BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2011 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file.
Bash also incorporates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE POSIX specification (IEEE
Standard 1003.1). Bash can be configured to be POSIX-conformant by default.
OPTIONS
All of the single-character shell options documented in the description of the set builtin command can be used as options
when the shell is invoked. In addition, bash interprets the following options when it is invoked:
-c string If the -c option is present, then commands are read from string. If there are arguments after the string, they
are assigned to the positional parameters, starting with $0.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell (see INVOCATION below).
-r If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).
-s If the -s option is present, or if no arguments remain after option processing, then commands are read from the
standard input. This option allows the positional parameters to be set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed on the standard output. These are the strings that
are subject to language translation when the current locale is not C or POSIX. This implies the -n option; no
commands will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt builtin (see SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of that option; +O unsets it. If shopt_option is not supplied, the
names and values of the shell options accepted by shopt are printed on the standard output. If the invocation
option is +O, the output is displayed in a format that may be reused as input.
-- A -- signals the end of options and disables further option processing. Any arguments after the -- are treated
as filenames and arguments. An argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options must appear on the command line before the single-
character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see the
description of the extdebug option to the shopt builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the system wide initialization file /etc/bash.bashrc and the standard personal
initialization file ~/.bashrc if the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the personal initialization files
~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these files when it is invoked as a login
shell (see INVOCATION below).
--norc Do not read and execute the system wide initialization file /etc/bash.bashrc and the personal initialization file
~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs from the POSIX standard to match the standard (posix
mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the standard output and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is
assumed to be the name of a file containing shell commands. If bash is invoked in this fashion, $0 is set to the name of
the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this
file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are
executed, the exit status is 0. An attempt is first made to open the file in the current directory, and, if no file is
found, then the shell searches the directories in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or one started with the --login option.
An interactive shell is one started without non-option arguments and without the -c option whose standard input and error
are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $-
includes i if bash is interactive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash
reports an error. Tildes are expanded in file names as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads
and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_pro‐
file, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is
readable. The --noprofile option may be used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash reads and executes commands from /etc/bash.bashrc and
~/.bashrc, if these files exist. This may be inhibited by using the --norc option. The --rcfile file option will force
bash to read and execute commands from file instead of /etc/bash.bashrc and ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the envi‐
ronment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute.
Bash behaves as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as pos‐
sible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive
shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that
order. The --noprofile option may be used to inhibit this behavior. When invoked as an interactive shell with the name
sh, bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file
to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup
files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any
other startup files. When invoked as sh, bash enters posix mode after the startup files are read.
When bash is started in posix mode, as with the --posix command line option, it follows the POSIX standard for startup
files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose
name is the expanded value. No other startup files are read.
Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed
by the remote shell daemon, usually rshd, or the secure shell daemon sshd. If bash determines it is being run in this
fashion, it reads and executes commands from ~/.bashrc and ~/.bashrc, if these files exist and are readable. It will not
do this if invoked as sh. The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to
force another file to be read, but rshd does not generally invoke the shell with those options or allow them to be speci‐
fied.
If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not
supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS,
CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the
real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is
not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known as a token.
name A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an
underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following symbols:
|| & && ; ;; ( ) | |&
RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when
unquoted and either the first word of a simple command (see SHELL GRAMMAR below) or the third word of a case or for com‐
mand:
! case do done elif else esac fi for function if in select then until while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and
terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero.
The remaining words are passed as arguments to the invoked command.
The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by one of the control operators | or |&. The format for a pipe‐
line is:
[time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2. This connection is performed
before any redirections specified by the command (see REDIRECTION below). If |& is used, the standard error of command is
connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the
standard error is performed after any redirections specified by the command.
The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail
is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or
zero if all commands exit successfully. If the reserved word ! precedes a pipeline, the exit status of that pipeline is
the logical negation of the exit status as described above. The shell waits for all commands in the pipeline to terminate
before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as user and system time consumed by its execution are
reported when the pipeline terminates. The -p option changes the output format to that specified by POSIX. When the shell
is in posix mode, it does not recognize time as a reserved word if the next token begins with a `-'. The TIMEFORMAT vari‐
able may be set to a format string that specifies how the timing information should be displayed; see the description of
TIMEFORMAT under Shell Variables below.
When the shell is in posix mode, time may be followed by a newline. In this case, the shell displays the total user and
system time consumed by the shell and its children. The TIMEFORMAT variable may be used to specify the format of the time
information.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated
by one of ;, &, or .
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The
shell does not wait for the command to finish, and the return status is 0. Commands separated by a ; are executed sequen‐
tially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command
executed.
AND and OR lists are sequences of one of more pipelines separated by the && and || control operators, respectively. AND
and OR lists are executed with left associativity. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.
An OR list has the form
command1 || command2
command2 is executed if and only if command1 returns a non-zero exit status. The return status of AND and OR lists is the
exit status of the last command executed in the list.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Variable assignments and
builtin commands that affect the shell's environment do not remain in effect after the command completes. The
return status is the exit status of list.
{ list; }
list is simply executed in the current shell environment. list must be terminated with a newline or semicolon.
This is known as a group command. The return status is the exit status of list. Note that unlike the metacharac‐
ters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized. Since
they do not cause a word break, they must be separated from list by whitespace or another shell metacharacter.
((expression))
The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the
expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let
"expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are
composed of the primaries described below under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are
not performed on the words between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic
expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such
as -f must be unquoted to be recognized as primaries.
When used with [[, the < and > operators sort lexicographically using the current locale.
See the description of the test builtin command (section SHELL BUILTIN COMMANDS below) for the handling of parameters (i.e.
missing parameters).
When the == and != operators are used, the string to the right of the operator is considered a pattern and matched accord‐
ing to the rules described below under Pattern Matching. If the shell option nocasematch is enabled, the match is per‐
formed without regard to the case of alphabetic characters. The return value is 0 if the string matches (==) or does not
match (!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.
An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the
right of the operator is considered an extended regular expression and matched accordingly (as in regex(3)). The return
value is 0 if the string matches the pattern, and 1 otherwise. If the regular expression is syntactically incorrect, the
conditional expression's return value is 2. If the shell option nocasematch is enabled, the match is performed without
regard to the case of alphabetic characters. Any part of the pattern may be quoted to force it to be matched as a string.
Substrings matched by parenthesized subexpressions within the regular expression are saved in the array variable
BASH_REMATCH. The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expres‐
sion. The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the
return value of the entire conditional expression.
for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of items. The variable name is set to each element of
this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once
for each positional parameter that is set (see PARAMETERS below). The return status is the exit status of the last
command that executes. If the expansion of the items following in results in an empty list, no commands are exe‐
cuted, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below under ARITHMETIC EVALUA‐
TION. The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2
evaluates to a non-zero value, list is executed and the arithmetic expression expr3 is evaluated. If any expression
is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list
that is executed, or false if any of the expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the
standard error, each preceded by a number. If the in word is omitted, the positional parameters are printed (see
PARAMETERS below). The PS3 prompt is then displayed and a line read from the standard input. If the line consists
of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is
empty, the words and prompt are displayed again. If EOF is read, the command completes. Any other value read
causes name to be set to null. The line read is saved in the variable REPLY. The list is executed after each
selection until a break command is executed. The exit status of select is the exit status of the last command exe‐
cuted in list, or zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules
as for pathname expansion (see Pathname Expansion below). The word is expanded using tilde expansion, parameter and
variable expansion, arithmetic substitution, command substitution, process substitution and quote removal. Each
pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, com‐
mand substitution, and process substitution. If the shell option nocasematch is enabled, the match is performed
without regard to the case of alphabetic characters. When a match is found, the corresponding list is executed. If
the ;; operator is used, no subsequent matches are attempted after the first pattern match. Using ;& in place of ;;
causes execution to continue with the list associated with the next set of patterns. Using ;;& in place of ;;
causes the shell to test the next pattern list in the statement, if any, and execute any associated list on a suc‐
cessful match. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command
executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is exe‐
cuted in turn, and if its exit status is zero, the corresponding then list is executed and the command completes.
Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed,
or zero if no condition tested true.
while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long as the last command in the list list-1 returns an
exit status of zero. The until command is identical to the while command, except that the test is negated; list-2
is executed as long as the last command in list-1 returns a non-zero exit status. The exit status of the while and
until commands is the exit status of the last command executed in list-2, or zero if none was executed.
Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell,
as if the command had been terminated with the & control operator, with a two-way pipe established between the executing
shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if
command is a simple command (see above); otherwise, it is interpreted as the first word of the simple command. When the
coproc is executed, the shell creates an array variable (see Arrays below) named NAME in the context of the executing
shell. The standard output of command is connected via a pipe to a file descriptor in the executing shell, and that file
descriptor is assigned to NAME[0]. The standard input of command is connected via a pipe to a file descriptor in the exe‐
cuting shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified
by the command (see REDIRECTION below). The file descriptors can be utilized as arguments to shell commands and redirec‐
tions using standard word expansions. The process ID of the shell spawned to execute the coprocess is available as the
value of the variable NAME_PID. The wait builtin command may be used to wait for the coprocess to terminate.
The return status of a coprocess is the exit status of command.
Shell Function Definitions
A shell function is an object that is called like a simple command and executes a compound command with a new set of posi‐
tional parameters. Shell functions are declared as follows:
name () compound-command [redirection]
function name [()] compound-command [redirection]
This defines a function named name. The reserved word function is optional. If the function reserved word is sup‐
plied, the parentheses are optional. The body of the function is the compound command compound-command (see Com‐
pound Commands above). That command is usually a list of commands between { and }, but may be any command listed
under Compound Commands above. compound-command is executed whenever name is specified as the name of a simple com‐
mand. Any redirections (see REDIRECTION below) specified when a function is defined are performed when the function
is executed. The exit status of a function definition is zero unless a syntax error occurs or a readonly function
with the same name already exists. When executed, the exit status of a function is the exit status of the last com‐
mand executed in the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is
enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # causes that word and all remaining characters on that
line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The
interactive_comments option is on by default in interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable
special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter
expansion.
Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion charac‐
ter, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows,
with the exception of . If a \ pair appears, and the backslash is not itself quoted, the \ is
treated as a line continuation (that is, it is removed from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may
not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception
of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double
quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or
. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expan‐
sion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the !
is not removed.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according to the cur‐
rent locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced,
the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below
under Special Parameters. A variable is a parameter denoted by a name. A variable has a value and zero or more
attributes. Attributes are assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be
unset only by using the unset builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and vari‐
able expansion, command substitution, arithmetic expansion, and quote removal (see EXPANSION below). If the variable has
its integer attribute set, then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used
(see Arithmetic Expansion below). Word splitting is not performed, with the exception of "$@" as explained below under
Special Parameters. Pathname expansion is not performed. Assignment statements may also appear as arguments to the alias,
declare, typeset, export, readonly, and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator can
be used to append to or add to the variable's previous value. When += is applied to a variable for which the integer
attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is
also evaluated. When += is applied to an array variable using compound assignment (see Arrays below), the variable's value
is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's
maximum index (for indexed arrays) or added as additional key-value pairs in an associative array. When applied to a
string-valued variable, value is expanded and appended to the variable's value.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters
are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Posi‐
tional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced
when a shell function is executed (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPAN‐
SION below).
Special Parameters
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands
to a single word with the value of each parameter separated by the first character of the IFS special variable.
That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS
is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening
separators.
@ Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each
parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion
occurs within a word, the expansion of the first parameter is joined with the beginning part of the original word,
and the expansion of the last parameter is joined with the last part of the original word. When there are no posi‐
tional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the exit status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the
shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not
the subshell.
! Expands to the process ID of the most recently executed background (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a
file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the
first argument after the string to be executed, if one is present. Otherwise, it is set to the file name used to
invoke bash, as given by argument zero.
_ At shell startup, set to the absolute pathname used to invoke the shell or shell script being executed as passed in
the environment or argument list. Subsequently, expands to the last argument to the previous command, after expan‐
sion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to
that command. When checking mail, this parameter holds the name of the mail file currently being checked.
Shell Variables
The following variables are set by the shell:
BASH Expands to the full file name used to invoke this instance of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument for the -s option to the
shopt builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in BASHOPTS are those reported as
on by shopt. If this variable is in the environment when bash starts up, each shell option in the list will be
enabled before reading any startup files. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This differs from $$ under certain circumstances, such as
subshells that do not require bash to be re-initialized.
BASH_ALIASES
An associative array variable whose members correspond to the internal list of aliases as maintained by the alias
builtin. Elements added to this array appear in the alias list; unsetting array elements cause aliases to be
removed from the alias list.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of the current bash execution call stack.
The number of parameters to the current subroutine (shell function or script executed with . or source) is at the
top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The
shell sets BASH_ARGC only when in extended debugging mode (see the description of the extdebug option to the shopt
builtin below)
BASH_ARGV
An array variable containing all of the parameters in the current bash execution call stack. The final parameter of
the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When
a subroutine is executed, the parameters supplied are pushed onto BASH_ARGV. The shell sets BASH_ARGV only when in
extended debugging mode (see the description of the extdebug option to the shopt builtin below)
BASH_CMDS
An associative array variable whose members correspond to the internal hash table of commands as maintained by the
hash builtin. Elements added to this array appear in the hash table; unsetting array elements cause commands to be
removed from the hash table.
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is executing a command as the result
of a trap, in which case it is the command executing at the time of the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was
invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was
called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line
number.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element
with index 0 is the portion of the string matching the entire regular expression. The element with index n is the
portion of the string matching the nth parenthesized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where the corresponding shell function names in the FUNC‐
NAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and
called from ${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell environment is spawned. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this instance of bash. The values assigned to
the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in
shell functions invoked by the programmable completion facilities (see Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the current completion function.
COMP_LINE
The current command line. This variable is available only in shell functions and external commands invoked by the
programmable completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning of the current command. If the current cursor
position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable
is available only in shell functions and external commands invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion attempted that caused a completion function to be
called: TAB, for normal completion, ?, for listing completions after successive tabs, !, for listing alternatives on
partial word completion, @, to list completions if the word is not unmodified, or %, for menu completion. This
variable is available only in shell functions and external commands invoked by the programmable completion facili‐
ties (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word separators when performing word completion. If
COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the current command line. The line is
split into words as readline would split it, using COMP_WORDBREAKS as described above. This variable is available
only in shell functions invoked by the programmable completion facilities (see Programmable Completion below).
