man bash

BASH(1) General Commands Manual BASH(1)

NAME
bash - GNU Bourne-Again SHell

SYNOPSIS
bash [options] [command_string | file]

DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read from the
standard input or from a file. Bash also incorporates useful features from the Korn and C
shells (ksh and csh).

Bash is intended to be a conformant implementation of the Shell and Utilities portion of
the IEEE POSIX specification (IEEE Standard 1003.1). Bash can be configured to be POSIX-
conformant by default.

OPTIONS
All of the single-character shell options documented in the description of the set
builtin command can be used as options when the shell is invoked. In addition, bash
interprets the following options when it is invoked:

-c If the -c option is present, then commands are read from the first non-option
argument command_string. If there are arguments after the command_string, 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 posi‐
tional 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 val‐
ues of the shell options accepted by shopt are printed on the standard output.
If the invocation option is +O, the output is displayed in a format that may be
reused as input.
-- A -- signals the end of options and disables further option processing. Any
arguments after the -- are treated as filenames and arguments. An argument of -
is equivalent to --.

Bash also interprets a number of multi-character options. These options must appear on
the command line before the single-character options to be recognized.

--debugger
Arrange for the debugger profile to be executed before the shell starts. Turns on
extended debugging mode (see the description of the extdebug option to the shopt
builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object) file
format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the system wide initialization file
/etc/bash.bashrc and the standard personal initialization file ~/.bashrc if the
shell is interactive (see INVOCATION below).

--login
Equivalent to -l.

--noediting
Do not use the GNU readline library to read command lines when the shell is inter‐
active.

--noprofile
Do not read either the system-wide startup file /etc/profile or any of the personal
initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default,
bash reads these files when it is invoked as a login shell (see INVOCATION below).

--norc Do not read and execute the system wide initialization file /etc/bash.bashrc and
the personal initialization file ~/.bashrc if the shell is interactive. This
option is on by default if the shell is invoked as sh.

--posix
Change the behavior of bash where the default operation differs from the POSIX
standard to match the standard (posix mode). See SEE ALSO below for a reference to
a document that details how posix mode affects bash's behavior.

--restricted
The shell becomes restricted (see RESTRICTED SHELL below).

--verbose
Equivalent to -v.

--version
Show version information for this instance of bash on the 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 com‐
mands. 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 com‐
mands from this file, then exits. Bash's exit status is the exit status of the last com‐
mand 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 filenames
as described below under Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-interactive shell with the
--login option, it first reads and executes commands from the file /etc/profile, if that
file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and
~/.profile, in that order, and reads and executes commands from the first one that exists
and is readable. The --noprofile option may be used when the shell is started to inhibit
this behavior.

When a login shell exits, bash reads and executes commands from the file ~/.bash_logout,
if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes
commands from /etc/bash.bashrc and ~/.bashrc, if these files exist. This may be inhibited
by using the --norc option. The --rcfile file option will force bash to read and execute
commands from file instead of /etc/bash.bashrc and ~/.bashrc.

When bash is started non-interactively, to run a shell script, for example, it looks for
the variable BASH_ENV in the environment, expands its value if it appears there, and uses
the expanded value as the name of a file to read and execute. Bash behaves as if the fol‐
lowing command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the filename.

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical
versions of sh as closely as possible, while conforming to the POSIX standard as well.
When invoked as an interactive login shell, or a non-interactive shell with the --login
option, it first attempts to read and execute commands from /etc/profile and ~/.profile,
in that order. The --noprofile option may be used to inhibit this behavior. When invoked
as an interactive shell with the name sh, bash looks for the variable ENV, expands its
value if it is defined, and uses the expanded value as the name of a file to read and exe‐
cute. Since a shell invoked as sh does not attempt to read and execute commands from any
other startup files, the --rcfile option has no effect. A non-interactive shell invoked
with the name sh does not attempt to read any other startup files. When invoked as sh,
bash enters posix mode after the startup files are read.

When bash is started in posix mode, as with the --posix command line option, it follows
the POSIX standard for startup files. In this mode, interactive shells expand the ENV
variable and commands are read and executed from the file whose name is the expanded
value. No other startup files are read.

Bash attempts to determine when it is being run with its standard input connected to a
network connection, as when executed by the remote shell daemon, usually rshd, or the
secure shell daemon sshd. If bash determines it is being run in this fashion, it reads
and executes commands from ~/.bashrc and ~/.bashrc, if these files exist and are readable.
It will not do this if invoked as sh. The --norc option may be used to inhibit this
behavior, and the --rcfile option may be used to force another file to be read, but nei‐
ther rshd nor sshd generally invoke the shell with those options or allow them to be spec‐
ified.

If the shell is started with the effective user (group) id not equal to the real user
(group) id, and the -p option is not supplied, no startup files are read, shell functions
are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
variables, if they appear in the environment, are ignored, and the effective user id is
set to the real user id. If the -p option is supplied at invocation, the startup behavior
is the same, but the effective user id is not reset.

DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known as a
token.
name A word consisting only of alphanumeric characters and underscores, and beginning
with an alphabetic character or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the following symbols:
|| & && ; ;; ( ) | |& <newline>

RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following words
are recognized as reserved when unquoted and either the first word of a simple command
(see SHELL GRAMMAR below) or the third word of a case or for command:

! case coproc do done elif else esac fi for function if in select then until while { }
time [[ ]]

SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-sepa‐
rated words and redirections, and terminated by a control operator. The first word speci‐
fies 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 termi‐
nated by signal n.

