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BASH(1) General Commands Manual BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [command_string | file]
COPYRIGHT
Bash is Copyright (C) 1989-2018 by the Free Software Foundation,
Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that exe‐
cutes 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 Stan‐
dard 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, including -o, can be used
as options when the shell is invoked. In addition, bash inter‐
prets the following options when it is invoked:
-c If the -c option is present, then commands are read from
the first non-option argument command_string. If there
are arguments after the command_string, the first argu‐
ment is assigned to $0 and any remaining arguments are
assigned to the positional parameters. The assignment
to $0 sets the name of the shell, which is used in warn‐
ing and error messages.
-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
or when reading input through a pipe.
-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 cur‐
rent 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 invo‐
cation 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-charac‐
ter 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 suc‐
cessfully.
--init-file file
--rcfile file
Execute commands from file instead of 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/pro‐
file 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 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). See SEE ALSO below for a reference to a doc‐
ument that details how posix mode affects bash's behavior.
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires
Produce the list of files that are required for the shell
script to run. This implies '-n' and is subject to the
same limitations as compile time error checking checking;
Command substitutions, Conditional expressions and eval
builtin are not parsed so some dependencies may be missed.
--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 direc‐
tory, and, if no file is found, then the shell searches the direc‐
tories 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
(unless -s is specified) 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 $-
includes i if bash is interactive, allowing a shell script or a
startup file to test this state.
The following paragraphs describe how bash executes its startup
files. If any of the files exist but cannot be read, bash reports
an error. Tildes are expanded in filenames as described below
under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-
interactive shell with the --login option, it first reads and exe‐
cutes commands from the file /etc/profile, if that file exists.
After reading that file, it looks for ~/.bash_profile,
~/.bash_login, and ~/.profile, in that order, and reads and exe‐
cutes commands from the first one that exists and is readable.
The --noprofile option may be used when the shell is started to
inhibit this behavior.
When an interactive login shell exits, or a non-interactive login
shell executes the exit builtin command, bash reads and executes
commands from the files ~/.bash_logout and /etc/bash.bash_logout,
if the files exists.
When an interactive shell that is not a login shell is started,
bash reads and executes commands from ~/.bashrc, if that file
exists. This may be inhibited by using the --norc option. The
--rcfile file option will force bash to read and execute commands
from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value
as the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the
filename.
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible,
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, if that file exists
and is 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
neither rshd nor sshd generally invoke the shell with those
options or allow them to be specified.
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 sup‐
plied, no startup files are read, shell functions are not inher‐
ited 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 newline
control operator
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>
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 command:
! case coproc 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 pipeline 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, command's standard error, in addition to its stan‐
dard output, is connected to command2's standard input through the
pipe; it is shorthand for 2>&1 |. This implicit redirection of
the standard error to the standard output 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 variable 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 new‐
line. In this case, the shell displays the total user and system
time consumed by the shell and its children. The TIMEFORMAT vari‐
able 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). See COMMAND EXECUTION ENVIRONMENT for a
description of a subshell environment. If the lastpipe option is
enabled using the shopt builtin (see the description of shopt
below), the last element of a pipeline may be run by the shell
process.
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 <newline>.
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. These are referred to as asynchronous commands. Commands sep‐
arated by a ; are executed sequentially; the shell waits for each
command to terminate in turn. The return status is the exit sta‐
tus of the last command executed.
AND and OR lists are sequences of one or 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 (success).
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. In most cases a list
in a command's description may be separated from the rest of the
command by one or more newlines, and may be followed by a newline
in place of a semicolon.
(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 metacharacters ( 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; oth‐
erwise 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 expan‐
sion, parameter and variable expansion, arithmetic expan‐
sion, 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 lexicographi‐
cally using the current locale.
When the == and != operators are used, the string to the
right of the operator is considered a pattern and matched
according to the rules described below under Pattern Match‐
ing, as if the extglob shell option were enabled. The =
operator is equivalent to ==. If the nocasematch shell
option is enabled, the match is performed without regard to
the case of alphabetic characters. The return value is 0
if the string matches (==) or does not match (!=) the pat‐
tern, and 1 otherwise. Any part of the pattern may be
quoted to force the quoted portion 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 a POSIX 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 nocasematch shell option is
enabled, the match is performed without regard to the case
of alphabetic characters. Any part of the pattern may be
quoted to force the quoted portion to be matched as a
string. Bracket expressions in regular expressions must be
treated carefully, since normal quoting characters lose
their meanings between brackets. If the pattern is stored
in a shell variable, quoting the variable expansion forces
the entire pattern 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 expression. 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 fol‐
lowing 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 accord‐
ing 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 expres‐
sion 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 executed 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 matching rules
described under Pattern Matching below. The word is
expanded using tilde expansion, parameter and variable
expansion, arithmetic expansion, command substitution,
process substitution and quote removal. Each pattern exam‐
ined is expanded using tilde expansion, parameter and vari‐
able expansion, arithmetic expansion, command substitution,
and process substitution. If the nocasematch shell option
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 pat‐
terns. 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 successful 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 corre‐
sponding 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 exe‐
cuted 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 coprocess 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 exe‐
cuting 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 executing shell, and that file descriptor is
assigned to NAME[1]. This pipe is established before any redirec‐
tions specified by the command (see REDIRECTION below). The file
descriptors can be utilized as arguments to shell commands and
redirections using standard word expansions. Other than those
created to execute command and process substitutions, the file
descriptors are not available in subshells. 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.
Since the coprocess is created as an asynchronous command, the
coproc command always returns success. 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 positional
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
supplied, 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, with one exception: If the
function reserved word is used, but the parentheses are not
supplied, the braces are required. compound-command is
executed whenever name is specified as the name of a simple
command. When in posix mode, name may not be the name of
one of the POSIX special builtins. 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 command 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 charac‐
ters or words to the shell. Quoting can be used to disable spe‐
cial 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 spe‐
cial 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 character, 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 <newline>. If a \<newline> pair appears, and the
backslash is not itself quoted, the \<newline> 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, !. When the shell is
in posix mode, the ! has no special meaning within double quotes,
even 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 fol‐
lowing characters: $, `, ", \, or <newline>. A double quote may
be quoted within double quotes by preceding it with a backslash.
If enabled, history expansion 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 $'string' are treated specially. The word
expands to string, with backslash-escaped characters replaced as
specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\? question mark
\nnn the eight-bit character whose value is the octal
value nnn (one to three octal digits)
\xHH the eight-bit character whose value is the hexadeci‐
mal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHHHHHH (one to eight hex
digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had
not been present.
A double-quoted string preceded by a dollar sign ($"string") will
cause the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign is ignored.
If the string is translated and replaced, the replacement is dou‐
ble-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 Spe‐
cial 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 COM‐
MANDS 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 variable expan‐
sion, 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 $((...)) expansion is not used (see Arithmetic Expan‐
sion below). Word splitting is not performed, with the exception
of "$@" 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 (declaration commands). When in posix
mode, these builtins may appear in a command after one or more
instances of the command builtin and retain these assignment
statement properties.
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. This includes
arguments to builtin commands such as declare that accept assign‐
ment statements (declaration commands). 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.
A variable can be assigned the nameref attribute using the -n
option to the declare or local builtin commands (see the descrip‐
tions of declare and local below) to create a nameref, or a refer‐
ence to another variable. This allows variables to be manipulated
indirectly. Whenever the nameref variable is referenced, assigned
to, unset, or has its attributes modified (other than using or
changing the nameref attribute itself), the operation is actually
performed on the variable specified by the nameref variable's
value. A nameref is commonly used within shell functions to refer
to a variable whose name is passed as an argument to the function.
For instance, if a variable name is passed to a shell function as
its first argument, running
declare -n ref=$1
inside the function creates a nameref variable ref whose value is
the variable name passed as the first argument. References and
assignments to ref, and changes to its attributes, are treated as
references, assignments, and attribute modifications to the vari‐
able whose name was passed as $1. If the control variable in a
for loop has the nameref attribute, the list of words can be a
list of shell variables, and a name reference will be established
for each word in the list, in turn, when the loop is executed.
Array variables cannot be given the nameref attribute. However,
nameref variables can reference array variables and subscripted
array variables. Namerefs can be unset using the -n option to the
unset builtin. Otherwise, if unset is executed with the name of a
nameref variable as an argument, the variable referenced by the
nameref variable will be unset.
Positional Parameters
A positional parameter is a parameter denoted by one or more dig‐
its, other than the single digit 0. Positional parameters are
assigned from the shell's arguments when it is invoked, and may be
reassigned using the set builtin command. Positional parameters
may not be assigned to with assignment statements. The positional
parameters are temporarily replaced when a shell function is exe‐
cuted (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit
is expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters
may only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one.
When the expansion is not within double quotes, each posi‐
tional parameter expands to a separate word. In contexts
where it is performed, those words are subject to further
word splitting and pathname expansion. When the expansion
occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first
character of the IFS special variable. That is, "$*" is
equivalent to "$1c$2c...", where c is the first character
of the value of the IFS variable. If IFS is unset, the
parameters are separated by spaces. If IFS is null, the
parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one.
In contexts where word splitting is performed, this expands
each positional parameter to a separate word; if not within
double quotes, these words are subject to word splitting.
In contexts where word splitting is not performed, this
expands to a single word with each positional parameter
separated by a space. When the expansion occurs within
double quotes, each parameter expands to a separate word.
That is, "$@" is equivalent to "$1" "$2" ... If the dou‐
ble-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter
is joined with the last part of the original word. When
there are no positional parameters, "$@" 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 invo‐
cation, 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 job most recently placed
into the background, whether executed as an asynchronous
command or using the bg builtin (see JOB CONTROL below).
