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