Pipelines
A pipeline is a sequence of one or more commands separated by one of the control operators
| or |&. The format for a pipeline is:

[time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]

The standard output of command is connected via a pipe to the standard input of command2.
This connection is performed before any redirections specified by the command (see REDI‐
RECTION below). If |& is used, command's standard error, in addition to its standard out‐
put, is connected to command2's standard input through the pipe; it is shorthand for 2>&1
|. This implicit redirection of the standard error to the standard output is performed
after any redirections specified by the command.

The return status of a pipeline is the exit status of the last command, unless the
pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-zero status, or zero if all com‐
mands exit successfully. If the reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical negation of the exit status as described above. The shell
waits for all commands in the pipeline to terminate before returning a value.

If the time reserved word precedes a pipeline, the elapsed as well as user and system time
consumed by its execution are reported when the pipeline terminates. The -p option
changes the output format to that specified by POSIX. When the shell is in posix mode, it
does not recognize time as a reserved word if the next token begins with a `-'. The TIME‐
FORMAT 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 optionally terminated by one of ;, &, or <newline>.

Of these list operators, && and || have equal precedence, followed by ; and &, which have
equal precedence.

A sequence of one or more newlines may appear in a list instead of a semicolon to delimit
commands.

If a command is terminated by the control operator &, the shell executes the command in
the background in a subshell. The shell does not wait for the command to finish, and the
return status is 0. 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, respectively. AND and OR lists are executed with left associativity. An AND
list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status. The return
status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands
A compound command is one of the following. In most cases a list in a command's descrip‐
tion may be separated from the rest of the command by one or more newlines, and may be
followed by a newline in place of a semicolon.

(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT
below). Variable assignments and builtin commands that affect the shell's environ‐
ment do not remain in effect after the command completes. The return status is the
exit status of list.

{ list; }
list is simply executed in the current shell environment. list must be terminated
with a newline or semicolon. This is known as a group command. The return status
is the exit status of list. Note that unlike the metacharacters ( and ), { and }
are reserved words and must occur where a reserved word is permitted to be recog‐
nized. Since they do not cause a word break, they must be separated from list by
whitespace or another shell metacharacter.

((expression))
The expression is evaluated according to the rules described below under ARITHMETIC
EVALUATION. If the value of the expression is non-zero, the return status is 0;
otherwise the return status is 1. This is exactly equivalent to let "expression".

[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression
expression. Expressions are composed of the primaries described below under CONDI‐
TIONAL EXPRESSIONS. Word splitting and pathname expansion are not performed on the
words between the [[ and ]]; tilde expansion, parameter and variable expansion,
arithmetic expansion, command substitution, process substitution, and quote removal
are performed. Conditional operators such as -f must be unquoted to be recognized
as primaries.

When used with [[, the < and > operators sort lexicographically using the current
locale.

See the description of the test builtin command (section SHELL BUILTIN COMMANDS below) for
the handling of parameters (i.e. missing parameters).

When the == and != operators are used, the string to the right of the operator is consid‐
ered a pattern and matched according to the rules described below under Pattern Matching,
as if the extglob shell option were enabled. The = operator is equivalent to ==. If the
shell option nocasematch is enabled, the match is performed without regard to the case of
alphabetic characters. The return value is 0 if the string matches (==) or does not match
(!=) the pattern, and 1 otherwise. Any part of the pattern may be quoted to force the
quoted portion to be matched as a string.

An additional binary operator, =~, is available, with the same precedence as == and !=.
When it is used, the string to the right of the operator is considered an extended regular
expression and matched accordingly (as in regex(3)). The return value is 0 if the string
matches the pattern, and 1 otherwise. If the regular expression is syntactically incor‐
rect, 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 the quoted portion to be matched as a string.
Bracket expressions in regular expressions must be treated carefully, since normal quoting
characters lose their meanings between brackets. If the pattern is stored in a shell
variable, quoting the variable expansion forces the entire pattern to be matched as a
string. Substrings matched by parenthesized subexpressions within the regular expression
are saved in the array variable BASH_REMATCH. The element of BASH_REMATCH with index 0 is
the portion of the string matching the entire regular expression. The element of
BASH_REMATCH with index n is the portion of the string matching the nth parenthesized sub‐
expression.

Expressions may be combined using the following operators, listed in decreasing order of
precedence:

( expression )
Returns the value of expression. This may be used to override the normal
precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is
sufficient to determine the return value of the entire conditional expression.

for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of items. The vari‐
able 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 posi‐
tional parameter that is set (see PARAMETERS below). The return status is the exit
status of the last command that executes. If the expansion of the items following
in results in an empty list, no commands are executed, and the return status is 0.

for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules
described below under ARITHMETIC EVALUATION. The arithmetic expression expr2 is
then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to
a non-zero value, list is executed and the arithmetic expression expr3 is evalu‐
ated. If any expression is omitted, it behaves as if it evaluates to 1. The
return value is the exit status of the last command in list that is executed, or
false if any of the expressions is invalid.

select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The set of
expanded words is printed on the standard error, each preceded by a number. If the
in word is omitted, the positional parameters are printed (see PARAMETERS below).
The PS3 prompt is then displayed and a line read from the standard input. If the
line consists of a number corresponding to one of the displayed words, then the
value of name is set to that word. If the line is empty, the words and prompt are
displayed again. If EOF is read, the command completes. Any other value read
causes name to be set to null. The line read is saved in the variable REPLY. The
list is executed after each selection until a break command is executed. The exit
status of select is the exit status of the last command executed in list, or zero
if no commands were executed.