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 filename 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 simple command exe‐
cuted in the foreground, after expansion. 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 filename 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.
Assignments to BASHPID have no effect. If BASHPID is
unset, it loses its special properties, even if it is sub‐
sequently reset.
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; however, unsetting array elements currently does not
cause aliases to be removed from the alias list. If
BASH_ALIASES is unset, it loses its special properties,
even if it is subsequently reset.
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). Setting extdebug after the shell has started to
execute a script, or referencing this variable when extde‐
bug is not set, may result in inconsistent values.
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). Setting
extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set, may
result in inconsistent values.
BASH_ARGV0
When referenced, this variable expands to the name of the
shell or shell script (identical to $0; see the description
of special parameter 0 above). Assignment to BASH_ARGV0
causes the value assigned to also be assigned to $0. If
BASH_ARGV0 is unset, it loses its special properties, even
if it is subsequently reset.
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; however, unsetting array elements currently
does not cause command names to be removed from the hash
table. If BASH_CMDS is unset, it loses its special proper‐
ties, even if it is subsequently reset.
BASH_COMMAND
The command currently being executed or about to be exe‐
cuted, 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_LOADABLES_PATH
A colon-separated list of directories in which the shell
looks for dynamically loadable builtins specified by the
enable command.
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 subex‐
pression. 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 within each subshell or subshell envi‐
ronment when the shell begins executing in that environ‐
ment. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version infor‐
mation 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 ver‐
sion).
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 cur‐
rent 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 Com‐
pletion 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 Pro‐
grammable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of com‐
pletion attempted that caused a completion function to be
called: TAB, for normal completion, ?, for listing comple‐
tions after successive tabs, !, for listing alternatives on
partial word completion, @, to list completions if the word
is not unmodified, or %, for menu completion. This vari‐
able is available only in shell functions and external com‐
mands invoked by the programmable completion facilities
(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 program‐
mable 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.
EPOCHREALTIME
Each time this parameter is referenced, it expands to the
number of seconds since the Unix Epoch (see time(3)) as a
floating point value with micro-second granularity.
Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME
is unset, it loses its special properties, even if it is
subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the
number of seconds since the Unix Epoch (see time(3)).
Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
EUID Expands to the effective user ID of the current user, ini‐
tialized at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell func‐
tions 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. If FUNCNAME is unset, it loses its special
properties, even if it is subsequently 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. If GROUPS is unset, it loses its special prop‐
erties, even if it is subsequently 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 substi‐
tutes 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 sys‐
tem type on which bash is executing, in the standard GNU
cpu-company-system format. The default is system-depen‐
dent.
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 system-
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 sin‐
gle 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 sec‐
onds 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_COMPAT
The value is used to set the shell's compatibility level.
See the description of the shopt builtin below under SHELL
BUILTIN COMMANDS for a description of the various compati‐
bility levels and their effects. The value may be a deci‐
mal number (e.g., 4.2) or an integer (e.g., 42) correspond‐
ing to the desired compatibility level. If BASH_COMPAT is
unset or set to the empty string, the compatibility level
is set to the default for the current version. If
BASH_COMPAT is set to a value that is not one of the valid
compatibility levels, the shell prints an error message and
sets the compatibility level to the default for the current
version. The valid compatibility levels correspond to the
compatibility options accepted by the shopt builtin
described below (for example, compat42 means that 4.2 and
42 are valid values). The current version is also a valid
value.
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 filename. PATH is not used to search for
the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descrip‐
tor, 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-sepa‐
rated list of directories in which the shell looks for des‐
tination directories specified by the cd command. A sample
value is ".:~:/usr".
CHILD_MAX
Set the number of exited child status values for the shell
to remember. Bash will not allow this value to be
decreased below a POSIX-mandated minimum, and there is a
maximum value (currently 8192) that this may not exceed.
The minimum value is system-dependent.
COLUMNS
Used by the select compound command to determine the termi‐
nal width when printing selection lists. Automatically set
if the checkwinsize option is enabled or in an interactive
shell upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible com‐
pletions generated by a shell function invoked by the pro‐
grammable completion facility (see Programmable Completion
below). Each array element contains one possible comple‐
tion.
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.
EXECIGNORE
A colon-separated list of shell patterns (see Pattern
Matching) defining the list of filenames to be ignored by
command search using PATH. Files whose full pathnames
match one of these patterns are not considered executable
files for the purposes of completion and command execution
via PATH lookup. This does not affect the behavior of the
[, test, and [[ commands. Full pathnames in the command
hash table are not subject to EXECIGNORE. Use this vari‐
able to ignore shared library files that have the exe‐
cutable bit set, but are not executable files. The pattern
matching honors the setting of the extglob shell option.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when perform‐
ing 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:~".
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 file
names to be ignored by pathname expansion. If a file name
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 char‐
acter 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 a 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, to contain no more than that num‐
ber of lines by removing the oldest entries. The history
file is also truncated to this size after writing it when a
shell exits. If the value is 0, the history file is trun‐
cated to zero size. Non-numeric values and numeric values
less than zero inhibit truncation. The shell sets the
default value to the value of HISTSIZE after reading any
startup files.
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. The pattern match‐
ing honors the setting of the extglob shell option.
HISTSIZE
The number of commands to remember in the command history
(see HISTORY below). If the value is 0, commands are not
saved in the history list. Numeric values less than zero
result in every command being saved on the history list
(there is no limit). The shell sets the default value to
500 after reading any startup files.
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 written
to the history file so they may be preserved across shell
sessions. This uses the history comment character to dis‐
tinguish timestamps from other history lines.
HOME The home directory of the current user; the default argu‐
ment for the cd builtin command. The value of this vari‐
able 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 com‐
plete a hostname. The list of possible hostname comple‐
tions may be changed while the shell is running; the next
time hostname 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 comple‐
tions. When HOSTFILE is unset, the hostname list is
cleared.
IFS The Internal Field Separator that is used for word split‐
ting after expansion and to split lines into words with the
read builtin command. The default value is
``<space><tab><newline>''.
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).
INSIDE_EMACS
If this variable appears in the environment when the shell
starts, bash assumes that it is running inside an Emacs
shell buffer and may disable line editing, depending on the
value of TERM.
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 dou‐
ble-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for num‐
ber formatting.
LC_TIME
This variable determines the locale category used for data
and time formatting.
LINES Used by the select compound command to determine the column
length for printing selection lists. Automatically set if
the checkwinsize option is enabled or in an interactive
shell 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 filenames to be checked for mail.
The message to be printed when mail arrives in a particular
file may be specified by separating the filename 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 can be configured to supply a default value for this
variable (there is no value by default), 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 gener‐
ated 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 sys‐
tem-dependent, and is set by the administrator who installs
bash. A common value is
``/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin''.
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 sup‐
plied. If it is set while the shell is running, bash
enables posix mode, as if the command set -o posix had been
executed. When the shell enters posix mode, it sets this
variable if it was not already set.
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 PROMPT‐
ING below). Characters removed are replaced with an ellip‐
sis.
PS0 The value of this parameter is expanded (see PROMPTING
below) and displayed by interactive shells after reading a
command and before the command is executed.
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 during
an execution trace. The first character of the expanded
value 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 pre‐
fixed 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 specified;
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 min‐
utes, 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\t%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 sec‐
onds when input is coming from a terminal. In an interac‐
tive shell, the value is interpreted as the number of sec‐
onds to wait for a line of input after issuing the primary
prompt. Bash terminates after waiting for that number of
seconds if a complete line of 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 candi‐
dates for resumption of an existing stopped job. There is
no ambiguity allowed; if there is more than one job begin‐
ning 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 char‐
acter, which is used as shorthand for re-running the previ‐
ous command entered, substituting one string for another in
the command. The default is `^'. The optional third char‐
acter 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 comment.
Arrays
Bash provides one-dimensional indexed and associative array vari‐
ables. 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.
Unless otherwise noted, indexed array indices must be non-negative
integers.
An indexed array is created automatically if any variable is
assigned to using the syntax name[subscript]=value. The subscript
is treated as an arithmetic expression that must evaluate to a
number. To explicitly declare an indexed array, use declare -a
name (see SHELL BUILTIN COMMANDS below). declare -a name[sub‐
script] 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 mem‐
bers of an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each value is of the form [sub‐
script]=string. Indexed array assignments do not require anything
but string. 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[subscript]=value
syntax introduced above. When assigning to an indexed array, if
name is subscripted by a negative number, that number is inter‐
preted as relative to one greater than the maximum index of name,
so negative indices count back from the end of the array, and an
index of -1 references the last element.
Any element of an array may be referenced using ${name[sub‐
script]}. 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 vari‐
able, 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 expansion 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[subscript]}. If subscript is * or @, the expansion is the
number of elements in the array. If the subscript used to refer‐
ence an element of an indexed array evaluates to a number less
than zero, it is interpreted as relative to one greater than the
maximum index of the array, so negative indices count back from
the end of the array, and an index of -1 references the last ele‐
ment.
Referencing an array variable without a subscript is equivalent to
referencing the array with a subscript of 0. Any reference to a
variable using a valid subscript is legal, and bash will create an
array if necessary.
An array variable is considered set if a subscript has been
assigned a value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as
the values. ${!name[@]} and ${!name[*]} expand to the indices
assigned in array variable name. The treatment when in double
quotes is similar to the expansion of the special parameters @ and
* within double quotes.
