BASH(1) General Commands Manual BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [command_string | file]
COPYRIGHT
Bash is Copyright (C) 1989-2022 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes com-
mands read from the standard input or from a file. Bash also incorpo-
rates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the Shell and
Utilities portion of the IEEE POSIX specification (IEEE Standard
1003.1). Bash can be configured to be POSIX-conformant by default.
OPTIONS
All of the single-character shell options documented in the description
of the set builtin command, including -o, can be used as options when
the shell is invoked. In addition, bash interprets the following op-
tions 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 argu-
ments after the command_string, the first argument is assigned
to $0 and any remaining arguments are assigned to the posi-
tional parameters. The assignment to $0 sets the name of the
shell, which is used in warning 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.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.
-D A list of all double-quoted strings preceded by $ is printed
on the standard output. These are the strings that are sub-
ject to language translation when the current locale is not C
or POSIX. This implies the -n option; no commands will be ex-
ecuted.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt
builtin (see SHELL BUILTIN COMMANDS below). If shopt_option
is present, -O sets the value of that option; +O unsets it.
If shopt_option is not supplied, the names and values of the
shell options accepted by shopt are printed on the standard
output. If the invocation option is +O, the output is dis-
played 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 file-
names and arguments. An argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options
must appear on the command line before the single-character options to
be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see the description of
the extdebug option to the shopt builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po
(portable object) file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the system wide initializa-
tion file /etc/bash.bashrc and the standard personal initializa-
tion file ~/.bashrc if the shell is interactive (see INVOCATION
below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when
the shell is interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or
any of the personal initialization files ~/.bash_profile,
~/.bash_login, or ~/.profile. By default, bash reads these files
when it is invoked as a login shell (see INVOCATION below).
--norc Do not read and execute the system wide initialization file
/etc/bash.bashrc and the personal initialization file ~/.bashrc
if the shell is interactive. This option is on by default if the
shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs
from the POSIX standard to match the standard (posix mode). See
SEE ALSO below for a reference to a document that details how
posix mode affects bash's behavior.
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the stan-
dard 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 parame-
ters are set to the remaining arguments. Bash reads and executes com-
mands 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 exe-
cuted, the exit status is 0. An attempt is first made to open the file
in the current directory, and, if no file is found, then the shell
searches the directories in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or
one started with the --login option.
An interactive shell is one started without non-option arguments (unless
-s is specified) and without the -c option, whose standard input and er-
ror are both connected to terminals (as determined by isatty(3)), or one
started with the -i option. PS1 is set and $- includes i if bash is in-
teractive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in filenames as described below under Tilde Expan-
sion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interac-
tive shell with the --login option, it first reads and executes commands
from the file /etc/profile, if that file exists. After reading that
file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in
that order, and reads and executes commands from the first one that ex-
ists and is readable. The --noprofile option may be used when the shell
is started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login shell
executes the exit builtin command, bash reads and executes commands from
the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash
reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if
these files exist. This may be inhibited by using the --norc option.
The --rcfile file option will force bash to read and execute commands
from file instead of /etc/bash.bashrc and ~/.bashrc.
When bash is started non-interactively, to run a shell script, for exam-
ple, it looks for the variable BASH_ENV in the environment, expands its
value if it appears there, and uses the expanded value as the name of a
file to read and execute. Bash behaves as if the following command were
executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file-
name.
If bash is invoked with the name sh, it tries to mimic the startup be-
havior of historical versions of sh as closely as possible, while con-
forming 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 in-
hibit 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 com-
mands 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 op-
tion, it follows the POSIX standard for startup files. In this mode,
interactive shells expand the ENV variable and commands are read and ex-
ecuted 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 historical
remote shell daemon, usually rshd, or the secure shell daemon sshd. If
bash determines it is being run non-interactively in this fashion, it
reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if
these files exist and are readable. It will not do this if invoked as
sh. The --norc option may be used to inhibit this behavior, and the
--rcfile option may be used to force another file to be read, but nei-
ther rshd nor sshd generally invoke the shell with those options or al-
low 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 supplied, no startup
files are read, shell functions are not inherited from the environment,
the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they ap-
pear 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 under-
scores, and beginning with an alphabetic character or an under-
score. 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 fol-
lowing 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 command (see SHELL GRAMMAR below), the third word of a
case or select command (only in is valid), or the third word of a for
command (only in and do are valid):
! case coproc do done elif else esac fi for function if in select then
until while { } time [[ ]]
SHELL GRAMMAR
This section describes the syntax of the various forms of shell com-
mands.
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]] [ ! ] command1 [ [|⎪|&] command2 ... ]
The standard output of command1 is connected via a pipe to the standard
input of command2. This connection is performed before any redirections
specified by the command1(see REDIRECTION below). If |& is used, com-
mand1's standard error, in addition to its standard 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 out-
put is performed after any redirections specified by command1.
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 re-
turning 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 recog-
nize 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 TIMEFOR-
MAT under Shell Variables below.
When the shell is in posix mode, time may be followed by a newline. In
this case, the shell displays the total user and system time consumed by
the shell and its children. The TIMEFORMAT variable may be used to
specify the format of the time information.
Each command in a multi-command pipeline, where pipes are created, is
executed in a subshell, which is a separate process. See COMMAND EXECU-
TION ENVIRONMENT for a description of subshells and a subshell environ-
ment. 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 when job control is not active.
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 re-
ferred to as asynchronous commands. Commands separated by a ; are exe-
cuted sequentially; the shell waits for each command to terminate in
turn. The return status is the exit status of the last command exe-
cuted.
AND and OR lists are sequences of one or more pipelines separated by the
&& and || control operators, respectively. AND and OR lists are exe-
cuted 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 (see COMMAND EXECUTION ENVIRONMENT
below for a description of a subshell environment). Variable as-
signments and builtin commands that affect the shell's environ-
ment 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 be-
low under ARITHMETIC EVALUATION. If the value of the expression
is non-zero, the return status is 0; otherwise the return status
is 1. The expression undergoes the same expansions as if it were
within double quotes, but double quote characters in expression
are not treated specially and are removed.
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the con-
ditional expression expression. Expressions are composed of the
primaries described below under CONDITIONAL EXPRESSIONS. The
words between the [[ and ]] do not undergo word splitting and
pathname expansion. The shell performs tilde expansion, parame-
ter and variable expansion, arithmetic expansion, command substi-
tution, process substitution, and quote removal on those words
(the expansions that would occur if the words were enclosed in
double quotes). Conditional operators such as -f must be un-
quoted to be recognized as primaries.
When used with [[, the < and > operators sort lexicographically
using the current locale.
See the description of the test builtin command (section SHELL BUILTIN
COMMANDS below) for the handling of parameters (i.e. missing parame-
ters).
When the == and != operators are used, the string to the right of the
operator is considered a pattern and matched according to the rules de-
scribed below under Pattern Matching, 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 pattern, 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 prece-
dence as == and !=. When it is used, the string to the right of the op-
erator is considered a POSIX extended regular expression and matched ac-
cordingly (using the POSIX regcomp and regexec interfaces usually de-
scribed 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 no-
casematch shell option is enabled, the match is performed without regard
to the case of alphabetic characters. If any part of the pattern is
quoted, the quoted portion is matched literally. This means every char-
acter in the quoted portion matches itself, instead of having any spe-
cial pattern matching meaning. If the pattern is stored in a shell
variable, quoting the variable expansion forces the entire pattern to be
matched literally. Treat bracket expressions in regular expressions
carefully, since normal quoting and pattern characters lose their mean-
ings between brackets.
The pattern will match if it matches any part of the string. Anchor the
pattern using the ^ and $ regular expression operators to force it to
match the entire string. The array variable BASH_REMATCH records which
parts of the string matched the pattern. The element of BASH_REMATCH
with index 0 contains the portion of the string matching the entire reg-
ular expression. Substrings matched by parenthesized subexpressions
within the regular expression are saved in the remaining BASH_REMATCH
indices. The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression. Bash sets BASH_RE-
MATCH in the global scope; declaring it as a local variable will lead to
unexpected results.
Expressions may be combined using the following operators, listed in de-
creasing order of precedence:
( expression )
Returns the value of expression. This may be used to
override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value
of expression1 is sufficient to determine the return value of the
entire conditional expression.
for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of
items. The variable name is set to each element of this list in
turn, and list is executed each time. If the in word is omitted,
the for command executes list once for each positional parameter
that is set (see PARAMETERS below). The return status is the
exit status of the last command that executes. If the expansion
of the items following in results in an empty list, no commands
are executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to
the rules described below under ARITHMETIC EVALUATION. The
arithmetic expression expr2 is then evaluated repeatedly until it
evaluates to zero. Each time expr2 evaluates to a non-zero
value, list is executed and the arithmetic expression expr3 is
evaluated. If any expression is omitted, it behaves as if it
evaluates to 1. The return value is the exit status of the last
command in list that is executed, or false if any of the expres-
sions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of
items, and 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). select
then displays the PS3 prompt and reads a line 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 select command completes and returns 1. 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 se-
lection 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 expan-
sion, parameter and variable expansion, arithmetic expansion,
command substitution, process substitution and quote removal.
Each pattern examined is expanded using tilde expansion, parame-
ter and variable expansion, arithmetic expansion, command substi-
tution, process substitution, and quote removal. If the nocase-
match shell option is enabled, the match is performed without re-
gard to the case of alphabetic characters. When a match is
found, the corresponding list is executed. If the ;; operator is
used, no subsequent matches are attempted after the first pattern
match. Using ;& in place of ;; causes execution to continue with
the list associated with the next set of patterns. Using ;;& in
place of ;; causes the shell to test the next pattern list in the
statement, if any, and execute any associated list on a success-
ful match, continuing the case statement execution as if the pat-
tern list had not matched. 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 executed in turn,
and if its exit status is zero, the corresponding then list is
executed and the command completes. Otherwise, the else list is
executed, if present. The exit status is the exit status of the
last command executed, or zero if no condition tested true.
while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long
as the last command in the list list-1 returns an exit status of
zero. The until command is identical to the while command, ex-
cept that the test is negated: list-2 is executed as long as the
last command in list-1 returns a non-zero exit status. The exit
status of the while and until commands is the exit status of the
last command executed in list-2, or zero if none was executed.
Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. A
coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the & control operator, with a two-way pipe es-
tablished between the executing shell and the coprocess.
The syntax for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. command may be either a simple
command or a compound command (see above). NAME is a shell variable
name. If NAME is not supplied, the default name is COPROC.
The recommended form to use for a coprocess is
coproc NAME { command [redirections]; }
This form is recommended because simple commands result in the coprocess
always being named COPROC, and it is simpler to use and more complete
than the other compound commands.
If command is a compound command, NAME is optional. The word following
coproc determines whether that word is interpreted as a variable name:
it is interpreted as NAME if it is not a reserved word that introduces a
compound command. If command is a simple command, NAME is not allowed;
this is to avoid confusion between NAME and 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 executing shell, and that file descriptor is assigned to NAME[0].
The standard input of command is connected via a pipe to a file descrip-
tor in the executing shell, and that file descriptor is assigned to
NAME[1]. This pipe is established before any redirections specified by
the command (see REDIRECTION below). The file descriptors can be uti-
lized 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 sub-
shells.
The process ID of the shell spawned to execute the coprocess is avail-
able 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:
fname () compound-command [redirection]
function fname [()] compound-command [redirection]
This defines a function named fname. The reserved word function
is optional. If the function reserved word is supplied, the
parentheses are optional. The body of the function is the com-
pound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above. If the
function reserved word is used, but the parentheses are not sup-
plied, the braces are recommended. compound-command is executed
whenever fname is specified as the name of a simple command.
When in posix mode, fname must be a valid shell name and may not
be the name of one of the POSIX special builtins. In default
mode, a function name can be any unquoted shell word that does
not contain $. Any redirections (see REDIRECTION below) speci-
fied 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 func-
tion 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 inter-
active_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 interac-
tive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or
words to the shell. Quoting can be used to disable special treatment
for special characters, to prevent reserved words from being recognized
as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special
meaning to the shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used (see HIS-
TORY 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 it-
self 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 fol-
lowed by one of the following 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).
Character sequences of the form $'string' are treated as a special vari-
ant of single quotes. The sequence expands to string, with backslash-
escaped characters in string replaced as specified by the ANSI C stan-
dard. Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\? question mark
\nnn the eight-bit character whose value is the octal value nnn
(one to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been
present.
A double-quoted string preceded by a dollar sign ($"string") will cause
the string to be translated according to the current locale. The get-
text infrastructure performs the lookup and translation, using the
LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN shell variables. If the cur-
rent locale is C or POSIX, if there are no translations available, or if
the string is not translated, the dollar sign is ignored. This is a
form of double quoting, so the string remains double-quoted by default,
whether or not it is translated and replaced. If the noexpand_transla-
tion option is enabled using the shopt builtin, translated strings are
single-quoted instead of double-quoted. See the description of shopt
below under SHELLBUILTINCOMMANDS.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a num-
ber, or one of the special characters listed below under Special Parame-
ters. A variable is a parameter denoted by a name. A variable has a
value and zero or more attributes. Attributes are assigned using the
declare builtin command (see declare below in SHELL BUILTIN COMMANDS).
A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
the unset builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion, com-
mand substitution, arithmetic expansion, and quote removal (see EXPAN-
SION below). If the variable has its integer attribute set, then value
is evaluated as an arithmetic expression even if the $((...)) expansion
is not used (see Arithmetic Expansion below). Word splitting and path-
name expansion are 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 com-
mand 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 assignment statements (dec-
laration commands). When += is applied to a variable for which the in-
teger attribute has been set, value is evaluated as an arithmetic ex-
pression and added to the variable's current value, which is also evalu-
ated. When += is applied to an array variable using compound assignment
(see Arrays below), the variable's value is not unset (as it is when us-
ing =), 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 descriptions of declare
and local below) to create a nameref, or a reference to another vari-
able. This allows variables to be manipulated indirectly. Whenever the
nameref variable is referenced, assigned to, unset, or has its attrib-
utes modified (other than using or changing the nameref attribute it-
self), 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, as-
signments, and attribute modifications to the variable whose name was
passed as $1. If the control variable in a for loop has the nameref at-
tribute, the list of words can be a list of shell variables, and a name
reference will be established for each word in the list, in turn, when
the loop is executed. Array variables cannot be given the nameref at-
tribute. However, nameref variables can reference array variables and
subscripted array variables. Namerefs can be unset using the -n option
to the unset builtin. Otherwise, if unset is executed with the name of
a nameref variable as an argument, the variable referenced by the
nameref variable will be unset.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits,
other than the single digit 0. Positional parameters are assigned from
the shell's arguments when it is invoked, and may be reassigned using
the set builtin command. Positional parameters may not be assigned to
with assignment statements. The positional parameters are temporarily
replaced when a shell function is executed (see FUNCTIONS below).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When
the expansion is not within double quotes, each positional para-
meter expands to a separate word. In contexts where it is per-
formed, those words are subject to further word splitting and
pathname expansion. When the expansion occurs within double
quotes, it expands to a single word with the value of each para-
meter separated by the first character of the IFS special vari-
able. That is, "$*" is equivalent to "$1c$2c...", where c is the
first character of the value of the IFS variable. If IFS is un-
set, the parameters are separated by spaces. If IFS is null, the
parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one. In con-
texts where word splitting is performed, this expands each posi-
tional parameter to a separate word; if not within double quotes,
these words are subject to word splitting. In contexts where
word splitting is not performed, this expands to a single word
with each positional parameter separated by a space. When the
expansion occurs within double quotes, each parameter expands to
a separate word. That is, "$@" is equivalent to "$1" "$2" ...