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pattern in
turn, using the same matching rules as for pathname expansion (see Pathname Expan‐
sion below). The word is expanded using tilde expansion, parameter and variable
expansion, arithmetic substitution, command substitution, process substitution and
quote removal. Each pattern examined is expanded using tilde expansion, parameter
and variable expansion, arithmetic substitution, command substitution, and process
substitution. If the shell option nocasematch is enabled, the match is performed
without regard to the case of alphabetic characters. When a match is found, the
corresponding list is executed. If the ;; operator is used, no subsequent matches
are attempted after the first pattern match. Using ;& in place of ;; causes execu‐
tion 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 com‐
mand executed, or zero if no condition tested true.

while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long as the last command
in the list list-1 returns an exit status of zero. The until command is identical
to the while command, except that the test is negated; list-2 is executed as long
as the last command in list-1 returns a non-zero exit status. The exit status of
the while and until commands is the exit status of the last command executed in
list-2, or zero if none was executed.

Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. A coprocess is exe‐
cuted asynchronously in a subshell, as if the command had been terminated with the & con‐
trol operator, with a two-way pipe established between the executing shell and the copro‐
cess.

The format for a coprocess is:

coproc [NAME] command [redirections]

This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC.
NAME must not be supplied if command is a simple command (see above); otherwise, it is
interpreted as the first word of the simple command. When the coprocess is executed, the
shell creates an array variable (see Arrays below) named NAME in the context of the exe‐
cuting 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 redirec‐
tions specified by the command (see REDIRECTION below). The file descriptors can be uti‐
lized as arguments to shell commands and redirections using standard word expansions. The
file descriptors are not available in subshells. The process ID of the shell spawned to
execute the coprocess is available as the value of the variable NAME_PID. The wait
builtin command may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command, the coproc command always
returns success. The return status of a coprocess is the exit status of command.

Shell Function Definitions
A shell function is an object that is called like a simple command and executes a compound
command with a new set of positional parameters. Shell functions are declared as follows:

name () compound-command [redirection]
function name [()] compound-command [redirection]
This defines a function named name. The reserved word function is optional. If
the function reserved word is supplied, the parentheses are optional. The body of
the function is the compound command compound-command (see Compound Commands
above). That command is usually a list of commands between { and }, but may be any
command listed under Compound Commands above. compound-command is executed when‐
ever name is specified as the name of a simple command. When in posix mode, name
may not be the name of one of the POSIX special builtins. Any redirections (see
REDIRECTION below) specified when a function is defined are performed when the
function is executed. The exit status of a function definition is zero unless a
syntax error occurs or a readonly function with the same name already exists. When
executed, the exit status of a function is the exit status of the last command exe‐
cuted in the body. (See FUNCTIONS below.)

COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments
option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word begin‐
ning with # causes that word and all remaining characters on that line to be ignored. An
interactive shell without the interactive_comments option enabled does not allow comments.
The interactive_comments option is on by default in interactive shells.

QUOTING
Quoting is used to remove the special meaning of certain characters or words to the shell.
Quoting can be used to disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent parameter expansion.

Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell
and must be quoted if it is to represent itself.

When the command history expansion facilities are being used (see HISTORY EXPANSION
below), the history expansion 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 con‐
tinuation (that is, it is removed from the input stream and effectively ignored).

Enclosing characters in single quotes preserves the literal value of each character within
the quotes. A single quote may not occur between single quotes, even when preceded by a
backslash.

Enclosing characters in double quotes preserves the literal value of all characters within
the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The
characters $ and ` retain their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of the following characters: $, `,
", \, or <newline>. A double quote may be quoted within double quotes by preceding it
with a backslash. If enabled, history expansion will be performed unless an ! appearing
in double quotes is escaped using a backslash. The backslash preceding the ! is not
removed.

The special parameters * and @ have special meaning when in double quotes (see PARAMETERS
below).

Words of the form $'string' are treated specially. The word expands to string, with back‐
slash-escaped characters replaced as specified by the ANSI C standard. Backslash escape
sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value nnn (one to three
digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two
hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH (one to eight hex digits)
\cx a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.

A double-quoted string preceded by a dollar sign ($"string") will cause the string to be
translated according to the current locale. If the current locale is C or POSIX, the dol‐
lar sign is ignored. If the string is translated and replaced, the replacement is double-
quoted.

PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of the
special characters listed below under Special Parameters. A variable is a parameter
denoted by a name. A variable has a value and zero or more attributes. Attributes are
assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS).

A parameter is set if it has been assigned a value. The null string is a valid value.
Once a variable is set, it may be unset only by using the unset builtin command (see SHELL
BUILTIN COMMANDS below).

A variable may be assigned to by a statement of the form

name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde
expansion, parameter and 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 $((...)) expansion is not
used (see Arithmetic Expansion below). Word splitting is not performed, with the excep‐
tion of "$@" as explained below under Special Parameters. Pathname expansion is not per‐
formed. Assignment statements may also appear as arguments to the alias, declare, type‐
set, export, readonly, and local builtin commands. When in posix mode, these builtins may
appear in a command after one or more instances of the command builtin and retain these
assignment statement properties.

In the context where an assignment statement is assigning a value to a shell variable or
array index, the += operator can be used to append to or add to the variable's previous
value. 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 assign‐
ment (see Arrays below), the variable's value is not unset (as it is when using =), and
new values are appended to the array beginning at one greater than the array's maximum
index (for indexed arrays) or added as additional key-value pairs in an associative array.
When applied to a string-valued variable, value is expanded and appended to the variable's
value.

