-In Shell Variables

The shell automatically sets built-in variables; they are typically used inside shell scripts. Built-in variables can use the variable substitution patterns shown previously. Note that the $ is not actually part of the variable name, although the variable is always referenced this way. The following are available in any Bourne-compatible shell:

$#

Number of command-line arguments.

$-

Options currently in effect (supplied on command line or to set). The shell sets some options automatically.

$?

Exit value of last executed command.

$$

Process number of the shell.

$!

Process number of last background command.

$0

First word; that is, the command name. This will have the full pathname if the command was found via a PATH search.

$n

Individual arguments on the command line (positional parameters). The Bourne shell allows only nine parameters to be referenced directly (n = 1–9); Bash allows n to be greater than 9 if specified as ${n}.

$*, $@

All arguments on the command line ($1 $2 …).

"$*"

All arguments on the command line as one string ("$1 $2…"). The values are separated by the first character in $IFS.

"$@"

All arguments on the command line, individually quoted ("$1" "$2" …).

Bash automatically sets the following additional variables:[2]

$_

Temporary variable; initialized to the pathname of the script or program being executed. Later, stores the last argument of the previous command. Also stores the name of the matching MAIL file during mail checks.

BASH

The full pathname used to invoke this instance of Bash.

BASHOPTS

A read-only, colon-separated list of shell options that are currently enabled. Each item in the list is a valid option for shopt -s. If this variable exists in the environment when Bash starts up, it sets the indicated options before executing any startup files.

BASHPID

The process ID of the current Bash process. In some cases, this can differ from $$.

BASH_ALIASES

Associative array variable. Each element holds an alias defined with the alias command. Adding an element to this array creates a new alias.

BASH_ARGC

Array variable. Each element holds the number of arguments for the corresponding function or dot-script invocation. Set only in extended debug mode, with shopt -s extdebug. It cannot be unset.

BASH_ARGV

An array variable similar to BASH_ARGC. Each element is one of the arguments passed to a function or dot-script. It functions as a stack, with values being pushed on at each call. Thus, the last element is the last argument to the most recent function or script invocation. Set only in extended debug mode, with shopt -s extdebug. It cannot be unset.

BASH_CMDS

Associative array variable. Each element refers to a command in the internal hash table maintained by the hash command. The index is the command name and the value is the full path to the command. Adding an element to this array adds a command to the hash table.

BASH_COMMAND

The command currently executing or about to be executed. Inside a trap handler, it is the command running when the trap was invoked.

BASH_EXECUTION_STRING

The string argument passed to the -c option.

BASH_LINENO

Array variable, corresponding to BASH_SOURCE and FUNCNAME. For any given function number i (starting at zero), ${FUNCNAME[i]} was invoked in file ${BASH_SOURCE[i]} on line ${BASH_LINENO[i]}. The information is stored with the most recent function invocation first. It cannot be unset.

BASH_REMATCH

Array variable, assigned by the =~ operator of the [[ ]] construct. Index zero is the text that matched the entire pattern. The other indices are the text matched by parenthesized subexpressions. This variable is read-only.

BASH_SOURCE

Array variable, containing source filenames. Each element corresponds to those in FUNCNAME and BASH_LINENO. It cannot be unset.

BASH_SUBSHELL

This variable is incremented by one each time a subshell or subshell environment is created.

BASH_VERSINFO[0]

The major version number, or release, of Bash.

BASH_VERSINFO[1]

The minor version number, or version, of Bash.

BASH_VERSINFO[2]

The patch level.

BASH_VERSINFO[3]

The build version.

BASH_VERSINFO[4]

The release status.

BASH_VERSINFO[5]

The machine type; same value as in $MACHTYPE.

BASH_VERSION

A string describing the version of Bash.

COMP_CWORD

For programmable completion. Index into COMP_WORDS, indicating the current cursor position.

COMP_KEY

For programmable completion. The key, or final key in a sequence, that caused the invocation of the current completion function.

COMP_LINE

For programmable completion. The current command line.

COMP_POINT

For programmable completion. The position of the cursor as a character index in $COMP_LINE.

COMP_TYPE

For programmable completion. A character describing the type of programmable completion. The character is one of Tab for normal completion, ? for a completions list after two Tabs, ! for the list of alternatives on partial word completion, @ for completions if the word is modified, or % for menu completion.

COMP_WORDBREAKS

For programmable completion. The characters that the readline library treats as word separators when doing word completion.

COMP_WORDS

For programmable completion. Array variable containing the individual words on the command line.

COPROC

Array variable that holds the file descriptors used for communicating with an unnamed coprocess. For more information, see the section “Coprocesses”.

DIRSTACK

Array variable, containing the contents of the directory stack as displayed by dirs. Changing existing elements modifies the stack, but only pushd and popd can add or remove elements from the stack.

EUID

Read-only variable with the numeric effective UID of the current user.

FUNCNAME

Array variable, containing function names. Each element corresponds to those in BASH_SOURCE and BASH_LINENO.

FUNCNEST

A value greater than zero defines the maximum function call nesting level. When exceeded, abort the current command.

GROUPS

Array variable, containing the list of numeric group IDs in which the current user is a member.

HISTCMD

The history number of the current command.

HOSTNAME

The name of the current host.

HOSTTYPE

A string that describes the host system.

LINENO

Current line number within the script or function.

MACHTYPE

A string that describes the host system in the GNU cpu-company-system format.

MAPFILE

Default array for the mapfile and readarray commands. See the entry for mapfile for more information.

OLDPWD

Previous working directory (set by cd, or inherited from the environment if it names a directory).

OPTARG

Value of argument to last option processed by getopts.

OPTIND

Numerical index of OPTARG.

OSTYPE

A string that describes the operating system.

PIPESTATUS

Array variable, containing the exit statuses of the commands in the most recent foreground pipeline. Note that a pipeline can contain only a single command.

PPID

Process number of this shell’s parent.

PWD

Current working directory (set by cd).

RANDOM[=n]

Generate a new random number with each reference; start with integer n, if given.

READLINE_LINE

For use with bind -x. The contents of the editing buffer are available in this variable.

READLINE_POINT

For use with bind -x. The index in $READLINE_LINE of the insertion point.

REPLY

Default reply; used by select and read.

SECONDS[=n]

Number of seconds since the shell was started, or, if n is given, number of seconds since the assignment + n.

SHELLOPTS

A read-only, colon-separated list of shell options (for set -o). If set in the environment at startup, Bash enables each option present in the list before reading any startup files.

SHLVL

Incremented by one every time a new Bash starts up.

UID

Read-only variable with the numeric real UID of the current user.

Many of these variables provide support for either programmable completion (see the section “Programmable Completion”) or for the Bash Debugger (see http://bashdb.sourceforge.net).

Other Shell Variables

The following variables are not automatically set by the shell, although many of them can influence the shell’s behavior. You typically set them in your .bash_profile or .profile file, where you can define them to suit your needs. Variables can be assigned values by issuing commands of the form:

variable=value

This list includes the type of value expected when defining these variables:

BASH_COMPAT

If set to a decimal or integer value (such as 4.3 or 43) that corresponds to a supported shell compatibility level, enables that compatibility level (e.g., 4.3 and 43 correspond to shopt -s compat43). If unset or set to the empty string, the compatibility is set to that of the current shell. The shopt command does not change this variable. This variable can be inherited from the environment.

BASH_ENV

If set at startup, names a file to be processed for initialization commands. The value undergoes parameter expansion, command substitution, and arithmetic expansion before being interpreted as a filename.

BASH_LOADABLES_PATH

One or more pathnames, delimited by colons, in which to search for dynamically loadable built-in commands specified by enable.

BASH_XTRACEFD=n

File descriptor to which Bash writes trace output (from set -x).

CDPATH=dirs

Directories searched by cd; allows shortcuts in changing directories; unset by default.

CHILD_MAX=n

Set the maximum number of child processes for which the shell will remember exit statuses. The maximum is 8192; the minimum is system-dependent.

COLUMNS=n

Screen’s column width; used in line edit modes and select lists. Defaults to current terminal width.

COMPREPLY=(words …)

Array variable from which Bash reads the possible completions generated by a completion function.

EMACS

If the value starts with t, Bash assumes it’s running in an Emacs buffer and disables line editing.

ENV=file

Name of script that is executed at startup in POSIX mode or when Bash is invoked as /bin/sh; useful for storing alias and function definitions. For example, ENV=$HOME/.shellrc.

EXECIGNORE=patlist

Colon-separated list of glob patterns describing the set of filenames to ignore when searching for executable files. Useful for ignoring shared library files which have execute permission. The value of the extglob shell option is honored.

FCEDIT=file

Editor used by fc command. The default is /bin/ed when Bash is in POSIX mode. Otherwise, the default is $EDITOR if set, vi if unset.

FIGNORE=patlist

Colon-separated list of suffixes describing the set of filenames to ignore when doing filename completion with the readline library.

GLOBIGNORE=patlist

Colon-separated list of patterns describing the set of filenames to ignore during pattern matching. The value of the nocasematch and extglob shell options are honored.

HISTCONTROL=list

Colon-separated list of values controlling how commands are saved in the history file. Recognized values are ignoredups, ignorespace, ignoreboth, and erasedups.

HISTFILE=file

File in which to store command history. Default value is ~/.bash_history.

HISTFILESIZE=n

Number of lines to be kept in the history file. This may be different from the number of commands. If zero, no commands are stored. If negative or nonnumeric, there is no limit. Default is 500.

HISTIGNORE=list

A colon-separated list of patterns that must match the entire command line. Matching lines are not saved in the history file. An unescaped & in a pattern matches the previous history line. The value of the extglob shell option is honored.

HISTSIZE=n

Number of history commands to be kept in the history list. If zero, no commands are stored. If negative or nonnumeric, there is no limit. Default is 500.

HISTTIMEFORMAT=string

A format string for strftime(3) to use for printing timestamps along with commands from the history command. If set (even if null), Bash saves timestamps in the history file along with the commands.

HOME=dir

Home directory; set by login (from the /etc/passwd file).

HOSTFILE=file

Name of a file in the same format as /etc/hosts that Bash should use to find hostnames for hostname completion.

IFS='chars'

Input field separators; default is space, Tab, and newline.

IGNOREEOF=n

Numeric value indicating how many successive EOF characters must be typed before Bash exits. If null or nonnumeric value, default is 10. Applies only to interactive shells.

INPUTRC=file

Initialization file for the readline library. This overrides the default value of ~/.inputrc.

LANG=locale

Default value for locale; used if no LC_* variables are set.

LC_ALL=locale

Current locale; overrides LANG and the other LC_* variables.

LC_COLLATE=locale

Locale to use for character collation (sorting order).

LC_CTYPE=locale

Locale to use for character class functions. (See the section “Filename Metacharacters”.)

LC_MESSAGES=locale

Locale to use for translating $"…" strings.

LC_NUMERIC=locale

Locale to use for the decimal-point character.

LC_TIME=locale

Locale to use for date and time formats.

LINES=n

Screen’s height; used for select lists. Defaults to current terminal height.

MAIL=file

Default file to check for incoming mail; set by login.

MAILCHECK=n

Number of seconds between mail checks; default is 60 (one minute).

MAILPATH=files

One or more files, delimited by colons, to check for incoming mail. Along with each file, you may supply an optional message that the shell prints when the file increases in size. Messages are separated from the filename by a ? character, and You have mail in $_ is the default message. $_ is replaced with the name of the file. For example, you might have MAILPATH="$MAIL?Candygram!:/etc/motd?New Login Message"

OPTERR=n

When set to 1 (the default value), Bash prints error messages from the built-in getopts command.

PATH=dirlist

One or more pathnames, delimited by colons, in which to search for commands to execute. The compiled-in default is /usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:.. The default for many systems is /bin:/usr/bin.

POSIXLY_CORRECT=string

When set at startup or while running, Bash enters POSIX mode, disabling behavior and modifying features that conflict with the POSIX standard.

PROMPT_COMMAND=command

If set, Bash executes this command each time before printing the primary prompt.

PROMPT_DIRTRIM=n

Indicates how many trailing directory components to retain for the \w or \W special prompt strings (see the section “Special Prompt Strings”). Elided components are replaced with an ellipsis.

PS0=string

String printed by interactive shells after reading a command but before executing it.

PS1=string

Primary prompt string; default is '\s-\v\$ '.

PS2=string

Secondary prompt (used in multiline commands); default is >.

PS3=string

Prompt string in select loops; default is #?.

PS4=string

Prompt string for execution trace (bash -x or set -x); default is +. Shells running as root do not inherit this variable from the environment.

SHELL=file

Name of user’s default shell (e.g., /bin/sh). Bash sets this if it’s not in the environment at startup.

TERM=string

Terminal type.

TIMEFORMAT=string

A format string for the output from the time keyword. See the bash(1) manual page for details.

TMOUT=n

If no command is typed after n seconds, exit the shell. Also affects the read command and the select loop.

TMPDIR=directory

Place temporary files created and used by the shell in directory.

auto_resume=list

Enable the use of simple strings for resuming stopped jobs. With a value of exact, the string must match a command name exactly. With a value of substring, it can match a substring of the command name.

histchars=chars

Two or three characters that control Bash’s csh-style history expansion. The first character signals a history event, the second is the “quick substitution” character, and the third indicates the start of a comment. The default value is !^#. See the section “C-Shell–Style History”.

Arrays

Bash provides two kinds of arrays: indexed arrays, where the indices are integers zero and above, and associative arrays, where the indices are strings.

Indexed arrays

Bash supports one-dimensional arrays. The first element is numbered zero. Bash has no limit on the number of elements. Arrays are initialized with a special form of assignment:

message=(hi there how are you today)

where the specified values become elements of the array. Individual elements may also be assigned:

message[0]=hi          # This is the hard way
message[1]=there
message[2]=how
message[3]=are
message[4]=you
message[5]=today

Declaring indexed arrays is not required. Any valid reference to a subscripted variable can create an array.

When referencing arrays, use the ${…} syntax. This isn’t needed when referencing arrays inside ((…)) (the form of let that does automatic quoting). Note that [ and ] are typed literally (i.e., they don’t stand for optional syntax).

Negative subscripts count from the last index plus one:

$ a=(0 1 2 3 4 5 6 7 8)     Create an indexed array
$ echo ${a[4]}              Use a positive index
4
$ echo ${a[-2]}             Use index: 8 + 1 − 2 = 7
7

Array substitutions

The variable substitutions for arrays and array elements are as follows:

${name[i]}

Use element i of array name; i can be any arithmetic expression as described in the section “Arithmetic Expressions”

${name}

Use element 0 of array name

${name[*]}, ${name[@]}

Use all elements of array name

${#name[*]}, ${#name[@]}

Use the number of elements in array name

Associative arrays

Bash provides associative arrays, where the indices are strings instead of numbers (as in awk). In this case, [ and ] act like double quotes. Associative arrays must be declared by using the -A option to the declare, local, and readonly commands. A special syntax allows assigning to multiple elements at once:

data=([joe]=30 [mary]=25)      Associative array assignment
message=([0]=hi [2]=there)     Indexed array assignment

Use ${data[joe]} and ${data[mary]} to retrieve the values.

The special expansions for retrieving all the indices of an associative array work just as they do for indexed arrays.

Special Prompt Strings

Bash processes the values of PS0, PS1, PS2, and PS4 for the following special escape sequences:

\a

An ASCII BEL character (octal 07).

\A

The current time in 24-hour HH:MM format.

\d

The date in “weekday month day” format.

\D{format}

The date as specified by the strftime(3) format format. The braces are required.

\e

An ASCII Escape character (octal 033).

\h

The hostname, up to the first period.

\H

The full hostname.

\j

The current number of jobs.

\l

The basename of the shell’s terminal device.

\n

A newline character.

\r

A carriage return character.

\s

The name of the shell (basename of $0).

\t

The current time in 24-hour HH:MM:SS format.

\T

The current time in 12-hour HH:MM:SS format.

\u

The current user’s username.

\v

The version of Bash.

\V

The release (version plus patchlevel) of Bash.

\w

The current directory, with $HOME abbreviated as ~. See also the description of the PROMPT_DIRTRIM variable.

\W

The basename of the current directory, with $HOME abbreviated as ~. See also the description of the PROMPT_DIRTRIM variable.

\!

The history number of this command (stored in the history).

\#

The command number of this command (count of commands executed by the current shell).

\$

If the effective UID is 0, a #; otherwise, a $.

\@

The current time in 12-hour a.m./p.m. format.

\nnn

The character represented by octal value nnn.

\\

A literal backslash.

\[

Start a sequence of nonprinting characters, such as for highlighting or changing colors in a terminal emulator.

\]

End a sequence of nonprinting characters.

The PS0, PS1, PS2, and PS4 variables undergo substitution for escape sequences, variable substitution, command substitution, and arithmetic substitution. The escape sequences are processed first, and then, if the promptvars shell option is enabled via the shopt command (the default), the substitutions are performed.