If the double-quoted expansion occurs within a word, the expan-
sion of the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter is
joined with the last part of the original word. When there are
no positional parameters, "$@" 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 fore-
ground pipeline.
- Expands to the current option flags as specified upon invocation,
by the set builtin command, or those set by the shell itself
(such as the -i option).
$ Expands to the process ID of the shell. In a subshell, it expands
to the process ID of the current shell, not the subshell.
! Expands to the process ID of the 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 com-
mands, $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.
Shell Variables
The following variables are set by the shell:
_ At shell startup, set to the pathname used to invoke the shell or
shell script being executed as passed in the environment or argu-
ment list. Subsequently, expands to the last argument to the
previous simple command executed in the foreground, after expan-
sion. Also set to the full pathname used to invoke each command
executed and placed in the environment exported to that command.
When checking mail, this parameter holds the name of the mail
file currently being checked.
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 dif-
fers 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 prop-
erties, even if it is subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to the in-
ternal list of aliases as maintained by the alias builtin. Ele-
ments added to this array appear in the alias list; however, un-
setting array elements currently does not cause aliases to be re-
moved 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
extdebug 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 sub-
routine call is at the top of the stack; the first parameter of
the initial call is at the bottom. When a subroutine is exe-
cuted, 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 be-
low). 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 in-
ternal 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 properties, even if it is subsequently reset.
BASH_COMMAND
The command currently being executed or about to be executed, un-
less 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.
If BASH_COMMAND is unset, it loses its special properties, even
if it is subsequently reset.
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 func-
tion). 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 com-
mand.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary op-
erator to the [[ conditional command. The element with index 0
is the portion of the string matching the entire regular expres-
sion. The element with index n is the portion of the string
matching the nth parenthesized subexpression.
BASH_SOURCE
An array variable whose members are the source filenames where
the corresponding shell function names in the FUNCNAME array
variable are defined. The shell function ${FUNCNAME[$i]} is de-
fined in the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The initial
value is 0. If BASH_SUBSHELL is unset, it loses its special
properties, even if it is subsequently reset.
BASH_VERSINFO
A readonly array variable whose members hold version information
for this instance of bash. The values assigned to the array mem-
bers are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of
bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current
cursor position. This variable is available only in shell func-
tions invoked by the programmable completion facilities (see Pro-
grammable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the cur-
rent completion function.
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the begin-
ning 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 com-
pletion facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called: TAB,
for normal completion, ?, for listing completions after succes-
sive tabs, !, for listing alternatives on partial word comple-
tion, @, to list completions if the word is not unmodified, or %,
for menu completion. This variable is available only in shell
functions and external commands invoked by the programmable com-
pletion 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 subse-
quently 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 in-
voked by the programmable completion facilities (see Programmable
Completion below).
COPROC An array variable (see Arrays below) created to hold the file de-
scriptors for output from and input to an unnamed coprocess (see
Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current con-
tents 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 EPOCHREAL-
TIME are ignored. If EPOCHREALTIME is unset, it loses its spe-
cial 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, initialized
at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0
is the name of any currently-executing shell function. The bot-
tom-most element (the one with the highest 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, ${FUNC-
NAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line
number ${BASH_LINENO[$i]}. The caller builtin displays the cur-
rent call stack using this information.
GROUPS An array variable containing the list of groups of which the cur-
rent user is a member. Assignments to GROUPS have no effect. If
GROUPS is unset, it loses its special properties, even if it is
subsequently reset.
HISTCMD
The history number, or index in the history list, of the current
command. Assignments to HISTCMD are ignored. If HISTCMD is un-
set, 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-depen-
dent.
LINENO Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to be
meaningful. If LINENO is unset, it loses its special properties,
even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system
type on which bash is executing, in the standard GNU cpu-company-
system format. The default is system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text
read by the mapfile builtin when no variable name is supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system
on which bash is executing. The default is 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 single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, it expands to a random
integer between 0 and 32767. Assigning a value to RANDOM ini-
tializes (seeds) the sequence of random numbers. If RANDOM is
unset, it loses its special properties, even if it is subse-
quently reset.
READLINE_ARGUMENT
Any numeric argument given to a readline command that was defined
using "bind -x" (see SHELL BUILTIN COMMANDS below) when it was
invoked.
READLINE_LINE
The contents of the readline line buffer, for use with "bind -x"
(see SHELL BUILTIN COMMANDS below).
READLINE_MARK
The position of the mark (saved insertion point) in the readline
line buffer, for use with "bind -x" (see SHELL BUILTIN COMMANDS
below). The characters between the insertion point and the mark
are often called the region.
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, it expands to the number
of seconds since shell invocation. 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.
The number of seconds at shell invocation and the current time
are always determined by querying the system clock. If SECONDS
is unset, it loses its special properties, even if it is subse-
quently 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 appear-
ing 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.
SRANDOM
This variable expands to a 32-bit pseudo-random number each time
it is referenced. The random number generator is not linear on
systems that support /dev/urandom or arc4random, so each returned
number has no relationship to the numbers preceding it. The ran-
dom number generator cannot be seeded, so assignments to this
variable have no effect. If SRANDOM is unset, it loses its spe-
cial properties, even if it is subsequently reset.
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 as-
signs 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
SHELL COMPATIBILITY MODE below for a description of the various
compatibility levels and their effects. The value may be a deci-
mal number (e.g., 4.2) or an integer (e.g., 42) corresponding to
the desired compatibility level. If BASH_COMPAT is unset or set
to the empty string, the compatibility level is set to the de-
fault 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 values correspond to
the compatibility levels described below under SHELL COMPATIBIL-
ITY MODE. For example, 4.2 and 42 are valid values that corre-
spond to the compat42 shopt option and set the compatibility
level to 42. 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, command 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 descriptor,
bash will write the trace output generated when set -x is enabled
to that file descriptor. The file descriptor is closed when
BASH_XTRACEFD is unset or assigned a new value. Unsetting
BASH_XTRACEFD or assigning it the empty string causes the trace
output to be sent to the standard error. Note that setting
BASH_XTRACEFD to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.
CDPATH The search path for the cd command. This is a colon-separated
list of directories in which the shell looks for destination di-
rectories specified by the cd command. A sample value is
".:~:/usr".
CHILD_MAX
Set the number of exited child status values for the shell to re-
member. 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-de-
pendent.
COLUMNS
Used by the select compound command to determine the terminal
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 completions
generated by a shell function invoked by the programmable comple-
tion facility (see Programmable Completion below). Each array
element contains one possible completion.
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 Expanded and executed similarly to BASH_ENV (see INVOCATION
above) when an interactive 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 pat-
terns 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 path-
names in the command hash table are not subject to EXECIGNORE.
Use this variable to ignore shared library files that have the
executable 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 performing
filename completion (see READLINE below). A filename whose suf-
fix matches one of the entries in FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~" (Quoting is
needed when assigning a value to this variable, which contains
tildes).
FUNCNEST
If set to a numeric value greater than 0, defines a maximum func-
tion nesting level. Function invocations that exceed this nest-
ing 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 character are not
saved in the history list. A value of ignoredups causes lines
matching the previous history entry to not be saved. A value of
ignoreboth is shorthand for ignorespace and ignoredups. A value
of erasedups causes all previous lines matching the current line
to be removed from the history list before that line is saved.
Any value not in the above list is ignored. If HISTCONTROL is
unset, or does not include a valid value, all lines read by the
shell parser are saved on the history list, subject to the value
of HISTIGNORE. The second and subsequent lines of a multi-line
compound command are not tested, and are added to the history re-
gardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HIS-
TORY 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 number of lines by re-
moving 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 truncated to zero size. Non-numeric val-
ues and numeric values less than zero inhibit truncation. The
shell sets the default value to the value of HISTSIZE after read-
ing 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 an-
chored 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 char-
acters, `&' matches the previous history line. `&' may be es-
caped using a backslash; the backslash is removed before attempt-
ing a match. The second and subsequent lines of a multi-line
compound command are not tested, and are added to the history re-
gardless of the value of HISTIGNORE. The pattern matching 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 com-
mand 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 for-
mat 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 distinguish timestamps from other
history lines.
HOME The home directory of the current user; the default argument for
the cd builtin command. The value of this variable is also used
when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that
should be read when the shell needs to complete a hostname. The
list of possible hostname completions may be changed while the
shell is running; the next time 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 completions.
When HOSTFILE is unset, the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting af-
ter 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 char-
acters on an input line before bash exits. If the variable ex-
ists but does not have a numeric value, or has no value, the de-
fault 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 de-
fault 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_ vari-
able 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 pat-
tern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number for-
matting.
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 ar-
rival of mail in the specified file or Maildir-format directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The de-
fault 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 vari-
able (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 generated by
the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTERR is initialized to 1 each time the shell is invoked or a
shell script is executed.
PATH The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory
name may appear as two adjacent colons, or as an initial or
trailing colon. The default path is system-dependent, and is set
by the administrator who installs bash. A common value is
``/usr/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 supplied. 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 this variable is set, and is an array, the value of each set
element is executed as a command prior to issuing each primary
prompt. If this is set but not an array variable, its value is
used as a command to execute instead.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the \w and \W prompt string escapes (see PROMPTING below). Char-
acters removed are replaced with an ellipsis.
PS0 The value of this parameter is expanded (see PROMPTING below) and
displayed by interactive shells after reading a command and be-
fore 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 This variable expands to the full pathname to the shell. If it
is not set when the shell starts, bash assigns to it the full
pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the time
reserved word should be displayed. The % character introduces an
escape sequence that is expanded to a time value or other infor-
mation. 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 af-
ter 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 minutes, of
the form MMmSS.FFs. The value of p determines whether or not the
fraction is included.
If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\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 de-
fault timeout for the read builtin. The select command termi-
nates if input does not arrive after TMOUT seconds when input is
coming from a terminal. In an interactive shell, the value is
interpreted as the number of seconds 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 com-
mands without redirections are treated as candidates for resump-
tion of an existing stopped job. There is no ambiguity allowed;
if there is more than one job beginning with the string typed,
the job most recently accessed is selected. The name of a
stopped job, in this context, is the command line used to start
it. If set to the value exact, the string supplied must match
the name of a stopped job exactly; if set to substring, the
string supplied needs to match a substring of the name of a
stopped job. The substring value provides functionality analo-
gous 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 char-
acter is the quick substitution character, which is used as
shorthand for re-running the previous command entered, substitut-
ing one string for another in the command. The default is `^'.
The optional third character is the character which indicates
that the remainder of the line is a comment when found as the
first character of a word, normally `#'. The history comment
character causes history substitution to be skipped for the re-
maining 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 variables.
Any variable may be used as an indexed array; the declare builtin will
explicitly declare an array. There is no maximum limit on the size of
an array, nor any requirement that members be indexed or assigned con-
tiguously. Indexed arrays are referenced using integers (including
arithmetic expressions) and are zero-based; associative arrays are ref-
erenced 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 de-
clare an indexed array, use declare -a name (see SHELL BUILTIN COMMANDS
below). declare -a name[subscript] is also accepted; the subscript is
ignored.
Associative arrays are created using declare -A name.
Attributes may be specified for an array variable using the declare and
readonly builtins. Each attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each value may be of the form [sub-
script]=string. Indexed array assignments do not require anything but
string. Each value in the list is expanded using all the shell expan-
sions described below under EXPANSION. When assigning to indexed ar-
rays, if the optional brackets and subscript are supplied, that index is
assigned to; otherwise the index of the element assigned is the last in-
dex assigned to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the words in a compound assign-
ment may be either assignment statements, for which the subscript is re-
quired, or a list of words that is interpreted as a sequence of alter-
nating keys and values: name=( key1 value1 key2 value2 ...). These are
treated identically to name=( [key1]=value1 [key2]=value2 ...). The
first word in the list determines how the remaining words are inter-
preted; all assignments in a list must be of the same type. When using
key/value pairs, the keys may not be missing or empty; a final missing
value is treated like the empty string.
This syntax is also accepted by the declare builtin. Individual array
elements may be assigned to using the name[subscript]=value syntax in-
troduced above. When assigning to an indexed array, if name is sub-
scripted by a negative number, that number is interpreted 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.
The += operator will append to an array variable when assigning using
the compound assignment syntax; see PARAMETERS above.
Any element of an array may be referenced using ${name[subscript]}. The
braces are required to avoid conflicts with pathname expansion. If sub-
script is @ or *, the word expands to all members of name. These sub-
scripts differ only when the word appears within double quotes. If the
word is double-quoted, ${name[*]} expands to a single word with the
value of each array member separated by the first character of the IFS
special variable, and ${name[@]} expands each element of name to a sepa-
rate word. When there are no array members, ${name[@]} expands to noth-
ing. 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[sub-
script]} expands to the length of ${name[subscript]}. If subscript is *
or @, the expansion is the number of elements in the array. If the sub-
script used to reference 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 element.
Referencing an array variable without a subscript is equivalent to ref-
erencing 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 nec-
essary.
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[subscript] de-
stroys the array element at index subscript, for both indexed and asso-
ciative 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, re-
moves the entire array. unset name[subscript], where subscript is * or
@, behaves differently depending on whether name is an indexed or asso-
ciative array. If name is an associative array, this unsets the element
with subscript * or @. If name is an indexed array, unset removes all
of the elements but does not remove the array itself.
When using a variable name with a subscript as an argument to a command,
such as with unset, without using the word expansion syntax described
above, the argument is subject to pathname expansion. If pathname ex-
pansion 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 associative ar-
ray. 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, command substitution,
arithmetic expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion; tilde expansion, parameter
and variable expansion, arithmetic expansion, and command substitution
(done in a left-to-right fashion); word splitting; and pathname expan-
sion.
On systems that can support it, there is an additional expansion avail-
able: process substitution. This is performed at the same time as
tilde, parameter, variable, and arithmetic expansion and command substi-
tution.
After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves (quote
removal).
