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BASH(1) General Commands Manual BASH(1)
bash - GNU Bourne-Again SHell
bash [options] [command_string | file]
Bash is Copyright (C) 1989-2025 by the Free Software Foundation,
Inc.
Bash is a command language interpreter that executes commands read
from the standard input, from a string, or from a file. It is a
reimplementation and extension of the Bourne shell, the historical
Unix command language interpreter. Bash also incorporates useful
features from the Korn and C shells (ksh and csh).
POSIX is the name for a family of computing standards based on
Unix. Bash is intended to be a conformant implementation of the
Shell and Utilities portion of the IEEE POSIX specification (IEEE
Standard 1003.1). Bash POSIX mode (hereafter referred to as posix
mode) changes the shell's behavior where its default operation
differs from the standard to strictly conform to the standard.
See SEE ALSO below for a reference to a document that details how
posix mode affects bash's behavior. Bash can be configured to be
POSIX-conformant by default.
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 options when it is invoked:
-c If the -c option is present, then commands are read from
the first non-option argument command_string. If there are
arguments after the command_string, the first argument is
assigned to $0 and any remaining arguments are assigned to
the positional parameters. The assignment to $0 sets the
name of the shell, which is used in 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, the shell reads commands from the
standard input. This option allows the positional
parameters to be set when invoking an interactive shell or
when reading input through a pipe.
-D Print a list of all double-quoted strings preceded by $ on
the standard output. These are the strings that are
subject to language translation when the current locale is
not C or POSIX. This implies the -n option; no commands
will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the
shopt builtin (see SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of that option;
+O unsets it. If shopt_option is not supplied, bash prints
the names and values of the shell options accepted by shopt
on the standard output. If the invocation option is +O,
the output is displayed in a format that may be reused as
input.
-- A -- signals the end of options and disables further option
processing. Any arguments after the -- are treated as a
shell script filename (see below) and arguments passed to
that script. 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 standard personal
initialization file ~/.bashrc if the shell is interactive
(see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines
when the shell is interactive.
--noprofile
Do not read either the system-wide startup file
/etc/profile or any of the personal initialization files
~/.bash_profile, ~/.bash_login, or ~/.profile. By default,
bash reads these files when it is invoked as a login shell
(see INVOCATION below).
--norc Do not read and execute the personal initialization file
~/.bashrc if the shell is interactive. This option is on
by default if the shell is invoked as sh.
--posix
Enable posix mode; change the behavior of bash where the
default operation differs from the POSIX standard to match
the standard.
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the
standard output and exit successfully.
If arguments remain after option processing, and neither the -c
nor the -s option has been supplied, the first argument is treated
as the name of a file containing shell commands (a shell script).
When bash is invoked in this fashion, $0 is set to the name of the
file, and the positional parameters are set to the remaining
arguments. Bash reads and executes commands from this file, then
exits. Bash's exit status is the exit status of the last command
executed in the script. If no commands are executed, the exit
status is 0. Bash first attempts to open the file in the current
directory, and, if no file is found, searches the directories in
PATH for the script.
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, and whose
standard input and standard error are both connected to terminals
(as determined by isatty(3)), or one started with the -i option.
Bash sets PS1 and $- includes i if the shell is interactive, so a
shell script or a startup file can test this state.
The following paragraphs describe how bash executes its startup
files. If any of the files exist but cannot be read, bash reports
an error. Tildes are expanded in filenames as described below
under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-
interactive shell with the --login option, it first reads and
executes commands from the file /etc/profile, if that file exists.
After reading that file, it looks for ~/.bash_profile,
~/.bash_login, and ~/.profile, in that order, and reads and
executes commands from the first one that exists and is readable.
The --noprofile option may be used when the shell is started to
inhibit this behavior.
When an interactive login shell exits, or a non-interactive login
shell executes the exit builtin command, bash reads and executes
commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started,
bash reads and executes commands from ~/.bashrc, if that file
exists. The --norc option inhibits this behavior. The --rcfile
file option causes bash to use file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value
as the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but does not use the value of the PATH variable to search for the
filename.
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible,
while conforming to the POSIX standard as well. When invoked as
an interactive login shell, or a non-interactive shell with the
--login option, it first attempts to read and execute commands
from /etc/profile and ~/.profile, in that order. The --noprofile
option inhibits this behavior. When invoked as an interactive
shell with the name sh, bash looks for the variable ENV, expands
its value if it is defined, and uses the expanded value as the
name of a file to read and execute. Since a shell invoked as sh
does not attempt to read and execute commands from any other
startup files, the --rcfile option has no effect. A non-
interactive shell invoked with the name sh does not attempt to
read any other startup files.
When invoked as sh, bash enters posix mode after reading the
startup files.
When bash is started in posix mode, as with the --posix command
line option, it follows the POSIX standard for startup files. In
this mode, interactive shells expand the ENV variable and read and
execute commands 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 and rarely-seen 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 ~/.bashrc, if that file exists and is readable. Bash does
not read this file if invoked as sh. The --norc option inhibits
this behavior, and the --rcfile option makes bash use a different
file instead of ~/.bashrc, but neither rshd nor sshd generally
invoke the shell with those options or allow them to be specified.
If the shell is started with the effective user (group) id not
equal to the real user (group) id, and the -p option is not
supplied, no startup files are read, shell functions are not
inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH,
and GLOBIGNORE variables, if they appear in the environment, are
ignored, and the effective user id is set to the real user id. If
the -p option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.
The following definitions are used throughout the rest of this
document.
blank A space or tab.
whitespace
A character belonging to the space character class in the
current locale, or for which isspace(3) returns true.
word A sequence of characters considered as a single unit by the
shell. Also known as a token.
name A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character or
an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of
the following:
| & ; ( ) < > space tab newline
control operator
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>
Reserved words 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 [[ ]]
This section describes the syntax of the various forms of shell
commands.
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 the pipeline operator, command1'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 output
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. If a pipeline is executed
synchronously, the shell waits for all commands in the pipeline to
terminate before returning a value.
If the time reserved word precedes a pipeline, the shell reports
the elapsed as well as user and system time consumed by its
execution when the pipeline terminates. The -p option changes the
output format to that specified by POSIX. When the shell is in
posix mode, it does not recognize time as a reserved word if the
next token begins with a “-”. The value of the TIMEFORMAT
variable is a format string that specifies how the timing
information should be displayed; see the description of TIMEFORMAT
below under Shell Variables.
When the shell is in posix mode, time may appear by itself as the
only word in a simple command. In this case, the shell displays
the total user and system time consumed by the shell and its
children. The TIMEFORMAT variable specifies 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 EXECUTION ENVIRONMENT for a description of subshells and a
subshell environment. If the lastpipe option is enabled using the
shopt builtin (see the description of shopt below), and job
control is not active, the last element of a pipeline may be run
by the shell process.
Lists
A list is a sequence of one or more pipelines separated by one of
the operators ;, &, &&, or ||, and optionally terminated by one of
;, &, or <newline>.
Of these list operators, && and || have equal precedence, followed
by ; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list instead of
a semicolon to delimit commands.
If a command is terminated by the control operator &, the shell
executes the command in the background in a subshell. The shell
does not wait for the command to finish, and the return status is
0. These are referred to as asynchronous commands. Commands
separated by a ; are executed sequentially; the shell waits for
each command to terminate in turn. The return status is the exit
status of the last command executed.
AND and OR lists are sequences of one or more pipelines separated
by the && and || control operators, respectively. AND and OR
lists are executed with left associativity. An AND list has the
form
command1 && command2
command2 is executed if, and only if, command1 returns an exit
status of zero (success).
An OR list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero
exit status. The return status of AND and OR lists is the exit
status of the last command executed in the list.
Compound Commands
A compound command is one of the following. In most cases a list
in a command's description may be separated from the rest of the
command by one or more newlines, and may be followed by a newline
in place of a semicolon.
(list) list is executed in a subshell (see COMMAND EXECUTION
ENVIRONMENT below for a description of a subshell
environment). Variable assignments and builtin commands
that affect the shell's environment do not remain in effect
after the command completes. The return status is the exit
status of list.
{ list; }
list is 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 arithmetic expression is evaluated according to the
rules described below under ARITHMETIC EVALUATION. If the
value of the expression is non-zero, the return status is
0; otherwise the return status is 1. The expression
undergoes the same expansions as if it were within double
quotes, but unescaped double quote characters in expression
are not treated specially and are removed. Since this can
potentially result in empty strings, this command treats
those as expressions that evaluate to 0.
[[ expression ]]
Evaluate the conditional expression expression and return a
status of zero (true) or non-zero (false). 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, parameter and variable expansion,
arithmetic expansion, command substitution, process
substitution, and quote removal on those words.
Conditional operators such as -f must be unquoted to be
recognized as primaries.
When used with [[, the < and > operators sort
lexicographically using the current locale.
When the == and != operators are used, the string to the
right of the operator is considered a pattern and matched
according to the rules described below under Pattern
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. If any part of the pattern is
quoted, the quoted portion is matched as a string: every
character in the quoted portion matches itself, instead of
having any special pattern matching meaning.
An additional binary operator, =~, is available, with the
same precedence as == and !=. When it is used, the string
to the right of the operator is considered a POSIX extended
regular expression and matched accordingly (using the POSIX
regcomp and regexec interfaces usually described in
regex(3)). The return value is 0 if the string matches the
pattern, and 1 otherwise. If the regular expression is
syntactically incorrect, the conditional expression's
return value is 2. If the nocasematch shell option is
enabled, the match is performed without regard to the case
of alphabetic characters.
If any part of the pattern is quoted, the quoted portion is
matched literally, as above. 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 meanings between
brackets.
The match succeeds if the pattern 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 regular 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_REMATCH 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 decreasing order of precedence:
( expression )
Returns the value of expression. This may be used
to override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
for name [ [ in word ... ] ; ] do list ; done
First, expand The list of words following in, generating a
list of items. Then, 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, evaluate the arithmetic expression expr1 according
to the rules described below under ARITHMETIC EVALUATION.
Then, repeatedly evaluate the arithmetic expression expr2
until it evaluates to zero. Each time expr2 evaluates to a
non-zero value, execute list and evaluate the arithmetic
expression expr3. 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
non-zero if any of the expressions is invalid.
Use the break and continue builtins (see SHELL BUILTIN
COMMANDS below) to control loop execution.
select name [ in word ] ; do list ; done
First, expand the list of words following in, generating a
list of items, and print the set of expanded words the
standard error, each preceded by a number. If the in word
is omitted, print the positional parameters (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
select sets the value of name to that word. If the line is
empty, select displays the words and prompt again. If EOF
is read, select completes and returns 1. Any other value
sets name to null. The line read is saved in the variable
REPLY. The list is executed after each selection until a
break command is executed. The exit status of select is
the exit status of the last command executed in list, or
zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it
against each pattern in turn, proceeding from first to
last, using the matching rules described under Pattern
Matching below. A pattern list is a set of one or more
patterns separated by , and the ) operator terminates the
pattern list. The word is expanded using tilde expansion,
parameter and variable expansion, arithmetic expansion,
command substitution, process substitution and quote
removal. Each pattern examined is expanded using tilde
expansion, parameter and variable expansion, arithmetic
expansion, command substitution, process substitution, and
quote removal. If the nocasematch shell option is enabled,
the match is performed without regard to the case of
alphabetic characters. A clause is a pattern list and an
associated list.
When a match is found, case executes the corresponding
list. If the ;; operator terminates the case clause, the
case command completes after the first match. Using ;& in
place of ;; causes execution to continue with the list
associated with the next pattern list. Using ;;& in place
of ;; causes the shell to test the next pattern list in the
statement, if any, and execute any associated list if the
match succeeds, continuing the case statement execution as
if the pattern 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 the last list executed.
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, except that the test is negated: list-2 is
executed as long as the last command in list-1 returns a
non-zero exit status. The exit status of the while and
until commands is the exit status of the last command
executed in list-2, or zero if none was executed.
Coprocesses
A coprocess is a shell command preceded by the coproc reserved
word. A coprocess is executed asynchronously in a subshell, as if
the command had been terminated with the & control operator, with
a two-way pipe established between the executing shell and the
coprocess.
The 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 preferred 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 descriptor 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 utilized as
arguments to shell commands and redirections using standard word
expansions. Other than those created to execute command and
process substitutions, the file descriptors are not available in
subshells.
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID. The wait builtin
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 compound 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 supplied, 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) specified when a function
is defined are performed when the function is executed.
The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already
exists. When executed, the exit status of a function is the exit
status of the last command executed in the body. (See FUNCTIONS
below.)
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is enabled (see
SHELL BUILTIN COMMANDS below), a word beginning with # introduces
a comment. A word begins at the beginning of a line, after
unquoted whitespace, or after an operator. The comment 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 enabled by default in interactive shells.
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to disable
special treatment for special characters, to prevent reserved
words from being recognized as such, and to prevent parameter
expansion.
Each of the metacharacters listed above under DEFINITIONS has
special meaning to the shell and must be quoted if it is to
represent itself.
When the command history expansion facilities are being used (see
HISTORY EXPANSION below), the history expansion character, usually
!, must be quoted to prevent history expansion.
There are four quoting mechanisms: the escape character, single
quotes, double quotes, and dollar-single quotes.
A non-quoted backslash (\) is the escape character. It preserves
the literal value of the next character that follows, removing any
special meaning it has, with the exception of <newline>. If a
\<newline> pair appears, and the backslash is not itself quoted,
the \<newline> is treated as a line continuation (that is, it is
removed from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value
of each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value
of all characters within the quotes, with the exception of $, `,
\, and, when history expansion is enabled, !. When the shell is
in posix mode, the ! has no special meaning within double quotes,
even when history expansion is enabled. The characters $ and `
retain their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of the
following characters: $, `, ", \, or <newline>. Backslashes
preceding characters without a special meaning are left
unmodified.
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 $'string' are treated as a special
variant of single quotes. The sequence expands to string, with
backslash-escaped characters in string replaced as specified by
the ANSI C standard. Backslash escape sequences, if present, are
decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\? question mark
\nnn The eight-bit character whose value is the octal
value nnn (one to three octal digits).
\xHH The eight-bit character whose value is the
hexadecimal value HH (one or two hex digits).
\uHHHH The Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHH (one to four hex digits).
\UHHHHHHHH
The Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHHHHHH (one to eight hex
digits).
\cx A control-x character.
The expanded result is single-quoted, as if the dollar sign had
not been present.
Translating Strings
A double-quoted string preceded by a dollar sign ($"string")
causes the string to be translated according to the current
locale. The gettext infrastructure performs the lookup and
translation, using the LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN
shell variables. If the current locale is C or POSIX, if there
are no translations available, or if the string is not translated,
the dollar sign is ignored, and the string is treated as double-
quoted as described above. 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_translation option is
enabled using the shopt builtin, translated strings are single-
quoted instead of double-quoted. See the description of shopt
below under SHELL BUILTIN COMMANDS.
A parameter is an entity that stores values. It can be a name, a
number, or one of the special characters listed below under
Special Parameters. A variable is a parameter denoted by a name.
A variable has a value and zero or more attributes. Attributes
are assigned using the declare builtin command (see declare below
in SHELL BUILTIN COMMANDS). The export and readonly builtins
assign specific attributes.
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 is assigned to using 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, command substitution, arithmetic expansion, and quote
removal (see EXPANSION below). If the variable has its integer
attribute set, then value is evaluated as an arithmetic expression
even if the $((...)) expansion is not used (see Arithmetic
Expansion below). Word splitting and pathname 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 command builtin and retain these assignment statement
properties.
In the context where an assignment statement is assigning a value
to a shell variable or array index, the “+=” operator appends to
or adds to the variable's previous value. This includes arguments
to declaration commands such as declare that accept assignment
statements. When “+=” is applied to a variable for which the
integer attribute has been set, the variable's current value and
value are each evaluated as arithmetic expressions, and the sum of
the results is assigned as the variable's value. The current
value is usually an integer constant, but may be an expression.
When “+=” is applied to an array variable using compound
assignment (see Arrays below), the variable's value is not unset
(as it is when using “=”), and new values are appended to the
array beginning at one greater than the array's maximum index (for
indexed arrays) or added as additional key-value pairs in an
associative array. When applied to a string-valued variable,
value is expanded and appended to the variable's value.
A variable can be assigned the nameref attribute using the -n
option to the declare or local builtin commands (see the
descriptions of declare and local below) to create a nameref, or a
reference to another variable. This allows variables to be
manipulated indirectly. Whenever the nameref variable is
referenced, assigned to, unset, or has its attributes modified
(other than using or changing the nameref attribute itself), the
operation is actually performed on the variable specified by the
nameref variable's value. A nameref is commonly used within shell
functions to refer to a variable whose name is passed as an
argument to the function. For instance, if a variable name is
passed to a shell function as its first argument, running
declare -n ref=$1
inside the function creates a local nameref variable ref whose
value is the variable name passed as the first argument.
References and assignments to ref, and changes to its attributes,
are treated as references, assignments, and attribute
modifications to the variable whose name was passed as $1. If the
control variable in a for loop has the nameref attribute, the list
of words can be a list of shell variables, and a name reference is
established for each word in the list, in turn, when the loop is
executed. Array variables cannot be given the nameref attribute.
However, nameref variables can reference array variables and
subscripted array variables. Namerefs can be unset using the -n
option to the unset builtin. Otherwise, if unset is executed with
the name of a nameref variable as an argument, the variable
referenced by the nameref variable is unset.
When the shell starts, it reads its environment and creates a
shell variable from each environment variable that has a valid
name, as described below (see ENVIRONMENT).
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).
Without braces, a digit following $ can only refer to one of the
first nine positional parameters ($1-$9) or the special parameter
$0 (see the next section).
Special Parameters
The shell treats several parameters specially. These parameters
may only be referenced; assignment to them is not allowed.
Special parameters are denoted by one of the following characters.
* ($*) Expands to the positional parameters, starting from
one. When the expansion is not within double quotes, each
positional parameter expands to a separate word. In
contexts where word expansions are performed, those words
are subject to further word splitting and pathname
expansion. When the expansion occurs within double quotes,
it expands to a single word with the value of each
parameter separated by the first character of the IFS
variable. That is, "$*" is equivalent to "$1c$2c...",
where c is the first character of the value of the IFS
variable. If IFS is unset, the parameters are separated by
spaces. If IFS is null, the parameters are joined without
intervening separators.
@ ($@) Expands to the positional parameters, starting from
one. In contexts where word splitting is performed, this
expands each positional parameter to a separate word; if
not within double quotes, these words are subject to word
splitting. In contexts where word splitting is not
performed, such as the value portion of an assignment
statement, this expands to a single word with each
positional parameter separated by a space. When the
expansion occurs within double quotes, and word splitting
is performed, each parameter expands to a separate word.
That is, "$@" is equivalent to "$1" "$2" ... If the
double-quoted expansion occurs within a word, the expansion
of the first parameter is joined with the expansion of the
beginning part of the original word, and the expansion of
the last parameter is joined with the expansion of the last
part of the original word. When there are no positional
parameters, "$@" and $@ expand to nothing (i.e., they are
removed).
# ($#) Expands to the number of positional parameters in
decimal.
? ($?) Expands to the exit status of the most recently
executed command.
- ($-) 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).
$ ($$) Expands to the process ID of the shell. In a
subshell, it expands to the process ID of the parent shell,
not the subshell.
! ($!)Expands to the process ID of the job most recently
placed into the background, whether executed as an
asynchronous command or using the bg builtin (see JOB
CONTROL below).
0 ($0) Expands to the name of the shell or shell script.
This is set at shell initialization. If bash is invoked
with a file of commands, $0 is set to the name of that
file. If bash is started with the -c option, then $0 is
set to the first argument after the string to be executed,
if one is present. Otherwise, it is set to the filename
used to invoke bash, as given by argument zero.
Shell Variables
The shell sets following variables:
_ ($_, an underscore) This has a number of meanings depending
on context. At shell startup, _ is set to the pathname
used to invoke the shell or shell script being executed as
passed in the environment or argument list. Subsequently,
it expands to the last argument to the previous simple
command executed in the foreground, after expansion. It is
also set to the full pathname used to invoke each command
executed and placed in the environment exported to that
command. When checking mail, $_ expands to the name of the
mail file currently being checked.
BASH Expands to the full filename used to invoke this instance
of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word
in the list is a valid argument for the -s option to the
shopt builtin command (see SHELL BUILTIN COMMANDS below).
The options appearing in BASHOPTS are those reported as on
by shopt. If this variable is in the environment when bash
starts up, the shell enables each option in the list before
reading any startup files. If this variable is exported,
child shells will enable each option in the list. This
variable is read-only.
BASHPID
Expands to the process ID of the current bash process.
This differs from $$ under certain circumstances, such as
subshells that do not require bash to be re-initialized.