In POSIX mode, things work differently. The values of PS1 and PS2 undergo parameter expansion, ! is replaced with the history number of the current command, and !! is replaced with a literal exclamation point.

Arithmetic Expressions

The let command performs integer arithmetic. The shell provides a way to substitute arithmetic values (for use as command arguments or in variables); base conversion is also possible:

$(( expr ))

Use the value of the enclosed arithmetic expression. Bash attempts to parse $((…)) as an arithmetic expression before attempting to parse it as a nested command substitution.

B#n

Interpret integer n in numeric base B. For example, 8#100 specifies the octal equivalent of decimal 64.

Operators

The shell uses the following arithmetic operators in decreasing order of precedence (most are from the C programming language):

Operator

Description

++ --

Auto-increment and auto-decrement, both prefix and postfix

+ -

Unary plus and minus

! ~

Logical negation and binary inversion (one’s complement)

**

Exponentiation

* / %

Multiplication, division, modulus (remainder)

+ -

Addition, subtraction

<< >>

Bitwise left shift, bitwise right shift

< <= > >=

Less than, less than or equal to, greater than, greater than or equal to

== !=

Equality, inequality (both evaluated left to right)

&

Bitwise AND

^

Bitwise exclusive OR

|

Bitwise OR

&&

Logical AND (short circuit)

||

Logical OR (short circuit)

?:

Inline conditional evaluation

= += -=

*= /= %=

<<= >>=

Assignment

&= ^= |=

,

Sequential expression evaluation

Notes

Because let and ((…)) are built in to the shell, they have access to variable values. It is not necessary to precede a variable’s name with a dollar sign in order to retrieve its value (doing so does work, of course).

The exit status of let is confusing. It’s zero (success) for a non-zero mathematical result, and non-zero (failure) for a zero mathematical result.

Examples

let "count=0" "i = i + 1"    Assign values to i and count
let "num % 2"                Exit successfully if num is odd
(( percent <= 0 &&           Test the range of a value
percent <= 100 ))
a=5 b=2                      Set some values
echo $(("a" + "b"))          Variables may be double-quoted

See the entry for let for more information and examples.

Command History

The shell lets you display or modify previous commands. Using the history command, you can manage the list of commands kept in the shell’s history; see the entry for history for more information. All shells for which history is enabled (with set -o history) save their history, not just interactive shells.

This section focuses on the facilities for editing stored commands. Commands in the history list can be modified using:

- Line-edit mode

- The fc command

- C-shell–style history

Line-Edit Mode

Line-edit mode emulates many features of the vi and Emacs editors. The history list is treated like a file. When the editor is invoked, you type editing keystrokes to move to the command line you want to execute. You can also change the line before executing it. When you’re ready to issue the command, press the Enter key.

Emacs editing mode is the default. To control command-line editing, you must use either set -o vi or set -o emacs; Bash does not use variables to specify the editor.

Note that the vi editing mode starts in input mode; to type a vi command, press the Escape key first.

Common editing keystrokes

vi

Emacs

Result

k

CTRL-p

Get previous command

j

CTRL-n

Get next command

/string

CTRL-r string

Get previous command containing string

h

CTRL-b

Move back one character

l

CTRL-f

Move forward one character

b

ESC-b

Move back one word

w

ESC-f

Move forward one word

X

DEL

Delete previous character

x

CTRL-d

Delete character under cursor

dw

ESC-d

Delete word forward

db

ESC-h

Delete word backward

xp

CTRL-t

Transpose two characters

Both editing modes allow you to use the cursor keys to move around within the saved history.

The fc Command

fc stands for either “find command” or “fix command,” because it does both jobs. Use fc -l to list history commands and fc -e to edit them. See the entry for fc for more information.

Examples

$ history           List the last 16 commands
$ fc -l 20 30       List commands 20 through 30
$ fc -l -5          List the last 5 commands
$ fc -l cat         List all commands since the last cat
$ fc -l 50          List all commands since command 50
$ fc -ln 5 > doit   Save command 5 to file doit
$ fc -e vi 5 20     Edit commands 5 through 20 using vi
$ fc -e emacs       Edit previous command using Emacs

Tip

Interactive line-editing is easier to use than fc, because you can move up and down in the saved command history using your favorite editor commands (as long as your favorite editor is either vi or Emacs!). You may also use the Up and Down arrow keys to traverse the command history.

C-Shell–Style History

Besides the interactive editing features and POSIX fc command, Bash supports a command-line editing mode similar to that of the Berkeley C shell (csh). It can be disabled using set +H. Many users prefer the interactive editing features, but for those whose “finger habits” are still those of csh, this feature comes in handy.

In POSIX mode, history expansion does not occur inside double quotes. It is always inhibited inside single quotes.

In Bash 5.0, set +H will become the default.

Event designators

Event designators mark a command-line word as a history substitution:

Command

Description

!

Begin a history substitution

!!

Previous command

!n

Command number n in history list

!-n

n th command back from current command

!string

Most recent command that starts with string

!?string[?]

Most recent command that contains string

#

Current command line up to this point (fairly useless)

^old^new^

Quick substitution; change string old to new in previous command, and execute modified command

Word substitution

Word specifiers allow you to retrieve individual words from previous command lines. They follow an initial event specifier, separated by a colon. The colon is optional if followed by any of the following: ^, $, *, -, or %.

Specifier

Description

:0

Command name

:n

Argument number n

^

First argument

$

Last argument

%

Argument matched by a !?string? search

:n-m

Arguments n through m

-m

Words 0 through m; same as :0-m

:n-

Arguments n through next-to-last

:n*

Arguments n through last; same as n-$

*

All arguments; same as ^-$ or 1-$

History modifiers

There are several ways to modify command and word substitutions. The printing, substitution, and quoting modifiers are shown in the following table:

Modifier

Description

:p

Display command, but don’t execute.

:s/old/new

Substitute string new for old, first instance only.

:gs/old/new

Substitute string new for old, all instances.

:as/old/new

Same as :gs.

:Gs/old/new

Like :gs, but apply the substitution to all the words in the command line.

:&

Repeat previous substitution (:s or ^ command), first instance only.

:g&

Repeat previous substitution, all instances.

:q

Quote a word list.

:x

Quote separate words.

The truncation modifiers are shown in the following table:

Modifier

Description

:r

Extract the first available pathname root (the portion before the last period).

:e

Extract the first available pathname extension (the portion after the last period).

:h

Extract the first available pathname header (the portion before the last slash).

:t

Extract the first available pathname tail (the portion after the last slash).

Programmable Completion

Bash and the readline library provide completion facilities, whereby you can type part of a command name, hit the Tab key, and Bash fills in part or all of the rest of the command or filename. Programmable completion lets you, as a shell programmer, write code to customize the list of possible completions that Bash will present for a particular partially entered word. This is accomplished through the combination of several facilities:

- The complete command allows you to provide a completion specification, or compspec, for individual commands. You specify, via various options, how to tailor the list of possible completions for the particular command. This is simple, but adequate for many needs. (See the entry for complete.)

- For more flexibility, you can use complete -F funcname command. This tells Bash to call funcname to provide the list of completions for command. You write the funcname function.

- Within the code for a -F function, the COMP* shell variables provide information about the current command line. COMPREPLY is an array into which the function places the final list of completion results.

- Also within the code for a -F function, you may use the compgen command to generate a list of results, such as “usernames that begin with a” or “all set variables.” The intent is that such results would be used with an array assignment:

…
COMPREPLY=( $( compgen options arguments ) )
…

Compspecs may be associated with either a full pathname for a command or, more commonly, an unadorned command name (/usr/bin/man versus plain man). Completions are attempted in the following order, based on the options provided to the complete command:

1. If completion is attempted on an empty input line, Bash applies the compspec given with complete -E. Otherwise, it proceeds to the next step.

2. Bash first identifies the command. If a pathname is used, Bash looks to see if a compspec exists for the full pathname. Otherwise, it sets the command name to the last component of the pathname, and searches for a compspec for the command name.

3. If a compspec exists, Bash uses it. If not, Bash uses the “default” compspec given with complete -D. If there is none, then Bash falls back to the default built-in completions.

4. Bash performs the action indicated by the compspec to generate a list of possible matches. Of this list, only those that have the word being completed as a prefix are used for the list of possible completions. For the -d and -f options, Bash uses the variable FIGNORE to filter out undesirable matches.

5. Bash generates filenames as specified by the -G option. GLOBIGNORE is not used to filter the results, but FIGNORE is.

6. Bash processes the argument string provided to -W. The string is split using the characters in $IFS. The resulting list provides the candidates for completion. This is often used to provide a list of options that a command accepts.

7. Bash runs functions and commands as specified by the -F and -C options. For both, Bash sets COMP_LINE and COMP_POINT as described in the section “Built-In Shell Variables”. For a shell function, COMP_WORDS and COMP_CWORD are also set.
Also, for both functions and commands, $1 is the name of the command whose arguments are being completed, $2 is the word being completed, and $3 is the word in front of the word being completed. Bash does not filter the results of the command or function:

a. Functions named with -F are run first. The function should set the COMPREPLY array to the list of possible completions. Bash retrieves the list from there.

b. Commands provided with -C are run next, in an environment equivalent to command substitution. The command should print the list of possible completions, one per line. An embedded newline should be escaped with a backslash.

8. Once the list is generated, Bash filters the results according to the -X option. The argument to -X is a pattern specifying files to exclude. By prefixing the pattern with a !, the sense is reversed, and the pattern instead specifies that only matching files should be retained in the list. The value of the nocasematch shell option is honored.
An & in the pattern is replaced with the text of the word being completed. Use \& to produce a literal &.

9. Finally, Bash prepends or appends any prefixes or suffixes supplied with the -P or -S options.

10. In the case that no matches were generated, if -o dirnames was used, Bash attempts directory name completion.

11. On the other hand, if -o plusdirs was provided, Bash adds the result of directory completion to the previously generated list.

12. Normally, when a compspec is provided, Bash’s default completions are not attempted, nor are the readline library’s default filename completions. However:

a. If the compspec produces no results and -o bashdefault was provided, then Bash attempts its default completions.

b. If neither the compspec nor the Bash default completions with -o bashdefault produced any results, and -o default was provided, then Bash has the readline library attempt its filename completions.

A compspec may be modified with the compopt command. When used without command names inside an executing completion, it affects the executing completion.

When a shell function used as a completion handler returns 124, Bash retries the completion process from the beginning. This is most useful with the default completion handler (complete -D) to dynamically build up a set of completions instead of loading a large set at startup. The bash(1) manpage has an example at the end of its Programmable Completion section.

Tip

Ian Macdonald has collected a large set of useful compspecs, often distributed as the file /etc/bash_completion. If your system does not have it, you may be able to install it with your system’s package manager. In the worst case, you can download it from http://bash-completion.alioth.debian.org/. It is worth reviewing.

Examples

Restrict files for the C compiler to C, C++, and assembler source files, and relocatable object files:

complete -f -X '!*.[Ccos]' gcc cc

For the man command, restrict expansions to things that have manpages:

# Simple example of programmable completion for manual
# pages.  A more elaborate example appears in the
# bash_completion file.
# Assumes   man [num] command   command syntax.

shopt -s extglob    # Enable extended pattern matching

# Define completion function
_man () {
# Local variables
local dir mandir=/usr/share/man

# Clear reply list
COMPREPLY=(  )

# If section number ...
if [[ ${COMP_WORDS[1]} = +([0-9]) ]]
then
# section provided: man 3 foo
# look in specified directory
dir=$mandir/man${COMP_WORDS[COMP_CWORD-1]}
else
# no section, default to commands
# look in command directories
dir=$mandir/'man[18]'
fi
COMPREPLY=( $(
# Generate raw file list
find $dir -type f |

# Remove leading directories
sed 's;..*/;;' |

# Remove trailing suffixes
sed 's/\.[0-9].*$//' |

# Keep those that match given prefix
grep "^${COMP_WORDS[$COMP_CWORD]}" |

# Sort final list
sort
) )
}

# Associate function with command
complete -F _man man

Job Control

Job control lets you place foreground jobs in the background, bring background jobs to the foreground, or suspend running jobs. All modern Unix systems — including Mac OS X, GNU/Linux and BSD systems — support it, so the job control features are automatically enabled. Many job control commands take a jobID as an argument, which can be specified as follows:

%n

Job number n

%s

Job whose command line starts with string s

%?s

Job whose command line contains string s

%%

Current job

%+

Current job (same as %%)

%

Current job (same as %%)

%-

Previous job

The shell provides the following job control commands (for more information on these commands, see the section “Built-In Commands”):

bg

Put the current job in the background.

fg

Put the current job in the foreground.

jobs

List active jobs.

kill

Terminate a job.

stty tostop

Stop background jobs if they try to send output to the terminal emulator. (Note that stty is not a built-in command.)

suspend

Suspend a job-control shell (such as one created by su).

wait

Wait for background jobs to finish.

CTRL-Z

Suspend a foreground job. Then use bg or fg. (Your terminal emulator may use something other than CTRL-Z as the suspend character, but this is unlikely.)

Shell Options

Bash provides a number of shell options, settings that you can change to modify the shell’s behavior. You control these options with the shopt command (see the entry for shopt).

The compatNN options are all mutually exclusive with each other. The compatibility level indicates a minimum level. For example, at compat40, the shell behaves like Bash 4.0 for the features affected by compatibility settings that changed after 4.0. Use of BASH_COMPAT is preferred.

The following descriptions describe the behavior when set. Options marked with a dagger (†) are enabled by default:

autocd

When the first word of a simple command cannot be executed, try to cd to it. If there is a function named cd, Bash will run it instead of calling the built-in cd.

cdable_vars

Treat a nondirectory argument to cd as a variable whose value is the directory to go to.

cdspell

Attempt spelling correction on each directory component of an argument to cd. Allowed in interactive shells only.

checkhash

Check that commands found in the hash table still exist before attempting to use them. If not, perform a normal PATH search.

checkjobs

When an attempt is made to exit a shell and there are stopped or running background jobs, the shell prints There are running jobs. and a list of jobs and their statuses. A second exit attempt (such as typing EOF again) causes the shell to exit.

checkwinsize

Check the window size after each command, and update LINES and COLUMNS if the size has changed. This works in both interactive and noninteractive shells.

cmdhist †

Save all lines of a multiline command in one history entry. This permits easy re-editing of multiline commands.

compat31

Restore the behavior of the =~ operator for the [[ ]] command whereby the righthand side is always treated as a regular expression to be matched. In addition, the < and > operators ignore the locale when doing string comparison.

compat32

Cause the < and > operators of the [[ ]] command to ignore the locale when doing string comparison. In addition, interrupting a command in the middle of a command list such as cmd1; cmd2; cmd3 does not abort execution of the entire list.

compat40

Cause the < and > operators of the [[ ]] command to ignore the locale when doing string comparison.

compat41

In POSIX mode, treat single quotes inside a double-quoted parameter expansion as quoting characters. There must be an even number of single quotes, and their contents are treated as quoted.

compat42

Do not process the replacement string in a pattern substitution word expansion using quote removal.

compat43

Do not print a warning message when using a quoted compound assignment in an argument to declare; treat word expansion errors as nonfatal, causing the current command to fail, even in POSIX mode; and do not reset the loop state inside a function, causing break and continue in a function to affect loops in the function’s caller.

complete_fullquote †

When performing filename completion, quote all shell metacharacters with preceding backslashes. If disabled, dollar signs (and possibly other characters) are not quoted, so that shell variable expansion occurs as expected.

direxpand

When performing filename completion, replace directory names with the results of word expansion, modifying the readline editing buffer.

dirspell

Attempt spelling correction on directory names during word completion if the name as given does not exist.

dotglob

Include filenames starting with a period in the results of filename expansion.

execfail

Do not exit a noninteractive shell if the command given to exec cannot be executed. Interactive shells do not exit in such a case, no matter the setting of this option.

expand_aliases †

Expand aliases created with alias. Disabled in noninteractive shells.

Enable behavior needed for debuggers:
extdebug

- declare -F displays the source filename and line number for each function name argument.

- When a command run by the DEBUG trap fails, the next command is skipped.

- When a command run by the DEBUG trap inside a shell function or script sourced with . (dot) or source returns with an exit status of 2, the shell simulates a call to return.

- BASH_ARGC and BASH_ARGV are set as described earlier.

- Function tracing is enabled. Command substitutions, shell functions, and subshells invoked via (…) inherit the DEBUG and RETURN traps.