Only brace expansion, word splitting, and pathname expansion can in-
crease the number of words of the expansion; other expansions expand a
single word to a single word. The only exceptions to this are the ex-
pansions 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 gener-
ated. This mechanism is similar to pathname expansion, but the file-
names generated need not exist. Patterns to be brace expanded take the
form of an optional preamble, followed by either a series of comma-sepa-
rated strings or a sequence expression between a pair of braces, fol-
lowed 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 letters, and incr, an optional increment, is
an integer. When integers are supplied, the expression expands to each
number between x and y, inclusive. Supplied integers may be prefixed
with 0 to force each term to have the same width. When either x or y
begins with a zero, the shell attempts to force all generated terms to
contain the same number of digits, zero-padding where necessary. When
letters are supplied, the expression expands to each character lexico-
graphically between x and y, inclusive, using the default C locale.
Note that both x and y must be of the same type (integer or letter).
When the increment is supplied, it is used as the difference between
each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any char-
acters special to other expansions are preserved in the result. It is
strictly textual. Bash does not apply any syntactic interpretation to
the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence ex-
pression. Any incorrectly formed brace expansion is left unchanged. A
{ or , may be quoted with a backslash to prevent its being considered
part of a brace expression. To avoid conflicts with parameter expan-
sion, the string ${ is not considered eligible for brace expansion, and
inhibits brace expansion until the closing }.
This construct is typically used as shorthand when the common prefix of
the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical ver-
sions 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 ex-
ample, a word entered to sh as file{1,2} appears identically in the out-
put. The same word is output as file1 file2 after expansion by bash.
If strict compatibility with sh is desired, start bash with the +B op-
tion or disable brace expansion with the +B option to the set command
(see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all of the
characters preceding the first unquoted slash (or all characters, if
there is no unquoted slash) are considered a tilde-prefix. If none of
the characters in the tilde-prefix are quoted, the characters in the
tilde-prefix following the tilde are treated as a possible login name.
If this login name is the null string, the tilde is replaced with the
value of the shell parameter HOME. If HOME is unset, the home directory
of the user executing the shell is substituted instead. Otherwise, the
tilde-prefix is replaced with the home directory associated with the
specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD re-
places the tilde-prefix. If the tilde-prefix is a `~-', the value of
the shell variable OLDPWD, if it is set, is substituted. If the charac-
ters following the tilde in the tilde-prefix consist of a number N, op-
tionally prefixed by a `+' or a `-', the tilde-prefix is replaced with
the corresponding element from the directory stack, as it would be dis-
played by the dirs builtin invoked with the tilde-prefix as an argument.
If the characters following the tilde in the tilde-prefix consist of a
number without a leading `+' or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word is
unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immedi-
ately following a : or the first =. In these cases, tilde expansion is
also performed. Consequently, one may use filenames with tildes in as-
signments to PATH, MAILPATH, and CDPATH, and the shell assigns the ex-
panded value.
Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (as described above under PARAMETERS) when they ap-
pear 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 substitution,
or arithmetic expansion. The parameter name or symbol to be expanded
may be enclosed in braces, which are optional but serve to protect the
variable to be expanded from characters immediately following it which
could be interpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not es-
caped by a backslash or within a quoted string, and not within an embed-
ded arithmetic expansion, command substitution, or parameter expansion.
${parameter}
The value of parameter is substituted. The braces are required
when parameter is a positional parameter with more than one
digit, or when parameter is followed by a character which is not
to be interpreted as part of its name. 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 pa-
rameter is not a nameref, it introduces a level of indirection. Bash
uses the value formed by expanding the rest of parameter as the new pa-
rameter; 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 refer-
enced by parameter instead of performing the complete indirect expan-
sion. The exceptions to this are the expansions of ${!prefix*} and
${!name[@]} described below. The exclamation point must immediately
follow the left brace in order to introduce indirection.
In each of the cases below, word is subject to tilde expansion, parame-
ter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the forms documented be-
low (e.g., :-), bash tests for a parameter that is unset or null. Omit-
ting the colon results in a test only for a parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion
of word is substituted. Otherwise, the value of parameter is
substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expan-
sion of word is assigned to parameter. The value of parameter is
then substituted. Positional parameters and special parameters
may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset,
the expansion of word (or a message to that effect if word is not
present) is written to the standard error and the shell, if it is
not interactive, exits. Otherwise, the value of parameter is
substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted.
${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 @ or *, an indexed array subscripted by @ or *,
or an associative array name, the results differ as described be-
low. If length is omitted, expands to the substring of the value
of parameter starting at the character specified by offset and
extending to the end of the value. length and offset are arith-
metic 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 parame-
ter. If length evaluates to a number less than zero, it is in-
terpreted 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 @ or *, the result is length positional parame-
ters beginning at offset. A negative offset is taken relative to
one greater than the greatest positional parameter, so an offset
of -1 evaluates to the last positional parameter. It is an ex-
pansion 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 ${para-
meter[offset]}. A negative offset is taken relative to one
greater than the maximum index of the specified array. It is an
expansion error if length evaluates to a number less than zero.
Substring expansion applied to an associative array produces un-
defined 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 pre-
fixed to the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables whose
names begin with prefix, separated by the first character of the
IFS special variable. When @ is used and the expansion appears
within double quotes, each variable name expands to a separate
word.
${!name[@]}
${!name[*]}
List of array keys. If name is an array variable, expands to the
list of array indices (keys) assigned in name. If name is not an
array, expands to 0 if name is set and null otherwise. When @ is
used and the expansion appears within double quotes, each key ex-
pands to a separate word.
${#parameter}
Parameter length. The length in characters of the value of para-
meter is substituted. If parameter is * or @, the value substi-
tuted 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 ar-
ray name subscripted by a negative number, that number is inter-
preted as relative to one greater than the maximum index of para-
meter, 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 Pat-
tern Matching below. If the pattern matches the beginning of the
value of parameter, then the result of the expansion is the ex-
panded 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 expan-
sion 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 re-
sultant 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 Pat-
tern Matching below. If the pattern matches a trailing portion
of the expanded value of parameter, then the result of the expan-
sion is the expanded value of parameter with the shortest match-
ing pattern (the ``%'' case) or the longest matching pattern (the
``%%'' case) deleted. If parameter is @ or *, the pattern re-
moval operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If parameter is an ar-
ray variable subscripted with @ or *, the pattern removal opera-
tion is applied to each member of the array in turn, and the ex-
pansion is the resultant list.
${parameter/pattern/string}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}
Pattern substitution. The pattern is expanded to produce a pat-
tern just as in pathname expansion. Parameter is expanded and
the longest match of pattern against its value is replaced with
string. string undergoes tilde expansion, parameter and variable
expansion, arithmetic expansion, command and process substitu-
tion, and quote removal. The match is performed using the rules
described under Pattern Matching below. In the first form above,
only the first match is replaced. If there are two slashes sepa-
rating parameter and pattern (the second form above), all matches
of pattern are replaced with string. If pattern is preceded by #
(the third form above), it must match at the beginning of the ex-
panded value of parameter. If pattern is preceded by % (the
fourth form above), it must match at the end of the expanded
value of parameter. If the expansion of string is null, matches
of pattern are deleted. If string is null, matches of pattern
are deleted and the / following pattern may be omitted.
If the patsub_replacement shell option is enabled using shopt,
any unquoted instances of & in string are replaced with the
matching portion of pattern.
Quoting any part of string inhibits replacement in the expansion
of the quoted portion, including replacement strings stored in
shell variables. Backslash will escape & in string; the back-
slash is removed in order to permit a literal & in the replace-
ment string. Backslash can also be used to escape a backslash;
\\ results in a literal backslash in the replacement. Users
should take care if string is double-quoted to avoid unwanted in-
teractions between the backslash and double-quoting, since back-
slash has special meaning within double quotes. Pattern substi-
tution performs the check for unquoted & after expanding string;
shell programmers should quote any occurrences of & they want to
be taken literally in the replacement and ensure any instances of
& they want to be replaced are unquoted.
If the nocasematch shell option is enabled, the match is per-
formed 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 re-
sultant list. If parameter is an array variable subscripted 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 alpha-
betic characters in parameter. The pattern is expanded to pro-
duce a pattern just as in pathname expansion. 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 ^ oper-
ator converts lowercase letters matching pattern to uppercase;
the , operator converts matching uppercase letters to lowercase.
The ^^ and ,, expansions convert each matched character in the
expanded value; the ^ and , expansions match and convert only the
first character in the expanded value. If pattern is omitted, it
is treated like a ?, which matches every character. If parameter
is @ or *, the case modification operation is applied to each po-
sitional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *,
the case modification operation is applied to each member of the
array in turn, and the expansion is the resultant list.
${parameter@operator}
Parameter transformation. The expansion is either a transforma-
tion of the value of parameter or information about parameter it-
self, depending on the value of operator. Each operator is a
single letter:
U The expansion is a string that is the value of parameter
with lowercase alphabetic characters converted to upper-
case.
u The expansion is a string that is the value of parameter
with the first character converted to uppercase, if it is
alphabetic.
L The expansion is a string that is the value of parameter
with uppercase alphabetic characters converted to lower-
case.
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 assignment
statement or declare command that, if evaluated, will
recreate parameter with its attributes and value.
K Produces a possibly-quoted version of the value of parame-
ter, except that it prints the values of indexed and asso-
ciative arrays as a sequence of quoted key-value pairs
(see Arrays above).
a The expansion is a string consisting of flag values repre-
senting parameter's attributes.
k Like the K transformation, but expands the keys and values
of indexed and associative arrays to separate words after
word splitting.
If parameter is @ or *, the operation is applied to each posi-
tional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *,
the operation is applied to each member of the array in turn, and
the expansion is the resultant 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 com-
mand name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command in a subshell environ-
ment and replacing the command substitution with the standard output of
the command, with any trailing newlines deleted. Embedded newlines are
not deleted, but they may be removed during word 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, backslash re-
tains its literal meaning except when followed by $, `, or \. The first
backquote not preceded by a backslash terminates the command substitu-
tion. When using the $(command) form, all characters between the paren-
theses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expansion
is:
$((expression))
The old format $[expression] is deprecated and will be removed in upcom-
ing versions of bash.
The expression undergoes the same expansions as if it were within double
quotes, but double quote characters in expression are not treated spe-
cially and are removed. All tokens in the expression undergo parameter
and variable expansion, command substitution, and quote removal. The
result is treated as the arithmetic expression to be evaluated. Arith-
metic 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 argument to the current command
as the result of the expansion. If the >(list) form is used, writing to
the file will provide input for list. If the <(list) form is used, the
file passed as an argument should be read to obtain the output of list.
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 substitu-
tion, and arithmetic expansion that did not occur within double quotes
for word splitting.
The shell treats each character of IFS as a delimiter, and splits the
results of the other expansions into words using these characters 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 charac-
ter). 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 commands
as empty strings. Unquoted implicit null arguments, resulting from the
expansion of parameters that have no values, are removed. If a parame-
ter with no value is expanded within double quotes, a null argument re-
sults 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 splitting 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 ap-
pears, and is not quoted, 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 filenames 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 with-
out regard to the case of alphabetic characters. Note that when using
range expressions like [a-z] (see below), letters of the other case may
be included, depending on the setting of LC_COLLATE. 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. In order to match the filenames ``.'' and
``..'', the pattern must begin with ``.'' (for example, ``.?''), even if
dotglob is set. If the globskipdots shell option is enabled, the file-
names ``.'' and ``..'' are never matched, even if the pattern begins
with a ``.''. When not matching pathnames, 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 contexts 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, globskipdots, failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file
names matching a pattern. If GLOBIGNORE is set, each matching file name
that also matches one of the patterns in GLOBIGNORE is removed from the
list of matches. 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, setting GLOBIGNORE to a non-null value has the
effect of enabling the dotglob shell option, so all other filenames be-
ginning with a ``.'' will match. To get the old behavior of ignoring
filenames beginning with a ``.'', make ``.*'' one of the patterns in
GLOBIGNORE. 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 character may not
occur in a pattern. A backslash escapes the following character; the
escaping backslash is discarded when matching. The special pattern
characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When the
globstar shell option is enabled, and * is used in a path-
name 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 expres-
sion; any character that falls between those two charac-
ters, inclusive, using the current locale's collating se-
quence and character set, is matched. If the first char-
acter following the [ is a ! or a ^ then any character
not enclosed is matched. The sorting order of characters
in range expressions, and the characters included in the
range, are determined by the current locale and the values
of the LC_COLLATE or LC_ALL shell variables, if set. To
obtain the traditional interpretation of range expres-
sions, where [a-d] is equivalent to [abcd], set value of
the LC_ALL shell variable to C, or enable the globasci-
iranges 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 letters, digits,
and the character _.
Within [ and ], an equivalence class can be specified us-
ing the syntax [=c=], which matches all characters with
the same collation weight (as defined by the current lo-
cale) as the character c.
Within [ and ], the syntax [.symbol.] matches the collat-
ing symbol symbol.
If the extglob shell option is enabled using the shopt builtin, the
shell recognizes several extended pattern matching operators. In the
following description, a pattern-list is a list of one or more patterns
separated by a |. Composite patterns may be formed using one or more of
the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Theextglob option changes the behavior of the parser, since the paren-
theses are normally treated as operators with syntactic meaning. To en-
sure that extended matching patterns are parsed correctly, make sure
that extglob is enabled before parsing constructs containing the pat-
terns, including shell functions and command substitutions.
When matching filenames, the dotglob shell option determines the set of
filenames that are tested: when dotglob is enabled, the set of filenames
includes all files beginning with ``.'', but ``.'' and ``..'' must be
matched by a pattern or sub-pattern that begins with a dot; when it is
disabled, the set does not include any filenames beginning with ``.''
unless the pattern or sub-pattern begins with a ``.''. As above, ``.''
only has a special meaning when matching filenames.
Complicated extended pattern matching against long strings is slow, es-
pecially 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 charac-
ters \, ', and " that did not result from one of the above expansions
are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected us-
ing a special notation interpreted by the shell. Redirection 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 environment. The following redirection opera-
tors may precede or appear anywhere within a simple command or may fol-
low 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 {varname} is supplied, the redirection
persists beyond the scope of the command, allowing the shell programmer
to manage the file descriptor's lifetime manually. The varredir_close
shell option manages this behavior.
In the following descriptions, if the file descriptor number is omitted,
and the first character of the redirection operator is <, the redirec-
tion refers to the standard input (file descriptor 0). If the first
character of the redirection operator is >, the redirection refers to
the standard output (file descriptor 1).
The word following the redirection operator in the following descrip-
tions, unless otherwise noted, is subjected to brace expansion, tilde
expansion, parameter and variable expansion, command substitution,
arithmetic expansion, quote removal, pathname expansion, and word split-
ting. 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 redirec-
tions, as described in the following table. If the operating 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 dupli-
cated.
/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 inter-
nally.
Note that the exec builtin command can make redirections take effect in
the current shell.
Redirecting Input
Redirection of input causes the file whose name results from the expan-
sion of word to be opened for reading on file descriptor n, or the stan-
dard 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 expan-
sion of word to be opened for writing on file descriptor n, or the stan-
dard 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 redirection operator is > and
the noclobber option to the set builtin command is not enabled, the
redirection is attempted even if the file named by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose name results
from the expansion of word to be opened for appending on file descriptor
n, or the standard output (file descriptor 1) if n is not specified. If
the file does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be redirected to the
file whose name is the expansion of word.
