bash Pocket Reference

Arnold Robbins

Beijing • Cambridge • Farnham • Köln • Sebastopol • Tokyo

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Chapter 1. The Bash Shell

This pocket reference covers Bash, particularly version 4.1, 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. The following topics are covered:

- History

- Overview of features

- Invoking the shell

- 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. Items which should be replaced with real data in examples and syntax descriptions are shown in constant width replaceable. New terms and emphasized items 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).

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 arguably 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)

- 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.

Options

-c str

Read commands from string str.

-D, --dump-strings

Print all $"…" strings in the program.

-i

Create an interactive shell (prompt for input).

-l, --login

Shell is 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 $HOME/.bash_profile) are read.

-r, --restricted

Create a restricted shell.

-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.

--debugger

Read the debugging profile at startup 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.

--verbose

Same as set -v; the shell prints lines as it reads them.

--version

Print a version message and exit.

-, --

End option processing.

See the entry for set for the remaining options.

Arguments

Arguments are assigned in order 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, etc. The name of the script is available as $0. The script file itself need not be executable, but it must be readable.

Syntax

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

- Special files

- Filename metacharacters

- Brace expansion

- 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:

1. /etc/profile. Executed automatically at login.

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

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

The getpwnam() and getpwuid() 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+, or LDAP, not your workstation password file.)

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.

This 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

Class

Characters matched

alnum

Alphanumeric characters

print

Printable characters

alpha

Alphabetic characters

punct

Punctuation characters

blank

Space or Tab

space

Whitespace characters

cntrl

Control characters

upper

Uppercase characters

digit

Decimal digits

word

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

graph

Nonspace characters

xdigit

Hexadecimal digits

lower

Lowercase characters

Examples

$ ls new*                List new and new.1
$ cat ch?                Match ch9 but not ch10
$ vi [D-R]*              Match files beginning with D through R
$ pr !(*.o|core) | lp    Print files non-object and non-core files

Caution

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.

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

Here, start, end, and incr are all integers. The shell expands them to the full range between start and end, increasing by incr if supplied. Bash ignores leading zeros on incr, always treating it as a decimal value.

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. The value of incr is treated as a plain integer, as returned by the C library strtol(3) routine. (Thus a leading zero on incr causes it to be treated as an octal 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. 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

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 the following escape sequences.

Sequence

Value

Sequence

Value

\a

Alert

\t

Tab

\b

Backspace

\v

Vertical tab

\cX

Control character X

\nnn

Octal value nnn

\e

Escape

\xnn

Hexadecimal value nn

\E

Escape

\’

Single quote

\f

Form feed

\"

Double quote

\n

Newline

\\

Backslash

\r

Carriage return

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

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 result of expression as 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 | lp

# Edit files found by grep
$ vi `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
$ grep XX file && lp 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.)

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  > f1 2> f2

Send standard output to file f1 and standard error to file f2.

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.

Bash also allows {variablename} instead of a file descriptor number in redirections. In such a case, the shell uses a file descriptor number greater than nine, and assigns the value to the named shell variable.

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 >( 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.

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.

Special filenames

Bash recognizes several special filenames in redirections and interprets them internally, even if you have such a file on your system:

/dev/stdin

A duplicate of file descriptor zero.

/dev/stdout

A duplicate of file descriptor one.

/dev/stderr

A duplicate of file descriptor two.

/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

$ cat part1 > book                  Copy part1 to book
$ cat part2 part3 >> book           Append part2 and part3k
$ mail tim < report                 Send report to the big boss
$ sed 's/^/XX /g' << END_ARCHIVE    Here document is sed's input
> 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 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 demonstrates how Bash assigns file descriptor numbers to named variables:

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

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

$ diff <(sort file1) <(sort file2)

[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.

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 just as are 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, etc. 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 value to the calling shell program. Be careful not to use exit from within a function unless you really wish to terminate the entire 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 share traps 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)

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 do not have to have names that are valid shell identifiers).

Variables

This section describes the following:

- Variable assignment

- Variable substitution

- 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 may 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
$ name+=" Robbins" ; echo $name         String variable
Arnold Robbins
$ declare -i jj ; jj=3+5 ; echo $jj     Integer variable
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.

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.

${var:+value}

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

${#var}

Use the length of var.

${#*}

Use the number of positional parameters.

${#@}

Same as previous.

${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 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 lower case. Applies only to the first character in the string.

${var,,pattern}

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

${!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.

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 beginning of the value.

${var/%pat/repl}

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

${!var}

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

Bash provides a special syntax that lets one variable indirectly reference another:

$ greet="hello, world"           Create initial variable
$ friendly_message=greet         Aliasing variable
$ echo ${!friendly_message}      Use the alias
hello, world

Examples

$ u=up d=down blank=         Assign values to 3 variables
(last is null)
$ echo ${u}root              Braces are needed here
uproot
$ echo ${u-$d}               Display value of u or d; u is set,
so print it
up
$ echo ${tmp-`date`}         If tmp not set, execute date
Mon Apr 12 14:33:16 EDT 2010
$ 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

Built-in Shell Variables

Built-in variables are automatically set by the shell and are typically used inside shell scripts. Built-in variables can make use of 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 it was found via a PATH search.

$n

Individual arguments on 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 command line ($1 $2 …).

"$*"

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

"$@"

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

Bash automatically sets the following additional variables:

$_

Temporary variable; initialized to pathname of script or program being executed. Later, stores the last argument of previous command. Also stores name of 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; removing an element removes the corresponding 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. 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. 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; removing an element removes the corresponding entry.