COPROC An array variable (see Arrays below) created to hold the file descriptors for output from and input to an unnamed
coprocess (see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the directory stack. Directories appear in
the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be
used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove
directories. Assignment to this variable will not change the current directory. If DIRSTACK is unset, it loses its
special properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in the execution call stack. The element
with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the high‐
est index) is "main". This variable exists only when a shell function is executing. Assignments to FUNCNAME have
no effect and return an error status. If FUNCNAME is unset, it loses its special properties, even if it is subse‐
quently reset.
This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of FUNCNAME has corresponding elements in
BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using
this information.
GROUPS An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have
no effect and return an error status. If GROUPS is unset, it loses its special properties, even if it is subse‐
quently reset.
HISTCMD
The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses its special
properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default
is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential
line number (starting with 1) within a script or function. When not in a script or function, the value substituted
is not guaranteed to be meaningful. If LINENO is unset, it loses its special properties, even if it is subsequently
reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which bash is executing, in the standard GNU
cpu-company-system format. The default is system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text read by the mapfile builtin when no variable name is
supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system on which bash is executing. The default is sys‐
tem-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values from the processes in the most-
recently-executed foreground pipeline (which may contain only a single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between 0 and 32767 is generated. The sequence of random
numbers may be initialized by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties,
even if it is subsequently reset.
READLINE_LINE
The contents of the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS below).
READLINE_POINT
The position of the insertion point in the readline line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS
below).
REPLY Set to the line of input read by the read builtin command when no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is
assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment
plus the value assigned. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid argument for the -o option to the
set builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in SHELLOPTS are those reported as on
by set -o. If this variable is in the environment when bash starts up, each shell option in the list will be
enabled before reading any startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell startup. This variable is readonly.
The following variables are used by the shell. In some cases, bash assigns a default value to a variable; these cases are
noted below.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is interpreted as a filename containing
commands to initialize the shell, as in ~/.bashrc. The value of BASH_ENV is subjected to parameter expansion, com‐
mand substitution, and arithmetic expansion before being interpreted as a file name. PATH is not used to search for
the resultant file name.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor, bash will write the trace output generated when set
-x is enabled to that file descriptor. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new
value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard
error. Note that setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result
in the standard error being closed.
CDPATH The search path for the cd command. This is a colon-separated list of directories in which the shell looks for des‐
tination directories specified by the cd command. A sample value is ".:~:/usr".
COLUMNS
Used by the select compound command to determine the terminal width when printing selection lists. Automatically
set upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated by a shell function invoked by the pro‐
grammable completion facility (see Programmable Completion below).
EMACS If bash finds this variable in the environment when the shell starts with value "t", it assumes that the shell is
running in an Emacs shell buffer and disables line editing.
ENV Similar to BASH_ENV; used when the shell is invoked in POSIX mode.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion (see READLINE below). A filename
whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value
is ".o:~" (Quoting is needed when assigning a value to this variable, which contains tildes).
FUNCNEST
If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that
exceed this nesting level will cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by pathname expansion. If a filename
matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list
of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the history list. If the list of values
includes ignorespace, lines which begin with a space character are not saved in the history list. A value of
ignoredups causes lines matching the previous history entry to not be saved. A value of ignoreboth is shorthand for
ignorespace and ignoredups. A value of erasedups causes all previous lines matching the current line to be removed
from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is
unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject
to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are not tested, and
are added to the history regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below). The default value is ~/.bash_history.
If unset, the command history is not saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is assigned a value, the history file
is truncated, if necessary, by removing the oldest entries, to contain no more than that number of lines. The
default value is 500. The history file is also truncated to this size after writing it when an interactive shell
exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each
pattern is anchored at the beginning of the line and must match the complete line (no implicit `*' is appended).
Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. In addition to the
normal shell pattern matching characters, `&' matches the previous history line. `&' may be escaped using a back‐
slash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound
command are not tested, and are added to the history regardless of the value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see HISTORY below). The default value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for strftime(3) to print the time stamp
associated with each history entry displayed by the history builtin. If this variable is set, time stamps are writ‐
ten to the history file so they may be preserved across shell sessions. This uses the history comment character to
distinguish timestamps from other history lines.
HOME The home directory of the current user; the default argument for the cd builtin command. The value of this variable
is also used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a
hostname. The list of possible hostname completions may be changed while the shell is running; the next time host‐
name completion is attempted after the value is changed, bash adds the contents of the new file to the existing
list. If HOSTFILE is set, but has no value, or does not name a readable file, bash attempts to read /etc/hosts to
obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the
read builtin command. The default value is ``''.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF character as the sole input. If set, the value is
the number of consecutive EOF characters which must be typed as the first characters on an input line before bash
exits. If the variable exists but does not have a numeric value, or has no value, the default value is 10. If it
does not exist, EOF signifies the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE below).
LANG Used to determine the locale category for any category not specifically selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results of pathname expansion, and determines the
behavior of range expressions, equivalence classes, and collating sequences within pathname expansion and pattern
matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior of character classes within pathname
expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select compound command to determine the column length for printing selection lists. Automatically set
upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file or directory name and the MAILPATH variable is not set, bash informs the user of
the arrival of mail in the specified file or Maildir-format directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for
mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is
not a number greater than or equal to zero, the shell disables mail checking.
MAILPATH
A colon-separated list of file names to be checked for mail. The message to be printed when mail arrives in a par‐
ticular file may be specified by separating the file name from the message with a `?'. When used in the text of the
message, $_ expands to the name of the current mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable, but the location of the user mail files that it uses is system
dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by the getopts builtin command (see SHELL BUILTIN COM‐
MANDS below). OPTERR is initialized to 1 each time the shell is invoked or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of directories in which the shell looks for commands
(see COMMAND EXECUTION below). A zero-length (null) directory name in the value of PATH indicates the current
directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. The
default path is system-dependent, and is set by the administrator who installs bash. A common value is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell enters posix mode before reading the startup
files, as if the --posix invocation option had been supplied. If it is set while the shell is running, bash enables
posix mode, as if the command set -o posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each primary prompt.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the number of trailing directory components to retain
when expanding the \w and \W prompt string escapes (see PROMPTING below). Characters removed are replaced with an
ellipsis.
PS1 The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default
value is ``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and used as the secondary prompt string. The default is ``> ''.
PS3 The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays dur‐
ing an execution trace. The first character of PS4 is replicated multiple times, as necessary, to indicate multiple
levels of indirection. The default is ``+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it is not set when the shell starts, bash
assigns to it the full pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed
with the time reserved word should be displayed. The % character introduces an escape sequence that is expanded to
a time value or other information. The escape sequences and their meanings are as follows; the braces denote
optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value
of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be speci‐
fied; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines
whether or not the fraction is included.
If this variable is not set, bash acts as if it had the value $'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the value is
null, no timing information is displayed. A trailing newline is added when the format string is displayed.
TMOUT If set to a value greater than zero, TMOUT is treated as the default timeout for the read builtin. The select com‐
mand terminates if input does not arrive after TMOUT seconds when input is coming from a terminal. In an interac‐
tive shell, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt.
Bash terminates after waiting for that number of seconds if input does not arrive.
TMPDIR If set, bash uses its value as the name of a directory in which bash creates temporary files for the shell's use.
auto_resume
This variable controls how the shell interacts with the user and job control. If this variable is set, single word
simple commands without redirections are treated as candidates for resumption of an existing stopped job. There is
no ambiguity allowed; if there is more than one job beginning with the string typed, the job most recently accessed
is selected. The name of a stopped job, in this context, is the command line used to start it. If set to the value
exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied
needs to match a substring of the name of a stopped job. The substring value provides functionality analogous to
the %? job identifier (see JOB CONTROL below). If set to any other value, the supplied string must be a prefix of
a stopped job's name; this provides functionality analogous to the %string job identifier.
histchars
The two or three characters which control history expansion and tokenization (see HISTORY EXPANSION below). The
first character is the history expansion character, the character which signals the start of a history expansion,
normally `!'. The second character is the quick substitution character, which is used as shorthand for re-running
the previous command entered, substituting one string for another in the command. The default is `^'. The optional
third character is the character which indicates that the remainder of the line is a comment when found as the first
character of a word, normally `#'. The history comment character causes history substitution to be skipped for the
remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a com‐
ment.
Arrays
Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the
declare builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement
that members be indexed or assigned contiguously. Indexed arrays are referenced using integers (including arithmetic
expressions) and are zero-based; associative arrays are referenced using arbitrary strings.
An indexed array is created automatically if any variable is assigned to using the syntax name[subscript]=value. The sub‐
script is treated as an arithmetic expression that must evaluate to a number. If subscript evaluates to a number less than
zero, it is used as an offset from one greater than the array's maximum index (so a subcript of -1 refers to the last ele‐
ment of the array). To explicitly declare an indexed array, use declare -a name (see SHELL BUILTIN COMMANDS below).
declare -a name[subscript] is also accepted; the subscript is ignored.
Associative arrays are created using declare -A name.
Attributes may be specified for an array variable using the declare and readonly builtins. Each attribute applies to all
members of an array.
Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each value is of the form
[subscript]=string. Indexed array assignments do not require the bracket and subscript. When assigning to indexed arrays,
if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned
is the last index assigned to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the subscript is required.
This syntax is also accepted by the declare builtin. Individual array elements may be assigned to using the name[sub‐
script]=value syntax introduced above.
Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with path‐
name expansion. If subscript is @ or *, the word expands to all members of name. These subscripts differ only when the
word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the value of
each array member separated by the first character of the IFS special variable, and ${name[@]} expands each element of name
to a separate word. When there are no array members, ${name[@]} expands to nothing. If the double-quoted expansion occurs
within a word, the expansion of the first parameter is joined with the beginning part of the original word, and the expan‐
sion of the last parameter is joined with the last part of the original word. This is analogous to the expansion of the
special parameters * and @ (see Special Parameters above). ${#name[subscript]} expands to the length of ${name[sub‐
script]}. If subscript is * or @, the expansion is the number of elements in the array. Referencing an array variable
without a subscript is equivalent to referencing the array with a subscript of 0.
An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index subscript. Care
must be taken to avoid unwanted side effects caused by pathname expansion. unset name, where name is an array, or unset
name[subscript], where subscript is * or @, removes the entire array.
The declare, local, and readonly builtins each accept a -a option to specify an indexed array and a -A option to specify an
associative array. If both options are supplied, -A takes precedence. The read builtin accepts a -a option to assign a
list of words read from the standard input to an array. The set and declare builtins display array values in a way that
allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion per‐
formed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion,
word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expansion, parameter, variable and arithmetic expansion and command sub‐
stitution (done in a left-to-right fashion), word splitting, and pathname expansion.
On systems that can support it, there is an additional expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion can change the number of words of the expansion; other expan‐
sions expand a single word to a single word. The only exceptions to this are the expansions of "$@" and "${name[@]}" as
explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expan‐
sion, but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble,
followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an
optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then
appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For
example, a{d,c,b}e expands into `ade ace abe'.
A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or single characters, and incr, an
optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y,
inclusive. Supplied integers may be prefixed with 0 to force each term to have the same width. When either x or y begins
with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where nec‐
essary. When characters are supplied, the expression expands to each character lexicographically between x and y, inclu‐
sive. Note that both x and y must be of the same type. When the increment is supplied, it is used as the difference
between each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in
the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or
the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a
valid sequence expression. Any incorrectly formed brace expansion is left unchanged. A { or , may be quoted with a back‐
slash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string
${ is not considered eligible for brace expansion.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the
above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical versions of sh. sh does not treat opening or closing
braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a
consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. The
same word is output as file1 file2 after expansion by bash. If strict compatibility with sh is desired, start bash with
the +B option or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all of the characters preceding the first unquoted slash (or all
characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix
are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login
name is the null string, the tilde is replaced with the value of the shell parameter HOME. If HOME is unset, the home
directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home
directory associated with the specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is a
`~-', the value of the shell variable OLDPWD, if it is set, is substituted. If the characters following the tilde in the
tilde-prefix consist of a number N, optionally prefixed by a `+' or a `-', the tilde-prefix is replaced with the corre‐
sponding element from the directory stack, as it would be displayed by the dirs builtin invoked with the tilde-prefix as an
argument. If the characters following the tilde in the tilde-prefix consist of a number without a leading `+' or `-', `+'
is assumed.
If the login name is invalid, or the tilde expansion fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a : or the first =. In these cases,
tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to PATH, MAILPATH, and
CDPATH, and the shell assigns the expanded value.
Parameter Expansion
The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or
symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from
characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and
not within an embedded arithmetic expansion, command substitution, or parameter expansion.
${parameter}
The value of parameter is substituted. The braces are required when parameter is a positional parameter with more
than one digit, or when parameter is followed by a character which is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses
the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and
that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect
expansion. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation
point must immediately follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic
expansion.
When not performing substring expansion, using the forms documented below, bash tests for a parameter that is unset or
null. Omitting the colon results in a test only for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of
parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of
parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect
if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise,
the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is sub‐
stituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of parameter starting at the character specified by offset.
If length is omitted, expands to the substring of parameter starting at the character specified by offset. length
and offset are arithmetic expressions (see ARITHMETIC EVALUATION below). If offset evaluates to a number less than
zero, the value is used as an offset from the end of the value of parameter. Arithmetic expressions starting with a
- must be separated by whitespace from the preceding : to be distinguished from the Use Default Values expansion.
If length evaluates to a number less than zero, and parameter is not @ and not an indexed or associative array, it
is interpreted as an offset from the end of the value of parameter rather than a number of characters, and the
expansion is the characters between the two offsets. If parameter is @, the result is length positional parameters
beginning at offset. If parameter is an indexed array name subscripted by @ or *, the result is the length members
of the array beginning with ${parameter[offset]}. A negative offset is taken relative to one greater than the maxi‐
mum index of the specified array. Substring expansion applied to an associative array produces undefined results.
Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the
:- expansion. Substring indexing is zero-based unless the positional parameters are used, in which case the index‐
ing starts at 1 by default. If offset is 0, and the positional parameters are used, $0 is prefixed to the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables whose names begin with prefix, separated by the first
character of the IFS special variable. When @ is used and the expansion appears within double quotes, each variable
name expands to a separate word.