Assignments to BASHPID have no effect. If BASHPID is
unset, it loses its special properties, even if it is
subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to
the internal list of aliases as maintained by the alias
builtin. Elements added to this array appear in the alias
list; however, unsetting array elements currently does not
remove aliases 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.
Assignments to BASH_ARGC have no effect, and it may not be
unset.
BASH_ARGV
An array variable containing all of the parameters in the
current bash execution call stack. The final parameter of
the last subroutine call is at the top of the stack; the
first parameter of the initial call is at the bottom. When
a subroutine is executed, the shell pushes the supplied
parameters onto BASH_ARGV. The shell sets BASH_ARGV only
when in extended debugging mode (see the description of the
extdebug option to the shopt builtin below). Setting
extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set, may
result in inconsistent values. Assignments to BASH_ARGV
have no effect, and it may not be unset.
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). Assigning a value to
BASH_ARGV0 sets $0 to the same value. If BASH_ARGV0 is
unset, it loses its special properties, even if it is
subsequently reset.
BASH_CMDS
An associative array variable whose members correspond to
the internal hash table of commands as maintained by the
hash builtin. Adding elements to this array makes them
appear in the hash table; however, unsetting array elements
currently does not remove command names from the hash
table. If BASH_CMDS is unset, it loses its special
properties, even if it is subsequently reset.
BASH_COMMAND
Expands to the command currently being executed or about to
be executed, unless the shell is executing a command as the
result of a trap, in which case it is the command executing
at the time of the trap. 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 function). Use LINENO to obtain the current
line number. Assignments to BASH_LINENO have no effect,
and it may not be unset.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the enable
command looks for dynamically loadable builtins.
BASH_MONOSECONDS
Each time this variable is referenced, it expands to the
value returned by the system's monotonic clock, if one is
available. If there is no monotonic clock, this is
equivalent to EPOCHSECONDS. If BASH_MONOSECONDS is unset,
it loses its special properties, even if it is subsequently
reset.
BASH_REMATCH
An array variable whose members are assigned by the =~
binary operator to the [[ conditional command. The element
with index 0 is the portion of the string matching the
entire regular expression. The element with index n is the
portion of the string matching the nth parenthesized
subexpression.
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 defined in the file ${BASH_SOURCE[$i]}
and called from ${BASH_SOURCE[$i+1]}. Assignments to
BASH_SOURCE have no effect, and it may not be unset.
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_TRAPSIG
Set to the signal number corresponding to the trap action
being executed during its execution. See the description
of trap under SHELL BUILTIN COMMANDS below for information
about signal numbers and trap execution.
BASH_VERSINFO
A readonly array variable whose members hold version
information for this instance of bash. The values assigned
to the array members are as follows:
BASH_VERSINFO[0]
The major version number (the release).
BASH_VERSINFO[1]
The minor version number (the version).
BASH_VERSINFO[2]
The patch level.
BASH_VERSINFO[3]
The build version.
BASH_VERSINFO[4]
The release status (e.g., beta).
BASH_VERSINFO[5]
The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance
of bash (e.g., 5.2.37(3)-release).
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the
current cursor position. This variable is available only
in shell functions invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the
current completion function. This variable is available
only in shell functions and external commands invoked by
the programmable completion facilities (see Programmable
Completion below).
COMP_LINE
The current command line. This variable is available only
in shell functions and external commands invoked by the
programmable completion facilities (see Programmable
Completion below).
COMP_POINT
The index of the current cursor position relative to the
beginning of the current command. If the current cursor
position is at the end of the current command, the value of
this variable is equal to ${#COMP_LINE}. This variable is
available only in shell functions and external commands
invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of
attempted completion that caused a completion function to
be called: TAB, for normal completion, ?, for listing
completions after successive tabs, !, for listing
alternatives on partial word completion, @, to list
completions if the word is not unmodified, or %, for menu
completion. This variable is available only in shell
functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as
word separators when performing word completion. If
COMP_WORDBREAKS is unset, it loses its special properties,
even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the
individual words in the current command line. The line is
split into words as readline would split it, using
COMP_WORDBREAKS as described above. This variable is
available only in shell functions invoked by the
programmable completion facilities (see Programmable
Completion below).
COPROC An array variable (see Arrays below) created to hold the
file descriptors for output from and input to an unnamed
coprocess (see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current
contents of the directory stack. Directories appear in the
stack in the order they are displayed by the dirs builtin.
Assigning to members of this array variable may be used to
modify directories already in the stack, but the pushd and
popd builtins must be used to add and remove directories.
Assigning to this variable does not change the current
directory. If DIRSTACK is unset, it loses its special
properties, even if it is subsequently reset.
EPOCHREALTIME
Each time this parameter is referenced, it expands to the
number of seconds since the Unix Epoch (see time(3)) as a
floating-point value with micro-second granularity.
Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME
is unset, it loses its special properties, even if it is
subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the
number of seconds since the Unix Epoch (see time(3)).
Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS
is unset, it loses its special properties, even if it is
subsequently reset.
EUID Expands to the effective user ID of the current user,
initialized at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell
functions currently in the execution call stack. The
element with index 0 is the name of any currently-executing
shell function. The bottom-most element (the one with the
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, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}.
The caller builtin displays the current call stack using
this information.
GROUPS An array variable containing the list of groups of which
the current user is a member. Assignments to GROUPS have
no effect. If GROUPS is unset, it loses its special
properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the
current command. Assignments to HISTCMD have no effect.
If HISTCMD is unset, it loses its special properties, even
if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the
type of machine on which bash is executing. The default is
system-dependent.
LINENO Each time this parameter is referenced, the shell
substitutes a decimal number representing the current
sequential line number (starting with 1) within a script or
function. When not in a script or function, the value
substituted is not guaranteed to be meaningful. If LINENO
is unset, it loses its special properties, even if it is
subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the
system type on which bash is executing, in the standard GNU
cpu-company-system format. The default is system-
dependent.
MAPFILE
An array variable (see Arrays below) created to hold the
text read by the mapfile builtin when no variable name is
supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the
getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the
getopts builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating
system on which bash is executing. The default is system-
dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of
exit status values from the commands in the most-recently-
executed foreground pipeline, which may consist of only a
simple command (see SHELL GRAMMAR above). Bash sets
PIPESTATUS after executing multi-element pipelines, timed
and negated pipelines, simple commands, subshells created
with the ( operator, the [[ and (( compound commands, and
after error conditions that result in the shell aborting
command execution.
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 initializes (seeds) the sequence of random numbers.
Seeding the random number generator with the same constant
value produces the same sequence of values. If RANDOM is
unset, it loses its special properties, even if it is
subsequently 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 at one-second resolution. If
SECONDS is unset, it loses its special properties, even if
it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word
in the list is a valid argument for the -o option to the
set builtin command (see SHELL BUILTIN COMMANDS below).
The options appearing in SHELLOPTS are those reported as on
by set -o. If this variable is in the environment when
bash starts up, the shell enables each option in the list
before reading any startup files. If this variable is
exported, child shells will enable each option in the list.
This variable is read-only.
SHLVL Incremented by one each time an instance of bash is
started.
SRANDOM
Each time it is referenced, this variable expands to a
32-bit pseudo-random number. The random number generator
is not linear on systems that support /dev/urandom or
arc4random(3), so each returned number has no relationship
to the numbers preceding it. The random number generator
cannot be seeded, so assignments to this variable have no
effect. If SRANDOM is unset, it loses its special
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 shell uses the following variables. In some cases, bash
assigns a default value to a variable; these cases are noted
below.
BASH_COMPAT
The value is used to set the shell's compatibility level.
See SHELL COMPATIBILITY MODE below for a description of the
various compatibility levels and their effects. The value
may be a decimal 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 default for the current
version. If BASH_COMPAT is set to a value that is not one
of the valid compatibility levels, the shell prints an
error message and sets the compatibility level to the
default for the current version. A subset of the valid
values correspond to the compatibility levels described
below under SHELL COMPATIBILITY MODE. For example, 4.2 and
42 are valid values that correspond 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 expanded value is interpreted as a filename
containing commands to initialize the shell before it reads
and executes commands from the script. 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 writes the trace output generated when
“set -x” is enabled to that file descriptor, instead of the
standard error. 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 where the shell looks for
directories specified as arguments to the cd command. A
sample value is “.:~:/usr”.
CHILD_MAX
Set the number of exited child status values for the shell
to remember. Bash will not allow this value to be
decreased below a POSIX-mandated minimum, and there is a
maximum value (currently 8192) that this may not exceed.
The minimum value is system-dependent.
COLUMNS
Used by the select compound command to determine the
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 completion 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 set of filenames to be ignored by
command search using PATH. Files whose full pathnames
match one of these patterns are not considered executable
files for the purposes of completion and command execution
via PATH lookup. This does not affect the behavior of the
[, test, and [[ commands. Full pathnames in the command
hash table are not subject to EXECIGNORE. Use this
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 suffix matches one of the entries in FIGNORE
is excluded from the list of matched filenames. A sample
value is “.o:~”.
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed
this nesting level 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. The pattern matching honors the setting of the
extglob shell option.
GLOBSORT
Controls how the results of pathname expansion are sorted.
The value of this variable specifies the sort criteria and
sort order for the results of pathname expansion. If this
variable is unset or set to the null string, pathname
expansion uses the historical behavior of sorting by name,
in ascending lexicographic order as determined by the
LC_COLLATE shell variable.
If set, a valid value begins with an optional +, which is
ignored, or -, which reverses the sort order from ascending
to descending, followed by a sort specifier. The valid
sort specifiers are name, numeric, size, mtime, atime,
ctime, and blocks, which sort the files on name, names in
numeric rather than lexicographic order, file size,
modification time, access time, inode change time, and
number of blocks, respectively. If any of the non-name
keys compare as equal (e.g., if two files are the same
size), sorting uses the name as a secondary sort key.
For example, a value of -mtime sorts the results in
descending order by modification time (newest first).
The numeric specifier treats names consisting solely of
digits as numbers and sorts them using their numeric value
(so “2” sorts before “10”, for example). When using
numeric, names containing non-digits sort after all the
all-digit names and are sorted by name using the
traditional behavior.
A sort specifier of nosort disables sorting completely;
bash returns the results in the order they are read from
the file system, ignoring any leading -.
If the sort specifier is missing, it defaults to name, so a
value of + is equivalent to the null string, and a value of
- sorts by name in descending order. Any invalid value
restores the historical sorting behavior.
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
not to 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, bash saves all lines
read by the shell parser on the history list, subject to
the value of HISTIGNORE. If the first line of a multi-line
compound command was saved, the second and subsequent lines
are not tested, and are added to the history regardless of
the value of HISTCONTROL. If the first line was not saved,
the second and subsequent lines of the command are not
saved either.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below). Bash assigns a default value of
~/.bash_history. If HISTFILE is unset or null, the shell
does not save the command history when it 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 the number
of history entries that total no more than that number of
lines by removing the oldest entries. If the history list
contains multi-line entries, the history file may contain
more lines than this maximum to avoid leaving partial
history entries. The history file is also truncated to
this size after writing it when a shell exits or by the
history builtin. If the value is 0, the history file is
truncated to zero size. Non-numeric values and numeric
values less than zero inhibit truncation. The shell sets
the default value to the value of HISTSIZE after reading
any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which
command lines should be saved on the history list. If a
command line matches one of the patterns in the value of
HISTIGNORE, it is not saved on the history list. Each
pattern is anchored at the beginning of the line and must
match the complete line (bash does not implicitly append a
“*”). Each pattern is tested against the line after the
checks specified by HISTCONTROL are applied. In addition
to the normal shell pattern matching characters, “&”
matches the previous history line. A backslash escapes the
“&”; the backslash is removed before attempting a match.
If the first line of a multi-line compound command was
saved, the second and subsequent lines are not tested, and
are added to the history regardless of the value of
HISTIGNORE. If the first line was not saved, the second
and subsequent lines of the command are not saved either.
The pattern matching honors the setting of the extglob
shell option.
HISTIGNORE subsumes some of the function of HISTCONTROL. A
pattern of “&” is identical to “ignoredups”, and a pattern
of “[ ]*” is identical to “ignorespace”. Combining these
two patterns, separating them with a colon, provides the
functionality of “ignoreboth”.
HISTSIZE
The number of commands to remember in the command history
(see HISTORY below). If the value is 0, commands are not
saved in the history list. Numeric values less than zero
result in every command being saved on the history list
(there is no limit). The shell sets the default value to
500 after reading any startup files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as
a format string for strftime(3) to print the time stamp
associated with each history entry displayed by the history
builtin. If this variable is set, the shell writes time
stamps 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, bash clears the
hostname list.
IFS The Internal Field Separator that is used for word
splitting after expansion and to split lines into words
with the read builtin command. Word splitting is described
below under EXPANSION. The default value is
“<space><tab><newline>”.
IGNOREEOF
Controls the action of an interactive shell on receipt of
an EOF character as the sole input. If set, the value is
the number of consecutive EOF characters which must be
typed as the first characters on an input line before bash
exits. If the variable is set but does not have a numeric
value, or the value is null, the default value is 10. If
it is unset, EOF signifies the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the
default of ~/.inputrc (see READLINE below).
INSIDE_EMACS
If this variable appears in the environment when the shell
starts, bash assumes that it is running inside an Emacs
shell buffer and may disable line editing, depending on the
value of TERM.
LANG Used to determine the locale category for any category not
specifically selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_
variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when
sorting the results of pathname expansion, and determines
the behavior of range expressions, equivalence classes, and
collating sequences within pathname expansion and pattern
matching.
LC_CTYPE
This variable determines the interpretation of characters
and the behavior of character classes within pathname
expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate
double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for
number formatting.
LC_TIME
This variable determines the locale category used for data
and time formatting.
LINES Used by the select compound command to determine the column
length for printing selection lists. Automatically set if
the checkwinsize option is enabled or in an interactive
shell upon receipt of a SIGWINCH.
MAIL If the value is set to a file or directory name and the
MAILPATH variable is not set, bash informs the user of the
arrival of mail in the specified file or Maildir-format
directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The
default is 60 seconds. When it is time to check for mail,
the shell does so before displaying the primary prompt. If
this variable is unset, or set to a value that is not a
number greater than or equal to zero, the shell disables
mail checking.
MAILPATH
A colon-separated list of filenames to be checked for mail.
The message to be printed when mail arrives in a particular
file may be specified by separating the filename from the
message with a “?”. When used in the text of the message,
$_ expands to the name of the current mailfile. For
example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash can be configured to supply a default value for this
variable (there is no value by default), but the location
of the user mail files that it uses is system dependent
(e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages
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). Characters 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 before the command is executed.
PS1 The value of this parameter is expanded (see PROMPTING
below) and used as the primary prompt string. The default
value is “\s-\v\$ ”.
PS2 The value of this parameter is expanded as with PS1 and
used as the secondary prompt string. The default is “> ”.
PS3 The value of this parameter is used as the prompt for the
select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the
value is printed before each command bash displays during
an execution trace. The first character of the expanded
value of PS4 is replicated multiple times, as necessary, to
indicate multiple levels of indirection. The default is
“+ ”.
SHELL 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 information. The escape
sequences and their meanings are as follows; the brackets
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.
time prints at most six digits after the decimal point;
values of p greater than 6 are changed to 6. If p is not
specified, time prints three digits after the decimal
point.
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, bash does not display any timing information. A
trailing newline is added when the format string is
displayed.
TMOUT If set to a value greater than zero, the read builtin uses
the value as its default timeout. The select command
terminates 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, simple
commands consisting of only a single word, without
redirections, are treated as candidates for resumption of
an existing stopped job. There is no ambiguity allowed; if
there is more than one job beginning with or containing the
word, this selects the most recently accessed job. The
name of a stopped job, in this context, is the command line
used to start it, as displayed by jobs. If set to the
value exact, the word must match the name of a stopped job
exactly; if set to substring, the word needs to match a
substring of the name of a stopped job. The substring
value provides functionality analogous to the %? job
identifier (see JOB CONTROL below). If set to any other
value (e.g., prefix), the word 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, quick substitution, and tokenization (see
HISTORY EXPANSION below). The first character is the
history expansion character, the character which begins a
history expansion, normally “!”. The second character is
the quick substitution character, normally “^”. When it
appears as the first character on the line, history
substitution repeats the previous command, replacing one
string with another. The optional third character is the
history comment character, normally “#”, which indicates
that the remainder of the line is a comment when it appears
as the first character of a word. The history comment
character disables history substitution for the remaining
words on the line. It does not necessarily cause the shell
parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional indexed and associative array
variables. Any variable may be used as an indexed array; the
declare builtin explicitly declares an array. There is no maximum
limit on the size of an array, nor any requirement that members be
indexed or assigned contiguously. Indexed arrays are referenced
using arithmetic expressions that must expand to an integer (see
ARITHMETIC EVALUATION below) and are zero-based; associative
arrays are referenced using arbitrary strings. Unless otherwise
noted, indexed array indices must be non-negative integers.
The shell performs parameter and variable expansion, arithmetic
expansion, command substitution, and quote removal on indexed
array subscripts. Since this can potentially result in empty
strings, subscript indexing treats those as expressions that
evaluate to 0.
The shell performs tilde expansion, parameter and variable
expansion, arithmetic expansion, command substitution, and quote
removal on associative array subscripts. Empty strings cannot be
used as associative array keys.
Bash automatically creates an indexed array 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 greater than or equal to zero. To explicitly
declare an indexed array, use
declare -a name
(see SHELL BUILTIN COMMANDS below).
declare -a name[subscript]
is also accepted; the subscript is ignored.
Associative arrays are created using
declare -A name
.
Attributes may be specified for an array variable using the
declare and readonly builtins. Each attribute applies to all
members of an array.
Arrays are assigned using compound assignments of the form
name=(value1 ... valuen), where each value may be of the form
[subscript]=string. Indexed array assignments do not require
anything but string. Each value in the list is expanded using the
shell expansions described below under EXPANSION, but values that
are valid variable assignments including the brackets and
subscript do not undergo brace expansion and word splitting, as
with individual variable assignments.
When assigning to indexed arrays, if the optional brackets and
subscript are supplied, that index is assigned to; otherwise the
index of the element assigned is the last index assigned to by the
statement plus one. Indexing starts at zero.
When assigning to an associative array, the words in a compound
assignment may be either assignment statements, for which the
subscript is required, or a list of words that is interpreted as a
sequence of alternating 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 interpreted; 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 introduced above.
When assigning to an indexed array, if name is subscripted 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 appends to an array variable when assigning
using the compound assignment syntax; see PARAMETERS above.
An array element is referenced using ${name[subscript]}. The
braces are required to avoid conflicts with pathname expansion.
If subscript is @ or *, the word expands to all members of name,
unless noted in the description of a builtin or word expansion.
These subscripts differ only when the word appears within double
quotes. If the word is double-quoted, ${name[*]} expands to a
single word with the value of each array member separated by the
first character of the IFS special variable, and ${name[@]}
expands each element of name to a separate word. When there are
no array members, ${name[@]} expands to nothing. If the double-
quoted expansion occurs within a word, the expansion of the first
parameter is joined with the beginning part of the expansion of
the original word, and the expansion of the last parameter is
joined with the last part of the expansion of the original word.
This is analogous to the expansion of the special parameters * and
@ (see Special Parameters above).
${#name[subscript]} expands to the length of ${name[subscript]}.
If subscript is * or @, the expansion is the number of elements in
the array.
If the subscript used to 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
referencing the array with a subscript of 0. Any reference to a
variable using a valid subscript is valid; bash creates an array
if necessary.
An array variable is considered set if a subscript has been
assigned a value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as
the values. ${!name[@]} and ${!name[*]} expand to the indices
assigned in array variable name. The treatment when in double
quotes is similar to the expansion of the special parameters @ and
* within double quotes.
The unset builtin is used to destroy arrays. unset
name[subscript] unsets the array element at index subscript, for
both indexed and associative arrays. Negative subscripts to
indexed arrays are interpreted as described above. Unsetting the
last element of an array variable does not unset the variable.
unset name, where name is an array, removes the entire array.
unset name[subscript] behaves differently depending on whether
name is an indexed or associative array when subscript is * or @.
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, (e.g., unset a[4]), the argument is
subject to pathname expansion. Quote the argument if pathname
expansion is not desired (e.g., unset 'a[4]').
The declare, local, and readonly builtins each accept a -a option
to specify an indexed array and a -A option to specify an
associative array. If both options are supplied, -A takes
precedence. The read builtin accepts a -a option to assign a list
of words read from the standard input to an array. The set and
declare builtins display array values in a way that allows them to
be reused as assignments. Other builtins accept array name
arguments as well (e.g., mapfile); see the descriptions of
individual builtins below for details. The shell provides a
number of builtin array variables.