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. 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. 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 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.

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).

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.

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 the Bash Debugger (see http://bashdb.sourceforge.net) or for programmable completion (see the section Programmable Completion).

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 use 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_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_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.

COLUMNS=n

Screen’s column width; used in line edit modes and select lists.

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.

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 patterns describing the set of filenames to ignore when doing filename completion.

GLOBIGNORE=patlist

Colon-separated list of patterns describing the set of filenames to ignore during pattern matching.

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.

HISTFILESIZE=n

Number of lines to be kept in the history file. This may be different from the number of commands.

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.

HISTSIZE=n

Number of history commands to be kept in the history file.

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 /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.

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.

MAIL=file

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

MAILCHECK=n

Number of seconds between mail checks; default is 600 (10 minutes).

MAILPATH=files

One or more files, delimited by a colon, 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 the default message is You have mail in $_. $_ is replaced with the name of the file. For example, you might have MAILPATH="$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 default for many systems is /bin:/usr/bin. On Solaris, the default is /usr/bin:. However, the standard startup scripts change it to /usr/bin:/usr/ucb:/etc:.

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.

PS1=string

Primary prompt string; default is $.

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 +.

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.

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

Enables 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 to:

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).

${name[i]}

Use element i of array name. i can be any arithmetic expression as described under let.

${name}

Use element 0 of array name.

${name[*]}

Use all elements of array name.

${name[@]}

Same as previous.

${#name[*]}

Use the number of elements in array name.

${#name[@]}

Same as previous.

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 works, too

The values would be retrieved as ${data[joe]} and ${data[mary]}.

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 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 on a terminal.

\]

End a sequence of nonprinting characters.

The 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.

Arithmetic Expressions

The let command performs arithmetic. Bash is restricted to 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.

B#n

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

Operators

The shell uses arithmetic operators from the C programming language, in decreasing order of precedence.

Operator

Description

++ --

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

+ -

Unary plus and minus

! ~

Logical negation and binary inversion (one’s complement)

**

Exponentiation[a]

* / %

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

[a] The ** operator is right-associative. Prior to version 3.1, it was left-associative.

Examples

let "count=0" "i = i + 1"       Assign values to i and count
let "num % 2"                   Exit successfully if num is even
(( percent >= 0 && \
percent <= 100 ))            Test the range of a value

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 history for more information. 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 command
$ 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

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.

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

nth 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 will fill 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, the variable FIGNORE is used 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.
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:

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.

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 can download it at http://freshmeat.net/projects/bashcompletion/. 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 bash_completion file.
# Assumes   man [num] command   command syntax.

shopt -s extglob    # Enable extended pattern matching

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

COMPREPLY=(  )                         # Clear reply list
if [[ ${COMP_WORDS[1]} = +([0-9]) ]]   # Have section no.
then
# section provided: man 3 foo
# look in specified directory
dir=$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 above)

%

Current job (same as above)

%-

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. (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 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 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.

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.

cmdhist †

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

compat31

Mutually exclusive with the compat32 and compat40 options. 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

Mutually exclusive with the compat31 and compat40 options. Cause the < and > operators of the [[ ]] command to ignore the locale when doing string comparison.

compat40

Mutually exclusive with the compat31 and compat32 options. Cause the < and > operators of the [[ ]] command to ignore the locale when doing string comparison. In addition, interrupting a command list such as cmd1; cmd2; cmd3 aborts execution of the entire list. (Prior to version 4.0, interrupting one command in a list did not prevent the following commands from executing.)

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.

extdebug

Enable behavior needed for debuggers:

- 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 +(…). (These were not in the Bourne shell and are not in POSIX; 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.

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.

interactive_comments †

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

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 PS1, PS2, and PS4.

restricted_shell

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

shift_verbose

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

sourcepath †

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

xpg_echo

Causes 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 until it finds a match:

1. Keywords such as if and for.

2. Aliases. 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.

3. 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.

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.

7. When a command is not found, if a function named command_not_found_handle exists, the shell will call 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:

cd () {                   Shell function; found before built-in 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 will send 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. 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.

Caution

As of version 4.1, 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:

$ coproc testproc (echo 1          Start a named coprocess
> read aline ; echo $aline)        in the background
[1] 5090
$ echo ${testproc[@]}              Show the file descriptors
63 60
$ echo $testproc_PID               Show the coprocess PID
5090
$ read out <&${testproc[0]}        Read the first line of coprocess
$ echo $out                        output and show it
1
$ echo foo >&${testproc[1]}        Send coprocess some input
$ read out2 <&${testproc[0]}       Read second output line
[1]+ Done  coproc testproc (echo 1; read aline; echo $aline)
$ echo $out2                       Show the second output line
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, $HOME/.profile, and the other startup files.

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, some of the reserved words used by multiline commands are also included.

Name

! — Invert the sense of the following pipeline.

Synopsis

! 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

Name

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

Synopsis

# 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.

Name

#!shell — Invoke the named interpreter to execute the script.

Synopsis

#!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. 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. For example:

#!/bin/sh

Name

: — Do-nothing command, used as a syntactic placeholder.

Synopsis

: [arguments]

Null command. Returns an exit status of 0. See this Example and the ones under 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

Name

. — Read and execute a file within the current shell.