There are two formats for redirecting standard output and standard er-
ror:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equiva-
lent 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 Descrip-
tors 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 cur-
rent 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, arithmetic
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 un-
quoted, all lines of the here-document are subjected to parameter expan-
sion, command substitution, and arithmetic expansion, the character se-
quence \<newline> is ignored, and \ must be used to quote the characters
\, $, and `.
If the redirection operator is <<-, then all leading tab characters are
stripped from input lines and the line containing delimiter. This al-
lows 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 expansion,
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 stan-
dard 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 de-
scriptor 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 dig-
its in word do not specify a file descriptor open for output, a redirec-
tion 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 output 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 standard
input (file descriptor 0) if n is not specified. digit is closed after
being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard
output (file descriptor 1) if n is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for
both reading and writing on file descriptor n, or on file descriptor 0
if n is not specified. If the file does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when it is used as
the first word of a simple command. The shell maintains a list of
aliases that may be set and unset with the alias and unalias builtin
commands (see SHELL BUILTIN COMMANDS below). The first word of each
simple command, if unquoted, is checked to see if it has an alias. If
so, that word is replaced by the text of the alias. The characters /,
$, `, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text may
contain any valid shell input, including shell metacharacters. The
first word of the replacement text is tested for aliases, but a word
that is identical to an alias being expanded is not expanded a second
time. This means that one may alias ls to ls -F, for instance, and bash
does not try to recursively expand the replacement text. If the last
character of the alias value is a blank, then the next command word fol-
lowing the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed with
the unalias command.
There is no mechanism for using arguments in the replacement text. If
arguments are needed, use a shell function (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the description of
shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat con-
fusing. Bash always reads at least one complete line of input, and all
lines that make up a compound command, before executing any of the com-
mands on that line or the compound command. Aliases are expanded when a
command is read, not when it is executed. Therefore, an alias defini-
tion 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 behav-
ior is also an issue when functions are executed. Aliases are expanded
when a function definition is read, not when the function is executed,
because a function definition is itself a command. As a consequence,
aliases defined in a function are not available until after that func-
tion is executed. To be safe, always put alias definitions on a sepa-
rate line, and do not use alias in compound commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR, stores
a series of commands for later execution. When the name of a shell
function is used as a simple command name, the list of commands associ-
ated with that function name is executed. Functions are executed in the
context of the current shell; no new process is created to interpret
them (contrast this with the execution of a shell script). When a func-
tion is executed, the arguments to the function become the positional
parameters during its execution. The special parameter # is updated to
reflect the change. Special parameter 0 is unchanged. The first ele-
ment 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 be-
tween a function and its caller with these exceptions: the DEBUG and RE-
TURN traps (see the description of the trap builtin under SHELL BUILTIN
COMMANDS below) are not inherited unless the function has been given the
trace attribute (see the description of the declare builtin below) or
the -o 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 (local variables). 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).
In the following description, the current scope is a currently- execut-
ing function. Previous scopes consist of that function's caller and so
on, back to the "global" scope, where the shell is not executing any
shell function. Consequently, a local variable at the current scope is
a variable declared using the local or declare builtins in the function
that is currently executing.
Local variables "shadow" variables with the same name declared at previ-
ous 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 variable 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 variable declaration "shadows", and the value that is re-
stored 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, shadow-
ing 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; otherwise 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 re-
main so (appearing as unset) 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 (see below how the lo-
calvar_unset shell option changes this behavior).
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 function
completes and execution resumes with the next command after the function
call. Any command associated with the RETURN trap is executed before
execution resumes. When a function completes, the values of the posi-
tional parameters and the special parameter # are restored to the values
they had prior to the function's execution.
Function names and definitions may be listed with the -f option to the
declare or typeset builtin commands. The -F option to declare or type-
set 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 child shell processes (those created when exe-
cuting a separate shell invocation) automatically have them defined with
the -f option to the export builtin. A function definition may be
deleted using the -f option to the unset builtin.
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 op-
erators is grouped into levels of equal-precedence operators. The lev-
els 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 per-
formed before the expression is evaluated. Within an expression, shell
variables may also be referenced by name without using the parameter ex-
pansion syntax. A shell variable that is null or unset evaluates to 0
when referenced by name without using the parameter expansion syntax.
The value of a variable is evaluated as an arithmetic expression when it
is referenced, or when a variable which has been given the integer at-
tribute using declare -i is assigned a value. A null value evaluates to
0. A shell variable need not have its integer attribute turned on to be
used in an expression.
Integer constants follow the C language definition, without suffixes or
character constants. Constants with a leading 0 are interpreted as oc-
tal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, num-
bers 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 specifying
n, if a non-digit is required, the digits greater than 9 are represented
by the lowercase letters, the uppercase letters, @, and _, in that or-
der. If base is less than or equal to 36, lowercase and uppercase let-
ters may be used interchangeably to represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence rules
above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test
and [ builtin commands to test file attributes and perform string and
arithmetic comparisons. The test and [ commands determine their behav-
ior based on the number of arguments; see the descriptions of those com-
mands for any other command-specific actions.
Expressions are formed from the following unary or binary primaries.
Bash handles several filenames specially when they are used in expres-
sions. If the operating system on which bash is running provides these
special files, bash will use them; otherwise it will emulate them inter-
nally 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 sym-
bolic links and operate on the target of the link, rather than the link
itself.
When used with [[, the < and > operators sort lexicographically using
the current locale. The test command sorts using ASCII ordering.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device and inode num-
bers.
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 op-
tions 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 refer-
ence.
-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 Com-
mands).
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
string1 > string2
True if string1 sorts after string2 lexicographically.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic
binary operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than
or equal to arg2, respectively. Arg1 and arg2 may be positive or
negative integers. When used with the [[ command, Arg1 and Arg2
are evaluated as arithmetic expressions (see ARITHMETIC EVALUA-
TION above).
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following ex-
pansions, assignments, and redirections, from left to right, in the fol-
lowing order.
1. The words that the parser has marked as variable assignments
(those preceding the command name) and redirections are saved for
later processing.
2. The words that are not variable assignments or redirections are
expanded. If any words remain after expansion, the first word is
taken to be the name of the command and the remaining words are
the arguments.
3. Redirections are performed as described above under REDIRECTION.
4. The text after the = in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, and quote removal before being assigned to the vari-
able.
If no command name results, the variable assignments affect the current
shell environment. In the case of such a command (one that consists
only of assignment statements and redirections), assignment statements
are performed before redirections. Otherwise, the variables are added
to the environment of the executed command and do not affect the current
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 af-
fect the current shell environment. A redirection error causes the com-
mand to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the expan-
sions contained a command substitution, the exit status of the command
is the exit status of the last command substitution performed. If there
were no command substitutions, the command exits with a status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple
command and an optional list of arguments, the 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 in-
voked as described above in FUNCTIONS. If the name does not match a
function, the shell searches for it in the list of shell builtins. If a
match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no
slashes, bash searches each element of the PATH for a directory contain-
ing 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 com-
mand_not_found_handle. If that function exists, it is invoked in a sep-
arate execution environment with the original command and the original
command's arguments as its arguments, and the function's exit status be-
comes the exit status of that subshell. If that function is not de-
fined, the shell prints an error message and returns an exit status of
127.
If the search is successful, or if the command name contains one or more
slashes, the shell executes the named program in a separate execution
environment. Argument 0 is set to the name given, and the remaining ar-
guments to the command are set to the arguments given, if any.
If this execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell script, a
file containing shell commands, and the shell creates a new instance of
itself 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 exe-
cutable format themselves. The arguments to the interpreter consist of
a single optional argument following the interpreter name on the first
line of the program, followed by the name of the program, followed by
the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:
• open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin
• the current working directory as set by cd, pushd, or popd, or
inherited by the shell at invocation
• the file creation mode mask as set by umask or inherited from the
shell's parent
• current traps set by trap
• shell parameters that are set by variable assignment or with set
or inherited from the shell's parent in the environment
• 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 con-
sists of the following. Unless otherwise noted, the values are inher-
ited from the shell.
• the shell's open files, plus any modifications and additions
specified by redirections to the command
• the current working directory
• the file creation mode mask
• shell variables and functions marked for export, along with vari-
ables exported for the command, passed in the environment
• 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.
A subshell is a copy of the shell process.
Command substitution, commands grouped with parentheses, and asynchro-
nous commands are invoked in a subshell environment that is a duplicate
of the shell environment, except that traps caught by the shell are re-
set to the values that the shell inherited from its parent at invoca-
tion. Builtin commands that are invoked as part of a pipeline are also
executed in a subshell environment. Changes made to the subshell envi-
ronment 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 de-
fault standard input for the command is the empty file /dev/null. Oth-
erwise, 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 en-
vironment. This is a list of name-value pairs, of the form name=value.
The shell provides several ways to manipulate the environment. On invo-
cation, 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 mod-
ified, the new value becomes part of the environment, replacing the old.
The environment inherited by any executed command consists of the
shell's initial environment, whose values may be modified in the shell,
less any pairs removed by the unset command, plus any additions via the
export and declare -x commands.
The environment for any simple command or function may be augmented tem-
porarily by prefixing it with parameter assignments, as described above
in PARAMETERS. These assignment statements affect only the environment
seen by that command.
If the -k option is set (see the set builtin command below), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.
When bash invokes an external command, the variable _ is set to the full
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 certain 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 terminates on a fatal
signal N, bash uses the value of 128+N as the exit status.
If a command is not found, the child process created to execute it re-
turns a status of 127. If a command is found but is not executable, the
return status is 126.
If a command fails because of an error during expansion or redirection,
the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful, and
non-zero (false) if an error occurs while they execute. All builtins
return an exit status of 2 to indicate incorrect usage, generally in-
valid options or missing arguments.
The exit status of the last command is available in the special parame-
ter $?.
Bash itself returns the exit status of the last command executed, unless
a syntax error occurs, in which case it exits with a non-zero value.
See also the exit builtin command below.
SIGNALS
When bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that kill 0 does not kill an interactive shell), and SIGINT
is caught and handled (so that the wait builtin is interruptible). In
all cases, bash ignores SIGQUIT. If job control is in effect, bash ig-
nores 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 ef-
fect, asynchronous commands ignore SIGINT and SIGQUIT in addition to
these inherited handlers. Commands run as a result of command substitu-
tion ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU,
and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exiting, an
interactive shell resends the SIGHUP to all jobs, running or stopped.
Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP.
To prevent the shell from 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 com-
mand completes. When bash is waiting for an asynchronous command via
the wait builtin, the reception of a signal for which a trap has been
set will cause the wait builtin to return immediately with an exit sta-
tus greater than 128, immediately after which the trap is executed.
When job control is not enabled, and bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals such
as SIGINT (usually generated by ^C) that users commonly intend to send
to that command. This happens because the shell and the command are in
the same process group as the terminal, and ^C sends SIGINT to all
processes in that process group.
When bash is running without job control enabled and receives SIGINT
while waiting for a foreground command, it waits until that foreground
command terminates and then decides what to do about the SIGINT:
1. If the command terminates due to the SIGINT, bash concludes that
the user meant to end the entire script, and acts on the SIGINT
(e.g., by running a SIGINT trap or exiting itself);
2. If the command does not terminate due to SIGINT, the program han-
dled the SIGINT itself and did not treat it as a fatal signal.
In that case, bash does not treat SIGINT as a fatal signal, ei-
ther, instead assuming that the SIGINT was used as part of the
program's normal operation (e.g., emacs uses it to abort editing
commands) or deliberately discarded. However, bash will run any
trap set on SIGINT, as it does with any other trapped signal it
receives while it is waiting for the foreground command to com-
plete, for compatibility.
JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the exe-
cution of processes and continue (resume) their execution at a later
point. A user typically employs this facility via an interactive inter-
face 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 cur-
rently executing jobs, which may be listed with the jobs command. When
bash starts a job asynchronously (in the background), it prints a line
that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the
last process in the pipeline associated with this job is 25647. All of
the processes in a single pipeline are members of the same job. Bash
uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control,
the operating system maintains the notion of a current terminal process
group ID. Members of this process group (processes whose process group
ID is equal to the current terminal process group ID) receive keyboard-
generated signals such as SIGINT. These processes are said to be in the
foreground. Background processes are those whose process group ID dif-
fers from the terminal's; such processes are immune to keyboard-gener-
ated signals. Only foreground processes are allowed to read from or, if
the user so specifies with stty tostop, write to the terminal. Back-
ground processes which attempt to read from (write to when stty tostop
is in effect) the terminal are sent a SIGTTIN (SIGTTOU) signal by the
kernel's terminal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports job control,
bash contains facilities to use it. Typing the suspend character (typi-
cally ^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 re-
turned to bash. The user may then manipulate the state of this job, us-
ing 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 charac-
ter % introduces a job specification (jobspec). Job number n may be re-
ferred 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 job whose command name be-
gins with ce. If a prefix matches more than one job, bash reports an
error. Using %?ce, on the other hand, refers to any job containing the
string ce in its command line. If the substring matches more than one
job, bash reports an error. The symbols %% and %+ refer to the shell's
notion of the current job, which is the last job stopped while it was in
the foreground or started in the background. The previous job may be
referenced using %-. If there is only a single job, %+ and %- can both
be used to refer to that job. In output pertaining to jobs (e.g., the
output of the jobs command), the current job is always flagged with a +,
and the previous job with a -. A single % (with no accompanying job
specification) also refers to the current job.
Simply naming a job can be used to bring it into the foreground: %1 is a
synonym for ``fg %1'', bringing job 1 from the background into the fore-
ground. Similarly, ``%1 &'' resumes job 1 in the background, equivalent
to ``bg %1''.
The shell learns immediately whenever a job changes state. Normally,
bash waits until it is about to print a prompt before reporting changes
in a job's status so as to not interrupt any other output. If the -b
option to the set builtin command is enabled, bash reports such changes
immediately. Any trap on SIGCHLD is executed for each child that exits.
If an attempt to exit bash is made while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin, run-
ning), 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 causes 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 customized by inserting a number of
backslash-escaped special characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format (e.g., "Tue May
26")
\D{format}
the format is passed to strftime(3) and the result is in-
serted into the prompt string; an empty format results in
a locale-specific time representation. The braces are re-
quired
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion
following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the value of the PWD shell variable ($PWD), with $HOME ab-
breviated with a tilde (uses the value of the PROMPT_DIR-
TRIM variable)
\W the basename of $PWD, 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 ex-
ecuted during the current shell session. After the string is decoded,
it is expanded via parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the promptvars
shell option (see the description of the shopt command under SHELL
BUILTIN COMMANDS below). This can have unwanted side effects if escaped
portions of the string appear within command substitution or contain
characters special to word expansion.