Expansion is performed on the command line after it has been split
into words. The shell performs these expansions: brace expansion,
tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, pathname
expansion, and quote removal.
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; pathname expansion; and quote removal.
On systems that can support it, there is an additional expansion
available: process substitution. This is performed at the same
time as tilde, parameter, variable, and arithmetic expansion and
command substitution.
Quote removal is always performed last. It removes quote
characters present in the original word, not ones resulting from
one of the other expansions, unless they have been quoted
themselves.
Only brace expansion, word splitting, and pathname expansion can
increase the number of words of the expansion; other expansions
expand a single word to a single word. The only exceptions to
this are the expansions of "$@" and "${name[@]}", and, in most
cases, $* and ${name[*]} as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism to generate arbitrary strings
sharing a common prefix and suffix, either of which can be empty.
This mechanism is similar to pathname expansion, but the filenames
generated need not exist. Patterns to be brace expanded are
formed from an optional preamble, followed by either a series of
comma-separated strings or a sequence expression between a pair of
braces, followed by an optional postscript. The preamble is
prefixed to each string contained within the braces, and the
postscript is then appended to each resulting string, expanding
left to right.
Brace expansions may be nested. The results of each expanded
string are not sorted; brace expansion preserves left to right
order. 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. If
either x or y begins with a zero, each generated term will contain
the same number of digits, zero-padding where necessary. When
letters are supplied, the expression expands to each character
lexicographically between x and y, inclusive, using the 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
characters special to other expansions are preserved in the
result. It is strictly textual. Bash does not apply any
syntactic interpretation to the context of the expansion or the
text between the braces.
A correctly-formed brace expansion must contain unquoted opening
and closing braces, and at least one unquoted comma or a valid
sequence expression. Any incorrectly formed brace expansion is
left unchanged.
A “{” or Q , may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with
parameter expansion, the string “${” is not considered eligible
for brace expansion, 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 versions of sh. sh does not treat opening or closing
braces specially when they appear as part of a word, and preserves
them in the output. Bash removes braces from words as a
consequence of brace expansion. For example, a word entered to sh
as “file{1,2}” appears identically in the output. Bash outputs
that word as “file1 file2” after brace expansion. Start bash with
the +B option or disable brace expansion with the +B option to the
set command (see SHELL BUILTIN COMMANDS below) for strict sh
compatibility.
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 tilde expands to the home directory of the
user executing the shell instead. Otherwise, the tilde-prefix is
replaced with the home directory associated with the specified
login name.
If the tilde-prefix is a “~+”, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is a “~-”, the
shell substitutes the value of the shell variable OLDPWD, if it is
set. If the characters following the tilde in the tilde-prefix
consist of a number N, optionally prefixed by a “+” or a “-”, the
tilde-prefix is replaced with the corresponding element from the
directory stack, as it would be displayed by the dirs builtin
invoked with the characters following the tilde in the tilde-
prefix as an argument. If the characters following the tilde in
the tilde-prefix consist of a number without a leading “+” or “-”,
tilde expansion assumes “+”.
The results of tilde expansion are treated as if they were quoted,
so the replacement is not subject to word splitting and pathname
expansion.
If the login name is invalid, or the tilde expansion fails, the
tilde-prefix is unchanged.
Bash checks each variable assignment for unquoted tilde-prefixes
immediately following a : or the first =, and performs tilde
expansion in these cases. Consequently, one may use filenames
with tildes in assignments to PATH, MAILPATH, and CDPATH, and the
shell assigns the expanded value.
Bash also performs tilde expansion on words satisfying the
conditions of variable assignments (as described above under
PARAMETERS) when they appear as arguments to simple commands.
Bash does not do this, except for the declaration commands listed
above, when in posix mode.
Parameter Expansion
The “$” character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name or
symbol to be expanded may be enclosed in braces, which are
optional but serve to protect the variable to be expanded from
characters immediately following it which could be interpreted as
part of the name.
When braces are used, the matching ending brace is the first “}”
not escaped by a backslash or within a quoted string, and not
within an embedded arithmetic expansion, command substitution, or
parameter expansion.
The basic form of parameter expansion is
${parameter}
which substitutes the value of parameter. 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 parameter is not a nameref, it introduces a level of
indirection. Bash uses the value formed by expanding the rest of
parameter as the new parameter; this new parameter is then
expanded and that value is used in the rest of the expansion,
rather than the expansion of the original parameter. This is
known as indirect expansion. The value is subject to tilde
expansion, parameter expansion, command substitution, and
arithmetic expansion. If parameter is a nameref, this expands to
the name of the parameter referenced by parameter instead of
performing the complete indirect expansion, for compatibility.
The exceptions to this are the expansions of ${!prefix*} and
${!name[@]} described below. The exclamation point must
immediately follow the left brace in order to introduce
indirection.
In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic
expansion.
When not performing substring expansion, using the forms
documented below (e.g., :-), bash tests for a parameter that is
unset or null. Omitting the colon tests only for a parameter that
is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the
expansion of word is substituted. Otherwise, the value of
parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter, and the
expansion is the final value of parameter. Positional
parameters and special parameters may not be assigned in
this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or
unset, the shell writes the expansion of word (or a message
to that effect if word is not present) to the standard
error and, if it is not interactive, exits with a non-zero
status. An interactive shell does not exit, but does not
execute the command associated with the expansion.
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. The value of parameter is not used.
${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 below. If :length is omitted
(the first form above), this expands to the substring of
the value of parameter starting at the character specified
by offset and extending to the end of the value. If offset
is omitted, it is treated as 0. If length is omitted, but
the colon after offset is present, it is treated as 0.
length and offset are arithmetic expressions (see
ARITHMETIC EVALUATION below).
If offset evaluates to a number less than zero, the value
is used as an offset in characters from the end of the
value of parameter. If length evaluates to a number less
than zero, it is interpreted as an offset in characters
from the end of the value of parameter rather than a number
of characters, and the expansion is the characters between
offset and that result. Note that a negative offset must
be separated from the colon by at least one space to avoid
being confused with the :- expansion.
If parameter is @ or *, the result is length positional
parameters 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 (or 0 if there are no positional
parameters). It is an expansion error if length evaluates
to a number less than zero.
If parameter is an indexed array name subscripted by @ or
*, the result is the length members of the array beginning
with ${parameter[offset]}. A negative offset is taken
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 undefined results.
Substring indexing is zero-based unless the positional
parameters are used, in which case the indexing starts at 1
by default. If offset is 0, and the positional parameters
are used, $0 is prefixed to the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables
whose names begin with prefix, separated by the first
character of the IFS special variable. When @ is used and
the expansion appears within double quotes, each variable
name expands to a separate word.
${!name[@]}
${!name[*]}
List of array keys. If name is an array variable, expands
to the list of array indices (keys) assigned in name. If
name is not an array, expands to 0 if name is set and null
otherwise. When @ is used and the expansion appears within
double quotes, each key expands to a separate word.
${#parameter}
Parameter length. Substitutes the length in characters of
the expanded value of parameter. If parameter is * or @,
the value substituted is the number of positional
parameters. If parameter is an array name subscripted by *
or @, the value substituted is the number of elements in
the array. If parameter is an indexed array name
subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum
index of parameter, so negative indices count back from the
end of the array, and an index of -1 references the last
element.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to
produce a pattern just as in pathname expansion, and
matched against the expanded value of parameter using the
rules described under Pattern Matching below. If the
pattern matches the beginning of the value of parameter,
then the result of the expansion is the expanded value of
parameter with the shortest matching pattern (the “#” case)
or the longest matching pattern (the “##” case) deleted.
If parameter is @ or *, the pattern removal operation is
applied to each positional parameter in turn, and the
expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the pattern removal
operation is applied to each member of the array in turn,
and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to
produce a pattern just as in pathname expansion, and
matched against the expanded value of parameter using the
rules described under Pattern Matching below. If the
pattern matches a trailing portion of the expanded value of
parameter, then the result of the expansion is the expanded
value of parameter with the shortest matching pattern (the
“%” case) or the longest matching pattern (the “%%” case)
deleted. If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If parameter is
an array variable subscripted with @ or *, the pattern
removal operation is applied to each member of the array in
turn, and the expansion is the resultant list.
${parameter/pattern/string}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}
Pattern substitution. The pattern is expanded to produce a
pattern and matched against the expanded value of parameter
as described under Pattern Matching below. The longest
match of pattern in the expanded value is replaced with
string. string undergoes tilde expansion, parameter and
variable expansion, arithmetic expansion, command and
process substitution, and quote removal.
In the first form above, only the first match is replaced.
If there are two slashes separating 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
expanded 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 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 escapes & in
string; the backslash is removed in order to permit a
literal & in the replacement 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 interactions
between the backslash and double-quoting, since backslash
has special meaning within double quotes. Pattern
substitution 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.
Like the pattern removal operators, double quotes
surrounding the replacement string quote the expanded
characters, while double quotes enclosing the entire
parameter substitution do not, since the expansion is
performed in a context that doesn't take any enclosing
double quotes into account.
If the nocasematch shell option is enabled, the match is
performed without regard to the case of alphabetic
characters.
If parameter is @ or *, the substitution operation is
applied to each positional parameter in turn, and the
expansion is the resultant list. If parameter is an array
variable 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
alphabetic characters in parameter. First, the pattern is
expanded to produce a pattern as described below under
Pattern Matching. Bash then examines characters in the
expanded value of parameter against pattern as described
below. If a character matches the pattern, its case is
converted. The pattern should not attempt to match more
than one character.
Using “^” converts lowercase letters matching pattern to
uppercase; “,” converts matching uppercase letters to
lowercase. The ^ and , variants examine the first
character in the expanded value and convert its case if it
matches pattern; the ^^ and ,, variants examine all
characters in the expanded value and convert each one that
matches pattern. If pattern is omitted, it is treated like
a ?, which matches every character.
If parameter is @ or *, the case modification operation is
applied to each positional parameter in turn, and the
expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the case 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
transformation of the value of parameter or information
about parameter itself, 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 uppercase.
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 lowercase.
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, recreates parameter with its attributes
and value.
K Produces a possibly-quoted version of the value of
parameter, except that it prints the values of
indexed and associative arrays as a sequence of
quoted key-value pairs (see Arrays above). The keys
and values are quoted in a format that can be reused
as input.
a The expansion is a string consisting of flag values
representing 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
positional 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
command itself. There are two standard forms:
$(command)
or (deprecated)
`command`.
Bash performs the expansion by executing command in a subshell
environment 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).
With the old-style backquote form of substitution, backslash
retains its literal meaning except when followed by $, `, or \.
The first backquote not preceded by a backslash terminates the
command substitution. When using the $(command) form, all
characters between the parentheses make up the command; none are
treated specially.
There is an alternate form of command substitution:
${c command;}
which executes command in the current execution environment and
captures its output, again with trailing newlines removed.
The character c following the open brace must be a space, tab,
newline, or |, and the close brace must be in a position where a
reserved word may appear (i.e., preceded by a command terminator
such as semicolon). Bash allows the close brace to be joined to
the remaining characters in the word without being followed by a
shell metacharacter as a reserved word would usually require.
Any side effects of command take effect immediately in the current
execution environment and persist in the current environment after
the command completes (e.g., the exit builtin exits the shell).
This type of command substitution superficially resembles
executing an unnamed shell function: local variables are created
as when a shell function is executing, and the return builtin
forces command to complete; however, the rest of the execution
environment, including the positional parameters, is shared with
the caller.
If the first character following the open brace is a |, the
construct expands to the value of the REPLY shell variable after
command executes, without removing any trailing newlines, and the
standard output of command remains the same as in the calling
shell. Bash creates REPLY as an initially-unset local variable
when command executes, and restores REPLY to the value it had
before the command substitution after command completes, as with
any local variable.
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, bash does not
perform word splitting and pathname expansion on the results.
Arithmetic Expansion
Arithmetic expansion evaluates an arithmetic expression and
substitutes the result. The format for arithmetic expansion is:
$((expression))
The expression undergoes the same expansions as if it were within
double quotes, but unescaped double quote characters in expression
are not treated specially 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. Since the way Bash handles
double quotes can potentially result in empty strings, arithmetic
expansion treats those as expressions that evaluate to 0.
Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below
under ARITHMETIC EVALUATION. If expression is invalid, bash
prints a message to standard error indicating failure, does not
perform the substitution, and does not execute the command
associated with the expansion.
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 provides input
for list. If the <(list) form is used, reading the file obtains
the output of list. No space may appear between the < or > and
the left parenthesis, otherwise the construct would be interpreted
as a redirection.
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
substitution, and arithmetic expansion that did not occur within
double quotes for word splitting. Words that were not expanded
are not split.
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.
An IFS whitespace character is whitespace as defined above (see
Definitions) that appears in the value of IFS. Space, tab, and
newline are always considered IFS whitespace, even if they don't
appear in the locale's space category.
If IFS is unset, field splitting acts as if its value were
<space><tab><newline>, and treats these characters as IFS
whitespace. If the value of IFS is null, no word splitting
occurs, but implicit null arguments (see below) are still removed.
Word splitting begins by removing sequences of IFS whitespace
characters from the beginning and end of the results of the
previous expansions, then splits the remaining words.
If the value of IFS consists solely of IFS whitespace, any
sequence of IFS whitespace characters delimits a field, so a field
consists of characters that are not unquoted IFS whitespace, and
null fields result only from quoting.
If IFS contains a non-whitespace character, then any character in
the value of IFS that is not IFS whitespace, along with any
adjacent IFS whitespace characters, delimits a field. This means
that adjacent non-IFS-whitespace delimiters produce a null field.
A sequence of IFS whitespace characters also delimits a field.
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. Expanding a parameter with no value within double
quotes produces a null field, which 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, word splitting removes the null argument
portion, leaving the non-null expansion. That is, the word “-d''”
becomes “-d” after word splitting and null argument removal.
Pathname Expansion
After word splitting, unless the -f option has been set, bash
scans each word for the characters *, ?, and [. If one of these
characters appears, and is not quoted, then the word is regarded
as a pattern, and replaced with a sorted list of filenames
matching the pattern (see Pattern Matching below) subject to the
value of the GLOBSORT shell variable.
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, bash
prints an error message and does not execute the command. If the
shell option nocaseglob is enabled, the match is performed without
regard to the case of alphabetic characters.
When a pattern is used for pathname expansion, the character “.”
at the start of a name or immediately following a slash must be
matched explicitly, unless the shell option dotglob is set. 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 filenames . and .. never
match, 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 beginning with a
“.” 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 GLOBIGNORE pattern matching honors the setting of the
extglob shell option.
The value of the GLOBSORT shell variable controls how the results
of pathname expansion are sorted, as described above under Shell
Variables.
Pattern Matching
Any character that appears in a pattern, other than the special
pattern characters described below, matches itself. The NUL
character may not occur in a pattern. A backslash escapes the
following character; the escaping backslash is discarded when
matching. The special pattern characters must be quoted if they
are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When
the globstar shell option is enabled, and * is used
in a pathname expansion context, two adjacent *s
used as a single pattern match all files and zero or
more directories and subdirectories. If followed by
a /, two adjacent *s match only directories and
subdirectories.
? Matches any single character.
[...] Matches any one of the characters enclosed between
the brackets. This is known as a bracket expression
and matches a single character. A pair of
characters separated by a hyphen denotes a range
expression; any character that falls between those
two characters, inclusive, using the current
locale's collating sequence and character set,
matches. If the first character following the [ is
a ! or a ^ then any character not within the range
matches. To match a -, include it as the first or
last character in the set. To match a ], include it
as the first character in the set.
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 expressions, where [a-d] is equivalent to
[abcd], set the value of the LC_COLLATE or LC_ALL
shell variables to C, or enable the globasciiranges
shell option.
Within a bracket expression, 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 a bracket expression, an equivalence class
can be specified using the syntax [=c=], which
matches all characters with the same collation
weight (as defined by the current locale) as the
character c.
Within a bracket expression, the syntax [.symbol.]
matches the collating 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.
The extglob option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic
meaning. To ensure that extended matching patterns are parsed
correctly, make sure that extglob is enabled before parsing
constructs containing the patterns, 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
“.”. If the globskipdots shell option is enabled, the filenames .
and .. never appear in the set. As above, “.” only has a special
meaning when matching filenames.
Complicated extended pattern matching against long strings is
slow, especially when the patterns contain alternations and the
strings contain multiple matches. Using separate matches against
shorter strings, or using arrays of strings instead of a single
long string, may be faster.
Quote Removal
After the preceding expansions, all unquoted occurrences of the
characters \, ', and " that did not result from one of the above
expansions are removed.
Before a command is executed, its input and output may be
redirected using 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. When used with
the exec builtin, redirections modify file handles in the current
shell execution environment. The following redirection operators
may precede or appear anywhere within a simple command or may
follow a command. Redirections are processed in the order they
appear, from left to right.
Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}. In this
case, for each redirection operator except >&- and <&-, the shell
allocates a file descriptor greater than or equal to 10 and
assigns it to varname. If {varname} precedes >&- or <&-, the
value of varname defines the file descriptor to close. If
{varname} is supplied, the redirection persists beyond the scope
of the command, which allows the shell programmer to manage the
file descriptor's lifetime manually without using the exec
builtin. 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 redirection refers to the standard input (file descriptor
0). If the first character of the redirection operator is “>”,
the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace
expansion, tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, quote removal,
pathname expansion, and word splitting. If it expands to more
than one word, bash reports an error.
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 directed to the standard output before the
standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in
redirections, as described in the following table. If the
operating system on which bash is running provides these special
files, bash uses them; otherwise it emulates them internally with
the behavior described below.
/dev/fd/fd
If fd is a valid integer, duplicate file descriptor
fd.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name, bash
attempts to open the corresponding TCP socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name, bash
attempts to open the corresponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used
with care, as they may conflict with file descriptors the shell
uses internally.
Redirecting Input
Redirecting input opens the file whose name results from the
expansion of word for reading on file descriptor n, or the
standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirecting output opens the file whose name results from the
expansion of word for writing on file descriptor n, or the
standard output (file descriptor 1) if n is not specified. If the
file does not exist it is created; if it does exist it is
truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the
set builtin command has been enabled, the redirection fails 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 is not enabled, bash attempts the redirection even if the
file named by word exists.
Appending Redirected Output
Redirecting output in this fashion opens the file whose name
results from the expansion of word 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 redirects both the standard output (file descriptor
1) and the standard error output (file descriptor 2) to the file
whose name is the expansion of word.
There are two formats for redirecting standard output and standard
error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically
equivalent to
>word 2>&1
When using the second form, word may not expand to a number or -.
If it does, other redirection operators apply (see Duplicating
File Descriptors below) for compatibility reasons.
Appending Standard Output and Standard Error
This construct appends both the standard output (file descriptor
1) and the standard error output (file descriptor 2) to the file
whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
Here Documents
This type of redirection instructs the shell to read input from
the current source until it reads a line containing only delimiter
(with no trailing blanks). All of the lines read up to that point
then become 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
The shell does not perform parameter and variable expansion,
command substitution, arithmetic expansion, or pathname expansion
on word.
If any part of word is quoted, the delimiter is the result of
quote removal on word, and the lines in the here-document are not
expanded. If word is unquoted, the delimiter is word itself, and
the here-document text is treated similarly to a double-quoted
string: all lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion, the
character sequence \<newline> is treated literally, and \ must be
used to quote the characters \, $, and `; however, double quote
characters have no special meaning.
If the redirection operator is <<-, then the shell strips all
leading tab characters from input lines and the line containing
delimiter. This allows here-documents within shell scripts to be
indented in a natural fashion.
If the delimiter is not quoted, the \<newline> sequence is treated
as a line continuation: the two lines are joined and the
backslash-newline is removed. This happens while reading the
here-document, before the check for the ending delimiter, so
joined lines can form the end delimiter.
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 standard input (or file descriptor
n if n is specified).
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to
one or more digits, file descriptor n is made to be a copy of that
file descriptor. It is a redirection error if the digits in word
do not specify a file descriptor open for input. If word
evaluates to -, file descriptor n is closed. If n is not
specified, this uses the standard input (file descriptor 0).
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is
not specified, this uses the standard output (file descriptor 1).
It is a redirection error if the digits in word do not specify a
file descriptor open for output. 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 -, this redirects
the standard output and standard error 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
opens the file whose name is the expansion of word 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 allow a string to be substituted for a word that is in a
position in the input where it can be the first word of a simple
command. Aliases have names and corresponding values that are set
and unset using the alias and unalias builtin commands (see SHELL
BUILTIN COMMANDS below).
If the shell reads an unquoted word in the right position, it
checks the word to see if it matches an alias name. If it
matches, the shell replaces the word with the alias value, and
reads that value as if it had been read instead of the word. The
shell doesn't look at any characters following the word before
attempting alias substitution.