- Error tracing is enabled. Command substitutions, shell functions, and subshells invoked via (…) inherit the ERR trap.

extglob

Enable extended pattern-matching facilities such as +(…). Enabled automatically in POSIX mode. (These were not in the original Bourne shell; thus Bash requires you to enable them if you want them.)

extquote †

Allow $'…' and $"…" within ${variable} expansions inside double quotes.

failglob

Cause patterns that do not match filenames to produce an error.

force_fignore †

When doing completion, ignore words matching the list of suffixes in FIGNORE, even if such words are the only possible completions.

globasciiranges

Expand ranges used in pattern matching bracket expressions as if they were in the "C" locale, ignoring the current locale’s collating sequence. This will be enabled by default in Bash 5.0.

globstar

Enable extended directory and subdirectory matching with the special ** pattern.

gnu_errfmt

Print error messages in the standard GNU format. Enabled automatically when Bash runs in an Emacs terminal window.

histappend

Append the history list to the file named by $HISTFILE upon exit, instead of overwriting the file.

histreedit

Allow a user to re-edit a failed csh-style history substitution with the readline library.

histverify

Place the results of csh-style history substitution into the readline library’s editing buffer instead of executing it directly, in case the user wishes to modify it further.

hostcomplete †

If using readline, attempt hostname completion when a word containing an @ is being completed.

huponexit

Send a SIGHUP to all running jobs upon exiting an interactive login shell.

inherit_errexit

Cause command substitution to inherit the value of set -e. Enabled automatically in POSIX mode.

interactive_comments †

Allow words beginning with # to start a comment in an interactive shell.

lastpipe

In non-job control shells, run the last command of a foreground pipeline in the current shell environment. All commands but the last run in subshells.

lithist

If cmdhist is also set, save multiline commands to the history file with newlines instead of semicolons.

login_shell

Set by the shell when it is a login shell. This is a read-only option.

mailwarn

Print the message The mail in mailfile has been read when a file being checked for mail has been accessed since the last time Bash checked it.

no_empty_cmd_completion

If using readline, do not search $PATH when a completion is attempted on an empty line, or a line consisting solely of whitespace.

nocaseglob

Ignore letter case when doing filename matching.

nocasematch

Ignore letter case when doing pattern matching for case and [[ ]].

nullglob

Expand patterns that do not match any files to the null string, instead of using the literal pattern as an argument.

progcomp †

Enable programmable completion.

promptvars †

Perform variable, command, and arithmetic substitution on the values of PS0, PS1, PS2, and PS4.

restricted_shell

Set by the shell when it is a restricted shell. This is a read-only option.

shift_verbose

Cause shift to print an error message when the shift count is greater than the number of positional parameters.

sourcepath †

Cause the . (dot) and source commands to search $PATH in order to find the file to read and execute.

xpg_echo

Cause echo to expand escape sequences, even without the -e or -E options.

Command Execution

When you type a command, Bash looks in the following places (in this order) until it finds a match:

1. Keywords such as if and for.

2. Aliases. In POSIX mode, you can’t define an alias whose name is a shell keyword, but you can define an alias that expands to a keyword (e.g., alias aslongas=while). When not in POSIX mode, Bash does allow you to define an alias for a shell keyword.
Normally, alias expansion is enabled only in interactive shells. POSIX mode shells always enable it.

3. POSIX shells only: Special built-ins like break and continue. The list of POSIX special built-ins is . (dot), :, break, continue, eval, exec, exit, export, readonly, return, set, shift, times, trap, and unset. Bash adds source.
An error from a POSIX special built-in causes noninteractive shells to exit.

4. Functions. When not in POSIX mode, Bash finds functions before all built-in commands.

5. Nonspecial built-ins such as cd and test.

6. Scripts and executable programs, for which the shell searches in the directories listed in the PATH environment variable. NOTE: In POSIX mode, tildes in $PATH elements are not expanded. Additionally, if a command in the hash table no longer exists, Bash re-searches $PATH.

7. When a command is not found, if a function named command_not_found_handle exists, the shell calls it, passing the command words as the function arguments.

The distinction between “special” built-in commands and nonspecial ones comes from POSIX. This distinction, combined with the command command, makes it possible to write functions that override shell built-ins, such as cd. For example:

# Shell function; found before built-in cd
cd () {
command cd "$@"     Use real cd to change directory
echo now in $PWD    Other stuff we want to do
}

If Bash exits due to receiving SIGHUP, or if the huponexit shell option is set, Bash sends a SIGHUP to all running child jobs. Use disown -h to prevent Bash from sending SIGHUP to a particular job.

Coprocesses

A coprocess is a process that runs in parallel with the shell and with which the shell can communicate. The shell starts the process in the background, connecting its standard input and output to a two-way pipe. (The coprocess’s standard error is not redirected.)

There are two syntaxes for running a coprocess:

coproc name non-simple command  Start a named coprocess

coproc command args             Start an unnamed coprocess

The shell creates an array variable named name to hold the file descriptors for communication with the coprocess. name[0] is the output of the coprocess (input to the controlling shell) and name[1] is the input to the coprocess (output from the shell). In addition, the variable name_PID holds the process-ID of the coprocess. When no name is supplied, the shell uses COPROC.

Note

There can be only one active coprocess at a time.

Example

The following example demonstrates the basic usage of the coproc keyword and the related variables:

# Start a named coprocess in the background
$ coproc testproc (echo 1
> read aline ; echo $aline)
[1] 5090

# Show the file descriptors
$ echo ${testproc[@]}
63 60

# Show the coprocess PID
$ echo $testproc_PID
5090

# Read the first line of coprocess output and show it
$ read out <&${testproc[0]}
$ echo $out
1

# Send coprocess some input
$ echo foo >&${testproc[1]}

# Read second output line
$ read out2 <&${testproc[0]}
[1]+ Done  coproc testproc (echo 1; read aline; echo $aline)

# Show the second output line
$ echo $out2
foo

Restricted Shells

A restricted shell is one that disallows certain actions, such as changing directory, setting PATH, or running commands whose names contain a / character.

The original V7 Bourne shell had an undocumented restricted mode. Later versions of the Bourne shell clarified the code and documented the facility. Bash also supplies a restricted mode. (See the manual page for the details.)

Shell scripts can still be run, since in that case the restricted shell calls the unrestricted version of the shell to run the script. This includes /etc/profile, ~/.profile, and the other startup files.

Tip

Restricted shells are not used much in practice, as they are difficult to set up correctly.

Built-In Commands

Examples to be entered as a command line are shown with the $ prompt. Otherwise, examples should be treated as code fragments that might be included in a shell script. For convenience, the reserved words used by multiline commands are also included.

Almost all built-in commands recognize the --help option and print a usage summary in response to it.

!

Invert the sense of the following pipeline.

! pipeline

Negate the sense of a pipeline. Returns an exit status of 0 if the pipeline exited nonzero, and an exit status of 1 if the pipeline exited zero. Typically used in if and while statements.

Example

This code prints a message if user jane is not logged on:

if ! who | grep jane > /dev/null
then
echo jane is not currently logged on
fi

#

Introduce a comment that runs to the end of the line.

# text …

Ignore all text that follows on the same line. # is used in shell scripts as the comment character and is not really a command.

#!shell

Invoke the named interpreter to execute the script.

#!shell [option]

Used as the first line of a script to invoke the named shell. Anything given on the rest of the line is passed as a single argument to the named shell. For example:

#!/bin/sh

Tip

This feature is typically implemented by the kernel, but may not be supported on some very old systems. Some systems have a limit of around 32 characters on the maximum length of shell.

:

Do-nothing command, used as a syntactic placeholder.

: [arguments]

Null command. Returns an exit status of 0. See this Example and the ones under the entry for case. The line is still processed for side effects, such as variable and command substitutions, or I/O redirection.

Example

Check whether someone is logged in:

if who | grep $1 > /dev/null
then :   # Do nothing if user is found
else echo "User $1 is not logged in"
fi

.

Read and execute a file within the current shell.

.  file [arguments]

Read and execute lines in file. file does not have to be executable but must reside in a directory searched by $PATH. If the sourcepath option is disabled, Bash does not search $PATH. The arguments are stored in the positional parameters. If file is not found in $PATH, Bash looks in the current directory for file. (Note that POSIX shells do not do so.) Bash removes zero (ASCII NUL) bytes from the contents of file before attempting to parse it. Noninteractive POSIX-mode shells exit if file is not found, unless prefixed with command. See also the entry for source.

When the -T option is set, traps on DEBUG are inherited, and any changes made to the DEBUG trap by file remain in place upon return to the calling shell. If -T is not set, DEBUG traps are saved and restored around the call to file, and the DEBUG trap is unset while file executes.

[[ ]]

Extended version of the test command.

[[ expression ]]

Same as test expression or [ expression ], except that [[ ]] allows additional operators. Word splitting and filename expansion are disabled. Note that the brackets ([ ]) are typed literally, and that they must be surrounded by whitespace. See the entry for test.

Additional Operators

&&

Logical AND of test expressions (short circuit).

||

Logical OR of test expressions (short circuit).

<

First string is lexically “less than” the second, based on the locale’s sorting order. (However, see the description of the compat31, compat32, and compat40 options in the section “Shell Options”.)

>

First string is lexically “greater than” the second, based on the locale’s sorting order. (However, see the description of the compat31, compat32, and compat40 options in the section “Shell Options”.)

name ( )

Define a shell function.

name () { commands; } [redirections]

Define name as a function. POSIX syntax. The function definition can be written on one line or across many. You may also provide the function keyword, an alternate form that works similarly. See the section “Functions”.

Example

$ countfiles () {
>     ls | wc -l
> }

When issued at the command line, countfiles displays the number of files in the current directory.

alias

Define and manage shell aliases.

alias [-p] [name[='cmd']]

Assign a shorthand name as a synonym for cmd. If ='cmd' is omitted, print the alias for name; if name is also omitted, print all aliases. If the alias value contains a trailing space, the next word on the command line also becomes a candidate for alias expansion. The BASH_ALIASES array provides programmatic access to all defined aliases; see the section “Built-In Shell Variables”. See also the entry for unalias.

Tip

In general, functions are preferred to aliases; they let you have local variables and are fully programmable.

Option

-p

Print the word alias before each alias.

Example

alias dir='echo ${PWD##*/}'

bg

Move a stopped job into the background.

bg [jobIDs]

Put current job or jobIDs in the background. See the section “Job Control”.

bind

Manage key bindings for the readline library.

bind [-m map] [options]
bind [-m map] [-q function] [-r sequence]
[-u function]
bind [-m map] -f file
bind [-m map] -x sequence:command
bind [-m map] sequence:function
bind readline-command

Manage the readline library. Nonoption arguments have the same form as in a .inputrc file.

Options

-f file

Read key bindings from file.

-l

List the names of all the readline functions.

-m map

Use map as the keymap. Available keymaps are emacs, emacs-ctlx, emacs-standard, emacs-meta, vi, vi-command, vi-insert, and vi-move. vi is the same as vi-command, and emacs is the same as emacs-standard.

-p

Print the current readline bindings such that they can be reread from a .inputrc file.

-P

Print the current readline bindings.

-q function

Query which keys invoke the readline function function.

-r sequence

Remove the binding for key sequence sequence.

-s

Print the current readline key sequence and macro bindings such that they can be reread from a .inputrc file.

-S

Print the current readline key sequence and macro bindings.

-u function

Unbind all keys that invoke the readline function function.

-v

Print the current readline variables such that they can be reread from a .inputrc file.

-V

Print the current readline variables.

-x sequence:command

Execute the shell command command whenever sequence is entered. The command may make use of and modify the READLINE_LINE and READLINE_POINT variables. Changes to these variables are reflected in the editing state.

-X

Print the current readline key sequences bound with -x such that they can be reread from a .inputrc file.

break

Exit from one or more loops.

break [n]

Exit from a for, while, select, or until loop (or break out of n nested loops).

builtin

Execute a built-in command, bypassing functions.

builtin command [arguments …]

Run the shell built-in command command with the given arguments. This allows you to bypass any functions that redefine a built-in command’s name. The command command is more portable.

Example

This function lets you do your own tasks when you change directory:

cd () {
builtin cd "$@"       # Actually change directory
pwd                   # Report location
}

caller

Print function or dot-file caller, for use with the Bash debugger.

caller [expression]

Print the line number and source filename of the current function call or dot file. With nonzero expression, print that element from the call stack. The most recent is zero. This command is for use by the Bash debugger.

case

Syntax for a case statement.

case value in
[(]pattern1) cmds1;;  # ;&  or  ;;& -- see text
[(]pattern2) cmds2;;
. . .
esac

Execute the first set of commands (cmds1) if value matches pattern1; execute the second set of commands (cmds2) if value matches pattern2, and so on. Be sure the last command in each set ends with ;;. value is typically a positional parameter or other shell variable. cmds are typically executable commands, shell programming commands, or variable assignments. Patterns can use file-generation metacharacters. Multiple patterns (separated by |) can be specified on the same line; in this case, the associated cmds are executed whenever value matches any of these patterns. See the Examples here and under the entry for eval.

The shell allows pattern to be preceded by an optional open parenthesis, as in (pattern). For some shell versions, it’s necessary for balancing parentheses inside a $( ) construct. Bash 4.0 and later do not require it. See also the nocasematch option in the section “Shell Options”.

Bash provides two additional special terminators for the cmds in a case statement: ;& causes execution to continue into the next set of cmds, and ;;& causes the next pattern list to be tested.

Examples

Check first command-line argument and take appropriate action:

case $1 in     # Match the first arg
no|yes) response=1;;
-[tT])  table=TRUE;;
*)      echo "unknown option"; exit 1;;
esac

Read user-supplied lines until user exits:

while true
do    printf "Type . to finish ==> "
read line
case "$line" in
.)  echo "Message done"
break ;;
*)  echo "$line" >> $message ;;
esac
done

cd

Change directory.

cd [-L] [-P [-e]] [-@] [dir]
cd [-L] [-P [-e]] [-@] [-]

With no arguments, change to home directory of user. Otherwise, change working directory to dir. Bash searches the directories given in $CDPATH first, and then looks in the current directory for dir. If dir is a relative pathname but is not in the current directory, then also search $CDPATH. A directory of - stands for the previous directory. This command exits with a failure status if PWD is read-only.

Options

-e

With -P, if the current directory cannot be determined, exit with a failure value.

-L

Use the logical path (what the user typed, including any symbolic links) for cd .. and the value of PWD. This is the default.

-P

Use the filesystem physical path for cd .. and the value of PWD.

-@

On systems supporting extended attributes, treat a file with extended attributes as a directory containing the file’s attributes.

Example

$ ls -ld /var/run                /var/run is a symbolic link
lrwxrwxrwx 1 root root 4 May  7 19:41 /var/run -> /run
$ cd -L /var/run                 Logical change directory
$ pwd                            Show location
/var/run                         Result is logical location
$ cd -P /var/run                 Physical change directory
$ pwd                            Show location
/run                             Result is physical location

command

Execute or print information about a built-in command.

command [-pvV] name [arg …]

Without -v or -V, execute name with given arguments. This command bypasses any aliases or functions that may be defined for name. When used with a special built-in, it prevents the built-in from exiting the script if it fails. In POSIX mode, assignments given as arguments to the alias, declare, export, local, readonly, and typeset commands still take effect, even when preceded by command.

Options

-p

Use a predefined default search path, not the current value of PATH.

-v

Print a description of how the shell interprets name.

-V

Print a more verbose description of how the shell interprets name.

Example

Create an alias for rm that gets the system’s version, and run it with the -i option:

$ alias 'rm=command -p rm -i'

compgen

Generate possible completions.

compgen [options] [string]

Generate possible completions for string according to the options. Options are those accepted by complete, except for -p and -r. For more information, see the entry for complete.

complete

Specify how to do completion for specific commands.

complete [-DE] [options] command …

Specify how to complete arguments for each command. This is discussed in the section “Programmable Completion”.

Options

-a

Same as -A alias.

-A type

Use type to specify a list of possible completions. The type may be one of the following:

alias

Alias names

arrayvar

Array variable names

binding

Bindings from the readline library

builtin

Shell built-in command names

command

Command names

directory

Directory names

disabled

Names of disabled shell built-in commands

enabled

Names of enabled shell built-in commands

export

Exported variables

file

Filenames

function

Names of shell functions

group

Group names

helptopic

Help topics as allowed by the help built-in command

hostname

Hostnames, as found in the file named by $HOSTFILE

job

Job names

keyword

Shell reserved keywords

running

Names of running jobs

service

Service names (from /etc/services)

setopt

Valid arguments for set -o

shopt

Valid option names for the shopt built-in command

signal

Signal names

stopped

Names of stopped jobs

user

Usernames

variable

Shell variable names

-b

Same as -A builtin.

-c

Same as -A command.

-C command

Run command in a subshell and use its output as the list of completions.

-d

Same as -A directory.

-D

Apply the rest of the options and parameters to the “default” completion, which is used when no other compspec can be found.

-e

Same as -A export.

-E

Apply the rest of the options and parameters to the “empty” completion, which is used when completion is attempted on an empty input line.