READLINE
This is the library that handles reading input when using an interactive
shell, unless the --noediting option is given at shell invocation. Line
editing is also used when using the -e option to the read builtin. By
default, the line editing commands are similar to those of Emacs. A vi-
style line editing interface is also available. Line editing can be en-
abled at any time using the -o emacs or -o vi options to the set builtin
(see SHELL BUILTIN COMMANDS below). To turn off line editing after the
shell is running, use the +o emacs or +o vi options to the set builtin.
Readline Notation
In this section, the Emacs-style notation is used to denote keystrokes.
Control keys are denoted by C-key, e.g., C-n means Control-N. Simi-
larly, meta keys are denoted by M-key, so M-x means Meta-X. (On key-
boards without a meta key, M-x means ESC x, i.e., press the Escape key
then the x key. This makes ESC the meta prefix. The combination M-C-x
means ESC-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. Commands which do not kill text
separate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initialization file
(the inputrc file). The name of this file is taken from the value of
the INPUTRC variable. If that variable is unset, the default is ~/.in-
putrc. If that file does not exist or cannot be read, the ultimate de-
fault is /etc/inputrc. When a program which uses the readline library
starts up, the initialization file is read, and the key bindings and
variables are set. There are only a few basic constructs allowed in the
readline initialization file. Blank lines are ignored. Lines beginning
with a # are comments. Lines beginning with a $ indicate conditional
constructs. Other lines denote key bindings and variable settings.
The default key-bindings may be changed with an inputrc file. Other
programs that use this library may add their own 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 univer-
sal-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 speci-
fied 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-argument,
M-DEL is bound to the function backward-kill-word, and C-o is bound to
run the macro expressed on the right hand side (that is, to insert the
text ``> output'' into the line).
In the second form, "keyseq":function-name or macro, keyseq differs from
keyname above in that strings denoting an entire key sequence may be
specified by placing the sequence within double quotes. Some GNU Emacs
style key escapes can be used, as in the following example, but the sym-
bolic character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function universal-argument.
C-x C-r is bound to the function re-read-init-file, and ESC [ 1 1 ~ is
bound to insert the text ``Function Key 1''.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
In addition to the GNU Emacs style escape sequences, a second set of
backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value nnn
(one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
When entering the text of a macro, single or double quotes must be used
to indicate a macro definition. Unquoted text is assumed to be a func-
tion 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 modi-
fied 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 behav-
ior. A variable may be set in the inputrc file with a statement of the
form
set variable-name value
or using the bind builtin command (see SHELL BUILTIN COMMANDS below).
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-insensi-
tive), and "1" are equivalent to On. All other values are equivalent to
Off. The variables and their default values are:
active-region-start-color
A string variable that controls the text color and background
when displaying the text in the active region (see the descrip-
tion of enable-active-region below). This string must not take
up any physical character positions on the display, so it should
consist only of terminal escape sequences. It is output to the
terminal before displaying the text in the active region. This
variable is reset to the default value whenever the terminal type
changes. The default value is the string that puts the terminal
in standout mode, as obtained from the terminal's terminfo de-
scription. A sample value might be "\e[01;33m".
active-region-end-color
A string variable that "undoes" the effects of active-re-
gion-start-color and restores "normal" terminal display appear-
ance after displaying text in the active region. This string
must not take up any physical character positions on the display,
so it should consist only of terminal escape sequences. It is
output to the terminal after displaying the text in the active
region. This variable is reset to the default value whenever the
terminal type changes. The default value is the string that re-
stores the terminal from standout mode, as obtained from the ter-
minal's terminfo description. A sample value might be "\e[0m".
bell-style (audible)
Controls what happens when readline wants to ring the terminal
bell. If set to none, readline never rings the bell. If set to
visible, readline uses a visible bell if one is available. If
set to audible, readline attempts to ring the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters
treated specially by the kernel's terminal driver to their read-
line 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 differ-
ent color. The color definitions are taken from the value of the
LS_COLORS environment variable. If there is a color definition
in $LS_COLORS for the custom suffix "readline-colored-completion-
prefix", readline uses this color for the common prefix instead
of its default.
colored-stats (Off)
If set to On, readline displays possible completions using dif-
ferent colors to indicate their file type. The color definitions
are taken from the value of the LS_COLORS environment variable.
comment-begin (``#'')
The string that is inserted when the readline insert-comment com-
mand 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 completion
in a case-insensitive fashion.
completion-map-case (Off)
If set to On, and completion-ignore-case is enabled, readline
treats hyphens (-) and underscores (_) as equivalent when per-
forming case-insensitive filename matching and completion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of possi-
ble completions that is displayed without modification. When set
to a value greater than zero, common prefixes longer than this
value are replaced with an ellipsis when displaying possible com-
pletions.
completion-query-items (100)
This determines when the user is queried about viewing the number
of possible completions generated by the possible-completions
command. It may be set to any integer value greater than or
equal to zero. If the number of possible completions is greater
than or equal to the value of this variable, readline will ask
whether or not the user wishes to view them; otherwise they are
simply listed on the terminal. A zero value means readline
should never ask; negative values are treated as zero.
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. This variable is
dependent on the LC_CTYPE locale category, and may change if the
locale is changed.
disable-completion (Off)
If set to On, readline will inhibit word completion. Completion
characters will be inserted into the line as if they had been
mapped to self-insert.
echo-control-characters (On)
When set to On, on operating systems that indicate they support
it, readline echoes a character corresponding to a signal gener-
ated from the keyboard.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings simi-
lar 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 primary 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-active-region (On)
The point is the current cursor position, and mark refers to a
saved cursor position. The text between the point and mark is
referred to as the region. When this variable is set to On,
readline allows certain commands to designate the region as ac-
tive. When the region is active, readline highlights the text in
the region using the value of the active-region-start-color,
which defaults to the string that enables the terminal's standout
mode. The active region shows the text inserted by bracketed-
paste and any matching text found by incremental and non-incre-
mental history searches.
enable-bracketed-paste (On)
When set to On, readline configures the terminal to insert each
paste into the editing buffer as a single string of characters,
instead of treating each character as if it had been read from
the keyboard. This prevents readline from executing any editing
commands bound to key sequences appearing in the pasted text.
enable-keypad (Off)
When set to On, readline will try to enable the application key-
pad when it is called. Some systems need this to enable the ar-
row keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta modifier key
the terminal claims to support when it is called. On many termi-
nals, 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 previous-his-
tory or next-history.
history-size (unset)
Set the maximum number of history entries saved in the history
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 display,
scrolling the input horizontally on a single screen line when it
becomes longer than the screen width rather than wrapping to a
new line. This setting is automatically enabled for terminals of
height 1.
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), re-
gardless of what the terminal claims it can support. 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. This variable is dependent on the LC_CTYPE locale
category, and may change if the locale is changed.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an incremental
search without subsequently executing the character as a command.
If this variable has not been given a value, the characters ESC
and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names
is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command,
and vi-insert. vi is equivalent to vi-command; emacs is equiva-
lent 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 re-
ceived within the timeout, readline will use the shorter but com-
plete key sequence. The value is specified in milliseconds, so a
value of 1000 means that readline will wait one second for addi-
tional input. If this variable is set to a value less than or
equal to zero, or to a non-numeric value, readline will wait un-
til another key is pressed to decide which key sequence to com-
plete.
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 direc-
tories have a slash appended (subject to the value of mark-direc-
tories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files
whose names begin with a `.' (hidden files) when performing file-
name completion. If set to Off, the leading `.' must be supplied
by the user in the filename to be completed.
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. This variable is dependent on the
LC_CTYPE locale category, and may change if the locale is
changed.
page-completions (On)
If set to On, readline uses an internal more-like pager to dis-
play 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 be-
fore returning when accept-line is executed. By default, history
lines may be modified and retain individual undo lists across
calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If
set to On, words which have more than one possible completion
cause the matches to be listed immediately instead of ringing the
bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a
fashion similar to show-all-if-ambiguous. If set to On, words
which have more than one possible completion without any possible
partial completion (the possible 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 indi-
cating 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, read-
line does not insert characters from the completion that match
characters after point in the word being completed, so portions
of the word following the cursor 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 primary 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 con-
trol 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 primary 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 con-
trol 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 listing possible comple-
tions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional com-
pilation 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 char-
acters are required to isolate it.
mode The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode. This may be used
in conjunction with the set keymap command, for instance,
to set bindings in the emacs-standard and emacs-ctlx
keymaps only if readline is starting out in emacs mode.
term The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by
the terminal's function keys. The word on the right side
of the = is tested against both the full name of the ter-
minal 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 version 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 op-
erator may be separated from the string version and from
the version number argument by whitespace.
application
The application construct is used to include application-
specific settings. Each program using the readline li-
brary sets the application name, and an initialization
file can test for a particular value. This could be used
to bind key sequences to functions useful for a specific
program. For instance, the following command adds a key
sequence that quotes the current or previous word in bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
variable
The variable construct provides simple equality tests for
readline variables and values. The permitted comparison
operators are =, ==, and !=. The variable name must be
separated from the comparison operator by whitespace; the
operator may be separated 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 history
(see HISTORY below) for lines containing a specified string. There are
two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the
search string. As each character of the search string is typed, read-
line displays the next entry from the history matching the string typed
so far. An incremental search requires only as many characters as
needed to find the desired history entry. The characters present in the
value of the isearch-terminators variable are used to terminate an in-
cremental search. If that variable has not been assigned a value the
Escape and Control-J characters will terminate an incremental search.
Control-G will abort an incremental search and restore the original
line. When the search is terminated, the history entry containing the
search string becomes the 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 his-
tory 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 accompany-
ing key sequence are unbound by default. In the following descriptions,
point refers to the current cursor position, and mark refers to a cursor
position saved by the set-mark command. The text between the point and
mark is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of
alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words
are composed of alphanumeric characters (letters and digits).
shell-forward-word
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word. Words
are delimited by non-quoted shell metacharacters.
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 ef-
fect 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 phys-
ical line or if the length of the current readline line is not
greater than the length of the prompt plus the screen width.
clear-display (M-C-l)
Clear the screen and, if possible, the terminal's scrollback
buffer, then redraw the current line, leaving the current line at
the top of the screen.
clear-screen (C-l)
Clear the screen, then redraw the current line, leaving the cur-
rent line at the top of the screen. With an argument, refresh
the current line without clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line
is non-empty, add it to the history list according to the state
of the HISTCONTROL variable. If the line is a modified history
line, then restore the history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in
the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in
the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently
being entered.
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 nu-
meric argument, if supplied, specifies the history entry to use
instead of the current line.
fetch-history
With a numeric argument, fetch that entry from the history list
and make it the current line. Without an argument, move back to
the first entry in the history list.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up'
through the history as necessary. This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down'
through the history as necessary. This is an incremental 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 characters
between the start of the current line and the point. This is a
non-incremental search.
history-search-backward
Search backward through the history for the string of characters
between the start of the current line and the point. This is a
non-incremental search.
history-substring-search-backward
Search backward through the history for the string of characters
between the start of the current line and the current cursor po-
sition (the point). The search string may match anywhere in a
history line. This is a non-incremental search.
history-substring-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a non-in-
cremental 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 ar-
gument n is computed, the argument is extracted as if the "!n"
history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word
of the previous history entry). With a numeric argument, behave
exactly like yank-nth-arg. Successive calls to yank-last-arg
move back through the history list, inserting the last word (or
the word specified by the argument to the first call) of each
line in turn. Any numeric argument supplied to these successive
calls determines the direction to move through the history. A
negative argument switches the direction through the history
(back or forward). 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 his-
tory expansion as well as all of the shell word expansions. See
HISTORY EXPANSION below for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY EX-
PANSION 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 expan-
sion.
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.
edit-and-execute-command (C-x C-e)
Invoke an editor on the current command line, and execute the re-
sult as shell commands. Bash attempts to invoke $VISUAL, $EDI-
TOR, 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 commonly is, see
above for the effects.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric ar-
gument, 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 cur-
sor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how
to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well. If point is at the end of
the line, then this transposes the two characters before point.
Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving
point over that word as well. If point is at the end of the
line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argu-
ment, uppercase the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative argu-
ment, lowercase the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative ar-
gument, capitalize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argu-
ment, switches to overwrite mode. With an explicit non-positive
numeric argument, switches to insert mode. This command affects
only emacs mode; vi mode does overwrite differently. Each call
to readline() starts in insert mode. In overwrite mode, charac-
ters bound to self-insert replace the text at point rather than
pushing the text to the right. Characters bound to back-
ward-delete-char replace the character before point with a space.
By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The
killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as
those used by backward-word.
shell-kill-word
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by shell-forward-word.
shell-backward-kill-word
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 char-
acter 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 bound-
aries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following
yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a
new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is fol-
lowed by digits, executing universal-argument again ends the nu-
meric argument, but is otherwise ignored. As a special case, if
this command is immediately followed by a character that is nei-
ther a digit nor minus sign, the argument count for the next com-
mand is multiplied 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 at-
tempts completion treating the text as a variable (if the text
begins with $), username (if the text begins with ~), hostname
(if the text begins with @), or command (including aliases and
functions) in turn. If none of these produces a match, filename
completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a
single match from the list of possible completions. Repeated ex-
ecution of menu-complete steps through the list of possible com-
pletions, inserting each match in turn. At the end of the list
of completions, the bell is rung (subject to the setting of
bell-style) and the original text is restored. An argument of n
moves n positions forward in the list of matches; a negative ar-
gument may be used to move backward through the list. This com-
mand is intended to be bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list
of possible completions, as if menu-complete had been given a
negative argument. This command is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char). If at the end of the line,
behaves identically to possible-completions. This command is un-
bound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating
it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating
it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating
it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating
it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating
it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible com-
pletions 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 charac-
ters 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 character. The be-
havior is undefined if x is already lowercase.
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 supplied,
the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is set
to the saved position, and the old cursor position is saved as
the mark.
character-search (C-])
A character is read and point is moved to the next occurrence of
that character. A negative argument searches for previous occur-
rences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence
of that character. A negative argument searches for subsequent
occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. Such sequences begin
with a Control Sequence Indicator (CSI), usually ESC-[. If this
sequence is bound to "\[", keys producing such sequences will
have no effect unless explicitly bound to a readline command, in-
stead of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline comment-be-
gin variable is inserted at the beginning of the current line.