${!name[@]}
${!name[*]}
List of array keys. If name is an array variable, expands to the list of array indices (keys) assigned in name. If
name is not an array, expands to 0 if name is set and null otherwise. When @ is used and the expansion appears
within double quotes, each key expands to a separate word.
${#parameter}
Parameter length. The length in characters of the value of parameter is substituted. If parameter is * or @, the
value substituted is the number of positional parameters. If parameter is an array name subscripted by * or @, the
value substituted is the number of elements in the array.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion. If the
pattern matches the beginning of the value of parameter, then the result of the expansion is the expanded value of
parameter with the shortest matching pattern (the ``#'' case) or the longest matching pattern (the ``##'' case)
deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and
the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal
operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to produce a pattern just as in pathname expansion. If the
pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching pattern (the ``%'' case) or the longest matching pattern (the
``%%'' case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parameter
in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the
pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter/pattern/string}
Pattern substitution. The pattern is expanded to produce a pattern just as in pathname expansion. Parameter is
expanded and the longest match of pattern against its value is replaced with string. If pattern begins with /, all
matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with #,
it must match at the beginning of the expanded value of parameter. If pattern begins with %, it must match at the
end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pat‐
tern may be omitted. If parameter is @ or *, the substitution operation is applied to each positional parameter in
turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the sub‐
stitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of alphabetic characters in parameter. The pattern is expanded
to produce a pattern just as in pathname expansion. The ^ operator converts lowercase letters matching pattern to
uppercase; the , operator converts matching uppercase letters to lowercase. The ^^ and ,, expansions convert each
matched character in the expanded value; the ^ and , expansions match and convert only the first character in the
expanded value. If pattern is omitted, it is treated like a ?, which matches every character. If parameter is @ or
*, the case modification operation is applied to each positional parameter in turn, and the expansion is the resul‐
tant list. If parameter is an array variable subscripted with @ or *, the case modification operation is applied to
each member of the array in turn, and the expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the
command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word split‐
ting. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $,
`, or \. The first backquote not preceded by a backslash terminates the command substitution. When using the $(command)
form, all characters between the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for
arithmetic expansion is:
$((expression))
The old format $[expression] is deprecated and will be removed in upcoming versions of bash.
The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated spe‐
cially. All tokens in the expression undergo parameter expansion, string expansion, command substitution, and quote
removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If expression is invalid,
bash prints a message indicating failure and no substitution occurs.
Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.
It takes the form of <(list) or >(list). The process list is run with its input or output connected to a FIFO or some file
in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the
>(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an
argument should be read to obtain the output of list.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitu‐
tion, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur
within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words on these
characters. If IFS is unset, or its value is exactly , the default, then sequences of , ,
and at the beginning and end of the results of the previous expansions are ignored, and any sequence of IFS char‐
acters not at the beginning or end serves to delimit words. If IFS has a value other than the default, then sequences of
the whitespace characters space and tab are ignored at the beginning and end of the word, as long as the whitespace charac‐
ter is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any
adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delim‐
iter. If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments, resulting from the expansion of parame‐
ters that have no values, are removed. If a parameter with no value is expanded within double quotes, a null argument
results and is retained.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of
these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of file
names matching the pattern. If no matching file names are found, and the shell option nullglob is not enabled, the word is
left unchanged. If the nullglob option is set, and no matches are found, the word is removed. If the failglob shell
option is set, and no matches are found, an error message is printed and the command is not executed. If the shell option
nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters. Note that when using
range expressions like [a-z] (see below), letters of the other case may be included, depending on the setting of LC_COL‐
LATE. When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immediately following
a slash must be matched explicitly, unless the shell option dotglob is set. When matching a pathname, the slash character
must always be matched explicitly. In other cases, the ``.'' character is not treated specially. See the description of
shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob, failglob, and dotglob shell
options.
The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern. If GLOBIGNORE is set, each
matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. The file names
``.'' and ``..'' are always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value
has the effect of enabling the dotglob shell option, so all other file names beginning with a ``.'' will match. To get
the old behavior of ignoring file names beginning with a ``.'', make ``.*'' one of the patterns in GLOBIGNORE. The dot‐
glob option is disabled when GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The
NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded
when matching. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When the globstar shell option is enabled, and * is used in a
pathname expansion context, two adjacent *s used as a single pattern will match all files and zero or more
directories and subdirectories. If followed by a /, two adjacent *s will match only directories and subdi‐
rectories.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated by a hyphen denotes a range
expression; any character that sorts between those two characters, inclusive, using the current locale's col‐
lating sequence and character set, is matched. If the first character following the [ is a ! or a ^ then
any character not enclosed is matched. The sorting order of characters in range expressions is determined by
the current locale and the value of the LC_COLLATE shell variable, if set. A - may be matched by including
it as the first or last character in the set. A ] may be matched by including it as the first character in
the set.
Within [ and ], character classes can be specified using the syntax [:class:], where class is one of the fol‐
lowing classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit
A character class matches any character belonging to that class. The word character class matches letters,
digits, and the character _.
Within [ and ], an equivalence class can be specified using the syntax [=c=], which matches all characters
with the same collation weight (as defined by the current locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt builtin, several extended pattern matching operators are recognized.
In the following description, a pattern-list is a list of one or more patterns separated by a |. Composite patterns may be
formed using one or more of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the
above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell.
Redirection may also be used to open and close files for the current shell execution environment. The following redirect‐
ion operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed
in the order they appear, from left to right.
Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}.
In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10
and assign it to varname. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to
close.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection opera‐
tor is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection
operator is >, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace
expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, pathname expan‐
sion, and word splitting. If it expands to more than one word, bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated from the standard output before
the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, bash
attempts to open a TCP connection to the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, bash
attempts to open a UDP connection to the corresponding socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the
shell uses internally.
Note that the exec builtin command can make redirections take effect in the current shell.
Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file
descriptor n, or the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]word
If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the redirection will fail
if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is >|,
or the redirection operator is > and the noclobber option to the set builtin command is not enabled, the redirection is
attempted even if the file named by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for
appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not
exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be
redirected to the file whose name is the expansion of word.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be
appended to the file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing only delimiter
(with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a com‐
mand.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any
characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are
not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitu‐
tion, and arithmetic expansion. In the latter case, the character sequence \ is ignored, and \ must be used to
quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing
delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
<<&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is
used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. As a special
case, if n is omitted, and word does not expand to one or more digits, the standard output and standard error are redi‐
rected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified.
digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on
file descriptor 0 if n is not specified. If the file does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell main‐
tains a list of aliases that may be set and unset with the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS
below). The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is
replaced by the text of the alias. The characters /, $, `, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may contain any valid shell input, including shell
metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias
being expanded is not expanded a second time. This means that one may alias ls to ls -F, for instance, and bash does not
try to recursively expand the replacement text. If the last character of the alias value is a blank, then the next command
word following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text. If arguments are needed, a shell function should be
used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see
the description of shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete
line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it
is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the
next line of input is read. The commands following the alias definition on that line are not affected by the new alias.
This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not
when the function is executed, because a function definition is itself a compound command. As a consequence, aliases
defined in a function are not available until after that function is executed. To be safe, always put alias definitions on
a separate line, and do not use alias in compound commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores a series of commands for later execution. When
the name of a shell function is used as a simple command name, the list of commands associated with that function name is
executed. Functions are executed in the context of the current shell; no new process is created to interpret them (con‐
trast this with the execution of a shell script). When a function is executed, the arguments to the function become the
positional parameters during its execution. The special parameter # is updated to reflect the change. Special parameter 0
is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is execut‐
ing.
All other aspects of the shell execution environment are identical between a function and its caller with these exceptions:
the DEBUG and RETURN traps (see the description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inherited
unless the function has been given the trace attribute (see the description of the declare builtin below) or the -o func‐
trace shell option has been enabled with the set builtin (in which case all functions inherit the DEBUG and RETURN traps),
and the ERR trap is not inherited unless the -o errtrace shell option has been enabled.
Variables local to the function may be declared with the local builtin command. Ordinarily, variables and their values are
shared between the function and its caller.
The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invo‐
cations that exceed the limit cause the entire command to abort.
If the builtin command return is executed in a function, the function completes and execution resumes with the next command
after the function call. Any command associated with the RETURN trap is executed before execution resumes. When a func‐
tion completes, the values of the positional parameters and the special parameter # are restored to the values they had
prior to the function's execution.
Function names and definitions may be listed with the -f option to the declare or typeset builtin commands. The -F option
to declare or typeset will list the function names only (and optionally the source file and line number, if the extdebug
shell option is enabled). Functions may be exported so that subshells automatically have them defined with the -f option
to the export builtin. A function definition may be deleted using the -f option to the unset builtin. Note that shell
functions and variables with the same name may result in multiple identically-named entries in the environment passed to
the shell's children. Care should be taken in cases where this may cause a problem.
Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the function call stack and restrict
the number of function invocations. By default, no limit is imposed on the number of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circumstances (see the let and declare builtin com‐
mands and Arithmetic Expansion). Evaluation is done in fixed-width integers with no check for overflow, though division by
0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the
C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an
expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable
that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a
variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the inte‐
ger attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its inte‐
ger attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers
take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and
n is a number in that base. If base# is omitted, then base 10 is used. The digits greater than 9 are represented by the
lowercase letters, the uppercase letters, @, and _, in that order. If base is less than or equal to 36, lowercase and
uppercase letters may be used interchangeably to represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the
precedence rules above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and
perform string and arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any
file argument to one of the primaries is of the form /dev/fd/n, then file descriptor n is checked. If the file argument to
one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is
checked.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link,
rather than the link itself.
When used with [[, the < and > operators sort lexicographically using the current locale. The test command sorts using
ASCII ordering.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
-o optname
True if the shell option optname is enabled. See the list of options under the description of the -o option to the
set builtin below.
-v varname
True if the shell variable varname is set (has been assigned a value).
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with the test command for POSIX conformance.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
string1 > string2
True if string1 sorts after string2 lexicographically.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true if arg1 is equal to,
not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1
and arg2 may be positive or negative integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to
right.
1. The words that the parser has marked as variable assignments (those preceding the command name) and redirections are
saved for later processing.
2. The words that are not variable assignments or redirections are expanded. If any words remain after expansion, the
first word is taken to be the name of the command and the remaining words are the arguments.
3. Redirections are performed as described above under REDIRECTION.
4. The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitu‐
tion, arithmetic expansion, and quote removal before being assigned to the variable.
If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are
added to the environment of the executed command and do not affect the current shell environment. If any of the assign‐
ments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection
error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If
one of the expansions contained a command substitution, the exit status of the command is the exit status of the last com‐
mand substitution performed. If there were no command substitutions, the command exits with a status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple command and an optional list of arguments, the follow‐
ing actions are taken.
If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name,
that function is invoked as described above in FUNCTIONS. If the name does not match a function, the shell searches for it
in the list of shell builtins. If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no slashes, bash searches each element of the PATH for
a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of exe‐
cutable files (see hash under SHELL BUILTIN COMMANDS below). A full search of the directories in PATH is performed only if
the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function
named command_not_found_handle. If that function exists, it is invoked with the original command and the original com‐
mand's arguments as its arguments, and the function's exit status becomes the exit status of the shell. If that function
is not defined, the shell prints an error message and returns an exit status of 127.
If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a
separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to
the arguments given, if any.
If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be
a shell script, a file containing shell commands. A subshell is spawned to execute it. This subshell reinitializes
itself, so that the effect is as if a new shell had been invoked to handle the script, with the exception that the loca‐
tions of commands remembered by the parent (see hash below under SHELL BUILTIN COMMANDS) are retained by the child.
If the program is a file beginning with #!, the remainder of the first line specifies an interpreter for the program. The
shell executes the specified interpreter on operating systems that do not handle this executable format themselves. The
arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the
program, followed by the name of the program, followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:
· open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin
· the current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation
· the file creation mode mask as set by umask or inherited from the shell's parent
· current traps set by trap
· shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the envi‐
ronment
· shell functions defined during execution or inherited from the shell's parent in the environment
· options enabled at invocation (either by default or with command-line arguments) or by set
· options enabled by shopt
· shell aliases defined with alias
· various process IDs, including those of background jobs, the value of $$, and the value of PPID
When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution envi‐
ronment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
· the shell's open files, plus any modifications and additions specified by redirections to the command
· the current working directory
· the file creation mode mask
· shell variables and functions marked for export, along with variables exported for the command, passed in the envi‐
ronment
· traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell
are ignored
A command invoked in this separate environment cannot affect the shell's execution environment.
Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment
that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell
inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a
subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.
Subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When not in
posix mode, bash clears the -e option in such subshells.
If a command is followed by a & and job control is not active, the default standard input for the command is the empty file
/dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.
ENVIRONMENT
When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of
the form name=value.
The shell provides several ways to manipulate the environment. On invocation, the shell scans its own environment and cre‐
ates a parameter for each name found, automatically marking it for export to child processes. Executed commands inherit
the environment. The export and declare -x commands allow parameters and functions to be added to and deleted from the
environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment,
replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose
values may be modified in the shell, less any pairs removed by the unset command, plus any additions via the export and
declare -x commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments,
as described above in PARAMETERS. These assignment statements affect only the environment seen by that command.
If the -k option is set (see the set builtin command below), then all parameter assignments are placed in the environment
for a command, not just those that precede the command name.
When bash invokes an external command, the variable _ is set to the full file name of the command and passed to that com‐
mand in its environment.
EXIT STATUS
The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit sta‐
tuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from
shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use spe‐
cial values to indicate specific failure modes.
For the shell's purposes, a command which exits with a zero exit status has succeeded. An exit status of zero indicates
success. A non-zero exit status indicates failure. When a command terminates on a fatal signal N, bash uses the value of
128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is
not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful, and non-zero (false) if an error occurs while they exe‐
cute. All builtins return an exit status of 2 to indicate incorrect usage.
Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with
a non-zero value. See also the exit builtin command below.
SIGNALS
When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill 0 does not kill an interactive
shell), and SIGINT is caught and handled (so that the wait builtin is interruptible). In all cases, bash ignores SIGQUIT.