-f

Same as -A file.

-F function

Run shell function function in the current shell. Upon its return, retrieve the list of completions from the COMPREPLY array.

-g

Same as -A group.

-G pattern

Expand pattern to generate completions.

-j

Same as -A job.

-k

Same as -A keyword.

-o option

Control the behavior of the completion specification. The value for option is one of the following:

bashdefault

Fall back to the normal Bash completions if no matches are produced.

default

Use the default readline completions if no matches are produced.

dirnames

Do directory name completion if no matches are produced.

filenames

Inform the readline library that the intended output is filenames, so the library can do any filename-specific processing, such as adding a trailing slash for directories or removing trailing spaces.

noquote

Inform the readline library that it should not quote completed words that are filenames.

nosort

Inform the readline library that it should not sort the list of completed words.

nospace

Inform the readline library that it should not append a space to words completed at the end of a line.

plusdirs

Attempt directory completion and add any results to the list of completions already generated.

-p

With no commands, print all completion settings in a way that can be reread.

-P prefix

Prepend prefix to each resulting string after all the other options have been applied.

-r

Remove the completion settings for the given commands, or all settings if no commands.

-s

Same as -A service.

-S suffix

Append suffix to each resulting string after all the other options have been applied.

-u

Same as -A user.

-v

Same as -A variable.

-W wordlist

Split wordlist (a single shell word) using $IFS. The generated list contains the members of the split list that matched the word being completed. Each member is then expanded using brace expansion, tilde expansion, parameter and variable expansion, command substitution, and arithmetic expansion. Shell quoting is respected.

-X pattern

Exclude filenames matching pattern from the filename completion list. With a leading !, the sense is reversed, and only filenames matching pattern are retained.

Tip

Courtesy of Chet Ramey. Use:

complete -A helptopic help

to restrict the set of possible completions for the help command to the set of help topics. Upon doing so, typing help and two Tabs causes the shell to display the list of help topics.

compopt

Print or change the completion options for a command.

compopt [-DE] [-o options] [+o options]
[commands …]

With no options, print the completion options for one or more commands, or for the currently executing completion when invoked without commands. With options, modify the existing compspecs for the given commands, or for the currently executing completion when invoked without commands.

Options

-D

Apply the options to the “default” completion.

-E

Apply the options to the “empty” completion.

-o option

Enable option, which is one of the valid options for the complete command.

+o option

Disable option, which is one of the valid options for the complete command.

continue

Skip the rest of the body of one or more loops.

continue [n]

Skip remaining commands in a for, while, select, or until loop, resuming with the next iteration of the loop (or skipping n nested loops).

declare

Declare shell variables and manage their attributes.

declare [options] [name[=value]]

Declare variables and manage their attributes. In function bodies, variables are local, as if declared with the local command. All options must be given first. See also the entry for typeset.

Options

-a

Each name is an indexed array.

-A

Each name is an associative array.

-f

Each name is a function.

-F

For functions, print just the function’s name and attributes, not the function definition (body).

-g

When used inside a function, declare the variable in the global scope, instead of at the local scope.

-i

Each variable is an integer; in an assignment, the value is evaluated as an arithmetic expression.

-l

Mark names to have their values converted to lowercase upon assignment.

-n

Each name is a nameref. See the section “Indirect Variables (namerefs)”.

-p

With no names, print all variables, their values, and attributes. With names, print the names, attributes, and values (if set) of the given variables. With -f, print function definitions.

-r

Mark names as read-only. Subsequent assignments will fail, and read-only variables cannot be unset.

-t

Apply the trace attribute to each name. Traced functions inherit the DEBUG trap. This attribute has no meaning for variables.

-u

Mark names to have their values converted to uppercase upon assignment.

-x

Mark names for export into the environment of child processes.

With a + instead of a -, the given attribute is disabled.

With no variable names, all variables having the given attribute(s) are printed in a form that can be reread as input to the shell.

Examples

$ declare -i val                 Make val an integer
$ val=4+7                        Evaluate value
$ echo $val                      Show result
11

$ declare -r z=42                Make z read-only
$ z=31                           Try to assign to it
bash: z: readonly variable       Assignment fails
$ echo $z
42

$ declare -p val z               Show attributes and values
declare -i val="11"
declare -r z="42"

dirs

Print or manage the directory stack.

dirs [-clpv] [+n] [-n]

Print the directory stack, which is managed with pushd and popd.

Options

+n

Print the nth entry from the left; first entry is zero.

-n

Print the nth entry from the right; first entry is zero.

-c

Clear (remove all entries from) the directory stack.

-l

Produce a longer listing, one that does not replace $HOME with ~.

-p

Print the directory stack, one entry per line.

-v

Print the directory stack, one entry per line, with each entry preceded by its index in the stack.

disown

Stop managing one or more jobs.

disown [-ahr] [job …]

Remove one or more jobs from the list of jobs managed by Bash. A job is either a job specification or a process-ID.

Options

-a

Remove all jobs. With -h, mark all jobs.

-h

Instead of removing jobs from the list of known jobs, mark them to not receive SIGHUP as described in the section “Command Execution”.

-r

With no jobs, remove (or mark) only running jobs.

do

Reserved word that starts the body of a loop.

do

Reserved word that precedes the command sequence in a for, while, until, or select statement.

done

Reserved word that ends the body of a loop.

done

Reserved word that ends a for, while, until, or select statement.

echo

Print command-line arguments to standard output.

echo [-eEn] [string]

Write string to standard output. This is the built-in version of the command. (It is likely that you also have a standalone executable program named echo for which you should see your local echo(1) man page.)

Options

If the xpg_echo shell option is set, along with POSIX mode (set -o posix), echo does not interpret any options.

-e

Enable interpretation of the escape sequences as described in the section “Escape Sequences”. They must be quoted (or escaped with a \) to prevent interpretation by the shell.

-E

Do not interpret escape sequences, even on systems where the default behavior of the built-in echo is to interpret them.

-n

Do not print the terminating newline.

Examples

$ echo "testing printer" | lpr
$ echo -e "Warning: ringing bell \a"

enable

Enable or disable shell built-in commands.

enable [-adnps] [-f file] [command …]

Enable or disable shell built-in commands. Disabling a built-in lets you use an external version of a command that would otherwise use a built-in version, such as echo or test.

Options

-a

For use with -p; print information about all built-in commands, disabled and enabled.

-d

Remove (delete) a built-in previously loaded with -f.

-f file

Load a new built-in command command from the shared library file file. The shell searches for file in the directories named in $BASH_LOADABLES_PATH.

-n

Disable the named built-in commands.

-p

Print a list of enabled built-in commands.

-s

Print only the POSIX special built-in commands. When combined with -f, the new built-in command becomes a POSIX special built-in.

esac

Reserved word that ends a case statement.

esac

Reserved word that ends a case statement.

eval

Rescan and execute an already-processed input line.

eval args

Typically, eval is used in shell scripts, and args is a line of code that contains shell variables. eval forces variable expansion to happen first and then runs the resulting command. This “double-scanning” is useful any time shell variables contain input/output redirection symbols, aliases, or other shell variables. (For example, redirection normally happens before variable expansion, so a variable containing redirection symbols must be expanded first using eval; otherwise, the redirection symbols remain uninterpreted.)

Example

This fragment of a shell script shows how eval constructs a command that is interpreted in the correct order:

for option
do
# Define where output goes
case "$option" in
save) out=' > $newfile' ;;
show) out=' | more' ;;
esac
done

eval sort $file $out

exec

Replace the current script or manage shell file descriptors.

exec [command args …]
exec [-a name] [-cl] [command args … ]
exec redirections …

Execute command in place of the current process (instead of creating a new process). With only redirections (see the section “Redirection using file descriptors”), exec is also useful for opening, closing, copying, or moving file descriptors. In this case, the script continues to run.

Options

-a

Use name for the value of command’s argv[0].

-c

Clear the environment before executing the program.

-l

Place a minus sign at the front of command’s argv[0], just as login(1) does.

Examples

trap 'exec 2>&-' 0   Close stderr when script exits (signal 0)

$ exec /bin/csh      Replace shell with C shell (bad idea)
$ exec < infile      Reassign standard input to infile

exit

Exit the shell script.

exit [n]

Exit a shell script with status n (e.g., exit 1). n can be 0 (success) or nonzero (failure). If n is not given, the shell’s exit status is that of the most recent command. exit can be issued at the command line to close a window (log out). Exit statuses can range in value from 0 to 255. Any trap on EXIT executes before the shell exits. Noninteractive login shells execute ~/.bash_logout if it exists. See also the section “Command Exit Status”.

Example

if [ $# -eq 0 ]
then
echo "Usage: $0 [-c] [-d] file(s)" 1>&2
exit 1               # Error status
fi

export

Export items or print information about exported items.

export [variables]
export [name=[value] …]
export -p
export [-fn] [name=[value] …]

Pass (export) the value of one or more shell variables, giving global meaning to the variables (which are local by default). For example, a variable defined in one shell script must be exported if its value is used in other programs called by the script. If no variables are given, export lists the variables exported by the current shell. The second form is the POSIX version, which is similar to the first form, except that you can set a variable name to a value before exporting it. export can also export functions.

Options

-f

Names refer to functions; the functions are exported into the environment.

-n

Remove the named variables or functions from the environment.

-p

Print declare -x before printing the names and values of exported variables. This allows saving a list of exported variables for rereading later. Print only the names of exported functions, not their bodies.

Examples

In the original Bourne shell, you would type:

TERM=vt100
export TERM

In Bash, you type this instead:

export TERM=vt100

false

Exit with a false (failure) return value.

false

Built-in command that exits with a false return value.

fc

Manage command-line history.

fc [options] [first [last]]
fc -e - [old=new] [command]
fc -s [old=new] [command]

Display or edit commands in the history list. (Use only one of -e, -l, or -s.)

first and last are numbers or strings specifying the range of commands to display or edit. If last is omitted, fc applies to a single command (specified by first). If both first and last are omitted, fc edits the previous command or lists the last 16. If first is -0, it relates to the current command.

The second form of fc takes a history command, replaces old with new, and executes the modified command. If no strings are specified, command is just re-executed. If no command is given either, the previous command is re-executed. command is a number or string like first. See the Examples in the section “Command History”. The third form is equivalent to the second form.

Options

-e editor

Invoke editor to edit the specified history commands. You must supply an editor name with -e. Without -e, fc invokes the default editor, which is set by the shell variable FCEDIT. If that variable is not set, Bash tries $EDITOR. If neither is set, the default editor is vi. Versions 3.1 and newer default to ed when in POSIX mode.

-e -

Execute (or redo) a history command; refer to the second syntax line above.

-l

List the specified command or range of commands, or list the last 16.

-n

Suppress command numbering from the -l listing.

-r

Reverse the order of the -l listing.

-s

Equivalent to -e -.

fg

Move a running or suspended background job into the foreground.

fg [jobIDs]

Bring current job or jobIDs to the foreground. See the section “Job Control”.

fi

Reserved word that ends an if statement.

fi

Reserved word that ends an if statement.

for

Start a loop over a list of values.

for x [in [list]]
do
commands
done

For variable x (in optional list of values), do commands. If in list is omitted, "$@" (the positional parameters) is assumed. If the expansion of list is empty, no commands execute.

Examples

Paginate files specified on the command line, and save each result:

for file
do
pr $file > $file.tmp
done

Same, but put entire loop into the background:

for file
do
pr $file > $file.tmp
done &

Search chapters for a list of words (like fgrep -f):

for item in $(cat program_list)
do
echo "Checking chapters for"
echo "references to program $item…"
grep -c "$item.[co]" chap*
done

Extract a one-word title from each file named on the command line and use it as the new filename:

for file
do
name=$(sed -n 's/NAME: //p' $file)
mv $file $name
done

for

Start an arithmetic loop.

for ((init; cond; incr))
do
commands
done

Arithmetic for loop, similar to C’s. Evaluate init. While cond is true, execute the body of the loop. Evaluate incr before retesting cond. Any one of the expressions may be omitted; a missing cond is treated as being true.

Example

Search for a phrase in each odd chapter:

for ((x=1; x <= 20; x += 2))
do
grep $1 chap$x
done

function

Define a shell function.

function name { commands; } [redirections]
function name () { commands; } [redirections]

Define name as a shell function. See the description of function semantics in the section “Functions”.

Example

Define a function to count files.

$ function countfiles {
>    ls | wc -l
> }

getopts

Process command-line options and arguments.

getopts string name [args]

Process command-line arguments (or args, if specified) and check for legal options. getopts is used in shell script loops and is intended to ensure standard syntax for command-line options.

Standard syntax dictates that command-line options begin with a -. Options can be stacked (i.e., consecutive letters can follow a single -). You end processing of options by specifying -- on the command line. string contains the option letters to be recognized by getopts when running the shell script. Valid options are processed in turn and stored in the shell variable name. If name is read-only, the command exits with a return value of 2.

If an option character in the options string is followed by a colon, the actual option must be followed by one or more arguments. (Multiple arguments must be given to the command as one shell word. This is done by quoting the arguments or separating them with commas. The script must be written to expect multiple arguments in this format.)

getopts uses the shell variables OPTARG, OPTIND, and OPTERR (see the section “Built-In Shell Variables”).

hash

Manage the table of previously found commands.

hash [-dlrt] [commands]
hash [-p file] [command]

As the shell finds commands along the search path ($PATH), it remembers the found locations in an internal hash table. The next time you enter a command, the shell uses the value stored in its hash table.

With no arguments, hash lists the current set of hashed commands. The display shows hits (the number of times the command has been called by the shell) and the command name. If the table is empty, then if Bash is in POSIX mode, hash prints nothing. Otherwise, it prints hash: hash table empty on standard output.

With commands, the shell adds those commands to the hash table. With no options and just commands, the shell resets the “hit count” associated with each command to zero.

The BASH_CMDS array provides programmatic access to all entries in the hash table; see the section “Built-In Shell Variables”.

Options

-d

Delete (remove) just the specified commands from the hash table.

-l

Produce output in a format that can be reread to rebuild the hash table.

-p file

Associate file with command in the hash table.

-r

Remove all commands from the hash table.

-t

With one name, print the full pathname of the command. With more than one name, print the name and the full path, in two columns.

Tip

Besides the -r option, the hash table is also cleared when PATH is assigned. Use PATH=$PATH to clear the hash table without affecting your search path. This is most useful if you have installed a new version of a command in a directory that is earlier in $PATH than the current version of the command.

help

Print command usage information.

help [-dms] [pattern]

Print usage information on standard output for each command that matches pattern. The information includes descriptions of each command’s options.

Options

-d

Print a brief description of what the command does.

-m

Print the full description of the command in a format similar to that of a Unix manual page.

-s

Print short (brief) usage information.

Examples

$ help -s cd                      Short help
cd: cd [-L|[-P [-e]] [-@]] [dir]

$ help true                       Full help
true: true
Return a successful result.

Exit Status:
Always succeeds.

history

Print command-line history.

history [count]
history [options]

Print commands in the history list or manage the history file. With no options or arguments, display the history list with command numbers. With a count argument, print only that many of the most recent commands.

Bash stores the command history for any shell where history is enabled (with set -o history) and HISTFILE is set, not just interactive shells.

Options

-a

Append new history lines (those executed since the history file was last written) to the history file. The write happens immediately.

-c

Clear the history list (remove all entries).

-d position

Delete the history item at position position.

-n

Read unread history lines from the history file and append them to the history list.

-p argument …

Perform csh-style history expansion on each argument, printing the results to standard output. The results are not saved in the history list.

-r

Read the history file and replace the history list with its contents.

-s argument …

Store the arguments in the history list, as a single entry.

-w

Write the current history list to the history file, overwriting it entirely. The write happens immediately.

Tip

Courtesy of Eli Zaretskii. You can use

export PROMPT_COMMAND='history -a'

to ensure that the history file is always up to date, even if the user did not log out gracefully and even if the session is still alive.

if

Syntax for an if-then-else statement.

if condition1
then commands1
[ elif condition2
then commands2 ]
.
.
.
[ else commands3 ]
fi

If condition1 is met, do commands1; otherwise, if condition2 is met, do commands2; if neither is met, do commands3. Conditions are often specified with the test and [[ ]] commands. See the entries for test, and [[ ]], for a full list of conditions, and see additional Examples under the entries for :, and exit.

Examples

Insert a 0 before numbers less than 10:

if [ $counter -lt 10 ]
then number=0$counter
else number=$counter
fi

Make a directory if it doesn’t exist:

if ! [ -d $dir ]
then
mkdir -m 775 $dir
fi

jobs

List running or stopped jobs.

jobs [options] [jobIDs]

List all running or stopped jobs, or list those specified by jobIDs. For example, you can check whether a long compilation or text formatting job is still running. Also useful before logging out. See the section “Job Control”.