If a numeric argument is supplied, this command acts as a toggle:
if the characters at the beginning of the line do not match the
value of comment-begin, the value is inserted, otherwise the
characters in comment-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 ar-
gument causes the comment character to be removed, the line will
be executed by the shell.
spell-correct-word (C-x s)
Perform spelling correction on the current word, treating it as a
directory or filename, in the same way as the cdspell shell op-
tion. Word boundaries are the same as those used by shell-for-
ward-word.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname expan-
sion, with an asterisk implicitly appended. This pattern is used
to generate a list of matching filenames for possible comple-
tions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expan-
sion, and the list of matching filenames is inserted, replacing
the word. If a numeric argument is supplied, an asterisk is ap-
pended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by glob-ex-
pand-word is displayed, and the line is redrawn. If a numeric
argument is supplied, an asterisk is appended before pathname ex-
pansion.
dump-functions
Print all of the functions and their key bindings to the readline
output stream. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an inputrc
file.
dump-variables
Print all of the settable readline variables and their values to
the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part of
an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied, the out-
put is formatted in such a way that it can be made part of an in-
putrc 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 com-
plete builtin (see SHELL BUILTIN COMMANDS below), the programmable com-
pletion 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 compspec 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 comp-
spec, bash attempts alias expansion on the command word as a final re-
sort, and attempts to find a compspec for the command word from any suc-
cessful 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 completion
as described above under Completing is performed.
First, the actions specified by the compspec are used. Only matches
which are prefixed by the word being completed are returned. When the
-f or -d option is used for filename or directory name completion, the
shell variable FIGNORE is used to filter the matches.
Any completions specified by a pathname expansion pattern to the -G op-
tion are generated next. The words generated by the pattern need not
match the word being completed. The GLOBIGNORE shell variable is not
used to filter the matches, but the FIGNORE variable is used.
Next, the string specified as the argument to the -W option is consid-
ered. The string is first split using the characters in the IFS special
variable as delimiters. Shell quoting is honored. Each word is then
expanded using brace expansion, tilde expansion, parameter and variable
expansion, command substitution, and arithmetic expansion, as described
above under EXPANSION. The results are split using the rules described
above under Word Splitting. The results of the expansion are prefix-
matched against the word being completed, and the matching words become
the possible completions.
After these matches have been generated, any shell function or command
specified with the -F and -C options is invoked. When the command or
function is invoked, the COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE
variables are assigned values as described above under Shell Variables.
If a shell function is being invoked, the COMP_WORDS and COMP_CWORD
variables are also set. When the function or command is invoked, the
first argument ($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 completions
against the word being completed is performed; the function or command
has complete freedom in generating the matches.
Any function specified with -F is invoked first. The function may use
any of the shell facilities, including the compgen builtin described be-
low, to generate the matches. It must put the possible completions in
the COMPREPLY array variable, one per array element.
Next, any command specified with the -C option is invoked in an environ-
ment equivalent to command substitution. It should print a list of com-
pletions, one per line, to the standard output. Backslash may be used
to escape a newline, if necessary.
After all of the possible completions are generated, any filter speci-
fied 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 com-
pletion that matches the pattern will be removed from the list. A lead-
ing ! 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 charac-
ters.
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 de-
fined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when the compspec was
defined, directory name completion is attempted and any matches are
added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions. The de-
fault bash completions are not attempted, and the readline default of
filename completion is disabled. If the -o bashdefault option was sup-
plied to complete when the compspec was defined, the bash default com-
pletions are attempted if the compspec generates no matches. If the -o
default option was supplied to complete when the compspec was defined,
readline's default completion will be performed if the compspec (and, if
attempted, the default bash completions) generate no matches.
When a compspec indicates that directory name completion is desired, the
programmable completion functions force readline to append a slash to
completed names which are symbolic links to directories, subject to the
value of the mark-directories readline variable, regardless of the set-
ting of the mark-symlinked-directories readline variable.
There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion specified
with complete -D. It's possible for shell functions executed as comple-
tion handlers to indicate that completion should be retried by returning
an exit status of 124. If a shell function returns 124, and changes the
compspec associated with the command on which completion is being at-
tempted (supplied as the first argument when the function is executed),
programmable completion restarts from the beginning, with an attempt to
find a new compspec for that command. This allows a set of completions
to be built dynamically as completion is attempted, rather than being
loaded all at once.
For instance, assuming that there is a library of compspecs, each kept
in a file corresponding to the name of the command, the following de-
fault completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default
HISTORY
When the -o history option to the set builtin is enabled, the shell pro-
vides access to the command history, the list of commands previously
typed. The value of the HISTSIZE variable is used as the number of com-
mands 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 vari-
able HISTFILE (default ~/.bash_history). The file named by the value of
HISTFILE is truncated, if necessary, to contain no more than the number
of lines specified by the value of HISTFILESIZE. If HISTFILESIZE is un-
set, 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 un-
der SHELL BUILTIN COMMANDS below), the lines are appended to the history
file, otherwise the history file is overwritten. If HISTFILE is unset,
or if the history file is unwritable, the history is not saved. If the
HISTTIMEFORMAT variable is set, time stamps are written to the history
file, marked with the history comment character, so they may be pre-
served 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 HISTFILE-
SIZE 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 trun-
cated.
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 history
list. The HISTCONTROL and HISTIGNORE variables may be set to cause the
shell to save only a subset of the commands entered. The cmdhist shell
option, if enabled, causes the shell to attempt to save each line of a
multi-line command in the same history entry, adding semicolons where
necessary to preserve syntactic correctness. The lithist shell option
causes the shell to save the command with embedded newlines instead of
semicolons. See the description of the shopt builtin below under SHELL
BUILTIN COMMANDS for information on 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 com-
mand (see SHELL BUILTIN COMMANDS below). Non-interactive shells do not
perform history expansion by default.
History expansions introduce words from the history list into the input
stream, making it easy to repeat commands, insert the arguments to a
previous command into the current input line, or fix errors in previous
commands quickly.
History expansion is performed immediately after a complete line is
read, before the shell breaks it into words, 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 substitution. The second is to select
portions of that line for inclusion into the current one. The line se-
lected 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
surrounded 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 fol-
lowing 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 op-
tion is enabled (see the description of the shopt builtin below), and
readline is being used, history substitutions are not immediately passed
to the shell parser. Instead, the expanded line is reloaded into the
readline editing buffer for further modification. If readline is being
used, and the histreedit shell option is enabled, a failed history sub-
stitution will be reloaded into the readline editing buffer for correc-
tion. 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 his-
tory timestamps when writing the history file.
Event Designators
An event designator is a reference to a command line entry in the his-
tory list. Unless the reference is absolute, events are relative to the
current position in the history list.
! Start a history substitution, except when followed by a blank,
newline, carriage return, = or ( (when the extglob shell option
is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command preceding the current position
in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current position
in the history list containing string. The trailing ? may be
omitted if string is followed immediately by a newline. If
string is missing, the string from the most recent search is
used; it is an error if there is no previous search string.
^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 ex-
pand to the zeroth word if there is only one word in the line.
% The first word matched by the most recent `?string?' search, if
the search string begins with a character that is part of a word.
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 x is miss-
ing, it defaults to 0.
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 `:'. These mod-
ify, or edit, the word or words selected from the history event.
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 substitutions.
x Quote the substituted words as with q, but break into words at
blanks and newlines. The q and x modifiers are mutually exclu-
sive; the last one supplied is used.
s/old/new/
Substitute new for the first occurrence of old in the event line.
Any character may be used as the delimiter 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 substitu-
tions took place, the last string in a !?string[?] search. If
new is null, each matching old is deleted.
& 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' or `&' 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, con-
tinue, let, and shift builtins accept and process arguments beginning
with - without requiring --. Other builtins that accept arguments but
are not specified as accepting options interpret arguments beginning
with - as invalid options and require -- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments
and performing any specified redirections. The return status is
zero.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell en-
vironment and return the exit status of the last command executed
from filename. If filename does not contain a slash, filenames
in PATH are used to find the directory containing filename, but
filename does not need to be executable. The file searched for
in PATH need not be executable. When bash is not in posix mode,
it searches the current directory if no file is found in PATH.
If the sourcepath option to the shopt builtin command is turned
off, the PATH is not searched. If any arguments are supplied,
they become the positional parameters when filename is executed.
Otherwise the positional parameters are unchanged. If the -T op-
tion is enabled, . inherits any trap on DEBUG; if it is not, any
DEBUG trap string is saved and restored around the call to ., and
. unsets the DEBUG trap while it executes. If -T is not set, and
the sourced file changes the DEBUG trap, the new value is re-
tained when . completes. The return status is the status of the
last command exited within the script (0 if no commands are exe-
cuted), and false if filename is not found or cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of
aliases in the form alias name=value on standard output. When
arguments are supplied, an alias is defined for each name whose
value is given. A trailing space in value causes the next word
to be checked for alias substitution when the alias is expanded.
For each name in the argument list for which no value is sup-
plied, 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 no-
tion of the current job is used. bg jobspec returns 0 unless run
when job control is disabled or, when run with job control en-
abled, 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
bind readline-command-line
Display current readline key and function bindings, bind a key
sequence to a readline function or macro, or set a readline vari-
able. Each non-option argument is a command as it would appear
in a readline initialization file such as .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
following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent
bindings. Acceptable keymap names are emacs, emacs-stan-
dard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and
vi-insert. vi is equivalent 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 en-
tered. When shell-command is executed, the shell sets the
READLINE_LINE variable to the contents of the readline
line buffer and the READLINE_POINT and READLINE_MARK vari-
ables to the current location of the insertion point and
the saved insertion point (the mark), respectively. The
shell assigns any numeric argument the user supplied to
the READLINE_ARGUMENT variable. If there was no argument,
that variable is not set. If the executed command changes
the value of any of READLINE_LINE, READLINE_POINT, or
READLINE_MARK, those new values will be reflected in the
editing state.
-X List all key sequences bound to shell commands and the as-
sociated 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, retaining the func-
tionality 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 func-
tion or a script executed with the . or source builtins). With-
out expr, caller displays the line number and source filename of
the current subroutine call. If a non-negative integer is sup-
plied as expr, caller displays the line number, subroutine name,
and source file corresponding to that position in the current ex-
ecution call stack. This extra information may be used, for ex-
ample, to print a stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a subrou-
tine call or expr does not correspond to a valid position in the
call stack.
cd [-L|[-P [-e]] [-@]] [dir]
Change the current directory to dir. if dir is not supplied, the
value of the HOME shell variable is the default. 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 separated by a colon (:). A null
directory name in CDPATH is the same as the current directory,
i.e., ``.''. If dir begins with a slash (/), then CDPATH is not
used. The -P option 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 processed by removing
the immediately previous pathname component 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 de-
termined after a successful 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 di-
rectory. An argument of - is converted to $OLDPWD before the di-
rectory 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 pathname of the new working
directory is written to the standard output. If the directory
change is successful, cd sets the value of the PWD environment
variable to the new directory name, and sets the OLDPWD environ-
ment variable to the value of the current working directory be-
fore the change. 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 op-
tion 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 nei-
ther option is supplied and an error occurred or command cannot
be found, the exit status is 127. Otherwise, the exit status of
the command builtin is the exit status of command.
compgen [option] [word]
Generate possible completion matches for word according to the
options, which may be any option accepted by the complete builtin
with the exception of -p and -r, and write the matches to the
standard output. When using the -F or -C options, the various
shell variables set by the programmable completion facilities,
while available, will not have useful values.
The matches will be generated in the same way as if the program-
mable completion code had generated them directly from a comple-
tion specification with the same flags. If word is specified,
only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or
no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action] [-G glob-
pat] [-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 speci-
fication for each name, or, if no names are supplied, all comple-
tion specifications. The -D option indicates that other supplied
options and actions should apply to the ``default'' command com-
pletion; that is, completion attempted on a command for which no
completion has previously been defined. The -E option indicates
that 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 supplied options and
actions should apply to completion on the initial 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 ap-
ply 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 necessary, the
-P and -S options) should be quoted to protect them from expan-
sion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the comp-
spec's behavior beyond the simple generation of comple-
tions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions
if the compspec generates no matches.
default Use readline's default filename completion if the
compspec generates no matches.
dirnames
Perform directory name completion if the compspec
generates no matches.
filenames
Tell readline that the compspec generates file-
names, so it can perform any filename-specific
processing (like adding a slash to directory
names, quoting special characters, or suppressing
trailing spaces). Intended to be used with shell
functions.
noquote Tell readline not to quote the completed words if
they are filenames (quoting filenames is the de-
fault).
nosort Tell readline not to sort the list of possible
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 specified as
-k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be speci-
fied as -v.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions. Arguments
are passed as with the -F option.
-F function
The shell function function is executed in the current
shell environment. When the function is executed, the
first argument ($1) is the name of the command whose ar-
guments 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 cur-
rent command line. When it finishes, the possible com-
pletions 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 comple-
tion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all
other options have been applied.
-W wordlist
The wordlist is split using the characters in the IFS
special variable as delimiters, and each resultant word
is expanded. 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 completions are the members of the resultant
list which match the word being completed.
-X filterpat
filterpat is a pattern as used for pathname expansion.
It is applied to the list of possible completions gener-
ated by the preceding options and arguments, and each
completion matching filterpat is removed from the list.
A leading ! in filterpat negates the pattern; in this
case, any completion not matching filterpat is removed.
The return value is true unless an invalid option is supplied, an
option other than -p or -r is supplied without a name argument,
an attempt is made to remove a completion specification for a
name for which no specification exists, or an error occurs adding
a completion specification.
compopt [-o option] [-DEI] [+o option] [name]
Modify completion options for each name according to the options,
or for the currently-executing completion if no names are sup-
plied. 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 complete builtin described above.
The -D option indicates that other supplied options should apply
to the ``default'' command completion; that is, completion at-
tempted on a command 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 initial 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 supplied, an
attempt is made to modify the options for a name for which no
completion specification exists, or an output error occurs.
continue [n]
Resume the next iteration of the enclosing for, while, until, or
select loop. If n is specified, resume at the nth enclosing
loop. n must be ≥ 1. If n is greater than the number of enclos-
ing 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 [-aAfFgiIlnrtux] [-p] [name[=value] ...]
typeset [-aAfFgiIlnrtux] [-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 at-
tributes specified by the additional options. If no other op-
tions are supplied with -p, declare will display the attributes
and values of all shell variables. The -f option will restrict
the display to shell functions. The -F option inhibits the dis-
play of function definitions; only the function name and attrib-
utes 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 -I option causes local variables
to inherit the attributes (except the nameref attribute) and
value of any existing variable with the same name at a surround-
ing scope. If there is no existing variable, the local variable
is initially unset. The following options can be used to re-
strict 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 evalua-
tion (see ARITHMETIC EVALUATION above) is performed when
the variable is assigned a value.
-l When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
-n Give each name the nameref attribute, making it a name
reference to another variable. That other variable is de-
fined by the value of name. All references, assignments,
and attribute modifications to name, except those using or
changing the -n attribute itself, are performed on the
variable referenced by name's value. The nameref at-
tribute cannot 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 functions in-
herit 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 envi-
ronment.