If job control is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Non-builtin commands run by bash have signal handlers set to the values inherited by the shell from its parent. When job
control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition to these inherited handlers. Com‐
mands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGT‐
STP.
The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs,
running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from send‐
ing the signal to a particular job, it should be removed from the jobs table with the disown builtin (see SHELL BUILTIN
COMMANDS below) or marked to not receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt, bash sends a SIGHUP to all jobs when an interactive login shell
exits.
If bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be exe‐
cuted until the command completes. When bash is waiting for an asynchronous command via the wait builtin, the reception of
a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than
128, immediately after which the trap is executed.
JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their exe‐
cution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the oper‐
ating system kernel's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which may be listed with the
jobs command. When bash starts a job asynchronously (in the background), it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this
job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the
basis for job control.
To facilitate the implementation of the user interface to job control, the operating system maintains the notion of a cur‐
rent terminal process group ID. Members of this process group (processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the fore‐
ground. Background processes are those whose process group ID differs from the terminal's; such processes are immune to
keyboard-generated signals. Only foreground processes are allowed to read from or, if the user so specifies with stty
tostop, write to the terminal. Background processes which attempt to read from (write to when stty tostop is in effect)
the terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel's terminal driver, which, unless caught, suspends the
process.
If the operating system on which bash is running supports job control, bash contains facilities to use it. Typing the sus‐
pend character (typically ^Z, Control-Z) while a process is running causes that process to be stopped and returns control
to bash. Typing the delayed suspend character (typically ^Y, Control-Y) causes the process to be stopped when it attempts
to read input from the terminal, and control to be returned to bash. The user may then manipulate the state of this job,
using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command
to kill it. A ^Z takes effect immediately, and has the additional side effect of causing pending output and typeahead to
be discarded.
There are a number of ways to refer to a job in the shell. The character % introduces a job specification (jobspec). Job
number n may be referred to as %n. A job may also be referred to using a prefix of the name used to start it, or using a
substring that appears in its command line. For example, %ce refers to a stopped ce job. If a prefix matches more than
one job, bash reports an error. Using %?ce, on the other hand, refers to any job containing the string ce in its command
line. If the substring matches more than one job, bash reports an error. The symbols %% and %+ refer to the shell's
notion of the current job, which is the last job stopped while it was in the foreground or started in the background. The
previous job may be referenced using %-. If there is only a single job, %+ and %- can both be used to refer to that job.
In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a +, and the
previous job with a -. A single % (with no accompanying job specification) also refers to the current job.
Simply naming a job can be used to bring it into the foreground: %1 is a synonym for ``fg %1'', bringing job 1 from the
background into the foreground. Similarly, ``%1 &'' resumes job 1 in the background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state. Normally, bash waits until it is about to print a prompt before
reporting changes in a job's status so as to not interrupt any other output. If the -b option to the set builtin command
is enabled, bash reports such changes immediately. Any trap on SIGCHLD is executed for each child that exits.
If an attempt to exit bash is made while jobs are stopped (or, if the checkjobs shell option has been enabled using the
shopt builtin, running), the shell prints a warning message, and, if the checkjobs option is enabled, lists the jobs and
their statuses. The jobs command may then be used to inspect their status. If a second attempt to exit is made without an
intervening command, the shell does not print another warning, and any stopped jobs are terminated.
PROMPTING
When executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the secondary
prompt PS2 when it needs more input to complete a command. Bash allows these prompt strings to be customized by inserting
a number of backslash-escaped special characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format (e.g., "Tue May 26")
\D{format}
the format is passed to strftime(3) and the result is inserted into the prompt string; an empty format
results in a locale-specific time representation. The braces are required
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated with a tilde (uses the value of the PROMPT_DIRTRIM
variable)
\W the basename of the current working directory, with $HOME abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into
the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually different: the history number of a command is its position in the
history list, which may include commands restored from the history file (see HISTORY below), while the command number is
the position in the sequence of commands executed during the current shell session. After the string is decoded, it is
expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of
the promptvars shell option (see the description of the shopt command under SHELL BUILTIN COMMANDS below).
READLINE
This is the library that handles reading input when using an interactive shell, unless the --noediting option is given at
shell invocation. Line editing is also used when using the -e option to the read builtin. By default, the line editing
commands are similar to those of Emacs. A vi-style line editing interface is also available. Line editing can be enabled
at any time using the -o emacs or -o vi options to the set builtin (see SHELL BUILTIN COMMANDS below). To turn off line
editing after the shell is running, use the +o emacs or +o vi options to the set builtin.
Readline Notation
In this section, the Emacs-style notation is used to denote keystrokes. Control keys are denoted by C-key, e.g., C-n means
Control-N. Similarly, meta keys are denoted by M-key, so M-x means Meta-X. (On keyboards without a meta key, M-x means
ESC x, i.e., press the Escape key then the x key. This makes ESC the meta prefix. The combination M-C-x means ESC-Con‐
trol-x, or press the Escape key then hold the Control key while pressing the x key.)
Readline commands may be given numeric arguments, which normally act as a repeat count. Sometimes, however, it is the sign
of the argument that is significant. Passing a negative argument to a command that acts in the forward direction (e.g.,
kill-line) causes that command to act in a backward direction. Commands whose behavior with arguments deviates from this
are noted below.
When a command is described as killing text, the text deleted is saved for possible future retrieval (yanking). The killed
text is saved in a kill ring. Consecutive kills cause the text to be accumulated into one unit, which can be yanked all at
once. Commands which do not kill text separate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initialization file (the inputrc file). The name of this file is taken
from the value of the INPUTRC variable. If that variable is unset, the default is ~/.inputrc. When a program which uses
the readline library starts up, the initialization file is read, and the key bindings and variables are set. There are
only a few basic constructs allowed in the readline initialization file. Blank lines are ignored. Lines beginning with a
# are comments. Lines beginning with a $ indicate conditional constructs. Other lines denote key bindings and variable
settings.
The default key-bindings may be changed with an inputrc file. Other programs that use this library may add their own com‐
mands and bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command universal-argument.
The following symbolic character names are recognized: RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.
In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a
macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple. All that is required is the name of the command or
the text of a macro and a key sequence to which it should be bound. The name may be specified in one of two ways: as a sym‐
bolic key name, possibly with Meta- or Control- prefixes, or as a key sequence.
When using the form keyname:function-name or macro, keyname is the name of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function universal-argument, M-DEL is bound to the function backward-kill-word,
and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text ``> output'' into the
line).
In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in that strings denoting an entire
key sequence may be specified by placing the sequence within double quotes. Some GNU Emacs style key escapes can be used,
as in the following example, but the symbolic character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function universal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text ``Function Key 1''.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
When entering the text of a macro, single or double quotes must be used to indicate a macro definition. Unquoted text is
assumed to be a function name. In the macro body, the backslash escapes described above are expanded. Backslash will
quote any other character in the macro text, including " and '.
Bash allows the current readline key bindings to be displayed or modified with the bind builtin command. The editing mode
may be switched during interactive use by using the -o option to the set builtin command (see SHELL BUILTIN COMMANDS
below).
Readline Variables
Readline has variables that can be used to further customize its behavior. A variable may be set in the inputrc file with
a statement of the form
set variable-name value
Except where noted, readline variables can take the values On or Off (without regard to case). Unrecognized variable names
are ignored. When a variable value is read, empty or null values, "on" (case-insensitive), and "1" are equivalent to On.
All other values are equivalent to Off. The variables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring the terminal bell. If set to none, readline never rings the bell.
If set to visible, readline uses a visible bell if one is available. If set to audible, readline attempts to ring
the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters treated specially by the kernel's terminal driver to
their readline equivalents.
comment-begin (``#'')
The string that is inserted when the readline insert-comment command is executed. This command is bound to M-# in
emacs mode and to # in vi command mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching and completion in a case-insensitive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of possible completions that is displayed without modifica‐
tion. When set to a value greater than zero, common prefixes longer than this value are replaced with an ellipsis
when displaying possible completions.
completion-query-items (100)
This determines when the user is queried about viewing the number of possible completions generated by the possi‐
ble-completions command. It may be set to any integer value greater than or equal to zero. If the number of possi‐
ble completions is greater than or equal to the value of this variable, the user is asked whether or not he wishes
to view them; otherwise they are simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth bit set to an ASCII key sequence by stripping the
eighth bit and prefixing an escape character (in effect, using escape as the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion. Completion characters will be inserted into the line as if
they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings similar to Emacs or vi. editing-mode can be set to
either emacs or vi.
echo-control-characters (On)
When set to On, on operating systems that indicate they support it, readline echoes a character corresponding to a
signal generated from the keyboard.
enable-keypad (Off)
When set to On, readline will try to enable the application keypad when it is called. Some systems need this to
enable the arrow keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta modifier key the terminal claims to support when it is called.
On many terminals, the meta key is used to send eight-bit characters.
expand-tilde (Off)
If set to On, tilde expansion is performed when readline attempts word completion.
history-preserve-point (Off)
If set to On, the history code attempts to place point at the same location on each history line retrieved with pre‐
vious-history or next-history.
history-size (0)
Set the maximum number of history entries saved in the history list. If set to zero, the number of entries in the
history list is not limited.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for display, scrolling the input horizontally on a single screen
line when it becomes longer than the screen width rather than wrapping to a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input (that is, it will not strip the high bit from the characters it
reads), regardless of what the terminal claims it can support. The name meta-flag is a synonym for this variable.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an incremental search without subsequently executing the character as
a command. If this variable has not been given a value, the characters ESC and C-J will terminate an incremental
search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names is emacs, emacs-standard, emacs-meta, emacs-ctlx,
vi, vi-command, and vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-standard. The default
value is emacs; the value of editing-mode also affects the default keymap.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are displayed with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to directories have a slash appended (subject to the value of
mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files whose names begin with a `.' (hidden files) when per‐
forming filename completion. If set to Off, the leading `.' must be supplied by the user in the filename to be com‐
pleted.
menu-complete-display-prefix (Off)
If set to On, menu completion displays the common prefix of the list of possible completions (which may be empty)
before cycling through the list.
output-meta (Off)
If set to On, readline will display characters with the eighth bit set directly rather than as a meta-prefixed
escape sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager to display a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches sorted horizontally in alphabetical order, rather than
down the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines before returning when accept-line is executed. By
default, history lines may be modified and retain individual undo lists across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If set to On, words which have more than one possible
completion cause the matches to be listed immediately instead of ringing the bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. If set
to On, words which have more than one possible completion without any possible partial completion (the possible com‐
pletions don't share a common prefix) cause the matches to be listed immediately instead of ringing the bell.
skip-completed-text (Off)
If set to On, this alters the default completion behavior when inserting a single match into the line. It's only
active when performing completion in the middle of a word. If enabled, readline does not insert characters from the
completion that match characters after point in the word being completed, so portions of the word following the cur‐
sor are not duplicated.
visible-stats (Off)
If set to On, a character denoting a file's type as reported by stat(2) is appended to the filename when listing
possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows
key bindings and variable settings to be performed as the result of tests. There are four parser directives used.
$if The $if construct allows bindings to be made based on the editing mode, the terminal being used, or the application
using readline. The text of the test extends to the end of the line; no characters are required to isolate it.
mode The mode= form of the $if directive is used to test whether readline is in emacs or vi mode. This may be
used in conjunction with the set keymap command, for instance, to set bindings in the emacs-standard and
emacs-ctlx keymaps only if readline is starting out in emacs mode.
term The term= form may be used to include terminal-specific key bindings, perhaps to bind the key sequences out‐
put by the terminal's function keys. The word on the right side of the = is tested against the both full
name of the terminal and the portion of the terminal name before the first -. This allows sun to match both
sun and sun-cmd, for instance.
application
The application construct is used to include application-specific settings. Each program using the readline
library sets the application name, and an initialization file can test for a particular value. This could be
used to bind key sequences to functions useful for a specific program. For instance, the following command
adds a key sequence that quotes the current or previous word in bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif This command, as seen in the previous example, terminates an $if command.
$else Commands in this branch of the $if directive are executed if the test fails.
$include
This directive takes a single filename as an argument and reads commands and bindings from that file. For example,
the following directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command history (see HISTORY below) for lines containing a specified
string. There are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the search string. As each character of the search string
is typed, readline displays the next entry from the history matching the string typed so far. An incremental search
requires only as many characters as needed to find the desired history entry. The characters present in the value of the
isearch-terminators variable are used to terminate an incremental search. If that variable has not been assigned a value
the Escape and Control-J characters will terminate an incremental search. Control-G will abort an incremental search and
restore the original line. When the search is terminated, the history entry containing the search string becomes the cur‐
rent line.
To find other matching entries in the history list, type Control-S or Control-R as appropriate. This will search backward
or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a
readline command will terminate the search and execute that command. For instance, a newline will terminate the search and
accept the line, thereby executing the command from the history list.
Readline remembers the last incremental search string. If two Control-Rs are typed without any intervening characters
defining a new search string, any remembered search string is used.
Non-incremental searches read the entire search string before starting to search for matching history lines. The search
string may be typed by the user or be part of the contents of the current line.
Readline Command Names
The following is a list of the names of the commands and the default key sequences to which they are bound. Command names
without an accompanying key sequence are unbound by default. In the following descriptions, point refers to the current
cursor position, and mark refers to a cursor position saved by the set-mark command. The text between the point and mark
is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words are composed of alphanumeric characters (letters and
digits).
shell-forward-word
Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word. Words are delimited by non-quoted shell metacharacters.
clear-screen (C-l)
Clear the screen leaving the current line at the top of the screen. With an argument, refresh the current line
without clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list according
to the state of the HISTCONTROL variable. If the line is a modified history line, then restore the history line to
its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up' through the history as necessary. This is an incremen‐
tal search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down' through the history as necessary. This is an incre‐
mental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line using a non-incremental search for a string sup‐
plied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search for a string supplied by the user.
history-search-forward
Search forward through the history for the string of characters between the start of the current line and the point.