The unset builtin is used to destroy arrays. unset name[sub‐
script] destroys the array element at index subscript, for both
indexed and associative arrays. Negative subscripts to indexed
arrays are interpreted as described above. Unsetting the last
element of an array variable does not unset the variable. unset
name, where name is an array, or unset name[subscript], where sub‐
script is * or @, removes the entire array.
When using a variable name with a subscript as an argument to a
command, such as with unset, without using the word expansion syn‐
tax described above, the argument is subject to pathname expan‐
sion. If pathname expansion is not desired, the argument should
be quoted.
The declare, local, and readonly builtins each accept a -a option
to specify an indexed array and a -A option to specify an associa‐
tive 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 performed: brace
expansion, tilde expansion, parameter and variable expansion, com‐
mand substitution, arithmetic expansion, word splitting, and path‐
name expansion.
The order of expansions is: brace expansion; tilde expansion,
parameter and variable expansion, arithmetic expansion, and com‐
mand substitution (done in a left-to-right fashion); word split‐
ting; and pathname expansion.
On systems that can support it, there is an additional expansion
available: process substitution. This is performed at the same
time as tilde, parameter, variable, and arithmetic expansion and
command substitution.
After these expansions are performed, quote characters present in
the original word are removed unless they have been quoted them‐
selves (quote removal).
Only brace expansion, word splitting, and pathname expansion can
increase the number of words of the expansion; other expansions
expand a single word to a single word. The only exceptions to
this are the expansions of "$@" and "${name[@]}", and, in most
cases, $* 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 expansion, 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 expres‐
sion 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 necessary. When characters are sup‐
plied, the expression expands to each character lexicographically
between x and y, inclusive, using the default C locale. Note that
both x and y must be of the same type. When the increment is sup‐
plied, 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 syntac‐
tic 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 backslash to pre‐
vent its being considered part of a brace expression. To avoid
conflicts with parameter expansion, the string ${ is not consid‐
ered eligible for brace expansion, and inhibits brace expansion
until the closing }.
This construct is typically used as shorthand when the common pre‐
fix 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 histori‐
cal 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 charac‐
ters, if there is no unquoted slash) are considered a tilde-pre‐
fix. 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 corresponding element from the direc‐
tory 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 lead‐
ing `+' 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 filenames
with tildes in assignments to PATH, MAILPATH, and CDPATH, and the
shell assigns the expanded value.
Bash also performs tilde expansion on words satisfying the condi‐
tions of variable assignments (as described above under PARAME‐
TERS) when they appear as arguments to simple commands. Bash does
not do this, except for the declaration commands listed above,
when in posix mode.
Parameter Expansion
The `$' character introduces parameter expansion, command substi‐
tution, 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 imme‐
diately 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 charac‐
ter which is not to be interpreted as part of its name.
The parameter is a shell parameter as described above
PARAMETERS) or an array reference (Arrays).
If the first character of parameter is an exclamation point (!),
and parameter is not a nameref, it introduces a level of indirec‐
tion. Bash uses the value formed by expanding the rest of parame‐
ter as the new parameter; this is then expanded and that value is
used in the rest of the expansion, rather than the expansion of
the original parameter. This is known as indirect expansion. The
value is subject to tilde expansion, parameter expansion, command
substitution, and arithmetic expansion. If parameter is a
nameref, this expands to the name of the parameter referenced by
parameter instead of performing the complete indirect expansion.
The exceptions to this are the expansions of ${!prefix*} and
${!name[@]} described below. The exclamation point must immedi‐
ately 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 expan‐
sion.
When not performing substring expansion, using the forms docu‐
mented below (e.g., :-), bash tests for a parameter that is unset
or null. Omitting the colon results in a test only for a parame‐
ter 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, noth‐
ing is substituted, otherwise the expansion of word is sub‐
stituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of
the value of parameter starting at the character specified
by offset. If parameter is @, an indexed array subscripted
by @ or *, or an associative array name, the results differ
as described below. If length is omitted, expands to the
substring of the value of parameter starting at the charac‐
ter specified by offset and extending to the end of the
value. 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 in characters from the end of the
value of parameter. If length evaluates to a number less
than zero, it is interpreted as an offset in characters
from the end of the value of parameter rather than a number
of characters, and the expansion is the characters between
offset and that result. Note that a negative offset must
be separated from the colon by at least one space to avoid
being confused with the :- expansion.
If parameter is @, the result is length positional parame‐
ters beginning at offset. A negative offset is taken rela‐
tive to one greater than the greatest positional parameter,
so an offset of -1 evaluates to the last positional parame‐
ter. It is an expansion error if length evaluates to a
number less than zero.
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 rel‐
ative to one greater than the maximum index of the speci‐
fied array. It is an expansion error if length evaluates
to a number less than zero.
Substring expansion applied to an associative array pro‐
duces undefined results.
Substring indexing is zero-based unless the positional
parameters are used, in which case the indexing 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 char‐
acter 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.
If parameter is an indexed array name subscripted by a neg‐
ative number, that number is interpreted as relative to one
greater than the maximum index of parameter, so negative
indices count back from the end of the array, and an index
of -1 references the last element.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to
produce a pattern just as in pathname expansion, and
matched against the expanded value of parameter using the
rules described under Pattern Matching below. If the pat‐
tern matches the beginning of the value of parameter, then
the result of the expansion is the expanded value of param‐
eter 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 oper‐
ation 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, and
matched against the expanded value of parameter using the
rules described under Pattern Matching below. If the pat‐
tern 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. The match is performed using the
rules described under Pattern Matching below. 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 parame‐
ter. If string is null, matches of pattern are deleted and
the / following pattern may be omitted. If the nocasematch
shell option is enabled, the match is performed without
regard to the case of alphabetic characters. 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 sub‐
scripted with @ or *, the substitution 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 expan‐
sion. Each character in the expanded value of parameter is
tested against pattern, and, if it matches the pattern, its
case is converted. The pattern should not attempt to match
more than one character. The ^ operator converts lowercase
letters matching pattern to uppercase; the , operator con‐
verts 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 resultant list. If parameter is
an array variable subscripted with @ or *, the case modifi‐
cation operation is applied to each member of the array in
turn, and the expansion is the resultant list.
${parameter@operator}
Parameter transformation. The expansion is either a trans‐
formation of the value of parameter or information about
parameter itself, depending on the value of operator. Each
operator is a single letter:
Q The expansion is a string that is the value of
parameter quoted in a format that can be reused as
input.
E The expansion is a string that is the value of
parameter with backslash escape sequences expanded
as with the $'...' quoting mechanism.
P The expansion is a string that is the result of
expanding the value of parameter as if it were a
prompt string (see PROMPTING below).
A The expansion is a string in the form of an assign‐
ment statement or declare command that, if evalu‐
ated, will recreate parameter with its attributes
and value.
a The expansion is a string consisting of flag values
representing parameter's attributes.
If parameter is @ or *, the operation is applied to each
positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable sub‐
scripted with @ or *, the operation is applied to each mem‐
ber of the array in turn, and the expansion is the resul‐
tant list.
The result of the expansion is subject to word splitting
and pathname expansion as described below.
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 in a subshell
environment and replacing the command substitution with the stan‐
dard output of the command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting. The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, back‐
slash 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 charac‐
ters between the parentheses make up the command; none are treated
specially.
Command substitutions may be nested. To nest when using the back‐
quoted 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 expression is treated as if it were within double quotes, but
a double quote inside the parentheses is not treated specially.
All tokens in the expression undergo parameter and variable expan‐
sion, command substitution, and quote removal. The result is
treated as the arithmetic expression to be evaluated. 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 allows a process's input or output to be
referred to using a filename. It takes the form of <(list) or
>(list). The process list is run asynchronously, and its input or
output appears as a filename. This filename is passed as an argu‐
ment 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. Process
substitution is supported on systems that support named pipes
(FIFOs) or the /dev/fd method of naming open files.
When available, process substitution is performed simultaneously
with parameter and variable expansion, command substitution, and
arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command sub‐
stitution, and arithmetic expansion that did not occur within dou‐
ble quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits
the results of the other expansions into words using these charac‐
ters as field terminators. If IFS is unset, or its value is
exactly <space><tab><newline>, the default, then sequences of
<space>, <tab>, and <newline> at the beginning and end of the
results of the previous expansions are ignored, and any sequence
of IFS characters 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, tab, and newline are ignored
at the beginning and end of the word, as long as the whitespace
character 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 delimiter. If
the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained and passed to com‐
mands as empty strings. Unquoted implicit null arguments, result‐
ing from the expansion of parameters that have no values, are
removed. If a parameter with no value is expanded within double
quotes, a null argument results and is retained and passed to a
command as an empty string. When a quoted null argument appears
as part of a word whose expansion is non-null, the null argument
is removed. That is, the word -d'' becomes -d after word split‐
ting and null argument removal.
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 filenames matching
the pattern (see Pattern Matching below). 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. 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. The filenames ``.'' and ``..''
must always be matched explicitly, even if dotglob is set. In
other cases, the ``.'' character is not treated specially. When
matching a pathname, the slash character must always be matched
explicitly by a slash in the pattern, but in other matching con‐
texts it can be matched by a special pattern character as
described below under Pattern Matching. 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 match‐
ing file name that also matches one of the patterns in GLOBIGNORE
is removed from the list of matches. If the nocaseglob option is
set, the matching against the patterns in GLOBIGNORE is performed
without regard to case. The filenames ``.'' and ``..'' are
always ignored when GLOBIGNORE is set and not null. However, set‐
ting GLOBIGNORE to a non-null value has the effect of enabling the
dotglob shell option, so all other filenames beginning with a
``.'' will match. To get the old behavior of ignoring filenames
beginning with a ``.'', make ``.*'' one of the patterns in GLO‐
BIGNORE. The dotglob option is disabled when GLOBIGNORE is unset.