Options

-l

List job IDs and process group IDs.

-n

List only jobs whose status changed since last notification.

-p

List process group IDs only.

-r

List running jobs only.

-s

List stopped jobs only.

-x cmd

Replace each job ID found in cmd with the associated process ID and then execute cmd.

kill

Send a signal to one or more jobs.

kill [options] IDs

Terminate each specified process ID or job ID. You must own the process or be a privileged user. This built-in is similar to the external kill command, but also allows symbolic job names. Stubborn processes can (usually) be killed using signal 9. See the section “Job Control”.

The command kill -l prints a list of the available signal names. The list varies by system architecture.

The signals and their numbers are defined in the C <signal.h> header file. This file may include others, thus the actual location of the signal definitions varies across systems.

Options

-l [n]

With no n, list the signal names. A numeric value n is interpreted as either a signal number, or as an exit status for a process terminated by a signal (128 + n). In both cases, kill prints the corresponding signal name.

-L [n]

Same as -l.

-n num

Send the given signal number.

-s name

Send the given signal name.

-signal

Send the given signal number (from <signal.h>) or signal name (from kill -l). With a signal number of 9, the kill is absolute.

In POSIX mode, you must leave off the SIG prefix for signal names, and kill -l prints all the signal names on a single line.

let

Perform arithmetic.

let expressions
(( expressions ))

Perform arithmetic as specified by one or more expressions. expressions consist of numbers, operators, and shell variables (which don’t need a preceding $). Expressions must be quoted if they contain spaces or other special characters. The ((…)) form does the quoting for you. For more information and examples, see the section “Arithmetic Expressions”. See also expr(1). If the final expression evaluates to zero, let returns 1 (failure); otherwise it returns zero (success).

Examples

Each of these examples adds 1 to variable i:

i=`expr $i + 1`         All Bourne shells
let i=i+1               Bash
let "i = i + 1"
(( i = i + 1 ))
(( i += 1 ))
(( i++ ))
if (( i % 2 == 0 )) …

local

Declare local variables inside shell functions.

local [options] [name[=value]]

Declare local variables for use inside functions. The options are the same as those accepted by declare; see the entry for declare for the full list. It is an error to use local outside a function body.

If name is -, the shell saves the values of the single letter options and restores them upon function exit.

logout

Exit the shell.

logout

Exit a login shell. Execute ~/.bash_logout if it exists. The command fails if the current shell is not a login shell.

mapfile

Read a file into a shell array.

mapfile [options] [array]

Read standard input into array, one line per element. If no array, use MAPFILE. An alternate file descriptor may be given with the -u option.

Options

-c count

Specifies the “quantum” for the -C option. The default value is 5,000.

-C command

Every “quantum” lines, evaluate command, passing it the index in array that is about to be assigned, and the line that is to be assigned. The quantum is set with the -c option. (This option is mainly used by the Bash debugger.)

-d delim

Use delim’s first character to terminate input lines instead of newline. If delim is the null string, use the zero byte (ASCII NUL) as the delimiter.

-n count

Read at most count lines. If count is zero, read all the lines.

-O index

Fill array starting at origin index. The default origin is zero.

-s count

Skip (ignore) the first count lines.

-t

Remove the trailing delimiter (usually a newline) from each line read.

-u n

Read from file descriptor n instead of from standard input.

popd

Pop a directory off of the directory stack.

popd [-n] [+count] [-count]

Pop the top directory off the directory stack (as shown by the dirs command), and change to the new top directory, or manage the directory stack.

Options

+count

Remove the item count entries from the left, as shown by dirs. Counting starts at zero. No directory change occurs.

-count

Remove the item count entries from the right, as shown by dirs. Counting starts at zero. No directory change occurs.

-n

Don’t change to the new top directory; just manipulate the stack.

printf

Do formatted printing of command-line arguments.

printf [-v var] format [val …]

Formatted printing, like the ANSI C printf() function (see printf(3)). Escape sequences in format are expanded as described in the section “Escape Sequences”. The format string is reused from the beginning if there are more values than format specifiers.

Option

-v var

Save the result in var instead of printing it to standard output, even if the result is empty. var may be an array element.

Additional Format Letters

Bash accepts these additional format letters:

%b

Expand escape sequences in argument strings. The set of escape sequences is those accepted by echo -e. (See the section “Escape Sequences”.)

%q

Print a quoted string that can be reread later on.

%(datefmt)T

Print time values using datefmt as the format control string for strftime(3). Use the current time if no argument is supplied.

pushd

Push a directory onto the directory stack.

pushd [-n] [directory]
pushd [-n] [+count] [-count]

Add directory to the directory stack, or rotate the directory stack. With no arguments, swap the top two entries on the stack, and change to the new top entry.

Options

+count

Rotate the stack so that the count’th item from the left, as shown by dirs, is the new top of the stack. Counting starts at zero. The new top becomes the current directory.

-count

Rotate the stack so that the count’th item from the right, as shown by dirs, is the new top of the stack. Counting starts at zero. The new top becomes the current directory.

-n

Don’t change to the new top directory; just manipulate the stack.

pwd

Print working directory.

pwd [-LP]

Print the current working directory on standard output. Exit with a failure status if PWD is read-only.

Options

Options give control over the use of logical versus physical treatment of the printed path. See also the entry for cd.

-L

Use the logical path (what the user typed, including any symbolic links) and the value of PWD for the current directory. This is the default.

-P

Use the filesystem physical path for the current directory.

read

Read data into one or more shell variables.

read [options] [variable1 [variable2 …]]

Read one line of standard input and assign each word to the corresponding variable, with all leftover words assigned to the last variable. If only one variable is specified, the entire line is assigned to that variable. Bash removes zero (ASCII NUL) bytes from the input. See the Examples here and under the entry for case. The return status is 0 unless EOF is reached. If no variables are given, input is stored in the REPLY variable. In a POSIX mode shell, trapped signals can interrupt read, in which case it returns 128 + the signal number and discards partially read input.

Options

-a array

Read into indexed array array. Report an error if array is a preexisting associative array.

-d delim

Read up to the first occurrence of delim, instead of newline. If delim is the null string, use the zero byte (ASCII NUL) as the delimiter.

-e

Use the readline library if reading from a terminal.

-i text

When using the readline library, put text into the initial editing buffer.

-n count

Read at most count bytes. If a delimiter character is seen before reading count bytes, stop reading further input.

-N count

Read at most count bytes. Delimiter characters in the data do not cause Bash to stop reading; instead they are included in the data that is read.

-p prompt

Print prompt before reading input.

-r

Raw mode; ignore \ as a line-continuation character.

-s

Read silently; characters are not echoed.

-t timeout

When reading from a terminal or pipe, if no data is entered after timeout seconds, return 1. This prevents an application from hanging forever, waiting for user input. Values for timeout may be fractional. If timeout is zero but data is available to be read, read returns successfully. Partial input read when the timeout expires is saved in variable1; the other variables are cleared. read returns greater than 128 if no data were read and the timeout expires.

-u [n]

Read input from file descriptor n (default is 0).

Examples

Read three variables:

$ read first last address
Sarah Caldwell 123 Main Street

$ echo -e "$last, $first\n$address"
Caldwell, Sarah
123 Main Street

Prompt yourself to enter two temperatures:

$ read -p "High low: " n1 n2
High low: 65 33

readarray

Read a file into a shell array.

readarray [options] [array]

Identical to the mapfile command. See the entry for mapfile for more information.

readonly

Mark variables as read only.

readonly [-aAfp] [variable[=value] …]

Prevent the specified shell variables from being assigned new values. An initial value may be supplied using the assignment syntax, but that value may not be changed subsequently. Read-only variables may not be unset.

Options

-a

Each variable must refer to an indexed array.

-A

Each variable must refer to an associative array.

-f

Each variable must refer to a function.

-p

Print readonly before printing the names and values of read-only variables. This allows saving a list of read-only variables for rereading later.

return

Return an exit status from a shell function.

return [n]

Use inside a function definition. Exit the function with status n or with the exit status of the previously executed command. If n is negative, precede it with --.

select

Present a menu of items for use in executing a block of code.

select x [in list]
do
commands
done

Display a list of menu items on standard error, numbered in the order they are specified in list. If no in list is given, items are taken from the command line (via "$@"). Following the menu is a prompt string (the value of PS3). At the $PS3 prompt, users select a menu item by typing its number, or they redisplay the menu by pressing the Enter key. User input is stored in the shell variable REPLY. If a valid item number is typed, the shell sets x to the chosen value and executes commands. Typing EOF terminates the loop.

Example

PS3="Select the item number: "
select event in Format Page View Exit
do
case "$event" in
Format) nroff $file | lpr;;
Page)   pr $file | lpr;;
View)   more $file;;
Exit)   exit 0;;
*   )   echo "Invalid selection";;
esac
done

The output of this script looks like this:

1. Format
2. Page
3. View
4. Exit
Select the item number:

set

Manage shell options and the script’s command-line parameters.

set [options arg1 arg2 …]

With no arguments, set prints the values of all variables known to the current shell. Options can be enabled (-option) or disabled (+option). Options can also be set when the shell is invoked. (See the section “Invoking the Shell”.) Nonoption arguments are assigned in order ($1, $2, etc.).

Options

-a

From now on, automatically mark variables for export after defining or changing them.

-b

Print job completion messages as soon as jobs terminate; don’t wait until the next prompt.

-B

Enable brace expansion. On by default.

-C

Prevent overwriting via > redirection; use >| to overwrite files.

-e

Exit if a command yields a nonzero exit status. The ERR trap executes before the shell exits. The exact behavior is complicated; see “set -e Details”, later in this entry.

-E

Cause shell functions, command substitutions, and subshells to inherit the ERR trap.

-f

Ignore filename metacharacters (e.g., * ? [ ]).

-h

Locate commands as they are defined. On by default. See the entry for hash.

-H

Enable csh-style history substitution. On by default. (Bash 5.0 will change this to be off by default.) See the section “C-Shell–Style History”.

-k

Assignment of environment variables (var=value) takes effect regardless of where they appear on the command line. Normally, assignments must precede the command name.

-m

Enable job control; background jobs execute in a separate process group. -m is usually set automatically; interactive shells enable it, scripts do not.

-n

Read commands but don’t execute them; useful for checking syntax. Interactive shells ignore this option.

+o [mode]

With mode, disable the given shell mode. Plain set +o prints the settings of all the current modes in a form that can be reread by the shell later.

-o [mode]

List shell modes, or turn on mode mode. Many modes can be set by other options. Modes are:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Set command-line editor to emacs.

errexit

Same as -e.

errtrace

Same as -E.

functrace

Same as -T.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history. On by default.

ignoreeof

Don’t process EOF signals. To exit the shell, type exit.

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f.

nolog

Omit function definitions from the history file. Accepted but ignored by Bash.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

pipefail

Change pipeline exit status to be that of the rightmost command that failed, or zero if all exited successfully.

posix

Change to POSIX mode.

privileged

Same as -p.

verbose

Same as -v.

vi

Set command-line editor to vi.

xtrace

Same as -x.

+p

Reset effective UID to real UID.

-p

Start up as a privileged user. Don’t read $ENV or $BASH_ENV, don’t import functions from the environment, and ignore the values of the BASHOPTS, CDPATH, GLOBIGNORE, and SHELLOPTS variables.

-P

Always use physical paths for cd and pwd.

-t

Exit after executing one command.

-T

Cause shell functions, command substitutions, and subshells to inherit the DEBUG and RETURN traps.

-u

In substitutions, treat unset variables as errors. However, references to $@ and $* are not errors when there are no positional parameters.

-v

Show each shell command line when read.

-x

Show commands and arguments when executed, preceded by the value of PS4. This provides step-by-step tracing of shell scripts.

-

Turn off -v and -x, and turn off option processing. Included for compatibility with older versions of the Bourne shell.

--

Used as the last option; -- turns off option processing so that arguments beginning with - are not misinterpreted as options. (For example, you can set $1 to -1.) If no arguments are given after --, unset the positional parameters.

Option Summary

Option

Same as

-a

-o allexport

-b

-o notify

-B

-o braceexpand

-C

-o noclobber

-e

-o errexit

-E

-o errtrace

-f

-o noglob

-h

-o hashall

-H

-o histexpand

-k

-o keyword

-m

-o monitor

-n

-o noexec

-o allexport

-a

-o braceexpand

-B

-o emacs

-o errexit

-e

-o errtrace

-E

-o functrace

-T

-o hashall

-h

-o history

-o histexpand

-H

-o ignoreeof

-o keyword

-k

-o monitor

-m

-o noclobber

-C

-o noexec

-n

-o noglob

-f

-o nolog

-o notify

-b

-o nounset

-u

-o onecmd

-t

-o physical

-P

-o pipefail

-o posix

-o privileged

-p

-o verbose

-v

-o vi

-o xtrace

-x

-p

-o privileged

-P

-o physical

-t

-o onecmd

-T

-o functrace

-u

-o nounset

-v

-o verbose

-x

-o xtrace

set -e Details

When set -e is enabled, the shell exits if one of the following fails: a pipeline (which can be just a single command); a subshell command in parentheses; or any of the commands in a group enclosed in braces.

In POSIX mode, shells created to run command substitutions inherit the setting of set -e; otherwise such shells inherit the setting of set -e based on the setting of the inherit_errexit shell option.

Failure of a command (non-zero exit status) does not cause an exit in the following cases: Any command in a list following while or until; the pipeline following if or elif; any command in an && or || list except the last; any command in a pipeline but the last; or if the sense of the command’s value is being inverted with !.

In general, shell programming experts consider set -e to be of little or no use in practical shell programming. It exists mostly for historical compatibility, and should not be used instead of careful programming to catch any and all errors that may occur.

Examples

set -- "$num" -20 -30   Set $1 to $num, $2 to -20, $3 to -30
set -vx                 Show each command twice; once when
read, and once when executed
set +x                  Stop command tracing
set -o noclobber        Prevent file overwriting
set +o noclobber        Allow file overwriting again

shift

Shift the command-line arguments left.

shift [n]

Shift positional arguments (e.g., $2 becomes $1). If n is given, shift to the left n places. Often used in while loops to iterate through the command-line arguments.

Example

shift $(($1 + $6))    Use expression result as shift count

shopt

Manage shell options.

shopt [-opqsu] [option]

Set or unset shell options. With no options or just -p, print the names and settings of the options. See the section “Shell Options” for a description of the various options.

Options

-o

Each option must be one of the shell option names for set -o, instead of the options listed in the section “Shell Options”.

-p

Print the option settings as shopt commands that can be reread later.

-q

Quiet mode. The exit status is zero if the given option is set, nonzero otherwise. With multiple options, all of them must be set for a zero exit status.

-s

Set the given options. With no options, print only those that are set.

-u

Unset the given options. With no options, print only those that are unset.

source

Read and execute a file within the current shell.

source file [arguments]

Identical to the . (dot) command; see the entry for . for more information.

suspend

Suspend the current shell.

suspend [-f]

Suspend the current shell. Often used to stop an su command.

Option

-f

Force the suspension, even if the shell is a login shell.

test

Evaluate conditions, for use in loops and conditionals.

test condition
[ condition ]
[[ condition ]]

Evaluate a condition and, if its value is true, return a zero exit status; otherwise, return a nonzero exit status. An alternate form of the command uses [ ] rather than the word test. An additional alternate form uses [[ ]], in which case word splitting and pathname expansion are not done (see the entry for [[ ]]). condition is constructed using the following expressions. Conditions are true if the description holds true.

File Conditions

-a file

file exists. (Deprecated; use -e instead.)

-b file

file exists and is a block special file.

-c file

file exists and is a character special file.

-d file

file exists and is a directory.

-e file

file exists. (Same as -a, for POSIX compatibility.)

-f file

file exists and is a regular file.

-g file

file exists, and its set-group-id bit is set.

-G file

file exists, and its group is the effective group ID.

-h file

file exists and is a symbolic link. (Same as -L.)

-k file

file exists, and its sticky bit is set.

-L file

file exists and is a symbolic link. (Same as -h.)

-N file

file exists and was modified after it was last read.

-O file

file exists, and its owner is the effective user ID.

-p file

file exists and is a named pipe (FIFO).

-r file

file exists and is readable.

-s file

file exists and has a size greater than zero.

-S file

file exists and is a socket.

-t [n]

The open file descriptor n is associated with a terminal device; default n is 1.

-u file

file exists, and its set-user-id bit is set.

-w file

file exists and is writable.

-x file

file exists and is executable.

f1 -ef f2

Files f1 and f2 are linked (refer to same file).

f1 -nt f2

File f1 is newer than f2.

f1 -ot f2

File f1 is older than f2.

String Conditions

string

string is not null.

-n s1

String s1 has nonzero length.