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, the shell
checks the next command word following the alias 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) instead.
Aliases are not expanded when the shell is not interactive, unless
the expand_aliases shell option is set using shopt (see the
description of shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one complete line
of input, and all lines that make up a compound command, before
executing any of the commands on that line or the compound
command. Aliases are expanded when a command is read, not when it
is executed. Therefore, an alias definition appearing on the same
line as another command does not take effect until the shell reads
the next line of input, and an alias definition in a compound
command does not take effect until the shell parses and executes
the entire compound command. The commands following the alias
definition on that line, or in the rest of a compound command, are
not affected by the new alias. This behavior is also an issue
when functions are executed. Aliases are expanded when a function
definition is read, not when the function is executed, because a
function definition is itself a command. As a consequence,
aliases defined in a function are not available until after that
function is executed. To be safe, always put alias definitions on
a separate line, and do not use alias in compound commands.
For almost every purpose, shell functions are preferable to
aliases.
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 shell
executes the list of commands associated with that function name.
Functions are executed in the context of the calling shell; there
is no new process created to interpret them (contrast this with
the execution of a shell script).
When a function is executed, the arguments to the function become
the positional parameters during its execution. The special
parameter # is updated to reflect the new positional parameters.
Special parameter 0 is unchanged. The first element of the
FUNCNAME variable is set to the name of the function while the
function is executing.
All other aspects of the shell execution environment are identical
between a function and its caller with these exceptions: the DEBUG
and RETURN traps (see the description of the trap builtin under
SHELL BUILTIN COMMANDS below) are not inherited unless the
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 are 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-
executing 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. 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
previous scopes. For instance, a local variable declared in a
function hides variables with the same name declared at previous
scopes, including global variables: references and assignments
refer to the local variable, leaving the variables at previous
scopes 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 restored when the
function returns.
For example, if a variable var is declared as local in function
func1, and func1 calls another function func2, references to var
made from within func2 resolve to the local variable var from
func1, shadowing any global variable named var.
The unset builtin also acts using the same dynamic scope: if a
variable is local to the current scope, unset unsets 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 remains 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 becomes
visible. If the unset acts on a variable at a previous scope, any
instance of a variable with that name that had been shadowed
becomes visible (see below how the localvar_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. If return is supplied a numeric
argument, that is the function's return status; otherwise the
function's return status is the exit status of the last command
executed before the return. Any command associated with the
RETURN trap is executed before execution resumes. When a function
completes, the values of the positional parameters and the special
parameter # are restored to the values they had prior to the
function's execution.
The -f option to the declare or typeset builtin commands lists
function names and definitions. The -F option to declare or
typeset lists 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 executing a separate shell invocation) automatically
have them defined with the -f option to the export builtin. The
-f option to the unset builtin deletes a function definition.
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, bash imposes no limit on the
number of recursive calls.
The shell allows arithmetic expressions to be evaluated, under
certain circumstances (see the let and declare builtin commands,
the (( compound command, the arithmetic for command, the [[
conditional command, and Arithmetic Expansion).
Evaluation is done in the largest fixed-width integers available,
with no check for overflow, though division by 0 is trapped and
flagged as an error. The operators and their precedence,
associativity, and values are the same as in the C language. The
following list of operators is grouped into levels of equal-
precedence operators. The levels are listed in order of
decreasing precedence.
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an
expression, shell variables may also be referenced by name without
using the parameter expansion syntax. This means you can use "x",
where x is a shell variable name, in an arithmetic expression, and
the shell will evaluate its value as an expression and use the
result. A shell variable that is null or unset evaluates to 0
when referenced by name in an expression.
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 attribute 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 octal numbers. A leading 0x or 0X denotes
hexadecimal. Otherwise, numbers take the form [base#]n, where the
optional base is a decimal number between 2 and 64 representing
the arithmetic base, and n is a number in that base. If base# is
omitted, then base 10 is used. When 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 order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between
10 and 35.
Operators are evaluated in precedence order. Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.
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 behavior based on the number of
arguments; see the descriptions of those commands for any other
command-specific actions.
Expressions are formed from the unary or binary primaries listed
below. Unary expressions are often used to examine the status of
a file or shell variable. Binary operators are used for string,
numeric, and file attribute comparisons.
Bash handles several filenames specially when they are used in
expressions. If the operating system on which bash is running
provides these special files, bash will use them; otherwise it
will emulate them internally with this behavior: If any file
argument to one of the primaries is of the form /dev/fd/n, then
bash checks file descriptor n. If the file argument to one of the
primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, bash
checks file descriptor 0, 1, or 2, respectively.
Unless otherwise specified, primaries that operate on files follow
symbolic links and operate on the target of the link, rather than
the link itself.
When used with [[, or when the shell is in posix mode, the < and >
operators sort lexicographically using the current locale. When
the shell is not in posix mode, 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
accessed.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
-o optname
True if the shell option optname is enabled. See the list
of options under the description of the -o option to the
set builtin below.
-v varname
True if the shell variable varname is set (has been
assigned a value). If varname is an indexed array variable
name subscripted by @ or *, this returns true if the array
has any set elements. If varname is an associative array
variable name subscripted by @ or *, this returns true if
an element with that key is set.
-R varname
True if the shell variable varname is set and is a name
reference.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with the
test command for POSIX conformance. When used with the [[
command, this performs pattern matching as described above
(Compound Commands).
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
string1 > string2
True if string1 sorts after string2 lexicographically.
file1 -ef file2
True if file1 and file2 refer to the same device and inode
numbers.
file1 -nt file2
True if file1 is newer (according to modification date)
than file2, or if file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and
file1 does not.
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 EVALUATION above).
Since the expansions the [[ command performs on arg1 and
arg2 can potentially result in empty strings, arithmetic
expression evaluation treats those as expressions that
evaluate to 0.
When the shell executes a simple command, it performs the
following expansions, assignments, and redirections, from left to
right, in the following 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 variable.
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
affect the current shell environment. A redirection error causes
the command to exit with a non-zero status.
If there is a command name left after expansion, execution
proceeds as described below. Otherwise, the command exits. If
one of the expansions contained a command substitution, the exit
status of the command is the exit status of the last command
substitution performed. If there were no command substitutions,
the command exits with a zero status.
After a command has been split into words, if it results in a
simple command and an optional list of arguments, the shell
performs the following actions.
If the command name contains no slashes, the shell attempts to
locate it. If there exists a shell function by that name, that
function is invoked as described above in FUNCTIONS. If the name
does not match a function, the shell searches for it in the list
of shell builtins. If a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and
contains no slashes, bash searches each element of the PATH for a
directory containing an executable file by that name. Bash uses a
hash table to remember the full pathnames of executable files (see
hash under SHELL BUILTIN COMMANDS below). Bash performs a full
search of the directories in PATH only if the command is not found
in the hash table. If the search is unsuccessful, the shell
searches for a defined shell function named
command_not_found_handle. If that function exists, it is invoked
in a separate execution environment with the original command and
the original command's arguments as its arguments, and the
function's exit status becomes the exit status of that subshell.
If that function is not defined, the shell prints an error message
and returns an exit status of 127.
If the search is successful, or if the command name contains one
or more slashes, the shell executes the named program in a
separate execution environment. Argument 0 is set to the name
given, and the remaining arguments to the command are set to the
arguments given, if any.
If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
shell script, a file containing shell commands, and the shell
creates a new instance of itself to execute it. Bash tries to
determine whether the file is a text file or a binary, and will
not execute files it determines to be binaries. This subshell
reinitializes itself, so that the effect is as if a new shell had
been invoked to handle the script, with the exception that the
locations of commands remembered by the parent (see hash below
under SHELL BUILTIN COMMANDS are retained by the child.
If the program is a file beginning with #!, the remainder of the
first line specifies an interpreter for the program. The shell
executes the specified interpreter on operating systems that do
not handle this executable format themselves. The arguments to
the interpreter consist of a single optional argument following
the interpreter name on the first line of the program, followed by
the name of the program, followed by the command arguments, if
any.
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 consists of the following. Unless otherwise noted, the
values are inherited from the shell.
• The shell's open files, plus any modifications and
additions specified by redirections to the command.
• The current working directory.
• The file creation mode mask.
• Shell variables and functions marked for export, along with
variables exported for the command, passed in the
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
asynchronous commands are invoked in a subshell environment that
is a duplicate of the shell environment, except that traps caught
by the shell are reset to the values that the shell inherited from
its parent at invocation. Builtin commands that are invoked as
part of a pipeline, except possibly in the last element depending
on the value of the lastpipe shell option, are also executed in a
subshell environment. Changes made to the subshell environment
cannot affect the shell's execution environment.
When the shell is in posix mode, subshells spawned to execute
command substitutions inherit the value of the -e option from
their parent shell. When not in posix mode, bash clears the -e
option in such subshells. See the description of the
inherit_errexit shell option below for how to control this
behavior when not in posix mode.
If a command is followed by a & and job control is not active, the
default standard input for the command is the empty file
/dev/null. Otherwise, the invoked command inherits the file
descriptors of the calling shell as modified by redirections.
When a program is invoked it is given an array of strings called
the environment. This is a list of name-value pairs, of the form
name=value.
The shell provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a
parameter for each name found, automatically marking it for export
to child processes. Executed commands inherit the environment.
The export, declare -x, and unset commands modify the environment
by adding and deleting parameters and functions. If the value of
a parameter in the environment is modified, the new value
automatically 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 or export -n commands,
plus any additions via the export and declare -x commands.
If any parameter assignments, as described above in PARAMETERS,
appear before a simple command, the variable assignments are part
of that command's environment for as long as it executes. These
assignment statements affect only the environment seen by that
command. If these assignments precede a call to a shell function,
the variables are local to the function and exported to that
function's children.
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 pathname of the command and passed to that command in its
environment.
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. So while an exit status of zero indicates
success, a non-zero exit status indicates failure.
When a command terminates on a fatal signal N, bash uses the value
of 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not
executable, the return status is 126.
If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful,
and non-zero (false) if an error occurs while they execute. All
builtins return an exit status of 2 to indicate incorrect usage,
generally invalid options or missing arguments.
The exit status of the last command is available in the special
parameter $?.
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.
When bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that kill 0 does not kill an interactive shell), and
catches and handles SIGINT (so that the wait builtin is
interruptible). When bash receives SIGINT, it breaks out of any
executing loops. In all cases, bash ignores SIGQUIT. If job
control is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
The trap builtin modifies the shell's signal handling, as
described below.
Non-builtin commands bash executes have signal handlers set to the
values inherited by the shell from its parent, unless trap sets
them to be ignored, in which case the child process will ignore
them as well. When job control is not in effect, asynchronous
commands ignore SIGINT and SIGQUIT in addition to these inherited
handlers. Commands run as a result of command substitution ignore
the keyboard-generated job control signals SIGTTIN, SIGTTOU, and
SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before
exiting, an interactive shell resends the SIGHUP to all jobs,
running or stopped. The shell sends SIGCONT to stopped jobs to
ensure that they receive the SIGHUP (see JOB CONTROL below for
more information about running and stopped jobs). To prevent the
shell from sending the signal to a particular job, remove it from
the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS
below) or mark it not to receive SIGHUP using disown -h.
If the huponexit shell option has been set using 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, it will not execute the trap until
the command completes. If bash is waiting for an asynchronous
command via the wait builtin, and it receives a signal for which a
trap has been set, the wait builtin will return immediately with
an exit status greater than 128, immediately after which the shell
executes the trap.
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. Since bash does not enable job control by default when the
shell is not interactive, this scenario is most common in non-
interactive shells.
When job control is enabled, and bash is waiting for a foreground
command to complete, the shell does not receive keyboard-generated
signals, because it is not in the same process group as the
terminal. This scenario is most common in interactive shells,
where bash attempts to enable job control by default. See JOB
CONTROL below for more information about process groups.
When job control is not enabled, and bash 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 send the SIGINT to the shell as
well, and acts on the SIGINT (e.g., by running a SIGINT
trap, exiting a non-interactive shell, or returning to the
top level to read a new command).
2. If the command does not terminate due to SIGINT, the
program handled the SIGINT itself and did not treat it as a
fatal signal. In that case, bash does not treat SIGINT as
a fatal signal, either, 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 complete, for
compatibility.
When job control is enabled, bash does not receive keyboard-
generated signals such as SIGINT while it is waiting for a
foreground command. An interactive shell does not pay attention
to the SIGINT, even if the foreground command terminates as a
result, other than noting its exit status. If the shell is not
interactive, and the foreground command terminates due to the
SIGINT, bash pretends it received the SIGINT itself (scenario 1
above), for compatibility.
Job control refers to the ability to selectively stop (suspend)
the execution of processes and continue (resume) their execution
at a later point. A user typically employs this facility via an
interactive interface supplied jointly by the operating system
kernel's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a table
of currently executing jobs, which the jobs command will display.
Each job has a job number, which jobs displays between brackets.
Job numbers start at 1. 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, each process has a process group ID, and the operating
system maintains the notion of a current terminal process group
ID. This terminal process group ID is associated with the
controlling terminal.
Processes that have the same process group ID are said to be part
of the same process group. Members of the foreground process
group (processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated signals such
as SIGINT. Processes in the foreground process group are said to
be foreground processes. Background processes are those whose
process group ID differs from the controlling terminal's; such
processes are immune to keyboard-generated signals. Only
foreground processes are allowed to read from or, if the user so
specifies with “stty tostop”, write to the controlling terminal.
The system sends a SIGTTIN (SIGTTOU) signal to background
processes which attempt to read from (write to when “tostop” is in
effect) the terminal, 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 (typically ^Z, Control-Z) while a process is running
stops that process and returns control to bash. Typing the
delayed suspend character (typically ^Y, Control-Y) causes the
process stop when it attempts to read input from the terminal, and
returns control to bash. The user then manipulates the state of
this job, using the bg command to continue it in the background,
the fg command to continue it in the foreground, or the kill
command to kill it. The suspend character takes effect
immediately, and has the additional side effect of discarding any
pending output and typeahead. To force a background process to
stop, or stop a process that's not associated with the current
terminal session, send it the SIGSTOP signal using kill.
There are a number of ways to refer to a job in the shell. The %
character introduces a job specification (jobspec).
Job number n may be referred to as %n. A job may also be referred
to using a prefix of the name used to start it, or using a
substring that appears in its command line. For example, %ce
refers to a job whose command name begins with ce. Using %?ce, on
the other hand, refers to any job containing the string ce in its
command line. If the prefix or substring matches more than one
job, bash reports an error.
The symbols %% and %+ refer to the shell's notion of the current
job. A single % (with no accompanying job specification) also
refers to the current job. %- refers to the previous job. When a
job starts in the background, a job stops while in the foreground,
or a job is resumed in the background, it becomes the current job.
The job that was the current job becomes the previous job. When
the current job terminates, the previous job becomes the current
job. 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 marked with a +,
and the previous job with a -.
Simply naming a job can be used to bring it into the foreground:
%1 is a synonym for “fg %1”, bringing job 1 from the background
into the foreground. Similarly, “%1 &” resumes job 1 in the
background, equivalent to “bg %1”.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt before
notifying the user about changes in a job's status so as to not
interrupt any other output, though it will notify of changes in a
job's status after a foreground command in a list completes,
before executing the next command in the list. If the -b option
to the set builtin command is enabled, bash reports status changes
immediately. Bash executes any trap on SIGCHLD for each child
that terminates.
When a job terminates and bash notifies the user about it, bash
removes the job from the table. It will not appear in jobs
output, but wait will report its exit status, as long as it's
supplied the process ID associated with the job as an argument.
When the table is empty, job numbers start over at 1.
If a user attempts to exit bash while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin,
running), the shell prints a warning message, and, if the
checkjobs option is enabled, lists the jobs and their statuses.
The jobs command may then be used to inspect their status. If the
user immediately attempts to exit again, without an intervening
command, bash does not print another warning, and terminates any
stopped jobs.
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.
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 examines the value of the array variable PROMPT_COMMAND just
before printing each primary prompt. If any elements in
PROMPT_COMMAND are set and non-null, Bash executes each value, in
numeric order, just as if it had been typed on the command line.
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 the prompt strings PS0, PS1, PS2, and PS4, to be
customized by inserting a number of backslash-escaped special
characters that are decoded as follows:
\a An ASCII bell character (07).
\d The date in “Weekday Month Date” format (e.g., “Tue
May 26”).
\D{format}
The format is passed to strftime(3) and the result
is inserted into the prompt string; an empty format
results in a locale-specific time representation.
The braces are required.
\e An ASCII escape character (033).
\h The hostname up to the first “.”.
\H The hostname.
\j The number of jobs currently managed by the shell.
\l The basename of the shell's terminal device name
(e.g., “ttys0”).
\n A newline.
\r A carriage return.
\s The name of the shell: the basename of $0 (the
portion following the final slash).
\t The current time in 24-hour HH:MM:SS format.
\T The current time in 12-hour HH:MM:SS format.
\@ The current time in 12-hour am/pm format.
\A The current time in 24-hour HH:MM format.
\u The username of the current user.
\v The bash version (e.g., 2.00).
\V The bash release, version + patch level (e.g.,
2.00.0)
\w The value of the PWD shell variable ($PWD), with
$HOME abbreviated with a tilde (uses the value of
the PROMPT_DIRTRIM 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 executed during the current shell
session. After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic expansion,
and quote removal, subject to the value of the promptvars shell
option (see the description of the shopt command under SHELL
BUILTIN COMMANDS below). This can have unwanted side effects if
escaped portions of the string appear within command substitution
or contain characters special to word expansion.
This is the library that handles reading input when using an
interactive shell, unless the --noediting option is supplied at
shell invocation. Line editing is also used when using the -e
option to the read builtin. By default, the line editing commands
are similar to those of emacs; a vi-style line editing interface
is also available. Line editing can be enabled at any time using
the -o emacs or -o vi options to the set builtin (see SHELL
BUILTIN COMMANDS below). To turn off line editing after the shell
is running, use the +o emacs or +o vi options to the set builtin.
Readline Notation
This section uses Emacs-style editing concepts and uses its
notation for keystrokes. Control keys are denoted by C-key, e.g.,
C-n means Control-N. Similarly, meta keys are denoted by M-key,
so M-x means Meta-X. The Meta key is often labeled “Alt” or
“Option”.
On keyboards without a Meta key, M-x means ESC x, i.e., press and
release the Escape key, then press and release the x key, in
sequence. This makes ESC the meta prefix. The combination M-C-x
means ESC Control-x: press and release the Escape key, then press
and hold the Control key while pressing the x key, then release
both.
On some keyboards, the Meta key modifier produces characters with
the eighth bit (0200) set. You can use the enable-meta-key
variable to control whether or not it does this, if the keyboard
allows it. On many others, the terminal or terminal emulator
converts the metafied key to a key sequence beginning with ESC as
described in the preceding paragraph.
If your Meta key produces a key sequence with the ESC meta prefix,
you can make M-key key bindings you specify (see Readline Key
Bindings below) do the same thing by setting the force-meta-prefix
variable.
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) makes
that command act in a backward direction. Commands whose behavior
with arguments deviates from this are noted below.
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. Readline has the concept of an active
region: when the region is active, readline redisplay highlights
the region using the value of the active-region-start-color
variable. The enable-active-region variable turns this on and
off. Several commands set the region to active; those 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 accumulate the deleted
text 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 shell variable. If that variable is unset,
the default is ~/.inputrc. If that file does not exist or cannot
be read, readline looks for /etc/inputrc. When a program that
uses the readline library starts up, readline reads the
initialization file and sets the key bindings and variables found
there, before reading any user input.
There are only a few basic constructs allowed in the inputrc 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 in this section may be changed using key
binding commands in the inputrc file. Programs that use the
readline library, including bash, may add their own commands and
bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command
universal-argument.
Key bindings may contain the following symbolic character names:
DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive
backspace), SPACE, SPC, 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). The
difference between a macro and a command is that a macro is
enclosed in single or double quotes.
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 key sequence may be specified in one of two ways: as a
symbolic key name, possibly with Meta- or Control- prefixes, or as
a key sequence composed of one or more characters enclosed in
double quotes. The key sequence and name are separated by a
colon. There can be no whitespace between the name and the colon.
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 none of the symbolic character names are
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 available when
specifying key sequences is
\C- A control prefix.
\M- Adding the meta prefix or converting the following
character to a meta character, as described below
under force-meta-prefix.
\e An escape character.
\\ Backslash.
\" Literal ", a double quote.
\' Literal ', a single quote.
In addition to the GNU Emacs style escape sequences, a second set
of backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn The eight-bit character whose value is the octal
value nnn (one to three digits).
\xHH The eight-bit character whose value is the
hexadecimal value HH (one or two hex digits).