The pattern matching honors the setting of the extglob shell
option.
Pattern Matching
Any character that appears in a pattern, other than the special
pattern characters described below, matches itself. The NUL char‐
acter may not occur in a pattern. A backslash escapes the follow‐
ing 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 subdirectories.
? 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 falls between those
two characters, inclusive, using the current
locale's collating 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 values of the LC_COLLATE or LC_ALL shell vari‐
ables, if set. To obtain the traditional interpre‐
tation of range expressions, where [a-d] is equiva‐
lent to [abcd], set value of the LC_ALL shell vari‐
able to C, or enable the globasciiranges shell
option. 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 following 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 let‐
ters, digits, and the character _.
Within [ and ], an equivalence class can be speci‐
fied using the syntax [=c=], which matches all char‐
acters 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 pat‐
terns
+(pattern-list)
Matches one or more occurrences of the given pat‐
terns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Complicated extended pattern matching against long strings is
slow, especially when the patterns contain alternations and the
strings contain multiple matches. Using separate matches against
shorter strings, or using arrays of strings instead of a single
long string, may be faster.
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 redi‐
rected using a special notation interpreted by the shell. Redi‐
rection allows commands' file handles to be duplicated, opened,
closed, made to refer to different files, and can change the files
the command reads from and writes to. Redirection may also be
used to modify file handles in the current shell execution envi‐
ronment. The following redirection 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 or equal to 10 and
assign it to varname. If >&- or <&- is preceded by {varname}, the
value of varname defines the file descriptor to close. If {var‐
name} is supplied, the redirection persists beyond the scope of
the command, allowing the shell programmer to manage the file
descriptor himself.
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is <,
the redirection refers to the standard input (file descriptor 0).
If the first character of the redirection operator is >, the redi‐
rection 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 expan‐
sion, tilde expansion, parameter and variable 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. If the operat‐
ing system on which bash is running provides these special files,
bash will use them; otherwise it will emulate them internally with
the behavior described below.
/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 the corresponding TCP 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 the corresponding UDP 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.
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
Redirecting Output
Redirection of output causes the file whose name results from the
expansion of word to be opened for writing 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; if it does exist it is
truncated to zero size.
The general format for redirecting output 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 redirect‐
ion operator is > and the noclobber option to the set builtin com‐
mand 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
When using the second form, word may not expand to a number or -.
If it does, other redirection operators apply (see Duplicating
File Descriptors below) for compatibility reasons.
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
(see Duplicating File Descriptors below).
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 (or file descriptor n if n is
specified) for a command.
The format of here-documents is:
[n]<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution, arith‐
metic expansion, or pathname expansion is performed on word. If
any part of word is 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 substitution, and arith‐
metic expansion, the character sequence \<newline> is ignored, and
\ must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab charac‐
ters are stripped from input lines and the line containing delim‐
iter. This allows here-documents within shell scripts to be
indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
[n]<<<word
The word undergoes tilde expansion, parameter and variable expan‐
sion, command substitution, arithmetic expansion, and quote
removal. Pathname expansion and word splitting are not performed.
The result is supplied as a single string, with a newline
appended, to the command on its standard input (or file descriptor
n if n is specified).
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to
one or more digits, the file descriptor denoted by n is made to be
a copy of that file descriptor. If the digits in word do not
specify a file descriptor open for input, a redirection error
occurs. If word evaluates to -, file descriptor n is closed. If
n is not specified, the standard input (file descriptor 0) is
used.
The operator
[n]>&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. If word evaluates to -, file
descriptor n is closed. As a special case, if n is omitted, and
word does not expand to one or more digits or -, the standard out‐
put and standard error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the stan‐
dard 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 stan‐
dard 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 maintains a
list of aliases that may be set and unset with the alias and una‐
lias 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 expan‐
sion.
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 some‐
what confusing. Bash always reads at least one complete line of
input, and all lines that make up a compound command, before exe‐
cuting any of the commands on that line or the compound command.
Aliases are expanded when a command is read, not when it is exe‐
cuted. 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 exe‐
cuted, because a function definition is itself a 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 func‐
tions.
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. Func‐
tions are executed in the context of the current shell; no new
process is created to interpret them (contrast this with the exe‐
cution of a shell script). When a function is executed, the argu‐
ments 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 executing.
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 func‐
tion has been given the trace attribute (see the description of
the declare builtin below) or the -o functrace 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. If a variable is
declared local, the variable's visible scope is restricted to that
function and its children (including the functions it calls).
Local variables "shadow" variables with the same name declared at
previous scopes. For instance, a local variable declared in a
function hides a global variable of the same name: references and
assignments refer to the local variable, leaving the global vari‐
able unmodified. When the function returns, the global variable
is once again visible.
The shell uses dynamic scoping to control a variable's visibility
within functions. With dynamic scoping, visible variables and
their values are a result of the sequence of function calls that
caused execution to reach the current function. The value of a
variable that a function sees depends on its value within its
caller, if any, whether that caller is the "global" scope or
another shell function. This is also the value that a local vari‐
able declaration "shadows", and the value that is restored when
the function returns.
For example, if a variable var is declared as local in function
func1, and func1 calls another function func2, references to var
made from within func2 will resolve to the local variable var from
func1, shadowing any global variable named var.
The unset builtin also acts using the same dynamic scope: if a
variable is local to the current scope, unset will unset it; oth‐
erwise the unset will refer to the variable found in any calling
scope as described above. If a variable at the current local
scope is unset, it will remain so until it is reset in that scope
or until the function returns. Once the function returns, any
instance of the variable at a previous scope will become visible.
If the unset acts on a variable at a previous scope, any instance
of a variable with that name that had been shadowed will become
visible.
The FUNCNEST variable, if set to a numeric value greater than 0,
defines a maximum function nesting level. Function invocations
that exceed the limit cause the entire command to abort.
If the builtin command return is executed in a function, the func‐
tion 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 function completes, the
values of the positional parameters and the special parameter #
are restored to the values they had prior to the function's execu‐
tion.
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 automati‐
cally 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.
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 commands,
the (( compound command, 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
- + unary minus and plus
++id --id
variable pre-increment and pre-decrement
! ~ 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 expres‐
sion, 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 vari‐
able which has been given the integer attribute using declare -i
is assigned a value. A null value evaluates to 0. A shell vari‐
able need not have its integer 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. When spec‐
ifying n, the digits greater than 9 are represented by the lower‐
case 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 per‐
form string and arithmetic comparisons. The test abd [ commands
determine their behavior based on the number of arguments; see the
descriptions of those commands for any other command-specific
actions.
Expressions are formed from the following unary or binary pri‐
maries. Bash handles several filenames specially when they are
used in expressions. If the operating system on which bash is
running provides these special files, bash will use them; other‐
wise it will emulate them internally with this behavior: 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 termi‐
nal.
-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).
-R varname
True if the shell variable varname is set and is a name
reference.
-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. When used with the [[
command, this performs pattern matching as described above
(Compound Commands).
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 arith‐
metic 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. When used with
the [[ command, Arg1 and Arg2 are evaluated as arithmetic
expressions (see ARITHMETIC EVALUATION above).
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the follow‐
ing expansions, assignments, and redirections, from left to right.
1. The words that the parser has marked as variable assign‐
ments (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 REDI‐
RECTION.
4. The text after the = in each variable assignment undergoes
tilde expansion, parameter expansion, command substitution,
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 cur‐
rent shell environment. If any of the assignments 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 pro‐
ceeds as described below. Otherwise, the command exits. If one
of the expansions contained a command substitution, the exit sta‐
tus of the command is the exit status of the last command substi‐
tution 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 sim‐
ple command and an optional list of arguments, the following
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 con‐
tains 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 executable 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_han‐
dle. If that function exists, it is invoked in a separate execu‐
tion environment 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 that subshell. 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 sepa‐
rate execution environment. Argument 0 is set to the name given,
and the remaining arguments to the command are set to the argu‐
ments given, if any.
If this execution fails because the file is not in executable for‐
mat, 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 locations 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 fol‐
lowing:
· open files inherited by the shell at invocation, as modi‐
fied 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 environment
that consists of the following. Unless otherwise noted, the val‐
ues are inherited from the shell.
· the shell's open files, plus any modifications and addi‐
tions 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 environ‐
ment
· 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 asyn‐
chronous 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 creates 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 aug‐
mented 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 com‐
mand, not just those that precede the command name.
When bash invokes an external command, the variable _ is set to
the full filename of the command and passed to that command 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 statuses
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 cer‐
tain circumstances, the shell will use special 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 termi‐
nates 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 exe‐
cutable, the return status is 126.
If a command fails because of an error during expansion or redi‐
rection, 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 execute. All
builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.
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 inter‐
ruptible). 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. Commands run as a result
of command substitution ignore the keyboard-generated job control
signals SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exit‐
ing, 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 sending 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 executed 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 immedi‐
ately 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 execution
at a later point. A user typically employs this facility via an
interactive interface supplied jointly by the operating 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 back‐
ground), 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 con‐
trol, the operating system maintains the notion of a current ter‐
minal 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 foreground. Background pro‐
cesses are those whose process group ID differs from the termi‐
nal'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 con‐
trol, bash contains facilities to use it. Typing the suspend
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 contain‐
ing 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 back‐
ground. 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 pre‐
vious job with a -. A single % (with no accompanying job specifi‐
cation) 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. Nor‐
mally, 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.