Synopsis

.  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 not enabled, Bash does not search $PATH. The arguments are stored in the positional parameters. If Bash is not in POSIX mode and file is not found in $PATH, Bash looks in the current directory for file. Bash removes NUL bytes from the contents of file before attempting to parse it. See also source.

Name

[[ ]] — Extended version of the test command.

Synopsis

[[ 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 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

name () — Define a shell function.

Synopsis

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 now displays the number of files in the current directory.

Name

alias — Define and manage shell aliases.

Synopsis

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 unalias.

Option

-p

Print the word alias before each alias.

Example

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

Name

bg — Move a stopped job into the background.

Synopsis

bg [jobIDs]

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

Name

bind — Manage key bindings for the readline library.

Synopsis

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.

Name

break — Exit from one or more loops.

Synopsis

break [n]

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

Name

builtin — Execute a built-in command, bypassing functions.

Synopsis

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
}

Name

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

Synopsis

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.

Name

case — Syntax for a case statement.

Synopsis

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, etc. Be sure the last command in each set ends with ;;. value is typically a positional parameter or other shell variable. cmds are typically Unix 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 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 4.1 do not require it. See also the nocasematch option in 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. ;;& 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 :        # Null command; always true
do    printf "Type . to finish ==> "
read line
case "$line" in
.)  echo "Message done"
break ;;
*)  echo "$line" >> $message ;;
esac
done

Name

cd — Change directory.

Synopsis

cd [-LP] [dir]
cd [-LP] [-]

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 $CDPATH is also searched. A directory of - stands for the previous directory.

Options

-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.

Example

$ ls -ld /usr/tmp                /usr/tmp is a symbolic link
lrwxrwxrwx 1 root root 10 Dec 26 2008 /usr/tmp ->../var/tmp
$ cd -L /usr/tmp                 Logical change directory
$ pwd                            Show location
/usr/tmp                         Result is logical location
$ cd -P /usr/tmp                 Physical change directory
$ pwd                            Show location
/var/tmp                         Result is physical location

Name

command — Execute or print information about a built-in command.

Synopsis

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.

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 will get the system’s version, and run it with the -i option:

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

Name

compgen — Generate possible completions.

Synopsis

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.

Name

complete — Specify how to do completion for specific commands.

Synopsis

complete [-DE] [options] command …

Specifies the way 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.

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 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.

Name

compopt — Print or change the completion options for a command.

Synopsis

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

With no options, print the completion options for 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.

Name

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

Synopsis

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).

Name

declare — Declare shell variables and manage their attributes.

Synopsis

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 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).

-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 lower case upon assignment.

-p

With no names, print all variables, their values, and attributes. With names, print the names, attributes, and values of the given variables. With -f, print function values and attributes.

-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 upper case 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"

Name

dirs — Print or manage the directory stack.

Synopsis

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

Remove all entries from (clear) 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.

Name

disown — Stop managing one or more jobs.

Synopsis

disown [-ahr] [job …]

Remove jobs from the list of jobs managed by Bash.

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.

Name

do — Reserved word that starts the body of a loop.

Synopsis

do

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

Name

done — Reserved word that ends the body of a loop.

Synopsis

done

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

Name

echo — Print command-line arguments to standard output.

Synopsis

echo [-eEn] [string]

Built-in version. Write string to standard output.

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 following escape sequences, which must be quoted (or escaped with a \) to prevent interpretation by the shell:

\a

Alert (ASCII BEL).

\b

Backspace.

\c

Suppress the terminating newline (same as -n).

\e

ASCII Escape character.

\f

Formfeed.

\n

Newline.

\r

Carriage return.

\t

Tab character.

\v

Vertical-tab character.

\\

Backslash.

\0nnn

ASCII character represented by octal number nnn, where nnn is zero, one, two, or three digits and is preceded by a 0.

\xHH

ASCII character represented by hexadecimal number HH, where HH is one or two hexadecimal digits.

-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" | lp
$ echo "Warning: ringing bell \a"

Name

enable — Enable or disable shell built-in commands.

Synopsis

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.

-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.

Name

esac — Reserved word that ends a case statement.

Synopsis

esac

Reserved word that ends a case statement.

Name

eval — Rescan and execute an already-processed input line.

Synopsis

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 right order:

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

eval sort $file $out

Name

exec — Replace the current script or manage shell file descriptors.

Synopsis

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, 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 argv[0].

-c

Clear the environment before executing the program.

-l

Place a minus sign at the front of 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
$ exec < infile        Reassign standard input to infile

Name

exit — Exit the shell script.

Synopsis

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 is executed before the shell exits.

Example

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

Name

export — Export variables or print information about exported variables.

Synopsis

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.

Options

-f

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

-n

Remove the named variables or functions from the environment.

-p

Print export before printing the names and values of exported variables. This allows saving a list of exported variables for rereading later.

Examples

In the original Bourne shell, you would type:

TERM=vt100
export TERM

In Bash, you could type this instead:

export TERM=vt100

Name

false — Exit with a false return value.

Synopsis

false

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

Name

fc — Manage command-line history.

Synopsis

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. 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. The default editor is set by the shell variable FCEDIT. If that variable is not set, the default is /bin/ed. (Bash defaults to vi; versions 3.1 and newer default to /bin/ed when in POSIX mode.) Bash tries $FCEDIT, then $EDITOR, and then /bin/ed.

-e -

Execute (or redo) a history command; refer to 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 -.

Name

fg — Move a running or suspended background job into the foreground.