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 as-
sign a value to a readonly variable, an attempt is made to assign
a value to an array variable without using the compound assign-
ment syntax (see Arrays above), one of the names is not a valid
shell variable name, an attempt is made to turn off readonly sta-
tus 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 direc-
tories. 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 list-
ing 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, prefix-
ing 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 supplied, the -a option means to remove
or mark all jobs; the -r option without a jobspec argument re-
stricts 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 newline. The
return status is 0 unless a write error occurs. If -n is speci-
fied, the trailing newline is suppressed. If the -e option is
given, interpretation of the following backslash-escaped charac-
ters is enabled. The -E option disables the interpretation of
these escape characters, even on systems where they are inter-
preted by default. The xpg_echo shell option may be used to dy-
namically determine whether or not echo expands these escape
characters by default. echo does not interpret -- to mean the
end of options. echo interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value nnn
(zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though
the shell normally searches for builtins before disk commands.
If -n is used, each name is disabled; otherwise, names are en-
abled. 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.
Bash will use the value of the BASH_LOADABLES_PATH variable as a
colon-separated list of directories in which to search for file-
name. The default is system-dependent. 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. If no options are supplied and a
name is not a shell builtin, enable will attempt to load name
from a shared object named name, as if the command were ``enable
-f name name . 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 com-
mand. 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 begin-
ning of the zeroth argument passed to command. This is what lo-
gin(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, un-
less the execfail shell option is enabled. In that case, it re-
turns 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 omitted, the
exit status is that of the last command executed. A trap on EXIT
is executed before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the envi-
ronment 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 variable is set to word. export returns
an exit status of 0 unless an invalid option is encountered, one
of the names is not a valid shell variable name, or -f is sup-
plied 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 negative number is used as an offset
from the current command number). When listing, a first or last
of 0 is equivalent to -1 and -0 is equivalent to the current com-
mand (usually the fc command); otherwise 0 is equivalent to -1
and -0 is invalid. If last is not specified, it is set to the
current command for listing (so that ``fc -l -10'' prints the
last 10 commands) and to first otherwise. If first is not speci-
fied, 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 vari-
able 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 in-
valid option is encountered or first or last specify history
lines out of range. If the -e option is supplied, the return
value is the value of the last command executed or failure if an
error occurs with the temporary file of commands. If the second
form is used, the return status is that of the command re-exe-
cuted, unless cmd does not specify a valid history line, in which
case fc returns failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the current job.
If jobspec is not present, the shell's notion of the current job
is used. The return value is that of the command placed into the
foreground, or failure if run when job control is disabled or,
when run with job control enabled, if jobspec does not specify a
valid job or jobspec specifies a job that was started without job
control.
getopts optstring name [arg ...]
getopts is used by shell procedures to parse positional parame-
ters. 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 vari-
able 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 re-
turn 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 supplied as arg values, getopts parses those in-
stead.
getopts can report errors in two ways. If the first character of
optstring is a colon, silent error reporting is used. In normal
operation, diagnostic messages are printed when invalid options
or missing option arguments are encountered. If the variable
OPTERR is set to 0, no error messages will be displayed, even if
the first character of optstring is not a colon.
If an invalid option is seen, getopts places ? into name and, if
not silent, prints an error message and unsets OPTARG. If
getopts is silent, the option character found is placed in OPTARG
and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent, a
question mark (?) is placed in name, OPTARG is unset, and a diag-
nostic message is printed. If getopts is silent, then a colon
(:) is placed in name and OPTARG is set to the option character
found.
getopts returns true if an option, specified or unspecified, is
found. It returns false if the end of options is encountered or
an error occurs.
hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, the full pathname of the command name
is determined by searching the directories in $PATH and remem-
bered. Any previously-remembered pathname is discarded. 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 op-
tion 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 for-
mat 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 supplied.
help [-dms] [pattern]
Display helpful information about builtin commands. If pattern
is specified, help gives detailed help on all commands matching
pattern; otherwise help for all the builtins and shell control
structures is printed.
-d Display a short description of each pattern
-m Display the description of each pattern in a manpage-like
format
-s Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -d start-end
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line num-
bers. Lines listed with a * have been modified. An argument of
n lists only the last n lines. If the shell variable HISTTIME-
FORMAT is set and not null, it is used as a format string for
strftime(3) to display the time stamp associated with each dis-
played history entry. No intervening 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 fol-
lowing meanings:
-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 negative 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 range of 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 his-
tory file.
-n Read the history lines not already read from the history
file into the current history list. These are lines ap-
pended to the history file since the beginning of the cur-
rent 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, over-
writing the history file's contents.
-p Perform history substitution on the following args and
display the result on the standard output. Does not store
the results in the history list. Each arg must be quoted
to disable normal history expansion.
-s Store the args in the history list as a single entry. The
last command in the history list is removed before the
args are added.
If the HISTTIMEFORMAT variable is set, the time stamp information
associated with each history entry is written to the history
file, marked with the history comment character. When the his-
tory file is read, lines beginning with the history comment char-
acter followed immediately by a digit are interpreted as time-
stamps for the following history 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 or range
is supplied as an argument to -d, or the history expansion sup-
plied as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the fol-
lowing meanings:
-l List process IDs in addition to the normal information.
-n Display information only about jobs that have changed sta-
tus 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 en-
countered 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 exe-
cutes 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-insensitive 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 argument of -l lists the signal
names. If any arguments are supplied when -l is given, the names
of the signals corresponding to the arguments are listed, and the
return status is 0. The exit_status argument to -l is a number
specifying either a signal number or the exit status of a process
terminated by a signal. 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 ARITH-
METIC EVALUATION above). If the last arg evaluates to 0, let re-
turns 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 original values when the function returns. The restore is
effected as if a series of set commands were executed to restore
the values that were in place before the function. With no
operands, local writes a list of local variables to the standard
output. It is an error to use local when not within a function.
The return status is 0 unless local is used outside a function,
an invalid name is supplied, or name is a readonly variable.
logout Exit a login shell.
mapfile [-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 vari-
able array, or from file descriptor fd if the -u option is sup-
plied. 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 in-
put 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 in-
dex 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 ele-
ment 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 ar-
ray is not an indexed array.
popd [-n] [+n] [-n]
Removes entries from the directory stack. The elements are num-
bered from 0 starting at the first directory listed by dirs.
With no arguments, popd removes the top directory from the stack,
and changes to the new top directory. Arguments, if supplied,
have the following meanings:
-n Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is ma-
nipulated.
+n Removes the nth entry counting from the left of the list
shown by dirs, starting with zero, from the stack. For
example: ``popd +0'' removes the first directory, ``popd
+1'' the second.
-n Removes the nth entry counting from the right of the list
shown by dirs, starting with zero. For example: ``popd
-0'' removes the last directory, ``popd -1'' the next to
last.
If the top element of the directory stack is modified, and the -n
option was not supplied, popd uses the cd builtin to change to
the directory at the top of the stack. If the cd fails, popd re-
turns a non-zero value.
Otherwise, popd returns false if an invalid option is encoun-
tered, the directory stack is empty, or a non-existent directory
stack entry is specified.
If the popd command is successful, bash runs dirs to show the fi-
nal contents of the directory stack, and the return status is 0.
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 as-
signed to the variable var rather than being printed to the stan-
dard output.
The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and
copied to the standard output, and format specifications, each of
which causes printing of the next successive argument. In addi-
tion to the standard printf(1) format specifications, printf in-
terprets 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.
%Q like %q, but applies any supplied precision to the argu-
ment before quoting it.
%(datefmt)T
causes printf to output the date-time string resulting
from using datefmt as a format string for strftime(3).
The corresponding argument is an integer representing the
number of seconds since the epoch. Two special argument
values may be used: -1 represents the current time, and -2
represents the time the shell was invoked. If no argument
is specified, conversion behaves as if -1 had been given.
This is an exception to the usual printf behavior.
The %b, %q, and %T directives all use the field width and preci-
sion arguments from the format specification and write that many
bytes from (or use that wide a field for) the expanded argument,
which usually contains more characters than the original.
Arguments to non-string format specifiers are treated as C con-
stants, except that a leading plus or minus sign is allowed, and
if the leading character is a single or double quote, the value
is the ASCII value of the following character.
The format is reused as necessary to consume all of the argu-
ments. 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 supplied. 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 current working
directory. With no arguments, pushd exchanges the top two ele-
ments of the directory stack. 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 (counting from
the left of the list shown by dirs, starting with zero) is
at the top.
-n Rotates the stack so that the nth directory (counting from
the right of the list shown by dirs, starting with zero)
is at the top.
dir Adds dir to the directory stack at the top
After the stack has been modified, if the -n option was not sup-
plied, pushd uses the cd builtin to change to the directory at
the top of the stack. If the cd fails, pushd returns a non-zero
value.
Otherwise, if no arguments are supplied, pushd returns 0 unless
the directory stack is empty. When rotating the directory stack,
pushd returns 0 unless the directory stack is empty or a non-ex-
istent directory stack element is specified.
If the pushd command is successful, bash runs dirs to show the
final contents of the directory stack.
pwd [-LP]
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P option
is supplied or the -o physical option to the set builtin command
is enabled. If the -L option is used, the pathname printed may
contain symbolic links. The return status is 0 unless an error
occurs while reading the name of the current directory or an in-
valid option is supplied.
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p
prompt] [-t timeout] [-u fd] [name ...]
One line is read from the standard input, or from the file de-
scriptor 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 second word to the second
name, and so on. If there are more words than names, the remain-
ing 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 re-
move 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 ig-
nored.
-d delim
The first character of delim is used to terminate the in-
put 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 pre-
viously active) editing settings, but uses readline's de-
fault filename completion.
-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 honors a delim-
iter if fewer than nchars characters are read before the
delimiter.
-N nchars
read returns after reading exactly nchars characters
rather than waiting for a complete line of input, unless
EOF is encountered or read times out. Delimiter charac-
ters 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 new-
line, 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 back-
slash is considered to be part of the line. In particu-
lar, a backslash-newline pair may not then be used as a
line continuation.
-s Silent mode. If input is coming from a terminal, charac-
ters are not echoed.
-t timeout
Cause read to time out and return failure if a complete
line of input (or a specified number of characters) 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 immediately,
without trying to read any data. The exit status is 0 if
input is available on the specified file descriptor, or
the read will return EOF, non-zero otherwise. The exit
status is greater than 128 if the timeout is exceeded.
-u fd Read input from file descriptor fd.
If no names are supplied, the line read, without the ending de-
limiter but otherwise unmodified, is assigned to the variable RE-
PLY. The exit status is zero, unless end-of-file is encountered,
read times out (in which case the status is greater than 128), a
variable assignment error (such as assigning to a readonly vari-
able) 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 read-
only names is printed. The other options may be used to restrict
the output to a subset of the set of readonly names. The -p op-
tion 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 speci-
fied by n to its caller. If n is omitted, the return status is
that of the last command executed in the function body. If re-
turn is executed by a trap handler, the last command used to de-
termine 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 outside a function and not during execution
of a script by . or source. Any command associated with the RE-
TURN 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, display the name and value of each shell vari-
able in a format that can be reused as input for setting or re-
setting 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 modified is
given the export attribute and marked for export to the
environment of subsequent commands.
-b Report the status of terminated background jobs immedi-
ately, 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 un-
til 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 com-
mand's return value is being inverted with !. If a com-
pound command other than a subshell returns a non-zero
status because a command failed while -e was being ig-
nored, 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
separately (see COMMAND EXECUTION ENVIRONMENT above), and
may cause subshells to exit before executing all the com-
mands 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 com-
mand or shell function sets -e while executing in a con-
text 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 processes run in a sepa-
rate 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 ig-
nored 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 inter-
face. 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 inter-
active shells.
ignoreeof
The effect is as if the shell command ``IG-
NOREEOF=10'' had been executed (see Shell Vari-
ables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all commands
in the pipeline exit successfully. This option
is disabled by default.
posix Change the behavior of bash where the default op-
eration 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 interface.
This also affects the editing interface used for
read -e.
xtrace Same as -x.
If -o is supplied with no option-name, the values of the
current options are printed. If +o is supplied with no
option-name, a series of set commands to recreate the
current option settings is displayed on the standard out-
put.
-p Turn on privileged mode. In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and GLOBIGNORE variables, if they ap-
pear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to
the real user (group) id, and the -p option is not sup-
plied, 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 effective user and group ids
to be set to the real user and group ids.
-r Enable restricted shell mode. This option cannot be un-
set once it has been set.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the spe-
cial parameters "@" and "*", or array variables sub-
scripted with "@" or "*", as an error when performing pa-
rameter expansion. If expansion is attempted on an unset
variable or parameter, the shell prints an error message,
and, if not interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command, case
command, select command, or arithmetic for command, dis-
play the expanded value of PS4, followed by the command
and its expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace Expansion
above). This is on by default.
-C If set, bash does not overwrite an existing file with the
>, >&, and <> redirection operators. This may be over-
ridden when creating output files by using the redirec-
tion operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions,
command substitutions, and commands executed in a sub-
shell environment. The ERR trap is normally not inher-
ited 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 struc-
ture instead. By default, bash follows the logical chain
of directories when performing commands which change the
current directory.
-T If set, any traps on DEBUG and RETURN are inherited by
shell functions, command substitutions, and commands exe-
cuted in a subshell environment. The DEBUG and RETURN
traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional
parameters are unset. Otherwise, the positional parame-
ters 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 op-
tions are turned off. If there are no args, the posi-
tional 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 un-
set. 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 parame-
ters 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 behav-
ior. 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 displayed, 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 op-
tions 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 multi-
ple 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, respectively. Unless
otherwise noted, the shopt options are disabled (unset) by de-
fault.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting op-
tions, the return status is zero unless an optname is not a valid
shell option.
The list of shopt options is:
assoc_expand_once
If set, the shell suppresses multiple evaluation of asso-
ciative array subscripts during arithmetic expression
evaluation, while executing builtins that can perform
variable assignments, and while executing builtins that
perform array dereferencing.
autocd If set, a command name that is the name of a directory is
executed as if it were the argument to the cd command.
This option is only used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command that is not
a directory is assumed to be the name of a variable whose
value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory com-
ponent in a cd command will be corrected. The errors
checked for are transposed characters, a missing charac-
ter, 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 ta-
ble exists before trying to execute it. If a hashed com-
mand 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 ex-
iting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each external
(non-builtin) command and, if necessary, updates the val-
ues of LINES and COLUMNS. This option is enabled by de-
fault.
cmdhist If set, bash attempts to save all lines of a multiple-
line command in the same history entry. This allows easy
re-editing of multi-line commands. This option is en-
abled by default, but only has an effect if command his-
tory is enabled, as described above under HISTORY.
compat31
compat32
compat40
compat41
compat42
compat43
compat44
compat50
These control aspects of the shell's compatibility mode
(see SHELL COMPATIBILITY MODE below).
complete_fullquote
If set, bash quotes all shell metacharacters in filenames
and directory names when performing completion. If not
set, bash removes metacharacters such as the dollar sign
from the set of characters that will be quoted in com-
pleted filenames when these metacharacters appear in
shell variable references 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 readline editing buffer. If
not set, bash attempts to preserve what the user typed.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name ini-
tially 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 can-
not 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 interac-
tive shells.
extdebug
If set at shell invocation, or in a shell startup file,
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 en-
abled:
1. The -F option to the declare builtin displays the
source file name and line number corresponding to
each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a
non-zero value, the next command is skipped and
not executed.