When the shell is waiting for a job or process using the wait
builtin, and job control is enabled, wait will return when the job
changes state. The -f option will force wait to wait until the job
or process terminates before returning.
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 displays PS0
after it reads a command but before executing it. Bash displays
PS4 as described above before tracing each command when the -x
option is enabled. Bash allows these prompt strings to be custom‐
ized 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 por‐
tion 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 abbrevi‐
ated with a tilde (uses the value of the PROMPT_DIR‐
TRIM 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 ses‐
sion. 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 simi‐
lar 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 COM‐
MANDS below). To turn off line editing after the shell is run‐
ning, 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 key‐
strokes. Control keys are denoted by C-key, e.g., C-n means Con‐
trol-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 pre‐
fix. The combination M-C-x means ESC-Control-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. Com‐
mands 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 con‐
structs allowed in the readline initialization file. Blank lines
are ignored. Lines beginning with a # are comments. Lines begin‐
ning 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 commands
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 symbolic 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-argu‐
ment, 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 dif‐
fers 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 recog‐
nized.
"\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 univer‐
sal-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 hexadeci‐
mal 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 state‐
ment 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 ter‐
minal 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 charac‐
ters treated specially by the kernel's terminal driver to
their readline equivalents.
blink-matching-paren (Off)
If set to On, readline attempts to briefly move the cursor
to an opening parenthesis when a closing parenthesis is
inserted.
colored-completion-prefix (Off)
If set to On, when listing completions, readline displays
the common prefix of the set of possible completions using
a different color. The color definitions are taken from
the value of the LS_COLORS environment variable.
colored-stats (Off)
If set to On, readline displays possible completions using
different colors to indicate their file type. The color
definitions are taken from the value of the LS_COLORS envi‐
ronment variable.
comment-begin (``#'')
The string that is inserted when the readline insert-com‐
ment command is executed. This command is bound to M-# in
emacs mode and to # in vi command mode.
completion-display-width (-1)
The number of screen columns used to display possible
matches when performing completion. The value is ignored
if it is less than 0 or greater than the terminal screen
width. A value of 0 will cause matches to be displayed one
per line. The default value is -1.
completion-ignore-case (Off)
If set to On, readline performs filename matching and com‐
pletion in a case-insensitive fashion.
completion-map-case (Off)
If set to On, and completion-ignore-case is enabled, read‐
line treats hyphens (-) and underscores (_) as equivalent
when performing case-insensitive filename matching and com‐
pletion.
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 pre‐
fixes 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 pos‐
sible 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 ter‐
minal.
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). The default is On, but
readline will set it to Off if the locale contains eight-
bit characters.
disable-completion (Off)
If set to On, readline will inhibit word completion. Com‐
pletion characters will be inserted into the line as if
they had been mapped to self-insert.
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.
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.
emacs-mode-string (@)
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the pri‐
mary prompt when emacs editing mode is active. The value
is expanded like a key binding, so the standard set of
meta- and control prefixes and backslash escape sequences
is available. Use the \1 and \2 escapes to begin and end
sequences of non-printing characters, which can be used to
embed a terminal control sequence into the mode string.
enable-bracketed-paste (Off)
When set to On, readline will configure the terminal in a
way that will enable it to insert each paste into the edit‐
ing buffer as a single string of characters, instead of
treating each character as if it had been read from the
keyboard. This can prevent pasted characters from being
interpreted as editing commands.
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 modi‐
fier 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 (unset)
Set the maximum number of history entries saved in the his‐
tory list. If set to zero, any existing history entries
are deleted and no new entries are saved. If set to a
value less than zero, the number of history entries is not
limited. By default, the number of history entries is set
to the value of the HISTSIZE shell variable. If an attempt
is made to set history-size to a non-numeric value, the
maximum number of history entries will be set to 500.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for dis‐
play, 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 eighth bit from the characters it
reads), regardless of what the terminal claims it can sup‐
port. The name meta-flag is a synonym for this variable.
The default is Off, but readline will set it to On if the
locale contains eight-bit characters.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an incremen‐
tal 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.
keyseq-timeout (500)
Specifies the duration readline will wait for a character
when reading an ambiguous key sequence (one that can form a
complete key sequence using the input read so far, or can
take additional input to complete a longer key sequence).
If no input is received within the timeout, readline will
use the shorter but complete key sequence. The value is
specified in milliseconds, so a value of 1000 means that
readline will wait one second for additional input. If
this variable is set to a value less than or equal to zero,
or to a non-numeric value, readline will wait until another
key is pressed to decide which key sequence to complete.
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. The default is Off, but readline will set
it to On if the locale contains eight-bit characters.
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 individ‐
ual undo lists across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion func‐
tions. If set to On, words which have more than one possi‐
ble 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 func‐
tions in a fashion similar to show-all-if-ambiguous. If
set to On, words which have more than one possible comple‐
tion without any possible partial completion (the possible
completions don't share a common prefix) cause the matches
to be listed immediately instead of ringing the bell.
show-mode-in-prompt (Off)
If set to On, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi
insertion. The mode strings are user-settable (e.g.,
emacs-mode-string).
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.
vi-cmd-mode-string ((cmd))
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the pri‐
mary prompt when vi editing mode is active and in command
mode. The value is expanded like a key binding, so the
standard set of meta- and control prefixes and backslash
escape sequences is available. Use the \1 and \2 escapes
to begin and end sequences of non-printing characters,
which can be used to embed a terminal control sequence into
the mode string.
vi-ins-mode-string ((ins))
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the pri‐
mary prompt when vi editing mode is active and in insertion
mode. The value is expanded like a key binding, so the
standard set of meta- and control prefixes and backslash
escape sequences is available. Use the \1 and \2 escapes
to begin and end sequences of non-printing characters,
which can be used to embed a terminal control sequence into
the mode string.
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 list‐
ing possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the condi‐
tional 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, after any comparison
operator,
extends to the end of the line; unless otherwise noted, 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-spe‐
cific key bindings, perhaps to bind the key
sequences output by the terminal's function keys.
The word on the right side of the = is tested
against both the 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.
version
The version test may be used to perform comparisons
against specific readline versions. The version
expands to the current readline version. The set of
comparison operators includes =, (and ==), !=, <=,
>=, <, and >. The version number supplied on the
right side of the operator consists of a major ver‐
sion number, an optional decimal point, and an
optional minor version (e.g., 7.1). If the minor
version is omitted, it is assumed to be 0. The
operator may be separated from the string version
and from the version number argument by whitespace.
application
The application construct is used to include appli‐
cation-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 func‐
tions 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
variable
The variable construct provides simple equality
tests for readline variables and values. The per‐
mitted comparison operators are =, ==, and !=. The
variable name must be separated from the comparison
operator by whitespace; the operator may be sepa‐
rated from the value on the right hand side by
whitespace. Both string and boolean variables may
be tested. Boolean variables must be tested against
the values on and off.
$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 his‐
tory (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 char‐
acters 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 incre‐
mental search and restore the original line. When the search is
terminated, the history entry containing the search string becomes
the current 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 com‐
posed 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 delim‐
ited 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.
previous-screen-line
Attempt to move point to the same physical screen column on
the previous physical screen line. This will not have the
desired effect if the current Readline line does not take
up more than one physical line or if point is not greater
than the length of the prompt plus the screen width.
next-screen-line
Attempt to move point to the same physical screen column on
the next physical screen line. This will not have the
desired effect if the current Readline line does not take
up more than one physical line or if the length of the cur‐
rent Readline line is not greater than the length of the
prompt plus the screen width.
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 for‐
ward 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 cur‐
rently 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 incre‐
mental 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 supplied
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 char‐
acters 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 char‐
acters between the start of the current line and the point.
This is a non-incremental search.
history-substring-search-backward
Search backward through the history for the string of char‐
acters between the start of the current line and the cur‐
rent cursor position (the point). The search string may
match anywhere in a history line. This is a non-incremen‐
tal search.
history-substring-search-forward
Search forward through the history for the string of char‐
acters between the start of the current line and the point.
The search string may match anywhere in a history line.
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 argu‐
ment, 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 argu‐
ment 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 argument
switches the direction through the history (back or for‐
ward). The history expansion facilities are used to
extract the last word, 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 expan‐
sions. 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. A numeric argument, if supplied, specifies the
history entry to use instead of the current line.
edit-and-execute-command (C-x C-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
end-of-file (usually C-d)
The character indicating end-of-file as set, for example,
by ``stty''. If this character is read when there are no
characters on the line, and point is at the beginning of
the line, Readline interprets it as the end of input and
returns EOF.
delete-char (C-d)
Delete the character at point. If this function is bound
to the same character as the tty EOF character, as C-d com‐
monly is, see above for the effects.
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, mov‐
ing 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 nega‐
tive 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 bound‐
aries 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
Kill from point to the end of the current word, or if
between words, to the end of the next word. Word bound‐
aries are the same as those used by shell-forward-word.
shell-backward-kill-word
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 com‐
mand 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 other‐
wise ignored. As a special case, if this command is imme‐
diately followed by a character that is neither a digit nor
minus sign, the argument count for the next command is mul‐
tiplied by four. The argument count is initially one, so
executing this function the first time makes the argument
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 $), username (if the text begins
with ~), hostname (if the text begins with @), or command
(including aliases and functions) in turn. If none of
these produces a match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would
have been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list
of possible completions, inserting each match in turn. At
the end of the list of completions, the bell is rung (sub‐
ject to the setting of bell-style) and the original text is
restored. An argument of n moves n positions forward in
the list of matches; a negative argument may be used to
move backward through the list. This command is intended
to be bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the
list of possible completions, as if menu-complete had been
given a negative argument. This command is unbound by
default.
delete-char-or-list
Deletes the character under the cursor if not at the begin‐
ning or end of the line (like delete-char). If at the end
of the line, behaves identically to possible-completions.