-z s1

String s1 has zero length.

s1 == s2

Strings s1 and s2 are identical. Inside [[ ]], s2 can be a wildcard pattern. Quote s2 to treat it literally. (See the section “Filename Metacharacters”. See also the nocasematch option in the section “Shell Options”.)

s1 = s2

Same as the == operator. Should be used with test and [ ] for compatibility with POSIX and other shells.

s1 != s2

Strings s1 and s2 are not identical. Inside [[ ]], s2 can be a wildcard pattern. Quote s2 to treat it literally.

s1 =~ s2

String s1 matches extended regular expression s2. Only available inside [[ ]]. Quote s2 to force string matching, instead of regular expression matching. Strings matched by parenthesized subexpressions are placed into elements of the BASH_REMATCH array. See the description of BASH_REMATCH in the section “Built-In Shell Variables”. See also the compat31, compat32, and compat40 options in the section “Shell Options”.

s1 < s2

String value of s1 precedes that of s2. With test and [ ], you must quote the < and Bash uses the machine’s sorting order (usually ASCII). With [[ ]], you don’t have to quote the < and Bash uses the locale’s sorting order.

s1 > s2

String value of s1 follows that of s2. With test and [ ], you must quote the > and Bash uses the machine’s sorting order (usually ASCII). With [[ ]], you don’t have to quote the > and Bash uses the locale’s sorting order.

Internal Shell Conditions

-o opt

Option opt for set -o is on.

-R var

Variable var has been assigned a value and is a nameref.

-v var

Variable var has been assigned a value. var may name an array element.

Integer Comparisons

n1 -eq n2

n1 equals n2.

n1 -ge n2

n1 is greater than or equal to n2.

n1 -gt n2

n1 is greater than n2.

n1 -le n2

n1 is less than or equal to n2.

n1 -lt n2

n1 is less than n2.

n1 -ne n2

n1 does not equal n2.

Combined Forms

(condition)

True if condition is true (used for grouping). For test and [ ], the parentheses should be quoted by a \. The [[ ]] form doesn’t require quoting the parentheses.

! condition

True if condition is false.

condition1 -a condition2

True if both conditions are true.

condition1 && condition2

True if both conditions are true. Short-circuit form. (Use only within [[ ]].)

condition1 -o condition2

True if either condition is true.

condition1 || condition2

True if either condition is true. Short-circuit form. (Use only within [[ ]].)

Examples

The following examples show the first line of various statements that might use a test condition:

while test $# -gt 0               While there are arguments

if [ $count -lt 10 ]              If $count is less than 10
if [ -d .git ]                    If the .git directory exists
if [ "$answer" != "y" ]           If the answer is not y
if [ ! -r "$1" -o ! -f "$1" ]     If the first argument is not
readable or a regular file
if ! [ -r "$1" ] || ! [ -f "$1" ] Same as previous

time

Time a command.

time [-p] [command]

Execute command and print the total elapsed time, user time, and system time (in seconds). Same as the external command time, except that the built-in version can also time other built-in commands as well as all commands in a pipeline. The current locale’s decimal point is used in the output.

With no command, print the elapsed user, system and real times for the shell and its children.

The value of the TIMEFORMAT variable controls the format of the output. See the bash(1) manual page for the details.

In POSIX mode, if the first argument begins with a minus sign, Bash treats time as a command, not as a keyword.

Option

-p

Print the timing summary in the format specified by POSIX.

times

Print accumulated CPU times.

times

Print accumulated user and system process times for the shell and the processes it has run.

trap

Manage the disposition of signals within a shell script.

trap [[commands] signals]
trap -l
trap -p

Execute commands if any signals are received. The second form lists all signals and their numbers, like kill -l. The third form prints the current trap settings in a form suitable for rereading later. Signals ignored at shell startup are included, but cannot be changed.

Common signals include EXIT (0), HUP (1), INT (2), and TERM (15). Multiple commands must be quoted as a group and separated by semicolons internally. If commands is the null string (i.e., trap "" signals), cause the shell to ignore signals. If commands are omitted entirely, reset processing of specified signals to the default action. If commands is “-”, reset signals to their initial defaults.

If both commands and signals are omitted, list the current trap assignments. See the Examples here and in the entry for exec.

Normally, trap prints signal names with a leading SIG prefix. In POSIX mode, it leaves off the prefix.

Note

The shell does not block additional occurrences of signals for traps that are running, allowing recursive trap invocations. Use with care!

Tip

In general, commands should be quoted using single quotes, so that any variable or other substitutions are delayed until the signal is handled. Otherwise, with double quotes, the expansions are evaluated earlier, when the trap command itself executes.

Signals

For standard signals, the shell allows you to use either the signal number or the signal name (with or without the SIG prefix). In addition, the shell supports “pseudo-signals,” signal names or numbers that aren’t real operating system signals but which direct the shell to perform a specific action. These signals and when they execute are:

DEBUG

Execution of any command.

ERR

Nonzero exit status.

EXIT

Exit from shell (usually when shell script finishes). Also for shells started for process substitution.

RETURN

A return is executed, or a script run with . (dot) or source finishes.

0

Same as EXIT, for historical compatibility with the Bourne shell.

Examples

trap "" INT     Ignore interrupts (signal 2)
trap INT        Obey interrupts again

Remove a $tmp file when the shell program exits, or if the user logs out, presses CTRL-C, or does a kill:

trap "rm -f $tmp; exit" EXIT HUP INT TERM  POSIX style
trap "rm -f $tmp; exit" 0 1 2 15           Original shell

Print a “clean up” message when the shell program receives signals SIGHUP, SIGINT, or SIGTERM:

trap 'echo Interrupt!  Cleaning up…' HUP INT TERM

true

Exit with a true (success) return value.

true

Built-in command that exits with a true return value.

type

Print the type of a command.

type [-afpPt] commands

Show whether each command name is an external command, a built-in command, an alias, a shell keyword, or a defined shell function.

Options

-a

Print all locations in $PATH that include command, including aliases and functions. Use -p together with -a to suppress aliases and functions.

-f

Suppress function lookup, as with command.

-p

If type -t would print file for a given command, print the full pathname for the corresponding executable file. Otherwise, print nothing.

-P

Like -p, but force a search of $PATH, even if type -t would not print file.

-t

Print a word describing each command. The word is one of alias, builtin, file, function, or keyword, depending upon the type of each command.

Example

$ type mv read if
mv is /bin/mv
read is a shell builtin
if is a shell keyword

typeset

Declare shell variables and manage their attributes.

typeset [options] [variable[=value …]]

Identical to declare. See the entry for declare.

ulimit

Manage various process limits.

ulimit [options] [n]

Print the value of one or more resource limits, or, if n is specified, set a resource limit to n. Resource limits can be either hard (-H) or soft (-S). By default, ulimit sets both limits or prints the soft limit. The options determine which resource is acted upon.

Options

-H

Hard limit. Anyone can lower a hard limit; only privileged users can raise it.

-S

Soft limit. Must be less than or equal to the hard limit.

-a

Print all limits.

-b

Maximum size of a socket buffer.

-c

Maximum size of core files. Default units are 1K-byte blocks. In POSIX mode, units are 512-byte blocks.

-d

Maximum kilobytes of data segment or heap.

-e

Maximum scheduling priority (nice value).

-f

Maximum size of files (the default option). Default units are 1K-byte blocks. In POSIX mode, units are 512-byte blocks.

-i

Maximum number of pending signals.

-k

Maximum number of kqueues. (Not effective on all systems.)

-l

Maximum size of address space that can be locked in memory.

-m

Maximum kilobytes of physical memory. (Not effective on all systems.)

-n

Maximum number of file descriptors.

-p

Size of pipe buffers. (Not effective on all systems.)

-P

Maximum number of pseudoterminals. (Not effective on all systems.)

-q

Maximum number of bytes in POSIX message queues.

-r

Maximum real-time scheduling priority.

-s

Maximum kilobytes of stack segment.

-t

Maximum CPU seconds.

-T

Maximum number of threads.

-u

Maximum number of processes a single user can have.

-v

Maximum kilobytes of virtual memory. (Not effective on all systems.)

-x

Maximum number of file locks.

umask

Display or set the process’s file creation mask.

umask [nnn]
umask [-pS] [mask]

Display the file creation mask or set the file creation mask to octal value nnn. The file creation mask determines which permission bits are turned off (e.g., umask 002 produces rw-rw-r--). For the second form, a symbolic mask represents permissions to keep.

Options

-p

Output is in a form that can be reread later by the shell.

-S

Print the current mask using symbolic notation.

unalias

Remove previously defined aliases.

unalias names
unalias -a

Remove names from the alias list. See also the entry for alias.

Option

-a

Remove all aliases.

unset

Remove variables or functions.

unset [options] names

Erase definitions of functions or variables listed in names. A name subscripted with an index (unset foo[2]) unsets the corresponding array element. An index of 0 unsets the related scalar variable.

Options

-f

Unset functions names.

-n

Unset nameref variables names. See the section “Indirect Variables (namerefs)”.

-v

Unset variables names (default).

until

Syntax for a loop that runs until a condition becomes true.

until condition
do
commands
done

Until condition is met, do commands. condition is often specified with the test command. See the Examples under the entries for case, and test.

wait

Wait for a process or job to complete.

wait [-n] [ID]

With no option or arguments, pause in execution until all background jobs complete and then return an exit status of zero. With ID, pause until the specified background process ID or job ID completes and then return its exit status. Note that the shell variable $! contains the process ID of the most recent background process.

With no arguments, wait’s behavior depends upon the setting of POSIX mode. Normally, wait waits for all backgrounded processes to finish, and then runs the SIGCHLD trap as many times as there were exited processes. In POSIX mode, an exiting child interrupts wait, causing it to exit with 128 + SIGCHLD. Bash attempts to run the SIGCHLD trap handler once for each exiting child, but it does not guarantee that it will do so.

Option

-n

Wait for any job to terminate and return its exit status.

Example

wait $!  Wait for most recent background process to finish

while

Syntax for a loop that runs while a condition remains true.

while condition
do
commands
done

While condition is met, do commands. condition is often specified with the test command. See the Examples under the entries for case, and test.

filename

Run an external command.

filename [arguments]

Read and execute commands from executable file filename, or execute a binary object file. If filename does not contain any slash characters, the shell searches for the file to execute in the directories listed in $PATH.

Resources

This section briefly describes other sources of information about or related to Bash.

Online Resources

http://ftp.gnu.org/gnu/bash

The top-level directory for Bash source code releases. Source code is usually made available as .tar.gz files, such as bash-4.4.tar.gz.

ftp://ftp.gnu.org/pub/gnu/bash/bash-4.4-patches

Patches for Bash 4.4 are in this directory.

http://www.gnu.org/software/bash/bash.html

http://tiswww.tis.cwru.edu/~chet/bash/bashtop.html

The two “home pages” for the Bash shell.

http://bashdb.sourceforge.net

The Bash debugger.

http://bash-completion.alioth.debian.org/

Ian Macdonald’s collected set of completion specifications.

http://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html

Full documentation on the effects of POSIX mode. Many of the differences are subtle and don’t affect most day-to-day uses of the shell.

http://www.opengroup.org/onlinepubs/9699919799

The online version of the POSIX standard.

http://tobold.org/article/rc

The rc shell for Unix systems.

Books

Newham, Cameron. Learning the bash Shell, Third Edition. Sebastopol: O’Reilly Media, 2005.

Robbins, Arnold, and Nelson H.F. Beebe. Classic Shell Scripting. Sebastopol: O’Reilly Media, 2005.

Acknowledgments

Thanks to Chet Ramey, the Bash maintainer, for providing access to early releases of Bash 4.4, for answering questions, and for yet again reviewing this reference. Thanks to Robert P.J. Day for again reviewing this work. Thanks to Eli Zaretskii for comments on the previous edition and for reviewing this edition. Thanks to Andy Oram at O’Reilly Media for his support of this update.

Acknowledgments from the First Edition

Thanks to Chet Ramey, the Bash maintainer, for providing access to early releases of Bash 4.1, and for once again reviewing this reference. Thanks to Robert P.J. Day for reviewing this edition. Thanks again to Mike Loukides at O’Reilly Media for his continued support of this project.

[1] With <, the file is opened read-only, and writes on the file descriptor will fail. With <>, the file is opened read-write; it is up to the application to actually take advantage of this.

[2] Not all variables are always set. For example, the COMP* variables only have values while programmable completion functions are running.

Index

Symbols

- ! (exclamation mark)

- != inequality operator, Operators

- event designators, Event designators

- filename metacharacter, Filename Metacharacters

- logical negation operator, Operators

- " (quotation marks, double)

- " " quoting, Quoting

- $" " quoting, Quoting

- escape sequence, Escape Sequences

- # (hash mark), comments, Quoting, Built-In Commands

- #!shell command, Built-In Commands

- $ (dollar sign)

- $( ) command substitution, Command Forms

- $(( )) arithmetic substitution, Command Forms

- ${ } variable substitution, Variable Substitution

- last argument, Word substitution

- shell variables, Built-In Shell Variables

- variable substitution, Quoting

- % (percent)

- argument matched by, Word substitution

- job control, Job Control

- modulus operator, Operators

- & (ampersand)

- && logical AND operator, Operators, Built-In Commands

- background execution, Quoting

- bitwise AND operator, Operators

- ' (quotation marks, single)

- $' ' quoting, Escape Sequences

- ' ' quoting, Quoting

- escape sequence, Escape Sequences

- () (parentheses), command grouping, Quoting

- * (asterisk)

- ** exponentiation operator, Operators

- ** filename metacharacters, Filename Metacharacters

- all arguments, Word substitution

- filename metacharacter, Filename Metacharacters

- multiplication operator, Operators

- + (plus sign)

- ++ auto-increment operator, Operators

- += operator, Variable Assignment

- addition operator, Operators

- filename metacharacter, Filename Metacharacters

- unary operator, Operators

- , (comma), sequential expression evaluation operator, Operators

- - (hyphen)

- -- auto-decrement operator, Operators

- filename metacharacter, Filename Metacharacters

- subtraction operator, Operators

- unary operator, Operators

- . (dot), read and execute files, Built-In Commands

- .bashrc file, Special Files

- .bash_history file, Other Shell Variables

- .bash_login file, Special Files

- .bash_logout file, Special Files, Built-In Commands, Built-In Commands

- .bash_profile file, Special Files

- shell variables, Other Shell Variables

- .inputrc file, Other Shell Variables, Built-In Commands, Built-In Commands, Built-In Commands, Built-In Commands, Built-In Commands

- .profile file, Invoking the Shell, Special Files, Restricted Shells

- shell variables, Other Shell Variables

- / (slash), division operator, Operators

- /etc/bash_completion file, Tip

- /etc/passwd file, Special Files, Other Shell Variables

- /etc/profile file, Invoking the Shell, Command-Line Options, Special Files, Restricted Shells

- : (colon)

- :0 command name, Word substitution

- :n argument number, Word substitution

- :… history and truncation modifiers, History modifiers

- do-nothing command, Built-In Commands

- ; (semicolon)

- ;& case terminator, Built-In Commands

- ;;& case terminator, Built-In Commands

- command separator, Quoting

- < (left angle bracket)

- << bitwise shift left operator, Operators

- <<= assignment operator, Operators

- <= less than or equal to operator, Operators

- less than operator, Operators, Built-In Commands

- <, > (angle brackets), redirection symbols, Quoting

- = (equals sign)

- == equality operator, Operators

- assignment operator, Variable Assignment

- > (right angle bracket)

- >= greater than or equal to operator, Operators

- >> bitwise shift right operator, Operators

- >>= assignment operator, Operators

- greater than operator, Operators, Built-In Commands

- ? (question mark)

- ?: inline conditional evaluation operator, Operators

- escape sequence, Escape Sequences

- filename metacharacter, Filename Metacharacters

- @ (at sign), filename metacharacter, Filename Metacharacters

- [ ] (brackets)

- filename metacharacters, Filename Metacharacters

- [[ ]] test command, Built-In Commands

- \ (backslash)