A variable can be assigned the nameref attribute using the -n option to the declare or
local builtin commands (see the descriptions of declare and local below) to create a
nameref, or a reference to another variable. This allows variables to be manipulated
indirectly. Whenever the nameref variable is referenced or assigned to, the operation is
actually performed on the variable specified by the nameref variable's value. A nameref
is commonly used within shell functions to refer to a variable whose name is passed as an
argument to the function. For instance, if a variable name is passed to a shell function
as its first argument, running
declare -n ref=$1
inside the function creates a nameref variable ref whose value is the variable name passed
as the first argument. References and assignments to ref are treated as references and
assignments to the variable whose name was passed as $1. If the control variable in a for
loop has the nameref attribute, the list of words can be a list of shell variables, and a
name reference will be established for each word in the list, in turn, when the loop is
executed. Array variables cannot be given the -n attribute. However, nameref variables
can reference array variables and subscripted array variables. Namerefs can be unset
using the -n option to the unset builtin. Otherwise, if unset is executed with the name
of a nameref variable as an argument, the variable referenced by the nameref variable will
be unset.

Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the single
digit 0. Positional parameters are assigned from the shell's arguments when it is
invoked, and may be reassigned using the set builtin command. Positional parameters may
not be assigned to with assignment statements. The positional parameters are temporarily
replaced when a shell function is executed (see FUNCTIONS below).

When a positional parameter consisting of more than a single digit is expanded, it must be
enclosed in braces (see EXPANSION below).

Special Parameters
The shell treats several parameters specially. These parameters may only be referenced;
assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion is not
within double quotes, each positional parameter expands to a separate word. In
contexts where it is performed, those words are subject to further word splitting
and pathname expansion. When the expansion occurs within double quotes, it expands
to a single word with the value of each parameter separated by the first character
of the IFS special variable. That is, "$*" is equivalent to "$1c$2c...", where c
is the first character of the value of the IFS variable. If IFS is unset, the
parameters are separated by spaces. If IFS is null, the parameters are joined
without intervening separators.
@ Expands to the positional parameters, starting from one. When the expansion occurs
within double quotes, each parameter expands to a separate word. That is, "$@" is
equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the beginning part of the orig‐
inal word, and the expansion of the last parameter is joined with the last part of
the original word. When there are no positional parameters, "$@" 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.
! Expands to the process ID of the job most recently placed into the background,
whether executed as an asynchronous command or using the bg builtin (see JOB CON‐
TROL below).
0 Expands to the name of the shell or shell script. This is set at shell initializa‐
tion. If bash is invoked with a file of commands, $0 is set to the name of that
file. If bash is started with the -c option, then $0 is set to the first argument
after the string to be executed, if one is present. Otherwise, it is set to the
filename used to invoke bash, as given by argument zero.
_ At shell startup, set to the absolute pathname used to invoke the shell or shell
script being executed as passed in the environment or argument list. Subsequently,
expands to the last argument to the previous command, after expansion. Also set to
the full pathname used to invoke each command executed and placed in the environ‐
ment exported to that command. When checking mail, this parameter holds the name
of the mail file currently being checked.

Shell Variables
The following variables are set by the shell:

BASH Expands to the full filename used to invoke this instance of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid
argument for the -s option to the shopt builtin command (see SHELL BUILTIN COMMANDS
below). The options appearing in BASHOPTS are those reported as on by shopt. If
this variable is in the environment when bash starts up, each shell option in the
list will be enabled before reading any startup files. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This differs from $$ under
certain circumstances, such as subshells that do not require bash to be re-initial‐
ized.
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 subrou‐
tine (shell function or script executed with . or source) is at the top of the
stack. When a subroutine is executed, the number of parameters passed is pushed
onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see
the description of the extdebug option to the shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in the current bash execution
call stack. The final parameter of the last subroutine call is at the top of the
stack; the first parameter of the initial call is at the bottom. When a subroutine
is executed, the parameters supplied are pushed onto BASH_ARGV. The shell sets
BASH_ARGV only when in extended debugging mode (see the description of the extdebug
option to the shopt builtin below)
BASH_CMDS
An associative array variable whose members correspond to the internal hash table
of commands as maintained by the hash builtin. Elements added to this array appear
in the hash table; unsetting array elements cause commands to be removed from the
hash table.
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell is
executing a command as the result of a trap, in which case it is the command exe‐
cuting 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 num‐
ber 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 match‐
ing the entire regular expression. The element with index n is the portion of the
string matching the nth parenthesized subexpression. This variable is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where the corresponding
shell function names in the FUNCNAME array variable are defined. The shell func‐
tion ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment when the shell
begins executing in that environment. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information for this instance
of bash. The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor position.
This variable is available only in shell functions invoked by the programmable com‐
pletion 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 Program‐
mable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning of the current
command. If the current cursor position is at the end of the current command, the
value of this variable is equal to ${#COMP_LINE}. This variable is available only
in shell functions and external commands invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion attempted that
caused a completion function to be called: TAB, for normal completion, ?, for list‐
ing completions after successive tabs, !, for listing alternatives on partial word
completion, @, to list completions if the word is not unmodified, or %, for menu
completion. This variable is available only in shell functions and external com‐
mands invoked by the programmable completion facilities (see Programmable Comple‐
tion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word separators when per‐
forming word completion. If COMP_WORDBREAKS is unset, it loses its special proper‐
ties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the cur‐
rent 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 func‐
tions invoked by the programmable completion facilities (see Programmable Comple‐
tion below).
COPROC An array variable (see Arrays below) created to hold the file descriptors for out‐
put from and input to an unnamed coprocess (see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the direc‐
tory stack. Directories appear in the stack in the order they are displayed by the
dirs builtin. Assigning to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd builtins must be used to
add and remove directories. Assignment to this variable will not change the cur‐
rent 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 exe‐
cution call stack. The element with index 0 is the name of any currently-executing
shell function. The bottom-most element (the one with the highest index) is
"main". This variable exists only when a shell function is executing. Assignments
to FUNCNAME have no effect 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 FUNC‐
NAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call
stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]}
at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call
stack using this information.
GROUPS An array variable containing the list of groups of which the current user is a mem‐
ber. 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.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on which
bash is executing. The default is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number rep‐
resenting the current sequential line number (starting with 1) within a script or
function. When not in a script or function, the value substituted is not guaran‐
teed to be meaningful. If LINENO is unset, it loses its special properties, even
if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which bash is
executing, in the standard GNU cpu-company-system format. The default is system-
dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text read by the mapfile
builtin when no variable name is supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin command (see
SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts builtin command (see
SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system on which bash is
executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values from
the processes in the most-recently-executed foreground pipeline (which may contain
only a single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, 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 sub‐
sequently 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 invoca‐
tion is returned. If a value is assigned to SECONDS, the value returned upon sub‐
sequent 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 sub‐
sequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a valid
argument for the -o option to the set builtin command (see SHELL BUILTIN COMMANDS
below). The options appearing in SHELLOPTS are those reported as on by set -o. If
this variable is in the environment when bash starts up, each shell option in the
list will be enabled before reading any startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell startup. This
variable is readonly.