This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as
a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
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 com‐
pletion matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing
the text against lines from the history list for possible
completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available
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 keyboard.
print-last-kbd-macro ()
Print the last keyboard macro defined in a format suitable
for the inputrc file.
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-lowercase-version (M-A, M-B, M-x, ...)
If the metafied character x is uppercase, run the command
that is bound to the corresponding metafied lowercase char‐
acter. The behavior is undefined if x is already lower‐
case.
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-<space>)
Set the mark to the point. If a numeric argument is sup‐
plied, 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 occur‐
rence of that character. A negative count searches for
previous 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 explic‐
itly 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 com‐
ment-begin variable is inserted at the beginning of the
current line. If a numeric argument is supplied, this com‐
mand 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 com‐
mand 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 pat‐
tern is used to generate a list of matching filenames 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 filenames 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 val‐
ues 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 command, the comp‐
spec is used to generate the list of possible completions for the
word. If the command word is a full pathname, 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. If there is no default compspec,
bash attempts alias expansion on the command word as a final
resort, and attempts to find a compspec for the command word from
any successful expansion.
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 com‐
pletion 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 direc‐
tory 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 vari‐
able 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 arith‐
metic 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 com‐
pletions.
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 ($1)
is the name of the command whose arguments are being completed,
the second argument ($2) is the word being completed, and the
third argument ($3) is the word preceding the word being completed
on the current command line. No filtering of the generated com‐
pletions against the word being completed is performed; the func‐
tion 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 possi‐
ble completions in the COMPREPLY array variable, one per array
element.
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. Back‐
slash may be used to escape a newline, if necessary.
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 completed. 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.
If the nocasematch shell option is enabled, the match is performed
without regard to the case of alphabetic characters.
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 comp‐
spec 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 com‐
pletions. 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 comp‐
spec generates no matches. If the -o default option was supplied
to complete when the compspec was defined, readline's default com‐
pletion 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-directo‐
ries 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 func‐
tion 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 com‐
pletion 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 fol‐
lowing default completion function would load completions dynami‐
cally:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return
124
}
complete -D -F _completion_loader -o bashdefault -o default
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 HISTFILE‐
SIZE. If HISTFILESIZE is unset, or set to null, a non-numeric
value, or a numeric value less than zero, the history file is not
truncated. When the history file is read, lines beginning with
the history comment character followed immediately by a digit are
interpreted as timestamps for the following history line. These
timestamps are optionally displayed depending on the value of the
HISTTIMEFORMAT variable. When a shell with history enabled 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 vari‐
able 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 unset, or set to null, a
non-numeric value, or a numeric value less than zero, the history
file is not truncated.
The builtin command fc (see SHELL BUILTIN COMMANDS below) may be
used to list or edit and re-execute a portion of the history 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 his‐
tory 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 COM‐
MANDS for information on setting 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 argu‐
ments 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, and is performed
on each line individually without taking quoting on previous lines
into account. It takes place in two parts. The first is to
determine which line from the history list to use during substitu‐
tion. 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, but the history expansion
character is also treated as quoted if it immediately precedes the
closing double quote in a double-quoted string.
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 expansion.
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 mod‐
ification. 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 writing 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 rela‐
tive 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
position 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 designator.
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 word. This is usually the last argument, but will
expand to the zeroth word if there is only one word in the
line.
% 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 filename component, leaving only the
head.
t Remove all leading filename 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 substitu‐
tions.
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, return, 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 argu‐
ments and performing any specified redirections. The
return status is zero.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current
shell environment and return the exit status of the last
command executed from filename. If filename does not con‐
tain a slash, filenames 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 argu‐
ments are supplied, they become the positional parameters
when filename is executed. Otherwise the positional param‐
eters are unchanged. If the -T option is enabled, source
inherits any trap on DEBUG; if it is not, any DEBUG trap
string is saved and restored around the call to source, and
source unsets the DEBUG trap while it executes. If -T is
not set, and the sourced file changes the DEBUG trap, the
new value is retained when source completes. The return
status is the status of the last command exited within the
script (0 if no commands are executed), and false if file‐
name 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 substi‐
tution 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] [-lpsvPSVX]
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 [-m keymap] keyseq: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 command
must be passed as a separate argument; e.g., '"\C-x\C-r":
re-read-init-file'. Options, if supplied, have the follow‐
ing meanings:
-m keymap
Use keymap as the keymap to be affected by the sub‐
sequent bindings. Acceptable keymap names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-move, vi-command, and vi-insert. vi is equiva‐
lent to vi-command (vi-move is also a synonym);
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 con‐
tents of the readline line buffer and the READ‐
LINE_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.
-X List all key sequences bound to shell commands and
the associated commands in a format that can be
reused as input.
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, retain‐
ing the functionality of the builtin within the function.
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 posi‐
tion in the call stack.
cd [-L|[-P [-e]] [-@]] [dir]
Change the current directory to dir. if dir is not sup‐
plied, the value of the HOME shell variable is the default.
Any additional arguments following dir are ignored. The
variable CDPATH defines the search path for the directory
containing dir: each directory name in CDPATH is searched
for dir. Alternative directory names in CDPATH are sepa‐
rated 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 causes cd to use the physical directory structure by
resolving symbolic links while traversing dir and before
processing instances of .. in dir (see also the -P option
to the set builtin command); the -L option forces symbolic
links to be followed by resolving the link after processing
instances of .. in dir. If .. appears in dir, it is pro‐
cessed by removing the immediately previous pathname compo‐
nent from dir, back to a slash or the beginning of dir. If
the -e option is supplied with -P, and the current working
directory cannot be successfully determined after a suc‐
cessful directory change, cd will return an unsuccessful
status. On systems that support it, the -@ option presents
the extended attributes associated with a file as a direc‐
tory. An argument of - is converted to $OLDPWD before the
directory change is attempted. If a non-empty directory
name from CDPATH is used, or if - is the first argument,
and the directory change is successful, the absolute path‐
name of the new working directory is written to the stan‐
dard 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 filename used to invoke command
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 sta‐
tus 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 program‐
mable 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 sup‐
plied, or no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action] [-G
globpat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...]
Specify how arguments to each name should be completed. If
the -p option is supplied, or if no options are supplied,
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 specifications. The -D
option indicates that other supplied options and actions
should apply to the ``default'' command completion; that
is, completion attempted on a command for which no comple‐
tion has previously been defined. The -E option indicates
that other supplied options and actions should apply to
``empty'' command completion; that is, completion attempted
on a blank line. The -I option indicates that other sup‐
plied options and actions should apply to completion on the
inital non-assignment word on the line, or after a command
delimiter such as ; or |, which is usually command name
completion. If multiple options are supplied, the -D
option takes precedence over -E, and both take precedence
over -I. If any of -D, -E, or -I are supplied, any other
name arguments are ignored; these completions only apply to
the case specified by the option.
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 neces‐
sary, 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
completions. comp-option may be one of:
bashdefault
Perform the rest of the default bash com‐
pletions 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 file‐
name-specific processing (like adding a
slash to directory names, quoting special
characters, or suppressing trailing spa‐
ces). Intended to be used with shell func‐
tions.
noquote Tell readline not to quote the completed
words if they are filenames (quoting file‐
names is the default).
nosort Tell readline not to sort the list of pos‐
sible completions alphabetically.
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 speci‐
fied 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 cur‐
rent shell environment. When the function is exe‐
cuted, the first argument ($1) is the name of the
command whose arguments are being completed, the
second argument ($2) is the word being completed,
and the third argument ($3) is the word preceding
the word being completed on the current command
line. 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 resul‐
tant word is expanded. Shell quoting is honored
within wordlist, in order to provide a mechanism
for the words to contain shell metacharacters or
characters in the value of IFS. The possible com‐
pletions are the members of the resultant list
which match the word being completed.
-X filterpat
filterpat is a pattern as used for pathname expan‐
sion. It is applied to the list of possible com‐
pletions 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 sup‐
plied, 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] [-DEI] [+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 completion.
The possible values of option are those valid for the com‐
plete builtin described above. The -D option indicates
that other supplied options should apply to the ``default''
command completion; that is, completion attempted on a com‐
mand for which no completion has previously been defined.
The -E option indicates that other supplied options should
apply to ``empty'' command completion; that is, completion
attempted on a blank line. The -I option indicates that
other supplied options should apply to completion on the
inital non-assignment word on the line, or after a command
delimiter such as ; or |, which is usually command name
completion.
The return value is true unless an invalid option is sup‐
plied, 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 [-aAfFgilnrtux] [-p] [name[=value] ...]
typeset [-aAfFgilnrtux] [-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,
other than -f and -F, 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 dis‐
play to shell functions. The -F option inhibits the dis‐
play 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 each name 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 per‐
formed 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.
-n Give each name the nameref attribute, making it a
name reference to another variable. That other
variable is defined by the value of name. All ref‐
erences, assignments, and attribute modifications to
name, except those using or changing the -n
attribute itself, are performed on the variable ref‐
erenced by name's value. The nameref attribute can‐
not be applied to array variables.
-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 func‐
tions 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 and +A may not be used to
destroy array variables and +r will not remove the readonly
attribute. When used in a function, declare and typeset
make 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. When
using -a or -A and the compound assignment syntax to create
array variables, additional attributes do not take effect
until subsequent assignments. 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 variable,
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 [-clpv] [+n] [-n]
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 command
removes entries from the list. The current directory is
always the first directory in the stack.