When entering the text of a macro, single or double quotes must be
used to indicate a macro definition. Unquoted text is assumed to
be a function name. The backslash escapes described above are
expanded in the macro body. Backslash quotes any other character
in the macro text, including " and '.
Bash will display or modify the current readline key bindings with
the bind builtin command. The -o emacs or -o vi options to the
set builtin (see SHELL BUILTIN COMMANDS below) change the editing
mode during interactive use.
Readline Variables
Readline has variables that can be used to further customize its
behavior. A variable may be set in the inputrc file with a
statement of the form
set variable-name value
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 readline reads a variable value, empty or null
values, “on” (case-insensitive), and “1” are equivalent to On.
All other values are equivalent to Off.
The bind -V command lists the current readline variable names and
values (see SHELL BUILTIN COMMANDS below).
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 description 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
description. A sample value might be “\e[01;33m”.
active-region-end-color
A string variable that “undoes” the effects of
active-region-start-color and restores “normal” terminal
display appearance 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 restores the terminal from standout mode, as obtained
from the terminal'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 that are treated specially by the kernel's
terminal driver to their readline equivalents. These
override the default readline bindings described here.
Type “stty -a” at a bash prompt to see your current
terminal settings, including the special control characters
(usually cchars).
blink-matching-paren (Off)
If set to On, readline attempts to briefly move the cursor
to an opening parenthesis when a closing parenthesis is
inserted.
colored-completion-prefix (Off)
If set to On, when listing completions, readline displays
the common prefix of the set of possible completions using
a different color. The color definitions are taken from
the value of the LS_COLORS environment variable. 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
different 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 the readline insert-comment command
inserts. 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 causes 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 performing case-insensitive filename
matching and completion.
completion-prefix-display-length(0)
The maximum length in characters of the common prefix of a
list of possible completions that is displayed without
modification. When set to a value greater than zero,
readline replaces common prefixes longer than this value
with an ellipsis when displaying possible completions. If
a completion begins with a period, and eadline is
completing filenames, it uses three underscores instead of
an ellipsis.
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 asks whether or not the user
wishes to view them; otherwise readline simply lists them
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 converts characters it reads that
have the eighth bit set to an ASCII key sequence by
clearing the eighth bit and prefixing it with an escape
character (converting the character to have the meta
prefix). The default is On, but readline sets it to Off if
the locale contains characters whose encodings may include
bytes with the eighth bit set. This variable is dependent
on the LC_CTYPE locale category, and may change if the
locale changes. This variable also affects key bindings;
see the description of force-meta-prefix below.
disable-completion (Off)
If set to On, readline inhibits word completion.
Completion characters are inserted into the line as if they
had been mapped to self-insert.
echo-control-characters (On)
When set to On, on operating systems that indicate they
support it, readline echoes a character corresponding to a
signal generated from the keyboard.
editing-mode (emacs)
Controls whether readline uses a set of key bindings
similar to Emacs or vi. editing-mode can be set to either
emacs or vi.
emacs-mode-string (@)
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the
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. The \1 and \2 escapes 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)
When this variable is set to On, readline allows certain
commands to designate the region as active. When the
region is active, readline highlights the text in the
region using the value of the active-region-start-color
variable, 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-incremental 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 is called
bracketed-paste mode; it prevents readline from executing
any editing commands bound to key sequences appearing in
the pasted text.
enable-keypad (Off)
When set to On, readline tries to enable the application
keypad when it is called. Some systems need this to enable
the arrow keys.
enable-meta-key (On)
When set to On, readline tries to enable any meta modifier
key the terminal claims to support. On many terminals, the
Meta key is used to send eight-bit characters; this
variable checks for the terminal capability that indicates
the terminal can enable and disable a mode that sets the
eighth bit of a character (0200) if the Meta key is held
down when the character is typed (a meta character).
expand-tilde (Off)
If set to On, readline performs tilde expansion when it
attempts word completion.
force-meta-prefix (Off)
If set to On, readline modifies its behavior when binding
key sequences containing \M- or Meta- (see Key Bindings
above) by converting a key sequence of the form \M-C or
Meta-C to the two-character sequence ESC C (adding the meta
prefix). If force-meta-prefix is set to Off (the default),
readline uses the value of the convert-meta variable to
determine whether to perform this conversion: if
convert-meta is On, readline performs the conversion
described above; if it is Off, readline converts C to a
meta character by setting the eighth bit (0200).
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-history 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, bash sets the maximum number of
history entries to the value of the HISTSIZE shell
variable. Setting history-size to a non-numeric value will
set the maximum number of history entries to 500.
horizontal-scroll-mode (Off)
Setting this variable 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 enables eight-bit input (that is, it
does not clear the eighth bit in the characters it reads),
regardless of what the terminal claims it can support. The
default is Off, but readline sets it to On if the locale
contains characters whose encodings may include bytes with
the eighth bit set. This variable is dependent on the
LC_CTYPE locale category, and its value may change if the
locale changes. The name meta-flag is a synonym for
input-meta.
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 terminate an
incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap
names is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-command, and vi-insert. vi is equivalent to vi-command;
emacs is equivalent to emacs-standard. The default value
is emacs; the value of editing-mode also affects the
default keymap.
keyseq-timeout (500)
Specifies the duration readline will wait for a character
when reading an ambiguous key sequence (one that can form a
complete key sequence using the input read so far, or can
take additional input to complete a longer key sequence).
If readline does not receive any input within the timeout,
it uses the shorter but complete key sequence. The value
is specified in milliseconds, so a value of 1000 means that
readline will wait one second for additional input. If
this variable is set to a value less than or equal to zero,
or to a non-numeric value, readline waits until another key
is pressed to decide which key sequence to complete.
mark-directories (On)
If set to On, completed directory names have a slash
appended.
mark-modified-lines (Off)
If set to On, readline displays history lines that have
been modified with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
directories have a slash appended, subject to the value of
mark-directories.
match-hidden-files (On)
This variable, when set to On, forces readline to match
files whose names begin with a “.” (hidden files) when
performing filename completion. If set to Off, the user
must include the leading “.” 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 displays characters with the eighth
bit set directly rather than as a meta-prefixed escape
sequence. The default is Off, but readline sets it to On
if the locale contains characters whose encodings may
include bytes with the eighth bit set. This variable is
dependent on the LC_CTYPE locale category, and its value
may change if the locale changes.
page-completions (On)
If set to On, readline uses an internal pager resembling
more(1) to display a screenful of possible completions at a
time.
prefer-visible-bell
See bell-style.
print-completions-horizontally (Off)
If set to On, readline displays completions with matches
sorted horizontally in alphabetical order, rather than down
the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history
lines before returning when executing accept-line. By
default, history lines may be modified and retain
individual undo lists across calls to readline.
search-ignore-case (Off)
If set to On, readline performs incremental and non-
incremental history list searches in a case-insensitive
fashion.
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
indicating the editing mode: emacs, vi command, or vi
insertion. The mode strings are user-settable (e.g.,
emacs-mode-string).
skip-completed-text (Off)
If set to On, this alters the default completion behavior
when inserting a single match into the line. It's only
active when performing completion in the middle of a word.
If enabled, readline does not insert characters from the
completion that match characters after point in the word
being completed, so portions of the word following the
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. The \1 and \2
escapes begin and end sequences of non-printing characters,
which can be used to embed a terminal control sequence into
the mode string.
vi-ins-mode-string ((ins))
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the
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. The \1 and \2
escapes begin and end sequences of non-printing characters,
which can be used to embed a terminal control sequence into
the mode string.
visible-stats (Off)
If set to On, a character denoting a file's type as
reported by stat(2) is appended to the filename when
listing possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the
conditional compilation features of the C preprocessor which
allows key bindings and variable settings to be performed as the
result of tests. There are four parser directives available.
$if The $if construct allows bindings to be made based on the
editing mode, the terminal being used, or the application
using readline. The text of the test, after any comparison
operator, extends to the end of the line; unless otherwise
noted, no characters are required to isolate it.
mode The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode. This may
be used in conjunction with the set keymap command,
for instance, to set bindings in the emacs-standard
and emacs-ctlx keymaps only if readline is starting
out in emacs mode.
term The term= form may be used to include terminal-
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 terminal and the
portion of the terminal name before the first -.
This allows xterm to match both xterm and
xterm-256color, 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 defaults to 0. The operator
may be separated from the string version and from
the version number argument by whitespace.
application
The application construct is used to include
application-specific settings. Each program using
the readline library sets the application name, and
an initialization file can test for a particular
value. This could be used to bind key sequences to
functions useful for a specific program. For
instance, the following command adds a key sequence
that quotes the current or previous word in bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
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. String and boolean variables may be
tested. Boolean variables must be tested against
the values on and off.
$else Commands in this branch of the $if directive are executed
if the test fails.
$endif This command, as seen in the previous example, terminates
an $if command.
$include
This directive takes a single filename as an argument and
reads commands and key bindings from that file. For
example, the following directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command
history (see HISTORY below) for lines containing a specified
string. There are two search modes: incremental and non-
incremental.
Incremental searches begin before the user has finished typing the
search string. As each character of the search string is typed,
readline displays the next entry from the history matching the
string typed so far. An incremental search requires only as many
characters as needed to find the desired history entry. When
using emacs editing mode, type C-r to search backward in the
history for a particular string. Typing C-s searches forward
through the history. The characters present in the value of the
isearch-terminators variable are used to terminate an incremental
search. If that variable has not been assigned a value, ESC and
C-j terminate an incremental search. C-g aborts an incremental
search and restores 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 C-r or
C-s as appropriate. This searches 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
terminates the search and executes that command. For instance, a
newline terminates the search and accepts the line, thereby
executing the command from the history list. A movement command
will terminate the search, make the last line found the current
line, and begin editing.
Readline remembers the last incremental search string. If two
C-rs are typed without any intervening characters defining a new
search string, readline uses any remembered search string.
Non-incremental searches read the entire search string before
starting to search for matching history entries. The search
string may be typed by the user or be part of the contents of the
current line.
Readline Command Names
The following is a list of the names of the commands and the
default key sequences to which they are bound. Command names
without an accompanying key sequence are unbound by default.
In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark command. The text between the point and mark is referred
to as the region. Readline has the concept of an active region:
when the region is active, readline redisplay highlights the
region using the value of the active-region-start-color variable.
The enable-active-region readline variable turns this on and off.
Several commands set the region to active; those are noted below.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line. This may also be
bound to the Home key on some keyboards.
end-of-line (C-e)
Move to the end of the line. This may also be bound to the
End key on some keyboards.
forward-char (C-f)
Move forward a character. This may also be bound to the
right arrow key on some keyboards.
backward-char (C-b)
Move back a character. This may also be bound to the left
arrow key on some keyboards.
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 (M-C-f)
Move forward to the end of the next word. Words are
delimited by non-quoted shell metacharacters.
shell-backward-word (M-C-b)
Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.
previous-screen-line
Attempt to move point to the same physical screen column on
the previous physical screen line. This will not have the
desired effect if the current readline line does not take
up more than one physical line or if point is not greater
than the length of the prompt plus the screen width.
next-screen-line
Attempt to move point to the same physical screen column on
the next physical screen line. This will not have the
desired effect if the current readline line does not take
up more than one physical line or if the length of the
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
current line at the top of the screen. With a numeric
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 and HISTIGNORE variables. If
the line is a modified history line, 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. This may also be bound to the up arrow
key on some keyboards.
next-history (C-n)
Fetch the next command from the history list, moving
forward in the list. This may also be bound to the down
arrow key on some keyboards.
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 as if a newline had
been entered, and fetch the next line relative to the
current line from the history for editing. A numeric
argument, if supplied, specifies the history entry to use
instead of the current line.
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. This command sets the region to the
matched text and activates the region.
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. This command sets the region to the
matched text and activates the region.
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. The search string may match anywhere in a
history line.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental
search for a string supplied by the user. The search
string may match anywhere in a history line.
history-search-backward
Search backward through the history for the string of
characters between the start of the current line and the
point. The search string must match at the beginning of a
history line. This is a non-incremental search. This may
be bound to the Page Up key on some keyboards.
history-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 must match at the beginning of a
history line. This is a non-incremental search. This may
be bound to the Page Down key on some keyboards.
history-substring-search-backward
Search backward 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-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-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually
the second word on the previous line) at point. With an
argument n, insert the nth word from the previous command
(the words in the previous command begin with word 0). A
negative argument inserts the nth word from the end of the
previous command. Once the argument n is computed, this
uses the history expansion facilities to extract the nth
word, 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). This uses the history expansion
facilities to extract the last word, as if the “!$” history
expansion had been specified.
shell-expand-line (M-C-e)
Expand the line by performing shell word expansions. This
performs alias and history expansion, $'string' and
$"string" quoting, tilde expansion, parameter and variable
expansion, arithmetic expansion, command and process
substitution, word splitting, and quote removal. An
explicit argument suppresses command and process
substitution. See HISTORY EXPANSION below for a
description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a
space. See HISTORY EXPANSION below for a description of
history expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES
above for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
edit-and-execute-command (C-x C-e)
Invoke an editor on the current command line, and execute
the result as shell commands. Bash attempts to invoke
$VISUAL, $EDITOR, and emacs as the editor, in that order.
Commands for Changing Text
end-of-file (usually C-d)
The character indicating end-of-file as set, for example,
by stty(1). 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. This may also be
bound to the Delete key on some keyboards.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a
numeric argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is
at the end of the line, in which case the character behind
the cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is
how to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
bracketed-paste-begin
This function is intended to be bound to the “bracketed
paste” escape sequence sent by some terminals, and such a
binding is assigned by default. It allows readline to
insert the pasted text as a single unit without treating
each character as if it had been read from the keyboard.
The pasted characters are inserted as if each one was bound
to self-insert instead of executing any editing commands.
Bracketed paste sets the region to the inserted text and
activates the region.
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 past that word as well. If point is at the
end of the line, this transposes the last two words on the
line.
shell-transpose-words (M-C-t)
Drag the word before point past the word after point,
moving point past that word as well. If the insertion
point is at the end of the line, this transposes the last
two words on the line. Word boundaries are the same as
shell-forward-word and shell-backward-word.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative
argument, uppercase the previous word, but do not move
point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative
argument, lowercase the previous word, but do not move
point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a
negative argument, capitalize the previous word, but do not
move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric
argument, switches to overwrite mode. With an explicit
non-positive numeric argument, switches to insert mode.
This command affects only emacs mode; vi mode does
overwrite differently. Each call to readline() starts in
insert mode.
In overwrite mode, characters bound to self-insert replace
the text at point rather than pushing the text to the
right. Characters bound to backward-delete-char replace
the character before point with a space. By default, this
command is unbound, but may be bound to the Insert key on
some keyboards.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the current line.
With a negative numeric argument, kill backward from the
cursor to the beginning of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the current line. With a
negative numeric argument, kill forward from the cursor to
the end of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line,
saving the killed text on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where
point is.
kill-word (M-d)
Kill from point to the end of the current word, or if
between words, to the end of the next word. Word
boundaries are the same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same
as those used by backward-word.
shell-kill-word (M-C-d)
Kill from point to the end of the current word, or if
between words, to the end of the next word. Word
boundaries are the same as those used by
shell-forward-word.
shell-backward-kill-word
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, saving the killed text on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash
character as the word boundaries, saving the killed text 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, so it can
be yanked immediately.
copy-backward-word
Copy the word before point to the kill buffer. The word
boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works
following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or
start a new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this
command is followed by one or more digits, optionally with
a leading minus sign, those digits define the argument. If
the command is followed by digits, executing
universal-argument again ends the numeric argument, but is
otherwise ignored. As a special case, if this command is
immediately followed by a character that is neither a digit
nor minus sign, the argument count for the next command is
multiplied by four. The argument count is initially one,
so executing this function the first time makes the
argument count four, a second time makes the argument count
sixteen, and so on.
Completing
complete (TAB)
Attempt to perform completion on the text before point.
Bash attempts completion by first checking for any
programmable completions for the command word (see
Programmable Completion below), otherwise 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, functions, and builtins) in
turn. If none of these produces a match, it falls back to
filename completion.
possible-completions (M-?)
List the possible completions of the text before point.
When displaying completions, readline sets the number of
columns used for display to the value of completion-
display-width, the value of the shell variable COLUMNS, or
the screen width, in that order.
insert-completions (M-*)
Insert all completions of the text before point that would
have been generated by possible-completions, separated by a
space.
menu-complete
Similar to complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeatedly executing menu-complete steps through the list
of possible completions, inserting each match in turn. At
the end of the list of completions, menu-complete rings the
bell (subject to the setting of bell-style) and restores
the original text. An argument of n moves n positions
forward in the list of matches; a negative argument moves
backward through the list. This command is intended to be
bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the
list of possible completions, as if menu-complete had been
given a negative argument. This command is unbound by
default.
export-completions
Perform completion on the word before point as described
above and write the list of possible completions to
readline's output stream using the following format,
writing information on separate lines:
• the number of matches N;
• the word being completed;
• S:E, where S and E are the start and end offsets of
the word in the readline line buffer; then
• each match, one per line
If there are no matches, the first line will be “0”, and
this command does not print any output after the S:E. If
there is only a single match, this prints a single line
containing it. If there is more than one match, this
prints the common prefix of the matches, which may be
empty, on the first line after the S:E, then the matches on
subsequent lines. In this case, N will include the first
line with the common prefix.
The user or application should be able to accommodate the
possibility of a blank line. The intent is that the user
or application reads N lines after the line containing S:E
to obtain the match list. 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). At the
end of the line, it behaves identically to
possible-completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as
a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as
a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point,
treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as
a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point,
treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as
a command name. Command completion attempts to match the
text against aliases, reserved words, shell functions,
shell builtins, and finally executable filenames, in that
order.
possible-command-completions (C-x !)
List the possible completions of the text before point,
treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the
text against history list entries 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
completions enclosed within braces so the list is available
to the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard
macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard
macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the
characters in the macro appear as if typed at the keyboard.
print-last-kbd-macro ()
Print the last keyboard macro defined in a format suitable
for the inputrc file.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate
any bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's
bell (subject to the setting of bell-style).
do-lowercase-version (M-A, M-B, M-x, ...)
If the metafied character x is uppercase, run the command
that is bound to the corresponding metafied lowercase
character. The behavior 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, set the mark to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. Set the current cursor
position to the saved position, then set the mark to the
old cursor position.
character-search (C-])
Read a character and move point to the next occurrence of
that character. A negative argument searches for previous
occurrences.
character-search-backward (M-C-])
Read a character and move point 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. CSI sequences
begin with a Control Sequence Indicator (CSI), usually ESC
[. If this sequence is bound to “\e[”, keys producing CSI
sequences have no effect unless explicitly bound to a
readline command, instead of inserting stray characters
into the editing buffer. This is unbound by default, but
usually bound to ESC [.
insert-comment (M-#)
Without a numeric argument, insert the value of the
readline comment-begin variable 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, insert the value; otherwise delete the
characters in comment-begin from the beginning of the line.
In either case, the line is accepted as if a newline had
been typed. The default value of comment-begin causes this
command to make the current line a shell comment. If a
numeric argument causes the comment character to be
removed, the line will be executed by the shell.
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 option. Word boundaries are the same as
those used by shell-forward-word.
glob-complete-word (M-g)
Treat the word before point as a pattern for pathname
expansion, with an asterisk implicitly appended, then use
the pattern to generate a list of matching file names for
possible completions.
glob-expand-word (C-x *)
Treat the word before point as a pattern for pathname
expansion, and insert the list of matching file names,
replacing the word. If a numeric argument is supplied,
append a * before pathname expansion.
glob-list-expansions (C-x g)
Display the list of expansions that would have been
generated by glob-expand-word and redisplay the line. If a
numeric argument is supplied, append a * before pathname
expansion.
dump-functions
Print all of the functions and their key bindings to the
readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made
part of an inputrc file.
dump-variables
Print all of the settable readline variables and their
values to the readline output stream. If a numeric
argument is supplied, the output is formatted in such a way
that it can be made part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and
the strings they output 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.
execute-named-command (M-x)
Read a bindable readline command name from the input and
execute the function to which it's bound, as if the key
sequence to which it was bound appeared in the input. If
this function is supplied with a numeric argument, it
passes that argument to the function it executes.
display-shell-version (C-x C-v)
Display version information about the current instance of
bash.
Programmable Completion
When a user attempts word completion for a command or an argument
to a command for which a completion specification (a compspec) has
been defined using the complete builtin (see SHELL BUILTIN
COMMANDS below), readline invokes the programmable completion
facilities.
First, bash identifies the command name. 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
the empty string (completion attempted at the beginning of an
empty line), bash uses any compspec defined with the -E option to
complete. The -I option to complete indicates that the command
word is the first non-assignment word on the line, or after a
command delimiter such as ; or |. This usually indicates command
name completion.