The following variables are used by the shell. In some cases, bash assigns a default
value to a variable; these cases are noted below.

BASH_COMPAT
The value is used to set the shell's compatibility level. See the description of
the shopt builtin below under SHELL BUILTIN COMMANDS for a description of the vari‐
ous compatibility levels and their effects. The value may be a decimal number
(e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility
level. If BASH_COMPAT is unset or set to the empty string, the compatibility level
is set to the default for the current version. If BASH_COMPAT is set to a value
that is not one of the valid compatibility levels, the shell prints an error mes‐
sage and sets the compatibility level to the default for the current version. The
valid compatibility levels correspond to the compatibility options accepted by the
shopt builtin described below (for example, compat42 means that 4.2 and 42 are
valid values). The current version is also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is inter‐
preted as a filename containing commands to initialize the shell, as in ~/.bashrc.
The value of BASH_ENV is subjected to parameter expansion, command substitution,
and arithmetic expansion before being interpreted as a filename. PATH is not used
to search for the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor, bash will write the
trace output generated when set -x is enabled to that file descriptor. The file
descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Unset‐
ting 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".
CHILD_MAX
Set the number of exited child status values for the shell to remember. Bash will
not allow this value to be decreased below a POSIX-mandated minimum, and there is a
maximum value (currently 8192) that this may not exceed. The minimum value is sys‐
tem-dependent.
COLUMNS
Used by the select compound command to determine the terminal width when printing
selection lists. Automatically set if the checkwinsize option is enabled or in an
interactive shell upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated by a
shell function invoked by the programmable completion facility (see Programmable
Completion below). Each array element contains one possible completion.
EMACS If bash finds this variable in the environment when the shell starts with value
"t", it assumes that the shell is running in an Emacs shell buffer and disables
line editing.
ENV Similar to BASH_ENV; used when the shell is invoked in POSIX mode.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename completion
(see READLINE below). A filename whose suffix matches one of the entries in FIG‐
NORE is excluded from the list of matched filenames. A sample value is ".o:~"
(Quoting is needed when assigning a value to this variable, which contains tildes).
FUNCNEST
If set to a numeric value greater than 0, defines a maximum function nesting level.
Function invocations that exceed this nesting level will cause the current command
to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of filenames to be ignored by
pathname expansion. If a filename matched by a pathname expansion pattern also
matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the history
list. If the list of values includes ignorespace, lines which begin with a space
character are not saved in the history list. A value of ignoredups causes lines
matching the previous history entry to not be saved. A value of ignoreboth is
shorthand for ignorespace and ignoredups. A value of erasedups causes all previous
lines matching the current line to be removed from the history list before that
line is saved. Any value not in the above list is ignored. If HISTCONTROL is
unset, or does not include a valid value, all lines read by the shell parser are
saved on the history list, subject to the value of HISTIGNORE. The second and sub‐
sequent lines of a multi-line compound command are not tested, and are added to the
history regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below). The
default value is ~/.bash_history. If unset, the command history is not saved when
a shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable is
assigned a value, the history file is truncated, if necessary, to contain no more
than that number of lines by removing the oldest entries. The history file is also
truncated to this size after writing it when a shell exits. If the value is 0, the
history file is truncated to zero size. Non-numeric values and numeric values less
than zero inhibit truncation. The shell sets the default value to the value of
HISTSIZE after reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should be
saved on the history list. Each pattern is anchored at the beginning of the line
and must match the complete line (no implicit `*' is appended). Each pattern is
tested against the line after the checks specified by HISTCONTROL are applied. In
addition to the normal shell pattern matching characters, `&' matches the previous
history line. `&' may be escaped using a backslash; the backslash is removed
before attempting a match. The second and subsequent lines of a multi-line com‐
pound 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). If
the value is 0, commands are not saved in the history list. Numeric values less
than zero result in every command being saved on the history list (there is no
limit). The shell sets the default value to 500 after reading any startup files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for
strftime(3) to print the time stamp associated with each history entry displayed by
the history builtin. If this variable is set, time stamps are written to the his‐
tory 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 comple‐
tions may be changed while the shell is running; the next time hostname completion
is attempted after the value is changed, bash adds the contents of the new file to
the existing list. If HOSTFILE is set, but has no value, or does not name a read‐
able 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>''.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF character as the
sole input. If set, the value is the number of consecutive EOF characters which
must be typed as the first characters on an input line before bash exits. If the
variable exists but does not have a numeric value, or has no value, the default
value is 10. If it does not exist, EOF signifies the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the default of ~/.inputrc
(see READLINE below).
LANG Used to determine the locale category for any category not specifically selected
with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_ variable specifying a
locale category.
LC_COLLATE
This variable determines the collation order used when sorting the results of path‐
name expansion, and determines the behavior of range expressions, equivalence
classes, and collating sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the behavior of char‐
acter classes within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-quoted strings pre‐
ceded by a $.
LC_NUMERIC
This variable determines the locale category used for number formatting.
LINES Used by the select compound command to determine the column length for printing
selection lists. Automatically set if the checkwinsize option is enabled or in an
interactive shell upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file or directory name and the MAILPATH variable is
not set, bash informs the user of the arrival of mail in the specified file or
Maildir-format directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The default is 60 seconds.
When it is time to check for mail, the shell does so before displaying the primary
prompt. If this variable is unset, or set to a value that is not a number greater
than or equal to zero, the shell disables mail checking.
MAILPATH
A colon-separated list of filenames to be checked for mail. The message to be
printed when mail arrives in a particular file may be specified by separating the
filename from the message with a `?'. When used in the text of the message, $_
expands to the name of the current mailfile. Example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash 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 indicates the current directory. A null direc‐
tory name may appear as two adjacent colons, or as an initial or trailing colon.
The default path is system-dependent, and is set by the administrator who installs
bash. A common value is
``/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin''.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell enters posix
mode before reading the startup files, as if the --posix invocation option had been
supplied. If it is set while the shell is running, bash enables posix mode, as if
the command set -o posix had been executed.
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 pri‐
mary 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 necessary, to indicate multiple levels of indirec‐
tion. The default is ``+ ''.
SHELL The full pathname to the shell is kept in this environment variable. If it is not
set when the shell starts, bash assigns to it the full pathname of the current
user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing
information for pipelines prefixed with the time reserved word should be displayed.
The % character introduces an escape sequence that is expanded to a time value or
other information. The escape sequences and their meanings are as follows; the
braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of fractional digits
after a decimal point. A value of 0 causes no decimal point or fraction to be out‐
put. At most three places after the decimal point may be specified; values of p
greater than 3 are changed to 3. If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs.
The value of p determines whether or not the fraction is included.