Synopsis

fg [jobIDs]

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

Name

fi — Reserved word that ends an if statement.

Synopsis

fi

Reserved word that ends an if statement.

Name

for — Start a loop over a list of values.

Synopsis

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 are executed.

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

Name

for — Start an arithmetic loop.

Synopsis

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

Name

function — Define a shell function.

Synopsis

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
> }

Name

getopts — Process command-line options and arguments.

Synopsis

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 -. 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 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 application must be written to expect multiple arguments in this format.) getopts uses the shell variables OPTARG, OPTIND, and OPTERR.

Name

hash — Manage the table of previously found commands.

Synopsis

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

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 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

Remove (delete) 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.

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.

Name

help — Print command usage information.

Synopsis

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 brief (short) usage information.

Examples

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

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

Name

history — Print command-line history.

Synopsis

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.

Options

-a

Append new history lines (those executed since the beginning of the session) to the history file.

-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 into 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.

Name

if — Syntax for an if-then-else statement.

Synopsis

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 test, and [[ ]], for a full list of conditions, and see additional Examples under :, 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

Name

jobs — List running or stopped jobs.

Synopsis

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.

-x cmd

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

Name

kill — Send a signal to one or more jobs.

Synopsis

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 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; for a PC-based system, it looks like this:

$ kill -l                 From Bash on GNU/Linux
1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL
5) SIGTRAP      6) SIGABRT      7) SIGBUS       8) SIGFPE
9) SIGKILL     10) SIGUSR1     11) SIGSEGV     12) SIGUSR2
13) SIGPIPE     14) SIGALRM     15) SIGTERM     16) SIGSTKFLT
17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU
25) SIGXFSZ     26) SIGVTALRM   27) SIGPROF     28) SIGWINCH
29) SIGIO       30) SIGPWR      31) SIGSYS      34) SIGRTMIN
35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3  38) SIGRTMIN+4
39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12
47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14
51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10
55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7  58) SIGRTMAX-6
59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

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

Options

-l [n]

List the signal names. (Used by itself.) 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.

-n num

Send the given signal number.

-s name

Send the given signal name.

-signal

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

Name

let — Perform arithmetic.

Synopsis

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).

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++ ))

Name

local — Declare local variables inside shell functions.

Synopsis

local [options] [name[=value]]

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

Name

logout — Exit the shell.

Synopsis

logout

Exit a login shell. The command fails if the current shell is not a login shell.

Name

mapfile — Read a file into a shell array.

Synopsis

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. The quantum is set with the -c option.

-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

Ignore the first count lines.

-t

Remove the trailing newline from each line read.

-u n

Read from file descriptor n instead of from standard input.

Name

popd — Pop a directory off of the directory stack.

Synopsis

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

-n

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

+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.

Name

printf — Do formatted printing of command-line arguments.

Synopsis

printf [-v var] format [val …]

Formatted printing, like the ANSI C printf function. 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. var may be an array element.

Additional Format Letters

Bash accepts these additional format letters:

%b

Expand escape sequences in strings (e.g., \t to tab, and so on).

%q

Print a quoted string that can be reread later on.

Name

pushd — Push a directory onto the directory stack.

Synopsis

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

-n

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

+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.

Name

pwd — Print working directory.

Synopsis

pwd [-LP]

Print the present working directory on standard output.

Options

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

-L

Use 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.

Name

read — Read data into one or more shell variables.

Synopsis

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. See the Examples here and under case. The return status is 0 unless EOF is reached. If no variables are given, input is stored in the REPLY variable.

Options

-a array

Read into indexed array array.

-d delim

Read up to the first occurrence of delim, instead of newline.

-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 will return 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 "$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

Name

readarray — Read a file into a shell array.

Synopsis

readarray [options] [array]

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

Name

readonly — Mark variables as read only.

Synopsis

readonly [-afp] [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.

Name

return — Return an exit status from a shell function.

Synopsis

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 --.

Name

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

Synopsis

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 | lp;;
Page)   pr $file | lp;;
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:

Name

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

Synopsis

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.) Arguments are assigned in order to $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. 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.

-n

Read commands but don’t execute; 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. This is 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 one command is executed.

-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 nonunset

-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 Bash clears the option in such subshells.

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 !.

Examples

set -- "$num" -20 -30   Set $1 to $num, $2 to -20, $3 to -30
set -vx                 Read each command line; show it;
execute it; show it again (with arguments)
set +x                  Stop command tracing
set -o noclobber        Prevent file overwriting
set +o noclobber        Allow file overwriting again

Name

shift — Shift the command-line arguments left.

Synopsis

shift [n]

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

Example

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

Name

shopt — Manage shell options.

Synopsis

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 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.

Name

source — Read and execute a file within the current shell.

Synopsis

source file [arguments]

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

Name

suspend — Suspend the current shell.

Synopsis

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.

Name

test — Evaluate conditions, for use in loops and conditionals.

Synopsis

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 [[ ]]). 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 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 Shell Options.

s1 < s2

String value of s1 precedes that of s2. Bash uses the locale’s sorting order. (Use only within [[ ]].)

s1 > s2

String value of s1 follows that of s2. Bash uses the locale’s sorting order. (Use only within [[ ]].)

Internal Shell Conditions

-o opt

Option opt for set -o is on.