If the command word is a full pathname, bash searches for a
compspec for the full pathname first. If there is no compspec for
the full pathname, bash attempts to find a compspec for the
portion following the final slash. If those searches do not
result in a compspec, or if there is no compspec for the command
word, bash uses any compspec defined with the -D option to
complete as the default. If there is no default compspec, bash
performs alias expansion on the command word as a final resort,
and attempts to find a compspec for the command word resulting
from any successful expansion.
If a compspec is not found, bash performs its default completion
as described above under Completing. Otherwise, once a compspec
has been found, bash uses it to generate the list of matching
words.
First, bash performs the actions specified by the compspec. This
only returns matches which are prefixes of the word being
completed. When the -f or -d option is used for filename or
directory name completion, bash uses the shell variable FIGNORE to
filter the matches.
Next, programmable completion generates matches specified by a
pathname expansion pattern supplied as an argument to the -G
option. The words generated by the pattern need not match the
word being completed. Bash uses the FIGNORE variable to filter
the matches, but does not use the GLOBIGNORE shell variable.
Next, completion considers the string specified as the argument to
the -W option. The string is first split using the characters in
the IFS special variable as delimiters. This honors shell quoting
within the string, in order to provide a mechanism for the words
to contain shell metacharacters or characters in the value of IFS.
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
possible completions.
After these matches have been generated, bash executes any shell
function or command specified with the -F and -C options. When
the command or function is invoked, bash assigns values to the
COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables as
described above under Shell Variables. If a shell function is
being invoked, bash also sets the COMP_WORDS and COMP_CWORD
variables. 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. There is no filtering of
the generated completions against the word being completed; the
function or command has complete freedom in generating the matches
and they do not need to match a prefix of the word.
Any function specified with -F is invoked first. The function may
use any of the shell facilities, including the compgen and compopt
builtins described below, 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
environment equivalent to command substitution. It should print a
list of completions, one per line, to the standard output.
Backslash will escape a newline, if necessary. These are added to
the set of possible completions.
After generating all of the possible completions, bash applies any
filter specified with the -X option to the completions in the
list. The filter is a pattern as used for pathname expansion; a &
in the pattern is replaced with the text of the word being
completed. A literal & may be escaped with a backslash; the
backslash is removed before attempting a match. Any completion
that matches the pattern is removed from the list. A leading !
negates the pattern; in this case bash removes any completion that
does not match the pattern. If the nocasematch shell option is
enabled, bash performs the match without regard to the case of
alphabetic characters.
Finally, programmable completion adds any prefix and suffix
specified with the -P and -S options, respectively, to each
completion, and returns the result to readline as the list of
possible completions.
If the previously-applied actions do not generate any matches, and
the -o dirnames option was supplied to complete when the compspec
was defined, bash attempts directory name completion.
If the -o plusdirs option was supplied to complete when the
compspec was defined, bash attempts directory name completion and
adds any matches to the set of possible completions.
By default, if a compspec is found, whatever it generates is
returned to the completion code as the full set of possible
completions. The default bash completions and the readline
default of filename completion are disabled. If the -o
bashdefault option was supplied to complete when the compspec was
defined, and the compspec generates no matches, bash attempts its
default completions. If the compspec and, if attempted, the
default bash completions generate no matches, and the -o default
option was supplied to complete when the compspec was defined,
programmable completion performs readline's default completion.
The options supplied to complete and compopt can control how
readline treats the completions. For instance, the -o fullquote
option tells readline to quote the matches as if they were
filenames. See the description of complete below for details.
When a compspec indicates that it wants directory name completion,
the programmable completion functions force readline to append a
slash to completed names which are symbolic links to directories,
subject to the value of the mark-directories readline variable,
regardless of the setting of the mark-symlinked-directories
readline variable.
There is some support for dynamically modifying completions. This
is most useful when used in combination with a default completion
specified with complete -D. It's possible for shell functions
executed as completion functions to indicate that completion
should be retried by returning an exit status of 124. If a shell
function returns 124, and changes the compspec associated with the
command on which completion is being attempted (supplied as the
first argument when the function is executed), programmable
completion restarts from the beginning, with an attempt to find a
new compspec for that command. This can be used to build a set of
completions dynamically as completion is attempted, rather than
loading them all at once.
For instance, assuming that there is a library of compspecs, each
kept in a file corresponding to the name of the command, the
following default completion function would load completions
dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" \
>/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader \
-o bashdefault -o default
When the -o history option to the set builtin is enabled, the
shell provides access to the command history, the list of commands
previously typed. The value of the HISTSIZE variable is used as
the number of commands to save in a history list: the shell saves
the text of the last HISTSIZE commands (default 500). 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, bash initializes the history list by reading history
entries from the file named by the HISTFILE variable (default
~/.bash_history). That file is referred to as the history file.
The history file is truncated, if necessary, to contain no more
than the number of history entries specified by the value of the
HISTFILESIZE variable. If HISTFILESIZE is unset, or set to null,
a non-numeric value, or a numeric value less than zero, the
history file is not truncated.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history line. These timestamps
are optionally displayed depending on the value of the
HISTTIMEFORMAT variable. When present, history timestamps delimit
history entries, making multi-line entries possible.
When a shell with history enabled exits, bash copies the last
$HISTSIZE entries from the history list to $HISTFILE. If the
histappend shell option is enabled (see the description of shopt
under SHELL BUILTIN COMMANDS below), bash appends the entries to
the history file, otherwise it overwrites the history file. If
HISTFILE is unset or null, or if the history file is unwritable,
the history is not saved. After saving the history, bash
truncates the history file to contain no more than HISTFILESIZE
lines as described above.
If the HISTTIMEFORMAT variable is set, the shell writes the
timestamp information associated with each history entry to the
history file, marked with the history comment character, so
timestamps are preserved across shell sessions. This uses the
history comment character to distinguish timestamps from other
history lines. As above, when using HISTTIMEFORMAT, the
timestamps delimit multi-line history entries.
The fc builtin command (see SHELL BUILTIN COMMANDS below) will
list or edit and re-execute a portion of the history list. The
history builtin can 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 are used
to save only a subset of the commands entered. If the cmdhist
shell option is enabled, the shell attempts 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 modifies cmdhist by saving 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.
The shell supports a history expansion feature that is similar to
the history expansion in csh. This section describes what syntax
features are available.
History expansion is enabled by default for interactive shells,
and can be disabled using the +H option to the set builtin command
(see SHELL BUILTIN COMMANDS below). Non-interactive shells do not
perform history expansion by default, but it can be enabled with
“set -H”.
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. The shell attempts to inform the
history expansion functions about quoting still in effect from
previous lines.
It takes place in two parts. The first is to determine which
history list entry to use during substitution. The second is to
select portions of that entry to include into the current one.
The entry selected from the history is the event, and the portions
of that entry that are acted upon are words. Various modifiers
are available to manipulate the selected words. The entry is
split into words in the same fashion as when reading input, so
that several metacharacter-separated words surrounded by quotes
are considered one word. The event designator selects the event,
the optional word designator selects words from the event, and
various optional modifiers are available to manipulate the
selected words.
History expansions are introduced by the appearance of the history
expansion character, which is ! by default. History expansions
may appear anywhere in the input, but do not nest.
Only backslash (\) and single quotes can quote the history
expansion character, but the history expansion character is also
treated as quoted if it immediately precedes the closing double
quote in a double-quoted string.
Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, =, and the other shell
metacharacters defined above.
There is a special abbreviation for substitution, active when the
quick substitution character (described above under histchars) is
the first character on the line. It selects the previous history
list entry, using an event designator equivalent to !!, and
substitutes one string for another in that entry. It is described
below under Event Designators. This is the only history expansion
that does not begin with the history expansion character.
Several shell options settable with the shopt builtin will modify
history expansion behavior (see the description of the shopt
builtin below).and If the histverify shell option is enabled, and
readline is being used, history substitutions are not immediately
passed to the shell parser. Instead, the expanded line is
reloaded into the readline editing buffer for further
modification. If readline is being used, and the histreedit shell
option is enabled, a failed history substitution is reloaded into
the readline editing buffer for correction.
The -p option to the history builtin command shows what a history
expansion will do before using it. The -s option to the history
builtin will add commands to the end of the history list without
actually executing them, so that they are available for subsequent
recall.
The shell allows control of the various characters used by the
history expansion mechanism (see the description of histchars
above under Shell Variables). The shell uses the history comment
character to mark history timestamps when writing the history
file.
Event Designators
An event designator is a reference to an entry in the history
list. The event designator consists of the portion of the word
beginning with the history expansion character and ending with the
word designator if present, or the end of the word. 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 history list entry n.
!-n Refer to the current entry minus n.
!! Refer to the previous entry. 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, this uses the string
from the most recent search; 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.
They are optional; if the word designator isn't supplied, the
history expansion uses the entire 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: word 1.
$ The last word. This is usually the last argument, but will
expand to the zeroth word if there is only one word in the
line.
% The first word matched by the most recent “?string?”
search, if the search string begins with a character that
is part of a word. By default, searches begin at the end
of each line and proceed to the beginning, so the first
word matched is the one closest to the end of the line.
x-y A range of words; “-y” abbreviates “0-y”.
* All of the words but the zeroth. This is a synonym for
“1-$”. It is not an error to use * if there is just one
word in the event; it expands to the empty string in that
case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word. If x is
missing, it defaults to 0.
If a word designator is supplied without an event specification,
the previous command is used as the event, equivalent to !!.
Modifiers
After the optional word designator, the expansion may include a
sequence of one or more of the following modifiers, each preceded
by a “:”. These modify, or edit, the word or words selected from
the history event.
h Remove a trailing pathname component, leaving only the
head.
t Remove all leading pathname 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
exclusive; expansion uses the last one supplied.
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. A single backslash quotes the
delimiter in old and new. If & appears in new, it is
replaced with old. A single backslash quotes the &. If
old is null, it is set to the last old substituted, or, if
no previous history substitutions took place, the last
string in a !?string[?] search. 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.
Unless otherwise noted, each builtin command documented in this
section as accepting options preceded by - accepts -- to signify
the end of the options. The :, true, false, and test/[ builtins
do not accept options and do not treat -- specially. The exit,
logout, return, break, continue, let, and shift builtins accept
and process arguments beginning with - without requiring --.
Other builtins that accept arguments but are not specified as
accepting options interpret arguments beginning with - as invalid
options and require -- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding
arguments and performing any specified redirections. The
return status is zero.
. [-p path] filename [arguments]
source [-p path] filename [arguments]
The . command (source) reads and execute commands from
filename in the current shell environment and returns the
exit status of the last command executed from filename.
If filename does not contain a slash, . searches for it.
If the -p option is supplied, . treats path as a colon-
separated list of directories in which to find filename;
otherwise, . uses the entries in PATH to find the directory
containing filename. filename does not need to be
executable. When bash is not in posix mode, it searches
the current directory if filename is not found in PATH, but
does not search the current directory if -p is supplied.
If the sourcepath option to the shopt builtin command is
turned off, . does not search PATH.
If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the
positional parameters are unchanged.
If the -T option 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 persists after . completes.
The return status is the status of the last command
executed from filename (0 if no commands are executed), and
non-zero if filename is not found or cannot be read.
alias [-p] [name[=value] ...]
With no arguments or with the -p option, alias prints the
list of aliases in the form alias name=value on standard
output. When arguments are supplied, define an alias 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 during command parsing. For
each name in the argument list for which no value is
supplied, print the name and value of the alias name.
alias returns true unless a name is given (without a
corresponding =value) 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 uses its notion of the current job. bg jobspec
returns 0 unless run when job control is disabled or, when
run with job control enabled, any specified jobspec was not
found or was started without job control.
bind [-m keymap] [-lsvSVX]
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] -p|-P [readline-command]
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 to a shell
command, or set a readline variable. Each non-option
argument is a key binding or 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'. In the following
descriptions, output available to be re-read is formatted
as commands that would appear in a readline initialization
file or that would be supplied as individual arguments to a
bind command. Options, if supplied, have the following
meanings:
-m keymap
Use keymap as the keymap to be affected by the
subsequent bindings. Acceptable keymap names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-move, vi-command, and vi-insert. vi is
equivalent to vi-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 used as an argument to a
subsequent bind command or in a readline
initialization file. If arguments remain after
option processing, bind treats them as readline
command names and restricts output to those names.
-P List current readline function names and bindings.
If arguments remain after option processing, bind
treats them as readline command names and restricts
output to those names.
-s Display readline key sequences bound to macros and
the strings they output in such a way that they can
be used as an argument to a subsequent bind command
or in a readline initialization file.
-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 used as an argument to a
subsequent bind command or in a readline
initialization file.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Display key sequences that invoke the named readline
function.
-u function
Unbind all key sequences bound to the named readline
function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq[: ]shell-command
Cause shell-command to be executed whenever keyseq
is entered. The separator between keyseq and
shell-command is either whitespace or a colon
optionally followed by whitespace. If the separator
is whitespace, shell-command must be enclosed in
double quotes and readline expands any of its
special backslash-escapes in shell-command before
saving it. If the separator is a colon, any
enclosing double quotes are optional, and readline
does not expand the command string before saving it.
Since the entire key binding expression must be a
single argument, it should be enclosed in single
quotes. 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 variables 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 associated commands in a format that can be
reused as an argument to a subsequent bind command.
The return value is 0 unless an unrecognized option is
supplied or an error occurred.
break [n]
Exit from within a for, while, until, or select loop. If n
is specified, break exits n enclosing loops. 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 shell-builtin, passing
it arguments, and return its exit status. This is useful
when defining a function whose name is the same as a shell
builtin, retaining the functionality of the builtin within
the function. The cd builtin is commonly redefined this
way. The return status is false if shell-builtin is not a
shell builtin command.
caller [expr]
Returns the context of any active subroutine call (a shell
function or a script executed with the . or source
builtins).
Without expr, caller displays the line number and source
filename of the current subroutine call. If a non-negative
integer is supplied as expr, caller displays the line
number, subroutine name, and source file corresponding to
that position in the current execution call stack. This
extra information may be used, for example, to print a
stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a
subroutine call or expr does not correspond to a valid
position in the call stack.
cd [-L] [-@] [dir]
cd -P [-e] [-@] [dir]
Change the current directory to dir. if dir is not
supplied, the value of the HOME shell variable is used as
dir. The variable CDPATH exists, and dir does not begin
with a slash (/), cd uses it as a search path: the shell
searches each directory name in CDPATH 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., “.”.
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 cd to follow symbolic links by
resolving the link after processing instances of .. in dir.
If .. appears in dir, cd processes it by removing the
immediately previous pathname component from dir, back to a
slash or the beginning of dir, and verifying that the
portion of dir it has processed to that point is still a
valid directory name after removing the pathname component.
If it is not a valid directory name, cd returns a non-zero
status. If neither -L nor -P is supplied, cd behaves as if
-L had been supplied.
If the -e option is supplied with -P, and cd cannot
successfully determine the current working directory after
a successful directory change, it returns a non-zero
status.
On systems that support it, the -@ option presents the
extended attributes associated with a file as a directory.
An argument of - is converted to $OLDPWD before attempting
the directory change.
If cd uses a non-empty directory name from CDPATH, or if -
is the first argument, and the directory change is
successful, cd writes the absolute pathname of the new
working directory 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 environment variable to the value of the
current working directory before the change.
The return value is true if the directory was successfully
changed; false otherwise.
command [-pVv] command [arg ...]
The command builtin runs command with args suppressing the
normal shell function lookup for command. Only builtin
commands or commands found in the PATH named command are
executed. If the -p option is supplied, the search for
command is performed using a default value for PATH that is
guaranteed to find all of the standard utilities.
If either the -V or -v option is supplied, command prints a
description of command. The -v option displays a single
word indicating the command or filename used to invoke
command; the -V option produces a more verbose description.
If the -V or -v option is supplied, the exit status is zero
if command was found, and non-zero if not. If neither
option is supplied and an error occurred or command cannot
be found, the exit status is 127. Otherwise, the exit
status of the command builtin is the exit status of
command.
compgen [-V varname] [option] [word]
Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete builtin with the exceptions of -p, -r, -D, -E, and
-I, and write the matches to the standard output.
If the -V option is supplied, compgen stores the generated
completions into the indexed array variable varname instead
of writing them to the standard output.
When using the -F or -C options, the various shell
variables set by the programmable completion facilities,
while available, will not have useful values.
The matches will be generated in the same way as if the
programmable completion code had generated them directly
from a completion specification with the same flags. If
word is specified, only those completions matching word
will be displayed or stored.
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 globpat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...]
Specify how arguments to each name should be completed.
If the -p option is supplied, or if no options or names are
supplied, print existing completion specifications in a way
that allows them to be reused as input. The -r option
removes a completion specification for each name, or, if no
names are supplied, all completion specifications.
The -D option indicates that other supplied options and
actions should apply to the “default” command completion;
that is, completion attempted on a command for which no
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 apply to the case specified by the option.
The process of applying these completion specifications
when attempting word completion 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 expansion before the complete builtin is
invoked.
-o comp-option
The comp-option controls several aspects of the
compspec's behavior beyond the simple generation of
completions. comp-option may be one of:
bashdefault
Perform the rest of the default bash
completions if the compspec generates no
matches.
default Use readline's default filename completion
if the compspec generates no matches.
dirnames
Perform directory name completion if the
compspec generates no matches.
filenames
Tell readline that the compspec generates
filenames, so it can perform any
filename-specific processing (such as
adding a slash to directory names, quoting
special characters, or suppressing trailing
spaces). This is intended to be used with
shell functions.
fullquote
Tell readline to quote all the completed
words even if they are not filenames.
noquote Tell readline not to quote the completed
words if they are filenames (quoting
filenames is the default).
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 generating any matches defined by the
compspec, attempt directory name completion
and add any matches 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 and directory names, similar to
readline's filename completion. May also
be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help
builtin.
hostname
Hostnames, as taken from the file specified
by the HOSTFILE shell variable.
job Job names, if job control is active. May
also be specified as -j.
keyword Shell reserved words. May also be
specified as -k.
running Names of running jobs, if job control is
active.
service Service names. May also be specified as
-s.
setopt Valid arguments for the -o option to the
set builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is
active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be
specified as -v.
-C command
command is executed in a subshell environment, and
its output is used as the possible completions.
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 arguments are being completed,
the second argument ($2) is the word being
completed, and the third argument ($3) is the word
preceding the word being completed on the current
command line. When function finishes, programmable
completion retrieves the possible completions from
the value of the COMPREPLY array variable.
-G globpat
Expand the pathname expansion pattern globpat to
generate the possible completions.
-P prefix
Add prefix to the beginning of each possible
completion after all other options have been
applied.
-S suffix
Append suffix to each possible completion after all
other options have been applied.
-W wordlist
Split the wordlist using the characters in the IFS
special variable as delimiters, and expand each
resulting word. 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 a
prefix of the word being completed.
-X filterpat
filterpat is a pattern as used for pathname
expansion. It is applied to the list of possible
completions generated by the preceding options and
arguments, and each completion matching filterpat
is removed from the list. A leading ! in filterpat
negates the pattern; in this case, any completion
not matching filterpat is removed.
The return value is true unless an invalid option is
supplied, an option other than -p, -r, -D, -E, or -I is
supplied without a name argument, an attempt is made to
remove a completion specification for a name for which no
specification exists, or an error occurs adding a
completion specification.
compopt [-o option] [-DEI] [+o option] [name]
Modify completion options for each name according to the
options, or for the currently-executing completion if no
names are supplied. If no options are supplied, 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; the -E option
indicates that other supplied options should apply to
“empty” command completion; and the -I option indicates
that other supplied options should apply to completion on
the initial word on the line. These are determined in the
same way as the complete builtin.
If multiple options are supplied, the -D option takes
precedence over -E, and both take precedence over -I.
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]
continue resumes the next iteration of the enclosing for,
while, until, or select loop. If n is specified, bash
resumes the nth enclosing loop. n must be ≥ 1. If n is
greater than the number of enclosing loops, the shell
resumes the last enclosing loop (the “top-level” loop).
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 or
functions. 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, declare will
display the attributes and values of all variables having
the attributes specified by the additional options. If no
other options are supplied with -p, declare will display
the attributes and values of all shell variables. The -f
option restricts the display to shell functions.
The -F option inhibits the display of function definitions;
only the function name and attributes are printed. If the
extdebug shell option is enabled using shopt, the source
file name and line number where 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 when declare is not executed in a
shell function.
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 surrounding
scope. If there is no existing variable, the local
variable is initially unset.
The following options can be used to restrict output to
variables with the specified attribute or to give variables
attributes:
-a Each name is an indexed array variable (see Arrays
above).
-A Each name is an associative array variable (see
Arrays above).
-f Each name refers to a shell function.
-i The variable is treated as an integer; arithmetic
evaluation (see ARITHMETIC EVALUATION above) is
performed when the variable is assigned a value.