If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'. If the value is null, no timing informa‐
tion is displayed. A trailing newline is added when the format string is dis‐
played.
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 terminal. In an interactive shell, the
value is interpreted as the number of seconds to wait for a line of input after
issuing the primary prompt. Bash terminates after waiting for that number of sec‐
onds if a complete line of input does not arrive.
TMPDIR If set, bash uses its value as the name of a directory in which bash creates tempo‐
rary files for the shell's use.
auto_resume
This variable controls how the shell interacts with the user and job control. If
this variable is set, single word simple commands without redirections are treated
as candidates for resumption of an existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning with the string typed, the job
most recently accessed is selected. The name of a stopped job, in this context, is
the command line used to start it. If set to the value exact, the string supplied
must match the name of a stopped job exactly; if set to substring, the string sup‐
plied needs to match a substring of the name of a stopped job. The substring value
provides functionality analogous to the %? job identifier (see JOB CONTROL below).
If set to any other value, the supplied string must be a prefix of a stopped job's
name; this provides functionality analogous to the %string job identifier.
histchars
The two or three characters which control history expansion and tokenization (see
HISTORY EXPANSION below). The first character is the history expansion character,
the character which signals the start of a history expansion, normally `!'. The
second character is the quick substitution character, which is used as shorthand
for re-running the previous command entered, substituting one string for another in
the command. The default is `^'. The optional third character is the character
which indicates that the remainder of the line is a comment when found as the first
character of a word, normally `#'. The history comment character causes history
substitution to be skipped for the remaining words on the line. It does not neces‐
sarily 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 arith‐
metic expressions) and are zero-based; associative arrays are referenced using arbitrary
strings. Unless otherwise noted, indexed array indices must be non-negative integers.

An indexed array is created automatically if any variable is assigned to using the syntax
name[subscript]=value. The subscript is treated as an arithmetic expression that must
evaluate to a number. To explicitly declare an indexed array, use declare -a name (see
SHELL BUILTIN COMMANDS below). declare -a name[subscript] is also accepted; the subscript
is ignored.

Associative arrays are created using declare -A name.

Attributes may be specified for an array variable using the declare and readonly builtins.
Each attribute applies to all members of an array.

Arrays are assigned to using compound assignments of the form name=(value1 ... valuen),
where each value is of the form [subscript]=string. Indexed array assignments do not
require anything but string. When assigning to indexed arrays, if the optional brackets
and subscript are supplied, that index is assigned to; otherwise the index of the element
assigned is the last index assigned to by the statement plus one. Indexing starts at
zero.

When assigning to an associative array, the subscript is required.

This syntax is also accepted by the declare builtin. Individual array elements may be
assigned to using the name[subscript]=value syntax introduced above. When assigning to an
indexed array, if name is subscripted by a negative number, that number is interpreted as
relative to one greater than the maximum index of name, so negative indices count back
from the end of the array, and an index of -1 references the last element.