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 using [[ ]] 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
while [ -n "$1" ]             While there are nonempty arguments
if [ $count -lt 10 ]          If $count is less than 10
if [ -d RCS ]                 If the RCS directory exists
if [ "$answer" != "y" ]       If the answer is not y
if [ ! -r "$1" -o ! -f "$1" ] If the first argument is not a
readable file or a regular file

Name

time — Time a command.

Synopsis

time 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.

Name

times — Print accumulated CPU times.

Synopsis

times

Print accumulated process times for user and system.

Name

trap — Manage the disposition of signals within a shell script.

Synopsis

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

Execute commands if any signals are received. The second form prints the current trap settings in a form suitable for rereading later. The third form lists all signals and their numbers, like kill -l.

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), signals are ignored by the shell. If commands are omitted entirely, reset processing of specified signals to the default action. If commands is “-”, reset signals to their initial defaults.

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 would be evaluated earlier, when the trap command itself is executed.

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

Signals

A list of signal names, numbers, and meanings were given in the entry for kill. The shell allows you to use either the signal number or the signal name (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 are:

DEBUG

Execution of any command.

ERR

Nonzero exit status.

EXIT

Exit from shell (usually when shell script finishes).

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 Bourne shell

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

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

Name

true — Exit with a true return value.

Synopsis

true

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

Name

type — Print the type of a command.

Synopsis

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 files. 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

Name

typeset — Declare shell variables and manage their attributes.

Synopsis

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

Identical to declare. See the entry for declare.

Name

ulimit — Manage various process limits.

Synopsis

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.

-l

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

-m

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

-n

Maximum number of file descriptors.

-p

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

-x

Maximum number of file locks.

Name

umask — Display or set the process’s file creation mask.

Synopsis

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

Display file creation mask or set 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.

Name

unalias — Remove previously defined aliases.

Synopsis

unalias names
unalias -a

Remove names from the alias list. See also alias.

Option

-a

Remove all aliases.

Name

unset — Remove variables or functions.

Synopsis

unset [options] names

Erase definitions of functions or variables listed in names.

Options

-f

Unset functions names.

-v

Unset variables names (default).

Name

until — Syntax for a loop that runs until a condition becomes true.

Synopsis

until condition
do
commands
done

Until condition is met, do commands. condition is often specified with the test command. See the Examples under case, and test.

Name

wait — Wait for a process or job to complete.

Synopsis

wait [ID]

With no ID, pause in execution until all background jobs complete (exit status 0 is returned), or pause until the specified background process ID or job ID completes (exit status of ID is returned). Note that the shell variable $! contains the process ID of the most recent background process.

Example

wait $!  Wait for most recent background process to finish

Name

while — Syntax for a loop that runs while a condition remains true.

Synopsis

while condition
do
commands
done

While condition is met, do commands. condition is often specified with the test command. See the Examples under case, and test.

Name

filename — Run an external command.

Synopsis

filename [arguments]

Read and execute commands from executable file filename, or execute a binary object file.

Resources

This section briefly describes other sources of information about 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-4.1.tar.gz.

ftp://ftp.gnu.org/pub/gnu/bash/bash-4.1-patches

Patches for Bash 4.1 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://freshmeat.net/projects/bashcompletion

Ian Macdonald’s collected set of completion specifications.

http://www.opengroup.org/onlinepubs/009695399

The online version of the POSIX standard.

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 4.1, 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.

Index

A note on the digital index

A link in an index entry is displayed as the section title in which that entry appears. Because some sections have multiple index markers, it is not unusual for an entry to have several links to the same section. Clicking on any link will take you directly to the place in the text in which the marker appears.

Symbols

! (exclamation mark)

!= inequality operator, Operators

event designators, Event designators

filename metacharacter, Filename Metacharacters

logical negation operator, Operators

" (quotation marks, double)

" " quoting, Quoting

escape sequence, Quoting

# (hash mark), comments, Quoting, #

#!shell command, #!shell

$ (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

$ENV file (read by Bash shell at startup), Special Files

% (percent)

argument matched by, Word substitution

job control, Job Control

modulus operator, Operators

& (ampersand)

&& logical AND operator, Additional Operators

background execution, Quoting

bitwise AND operator, Operators

logical AND operator, Operators

' (quotation marks, single)

escape sequence, Quoting

quoting, Quoting

() (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

subtraction operator, Operators

unary operator, Operators

. (dot), read and execute files, .

.bash_profile file, shell variables, Other Shell Variables

.profile file, shell variables, Other Shell Variables

/ (slash), division operator, Operators

/etc/bash_completion file, Programmable Completion

/etc/passwd file, Special Files

/etc/profile file, Special Files

: (colon)

:0 command name, Word substitution

:n argument number, Word substitution

:… history and truncation modifiers, History modifiers

do-nothing command, :

; (semicolon), command separator, Quoting

< (left angle bracket), Additional Operators

<< bitwise shift left operator, Operators

<<= assignment operator, Operators

less than operator, Operators

< > (angle brackets), redirection symbols, Quoting

= (equals sign), == equality operator, Operators

> (right angle bracket)

>= greater than or equal to operator, Operators

>> bitwise shift right operator, Operators

>>= assignment operator, Operators

greater than operator, Operators, Additional Operators

? (question mark)

?: inline conditional evaluation operator, Operators

filename metacharacter, Filename Metacharacters

@ (at sign), filename metacharacter, Filename Metacharacters

[ ] (brackets)

filename metacharacters, Filename Metacharacters

[[:word:]] filename metacharacter, Filename Metacharacters

[[ ]] test command, [[ ]]