-l When the variable is assigned a value, all upper-
case characters are converted to lower-case. The
upper-case attribute is disabled.
-n Give each name the nameref attribute, making it a
name reference to another variable. That other
variable is defined by the value of name. All
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 attribute
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 inherit the DEBUG and RETURN traps from
the calling shell. The trace attribute has no
special meaning for variables.
-u When the variable is assigned a value, all lower-
case characters are converted to upper-case. The
lower-case attribute is disabled.
-x Mark each name for export to subsequent commands via
the environment.
Using “+” instead of “-” turns off the specified attribute
instead, with the exceptions that +a and +A may not be used
to destroy array variables and +r will not remove the
readonly attribute.
When used in a function, declare and typeset make each name
local, as with the local command, unless the -g option is
supplied. If a variable name is followed by =value, the
value of the variable is set to value. When using -a or -A
and the compound assignment syntax to create array
variables, additional attributes do not take effect until
subsequent assignments.
The return value is 0 unless an invalid option is
encountered, an attempt is made to define a function using
“-f foo=bar”, an attempt is made to assign a value to a
readonly variable, an attempt is made to assign a value to
an array variable without using the compound assignment
syntax (see Arrays above), one of the names is not a valid
shell variable name, an attempt is made to turn off
readonly status for a readonly variable, an attempt is made
to turn off array status for an array variable, or an
attempt is made to display a non-existent function with -f.
dirs [-clpv] [+n] [-n]
Without options, display the list of currently remembered
directories. The default display is on a single line with
directory names separated by spaces. Directories are added
to the list with the pushd command; the popd command
removes entries from the list. The current directory is
always the first directory in the stack.
Options, if supplied, have the following meanings:
-c Clears the directory stack by deleting all of the
entries.
-l Produces a listing using full pathnames; the default
listing format uses a tilde to denote the home
directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
+n Displays the nth entry counting from the left of the
list shown by dirs when invoked without options,
starting with zero.
-n Displays the nth entry counting from the right of
the list shown by dirs when invoked without options,
starting with zero.
The return value is 0 unless an invalid option is supplied
or n indexes beyond the end of the directory stack.
disown [-ar] [-h] [id ...]
Without options, remove each id from the table of active
jobs. Each id may be a job specification jobspec or a
process ID pid; if id is a pid, disown uses the job
containing pid as jobspec.
If the -h option is supplied, disown does not remove the
jobs corresponding to each id from the jobs table, but
rather marks them so the shell does not send SIGHUP to the
job if the shell receives a SIGHUP.
If no id is supplied, the -a option means to remove or mark
all jobs; the -r option without an id argument removes or
marks running jobs. If no id is supplied, and neither the
-a nor the -r option is supplied, disown removes or marks
the current job.
The return value is 0 unless an id 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 specified, the trailing newline is not
printed.
If the -e option is given, echo interprets the following
backslash-escaped characters. The -E option disables
interpretation of these escape characters, even on systems
where they are interpreted by default. The xpg_echo shell
option determines whether or not echo interprets any
options and expands these escape characters. 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).
echo writes any unrecognized backslash-escaped characters
unchanged.
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a
builtin allows an executable file 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 files.
If -n is supplied, each name is disabled; otherwise, names
are enabled. For example, to use the test binary found
using PATH instead of the shell builtin version, run
“enable -n test”.
If no name arguments are supplied, or if the -p option is
supplied, print a list of shell builtins. With no other
option arguments, the list consists of all enabled shell
builtins. If -n is supplied, print only disabled builtins.
If -a is supplied, the list printed includes all builtins,
with an indication of whether or not each is enabled. The
-s option means to restrict the output to the POSIX special
builtins.
The -f option means to load the new builtin command name
from shared object filename, on systems that support
dynamic loading. If filename does not contain a slash,
Bash will use the value of the BASH_LOADABLES_PATH variable
as a colon-separated list of directories in which to search
for filename. The default for BASH_LOADABLES_PATH is
system-dependent, and may include “.” to force a search of
the current directory. The -d option will delete a builtin
previously loaded with -f. If -s is used with -f, the new
builtin becomes a POSIX special builtin.
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 ...]
Concatenate the args together into a single command,
separating them with spaces. Bash then reads and execute
this command, and returns its exit status as the return
status 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 without
creating a new process. command cannot be a shell builtin
or function. The arguments become the arguments to
command. If the -l option is supplied, the shell places a
dash at the beginning of the zeroth argument passed to
command. This is what login(1) does. The -c option causes
command to be executed with an empty environment. If -a is
supplied, the shell passes name as the zeroth argument to
the executed command.
If command cannot be executed for some reason, a non-
interactive shell exits, unless the execfail shell option
is enabled. In that case, it returns a non-zero status.
An interactive shell returns a non-zero status 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. Any trap on EXIT is executed before the shell
terminates.
export [-fn] [name[=value]] ...
export -p [-f]
The supplied names are marked for automatic export to the
environment of subsequently executed commands. If the -f
option is given, the names refer to functions.
The -n option unexports, or removes the export attribute,
from each name. If no names are given, or if only the -p
option is supplied, export displays a list of names of all
exported variables on the standard output. Using -p and -f
together displays exported functions. The -p option
displays output in a form that may be reused as input.
export allows the value of a variable to be set when it is
exported or unexported by following the variable name with
=value. This sets the value of the variable to value while
modifying the export attribute. export returns an exit
status of 0 unless an invalid option is encountered, one of
the names is not a valid shell variable name, or -f is
supplied with a name that is not a function.
false Does nothing; returns a non-zero status.
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 command (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
specified, it is set to the previous command for editing
and -16 for listing.
If the -l option is supplied, the commands are listed on
the standard output. The -n option suppresses the command
numbers when listing. The -r option reverses the order of
the commands.
Otherwise, fc invokes the editor named by ename on a file
containing those commands. If ename is not supplied, fc
uses the value of the FCEDIT variable, and the value of
EDITOR if FCEDIT is not set. If neither variable is set,
fc uses vi. When editing is complete, fc reads the file
containing the edited commands and echoes and executes
them.
In the second form, fc re-executes command after replacing
each instance of pat with rep. Command is interpreted the
same as first above.
A useful alias to use with fc 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 zero unless
an invalid option is encountered or first or last specify
history lines out of range. When editing and re-executing
a file of commands, the return value is the value of the
last command executed or failure if an error occurs with
the temporary file. If the second form is used, the return
status is that of the re-executed command, unless cmd does
not specify a valid history entry, in which case fc returns
a non-zero status.
fg [jobspec]
Resume jobspec in the foreground, and make it the current
job. If jobspec is not present, fg uses the shell's notion
of the current job. 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 scripts and functions to parse
positional parameters and obtain options and their
arguments. optstring contains the option characters to be
recognized; if a character is followed by a colon, the
option is expected to have an argument, which should be
separated from it by white space. The colon and question
mark characters may not be used as option characters.
Each time it is invoked, getopts places the next option in
the shell variable name, initializing name if it does not
exist, and the index of the next argument to be processed
into the variable OPTIND. OPTIND is initialized to 1 each
time the shell or a shell script is invoked. When an
option requires an argument, getopts places that argument
into the variable OPTARG.
The shell does not reset OPTIND automatically; it must be
manually reset between multiple calls to getopts within the
same shell invocation to use a new set of parameters.
When it reaches the end of options, getopts exits with a
return value greater than zero. OPTIND is set to the index
of the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if
more arguments are supplied as arg values, getopts parses
those instead.
getopts can report errors in two ways. If the first
character of optstring is a colon, getopts uses silent
error reporting. In normal operation, getopts prints
diagnostic messages when it encounters invalid options or
missing option arguments. If the variable OPTERR is set to
0, getopts does not display any error messages, even if the
first character of optstring is not a colon.
If getopts detects an invalid option, it places ? into name
and, if not silent, prints an error message and unsets
OPTARG. If getopts is silent, it assigns the option
character found to OPTARG and does not print a diagnostic
message.
If a required argument is not found, and getopts is not
silent, it sets the value of name to a question mark (?),
unsets OPTARG, and prints a diagnostic message. If getopts
is silent, it sets the value of name to a colon (:) and
sets OPTARG 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, it remembers the full pathname
of the command name as determined by searching the
directories in $PATH. Any previously-remembered pathname
associated with name is discarded. If the -p option is
supplied, hash uses filename as the full pathname of the
command.
The -r option causes the shell to forget all remembered
locations. Assigning to the PATH variable also clears all
hashed filenames. The -d option causes the shell to forget
the remembered location of each name.
If the -t option is supplied, hash prints the full pathname
corresponding to each name. If multiple name arguments are
supplied with -t, hash prints the name before the
corresponding hashed full pathname. The -l option displays
output in a format that may be reused as input.
If no arguments are given, or if only -l is supplied, hash
prints information about remembered commands. The -t, -d,
and -p options (the options that act on the name arguments)
are mutually exclusive. Only one will be active. If more
than one is supplied, -t has higher priority than -p, and
both have higher priority than -d.
The return status is zero 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 as described below; otherwise it
displays a list of all the builtins and shell compound
commands.
Options, if supplied, have the follow meanings:
-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
If pattern contains pattern matching characters (see
Pattern Matching above) it's treated as a shell pattern and
help prints the description of each help topic matching
pattern.
If not, and pattern exactly matches the name of a help
topic, help prints the description associated with that
topic. Otherwise, help performs prefix matching and prints
the descriptions of all matching help topics.
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
numbers. Entries prefixed with a * have been modified. An
argument of n lists only the last n entries. If the shell
variable HISTTIMEFORMAT is set and not null, it is used as
a format string for strftime(3) to display the time stamp
associated with each displayed history entry. If history
uses HISTTIMEFORMAT, it does not print an intervening space
between the formatted time stamp and the history entry.
If filename is supplied, history uses it as the name of the
history file; if not, it uses the value of HISTFILE. If
filename is not supplied and HISTFILE is unset or null, the
-a, -n, -r, and -w options have no effect.
Options, if supplied, have the following meanings:
-c Clear the history list by deleting all the entries.
This can be used with the other options to replace
the history list.
-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 history file.
-n Read the history lines not already read from the
history file and add them to the current history
list. These are lines appended to the history file
since the beginning of the current bash session.
-r Read the history file and append its contents to the
current history list.
-w Write the current history list to the history file,
overwriting the history file.
-p Perform history substitution on the following args
and display the result on the standard output,
without storing 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 adding the args.
If the HISTTIMEFORMAT variable is set, history writes the
time stamp information associated with each history entry
to the history file, marked with the history comment
character as described above. 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 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 supplied as an
argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the
following meanings:
-l List process IDs in addition to the normal
information.
-n Display information only about jobs that have
changed status since the user was last notified of
their 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 supplied, jobs restricts output to
information about that job. The return status is 0 unless
an invalid option is encountered or an invalid jobspec is
supplied.
If the -x option is supplied, jobs replaces any jobspec
found in command or args with the corresponding process
group ID, and executes command, passing it args, returning
its exit status.
kill [-s sigspec | -n signum | -sigspec] id [ ... ]
kill -l|-L [sigspec | exit_status]
Send the signal specified by sigspec or signum to the
processes named by each id. Each id may be a job
specification jobspec or a process ID pid. 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 supplied, then kill sends
SIGTERM.
The -l option lists the signal names. If any arguments are
supplied when -l is given, kill lists the names of the
signals corresponding to the arguments, 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; if it is supplied, kill
prints the name of the signal that caused the process to
terminate. kill assumes that process exit statuses are
greater than 128; anything less than that is a signal
number. 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 evaluated as an arithmetic expression (see
ARITHMETIC EVALUATION above). If the last arg evaluates to
0, let returns 1; otherwise let returns 0.
local [option] [name[=value] ... | - ]
For each argument, create a local variable named name and
assign it 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. It is an
error to use local when not within a function.
If name is -, it makes the set of shell options local to
the function in which local is invoked: any shell options
changed using the set builtin inside the function after the
call to local are restored to their original values when
the function returns. The restore is performed 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.
The return status is 0 unless local is used outside a
function, an invalid name is supplied, or name is a
readonly variable.
logout [n]
Exit a login shell, returning a status of n to the shell's
parent.
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, or from file descriptor
fd if the -u option is supplied, into the indexed array
variable array. The variable MAPFILE is the default array.
Options, if supplied, have the following meanings:
-d Use the first character of delim to terminate each
input line, rather than newline. If delim is the
empty string, mapfile will terminate a line when it
reads a NUL character.
-n Copy at most count lines. If count is 0, copy all
lines.
-O Begin assigning to array at index origin. The
default index is 0.
-s Discard the first count lines read.
-t Remove a trailing delim (default newline) from each
line read.
-u Read lines from file descriptor fd instead of the
standard input.
-C Evaluate callback each time quantum lines are read.
The -c option specifies quantum.
-c Specify the number of lines read between each call
to callback.
If -C is specified without -c, the default quantum is 5000.
When callback is evaluated, it is supplied the index of the
next array element to be assigned and the line to be
assigned to that element as additional arguments. callback
is evaluated after the line is read but before the array
element is assigned.
If not supplied with an explicit origin, mapfile will clear
array before assigning to it.
mapfile returns zero unless an invalid option or option
argument is supplied, array is invalid or unassignable, or
if array is not an indexed array.
popd [-n] [+n] [-n]
Remove entries from the directory stack. The elements are
numbered from 0 starting at the first directory listed by
dirs, so popd is equivalent to “popd +0.” 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 Suppress the normal change of directory when
removing directories from the stack, only manipulate
the stack.
+n Remove 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 Remove 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 returns a non-zero value.
Otherwise, popd returns false if an invalid option is
supplied, the directory stack is empty, or n specifies a
non-existent directory stack entry.
If the popd command is successful, bash runs dirs to show
the final 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 assigns the
output to the variable var rather than printing it to the
standard output.
The format is a character string which contains three types
of objects: plain characters, which are simply copied to
standard output, character escape sequences, which are
converted and copied to the standard output, and format
specifications, each of which causes printing of the next
successive argument. In addition to the standard printf(3)
format characters cCsSndiouxXeEfFgGaA, printf interprets
the following additional format specifiers:
%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
and %Q use the $'' quoting style if any characters
in the argument string require it, and backslash
quoting otherwise. If the format string uses the
printf alternate form, these two formats quote the
argument string using single quotes.
%Q like %q, but applies any supplied precision to the
argument 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. This format specifier recognizes two special
argument values: -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 supplied. This is an exception to the
usual printf behavior.
The %b, %q, and %T format specifiers all use the field
width and precision 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.
The %n format specifier accepts a corresponding argument
that is treated as a shell variable name.
The %s and %c format specifiers accept an l (long)
modifier, which forces them to convert the argument string
to a wide-character string and apply any supplied field
width and precision in terms of characters, not bytes. The
%S and %C format specifiers are equivalent to %ls and %lc,
respectively.
Arguments to non-string format specifiers are treated as C
constants, except that a leading plus or minus sign is
allowed, and if the leading character is a single or double
quote, the value is the numeric value of the following
character, using the current locale.
The format is reused as necessary to consume all of the
arguments. If the format requires more arguments than are
supplied, the extra format specifications behave as if a
zero value or null string, as appropriate, had been
supplied. The return value is zero on success, non-zero if
an invalid option is supplied or a write or assignment
error occurs.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
Add a directory to the top of the directory stack, or
rotate the stack, making the new top of the stack the
current working directory. With no arguments, pushd
exchanges the top two elements of the directory stack.
Arguments, if supplied, have the following meanings:
-n Suppress the normal change of directory when
rotating or adding directories to the stack, only
manipulate the stack.
+n Rotate 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
supplied, 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 zero
unless the directory stack is empty. When rotating the
directory stack, pushd returns zero unless the directory
stack is empty or n specifies a non-existent directory
stack element.
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 invalid option is
supplied.
read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars] [-N
nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
Read one line from the standard input, or from the file
descriptor fd supplied as an argument to the -u option,
split it into words as described above under Word
Splitting, and assign the first word to the first name, the
second word to the second name, and so on. If there are
more words than names, the remaining words and their
intervening delimiters are assigned to the last name. If
there are fewer words read from the input stream than
names, the remaining names are assigned empty values. The
characters in the value of the IFS variable 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 (\) removes any special meaning for
the next character read and is used for line continuation.
Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the
array variable aname, starting at 0. aname is unset
before any new values are assigned. Other name
arguments are ignored.
-d delim
The first character of delim terminates the input
line, rather than newline. If delim is the empty
string, read will terminate a line when it reads a
NUL character.
-e If the standard input is coming from a terminal,
read uses readline (see READLINE above) to obtain
the line. Readline uses the current (or default, if
line editing was not previously active) editing
settings, but uses readline's default filename
completion.
-E If the standard input is coming from a terminal,
read uses readline (see READLINE above) to obtain
the line. Readline uses the current (or default, if
line editing was not previously active) editing
settings, but uses bash's default completion,
including programmable completion.
-i text
If readline is being used to read the line, read
places text into the editing buffer before editing
begins.
-n nchars
read returns after reading nchars characters rather
than waiting for a complete line of input, unless it
encounters EOF or read times out, but honors a
delimiter if it reads fewer than nchars characters
before the delimiter.
-N nchars
read returns after reading exactly nchars characters
rather than waiting for a complete line of input,
unless it encounters EOF or read times out. Any
delimiter characters in the input are not treated
specially and do not cause read to return until it
has read nchars characters. The result is not split
on the characters in IFS; the intent is that the
variable is assigned exactly the characters read
(with the exception of backslash; see the -r option
below).
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input, but
only if input is coming from a terminal.
-r Backslash does not act as an escape character. The
backslash is considered to be part of the line. In
particular, a backslash-newline pair may not then be
used as a line continuation.
-s Silent mode. If input is coming from a terminal,
characters are not echoed.
-t timeout
Cause read to time out and return failure if it does
not read a complete line of input (or a specified
number of characters) 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, it saves any partial input read into the
specified variable name, and the exit status is
greater than 128. If timeout is 0, read returns
immediately, without trying to read any data. In
this case, the exit status is 0 if input is
available on the specified file descriptor, or the
read will return EOF, non-zero otherwise.
-u fd Read input from file descriptor fd instead of the
standard input.
Other than the case where delim is the empty string, read
ignores any NUL characters in the input.
If no names are supplied, read assigns the line read,
without the ending delimiter but otherwise unmodified, to
the variable REPLY.
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 variable) occurs, or an invalid file descriptor is
supplied as the argument to -u.
readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these
names may not be changed by subsequent assignment or unset.
If the -f option is supplied, each name refers to a shell
function. 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 supplied, or if the
-p option is supplied, print a list of all readonly names.
The other options may be used to restrict the output to a
subset of the set of readonly names. The -p option
displays output in a format that may be reused as input.
readonly allows the value of a variable to be set at the
same time the readonly attribute is changed by following
the variable name with =value. This sets the value of the
variable is to value while modifying the readonly
attribute.
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]
Stop executing a shell function or sourced file and return
the value specified by n to its caller. If n is omitted,
the return status is that of the last command executed. If
return is executed by a trap handler, the last command used
to determine the status is the last command executed before
the trap handler. If return is executed during a DEBUG
trap, the last command used to determine the status is the
last command executed by the trap handler before return was
invoked.
When return is used to terminate execution of a script
being executed 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.
Any command associated with the RETURN trap is executed
before execution resumes after the function or script.
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.
set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]
set -o
set +o Without options, display the name and value of each shell
variable in a format that can be reused as input for
setting or resetting the currently-set variables. Read-
only variables cannot be reset. In posix mode, only shell
variables are listed. The output is sorted according to
the current locale. When options are specified, they set
or unset shell attributes. Any arguments remaining after
option processing are treated as values for the positional
parameters and are assigned, in order, to $1, $2, ..., $n.
Options, if specified, have the following meanings:
-a Each variable or function that is created or
modified is given the export attribute and marked
for export to the environment of subsequent
commands.
-b Report the status of terminated background jobs
immediately, rather than before the next primary
prompt or after a foreground command terminates.
This is effective only when job control is enabled.
-e Exit immediately if a pipeline (which may consist
of a single simple command), a list, or a compound
command (see SHELL GRAMMAR above), exits with a
non-zero status. The shell does not exit if the
command that fails is part of the command list
immediately following a while or until reserved
word, 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 (subject to the state of the pipefail shell
option), or if the command's return value is being
inverted with !. If a compound command other than
a subshell returns a non-zero status because a
command failed while -e was being ignored, the
shell does not exit. A trap on ERR, if set, is
executed before the shell exits. This option
applies to the shell environment and each subshell
environment separately (see COMMAND EXECUTION
ENVIRONMENT above), and may cause subshells to exit
before executing all the commands in the subshell.