- escape sequence, Escape Sequences

- prompt strings, Special Prompt Strings

- quoting, Quoting, Quoting

- \! prompt string, Special Prompt Strings

- \" escape sequence, Escape Sequences

- \# prompt string, Special Prompt Strings

- \$ prompt string, Special Prompt Strings

- \& in completion patterns, Programmable Completion

- \0 escape sequence, Escape Sequences

- \? escape sequence, Escape Sequences

- \@ prompt string, Special Prompt Strings

- \a escape sequence, Escape Sequences

- \a prompt string, Special Prompt Strings

- \A prompt string, Special Prompt Strings

- \b escape sequence, Escape Sequences

- \c escape sequence, Escape Sequences

- \cX escape sequence, Escape Sequences

- \d prompt string, Special Prompt Strings

- \D prompt string, Special Prompt Strings

- \e escape sequence, Escape Sequences

- \E escape sequence, Escape Sequences

- \e prompt string, Special Prompt Strings

- \f escape sequence, Escape Sequences

- \h prompt string, Special Prompt Strings

- \H prompt string, Special Prompt Strings

- \J prompt string, Special Prompt Strings

- \l prompt string, Special Prompt Strings

- \n escape sequence, Escape Sequences

- \n prompt string, Special Prompt Strings

- \nnn escape sequence, Escape Sequences

- \nnn prompt string, Special Prompt Strings

- \r escape sequence, Escape Sequences

- \r prompt string, Special Prompt Strings

- \s prompt string, Special Prompt Strings

- \t escape sequence, Escape Sequences

- \t prompt string, Special Prompt Strings

- \T prompt string, Special Prompt Strings

- \u escape sequence, Escape Sequences

- \U escape sequence, Escape Sequences

- \u prompt string, Special Prompt Strings

- \v escape sequence, Escape Sequences

- \v prompt string, Special Prompt Strings

- \V prompt string, Special Prompt Strings

- \w prompt string, Other Shell Variables, Special Prompt Strings

- \W prompt string, Other Shell Variables, Special Prompt Strings

- \x escape sequence, Escape Sequences

- \[ prompt string, Special Prompt Strings

- \\ escape sequence, Escape Sequences

- \\ prompt string, Special Prompt Strings

- \] prompt string, Special Prompt Strings

- \’ escape sequence, Escape Sequences

- ^ (caret)

- bitwise exclusive OR operator, Operators

- first argument, Word substitution

- ` (backquote), command substitution, Quoting

- | (pipe character)

- bitwise OR operator, Operators

- pipe command, Built-In Commands

- quoting, Quoting

- || logical OR operator, Operators, Built-In Commands

- ~ (tilde), filename metacharacter, Filename Metacharacters

A

- addition operator, Operators

- alert escape sequence, Escape Sequences

- alias command, Built-In Commands

- aliases, removing, Built-In Commands

- alnum class, Filename Metacharacters

- alpha class, Filename Metacharacters

- alphabetic characters, filename metacharacter, Filename Metacharacters

- alphanumeric characters, filename metacharacter, Filename Metacharacters

- ampersand (&)

- && logical AND operator, Operators, Built-In Commands

- background execution, Quoting

- bitwise AND operator, Operators

- AND operators, Operators

- angle brackets (<, >), redirection symbols, Quoting

- arguments

- Bash shell, Arguments

- job control commands, Job Control

- printing, Built-In Commands, Built-In Commands

- shifting command-line arguments left, Built-In Commands

- arithmetic operations, let command, Arithmetic Expressions, Built-In Commands

- arithmetic operators, Arithmetic Expressions

- arrays

- associative arrays, Associative arrays

- indexed arrays, Indexed arrays

- reading files into, Built-In Commands, Built-In Commands

- ASCII characters, filename metacharacter, Filename Metacharacters

- ascii class, Filename Metacharacters

- associative arrays, Associative arrays

- asterisk (*)

- ** exponentiation operator, Operators

- ** filename metacharacters, Filename Metacharacters

- all arguments, Word substitution

- filename metacharacter, Filename Metacharacters

- multiplication operator, Operators

- at sign (@), filename metacharacter, Filename Metacharacters

- attributes, shell variables, Built-In Commands

- auto-decrement operator, Operators

- autocd shell option, Shell Options

- auto_resume shell variable, Other Shell Variables

B

- background jobs

- putting current job into background, Built-In Commands

- running or suspending, Built-In Commands

- backquote (`), command substitution, Quoting

- backslash (\)

- escape sequence, Escape Sequences

- prompt strings, Special Prompt Strings

- quoting, Quoting, Quoting

- Bash (Bourne-Again shell)

- arithmetic expressions, Arithmetic Expressions

- built-in commands, Built-In Commands-Built-In Commands

- command execution, Command Execution

- command exit status, Command Exit Status-Command Exit Status

- command history, Command History-History modifiers

- coprocesses, Coprocesses

- features, Overview of Features

- functions, Functions

- history of, History

- invoking Bash shell, Invoking the Shell

- job control, Job Control

- options, Shell Options-Shell Options

- programmable completion, Programmable Completion-Examples

- restricted shells, Restricted Shells

- syntax, Syntax-Examples

- variables, Variables-Special Prompt Strings

- BASH shell variable, Built-In Shell Variables

- BASHOPTS shell variable, Built-In Shell Variables

- BASHPID shell variable, Built-In Shell Variables

- .bashrc file, Special Files

- BASH_ALIASES shell variable, Built-In Shell Variables

- BASH_ARGC shell variable, Built-In Shell Variables

- BASH_ARGV shell variable, Built-In Shell Variables

- BASH_CMDS shell variable, Built-In Shell Variables

- BASH_COMMAND shell variable, Built-In Shell Variables

- BASH_COMPAT shell variable, Other Shell Variables

- BASH_ENV shell variable, Other Shell Variables

- BASH_EXECUTION_STRING shell variable, Built-In Shell Variables

- .bash_history file, Other Shell Variables

- BASH_LINENO shell variable, Built-In Shell Variables

- BASH_LOADABLES_PATH shell variable, Other Shell Variables

- .bash_login file, Special Files

- .bash_logout file, Special Files, Built-In Commands, Built-In Commands

- .bash_profile file, Special Files

- shell variables, Other Shell Variables

- BASH_REMATCH shell variable, Built-In Shell Variables

- BASH_SOURCE shell variable, Built-In Shell Variables

- BASH_SUBSHELL shell variable, Built-In Shell Variables

- BASH_VERSINFO shell variable, Built-In Shell Variables

- BASH_VERSION shell variable, Built-In Shell Variables

- BASH_XTRACEFD shell variable, Other Shell Variables

- Berkeley C shell, history of, History

- bg command, Job Control, Built-In Commands

- bind command, Built-In Commands

- bitwise AND operator, Operators

- bitwise exclusive OR operator, Operators

- bitwise OR operator, Operators

- bitwise shift left operator, Operators

- bitwise shift right operator, Operators

- blank class, Filename Metacharacters

- Bourne shell, history of, History

- brace expansion, syntax, Brace Expansion

- brackets [ ]

- filename metacharacters, Filename Metacharacters

- [[ ]] test command, Built-In Commands

- break command, Built-In Commands

- built-in shell variables, Built-In Shell Variables

- builtin command, Built-In Commands

C

- caller command, Built-In Commands

- caret (^)

- bitwise exclusive OR operator, Operators

- first argument, Word substitution

- carriage return escape sequence, Escape Sequences

- case command, Built-In Commands

- case statements, esac command, Built-In Commands

- cd command, Built-In Commands

- cdable_vars shell option, Shell Options

- CDPATH shell variable, Other Shell Variables

- cdspell shell option, Shell Options

- checkhash shell option, Shell Options

- checkjobs shell option, Shell Options

- checkwinsize shell option, Shell Options

- CHILD_MAX shell variable, Other Shell Variables

- cmdhist shell option, Shell Options

- cntrl class, Filename Metacharacters

- colon (:)

- :0 command name, Word substitution

- :n argument number, Word substitution

- :… history and truncation modifiers, History modifiers

- do-nothing command, Built-In Commands

- COLUMNS shell variable, Other Shell Variables

- comma (,), sequential expression evaluation operator, Operators

- command command, Built-In Commands

- command exit status, Command Exit Status-Command Exit Status

- command forms, syntax, Command Forms

- command history, Command History-History modifiers

- C-shell–style history, C-Shell–Style History

- fc command, The fc Command

- line-edit mode, Line-Edit Mode

- command line

- editor, Line-Edit Mode

- history, Built-In Commands, Built-In Commands

- manipulation in Bash, Line-Edit Mode

- options, Built-In Commands

- printing arguments, Built-In Commands

- script parameters, Built-In Commands

- commands, Built-In Commands-Built-In Commands

- #!shell command, Built-In Commands

- : do-nothing command, Built-In Commands

- bg command, Job Control, Built-In Commands

- bind command, Built-In Commands

- break command, Built-In Commands

- builtin command, Built-In Commands

- caller command, Built-In Commands

- case command, Built-In Commands

- cd command, Built-In Commands

- command command, Built-In Commands

- compgen command, Programmable Completion, Built-In Commands

- complete command, Programmable Completion, Built-In Commands

- compopt command, Programmable Completion, Built-In Commands

- continue command, Built-In Commands

- declare command, Built-In Commands

- dirs command, Built-In Commands

- disown command, Built-In Commands

- do command, Built-In Commands

- done command, Built-In Commands

- echo command, Built-In Commands

- enable command, Built-In Commands

- enabling and disabling, Built-In Commands

- esac command, Built-In Commands

- eval command, Built-In Commands

- exec command, Built-In Commands

- executing, Command Execution

- exit command, Built-In Commands

- export command, Built-In Commands

- external commands, Built-In Commands

- false command, Built-In Commands

- fc command, The fc Command, Built-In Commands

- fg command, Job Control, Built-In Commands

- fi command, Built-In Commands

- filename command, Built-In Commands

- for command, Built-In Commands

- function command, Built-In Commands

- getopts command, Built-In Commands

- hash command, Built-In Commands

- help command, Built-In Commands

- history command, Command History, Built-In Commands

- if command, Built-In Commands

- job control, Job Control

- jobs command, Job Control, Built-In Commands

- kill command, Job Control, Built-In Commands

- let command, Built-In Commands

- local command, Built-In Commands

- logout command, Built-In Commands

- man command, Examples

- mapfile command, Built-In Commands

- name ( ) command, Built-In Commands

- popd command, Built-In Commands

- printf command, Built-In Commands

- pushd command, Built-In Commands

- pwd command, Built-In Commands

- read command, Built-In Commands, Built-In Commands

- readonly command, Built-In Commands

- return command, Functions, Built-In Commands

- select command, Built-In Commands

- set command, Built-In Commands-Built-In Commands

- shift command, Built-In Commands

- shopt command, Shell Options, Built-In Commands

- source command, Built-In Commands

- stty command, Job Control

- suspend command, Job Control, Built-In Commands

- test command, Built-In Commands-Built-In Commands

- time command, Built-In Commands

- times command, Built-In Commands

- trap command, Built-In Commands

- true command, Built-In Commands

- type command, Built-In Commands

- typeset command, Built-In Commands

- ulimit command, Built-In Commands

- umask command, Built-In Commands

- unalias command, Built-In Commands

- unset command, Built-In Commands

- until command, Built-In Commands

- wait command, Job Control, Built-In Commands

- while command, Built-In Commands

- [[ ]] test command, Built-In Commands

- comments, # (hash mark), Quoting, Built-In Commands

- comparisons, integers, Built-In Commands

- compat31 shell option, Shell Options

- compat32 shell option, Shell Options

- compat40 shell option, Shell Options

- compat41 shell option, Shell Options

- compat42 shell option, Shell Options

- compat43 shell option, Shell Options

- compgen command, Programmable Completion, Built-In Commands

- complete command, Programmable Completion, Built-In Commands, Built-In Commands

- complete_fullquote shell option, Shell Options

- completion facilities, The fc Command, Programmable Completion, Built-In Commands

- completion, programmable, Programmable Completion-Examples

- compopt command, Programmable Completion, Built-In Commands

- COMPREPLY shell variable, Other Shell Variables

- compspecs, programmable completion, Programmable Completion

- COMP_CWORD shell variable, Built-In Shell Variables

- COMP_KEY shell variable, Built-In Shell Variables

- COMP_LINE shell variable, Built-In Shell Variables

- COMP_POINT shell variable, Built-In Shell Variables

- COMP_TYPE shell variable, Built-In Shell Variables

- COMP_WORDBREAKS shell variable, Built-In Shell Variables

- COMP_WORDS shell variable, Built-In Shell Variables

- conditions, evaluating, Built-In Commands

- continue command, Built-In Commands

- control characters, filename metacharacter, Filename Metacharacters

- COPROC shell variable, Built-In Shell Variables

- coprocesses, Coprocesses

- CPU times, Built-In Commands

- csh (Berkeley C shell), history, History

- CTRL-Z job control command, Job Control, Job Control

D

- DEBUG trap, Functions

- decimal digits, filename metacharacter, Filename Metacharacters

- declare command, Built-In Commands

- declaring

- variables, Built-In Commands, Built-In Commands

- digit class, Filename Metacharacters

- directories

- changing, Built-In Commands

- popping directories off directory stack, Built-In Commands

- pushing directories onto directory stack, Built-In Commands

- search path for changing, Other Shell Variables

- working directories, Built-In Commands

- directory stack

- popping directories off directory stack, Built-In Commands

- printing or managing, Built-In Commands

- pushing directories onto directory stack, Built-In Commands

- direxpand shell option, Shell Options

- dirs command, Built-In Commands

- dirspell shell option, Shell Options

- DIRSTACK shell variable, Built-In Shell Variables

- disabling commands, Built-In Commands

- disown command, Built-In Commands

- division operator, Operators

- do command, Built-In Commands

- do-nothing command (:), Built-In Commands

- dollar sign ($)

- $( ) command substitution, Command Forms

- $(( )) arithmetic substitution, Command Forms

- ${ } variable substitution, Variable Substitution

- last argument, Word substitution

- shell variables, Built-In Shell Variables

- variable substitution, Quoting

- done command, Built-In Commands

- dot (.), read and execute files, Built-In Commands

- dotglob shell option, Shell Options

- dynamic scoping, Functions

E

- echo command, Built-In Commands

- editing, keyboard shortcuts for, Line-Edit Mode

- Emacs editor, Bash command-line editing mode, Line-Edit Mode

- EMACS shell variable, Other Shell Variables

- enable command, Built-In Commands

- $ENV file (read by Bash shell at startup), Special Files

- ENV shell variable, Other Shell Variables

- equals sign (=)

- == equality operator, Operators

- assignment operator, Variable Assignment

- ERR trap, Functions

- esac command, Built-In Commands

- escape sequences, quoted text ($' '), Quoting

- EUID shell variable, Built-In Shell Variables

- eval command, Built-In Commands

- evaluating conditions, Built-In Commands

- event designators, Event designators

- exclamation mark (!)

- != inequality operator, Operators

- event designators, Event designators

- filename metacharacter, Filename Metacharacters

- logical negation operator, Operators

- exec command, Built-In Commands

- execfail shell option, Shell Options

- EXECIGNORE shell variable, Other Shell Variables

- executing

- commands, Command Execution

- files, Built-In Commands, Built-In Commands

- input lines, Built-In Commands

- exit command, Built-In Commands

- exit status, Command Exit Status

- $? variable, Command Exit Status

- nonzero, Command Exit Status

- of shell functions, Built-In Commands

- success and failure, Command Exit Status

- true and false, Command Exit Status

- zero, Command Exit Status

- EXIT trap, Functions

- exiting

- kill command, Built-In Commands, Built-In Commands

- shell, Built-In Commands

- shell scripts, Built-In Commands

- with a false return value, Built-In Commands

- with a true return value, Built-In Commands

- expand_aliases shell option, Shell Options

- exponentiation operator, Operators

- export command, Built-In Commands

- expressions, arithmetic, Arithmetic Expressions

- extdebug shell option, Shell Options

- extglob shell option, Other Shell Variables, Other Shell Variables, Other Shell Variables, Shell Options

- extquote shell option, Shell Options

F

- failglob shell option, Shell Options

- false command, Built-In Commands

- fc command, The fc Command, Built-In Commands

- FCEDIT shell variable, Other Shell Variables, Built-In Commands

- fg command, Job Control, Built-In Commands

- fi command, Built-In Commands

- FIGNORE shell variable, Other Shell Variables

- filename command, Built-In Commands

- filenames

- metacharacters, Filename Metacharacters

- redirection forms, Special filenames

- files

- creation mask, Built-In Commands

- evaluating conditions, Built-In Commands

- reading and executing, Built-In Commands, Built-In Commands

- reading into arrays, Built-In Commands, Built-In Commands

- for command, Built-In Commands

- force_fignore shell option, Shell Options

- formfeed escape sequence, Escape Sequences

- forms, combined forms, Built-In Commands

- (see also redirection forms)