3. If the command run by the DEBUG trap returns a
value of 2, and the shell is executing in a sub-
routine (a shell function or a shell script exe-
cuted by the . or source builtins), the shell sim-
ulates a call to return.
4. BASH_ARGC and BASH_ARGV are updated as described
in their descriptions above).
5. Function tracing is enabled: command substitution,
shell functions, and subshells invoked with ( com-
mand ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution,
shell functions, and subshells invoked with ( com-
mand ) inherit the ERR trap.
extglob If set, the extended pattern matching features described
above under Pathname Expansion are enabled.
extquote
If set, $'string' and $"string" quoting is performed
within ${parameter} expansions enclosed in double quotes.
This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell vari-
able cause words to be ignored when performing word com-
pletion 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 performing compar-
isons. 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.
globskipdots
If set, pathname expansion will never match the filenames
``.'' and ``..'', even if the pattern begins with a
``.''. This option is enabled by default.
globstar
If set, the pattern ** used in a pathname expansion con-
text will match all files and zero or more directories
and subdirectories. If the pattern is followed by a /,
only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU error message format.
histappend
If set, the history list is appended to the file named by
the value of the HISTFILE variable when the shell exits,
rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of his-
tory 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 inter-
active 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 op-
tion is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the
last command of a pipeline not executed in the background
in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded 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 at-
tribute is not inherited.
localvar_unset
If set, calling unset on local variables in previous
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 cur-
rent 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 dis-
played.
no_empty_cmd_completion
If set, and readline is being used, bash will not attempt
to search the PATH for possible completions when comple-
tion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive fash-
ion when performing matching while executing case or [[
conditional commands, when performing pattern substitu-
tion word expansions, or when filtering possible comple-
tions as part of programmable completion.
noexpand_translation
If set, bash encloses the translated results of $"..."
quoting in single quotes instead of double quotes. If
the string is not translated, this has no effect.
nullglob
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string,
rather than themselves.
patsub_replacement
If set, bash expands occurrences of & in the replacement
string of pattern substitution to the text matched by the
pattern, as described under Parameter Expansion above.
This option is enabled by default.
progcomp
If set, the programmable completion facilities (see Pro-
grammable 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 programmable completion using
the command word resulting from the expanded alias.
promptvars
If set, prompt strings undergo parameter expansion, com-
mand substitution, arithmetic expansion, and quote re-
moval 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 positional parame-
ters.
sourcepath
If set, the . (source) builtin uses the value of PATH to
find the directory containing the file supplied as an ar-
gument. This option is enabled by default.
varredir_close
If set, the shell automatically closes file descriptors
assigned using the {varname} redirection syntax (see
REDIRECTION above) instead of leaving them open when the
command completes.
xpg_echo
If set, the echo builtin expands backslash-escape se-
quences by default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT
signal. A login shell, or a shell without job control enabled,
cannot be suspended; the -f option can be used to override this
and force the suspension. The return status is 0 unless the
shell is a login shell or job control is not enabled and -f is
not supplied.
test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending on the evalua-
tion of the conditional expression expr. Each operator and
operand must be a separate argument. Expressions are composed of
the primaries described above under CONDITIONAL 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 argu-
ment is one of the unary conditional operators listed
above under CONDITIONAL EXPRESSIONS, the expression is
true if the unary test is true. If the first argument is
not a valid unary conditional operator, the expression is
false.
3 arguments
The following conditions are applied in the order listed.
If the second argument is one of the binary 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 second and third arguments. If the first argument is
exactly ( and the third argument is exactly ), the result
is the one-argument test of the second argument. Other-
wise, the expression is false.
4 arguments
The following conditions are applied in the order listed.
If the first argument is !, the result is the negation of
the three-argument expression composed of the remaining
arguments. the two-argument test using the second and
third arguments. If the first argument is exactly ( and
the fourth argument is exactly ), the result is the two-
argument test of the second and third arguments. Other-
wise, 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 prece-
dence using the rules listed above.
When used with test or [, the < and > operators sort lexicograph-
ically 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 re-
ceives signal(s) sigspec. If arg is absent (and there is a sin-
gle sigspec) or -, each specified signal is reset to its original
disposition (the value it had upon entrance to the shell). If
arg is the null string the signal specified by each sigspec is
ignored by the shell and by the commands it invokes. If arg is
not present and -p has been supplied, then the trap commands as-
sociated 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 com-
mand executes in a shell function (see SHELL GRAMMAR above). Re-
fer to the description of the extdebug option to the shopt
builtin for details of its effect on the DEBUG trap. If a
sigspec is RETURN, the command arg is executed each time a shell
function or a script executed with the . or source builtins fin-
ishes 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 or re-
set. Trapped signals that are not being ignored are reset to
their original values in a subshell or subshell environment when
one is created. The return status is false if any sigspec is in-
valid; otherwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if
used as a command name. If the -t option is used, type prints a
string which is one of alias, keyword, function, builtin, or file
if name is an alias, shell reserved word, function, builtin, or
disk file, respectively. If the name is not found, then nothing
is printed, and an exit status of false is returned. If the -p
option is used, type either returns the name of the disk file
that would be executed if name were specified as a command name,
or nothing if ``type -t name'' would not return file. The -P op-
tion 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 necessarily the file that appears
first in PATH. If the -a option is used, type prints all of the
places that contain an executable named name. This includes
aliases and functions, if and only if the -p option is not also
used. The table of hashed commands is not consulted when using
-a. The -f option suppresses shell function lookup, as with the
command builtin. type returns true if all of the arguments are
found, false if any are not found.
ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control. The
-H and -S options specify that the hard or soft limit is set for
the given resource. A hard limit cannot be increased by a non-
root user once it is set; a soft limit may be increased up to the
value of the hard limit. If neither -H nor -S is specified, both
the soft and hard limits are set. The value of limit can be a
number in the unit specified for the resource or one of the spe-
cial 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, if ap-
propriate, are printed before the value. Other options are in-
terpreted as follows:
-a All current limits are reported; no limits are set
-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 single user
-v The maximum amount of virtual memory available to the
shell and, on some systems, to its children
-x The maximum number of file locks
-P The maximum number of pseudoterminals
-R The maximum time a real-time process can run before block-
ing, in microseconds
-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 increments, except for
-t, which is in seconds; -R, which is in microseconds; -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 in-
valid option or argument is supplied, or an error occurs while
setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins with
a digit, it is interpreted as an octal number; otherwise 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 suc-
cessfully changed or if no mode argument was supplied, and false
otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a is sup-
plied, 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 function. 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 variable 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,
a function with that name, if any, is unset. Each unset variable
or function is removed from the environment passed to subsequent
commands. If any of BASH_ALIASES, BASH_ARGV0, BASH_CMDS,
BASH_COMMAND, BASH_SUBSHELL, BASHPID, COMP_WORDBREAKS, DIRSTACK,
EPOCHREALTIME, EPOCHSECONDS, FUNCNAME, GROUPS, HISTCMD, LINENO,
RANDOM, SECONDS, or SRANDOM are unset, they lose their special
properties, even if they are subsequently reset. The exit status
is true unless a name is readonly or may not be unset.
wait [-fn] [-p varname] [id ...]
Wait for each specified child process and return its termination
status. Each id may be a process ID or a job specification; if a
job spec is given, all processes in that job's pipeline are
waited for. If id is not given, wait waits for all running back-
ground jobs and the last-executed process substitution, if its
process id is the same as $!, and the return status is zero. If
the -n option is supplied, wait waits for a single job from the
list of ids or, if no ids are supplied, any job, to complete and
returns its exit status. If none of the supplied arguments is a
child of the shell, or if no arguments are supplied and the shell
has no unwaited-for children, the exit status is 127. If the -p
option is supplied, the process or job identifier of the job for
which the exit status is returned is assigned to the variable
varname named by the option argument. The variable will be unset
initially, before any assignment. This is useful only when the
-n option is supplied. Supplying the -f option, when job control
is enabled, forces wait to wait for id to terminate before re-
turning its status, instead of returning when it changes status.
If id specifies a non-existent process or job, the return status
is 127. If wait is interrupted by a signal, the return status
will be greater than 128, as described under SIGNALS above. Oth-
erwise, the return status is the exit status of the last process
or job waited for.
SHELL COMPATIBILITY MODE
Bash-4.0 introduced the concept of a shell compatibility level, speci-
fied as a set of options to the shopt builtin ( compat31, compat32, com-
pat40, compat41, and so on). There is only one current compatibility
level -- each option is mutually exclusive. The compatibility level is
intended to allow users to select behavior from previous versions that
is incompatible with newer versions while they migrate scripts to use
current features and behavior. It's intended to be a temporary solution.
This section does not mention behavior that is standard for a particular
version (e.g., setting compat32 means that quoting the rhs of the regexp
matching operator quotes special regexp characters in the word, which is
default behavior in bash-3.2 and subsequent versions).
If a user enables, say, compat32, it may affect the behavior of other
compatibility levels up to and including the current compatibility
level. The idea is that each compatibility level controls behavior that
changed in that version of bash, but that behavior may have been present
in earlier versions. For instance, the change to use locale-based com-
parisons with the [[ command came in bash-4.1, and earlier versions used
ASCII-based comparisons, so enabling compat32 will enable ASCII-based
comparisons as well. That granularity may not be sufficient for all
uses, and as a result users should employ compatibility levels care-
fully. Read the documentation for a particular feature to find out the
current behavior.
Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value as-
signed to this variable (a decimal version number like 4.2, or an inte-
ger corresponding to the compatNN option, like 42) determines the com-
patibility level.
Starting with bash-4.4, Bash has begun deprecating older compatibility
levels. Eventually, the options will be removed in favor of BASH_COM-
PAT.
Bash-5.0 is the final version for which there will be an individual
shopt option for the previous version. Users should use BASH_COMPAT on
bash-5.0 and later versions.
The following table describes the behavior changes controlled by each
compatibility level setting. The compatNN tag is used as shorthand for
setting the compatibility level to NN using one of the following mecha-
nisms. For versions prior to bash-5.0, the compatibility level may be
set using the corresponding compatNN shopt option. For bash-4.3 and
later versions, the BASH_COMPAT variable is preferred, and it is re-
quired for bash-5.1 and later versions.
compat31
• quoting the rhs of the [[ command's regexp matching opera-
tor (=~) has no special effect
compat32
• interrupting a command list such as "a ; b ; c" causes the
execution of the next command in the list (in bash-4.0 and
later versions, the shell acts as if it received the in-
terrupt, so interrupting one command in a list aborts the
execution of the entire list)
compat40
• the < and > operators to the [[ command do not consider
the current locale when comparing strings; they use ASCII
ordering. Bash versions prior to bash-4.1 use ASCII col-
lation and strcmp(3); bash-4.1 and later use the current
locale's collation sequence and strcoll(3).
compat41
• in posix mode, time may be followed by options and still
be recognized as a reserved word (this is POSIX interpre-
tation 267)
• in posix mode, the parser requires that an even number of
single quotes occur in the word portion of a double-quoted
parameter expansion and treats them specially, so that
characters within the single quotes are considered quoted
(this is POSIX interpretation 221)
compat42
• the replacement string in double-quoted pattern substitu-
tion does not undergo quote removal, as it does in ver-
sions after bash-4.2
• in posix mode, single quotes are considered special when
expanding the word portion of a double-quoted parameter
expansion and can be used to quote a closing brace or
other special character (this is part of POSIX interpreta-
tion 221); in later versions, single quotes are not spe-
cial within double-quoted word expansions
compat43
• the shell does not print a warning message if an attempt
is made to use a quoted compound assignment as an argument
to declare (e.g., declare -a foo='(1 2)'). Later versions
warn that this usage is deprecated
• word expansion errors are considered non-fatal errors that
cause the current command to fail, even in posix mode (the
default behavior is to make them fatal errors that cause
the shell to exit)
• when executing a shell function, the loop state (while/un-
til/etc.) is not reset, so break or continue in that
function will break or continue loops in the calling con-
text. Bash-4.4 and later reset the loop state to prevent
this
compat44
• the shell sets up the values used by BASH_ARGV and
BASH_ARGC so they can expand to the shell's positional pa-
rameters even if extended debugging mode is not enabled
• a subshell inherits loops from its parent context, so
break or continue will cause the subshell to exit.
Bash-5.0 and later reset the loop state to prevent the
exit
• variable assignments preceding builtins like export and
readonly that set attributes continue to affect variables
with the same name in the calling environment even if the
shell is not in posix mode
compat50
• Bash-5.1 changed the way $RANDOM is generated to introduce
slightly more randomness. If the shell compatibility level
is set to 50 or lower, it reverts to the method from
bash-5.0 and previous versions, so seeding the random num-
ber generator by assigning a value to RANDOM will produce
the same sequence as in bash-5.0
• If the command hash table is empty, bash versions prior to
bash-5.1 printed an informational message to that effect,
even when producing output that can be reused as input.
Bash-5.1 suppresses that message when the -l option is
supplied.
compat51
• The unset builtin treats attempts to unset array sub-
scripts @ and * differently depending on whether the array
is indexed or associative, and differently than in previ-
ous versions.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at
invocation, the shell becomes restricted. A restricted shell is used to
set up an environment more controlled than the standard shell. It be-
haves identically to bash with the exception that the following are dis-
allowed or not performed:
• changing directories with cd
• setting or unsetting the values of SHELL, PATH, HISTFILE, 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
history 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 >> redirec-
tion 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 shopt -u re-
stricted_shell.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see COM-
MAND 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.bashrc
The systemwide per-interactive-shell startup file
/etc/bash.bash.logout
The systemwide login shell cleanup file, executed when a login
shell exits
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login
shell exits
~/.bash_history
The default value of HISTFILE, the file in which bash saves the
command history
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet.ramey@case.edu
BUG REPORTS
If you find a bug in bash, you should report it. But first, you should
make sure that it really is a bug, and that it appears in the latest
version of bash. The latest version is always available from
ftp://ftp.gnu.org/pub/gnu/bash/ and http://git.savan-
nah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz.
Once you have determined that a bug actually exists, use the bashbug
command to submit a bug report. If you have a fix, you are encouraged
to mail that as well! Suggestions and `philosophical' bug reports may
be mailed to bug-bash@gnu.org or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into the template it
provides for filing a bug report.
Comments and bug reports concerning this manual page should be directed
to chet.ramey@case.edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and traditional versions
of sh, mostly because of the POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command sequences of the form `a ; b ; c' are not
handled gracefully when process suspension is attempted. When a process
is stopped, the shell immediately executes the next command in the se-
quence. It suffices to place the sequence of commands between parenthe-
ses 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 5.2 2022 September 19 BASH(1)
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