Any element of an array may be referenced using ${name[subscript]}. The braces are
required to avoid conflicts with pathname expansion. If subscript is @ or *, the word
expands to all members of name. These subscripts differ only when the word appears within
double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the
value of each array member separated by the first character of the IFS special variable,
and ${name[@]} expands each element of name to a separate word. When there are no array
members, ${name[@]} expands to nothing. If the double-quoted expansion occurs within a
word, the expansion of the first parameter is joined with the beginning part of the origi‐
nal word, and the expansion of the last parameter is joined with the last part of the
original word. This is analogous to the expansion of the special parameters * and @ (see
Special Parameters above). ${#name[subscript]} expands to the length of ${name[sub‐
script]}. If subscript is * or @, the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equivalent to referencing the array
with a subscript of 0. If the subscript used to reference an element of an indexed array
evaluates to a number less than zero, it is interpreted as relative to one greater than
the maximum index of the array, so negative indices count back from the end of the array,
and an index of -1 references the last element.

An array variable is considered set if a subscript has been assigned a value. The null
string is a valid value.

It is possible to obtain the keys (indices) of an array as well as the values.
${!name[@]} and ${!name[*]} expand to the indices assigned in array variable name. The
treatment when in double quotes is similar to the expansion of the special parameters @
and * within double quotes.

The unset builtin is used to destroy arrays. unset name[subscript] destroys the array
element at index subscript. Negative subscripts to indexed arrays are interpreted as
described above. 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 vari‐
able expansion, command substitution, arithmetic expansion, word splitting, and pathname
expansion.

The order of expansions is: brace expansion; tilde expansion, parameter and variable
expansion, arithmetic expansion, and command substitution (done in a left-to-right fash‐
ion); word splitting; and pathname expansion.

On systems that can support it, there is an additional expansion available: process sub‐
stitution. This is performed at the same time as tilde, parameter, variable, and arith‐
metic expansion and command 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 mecha‐
nism is similar to pathname expansion, but the filenames generated need not exist. Pat‐
terns 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, fol‐
lowed by an optional postscript. The preamble is prefixed to each string contained within
the braces, and the postscript is then appended to each resulting string, expanding left
to right.

Brace expansions may be nested. The results of each expanded string are not sorted; left
to right order is preserved. For example, a{d,c,b}e expands into `ade ace abe'.

A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or
single characters, and incr, an optional increment, is an integer. When integers are sup‐
plied, the expression expands to each number between x and y, inclusive. Supplied inte‐
gers 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 expres‐
sion expands to each character lexicographically between x and y, inclusive, using the
default C locale. Note that both x and y must be of the same type. When the increment is
supplied, it is used as the difference between each term. The default increment is 1 or
-1 as appropriate.

Brace expansion is performed before any other expansions, and any characters special to
other expansions are preserved in the result. It is strictly textual. Bash does not
apply any syntactic interpretation to the context of the expansion or the text between the
braces.

A correctly-formed brace expansion must contain unquoted opening and closing braces, and
at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace
expansion is left unchanged. A { or , may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with parameter expansion, the
string ${ is not considered eligible for brace expansion.

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 out‐
put. The same word is output as file1 file2 after expansion by bash. If strict compati‐
bility with sh is desired, start bash with the +B option or disable brace expansion with
the +B option to the set command (see SHELL BUILTIN COMMANDS below).

Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all of the characters preceding
the first unquoted slash (or all characters, if there is no unquoted slash) are considered
a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters
in the tilde-prefix following the tilde are treated as a possible login name. If this
login name is the null string, the tilde is replaced with the value of the shell parameter
HOME. If HOME is unset, the home directory of the user executing the shell is substituted
instead. Otherwise, the tilde-prefix is replaced with the home directory associated with
the specified login name.

If the tilde-prefix is a `~+', the value of the shell variable PWD replaces the tilde-pre‐
fix. 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 filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell
assigns the expanded value.

Parameter Expansion
The `$' character introduces parameter expansion, command substitution, or arithmetic
expansion. The parameter name or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be expanded from characters immediately
following it which could be interpreted as part of the name.

When braces are used, the matching ending brace is the first `}' not escaped by a back‐
slash or within a quoted string, and not within an embedded arithmetic expansion, command
substitution, or parameter expansion.

${parameter}
The value of parameter is substituted. The braces are required when parameter is a
positional parameter with more than one digit, or when parameter is followed by a
character which is not to be interpreted as part of its name. The parameter is a
shell parameter as described above PARAMETERS) or an array reference (Arrays).

If the first character of parameter is an exclamation point (!), it introduces a level of
variable indirection. Bash uses the value of the variable formed from the rest of parame‐
ter as the name of the variable; this variable is then expanded and that value is used in
the rest of the substitution, rather than the value of parameter itself. This is known as
indirect expansion. The exceptions to this are the expansions of ${!prefix*} and
${!name[@]} described below. The exclamation point must immediately follow the left brace
in order to introduce indirection.

In each of the cases below, word is subject to tilde expansion, parameter expansion, com‐
mand substitution, and arithmetic expansion.

When not performing substring expansion, using the forms documented below (e.g., :-), bash
tests for a parameter that is unset or null. Omitting the colon results in a test only
for a parameter that is unset.

${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is sub‐
stituted. Otherwise, the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is
assigned to parameter. The value of parameter is then substituted. Positional
parameters and special parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of
word (or a message to that effect if word is not present) is written to the stan‐
dard error and the shell, if it is not interactive, exits. Otherwise, the value of
parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, other‐
wise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of the value of parameter
starting at the character specified by offset. If parameter is @, an indexed array
subscripted by @ or *, or an associative array name, the results differ as
described below. If length is omitted, expands to the substring of the value of
parameter starting at the character specified by offset and extending to the end of
the value. length and offset are arithmetic expressions (see ARITHMETIC EVALUATION
below).