If a compound command or shell function executes in
a context where -e is being ignored, none of the
commands executed within the compound command or
function body will be affected by the -e setting,
even if -e is set and a command returns a failure
status. If a compound command or shell function
sets -e while executing in a context where -e is
ignored, that setting will not have any effect
until the compound command or the command
containing 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 separate process group. When a
background job completes, the shell prints a line
containing its exit status.
-n Read commands but do not execute them. This may be
used to check a shell script for syntax errors.
This is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing
interface. This is enabled by default when
the shell is interactive, unless the shell
is started with the --noediting option.
This also affects the editing interface
used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above
under HISTORY. This option is on by
default in interactive shells.
ignoreeof
The effect is as if the shell command
“IGNOREEOF=10” had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is
the value of the last (rightmost) command
to exit with a non-zero status, or zero if
all commands in the pipeline exit
successfully. This option is disabled by
default.
posix Enable posix mode; change the behavior of
bash where the default operation differs
from the POSIX standard to match the
standard. 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, set prints
the current shell option settings. If +o is
supplied with no option-name, set prints a series
of set commands to recreate the current option
settings on the standard output.
-p Turn on privileged mode. In this mode, the shell
does not read the $ENV and $BASH_ENV files, shell
functions are not inherited from the environment,
and the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
variables, if they appear in the environment, are
ignored. If the shell is started with the
effective user (group) id not equal to the real
user (group) id, and the -p option is not supplied,
these actions are taken and the 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 unset once it has been set.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the
special parameters “@” and “*”, or array variables
subscripted with “@” or “*”, as an error when
performing parameter expansion. If expansion is
attempted on an unset variable or parameter, the
shell prints an error message, and, if not
interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command,
case command, select command, or arithmetic for
command, display the expanded value of PS4,
followed by the command and its expanded arguments
or associated word list, to the standard error.
-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.
Using the redirection operator >| instead of > will
override this and force the creation of an output
file.
-E If set, any trap on ERR is inherited by shell
functions, command substitutions, and commands
executed in a subshell environment. The ERR trap
is normally not inherited in such cases.
-H Enable ! style history substitution. This option
is on by default when the shell is interactive.
-P If set, the shell does not resolve symbolic links
when executing commands such as cd that change the
current working directory. It uses the physical
directory structure instead. By default, bash
follows the logical chain of directories when
performing commands which change the current
directory.
-T If set, any traps on DEBUG and RETURN are inherited
by shell functions, command substitutions, and
commands executed in a subshell environment. The
DEBUG and RETURN traps are normally not inherited
in such cases.
-- If no arguments follow this option, unset the
positional parameters. Otherwise, set the
positional parameters to the args, even if some of
them begin with a -.
- Signal the end of options, and assign all remaining
args to the positional parameters. The -x and -v
options are turned off. If there are no args, the
positional parameters remain unchanged.
The options are off by default unless otherwise noted.
Using + rather than - causes these options to be turned
off. The options can also be specified as arguments to an
invocation of the shell. The current set of options may be
found in $-. The return status is always zero unless an
invalid option is encountered.
shift [n]
Rename positional parameters from n+1 ... to $1 ....
Parameters represented by the numbers $# down to $#-n+1 are
unset. n must be a non-negative number less than or equal
to $#. If n is 0, no parameters are changed. If n is not
given, it is assumed to be 1. If n is greater than $#, the
positional parameters are not changed. The return status
is greater than zero if n is greater than $# or less than
zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of settings controlling optional shell
behavior. The settings can be either those listed below,
or, if the -o option is used, those available with the -o
option to the set builtin command.
With no options, or with the -p option, display a list of
all settable options, with an indication of whether or not
each is set; if any optnames are supplied, the output is
restricted to those options. The -p option displays output
in a form that may be reused as input.
Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return
status indicates whether the optname is set or
unset. If multiple optname arguments are supplied
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 default.
The return status when listing options is zero if all
optnames are enabled, non-zero otherwise. When setting or
unsetting options, the return status is zero unless an
optname is not a valid shell option.
The list of shopt options is:
array_expand_once
If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts during
arithmetic expression evaluation, while executing
builtins that can perform variable assignments, and
while executing builtins that perform array
dereferencing.
assoc_expand_once
Deprecated; a synonym for array_expand_once.
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.
bash_source_fullpath
If set, filenames added to the BASH_SOURCE array
variable are converted to full pathnames (see Shell
Variables above).
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, the cd command attempts to correct minor
errors in the spelling of a directory component.
Minor errors include transposed characters, a
missing character, and one extra character. If cd
corrects the directory name, it prints the
corrected filename, and the command proceeds. This
option is only used by interactive shells.
checkhash
If set, bash checks that a command found in the
hash table exists before trying to execute it. If
a hashed command no longer exists, bash performs a
normal path search.
checkjobs
If set, bash lists the status of any stopped and
running jobs before exiting an interactive shell.
If any jobs are running, bash defers the exit until
a second exit is attempted without an intervening
command (see JOB CONTROL above). The shell always
postpones exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each
external (non-builtin) command and, if necessary,
updates the values of LINES and COLUMNS, using the
file descriptor associated with the standard error
if it is a terminal. This option is enabled by
default.
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 enabled by default, but only has an
effect if command history is enabled, as described
above under HISTORY.
compat31
compat32
compat40
compat41
compat42
compat43
compat44
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 completed
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 initially supplied does not exist.
dotglob If set, bash includes filenames beginning with a
“.” in the results of pathname expansion. The
filenames . and .. must always be matched
explicitly, even if dotglob is set.
execfail
If set, a non-interactive shell will not exit if it
cannot execute the file specified as an argument to
the exec builtin. An interactive shell does not
exit if exec fails.
expand_aliases
If set, aliases are expanded as described above
under ALIASES. This option is enabled by default
for interactive shells.
extdebug
If set at shell invocation, 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 enabled:
1. The -F option to the declare builtin
displays the source file name and line
number corresponding to each function name
supplied as an argument.
2. If the command run by the DEBUG trap returns
a non-zero value, the next command is
skipped and not executed.
3. If the command run by the DEBUG trap returns
a value of 2, and the shell is executing in
a subroutine (a shell function or a shell
script executed by the . or source
builtins), the shell simulates a call to
return.
4. BASH_ARGC and BASH_ARGV are updated as
described in their descriptions above).
5. Function tracing is enabled: command
substitution, shell functions, and subshells
invoked with ( command ) inherit the DEBUG
and RETURN traps.
6. Error tracing is enabled: command
substitution, shell functions, and subshells
invoked with ( command ) inherit the ERR
trap.
extglob If set, enable the extended pattern matching
features described above under Pathname Expansion.
extquote
If set, $'string' and $"string" quoting is
performed within ${parameter} expansions enclosed
in double quotes. This option is enabled by
default.
failglob
If set, patterns which fail to match filenames
during pathname expansion result in an expansion
error.
force_fignore
If set, the suffixes specified by the FIGNORE shell
variable cause words to be ignored when performing
word completion even if the ignored words are the
only possible completions. See Shell Variables
above for a description of FIGNORE. This option is
enabled by default.
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 comparisons. That is, pattern matching
does not take the current locale's collating
sequence 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
context 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, the user is
given the opportunity to re-edit a failed history
substitution.
histverify
If set, and readline is being used, the results of
history substitution are not immediately passed to
the shell parser. Instead, the resulting line is
loaded into the readline editing buffer, allowing
further modification.
hostcomplete
If set, and readline is being used, bash will
attempt to perform hostname completion when a word
containing a @ is being completed (see Completing
under READLINE above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an
interactive login shell exits.
inherit_errexit
If set, command substitution inherits the value of
the errexit option, instead of unsetting it in the
subshell environment. This option is enabled when
posix mode is enabled.
interactive_comments
In an interactive shell, a word beginning with #
causes that word and all remaining characters on
that line to be ignored, as in a non-interactive
shell (see COMMENTS above). This option is enabled
by default.
lastpipe
If set, and job control is not active, the shell
runs the last command of a pipeline not executed in
the background in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-
line commands are saved to the history with
embedded newlines rather than using semicolon
separators where possible.
localvar_inherit
If set, local variables inherit the value and
attributes of a variable of the same name that
exists at a previous scope before any new value is
assigned. The nameref attribute is not inherited.
localvar_unset
If set, calling unset on local variables in
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 current function
scope.
login_shell
The shell sets this option if it is started as a
login shell (see INVOCATION above). The value may
not be changed.
mailwarn
If set, and a file that bash is checking for mail
has been accessed since the last time it was
checked, bash displays the message “The mail in
mailfile has been read”.
no_empty_cmd_completion
If set, and readline is being used, bash does not
search PATH for possible completions when
completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a
case-insensitive fashion when performing pathname
expansion (see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing
case or [[ conditional commands, when performing
pattern substitution word expansions, or when
filtering possible completions 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, pathname expansion patterns which match no
files (see Pathname Expansion above) expand to
nothing and are removed, rather than expanding to
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, enable the programmable completion
facilities (see Programmable Completion above).
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,
command substitution, arithmetic expansion, and
quote removal after being expanded as described in
PROMPTING above. This option is enabled by
default.
restricted_shell
The shell sets this option if it is started in
restricted mode (see RESTRICTED SHELL below). The
value may not be changed. This is not reset when
the startup files are executed, allowing the
startup files to discover whether or not a shell is
restricted.
shift_verbose
If set, the shift builtin prints an error message
when the shift count exceeds the number of
positional parameters.
sourcepath
If set, the . (source) builtin uses the value of
PATH to find the directory containing the file
supplied as an argument when the -p option is not
supplied. 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
sequences by default. If the posix shell option is
also enabled, echo does not interpret any options.
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 will
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
evaluation of the conditional expression expr. Each
operator and operand must be a separate argument.
Expressions are composed of the primaries described above
under CONDITIONAL 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. test uses
operator precedence 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 normal operator precedence.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of
rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument
is not null.
2 arguments
If the first argument is !, the expression is true
if and only if the second argument is null. If the
first argument is one of the unary conditional
operators listed above under CONDITIONAL
EXPRESSIONS, the expression is true if the unary
test is true. If the first argument is not a valid
unary conditional operator, the expression is false.
3 arguments
The following conditions are applied in the order
listed. If the second argument is one of the binary
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. Otherwise, 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. 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. Otherwise, the
expression is parsed and evaluated according to
precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to
precedence using the rules listed above.
When the shell is in posix mode, or if the expression is
part of the [[ command, the < and > operators sort using
the current locale. If the shell is not in posix mode, the
test and [ commands sort lexicographically using ASCII
ordering.
The historical operator-precedence parsing with 4 or more
arguments can lead to ambiguities when it encounters
strings that look like primaries. The POSIX standard has
deprecated the -a and -o primaries and enclosing
expressions within parentheses. Scripts should no longer
use them. It's much more reliable to restrict test
invocations to a single primary, and to replace uses of -a
and -o with the shell's && and || list operators.
times Print the accumulated user and system times for the shell
and for processes run from the shell. The return status is
0.
trap [-lpP] [[action] sigspec ...]
The action is a command that is read and executed when the
shell receives any of the signals sigspec. If action is
absent (and there is a single sigspec) or -, each specified
sigspec is reset to the value it had when the shell was
started. If action is the null string the signal specified
by each sigspec is ignored by the shell and by the commands
it invokes.
If no arguments are supplied, trap displays the actions
associated with each trapped signal as a set of trap
commands that can be reused as shell input to restore the
current signal dispositions. If -p is given, and action is
not present, then trap displays the actions associated with
each sigspec or, if none are supplied, for all trapped
signals, as a set of trap commands that can be reused as
shell input to restore the current signal dispositions.
The -P option behaves similarly, but displays only the
actions associated with each sigspec argument. -P requires
at least one sigspec argument. The -P or -p options may be
used in a subshell environment (e.g., command substitution)
and, as long as they are used before trap is used to change
a signal's handling, will display the state of its parent's
traps.
The -l option prints 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 -l is supplied with no sigspec arguments, it prints a
list of valid signal names.
If a sigspec is EXIT (0), action is executed on exit from
the shell. If a sigspec is DEBUG, action is executed
before every simple command, for command, case command,
select command, (( arithmetic command, [[ conditional
command, arithmetic for command, and before the first
command executes in a shell function (see SHELL GRAMMAR
above). Refer to the description of the extdebug shell
option (see shopt above) for details of its effect on the
DEBUG trap. If a sigspec is RETURN, action is executed
each time a shell function or a script executed with the .
or source builtins finishes executing.
If a sigspec is ERR, action 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 reserved word, 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 (subject
to the state of the pipefail shell option), or if the
command's return value is being inverted using !. These
are the same conditions obeyed by the errexit (-e) option.
When the shell is not interactive, signals ignored upon
entry to the shell cannot be trapped or reset. Interactive
shells permit trapping signals ignored on entry. Trapped
signals that are not being ignored are reset to their
original values in a subshell or subshell environment when
one is created. The return status is false if any sigspec
is invalid; otherwise trap returns true.
true Does nothing, returns a 0 status.
type [-aftpP] name [name ...]
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
executable file, respectively. If the name is not found,
type prints nothing and returns a non-zero exit status.
If the -p option is used, type either returns the pathname
of the executable file that would be found by searching
$PATH for name or nothing if “type -t name” would not
return file. The -P option forces a PATH search for each
name, even if “type -t name” would not return file. If
name is present in the table of hashed commands, -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 a command named name. This includes aliases,
reserved words, functions, and builtins, but the path
search options (-p and -P) can be supplied to restrict the
output to executable files. type does not consult the
table of hashed commands when using -a with -p, and only
performs a PATH search for name.
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 it starts, on systems that allow such
control.
The -H and -S options specify whether 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, ulimit sets both the
soft and hard limits.
The value of limit can be a number in the unit specified
for the resource or one of the special values hard, soft,
or unlimited, which stand for the current hard limit, the
current soft limit, and no limit, respectively. If limit
is omitted, ulimit prints the current value of the soft
limit of the resource, unless the -H option is given. When
more than one resource is specified, the limit name and
unit, if appropriate, are printed before the value. Other
options are interpreted as follows:
-a Report all current limits; 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
blocking, in microseconds.
-T The maximum number of threads.
If limit is supplied, and the -a option is not used, limit
is the new value of the specified resource. If no option
is supplied, 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 invalid option or argument is supplied, or an
error occurs while setting a new limit.
umask [-p] [-S] [mode]
Set the user file-creation mask 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, umask
prints the current value of the mask. The -S option
without a mode argument prints the mask in a symbolic
format; 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
zero if the mode was successfully changed or if no mode
argument was supplied, and non-zero otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a
is supplied, remove all alias definitions. 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. 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.
Read-only variables and functions may not be unset. When
variables or functions are removed, they are also removed
from the environment passed to subsequent commands. 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. Some shell variables may not be
unset. 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 id and return the
termination status of the last id. Each id may be a
process ID pid or a job specification jobspec; if a jobspec
is supplied, wait waits for all processes in the job.
If no options or ids are supplied, wait waits for all
running background 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 any one of the
given ids or, if no ids are supplied, any job or process
substitution, to complete and returns its exit status. If
none of the supplied ids is a child of the shell, or if no
ids are supplied and the shell has no unwaited-for
children, the exit status is 127.
If the -p option is supplied, wait assigns the process or
job identifier of the job for which the exit status is
returned to the variable varname named by the option
argument. The variable, which cannot be readonly, will be
unset initially, before any assignment. This is useful
only when used with the -n option.
Supplying the -f option, when job control is enabled,
forces wait to wait for each id to terminate before
returning its status, instead of returning when it changes
status.
If none of the ids specify one of the shell's active child
processes, the return status is 127. If wait is
interrupted by a signal, any varname will remain unset, and
the return status will be greater than 128, as described
under SIGNALS above. Otherwise, the return status is the
exit status of the last id.
Bash-4.0 introduced the concept of a shell compatibility level,
specified as a set of options to the shopt builtin (compat31,
compat32, compat40, 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
right hand side 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 comparisons 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
carefully. 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
assigned to this variable (a decimal version number like 4.2, or
an integer corresponding to the compatNN option, like 42)
determines the compatibility level.
Starting with bash-4.4, bash began deprecating older compatibility
levels. Eventually, the options will be removed in favor of
BASH_COMPAT.
Bash-5.0 was the final version for which there was an individual
shopt option for the previous version. BASH_COMPAT is the only
mechanism to control the compatibility level in versions newer
than bash-5.0.
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 mechanisms. 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 required for bash-5.1 and later
versions.
compat31
• Quoting the rhs of the [[ command's regexp matching
operator (=~) has no special effect.
compat32
• The < and > operators to the [[ command do not
consider the current locale when comparing strings;
they use ASCII ordering.
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 collation 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 interpretation 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
substitution does not undergo quote removal, as it
does in versions 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 interpretation 221); in later
versions, single quotes are not special within
double-quoted word expansions.
compat43
• 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/until/etc.) is not reset, so break or
continue in that function will break or continue
loops in the calling context. 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 parameters 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 number 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
subscripts @ and * differently depending on whether
the array is indexed or associative, and differently
than in previous versions.
• Arithmetic commands ( ((...)) ) and the expressions
in an arithmetic for statement can be expanded more
than once.
• Expressions used as arguments to arithmetic
operators in the [[ conditional command can be
expanded more than once.
• The expressions in substring parameter brace
expansion can be expanded more than once.
• The expressions in the $((...)) word expansion can
be expanded more than once.
• Arithmetic expressions used as indexed array
subscripts can be expanded more than once.
• test -v, when given an argument of A[@], where A is
an existing associative array, will return true if
the array has any set elements. Bash-5.2 will look
for and report on a key named @.
• The ${parameter[:]=value} word expansion will return
value, before any variable-specific transformations
have been performed (e.g., converting to lowercase).
Bash-5.2 will return the final value assigned to the
variable.
• Parsing command substitutions will behave as if
extended globbing (see the description of the shopt
builtin above) is enabled, so that parsing a command
substitution containing an extglob pattern (say, as
part of a shell function) will not fail. This
assumes the intent is to enable extglob before the
command is executed and word expansions are
performed. It will fail at word expansion time if
extglob hasn't been enabled by the time the command
is executed.
compat52
• The test builtin uses its historical algorithm to
parse parenthesized subexpressions when given five
or more arguments.
• If the -p or -P option is supplied to the bind
builtin, bind treats any arguments remaining after
option processing as bindable command names, and
displays any key sequences bound to those commands,
instead of treating the arguments as key sequences
to bind.
If bash is started with the name rbash, or the -r option is
supplied at invocation, the shell becomes restricted. A
restricted shell is used to set up an environment more controlled
than the standard shell. It behaves identically to bash with the
exception that the following are disallowed or not performed:
• Changing directories with cd.
• Setting or unsetting the values of SHELL, PATH, HISTFILE,
ENV, or BASH_ENV.
• Specifying command names containing /.
• Specifying a filename containing a / as an argument to the
. builtin command.
• Using the -p option to the . builtin command to specify a
search path.
• 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 values of BASHOPTS and SHELLOPTS from the shell
environment at startup.
• Redirecting output using the >, >|, <>, >&, &>, and >>
redirection operators.
• Using the exec builtin command to replace the shell with
another command.
• Adding or deleting builtin commands with the -f and -d
options to the enable builtin command.
• Using the enable builtin command to enable disabled shell
builtins.
• Specifying the -p option to the command builtin command.
• Turning off restricted mode with set +r or shopt -u
restricted_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
COMMAND EXECUTION above), rbash turns off any restrictions in the
shell spawned to execute the script.
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/9799919799/
http://tiswww.case.edu/~chet/bash/POSIX — a description of posix
mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login
shells
~/.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
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet.ramey@case.edu
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.savannah.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! You may send suggestions and
“philosophical” bug reports to bug-bash@gnu.org or post them 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 behavior
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.
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 lists of the form “a ; b ; c” are
not handled gracefully when combined with process suspension.
When a process is stopped, the shell immediately executes the next
command in the list or breaks out of any existing loops. It
suffices to enclose the command in parentheses to force it into a
subshell, which may be stopped as a unit, or to start the command
in the background and immediately bring it into the foreground.
Array variables may not (yet) be exported.
This page is part of the bash (Bourne again shell) project.
Information about the project can be found at
⟨http://www.gnu.org/software/bash/⟩. If you have a bug report for
this manual page, see ⟨http://www.gnu.org/software/bash/⟩. This
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GNU Bash 5.3 2025 April 7 BASH(1)
Pages that refer to this page: getopt(1), intro(1), kill(1), memusage(1), pmdabash(1), pv(1), quilt(1), systemctl(1), systemd-notify(1), systemd-run(1), time(1), setpgid(2), history(3), readline(3), strcmp(3), termios(3), ulimit(3), core(5), credentials(7), environ(7), suffixes(7), time_namespaces(7), cupsenable(8), dpkg-fsys-usrunmess(8), wg(8), wg-quick(8)
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