This is a non-incremental search.
history-search-backward
Search backward through the history for the string of characters between the start of the current line and the
point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the second word on the previous line) at point. With an
argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A
negative argument inserts the nth word from the end of the previous command. Once the argument n is computed, the
argument is extracted as if the "!n" history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word of the previous history entry). With a numeric
argument, behave exactly like yank-nth-arg. Successive calls to yank-last-arg move back through the history list,
inserting the last word (or the word specified by the argument to the first call) of each line in turn. Any numeric
argument supplied to these successive calls determines the direction to move through the history. A negative argu‐
ment switches the direction through the history (back or forward). The history expansion facilities are used to
extract the last argument, as if the "!$" history expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and history expansion as well as all of the shell word
expansions. See HISTORY EXPANSION below for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a space. See HISTORY EXPANSION below for a description of
history expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line relative to the current line from the history for
editing. Any argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke
$VISUAL, $EDITOR, and emacs as the editor, in that order.
Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the beginning of the line, there are no characters in the line, and
the last character typed was not bound to delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at the end of the line, in which case the character
behind the cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at point, moving point forward as well. If point is at
the end of the line, then this transposes the two characters before point. Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving point over that word as well. If point is at the end
of the line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move
point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative argument, lowercase the previous word, but do not move
point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative argument, capitalize the previous word, but do not move
point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argument, switches to overwrite mode. With an explicit
non-positive numeric argument, switches to insert mode. This command affects only emacs mode; vi mode does over‐
write differently. Each call to readline() starts in insert mode. In overwrite mode, characters bound to
self-insert replace the text at point rather than pushing the text to the right. Characters bound to back‐
ward-delete-char replace the character before point with a space. By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point is.
kill-word (M-d)
Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries
are the same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those used by backward-word.
shell-kill-word (M-d)
Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries
are the same as those used by shell-forward-word.
shell-backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary. The killed text is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash character as the word boundaries. The killed text is
saved on the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is followed by one or more digits, optionally with a
leading minus sign, those digits define the argument. If the command is followed by digits, executing univer‐
sal-argument again ends the numeric argument, but is otherwise ignored. As a special case, if this command is imme‐
diately followed by a character that is neither a digit or minus sign, the argument count for the next command is
multiplied by four. The argument count is initially one, so executing this function the first time makes the argu‐
ment count four, a second time makes the argument count sixteen, and so on.
Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash attempts completion treating the text as a variable
(if the text begins with $), username (if the text begins with ~), hostname (if the text begins with @), or command
(including aliases and functions) in turn. If none of these produces a match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a single match from the list of possible comple‐
tions. Repeated execution of menu-complete steps through the list of possible completions, inserting each match in
turn. At the end of the list of completions, the bell is rung (subject to the setting of bell-style) and the origi‐
nal text is restored. An argument of n moves n positions forward in the list of matches; a negative argument may be
used to move backward through the list. This command is intended to be bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list of possible completions, as if menu-complete had
been given a negative argument. This command is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or end of the line (like delete-char). If at the end
of the line, behaves identically to possible-completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a command name. Command completion attempts to match
the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that
order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text against lines from the history list for possible
completion matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the text against lines from the history list for possi‐
ble completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible completions enclosed within braces so the list is avail‐
able to the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the key‐
board.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell (subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the command that is bound to the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the undo command enough times to return the line to its
initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-)
Set the mark to the point. If a numeric argument is supplied, the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is set to the saved position, and the old cursor position
is saved as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence of that character. A negative count searches for pre‐
vious occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence of that character. A negative count searches for
subsequent occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. Such
sequences begin with a Control Sequence Indicator (CSI), usually ESC-[. If this sequence is bound to "\[", keys
producing such sequences will have no effect unless explicitly bound to a readline command, instead of inserting
stray characters into the editing buffer. This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline comment-begin variable is inserted at the beginning of the
current line. If a numeric argument is supplied, this command acts as a toggle: if the characters at the beginning
of the line do not match the value of comment-begin, the value is inserted, otherwise the characters in com‐
ment-begin are deleted from the beginning of the line. In either case, the line is accepted as if a newline had
been typed. The default value of comment-begin causes this command to make the current line a shell comment. If a
numeric argument causes the comment character to be removed, the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname expansion, with an asterisk implicitly appended. This
pattern is used to generate a list of matching file names for possible completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expansion, and the list of matching file names is
inserted, replacing the word. If a numeric argument is supplied, an asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by glob-expand-word is displayed, and the line is redrawn. If
a numeric argument is supplied, an asterisk is appended before pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and their values to the readline output stream. If a numeric argument
is supplied, the output is formatted in such a way that it can be made part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the strings they output. If a numeric argument is sup‐
plied, the output is formatted in such a way that it can be made part of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
Programmable Completion
When word completion is attempted for an argument to a command for which a completion specification (a compspec) has been
defined using the complete builtin (see SHELL BUILTIN COMMANDS below), the programmable completion facilities are invoked.
First, the command name is identified. If the command word is the empty string (completion attempted at the beginning of
an empty line), any compspec defined with the -E option to complete is used. If a compspec has been defined for that com‐
mand, the compspec is used to generate the list of possible completions for the word. If the command word is a full path‐
name, a compspec for the full pathname is searched for first. If no compspec is found for the full pathname, an attempt is
made to find a compspec for the portion following the final slash. If those searches do not result in a compspec, any
compspec defined with the -D option to complete is used as the default.
Once a compspec has been found, it is used to generate the list of matching words. If a compspec is not found, the default
bash completion as described above under Completing is performed.
First, the actions specified by the compspec are used. Only matches which are prefixed by the word being completed are
returned. When the -f or -d option is used for filename or directory name completion, the shell variable FIGNORE is used
to filter the matches.
Any completions specified by a pathname expansion pattern to the -G option are generated next. The words generated by the
pattern need not match the word being completed. The GLOBIGNORE shell variable is not used to filter the matches, but the
FIGNORE variable is used.
Next, the string specified as the argument to the -W option is considered. The string is first split using the characters
in the IFS special variable as delimiters. Shell quoting is honored. Each word is then expanded using brace expansion,
tilde expansion, parameter and variable expansion, command substitution, and arithmetic expansion, as described above under
EXPANSION. The results are split using the rules described above under Word Splitting. The results of the expansion are
prefix-matched against the word being completed, and the matching words become the possible completions.
After these matches have been generated, any shell function or command specified with the -F and -C options is invoked.
When the command or function is invoked, the COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables are assigned values
as described above under Shell Variables. If a shell function is being invoked, the COMP_WORDS and COMP_CWORD variables
are also set. When the function or command is invoked, the first argument is the name of the command whose arguments are
being completed, the second argument is the word being completed, and the third argument is the word preceding the word
being completed on the current command line. No filtering of the generated completions against the word being completed is
performed; the function or command has complete freedom in generating the matches.
Any function specified with -F is invoked first. The function may use any of the shell facilities, including the compgen
builtin described below, to generate the matches. It must put the possible completions in the COMPREPLY array variable.
Next, any command specified with the -C option is invoked in an environment equivalent to command substitution. It should
print a list of completions, one per line, to the standard output. Backslash may be used to escape a newline, if neces‐
sary.
After all of the possible completions are generated, any filter specified with the -X option is applied to the list. The
filter is a pattern as used for pathname expansion; a & in the pattern is replaced with the text of the word being com‐
pleted. A literal & may be escaped with a backslash; the backslash is removed before attempting a match. Any completion
that matches the pattern will be removed from the list. A leading ! negates the pattern; in this case any completion not
matching the pattern will be removed.
Finally, any prefix and suffix specified with the -P and -S options are added to each member of the completion list, and
the result is returned to the readline completion code as the list of possible completions.
If the previously-applied actions do not generate any matches, and the -o dirnames option was supplied to complete when the
compspec was defined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when the compspec was defined, directory name completion is attempted
and any matches are added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned to the completion code as the full set of possible
completions. The default bash completions are not attempted, and the readline default of filename completion is disabled.
If the -o bashdefault option was supplied to complete when the compspec was defined, the bash default completions are
attempted if the compspec generates no matches. If the -o default option was supplied to complete when the compspec was
defined, readline's default completion will be performed if the compspec (and, if attempted, the default bash completions)
generate no matches.
When a compspec indicates that directory name completion is desired, the programmable completion functions force readline
to append a slash to completed names which are symbolic links to directories, subject to the value of the mark-directories
readline variable, regardless of the setting of the mark-symlinked-directories readline variable.
There is some support for dynamically modifying completions. This is most useful when used in combination with a default
completion specified with complete -D. It's possible for shell functions executed as completion handlers to indicate that
completion should be retried by returning an exit status of 124. If a shell function returns 124, and changes the compspec
associated with the command on which completion is being attempted (supplied as the first argument when the function is
executed), programmable completion restarts from the beginning, with an attempt to find a new compspec for that command.
This allows a set of completions to be built dynamically as completion is attempted, rather than being loaded all at once.
For instance, assuming that there is a library of compspecs, each kept in a file corresponding to the name of the command,
the following default completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader
HISTORY
When the -o history option to the set builtin is enabled, the shell provides access to the command history, the list of
commands previously typed. The value of the HISTSIZE variable is used as the number of commands to save in a history list.
The text of the last HISTSIZE commands (default 500) is saved. The shell stores each command in the history list prior to
parameter and variable expansion (see EXPANSION above) but after history expansion is performed, subject to the values of
the shell variables HISTIGNORE and HISTCONTROL.
On startup, the history is initialized from the file named by the variable HISTFILE (default ~/.bash_history). The file
named by the value of HISTFILE is truncated, if necessary, to contain no more than the number of lines specified by the
value of HISTFILESIZE. When the history file is read, lines beginning with the history comment character followed immedi‐
ately by a digit are interpreted as timestamps for the preceding history line. These timestamps are optionally displayed
depending on the value of the HISTTIMEFORMAT variable. When an interactive shell exits, the last $HISTSIZE lines are
copied from the history list to $HISTFILE. If the histappend shell option is enabled (see the description of shopt under
SHELL BUILTIN COMMANDS below), the lines are appended to the history file, otherwise the history file is overwritten. If
HISTFILE is unset, or if the history file is unwritable, the history is not saved. If the HISTTIMEFORMAT variable is set,
time stamps are written to the history file, marked with the history comment character, so they may be preserved across
shell sessions. This uses the history comment character to distinguish timestamps from other history lines. After saving
the history, the history file is truncated to contain no more than HISTFILESIZE lines. If HISTFILESIZE is not set, no
truncation is performed.
The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used to list or edit and re-execute a portion of the his‐
tory list. The history builtin may be used to display or modify the history list and manipulate the history file. When
using command-line editing, search commands are available in each editing mode that provide access to the history list.
The shell allows control over which commands are saved on the history list. The HISTCONTROL and HISTIGNORE variables may
be set to cause the shell to save only a subset of the commands entered. The cmdhist shell option, if enabled, causes the
shell to attempt to save each line of a multi-line command in the same history entry, adding semicolons where necessary to
preserve syntactic correctness. The lithist shell option causes the shell to save the command with embedded newlines
instead of semicolons. See the description of the shopt builtin below under SHELL BUILTIN COMMANDS for information on set‐
ting and unsetting shell options.
HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the history expansion in csh. This section describes
what syntax features are available. This feature is enabled by default for interactive shells, and can be disabled using
the +H option to the set builtin command (see SHELL BUILTIN COMMANDS below). Non-interactive shells do not perform history
expansion by default.
History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert
the arguments to a previous command into the current input line, or fix errors in previous commands quickly.
History expansion is performed immediately after a complete line is read, before the shell breaks it into words. It takes
place in two parts. The first is to determine which line from the history list to use during substitution. The second is
to select portions of that line for inclusion into the current one. The line selected from the history is the event, and
the portions of that line that are acted upon are words. Various modifiers are available to manipulate the selected words.
The line is broken into words in the same fashion as when reading input, so that several metacharacter-separated words sur‐
rounded by quotes are considered one word. History expansions are introduced by the appearance of the history expansion
character, which is ! by default. Only backslash (\) and single quotes can quote the history expansion character.
Several characters inhibit history expansion if found immediately following the history expansion character, even if it is
unquoted: space, tab, newline, carriage return, and =. If the extglob shell option is enabled, ( will also inhibit expan‐
sion.
Several shell options settable with the shopt builtin may be used to tailor the behavior of history expansion. If the
histverify shell option is enabled (see the description of the shopt builtin below), and readline is being used, history
substitutions are not immediately passed to the shell parser. Instead, the expanded line is reloaded into the readline
editing buffer for further modification. If readline is being used, and the histreedit shell option is enabled, a failed
history substitution will be reloaded into the readline editing buffer for correction. The -p option to the history
builtin command may be used to see what a history expansion will do before using it. The -s option to the history builtin
may be used to add commands to the end of the history list without actually executing them, so that they are available for
subsequent recall.
The shell allows control of the various characters used by the history expansion mechanism (see the description of
histchars above under Shell Variables). The shell uses the history comment character to mark history timestamps when writ‐
ing the history file.
Event Designators
An event designator is a reference to a command line entry in the history list. Unless the reference is absolute, events
are relative to the current position in the history list.
! Start a history substitution, except when followed by a blank, newline, carriage return, = or ( (when the extglob
shell option is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command preceding the current position in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current postition in the history list containing string. The trail‐
ing ? may be omitted if string is followed immediately by a newline.
^string1^string2^
Quick substitution. Repeat the previous command, replacing string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the event. A : separates the event specification from the word des‐
ignator. It may be omitted if the word designator begins with a ^, $, *, -, or %. Words are numbered from the beginning
of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single
spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for `1-$'. It is not an error to use * if there is just one
word in the event; the empty string is returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specification, the previous command is used as the event.
Modifiers
After the optional word designator, there may appear a sequence of one or more of the following modifiers, each preceded by
a `:'.
h Remove a trailing file name component, leaving only the head.
t Remove all leading file name components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the event line. Any delimiter can be used in place of /. The
final delimiter is optional if it is the last character of the event line. The delimiter may be quoted in old and
new with a single backslash. If & appears in new, it is replaced by old. A single backslash will quote the &. If
old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last
string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is used in conjunction with `:s' (e.g.,
`:gs/old/new/') or `:&'. If used with `:s', any delimiter can be used in place of /, and the final delimiter is
optional if it is the last character of the event line. An a may be used as a synonym for g.
G Apply the following `s' modifier once to each word in the event line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to
signify the end of the options. The :, true, false, and test builtins do not accept options and do not treat -- specially.