\ (backslash)

escape sequence, Quoting

prompt strings, Special Prompt Strings

quoting, Quoting, Quoting

\ escape sequence, Quoting

\0 octal number escape sequence, Options

\a escape sequence, Quoting, Options

\b escape sequence, Quoting, Options

\c escape sequence, Quoting, Options

\e escape sequence, Quoting, Options

\E escape sequence, Quoting

\f escape sequence, Quoting, Options

\n escape sequence, Quoting, Options

\r escape sequence, Quoting, Options

\t escape sequence, Quoting, Options

\v escape sequence, Quoting, Options

\x escape sequence, Quoting

\\ backslash escape sequence, Options

\\ escape sequence, Quoting

\’ escape sequence, Quoting

\” escape sequence, Quoting

^ (caret)

bitwise exclusive OR operator, Operators

first argument, Word substitution

` (backquote), command substitution, Quoting

| (pipe character)

|| logical OR operator, Operators, Additional Operators

bitwise OR operator, Operators

pipe command,

quoting, Quoting

~ (tilde), filename metacharacter, Filename Metacharacters

A

addition operator, Operators

alias command, alias

aliases, removing, unalias

alnum class, Filename Metacharacters

alpha class, Filename Metacharacters

alphabetic characters, filename metacharacter, Filename Metacharacters

alphanumeric characters, filename metacharacter, Filename Metacharacters

ampersand (&)

&& logical AND operator, Additional Operators

background execution, Quoting

bitwise AND operator, Operators

logical AND operator, Operators

AND operators, Operators

angle brackets (< >), redirection symbols, Quoting

arguments

Bash shell, Arguments

job control commands, Job Control

printing, echo, printf

shifting command-line arguments left, shift

arithmetic operations, let command, let

arithmetic operators, Arithmetic Expressions

arrays

associative arrays, Associative arrays

indexed arrays, Indexed arrays

reading files into, mapfile, readarray

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, declare

auto-decrement operator, Operators

auto_resume shell variable, Other Shell Variables

B

background jobs

putting current job into background,

running or suspending, fg

backquote (`), command substitution, Quoting

backslash (\)

escape sequence, Quoting

prompt strings, Special Prompt Strings

quoting, Quoting, Quoting

Bash (Bourne-Again shell)

arithmetic expressions, Arithmetic Expressions

built-in commands, Built-in Commands–

command execution, Command Execution

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_ENV shell variable, Other Shell Variables

BASH_EXECUTION_STRING shell variable, Built-in Shell Variables

BASH_LINENO shell variable, Built-in 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, bg

bind command, bind

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

[[:word:]] filename metacharacter, Filename Metacharacters

[[ ]] test command, [[ ]]

break command, break

built-in shell variables, Built-in Shell Variables

builtin command, builtin

C

caller command, caller

caret (^)

bitwise exclusive OR operator, Operators

first argument, Word substitution

carriage return escape sequence, Quoting

case command, case

case statements, esac command, esac

cd command, cd

CDPATH shell variable, Other Shell Variables

cntrl class, Filename Metacharacters

colon (:)

:0 command name, Word substitution

:n argument number, Word substitution

:… history and truncation modifiers, History modifiers

do-nothing command, :

COLUMNS shell variable, Other Shell Variables

comma (,), sequential expression evaluation operator, Operators

command command, command

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, fc, history

manipulation in Bash, Line-Edit Mode

options, getopts

printing arguments, printf

script parameters, set

commands, Built-in Commands–

#!shell command, #!shell

: do-nothing command, :

bg command, Job Control, bg

bind command, bind

break command, break

builtin command, builtin

caller command, caller

case command, case

cd command, cd

command command, command

compgen command, Programmable Completion, compgen

complete command, Programmable Completion, complete

compopt command, Programmable Completion, compopt

continue command, continue

declare command, declare

dirs command, dirs

disown command, disown

do command, do

done command, done

echo command, echo

enable command, enable

enabling and disabling, enable

esac command, esac

eval command, eval

exec command, exec

executing, Command Execution

exit command, exit

export command, export

external commands, filename

false command, false

fc command, The fc Command, fc

fg command, Job Control, fg

fi command, fi

filename command, filename

for command, for

funcname command, Programmable Completion

function command, function

getopts command, getopts

hash command, hash

help command, help

history command, Command History, history

if command, if

job control, Job Control

jobs command, Job Control, jobs

kill command, Job Control, kill

let command, let

local command, local

logout command, logout

man command, Examples

mapfile command, mapfile

name () command, name (), alias

popd command, popd

printf command, printf

pushd command, pushd

pwd command, pwd

read command, read, readarray

readonly command, readonly

return command, Functions, return

select command, select

set command, set–set -e Details

shift command, shift

shopt command, Shell Options, shopt

source command, source

stty command, Job Control

suspend command, Job Control, suspend

test command, test–Examples

time command, time

times command, times

trap command, trap

true command, true

type command, type

typeset command, typeset

ulimit command, ulimit

umask command, umask

unalias command, unalias

unset command, unset

until command, until

wait command, Job Control, wait

while command, while

[[ ]] test command, [[ ]]