-c Clears the directory stack by deleting all of the
entries.
-l Produces a listing using full pathnames; 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.
+n Displays the nth entry counting from the left of the
list shown by dirs when invoked without options,
starting with zero.
-n Displays the nth entry counting from the right of
the list shown by dirs when invoked without options,
starting with zero.
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 ... | pid ... ]
Without options, remove each jobspec from the table of
active jobs. If jobspec is not present, and neither the -a
nor the -r option is supplied, 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 sup‐
plied, the -a option means to remove or mark all jobs; the
-r option without a jobspec argument restricts operation 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 new‐
line. The return status is 0 unless a write error occurs.
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 hexadeci‐
mal 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 com‐
mand. 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
execfail shell option is enabled. In that case, it returns
failure. An interactive shell returns failure if the file
cannot be executed. A subshell exits unconditionally if
exec fails. 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 omit‐
ted, 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 names
of all exported variables 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 vari‐
able 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]
The first form selects a range of commands from first to
last from the history list and displays or edits and re-
executes them. 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 nega‐
tive number is used as an offset from the current command
number). If last is not specified, it is set to the cur‐
rent 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 listing.
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 commands
are echoed and executed.
In the second form, command is re-executed after each
instance of pat is replaced by rep. Command is interpreted
the same as first above. 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 his‐
tory lines out of range. If the -e option is supplied, the
return value is the value of the last command executed or
failure if an error occurs with the temporary file of com‐
mands. If the second form is used, the return status is
that of the command re-executed, unless cmd does not spec‐
ify 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 com‐
mand 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
separated 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 char‐
acter 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 unspeci‐
fied, 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 dis‐
carded. If the -p option is supplied, no path search is
performed, and filename is used as the full filename 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 man‐
page-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 -d start-end
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 interven‐
ing 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 mean‐
ings:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset. If
offset is negative, it is interpreted as relative to
one greater than the last history position, so nega‐
tive indices count back from the end of the history,
and an index of -1 refers to the current history -d
command.
-d start-end
Delete the history entries between positions start
and end, inclusive. Positive and negative values
for start and end are interpreted as described
above.
-a Append the ``new'' history lines to the history
file. These are history lines entered since the
beginning of the current bash session, but not
already appended 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 append
them to the current history list.
-w Write the current history list 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 char‐
acter. When the history file is read, lines beginning with
the history comment character followed immediately by a
digit are interpreted as timestamps for the following his‐
tory entry. 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 argu‐
ment to -d, or the history expansion supplied as an argu‐
ment 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 informa‐
tion.
-n Display information only about jobs that have
changed status since the user was last notified of
their status.
-p List only the process ID of the job's process group
leader.
-r Display only running jobs.
-s Display only 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|-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-insensi‐
tive signal name such as SIGKILL (with or without the SIG
prefix) or a signal number; signum is a signal number. If
sigspec is not present, then SIGTERM is assumed. An argu‐
ment of -l lists the signal names. If any arguments are
supplied when -l is given, the names of the signals corre‐
sponding to the arguments are listed, and the return status
is 0. The exit_status argument to -l is a number specify‐
ing either a signal number or the exit status of a process
terminated by a signal. The -L option is equivalent to -l.
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. If name is
-, the set of shell options is made local to the function
in which local is invoked: shell options changed using the
set builtin inside the function are restored to their orig‐
inal values when the function returns. With no operands,
local writes a list of local variables to the standard out‐
put. It is an error to use local when not within a func‐
tion. The return status is 0 unless local is used outside
a function, an invalid name is supplied, or name is a read‐
only variable.
logout Exit a login shell.
mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd]
[-C callback] [-c quantum] [array]
readarray [-d delim] [-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:
-d The first character of delim is used to terminate
each input line, rather than newline. If delim is
the empty string, mapfile will terminate a line when
it reads a NUL character.
-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 delim (default 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 argu‐
ments, removes the top directory from the stack, and per‐
forms a cd to the new top directory. Arguments, if sup‐
plied, 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 exam‐
ple: ``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 exam‐
ple: ``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 output
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 con‐
verted and copied to the standard output, and format speci‐
fications, each of which causes printing of the next suc‐
cessive 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 in the same way as
echo -e.
%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 result‐
ing from using datefmt as a format string for strf‐
time(3). The corresponding argument is an integer
representing the number of seconds since the epoch.
Two special argument values may be used: -1 repre‐
sents the current time, and -2 represents the time
the shell was invoked. If no argument is specified,
conversion behaves as if -1 had been given. This is
an exception to the usual printf behavior.
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 char‐
acter.
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, pushd 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
rotating or adding directories to the stack, so that
only the stack is manipulated.
+n Rotates the stack so that the nth directory (count‐
ing from the left of the list shown by dirs, start‐
ing with zero) is at the top.
-n Rotates the stack so that the nth directory (count‐
ing from the right of the list shown by dirs, start‐
ing with zero) is at the top.
dir Adds dir to the directory stack at the top, making
it the new current working directory as if it had
been supplied as the argument to the cd builtin.
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 direc‐
tory 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 direc‐
tory. 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 sup‐
plied.
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,
split into words as described above under Word Splitting,
and the first word is assigned to the first name, the sec‐
ond word to the second name, and so on. If there are more
words than names, the remaining words and their intervening
delimiters are 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 using the same
rules the shell uses for expansion (described above under
Word Splitting). 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. If delim is
the empty string, read will terminate a line when it
reads a NUL character.
-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 set‐
tings, but uses Readline's default filename comple‐
tion.
-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 hon‐
ors 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. Delim‐
iter characters encountered in the input are not
treated specially and do not cause read to return
until nchars characters are read. The result is not
split on the characters in IFS; the intent is that
the variable is assigned exactly the characters read
(with the exception of backslash; see the -r option
below).
-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
particular, a backslash-newline pair may not then 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 com‐
plete line of input (or a specified number of char‐
acters) 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
reading from regular files. If read times out, read
saves any partial input read into the specified
variable name. If timeout is 0, read returns imme‐
diately, without trying to read any data. The exit
status is 0 if input is available on the specified
file descriptor, non-zero otherwise. The exit sta‐
tus 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 exit status is zero, unless end-of-
file is encountered, read times out (in which case the sta‐
tus is greater than 128), a variable assignment error (such
as assigning to a readonly variable) occurs, or an invalid
file descriptor 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 variables
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 stop executing and return the value
specified by n to its caller. If n is omitted, the return
status is that of the last command executed in the function
body. If return is executed by a trap handler, the last
command used to determine the status is the last command
executed before the trap handler. If return is executed
during a DEBUG trap, the last command used to determine the
status is the last command executed by the trap handler
before return was invoked. If return is 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 n is supplied, the return value is its
least significant 8 bits. The return status is non-zero if
return is supplied a non-numeric argument, or is used out‐
side a function and not during execution of a script by .
or source. 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 Each variable or function that is created or modi‐
fied is given the export attribute and marked 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 list, or a compound
command (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
!. If a compound command other than a subshell
returns a non-zero status because a command failed
while -e was being ignored, the shell does not
exit. A trap on ERR, if set, is executed before
the shell exits. This option applies to the shell
environment and each subshell environment sepa‐
rately (see COMMAND EXECUTION ENVIRONMENT above),
and may cause subshells to exit before executing
all the commands in the subshell.
If a compound command or shell function executes in
a context where -e is being ignored, none of the
commands executed within the compound command or
function body will be affected by the -e setting,
even if -e is set and a command returns a failure
status. If a compound command or shell function
sets -e while executing in a context where -e is
ignored, that setting will not have any effect
until the compound command or the command contain‐
ing the function call completes.
-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). All pro‐
cesses run in a separate process group. When a
background job completes, the shell prints a line
containing its exit status.
-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 interactive 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 success‐
fully. 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). See SEE ALSO below for a reference
to a document that details how posix mode
affects bash's behavior.
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing inter‐
face. This also affects the editing inter‐
face 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 sup‐
plied with no option-name, a series of set commands
to recreate the current option settings is dis‐
played on the standard output.
-p Turn on privileged mode. In this mode, the $ENV
and $BASH_ENV files are not processed, shell func‐
tions 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 effec‐
tive user (group) id not equal to the real user
(group) id, and the -p option is not supplied,
these actions are taken and the effective 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 effec‐
tive 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
performing parameter expansion. If expansion is
attempted on an unset variable or parameter, the
shell prints an error message, and, if not interac‐
tive, 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, fol‐
lowed 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 func‐
tions, command substitutions, and commands executed
in a subshell environment. The ERR trap is nor‐
mally 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 resolve symbolic links
when executing commands such as cd that change the
current working directory. It uses the physical
directory structure instead. By default, bash fol‐
lows the logical chain of directories when perform‐
ing commands which change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited
by shell functions, command substitutions, and com‐
mands 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 posi‐
tional parameters are unset. Otherwise, the posi‐
tional 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 settings controlling optional shell
behavior. The settings can be either those listed below,
or, if the -o option is used, those available with the -o
option to the set builtin command. With no options, or
with the -p option, a list of all settable options is dis‐
played, with an indication of whether or not each is set;
if optnames are supplied, the output is restricted to those
options. 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, shopt
shows only those options which are set or unset, respec‐
tively. Unless otherwise noted, the shopt options are dis‐
abled (unset) by default.
The return status when listing options is zero if all opt‐
names are enabled, non-zero otherwise. When setting or
unsetting options, the return status is zero unless an opt‐
name is not a valid shell option.