The exit, logout, break, continue, let, and shift builtins accept and process arguments beginning with - without requiring
--. Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with -
as invalid options and require -- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments and performing any specified redirections. A zero
exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and return the exit status of the last com‐
mand executed from filename. If filename does not contain a slash, file names in PATH are used to find the direc‐
tory containing filename. The file searched for in PATH need not be executable. When bash is not in posix mode,
the current directory is searched if no file is found in PATH. If the sourcepath option to the shopt builtin com‐
mand is turned off, the PATH is not searched. If any arguments are supplied, they become the positional parameters
when filename is executed. Otherwise the positional parameters are unchanged. The return status is the status of
the last command exited within the script (0 if no commands are executed), and false if filename is not found or
cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of aliases in the form alias name=value on standard
output. When arguments are supplied, an alias is defined for each name whose value is given. A trailing space in
value causes the next word to be checked for alias substitution when the alias is expanded. For each name in the
argument list for which no value is supplied, the name and value of the alias is printed. Alias returns true unless
a name is given for which no alias has been defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started with &. If jobspec is not present,
the shell's notion of the current job is used. bg jobspec returns 0 unless run when job control is disabled or,
when run with job control enabled, any specified jobspec was not found or was started without job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a key sequence to a readline function or macro, or set a
readline variable. Each non-option argument is a command as it would appear in .inputrc, but each binding or com‐
mand must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'. Options, if supplied, have the
following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-com‐
mand; emacs is equivalent to emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way that they can be re-read.
-P List current readline function names and bindings.
-s Display readline key sequences bound to macros and the strings they output in such a way that they can be re-
read.
-S Display readline key sequences bound to macros and the strings they output.
-v Display readline variable names and values in such a way that they can be re-read.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is entered. When shell-command is executed, the shell
sets the READLINE_LINE variable to the contents of the readline line buffer and the READLINE_POINT variable
to the current location of the insertion point. If the executed command changes the value of READLINE_LINE
or READLINE_POINT, those new values will be reflected in the editing state.
The return value is 0 unless an unrecognized option is given or an error occurred.
break [n]
Exit from within a for, while, until, or select loop. If n is specified, break n levels. n must be ≥ 1. If n is
greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless n is not
greater than or equal to 1.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and return its exit status. This is useful when defining
a function whose name is the same as a shell builtin, retaining the functionality of the builtin within the func‐
tion. The cd builtin is commonly redefined this way. The return status is false if shell-builtin is not a shell
builtin command.
caller [expr]
Returns the context of any active subroutine call (a shell function or a script executed with the . or source
builtins). Without expr, caller displays the line number and source filename of the current subroutine call. If a
non-negative integer is supplied as expr, caller displays the line number, subroutine name, and source file corre‐
sponding to that position in the current execution call stack. This extra information may be used, for example, to
print a stack trace. The current frame is frame 0. The return value is 0 unless the shell is not executing a sub‐
routine call or expr does not correspond to a valid position in the call stack.
cd [-L|[-P [-e]]] [dir]
Change the current directory to dir. The variable HOME is the default dir. The variable CDPATH defines the search
path for the directory containing dir. Alternative directory names in CDPATH are separated by a colon (:). A null
directory name in CDPATH is the same as the current directory, i.e., ``.''. If dir begins with a slash (/), then
CDPATH is not used. The -P option says to use the physical directory structure instead of following symbolic links
(see also the -P option to the set builtin command); the -L option forces symbolic links to be followed. If the -e
option is supplied with -P, and the current working directory cannot be successfully determined after a successful
directory change, cd will return an unsuccessful status. An argument of - is equivalent to $OLDPWD. If a non-empty
directory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the abso‐
lute pathname of the new working directory is written to the standard output. The return value is true if the
directory was successfully changed; false otherwise.
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell function lookup. Only builtin commands or commands found in the
PATH are executed. If the -p option is given, the search for command is performed using a default value for PATH
that is guaranteed to find all of the standard utilities. If either the -V or -v option is supplied, a description
of command is printed. The -v option causes a single word indicating the command or file name used to invoke com‐
mand to be displayed; the -V option produces a more verbose description. If the -V or -v option is supplied, the
exit status is 0 if command was found, and 1 if not. If neither option is supplied and an error occurred or command
cannot be found, the exit status is 127. Otherwise, the exit status of the command builtin is the exit status of
command.
compgen [option] [word]
Generate possible completion matches for word according to the options, which may be any option accepted by the com‐
plete builtin with the exception of -p and -r, and write the matches to the standard output. When using the -F or
-C options, the various shell variables set by the programmable completion facilities, while available, will not
have useful values.
The matches will be generated in the same way as if the programmable completion code had generated them directly
from a completion specification with the same flags. If word is specified, only those completions matching word
will be displayed.
The return value is true unless an invalid option is supplied, or no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G globpat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DE] [name ...]
Specify how arguments to each name should be completed. If the -p option is supplied, or if no options are sup‐
plied, existing completion specifications are printed in a way that allows them to be reused as input. The -r
option removes a completion specification for each name, or, if no names are supplied, all completion specifica‐
tions. The -D option indicates that the remaining options and actions should apply to the ``default'' command com‐
pletion; that is, completion attempted on a command for which no completion has previously been defined. The -E
option indicates that the remaining options and actions should apply to ``empty'' command completion; that is, com‐
pletion attempted on a blank line.
The process of applying these completion specifications when word completion is attempted is described above under
Programmable Completion.
Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X options (and, if nec‐
essary, the -P and -S options) should be quoted to protect them from expansion before the complete builtin is
invoked.
-o comp-option
The comp-option controls several aspects of the compspec's behavior beyond the simple generation of comple‐
tions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions if the compspec generates no matches.
default Use readline's default filename completion if the compspec generates no matches.
dirnames
Perform directory name completion if the compspec generates no matches.
filenames
Tell readline that the compspec generates filenames, so it can perform any filename-specific pro‐
cessing (like adding a slash to directory names, quoting special characters, or suppressing trailing
spaces). Intended to be used with shell functions.
nospace Tell readline not to append a space (the default) to words completed at the end of the line.
plusdirs
After any matches defined by the compspec are generated, directory name completion is attempted and
any matches are added to the results of the other actions.
-A action
The action may be one of the following to generate a list of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the HOSTFILE shell variable.
job Job names, if job control is active. May also be specified as -j.
keyword Shell reserved words. May also be specified as -k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set builtin.
shopt Shell option names as accepted by the shopt builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be specified as -v.
-C command
command is executed in a subshell environment, and its output is used as the possible completions.
-F function
The shell function function is executed in the current shell environment. When it finishes, the possible
completions are retrieved from the value of the COMPREPLY array variable.
-G globpat
The pathname expansion pattern globpat is expanded to generate the possible completions.
-P prefix
prefix is added at the beginning of each possible completion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all other options have been applied.
-W wordlist
The wordlist is split using the characters in the IFS special variable as delimiters, and each resultant
word is expanded. The possible completions are the members of the resultant list which match the word being
completed.
-X filterpat
filterpat is a pattern as used for pathname expansion. It is applied to the list of possible completions
generated by the preceding options and arguments, and each completion matching filterpat is removed from the
list. A leading ! in filterpat negates the pattern; in this case, any completion not matching filterpat is
removed.
The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a
name argument, an attempt is made to remove a completion specification for a name for which no specification exists,
or an error occurs adding a completion specification.
compopt [-o option] [-DE] [+o option] [name]
Modify completion options for each name according to the options, or for the currently-executing completion if no
names are supplied. If no options are given, display the completion options for each name or the current comple‐
tion. The possible values of option are those valid for the complete builtin described above. The -D option indi‐
cates that the remaining options should apply to the ``default'' command completion; that is, completion attempted
on a command for which no completion has previously been defined. The -E option indicates that the remaining
options should apply to ``empty'' command completion; that is, completion attempted on a blank line.
The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a name
for which no completion specification exists, or an output error occurs.
continue [n]
Resume the next iteration of the enclosing for, while, until, or select loop. If n is specified, resume at the nth
enclosing loop. n must be ≥ 1. If n is greater than the number of enclosing loops, the last enclosing loop (the
``top-level'' loop) is resumed. The return value is 0 unless n is not greater than or equal to 1.
declare [-aAfFgilrtux] [-p] [name[=value] ...]
typeset [-aAfFgilrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are given then display the values of variables. The -p
option will display the attributes and values of each name. When -p is used with name arguments, additional options
are ignored. When -p is supplied without name arguments, it will display the attributes and values of all variables
having the attributes specified by the additional options. If no other options are supplied with -p, declare will
display the attributes and values of all shell variables. The -f option will restrict the display to shell func‐
tions. The -F option inhibits the display of function definitions; only the function name and attributes are
printed. If the extdebug shell option is enabled using shopt, the source file name and line number where the func‐
tion is defined are displayed as well. The -F option implies -f. The -g option forces variables to be created or
modified at the global scope, even when declare is executed in a shell function. It is ignored in all other cases.
The following options can be used to restrict output to variables with the specified attribute or to give variables
attributes:
-a Each name is an indexed array variable (see Arrays above).
-A Each name is an associative array variable (see Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION above) is performed
when the variable is assigned a value.
-l When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case
attribute is disabled.
-r Make names readonly. These names cannot then be assigned values by subsequent assignment statements or
unset.
-t Give each name the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling
shell. The trace attribute has no special meaning for variables.
-u When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case
attribute is disabled.
-x Mark names for export to subsequent commands via the environment.
Using `+' instead of `-' turns off the attribute instead, with the exceptions that +a may not be used to destroy an
array variable and +r will not remove the readonly attribute. When used in a function, makes each name local, as
with the local command, unless the -g option is supplied, If a variable name is followed by =value, the value of the
variable is set to value. The return value is 0 unless an invalid option is encountered, an attempt is made to
define a function using ``-f foo=bar'', an attempt is made to assign a value to a readonly variable, an attempt is
made to assign a value to an array variable without using the compound assignment syntax (see Arrays above), one of
the names is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly vari‐
able, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-
existent function with -f.
dirs [+n] [-n] [-clpv]
Without options, displays the list of currently remembered directories. The default display is on a single line
with directory names separated by spaces. Directories are added to the list with the pushd command; the popd com‐
mand removes entries from the list.
+n Displays the nth entry counting from the left of the list shown by dirs when invoked without options, start‐
ing with zero.
-n Displays the nth entry counting from the right of the list shown by dirs when invoked without options, start‐
ing with zero.
-c Clears the directory stack by deleting all of the entries.
-l Produces a longer listing; the default listing format uses a tilde to denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, prefixing each entry with its index in the stack.
The return value is 0 unless an invalid option is supplied or n indexes beyond the end of the directory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of active jobs. If jobspec is not present, and neither -a
nor -r is supplied, the shell's notion of the current job is used. If the -h option is given, each jobspec is not
removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no
jobspec is present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is
supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts opera‐
tion to running jobs. The return value is 0 unless a jobspec does not specify a valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The return status is always 0. If -n is specified,
the trailing newline is suppressed. If the -e option is given, interpretation of the following backslash-escaped
characters is enabled. The -E option disables the interpretation of these escape characters, even on systems where
they are interpreted by default. The xpg_echo shell option may be used to dynamically determine whether or not echo
expands these escape characters by default. echo does not interpret -- to mean the end of options. echo interprets
the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value nnn (zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a
shell builtin to be executed without specifying a full pathname, even though the shell normally searches for
builtins before disk commands. If -n is used, each name is disabled; otherwise, names are enabled. For example, to
use the test binary found via the PATH instead of the shell builtin version, run ``enable -n test''. The -f option
means to load the new builtin command name from shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin previously loaded with -f. If no name arguments are given, or if the -p option
is supplied, a list of shell builtins is printed. With no other option arguments, the list consists of all enabled
shell builtins. If -n is supplied, only disabled builtins are printed. If -a is supplied, the list printed
includes all builtins, with an indication of whether or not each is enabled. If -s is supplied, the output is
restricted to the POSIX special builtins. The return value is 0 unless a name is not a shell builtin or there is an
error loading a new builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a single command. This command is then read and executed by the
shell, and its exit status is returned as the value of eval. If there are no args, or only null arguments, eval
returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process is created. The arguments become the arguments to
command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to
command. This is what login(1) does. The -c option causes command to be executed with an empty environment. If -a
is supplied, the shell passes name as the zeroth argument to the executed command. If command cannot be executed
for some reason, a non-interactive shell exits, unless the shell option execfail is enabled, in which case it
returns failure. An interactive shell returns failure if the file cannot be executed. If command is not specified,
any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the
return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the last command executed.
A trap on EXIT is executed before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the environment of subsequently executed commands. If the -f
option is given, the names refer to functions. If no names are given, or if the -p option is supplied, a list of
all names that are exported in this shell is printed. The -n option causes the export property to be removed from
each name. If a variable name is followed by =word, the value of the variable is set to word. export returns an
exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f
is supplied with a name that is not a function.
fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from first to last is selected from the history list. First
and last may be specified as a string (to locate the last command beginning with that string) or as a number (an
index into the history list, where a negative number is used as an offset from the current command number). If last
is not specified it is set to the current command for listing (so that ``fc -l -10'' prints the last 10 commands)
and to first otherwise. If first is not specified it is set to the previous command for editing and -16 for list‐
ing.
The -n option suppresses the command numbers when listing. The -r option reverses the order of the commands. If
the -l option is given, the commands are listed on standard output. Otherwise, the editor given by ename is invoked
on a file containing those commands. If ename is not given, the value of the FCEDIT variable is used, and the value
of EDITOR if FCEDIT is not set. If neither variable is set, vi is used. When editing is complete, the edited com‐
mands are echoed and executed.
In the second form, command is re-executed after each instance of pat is replaced by rep. A useful alias to use
with this is ``r="fc -s"'', so that typing ``r cc'' runs the last command beginning with ``cc'' and typing ``r'' re-
executes the last command.
If the first form is used, the return value is 0 unless an invalid option is encountered or first or last specify
history lines out of range. If the -e option is supplied, the return value is the value of the last command exe‐
cuted or failure if an error occurs with the temporary file of commands. If the second form is used, the return
status is that of the command re-executed, unless cmd does not specify a valid history line, in which case fc
returns failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the current job. If jobspec is not present, the shell's notion of the
current job is used. The return value is that of the command placed into the foreground, or failure if run when job
control is disabled or, when run with job control enabled, if jobspec does not specify a valid job or jobspec speci‐
fies a job that was started without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional parameters. optstring contains the option characters to be
recognized; if a character is followed by a colon, the option is expected to have an argument, which should be sepa‐
rated from it by white space. The colon and question mark characters may not be used as option characters. Each
time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not
exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each
time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into
the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple
calls to getopts within the same shell invocation if a new set of parameters is to be used.