comments, # (hash mark), Quoting, #

comparisons, integers, Integer Comparisons

compgen command, Programmable Completion, compgen

complete command, Programmable Completion, , complete

completion facilities, The fc Command, Programmable Completion,

compopt command, Programmable Completion, compopt

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, test

continue command, continue

control characters, filename metacharacter, Filename Metacharacters

COPROC shell variable, Built-in Shell Variables

coprocesses, Coprocesses

CPU times, times

csh (Berkeley C shell), history, History

CTRL-Z command, Job Control, Job Control

D

DEBUG trap, Functions

decimal digits, filename metacharacter, Filename Metacharacters

declare command, declare

declaring

shell variables, declare

variables, typeset

digit class, Filename Metacharacters

directories

changing, cd

popping directories off directory stack, popd

pushing directories onto directory stack, pushd

working directories, pwd

directory stack

popping directories off directory stack, popd

printing or managing, dirs

pushing directories onto directory stack, pushd

dirs command, dirs

DIRSTACK shell variable, Built-in Shell Variables

disabling commands, enable

disown command, disown

division operator, Operators

do command, do

do-nothing command (:), :

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, done

dot (.), read and execute files, .

E

echo command, echo

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, enable

ENV shell variable, Other Shell Variables

equals sign (=), == equality operator, Operators

ERR trap, Functions

esac command, esac

escape sequence, Quoting

escape sequences, quoted text, Quoting

EUID shell variable, Built-in Shell Variables

eval command, eval

evaluating conditions, test

event designators, Event designators

exclamation mark (!)

!= inequality operator, Operators

event designators, Event designators

filename metacharacter, Filename Metacharacters

logical negation operator, Operators

exec command, exec

executing

commands, Command Execution

files, ., source

input lines, eval

exit command, exit

exit status of shell functions, return

EXIT trap, Functions

exiting

with false return values, false

kill command, kill, kill

shell, logout

shell scripts, exit

with a true return value, true

exponentiation operator, Operators

export command, export

expressions, arithmetic, Arithmetic Expressions

F

false command, false

fc command, The fc Command, fc

FCEDIT shell variable, Other Shell Variables

fg command, Job Control, fg

fi command, fi

FIGNORE shell variable, Other Shell Variables

filename command, filename

filenames

metacharacters, Filename Metacharacters

redirection forms, Special filenames

files

creation mask, umask

evaluating conditions,

reading and executing, ., source

reading into arrays, mapfile, readarray

for command, for

form feed escape sequence, Quoting

forms, combined forms, Combined Forms

(see also redirection forms)

funcname command, Programmable Completion

FUNCNAME shell variable, Built-in Shell Variables

function command, function

functions

about, Functions

defining, name ()

exit status of, return

getpwnam() function, Special Files

getpwuid() function, Special Files

removing, unset

traps, Functions

G

getconf command,

getopts command, getopts

getpwnam() function, Special Files

getpwuid() function, Special Files

GLOBIGNORE shell variable, Other Shell Variables

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, hash

hash mark (#), comments, Quoting, #

help command, help

here document, Simple redirection

here string, Simple redirection

hexadecimal escape sequence, Quoting

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, fc, history

history modifiers, History modifiers

history command, Command History, history

HISTSIZE shell variable, Other Shell Variables

HISTTIMEFORMAT shell variable, Other Shell Variables

HOME shell variable, Other Shell Variables

HOSTFILE shell variable, Other Shell Variables

HOSTNAME, Built-in Shell Variables

HOSTTYPE shell variable, Built-in Shell Variables

hyphen (-)

-- auto-decrement operator, Operators

subtraction operator, Operators

unary operator, Operators

I

if statement, fi, if

IFS shell variable, Other Shell Variables

IGNOREEOF shell variable, Other Shell Variables

indexed arrays, Indexed arrays

indirect variables, Variable Substitution

inequality operator, Operators

inline conditional evaluation operator, Operators

input lines, rescan or execute, eval

INPUTRC shell variable, Other Shell Variables

integers, comparisons, Integer Comparisons

interpreters, invoking, #!shell

invoking

Bash shell, Invoking the Shell

interpreters, #!shell

J

job control, Job Control

jobID argument, Job Control

jobs

background jobs, , fg

list running or stopped jobs, jobs

stop managing, disown

stopping, kill

wait command, wait

jobs command, Job Control, jobs

K

keyboard shortcuts for editing, Line-Edit Mode

kill command, Job Control, kill

L

LANG shell variable, Other Shell Variables

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 (<), Additional Operators

<< bitwise shift left operator, Operators

<<= assignment operator, Operators

<= less than or equal to operator, Operators

less than operator, Operators, Operators

less than operator, Operators

less than or equal to operator, Operators

let command, let

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

local command (local variables), local

logical AND operator, Operators

logical negation operator, Operators

logical OR operator, Operators

logout command, logout

loops

breaking out,

continuing, continue

do command, do

done command, done

for command, for

until command, until

while command, while

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

man command, Examples

mapfile command, mapfile

MAPFILE shell variable, Built-in Shell Variables

masks, file creation mask, umask

metacharacters, Bash shell filenames, Filename Metacharacters

modulus operator, Operators

multiple redirection, Multiple redirection

multiplication operator, Operators

N

name () command, name ()

newline escape sequence, Quoting

newline word separator, Quoting

nonspace characters, filename metacharacter, Filename Metacharacters

O

octal value escape sequence, Quoting

OLDPWD shell variable, Built-in Shell Variables

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

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,

|| logical OR operator, Operators, Additional Operators

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, popd,

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, printf

printing

command usage information, help

command-line history, history

working directories, pwd

process substitution, Process substitution

.profile file, Special Files

programmable completion, Programmable Completion–Examples

prompt strings, Special Prompt Strings

PROMPT_COMMAND shell variable, Other Shell Variables

PROMPT_DIRTRIM shell variable, Other Shell Variables

PS1-PS4 shell variable, Other Shell Variables

punctuation characters, filename metacharacter, Filename Metacharacters

pushd command, pushd

pwd command, pwd

PWD shell variable, Built-in Shell Variables

Q

question mark (?)