- FUNCNAME shell variable, Built-In Shell Variables

- FUNCNEST shell variable, Built-In Shell Variables

- function command, Built-In Commands

- functions

- about, Functions

- defining, Built-In Commands

- exit status of, Built-In Commands

- getpwnam() C function, Special Files

- getpwuid() C function, Special Files

- parsing, Tip

- removing, Built-In Commands

- traps, Functions

- variable scoping, Functions

G

- getconf command, Built-In Commands

- getopts command, Built-In Commands

- getpwnam() C function, Special Files

- getpwuid() C function, Special Files

- globasciiranges shell option, Shell Options

- GLOBIGNORE shell variable, Other Shell Variables

- globstar shell option, Shell Options

- gnu_errfmt shell option, Shell Options

- graph class, Filename Metacharacters

- greater than operator, Operators

- greater than or equal to operator, Operators

- GROUPS shell variable, Built-In Shell Variables

H

- hash command, Built-In Commands

- hash mark (#), comments, Quoting, Built-In Commands

- help command, Built-In Commands

- here document, Simple redirection

- here string, Simple redirection

- hexadecimal escape sequence, Escape Sequences

- histappend shell option, Shell Options

- histchars shell variable, Other Shell Variables

- HISTCMD shell variable, Built-In Shell Variables

- HISTCONTROL shell variable, Other Shell Variables

- HISTFILE shell variable, Other Shell Variables

- HISTFILESIZE shell variable, Other Shell Variables

- HISTIGNORE shell variable, Other Shell Variables

- history

- command history, Command History-History modifiers, Built-In Commands, Built-In Commands

- history modifiers, History modifiers

- history command, Command History, Built-In Commands

- histreedit shell option, Shell Options

- HISTSIZE shell variable, Other Shell Variables

- HISTTIMEFORMAT shell variable, Other Shell Variables

- histverify shell option, Shell Options

- HOME shell variable, Other Shell Variables

- hostcomplete shell option, Shell Options

- HOSTFILE shell variable, Other Shell Variables

- HOSTNAME shell variable, Built-In Shell Variables

- HOSTTYPE shell variable, Built-In Shell Variables

- huponexit shell option, Shell Options

- hyphen (-)

- -- auto-decrement operator, Operators

- option terminator, Command-Line Options

- subtraction operator, Operators

- unary operator, Operators

I

- if statement, Built-In Commands, Built-In Commands

- IFS shell variable, Other Shell Variables

- IGNOREEOF shell variable, Other Shell Variables

- indexed arrays, Indexed arrays

- indirect variables, Indirect Variables (namerefs)

- inequality operator, Operators

- inherit_errexit shell option, Shell Options, Built-In Commands

- inline conditional evaluation operator, Operators

- input lines, rescan or execute, Built-In Commands

- .inputrc file, Other Shell Variables, Built-In Commands, Built-In Commands, Built-In Commands, Built-In Commands, Built-In Commands

- INPUTRC shell variable, Other Shell Variables

- integers, comparisons, Built-In Commands

- interactive_comments shell option, Shell Options

- interpreters, invoking, Built-In Commands

- invoking

- Bash shell, Invoking the Shell

- interpreters, Built-In Commands

J

- job control, Job Control

- jobID argument, Job Control

- jobs

- background jobs, Built-In Commands, Built-In Commands

- list running or stopped jobs, Built-In Commands

- stop managing, Built-In Commands

- stopping, Built-In Commands

- wait command, Built-In Commands

- jobs command, Job Control, Built-In Commands

K

- keyboard shortcuts for editing, Line-Edit Mode

- kill command, Job Control, Built-In Commands

L

- LANG shell variable, Other Shell Variables

- lastpipe shell option, Shell Options

- LC_ALL shell variable, Other Shell Variables

- LC_COLLATE shell variable, Other Shell Variables

- LC_CTYPE shell variable, Other Shell Variables

- LC_MESSAGES shell variable, Other Shell Variables

- LC_NUMERIC shell variable, Other Shell Variables

- LC_TIME shell variable, Other Shell Variables

- left angle bracket (<)

- << bitwise shift left operator, Operators

- <<= assignment operator, Operators

- <= less than or equal to operator, Operators

- less than operator, Operators, Built-In Commands

- less than operator, Operators

- less than or equal to operator, Operators

- let command, Arithmetic Expressions, Built-In Commands

- line-edit mode (command history), Line-Edit Mode

- LINENO shell variable, Built-In Shell Variables

- LINES shell variable, Other Shell Variables

- /bin/sh, link to Bash, Invoking the Shell

- lithist shell option, Shell Options

- local command (local variables), Built-In Commands

- logical AND operator, Operators, Built-In Commands

- logical negation operator, Operators

- logical OR operator, Operators

- login_shell shell option, Shell Options

- logout command, Built-In Commands

- loops

- breaking out, Built-In Commands

- continuing, Built-In Commands

- do command, Built-In Commands

- done command, Built-In Commands

- for command, Built-In Commands

- select command, Built-In Commands

- until command, Built-In Commands

- while command, Built-In Commands

- lower class, Filename Metacharacters

- lowercase characters, filename metacharacter, Filename Metacharacters

M

- MACHTYPE shell variable, Built-In Shell Variables

- MAIL shell variable, Other Shell Variables

- MAILCHECK shell variable, Other Shell Variables

- MAILPATH shell variable, Other Shell Variables

- mailwarn shell option, Shell Options

- man command, Examples

- mapfile command, Built-In Commands

- MAPFILE shell variable, Built-In Shell Variables

- masks, file creation mask, Built-In Commands

- metacharacters, Bash shell filenames, Filename Metacharacters

- modulus operator, Operators

- multiple redirection, Multiple redirection

- multiplication operator, Operators

N

- nameref variables, Indirect Variables (namerefs)

- newline

- escape sequence, Escape Sequences

- word separator, Quoting

- nocaseglob shell option, Shell Options

- nocasematch shell option, Variable Substitution, Other Shell Variables, Programmable Completion, Shell Options, Built-In Commands

- nonspace characters, filename metacharacter, Filename Metacharacters

- nonzero exit status, Command Exit Status

- no_empty_cmd_completion shell option, Shell Options

- nullglob shell option, Shell Options

O

- octal value escape sequence, Escape Sequences, Escape Sequences

- OLDPWD shell variable, Built-In Shell Variables

- OpenVMS, The Bash Shell

- operators

- += operator, Variable Assignment

- arithmetic operators, Arithmetic Expressions

- OPTARG shell variable, Built-In Shell Variables

- OPTERR shell variable, Other Shell Variables

- OPTIND shell variable, Built-In Shell Variables

- OR operators, Operators

- OSTYPE shell variable, Built-In Shell Variables

P

- parentheses (), command grouping, Quoting

- parsing

- of functions, Tip

- of scripts, Tip

- PATH shell variable, Other Shell Variables

- percent (%)

- argument matched by, Word substitution

- job control, Job Control

- modulus operator, Operators

- pipe character (|)

- bitwise OR operator, Operators

- pipe command, Built-In Commands

- || logical OR operator, Operators, Built-In Commands

- PIPESTATUS shell variable, Built-In Shell Variables

- plus sign (+)

- ++ auto-increment operator, Operators

- += operator, Variable Assignment

- addition operator, Operators

- filename metacharacter, Filename Metacharacters

- unary operator, Operators

- popd command, Built-In Commands, Built-In Commands

- POSIX differences

- aliases for keywords, Command Execution

- command execution order, Command Execution

- command line assignments, Built-In Commands

- echo command, Built-In Commands

- extglob shell option, Shell Options

- fc command, Other Shell Variables, Built-In Commands

- function naming, Functions

- hash command, Built-In Commands

- history expansion, C-Shell–Style History

- inherit_errexit shell option, Shell Options

- kill command, Built-In Commands

- process substitution, Process substitution

- prompting, Special Prompt Strings

- read command, Built-In Commands

- searching $PATH, Built-In Commands

- set -e inheritance, Built-In Commands

- startup file, Special Files

- tilde in $PATH, Command Execution

- time command, Built-In Commands

- trap command, Built-In Commands

- ulimit command, Built-In Commands, Built-In Commands

- variable substitution and nested quotes, Variable Substitution

- wait command, Built-In Commands

- POSIXLY_CORRECT shell variable, Other Shell Variables

- postfix texts (brace expansion), Brace Expansion

- PPID shell variable, Built-In Shell Variables

- prefix texts (brace expansion), Brace Expansion

- printable characters, filename metacharacter, Filename Metacharacters

- printf command, Built-In Commands

- printing

- command usage information, Built-In Commands

- command-line history, Built-In Commands

- working directories, Built-In Commands

- process substitution, Process substitution

- .profile file, Invoking the Shell, Special Files, Restricted Shells

- shell variables, Other Shell Variables

- progcomp shell option, Shell Options

- programmable completion, Programmable Completion-Examples

- prompt strings, Special Prompt Strings

- promptvars shell option, Shell Options

- PROMPT_COMMAND shell variable, Other Shell Variables

- PROMPT_DIRTRIM shell variable, Other Shell Variables

- PS0-PS4 shell variables, Other Shell Variables

- punctuation characters, filename metacharacter, Filename Metacharacters

- pushd command, Built-In Commands

- pwd command, Built-In Commands

- PWD shell variable, Built-In Shell Variables

Q

- question mark (?)

- ?: inline conditional evaluation operator, Operators

- escape sequence, Escape Sequences

- filename metacharacter, Filename Metacharacters

- quotation marks, double (")

- " " quoting, Quoting

- $" " quoting, Quoting

- escape sequence, Escape Sequences

- quotation marks, single (')

- $' ' quoting, Escape Sequences

- ' ' quoting, Quoting

- escape sequence, Escape Sequences

- quoting, syntax, Quoting

R

- RANDOM shell variable, Built-In Shell Variables

- read command, Built-In Commands

- readarray command, Built-In Commands, Built-In Commands

- reading, files, Built-In Commands, Built-In Commands, Built-In Commands, Built-In Commands

- readline library, bindings, Built-In Commands

- READLINE_LINE shell variable, Built-In Shell Variables

- READLINE_POINT shell variable, Built-In Shell Variables

- readonly command, Built-In Commands

- redirection forms, Redirection Forms-Examples

- multiple redirection, Multiple redirection

- process substitution, Process substitution

- redirection using file descriptors, Redirection using file descriptors

- simple redirection, Simple redirection

- special filenames, Special filenames

- standard error, Redirection Forms

- standard input, Redirection Forms

- standard output, Redirection Forms

- referencing arrays, Indexed arrays

- removing

- aliases, Built-In Commands

- functions, Built-In Commands

- variables, Built-In Commands

- REPLY shell variable, Built-In Shell Variables, Built-In Commands

- rescanning input lines, Built-In Commands

- resources, Resources

- restricted shells, Restricted Shells

- restricted_shell shell option, Shell Options

- return command, Functions, Built-In Commands

- RETURN trap, Functions

- return values

- exiting with a false return value, Built-In Commands

- exiting with a true return value, Built-In Commands

- right angle bracket (>)

- >= greater than or equal to operator, Operators

- >> bitwise shift right operator, Operators

- >>= assignment operator, Operators

- greater than operator, Operators, Built-In Commands

S

- scoping

- dynamic, Functions

- variables, Functions

- scripts

- command-line parameters, Built-In Commands

- exiting, Built-In Commands

- parsing, Tip

- replacing, Built-In Commands

- signals, Built-In Commands

- select command, Built-In Commands

- semicolon (;)

- ;& case terminator, Built-In Commands

- ;;& case terminator, Built-In Commands

- command separator, Quoting

- set command, Built-In Commands-Built-In Commands

- sh, invoking Bash as, Invoking the Shell

- SHELL shell variable, Other Shell Variables

- SHELLOPTS shell variable, Built-In Shell Variables

- shells, History

- (see also Bash)

- Bourne shell, History

- declaring variables, Built-In Commands

- evaluating conditions, Built-In Commands

- invoking Bash shell, Invoking the Shell

- managing options, Built-In Commands

- options, Shell Options-Shell Options, Built-In Commands

- restricted shells, Restricted Shells

- suspending, Built-In Commands

- shift command, Built-In Commands

- shift_verbose shell option, Shell Options

- SHLVL shell variable, Built-In Shell Variables

- shopt command, Shell Options, Built-In Commands

- signal-based traps, Functions

- signals, shell scripts, Built-In Commands

- slash (/), division operator, Operators

- source command, Built-In Commands

- sourcepath shell option, Shell Options, Built-In Commands

- space character

- filename metacharacter, Filename Metacharacters

- word separator, Quoting

- stacks, directory stack, Built-In Commands, Built-In Commands, Built-In Commands

- standard error, redirection forms, Redirection Forms

- standard input, redirection forms, Redirection Forms

- standard output, redirection forms, Redirection Forms

- status, exit, Command Exit Status

- stopping jobs, Built-In Commands

- (see also exiting)

- strings

- completions, Built-In Commands

- default variable value, Variable Assignment

- evaluating conditions, Built-In Commands

- prompt strings, Special Prompt Strings

- stty command, Job Control

- substitution

- arithmetic, Command Forms, Arithmetic Expressions

- command, Command Forms

- PS0-PS4 shell variables, Special Prompt Strings

- variables, Variable Substitution

- word substitution, Word substitution

- subtraction operator, Operators

- suspend command, Job Control, Built-In Commands

- syntax, Syntax-Examples

- Bash arguments, Arguments

- brace expansion, Brace Expansion

- command forms, Command Forms

- filename metacharacters, Filename Metacharacters

- quoting, Quoting

- redirection forms, Redirection Forms-Examples

- special files, Special Files

- System V, Bourne shell, History

T

- tab character, filename metacharacter, Filename Metacharacters

- tab escape sequence, Escape Sequences

- tab word separator, Quoting

- TERM shell variable, Other Shell Variables

- test command, Built-In Commands, Built-In Commands-Built-In Commands

- tilde (~), filename metacharacter, Filename Metacharacters

- time command, Built-In Commands

- TIMEFORMAT shell variable, Other Shell Variables

- times command, Built-In Commands

- tip

- #! line length limit, Built-In Commands

- /etc/bash_completion file, Tip

- avoid restricted shells, Tip

- changing script parsing, Tip

- clearing the hash table, Built-In Commands

- exit within functions, Tip

- interactive editing vs. fc command, Tip

- keeping the history file up-to-date, Built-In Commands

- prefer functions to aliases, Built-In Commands

- quoting with the trap command, Built-In Commands

- restricting completion to help topics, Built-In Commands

- TMOUT shell variable, Other Shell Variables

- TMPDIR shell variable, Other Shell Variables

- trap command, Built-In Commands

- traps, list of, Functions

- true command, Built-In Commands

- truncation modifiers, History modifiers

- type command, Built-In Commands

- typeset command, Built-In Commands

U

- UID shell variable, Built-In Shell Variables

- ulimit command, Built-In Commands

- umask command, Built-In Commands

- unalias command, Built-In Commands

- unary operators, Operators

- unset command, Built-In Commands

- until command, Built-In Commands

- upper class, Filename Metacharacters

- uppercase characters, filename metacharacter, Filename Metacharacters

V

- variable scoping, Functions

- variables, Variables-Special Prompt Strings

- arrays, Arrays

- assignment, Variable Assignment

- built-in shell variables, Built-In Shell Variables

- declaring, Built-In Commands, Built-In Commands

- exporting or printing info about, Built-In Commands

- local variables, Built-In Commands

- other shell variables, Other Shell Variables

- prompt strings, Special Prompt Strings

- read-only, Built-In Commands

- reading, Built-In Commands

- removing, Built-In Commands

- substitution, Variable Substitution

- vertical tab escape sequence, Escape Sequences

- vi editor, Bash command-line editing mode, Line-Edit Mode

- VMS, The Bash Shell

W

- wait command, Job Control, Built-In Commands

- while command, Built-In Commands

- whitespace characters, filename metacharacter, Filename Metacharacters

- word substitution, Word substitution

- working directories, printing, Built-In Commands

X

- xpg_echo shell option, Shell Options

Z

- zero exit status, Command Exit Status

About the Author

Arnold Robbins is a professional programmer and technical author who has worked with Unix systems since 1980 and has been using Awk since 1987. As a member of the POSIX 1003.2 balloting group, he helped shape the POSIX standard for Awk. Arnold is the maintainer of GNU Awk (gawk) and its documentation. He is the author of the fourth edition of Effective awk Programming and the coauthor of Classic Shell Scripting (both published by O’Reilly).

1. 1. The Bash Shell

1. Conventions

2. History

3. Overview of Features

4. Invoking the Shell

1. Command-Line Options

2. Arguments

5. Command Exit Status

6. Syntax

1. Special Files

2. Filename Metacharacters

3. Brace Expansion

4. Escape Sequences

5. Quoting

6. Command Forms

7. Redirection Forms

7. Functions

1. Tip

8. Variables

1. Variable Assignment

2. Variable Substitution

3. Indirect Variables (namerefs)

4. Built-In Shell Variables

5. Other Shell Variables

6. Arrays

7. Special Prompt Strings

9. Arithmetic Expressions

1. Operators

2. Notes

3. Examples

10. Command History

1. Line-Edit Mode

2. The fc Command

3. C-Shell–Style History

11. Programmable Completion

1. Tip

2. Examples

12. Job Control

13. Shell Options

14. Command Execution

15. Coprocesses

1. Example

16. Restricted Shells

1. Tip

17. Built-In Commands

18. Resources

1. Online Resources

2. Books

19. Acknowledgments

20. Acknowledgments from the First Edition

2. Index