When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the
index of the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if more arguments are given in args, getopts parses those
instead.
getopts can report errors in two ways. If the first character of optstring is a colon, silent error reporting is
used. In normal operation diagnostic messages are printed when invalid options or missing option arguments are
encountered. If the variable OPTERR is set to 0, no error messages will be displayed, even if the first character
of optstring is not a colon.
If an invalid option is seen, getopts places ? into name and, if not silent, prints an error message and unsets
OPTARG. If getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent, a question mark (?) is placed in name, OPTARG is
unset, and a diagnostic message is printed. If getopts is silent, then a colon (:) is placed in name and OPTARG is
set to the option character found.
getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is
encountered or an error occurs.
hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, the full pathname of the command name is determined by searching the directories in $PATH
and remembered. Any previously-remembered pathname is discarded. If the -p option is supplied, no path search is
performed, and filename is used as the full file name of the command. The -r option causes the shell to forget all
remembered locations. The -d option causes the shell to forget the remembered location of each name. If the -t
option is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are
supplied with -t, the name is printed before the hashed full pathname. The -l option causes output to be displayed
in a format that may be reused as input. If no arguments are given, or if only -l is supplied, information about
remembered commands is printed. The return status is true unless a name is not found or an invalid option is sup‐
plied.
help [-dms] [pattern]
Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all com‐
mands matching pattern; otherwise help for all the builtins and shell control structures is printed.
-d Display a short description of each pattern
-m Display the description of each pattern in a manpage-like format
-s Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line numbers. Lines listed with a * have been modified. An
argument of n lists only the last n lines. If the shell variable HISTTIMEFORMAT is set and not null, it is used as
a format string for strftime(3) to display the time stamp associated with each displayed history entry. No inter‐
vening blank is printed between the formatted time stamp and the history line. If filename is supplied, it is used
as the name of the history file; if not, the value of HISTFILE is used. Options, if supplied, have the following
meanings:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a Append the ``new'' history lines (history lines entered since the beginning of the current bash session) to
the history file.
-n Read the history lines not already read from the history file into the current history list. These are lines
appended to the history file since the beginning of the current bash session.
-r Read the contents of the history file and use them as the current history.
-w Write the current history to the history file, overwriting the history file's contents.
-p Perform history substitution on the following args and display the result on the standard output. Does not
store the results in the history list. Each arg must be quoted to disable normal history expansion.
-s Store the args in the history list as a single entry. The last command in the history list is removed before
the args are added.
If the HISTTIMEFORMAT variable is set, the time stamp information associated with each history entry is written to
the history file, marked with the history comment character. When the history file is read, lines beginning with
the history comment character followed immediately by a digit are interpreted as timestamps for the previous history
line. The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the
history file, an invalid offset is supplied as an argument to -d, or the history expansion supplied as an argument
to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the following meanings:
-l List process IDs in addition to the normal information.
-n Display information only about jobs that have changed status since the user was last notified of their sta‐
tus.
-p List only the process ID of the job's process group leader.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.
If jobspec is given, output is restricted to information about that job. The return status is 0 unless an invalid
option is encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found in command or args with the corresponding process
group ID, and executes command passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes named by pid or jobspec. sigspec is either a case-
insensitive signal name such as SIGKILL (with or without the SIG prefix) or a signal number; signum is a signal num‐
ber. If sigspec is not present, then SIGTERM is assumed. An argument of -l lists the signal names. If any argu‐
ments are supplied when -l is given, the names of the signals corresponding to the arguments are listed, and the
return status is 0. The exit_status argument to -l is a number specifying either a signal number or the exit status
of a process terminated by a signal. kill returns true if at least one signal was successfully sent, or false if an
error occurs or an invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see ARITHMETIC EVALUATION above). If the last arg evaluates
to 0, let returns 1; 0 is returned otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is created, and assigned value. The option can be any of the options
accepted by declare. When local is used within a function, it causes the variable name to have a visible scope
restricted to that function and its children. With no operands, local writes a list of local variables to the stan‐
dard output. It is an error to use local when not within a function. The return status is 0 unless local is used
outside a function, an invalid name is supplied, or name is a readonly variable.
logout Exit a login shell.
mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the -u
option is supplied. The variable MAPFILE is the default array. Options, if supplied, have the following meanings:
-n Copy at most count lines. If count is 0, all lines are copied.
-O Begin assigning to array at index origin. The default index is 0.
-s Discard the first count lines read.
-t Remove a trailing newline from each line read.
-u Read lines from file descriptor fd instead of the standard input.
-C Evaluate callback each time quantum lines are read. The -c option specifies quantum.
-c Specify the number of lines read between each call to callback.
If -C is specified without -c, the default quantum is 5000. When callback is evaluated, it is supplied the index of
the next array element to be assigned and the line to be assigned to that element as additional arguments. callback
is evaluated after the line is read but before the array element is assigned.
If not supplied with an explicit origin, mapfile will clear array before assigning to it.
mapfile returns successfully unless an invalid option or option argument is supplied, array is invalid or
unassignable, or if array is not an indexed array.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments, removes the top directory from the stack, and performs
a cd to the new top directory. Arguments, if supplied, have the following meanings:
-n Suppresses the normal change of directory when removing directories from the stack, so that only the stack is
manipulated.
+n Removes the nth entry counting from the left of the list shown by dirs, starting with zero. For example:
``popd +0'' removes the first directory, ``popd +1'' the second.
-n Removes the nth entry counting from the right of the list shown by dirs, starting with zero. For example:
``popd -0'' removes the last directory, ``popd -1'' the next to last.
If the popd command is successful, a dirs is performed as well, and the return status is 0. popd returns false if
an invalid option is encountered, the directory stack is empty, a non-existent directory stack entry is specified,
or the directory change fails.
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of the format. The -v option causes the out‐
put to be assigned to the variable var rather than being printed to the standard output.
The format is a character string which contains three types of objects: plain characters, which are simply copied to
standard output, character escape sequences, which are converted and copied to the standard output, and format spec‐
ifications, each of which causes printing of the next successive argument. In addition to the standard printf(1)
format specifications, printf interprets the following extensions:
%b causes printf to expand backslash escape sequences in the corresponding argument (except that \c terminates
output, backslashes in \', \", and \? are not removed, and octal escapes beginning with \0 may contain up to
four digits).
%q causes printf to output the corresponding argument in a format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting from using datefmt as a format string for strftime(3).
The corresponding argument is an integer representing the number of seconds since the epoch. Two special
argument values may be used: -1 represents the current time, and -2 represents the time the shell was
invoked.
Arguments to non-string format specifiers are treated as C constants, except that a leading plus or minus sign is
allowed, and if the leading character is a single or double quote, the value is the ASCII value of the following
character.
The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are
supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been sup‐
plied. The return value is zero on success, non-zero on failure.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
Adds a directory to the top of the directory stack, or rotates the stack, making the new top of the stack the cur‐
rent working directory. With no arguments, exchanges the top two directories and returns 0, unless the directory
stack is empty. Arguments, if supplied, have the following meanings:
-n Suppresses the normal change of directory when adding directories to the stack, so that only the stack is
manipulated.
+n Rotates the stack so that the nth directory (counting from the left of the list shown by dirs, starting with
zero) is at the top.
-n Rotates the stack so that the nth directory (counting from the right of the list shown by dirs, starting with
zero) is at the top.
dir Adds dir to the directory stack at the top, making it the new current working directory.
If the pushd command is successful, a dirs is performed as well. If the first form is used, pushd returns 0 unless
the cd to dir fails. With the second form, pushd returns 0 unless the directory stack is empty, a non-existent
directory stack element is specified, or the directory change to the specified new current directory fails.
pwd [-LP]
Print the absolute pathname of the current working directory. The pathname printed contains no symbolic links if
the -P option is supplied or the -o physical option to the set builtin command is enabled. If the -L option is
used, the pathname printed may contain symbolic links. The return status is 0 unless an error occurs while reading
the name of the current directory or an invalid option is supplied.
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
One line is read from the standard input, or from the file descriptor fd supplied as an argument to the -u option,
and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words
and their intervening separators assigned to the last name. If there are fewer words read from the input stream
than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into
words. The backslash character (\) may be used to remove any special meaning for the next character read and for
line continuation. Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array variable aname, starting at 0. aname is unset
before any new values are assigned. Other name arguments are ignored.
-d delim
The first character of delim is used to terminate the input line, rather than newline.
-e If the standard input is coming from a terminal, readline (see READLINE above) is used to obtain the line.
Readline uses the current (or default, if line editing was not previously active) editing settings.
-i text
If readline is being used to read the line, text is placed into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather than waiting for a complete line of input, but honor a
delimiter if fewer than nchars characters are read before the delimiter.
-N nchars
read returns after reading exactly nchars characters rather than waiting for a complete line of input, unless
EOF is encountered or read times out. Delimiter characters encountered in the input are not treated spe‐
cially and do not cause read to return until nchars characters are read.
-p prompt
Display prompt on standard error, without a trailing newline, before attempting to read any input. The
prompt is displayed only if input is coming from a terminal.
-r Backslash does not act as an escape character. The backslash is considered to be part of the line. In par‐
ticular, a backslash-newline pair may not be used as a line continuation.
-s Silent mode. If input is coming from a terminal, characters are not echoed.
-t timeout
Cause read to time out and return failure if a complete line of input is not read within timeout seconds.
timeout may be a decimal number with a fractional portion following the decimal point. This option is only
effective if read is reading input from a terminal, pipe, or other special file; it has no effect when read‐
ing from regular files. If timeout is 0, read returns success if input is available on the specified file
descriptor, failure otherwise. The exit status is greater than 128 if the timeout is exceeded.
-u fd Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the variable REPLY. The return code is zero, unless end-of-
file is encountered, read times out (in which case the return code is greater than 128), or an invalid file descrip‐
tor is supplied as the argument to -u.
readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these names may not be changed by subsequent assignment. If the
-f option is supplied, the functions corresponding to the names are so marked. The -a option restricts the vari‐
ables to indexed arrays; the -A option restricts the variables to associative arrays. If both options are supplied,
-A takes precedence. If no name arguments are given, or if the -p option is supplied, a list of all readonly names
is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The -p
option causes output to be displayed in a format that may be reused as input. If a variable name is followed by
=word, the value of the variable is set to word. The return status is 0 unless an invalid option is encountered,
one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.
return [n]
Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the
last command executed in the function body. If used outside a function, but during execution of a script by the .
(source) command, it causes the shell to stop executing that script and return either n or the exit status of the
last command executed within the script as the exit status of the script. If used outside a function and not during
execution of a script by ., the return status is false. Any command associated with the RETURN trap is executed
before execution resumes after the function or script.
set [--abefhkmnptuvxBCEHPT] [-o option-name] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg ...]
Without options, the name and value of each shell variable are displayed in a format that can be reused as input for
setting or resetting the currently-set variables. Read-only variables cannot be reset. In posix mode, only shell
variables are listed. The output is sorted according to the current locale. When options are specified, they set
or unset shell attributes. Any arguments remaining after option processing are treated as values for the positional
parameters and are assigned, in order, to $1, $2, ... $n. Options, if specified, have the following meanings:
-a Automatically mark variables and functions which are modified or created for export to the environment of
subsequent commands.
-b Report the status of terminated background jobs immediately, rather than before the next primary prompt.
This is effective only when job control is enabled.
-e Exit immediately if a pipeline (which may consist of a single simple command), a subshell command enclosed
in parentheses, or one of the commands executed as part of a command list enclosed by braces (see SHELL
GRAMMAR above) exits with a non-zero status. The shell does not exit if the command that fails is part of
the command list immediately following a while or until keyword, part of the test following the if or elif
reserved words, part of any command executed in a && or || list except the command following the final && or
||, any command in a pipeline but the last, or if the command's return value is being inverted with !. A
trap on ERR, if set, is executed before the shell exits. This option applies to the shell environment and
each subshell environment separately (see COMMAND EXECUTION ENVIRONMENT above), and may cause subshells to
exit before executing all the commands in the subshell.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up for execution. This is enabled by default.
-k All arguments in the form of assignment statements are placed in the environment for a command, not just
those that precede the command name.
-m Monitor mode. Job control is enabled. This option is on by default for interactive shells on systems that
support it (see JOB CONTROL above). Background processes run in a separate process group and a line con‐
taining their exit status is printed upon their completion.
-n Read commands but do not execute them. This may be used to check a shell script for syntax errors. This is
ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing interface. This is enabled by default when the shell is
interactive, unless the shell is started with the --noediting option. This also affects the editing
interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under HISTORY. This option is on by default in interac‐
tive shells.
ignoreeof
The effect is as if the shell command ``IGNOREEOF=10'' had been executed (see Shell Variables
above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a
non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled
by default.
posix Change the behavior of bash where the default operation differs from the POSIX standard to match the
standard (posix mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface. This also affects the editing interface used for
read -e.
xtrace Same as -x.
If -o is supplied with no option-name, the values of the current options are printed. If +o is supplied
with no option-name, a series of set commands to recreate the current option settings is displayed on the
standard output.
-p Turn on privileged mode. In this mode, the $ENV and $BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored. If the shell is started with the effective user (group) id not
equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effec‐
tive user id is set to the real user id. If the -p option is supplied at startup, the effective user id is
not reset. Turning this option off causes the effective user and group ids to be set to the real user and
group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the special parameters "@" and "*" as an error when perform‐
ing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an
error message, and, if not interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command, case command, select command, or arithmetic for command,
display the expanded value of PS4, followed by the command and its expanded arguments or associated word
list.
-B The shell performs brace expansion (see Brace Expansion above). This is on by default.
-C If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This may be
overridden when creating output files by using the redirection operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a
subshell environment. The ERR trap is normally not inherited in such cases.
-H Enable ! style history substitution. This option is on by default when the shell is interactive.
-P If set, the shell does not follow symbolic links when executing commands such as cd that change the current
working directory. It uses the physical directory structure instead. By default, bash follows the logical
chain of directories when performing commands which change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands
executed in a subshell environment. The DEBUG and RETURN traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional
parameters are set to the args, even if some of them begin with a -.
- Signal the end of options, cause all remaining args to be assigned to the positional parameters. The -x and
-v options are turned off. If there are no args, the positional parameters remain unchanged.