?: inline conditional evaluation operator, Operators

filename metacharacter, Filename Metacharacters

quotation marks, double (“)

" " quoting, Quoting

escape sequence, Quoting

quotation marks, single (')

escape sequence, Quoting

quoting, Quoting

quoting, syntax, Quoting

R

RANDOM shell variable[ ], Built-in Shell Variables

read command, read

readarray command, readarray,

reading, files, ., mapfile, source

readline library, bindings, bind

READLINE_LINE shell variable, Built-in Shell Variables

READLINE_POINT shell variable, Built-in Shell Variables

readonly command, readonly

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

referencing arrays, Indexed arrays

removing

aliases, unalias

functions, unset

variables, unset

REPLY shell variable, Built-in Shell Variables

rescanning input lines, eval

resources, Resources

restricted shells, Restricted Shells

return command, Functions, return

RETURN trap, Functions

return values

exiting with a true return value, true

exiting with false, false

right angle bracket (>)

>= greater than or equal to operator, Operators

>> bitwise shift right operator, Operators

>>= assignment operator, Operators

greater than operator, Operators, Additional Operators

S

scripts

command-line parameters, set

exiting, exit

replacing, exec

signals, trap

select command, select

semicolon (;), command separator, Quoting

set command, set–set -e Details

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, declare

evaluating conditions, Internal Shell Conditions

invoking Bash shell, Invoking the Shell

managing options, shopt

options, Shell Options–Shell Options, set

restricted shells, Restricted Shells

suspending, suspend

shift command, shift

SHLVL shell variable, Built-in Shell Variables

shopt command, Shell Options, shopt

signal-based traps, Functions

signals, shell scripts, trap

slash (/), division operator, Operators

source command, source

space character, filename metacharacter, Filename Metacharacters

space word separator, Quoting

stacks, directory stack, dirs, popd, pushd

standard error, redirection forms, Redirection Forms

standard input, redirection forms, Redirection Forms

standard output, redirection forms, Redirection Forms

stopping jobs, kill

(see also exiting)

strings

completions,

default variable value, Variable Assignment

evaluating conditions, String Conditions

prompt strings, Special Prompt Strings

stty command, Job Control

substitution

PS1-PS4 variables, Special Prompt Strings

variables, Variable Substitution

word substitution, Word substitution

subtraction operator, Operators

suspend command, Job Control, suspend

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, Quoting

tab word separator, Quoting

TERM, Other Shell Variables

test command, [[ ]], test–Examples

tilde (~), filename metacharacter, Filename Metacharacters

time command, time

TIMEFORMAT shell variable, Other Shell Variables

times command, times

TMOUT shell variable, Other Shell Variables

TMPDIR shell variable, Other Shell Variables

trap command, trap

traps, list of, Functions

true command, true

truncation modifiers, History modifiers

type command, type

typeset command, typeset

U

UID shell variable, Built-in Shell Variables

ulimit command, ulimit

umask command, umask

unalias command, unalias

unary operator, Operators

unset command, unset

until command, until

uppercase characters, filename metacharacter, Filename Metacharacters

V

variables, Variables–Special Prompt Strings

arrays, Arrays

assignment, Variable Assignment

built-in shell variables, Built-in Shell Variables

declaring, declare, typeset

exporting or printing info about, export

local variables, local

other shell variables, Other Shell Variables

prompt strings, Special Prompt Strings

read-only, readonly

reading, read

removing, unset

substitution, Variable Substitution

vertical tab escape sequence, Quoting

vi editor, line-edit mode, Line-Edit Mode

W

wait command, Job Control, wait

while command, while

whitespace characters, filename metacharacter, Filename Metacharacters

word substitution, Word substitution

working directories, printing, pwd

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 currently the maintainer of gawk and its documentation. He is coauthor of the sixth edition of O'Reilly's Learning the vi Editor.

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bash Pocket Reference

Arnold Robbins

Editor

Mike Loukides

Copyright © 2010 Arnold Robbins

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bash Pocket Reference

Table of Contents

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1. The Bash Shell

Conventions

History

Overview of Features

Invoking the Shell

Options

Arguments

Syntax

Special Files

Filename Metacharacters

Examples

Brace Expansion

Examples

Quoting

Examples

Command Forms

Examples

Redirection Forms

Simple redirection

Redirection using file descriptors

Multiple redirection

Process substitution

Special filenames

Examples

Functions

Variables

Variable Assignment

Variable Substitution

Examples

Built-in Shell Variables

Other Shell Variables

Arrays

Indexed arrays

Associative arrays

Special Prompt Strings

Arithmetic Expressions

Operators

Examples

Command History

Line-Edit Mode

Common editing keystrokes

The fc Command

Examples

C-Shell–Style History

Event designators

Word substitution

History modifiers

Programmable Completion

Examples

Job Control

Shell Options

Command Execution

Coprocesses

Example

Restricted Shells

Built-in Commands

Resources

Online Resources

Books

Acknowledgments

Index

About the Author

Special Upgrade Offer

Copyright