The list of shopt options is:
assoc_expand_once
If set, the shell suppresses multiple evaluation of
associative array subscripts during arithmetic
expression evaluation, while executing builtins
that can perform variable assignments, and while
executing builtins that perform array dereferenc‐
ing.
autocd If set, a command name that is the name of a direc‐
tory is executed as if it were the argument to the
cd command. This option is only used by interac‐
tive 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
variable 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 filename is
printed, and the command proceeds. This option is
only used by interactive 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 intervening 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
external (non-builtin) command and, if necessary,
updates the values of LINES and COLUMNS. This
option is enabled by default.
cmdhist If set, bash attempts to save all lines of a multi‐
ple-line command in the same history entry. This
allows easy re-editing of multi-line commands.
This option is enabled by default, but only has an
effect if command history is enabled, as described
above under HISTORY.
compat31
If set, bash changes its behavior to that of ver‐
sion 3.1 with respect to quoted arguments to the [[
conditional command's =~ operator and locale-spe‐
cific string comparison when using the [[ condi‐
tional command's < and > operators. Bash versions
prior to bash-4.1 use ASCII collation and str‐
cmp(3); bash-4.1 and later use the current locale's
collation sequence and strcoll(3).
compat32
If set, bash changes its behavior to that of ver‐
sion 3.2 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. Bash ver‐
sions 3.2 and earlier continue with the next com‐
mand in the list after one terminates due to an
interrupt.
compat40
If set, bash changes its behavior to that of ver‐
sion 4.0 with respect to locale-specific string
comparison when using the [[ conditional command's
< and > operators (see description of compat31) and
the effect of interrupting a command list. Bash
versions 4.0 and later interrupt the list as if the
shell received the interrupt; previous versions
continue with the next command in the 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 ver‐
sions.
compat42
If set, bash does not process the replacement
string in the pattern substitution word expansion
using quote removal.
compat43
If set, bash does not print a warning message if an
attempt is made to use a quoted compound array
assignment as an argument to declare, makes word
expansion errors non-fatal errors that cause the
current command to fail (the default behavior is to
make them fatal errors that cause the shell to
exit), and does not reset the loop state when a
shell function is executed (this allows break or
continue in a shell function to affect loops in the
caller's context).
compat44
If set, bash saves the positional parameters to
BASH_ARGV and BASH_ARGC before they are used,
regardless of whether or not extended debugging
mode is enabled.
complete_fullquote
If set, bash quotes all shell metacharacters in
filenames and directory names when performing com‐
pletion. If not set, bash removes metacharacters
such as the dollar sign from the set of characters
that will be quoted in completed filenames when
these metacharacters appear in shell variable ref‐
erences in words to be completed. This means that
dollar signs in variable names that expand to
directories will not be quoted; however, any dollar
signs appearing in filenames will not be quoted,
either. This is active only when bash is using
backslashes to quote completed filenames. This
variable is set by default, which is the default
bash behavior in versions through 4.2.
direxpand
If set, bash replaces directory names with the
results of word expansion when performing filename
completion. This changes the contents of the read‐
line editing buffer. If not set, bash attempts to
preserve what the user typed.
dirspell
If set, bash attempts spelling correction on direc‐
tory 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. The
filenames ``.'' and ``..'' must always be matched
explicitly, even if dotglob is set.
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 interactive shells.
extdebug
If set at shell invocation, arrange to execute the
debugger profile before the shell starts, identical
to the --debugger option. If set after invocation,
behavior intended for use by debuggers is enabled:
1. The -F option to the declare builtin dis‐
plays 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), the shell simulates a call to
return.
4. BASH_ARGC and BASH_ARGV are updated as
described in their descriptions above.
5. Function tracing is enabled: command substi‐
tution, shell functions, and subshells
invoked with ( command ) inherit the DEBUG
and RETURN traps.
6. Error tracing is enabled: command substitu‐
tion, shell functions, and subshells invoked
with ( command ) 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 per‐
formed within ${parameter} expansions enclosed in
double quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames dur‐
ing 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.
globasciiranges
If set, range expressions used in pattern matching
bracket expressions (see Pattern Matching above)
behave as if in the traditional C locale when per‐
forming comparisons. That is, the current locale's
collating sequence is not taken into account, so b
will not collate between A and B, and upper-case
and lower-case ASCII characters will collate
together.
globstar
If set, the pattern ** used in a pathname expansion
context will match all files and zero or more
directories and subdirectories. If the pattern is
followed by a /, only directories and subdirecto‐
ries 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 substi‐
tution.
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 modification.
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.
inherit_errexit
If set, command substitution inherits the value of
the errexit option, instead of unsetting it in the
subshell environment. This option is enabled when
posix mode is enabled.
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 embed‐
ded newlines rather than using semicolon separators
where possible.
localvar_inherit
If set, local variables inherit the value and
attributes of a variable of the same name that
exists at a previous scope before any new value is
assigned. The nameref attribute is not inherited.
localvar_unset
If set, calling unset on local variables in previ‐
ous function scopes marks them so subsequent
lookups find them unset until that function
returns. This is identical to the behavior of
unsetting local variables at the current function
scope.
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-insensi‐
tive fashion when performing pathname expansion
(see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing
case or [[ conditional commands, when performing
pattern substitution word expansions, or when fil‐
tering possible completions as part of programmable
completion.
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.
progcomp_alias
If set, and programmable completion is enabled,
bash treats a command name that doesn't have any
completions as a possible alias and attempts alias
expansion. If it has an alias, bash attempts pro‐
grammable completion using the command word result‐
ing from the expanded alias.
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 discover 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 posi‐
tional parameters.
sourcepath
If set, the source (.) builtin uses the value of
PATH to find the directory containing the file sup‐
plied 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 suspen‐
sion. 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 (true) or 1 (false) depending on the
evaluation of the conditional expression expr. Each opera‐
tor and operand must be a separate argument. Expressions
are composed of the primaries described above under CONDI‐
TIONAL EXPRESSIONS. 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 oper‐
ators listed above under CONDITIONAL EXPRESSIONS,
the expression is true if the unary test is true.
If the first argument is not a valid unary condi‐
tional 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
conditional 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 sec‐
ond 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 argu‐
ment. Otherwise, the expression is false.
4 arguments
If the first argument is !, the result is the nega‐
tion 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 lexi‐
cographically 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 sup‐
plied, 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 <signal.h>, 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 com‐
mand arg is executed each time a shell function or a script
executed with the . or source builtins finishes executing.
If a sigspec is ERR, the command arg is executed whenever a
pipeline (which may consist of a single simple command), a
list, or a compound command returns 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
immediately following a while or until keyword, part of the
test in an if statement, part of a 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 using !. These are the same
conditions obeyed by the errexit (-e) option.
Signals ignored upon entry to the shell cannot be trapped,
reset or listed. 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 sta‐
tus is false if any sigspec is invalid; otherwise trap
returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be inter‐
preted 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, respec‐
tively. 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, which is not nec‐
essarily 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 func‐
tions, 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 [-HSabcdefiklmnpqrstuvxPT [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 num‐
ber 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
specified, 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
-k The maximum number of kqueues that may be allocated
-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 sin‐
gle 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
-P The maximum number of pseudoterminals
-T The maximum number of threads
If limit is given, and the -a option is not used, limit is
the new value of the specified resource. If no option is
given, then -f is assumed. Values are in 1024-byte incre‐
ments, except for -t, which is in seconds; -p, which is in
units of 512-byte blocks; -P, -T, -b, -k, -n, and -u, which
are unscaled values; and, when in posix mode, -c and -f,
which are in 512-byte increments. The return status is 0
unless an invalid option or argument is supplied, or an
error occurs while setting a new limit. In POSIX Mode
512-byte blocks are used for the `-c' and `-f' options.
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; other‐
wise it is interpreted as a symbolic mode mask similar to
that accepted by chmod(1). If mode is omitted, the current
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 oth‐
erwise.
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] [-n] [name ...]
For each name, remove the corresponding variable or func‐
tion. If the -v option is given, each name refers to a
shell variable, and that variable is removed. Read-only
variables may not be unset. If -f is specified, each name
refers to a shell function, and the function definition is
removed. If the -n option is supplied, and name is a vari‐
able with the nameref attribute, name will be unset rather
than the variable it references. -n has no effect if the
-f option is supplied. If no options are supplied, each
name refers to a variable; if there is no variable by that
name, any function with that name is unset. Each unset
variable or function is removed from the environment passed
to subsequent commands. If any of COMP_WORDBREAKS, RANDOM,
SECONDS, LINENO, HISTCMD, FUNCNAME, 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 [-fn] [id ...]
Wait for each specified child process and return its termi‐
nation status. Each id may be a process ID or a job speci‐
fication; if a job spec is given, all processes in that
job's pipeline are waited for. If id is not given, all
currently active child processes are waited for, and the
return status is zero. If the -n option is supplied, wait
waits for any job to terminate and returns its exit status.
If the -f option is supplied, and job control is enabled,
wait forces id to terminate before returning its status,
instead of returning when it changes status. If id speci‐
fies 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 sup‐
plied 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 filename 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 >> re‐
direction 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 restrictions 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 --
http://pubs.opengroup.org/onlinepubs/9699919799/
http://tiswww.case.edu/~chet/bash/POSIX -- a description of posix
mode
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.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 avail‐
able from ftp://ftp.gnu.org/pub/gnu/bash/.
Once you have determined that a bug actually exists, use the bash‐
bug 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 tem‐
plate 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 stop‐
pable/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 5.0 2018 December 7 BASH(1)