The options are off by default unless otherwise noted. Using + rather than - causes these options to be turned off.
The options can also be specified as arguments to an invocation of the shell. The current set of options may be
found in $-. The return status is always true unless an invalid option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed to $1 .... Parameters represented by the numbers $# down to
$#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed.
If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The
return status is greater than zero if n is greater than $# or less than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. With no options, or with the -p option, a list
of all settable options is displayed, with an indication of whether or not each is set. The -p option causes output
to be displayed in a form that may be reused as input. Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status indicates whether the optname is set or unset. If
multiple optname arguments are given with -q, the return status is zero if all optnames are enabled; non-zero
otherwise.
-o Restricts the values of optname to be those defined for the -o option to the set builtin.
If either -s or -u is used with no optname arguments, the display is limited to those options which are set or
unset, respectively. Unless otherwise noted, the shopt options are disabled (unset) by default.
The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or
unsetting options, the return status is zero unless an optname is not a valid shell option.
The list of shopt options is:
autocd If set, a command name that is the name of a directory is executed as if it were the argument to the cd com‐
mand. This option is only used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command that is not a directory is assumed to be the name of a vari‐
able whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory component in a cd command will be corrected. The errors
checked for are transposed characters, a missing character, and one character too many. If a correction is
found, the corrected file name is printed, and the command proceeds. This option is only used by interac‐
tive shells.
checkhash
If set, bash checks that a command found in the hash table exists before trying to execute it. If a hashed
command no longer exists, a normal path search is performed.
checkjobs
If set, bash lists the status of any stopped and running jobs before exiting an interactive shell. If any
jobs are running, this causes the exit to be deferred until a second exit is attempted without an interven‐
ing command (see JOB CONTROL above). The shell always postpones exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each command and, if necessary, updates the values of LINES and
COLUMNS.
cmdhist If set, bash attempts to save all lines of a multiple-line command in the same history entry. This allows
easy re-editing of multi-line commands.
compat31
If set, bash changes its behavior to that of version 3.1 with respect to quoted arguments to the [[ condi‐
tional command's =~ operator.
compat32
If set, bash changes its behavior to that of version 3.2 with respect to locale-specific string comparison
when using the [[ conditional command's < and > operators. Bash versions prior to bash-4.1 use ASCII colla‐
tion and strcmp(3); bash-4.1 and later use the current locale's collation sequence and strcoll(3).
compat40
If set, bash changes its behavior to that of version 4.0 with respect to locale-specific string comparison
when using the [[ conditional command's < and > operators (see previous item) and the effect of interrupting
a command list.
compat41
If set, bash, when in posix mode, treats a single quote in a double-quoted parameter expansion as a special
character. The single quotes must match (an even number) and the characters between the single quotes are
considered quoted. This is the behavior of posix mode through version 4.1. The default bash behavior
remains as in previous versions.
direxpand
If set, bash replaces directory names with the results of word expansion when performing filename comple‐
tion. This changes the contents of the readline editing buffer. If not set, bash attempts to preserve what
the user typed.
dirspell
If set, bash attempts spelling correction on directory names during word completion if the directory name
initially supplied does not exist.
dotglob If set, bash includes filenames beginning with a `.' in the results of pathname expansion.
execfail
If set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the
exec builtin command. An interactive shell does not exit if exec fails.
expand_aliases
If set, aliases are expanded as described above under ALIASES. This option is enabled by default for inter‐
active shells.
extdebug
If set, behavior intended for use by debuggers is enabled:
1. The -F option to the declare builtin displays the source file name and line number corresponding to
each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not
executed.
3. If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine
(a shell function or a shell script executed by the . or source builtins), a call to return is simu‐
lated.
4. BASH_ARGC and BASH_ARGV are updated as described in their descriptions above.
5. Function tracing is enabled: command substitution, shell functions, and subshells invoked with (
command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution, shell functions, and subshells invoked with ( com‐
mand ) inherit the ERR trap.
extglob If set, the extended pattern matching features described above under Pathname Expansion are enabled.
extquote
If set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in double
quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word
completion even if the ignored words are the only possible completions. See SHELL VARIABLES above for a
description of FIGNORE. This option is enabled by default.
globstar
If set, the pattern ** used in a pathname expansion context will match all files and zero or more directo‐
ries and subdirectories. If the pattern is followed by a /, only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard GNU error message format.
histappend
If set, the history list is appended to the file named by the value of the HISTFILE variable when the shell
exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the opportunity to re-edit a failed history substitu‐
tion.
histverify
If set, and readline is being used, the results of history substitution are not immediately passed to the
shell parser. Instead, the resulting line is loaded into the readline editing buffer, allowing further mod‐
ification.
hostcomplete
If set, and readline is being used, bash will attempt to perform hostname completion when a word containing
a @ is being completed (see Completing under READLINE above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to cause that word and all remaining characters on that line to be
ignored in an interactive shell (see COMMENTS above). This option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the last command of a pipeline not executed in the
background in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-line commands are saved to the history with embedded new‐
lines rather than using semicolon separators where possible.
login_shell
The shell sets this option if it is started as a login shell (see INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has been accessed since the last time it was checked, the
message ``The mail in mailfile has been read'' is displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not attempt to search the PATH for possible completions when
completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive fashion when performing pathname expansion (see Path‐
name Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive fashion when performing matching while executing case or
[[ conditional commands.
nullglob
If set, bash allows patterns which match no files (see Pathname Expansion above) to expand to a null string,
rather than themselves.
progcomp
If set, the programmable completion facilities (see Programmable Completion above) are enabled. This option
is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING above. This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted mode (see RESTRICTED SHELL below). The value may
not be changed. This is not reset when the startup files are executed, allowing the startup files to dis‐
cover whether or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when the shift count exceeds the number of positional
parameters.
sourcepath
If set, the source (.) builtin uses the value of PATH to find the directory containing the file supplied as
an argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT signal. A login shell cannot be suspended; the -f
option can be used to override this and force the suspension. The return status is 0 unless the shell is a login
shell and -f is not supplied, or if job control is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expr. Each operator and operand
must be a separate argument. Expressions are composed of the primaries described above under CONDITIONAL EXPRES‐
SIONS. test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of
options.
Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation
depends on the number of arguments; see below. Operator precedence is used when there are five or more arguments.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
If the first argument is !, the expression is true if and only if the second argument is null. If the first
argument is one of the unary conditional operators listed above under CONDITIONAL EXPRESSIONS, the expression
is true if the unary test is true. If the first argument is not a valid unary conditional operator, the
expression is false.
3 arguments
The following conditions are applied in the order listed. If the second argument is one of the binary condi‐
tional operators listed above under CONDITIONAL EXPRESSIONS, the result of the expression is the result of
the binary test using the first and third arguments as operands. The -a and -o operators are considered
binary operators when there are three arguments. If the first argument is !, the value is the negation of
the two-argument test using the second and third arguments. If the first argument is exactly ( and the third
argument is exactly ), the result is the one-argument test of the second argument. Otherwise, the expression
is false.
4 arguments
If the first argument is !, the result is the negation of the three-argument expression composed of the
remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the
rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence using the rules listed above.
When used with test or [, the < and > operators sort lexicographically using ASCII ordering.
times Print the accumulated user and system times for the shell and for processes run from the shell. The return status
is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell receives signal(s) sigspec. If arg is absent (and there
is a single sigspec) or -, each specified signal is reset to its original disposition (the value it had upon
entrance to the shell). If arg is the null string the signal specified by each sigspec is ignored by the shell and
by the commands it invokes. If arg is not present and -p has been supplied, then the trap commands associated with
each sigspec are displayed. If no arguments are supplied or if only -p is given, trap prints the list of commands
associated with each signal. The -l option causes the shell to print a list of signal names and their corresponding
numbers. Each sigspec is either a signal name defined in , or a signal number. Signal names are case
insensitive and the SIG prefix is optional.
If a sigspec is EXIT (0) the command arg is executed on exit from the shell. If a sigspec is DEBUG, the command arg
is executed before every simple command, for command, case command, select command, every arithmetic for command,
and before the first command executes in a shell function (see SHELL GRAMMAR above). Refer to the description of
the extdebug option to the shopt builtin for details of its effect on the DEBUG trap. If a sigspec is RETURN, the
command arg is executed each time a shell function or a script executed with the . or source builtins finishes exe‐
cuting.
If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status, subject to
the following conditions. The ERR trap is not executed if the failed command is part of the command list immedi‐
ately following a while or until keyword, part of the test in an if statement, part of a command executed in a && or
|| list, or if the command's return value is being inverted via !. These are the same conditions obeyed by the
errexit option.
Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored are
reset to their original values in a subshell or subshell environment when one is created. The return status is
false if any sigspec is invalid; otherwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if used as a command name. If the -t option is used,
type prints a string which is one of alias, keyword, function, builtin, or file if name is an alias, shell reserved
word, function, builtin, or disk file, respectively. If the name is not found, then nothing is printed, and an exit
status of false is returned. If the -p option is used, type either returns the name of the disk file that would be
executed if name were specified as a command name, or nothing if ``type -t name'' would not return file. The -P
option forces a PATH search for each name, even if ``type -t name'' would not return file. If a command is hashed,
-p and -P print the hashed value, not necessarily the file that appears first in PATH. If the -a option is used,
type prints all of the places that contain an executable named name. This includes aliases and functions, if and
only if the -p option is not also used. The table of hashed commands is not consulted when using -a. The -f option
suppresses shell function lookup, as with the command builtin. type returns true if all of the arguments are found,
false if any are not found.
ulimit [-HSTabcdefilmnpqrstuvx [limit]]
Provides control over the resources available to the shell and to processes started by it, on systems that allow
such control. The -H and -S options specify that the hard or soft limit is set for the given resource. A hard
limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the
hard limit. If neither -H nor -S is specified, both the soft and hard limits are set. The value of limit can be a
number in the unit specified for the resource or one of the special values hard, soft, or unlimited, which stand for
the current hard limit, the current soft limit, and no limit, respectively. If limit is omitted, the current value
of the soft limit of the resource is printed, unless the -H option is given. When more than one resource is speci‐
fied, the limit name and unit are printed before the value. Other options are interpreted as follows:
-a All current limits are reported
-b The maximum socket buffer size
-c The maximum size of core files created
-d The maximum size of a process's data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and its children
-i The maximum number of pending signals
-l The maximum size that may be locked into memory
-m The maximum resident set size (many systems do not honor this limit)
-n The maximum number of open file descriptors (most systems do not allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be set)
-q The maximum number of bytes in POSIX message queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a single user
-v The maximum amount of virtual memory available to the shell and, on some systems, to its children
-x The maximum number of file locks
-T The maximum number of threads
If limit is given, it is the new value of the specified resource (the -a option is display only). If no option is
given, then -f is assumed. Values are in 1024-byte increments, except for -t, which is in seconds, -p, which is in
units of 512-byte blocks, and -T, -b, -n, and -u, which are unscaled values. The return status is 0 unless an
invalid option or argument is supplied, or an error occurs while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins with a digit, it is interpreted as an octal number; oth‐
erwise it is interpreted as a symbolic mode mask similar to that accepted by chmod(1). If mode is omitted, the cur‐
rent value of the mask is printed. The -S option causes the mask to be printed in symbolic form; the default output
is an octal number. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused
as input. The return status is 0 if the mode was successfully changed or if no mode argument was supplied, and
false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a is supplied, all alias definitions are removed. The
return value is true unless a supplied name is not a defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or function. If no options are supplied, or the -v option is
given, each name refers to a shell variable. Read-only variables may not be unset. If -f is specified, each name
refers to a shell function, and the function definition is removed. Each unset variable or function is removed from
the environment passed to subsequent commands. If any of COMP_WORDBREAKS, RANDOM, SECONDS, LINENO, HISTCMD, FUNC‐
NAME, GROUPS, or DIRSTACK are unset, they lose their special properties, even if they are subsequently reset. The
exit status is true unless a name is readonly.
wait [n ...]
Wait for each specified process and return its termination status. Each n may be a process ID or a job specifica‐
tion; if a job spec is given, all processes in that job's pipeline are waited for. If n is not given, all currently
active child processes are waited for, and the return status is zero. If n specifies a non-existent process or job,
the return status is 127. Otherwise, the return status is the exit status of the last process or job waited for.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted. A
restricted shell is used to set up an environment more controlled than the standard shell. It behaves identically to bash
with the exception that the following are disallowed or not performed:
· changing directories with cd
· setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
· specifying command names containing /
· specifying a file name containing a / as an argument to the . builtin command
· specifying a filename containing a slash as an argument to the -p option to the hash builtin command
· importing function definitions from the shell environment at startup
· parsing the value of SHELLOPTS from the shell environment at startup
· redirecting output using the >, >|, <>, >&, &>, and >> redirection operators
· using the exec builtin command to replace the shell with another command
· adding or deleting builtin commands with the -f and -d options to the enable builtin command
· using the enable builtin command to enable disabled shell builtins
· specifying the -p option to the command builtin command
· turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see COMMAND EXECUTION above), rbash turns off any restric‐
tions in the shell spawned to execute the script.
SEE ALSO
Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
/etc/bash.bashrc
The systemwide per-interactive-shell startup file
/etc/bash.bash.logout
The systemwide login shell cleanup file, executed when a login shell exits
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login shell exits
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet.ramey@case.edu
BUG REPORTS
If you find a bug in bash, you should report it. But first, you should make sure that it really is a bug, and that it
appears in the latest version of bash. The latest version is always available from ftp://ftp.gnu.org/pub/gnu/bash/.
Once you have determined that a bug actually exists, use the bashbug command to submit a bug report. If you have a fix,
you are encouraged to mail that as well! Suggestions and `philosophical' bug reports may be mailed to bug-bash@gnu.org or
posted to the Usenet newsgroup gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into the template it provides for filing a bug report.
Comments and bug reports concerning this manual page should be directed to chet.ramey@case.edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and traditional versions of sh, mostly because of the POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command sequences of the form `a ; b ; c' are not handled gracefully when process suspension is
attempted. When a process is stopped, the shell immediately executes the next command in the sequence. It suffices to
place the sequence of commands between parentheses to force it into a subshell, which may be stopped as a unit.
Array variables may not (yet) be exported.
There may be only one active coprocess at a time.
GNU Bash-4.2 2010 December 28 BASH(1)
|