If offset evaluates to a number less than zero, the value is used as an offset in
characters from the end of the value of parameter. If length evaluates to a number
less than zero, it is interpreted as an offset in characters from the end of the
value of parameter rather than a number of characters, and the expansion is the
characters between offset and that result. Note that a negative offset must be
separated from the colon by at least one space to avoid being confused with the :-
expansion.

If parameter is @, the result is length positional parameters beginning at offset.
A negative offset is taken relative to one greater than the greatest positional
parameter, so an offset of -1 evaluates to the last positional parameter. It is an
expansion error if length evaluates to a number less than zero.

If parameter is an indexed array name subscripted by @ or *, the result is the
length members of the array beginning with ${parameter[offset]}. A negative offset
is taken relative to one greater than the maximum index of the specified array. It
is an expansion error if length evaluates to a number less than zero.

Substring expansion applied to an associative array produces undefined results.

Substring indexing is zero-based unless the positional parameters are used, in
which case the indexing starts at 1 by default. If offset is 0, and the positional
parameters are used, $0 is prefixed to the list.

${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables whose names begin with
prefix, separated by the first character of the IFS special variable. When @ is
used and the expansion appears within double quotes, each variable name expands to
a separate word.

${!name[@]}
${!name[*]}
List of array keys. If name is an array variable, expands to the list of array
indices (keys) assigned in name. If name is not an array, expands to 0 if name is
set and null otherwise. When @ is used and the expansion appears within double
quotes, each key expands to a separate word.

${#parameter}
Parameter length. The length in characters of the value of parameter is substi‐
tuted. If parameter is * or @, the value substituted is the number of positional
parameters. If parameter is an array name subscripted by * or @, the value substi‐
tuted is the number of elements in the array. If parameter is an indexed array
name subscripted by a negative number, that number is interpreted as relative to
one greater than the maximum index of parameter, so negative indices count back
from the end of the array, and an index of -1 references the last element.

${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches the beginning of the value of param‐
eter, then the result of the expansion is the expanded value of parameter with the
shortest matching pattern (the ``#'' case) or the longest matching pattern (the
``##'' case) deleted. If parameter is @ or *, the pattern removal operation is
applied to each positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *, the pattern
removal operation is applied to each member of the array in turn, and the expansion
is the resultant list.

${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches a trailing portion of the expanded
value of parameter, then the result of the expansion is the expanded value of
parameter with the shortest matching pattern (the ``%'' case) or the longest match‐
ing pattern (the ``%%'' case) deleted. If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted with @ or *, the
pattern removal operation is applied to each member of the array in turn, and the
expansion is the resultant list.

${parameter/pattern/string}
Pattern substitution. The pattern is expanded to produce a pattern just as in
pathname expansion. Parameter is expanded and the longest match of pattern against
its value is replaced with string. If pattern begins with /, all matches of pat‐
tern are replaced with string. Normally only the first match is replaced. If pat‐
tern 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
operation is applied to each member of the array in turn, and the expansion is the
resultant list.

${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of alphabetic characters in
parameter. The pattern is expanded to produce a pattern just as in pathname expan‐
sion. Each character in the expanded value of parameter is tested against pattern,
and, if it matches the pattern, its case is converted. The pattern should not
attempt to match more than one character. The ^ operator converts lowercase let‐
ters matching pattern to uppercase; the , operator converts matching uppercase let‐
ters to lowercase. The ^^ and ,, expansions convert each matched character in the
expanded value; the ^ and , expansions match and convert only the first character
in the expanded value. If pattern is omitted, it is treated like a ?, which
matches every character. If parameter is @ or *, the case modification operation
is applied to each positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *, the case modifi‐
cation operation is applied to each member of the array in turn, and the expansion
is the resultant list.

Command Substitution
Command substitution allows the output of a command to replace the command name. There
are two forms:

$(command)
or
`command`

Bash performs the expansion by executing command and replacing the command substitution
with the standard output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word splitting. The command sub‐
stitution $(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 back‐
slash 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 substitu‐
tion of the result. The format for arithmetic expansion is:

$((expression))

The old format $[expression] is deprecated and will be removed in upcoming versions of
bash.

The expression is treated as if it were within double quotes, but a double quote inside
the parentheses is not treated specially. All tokens in the expression undergo parameter
and variable expansion, command substitution, and quote removal. The result is treated as
the arithmetic expression to be evaluated. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below under ARITHMETIC EVALUA‐
TION. If expression is invalid, bash prints a message indicating failure and no substitu‐
tion occurs.

Process Substitution
Process substitution is supported on systems that support named pipes (FIFOs) or the
/dev/fd method of naming open files. It takes the form of <(list) or >(list). The
process list is run with its input or output connected to a FIFO or some file in /dev/fd.
The name of this file is passed as an argument to the current command as the result of the
expansion. If the >(list) form is used, writing to the file will provide input for list.
If the <(list) form is used, the file passed as an argument should be read to obtain the
output of list.

When available, process substitution is performed simultaneously with parameter and vari‐
able expansion, command substitution, and arithmetic expansion.

Word Splitting
The shell scans the results of parameter expansion, command substitution, and arithmetic
expansion that did not occur within double quotes for word splitting.

The shell treats each character of IFS as a delimiter, and splits the results of the other
expansions into words using these characters as field terminators. If IFS is unset, or
its value is exactly <space><tab><newline>, the default, then sequences of <space>, <tab>,
and <newline> at the beginning and end of the results of the previous expansions are
ignored, and any sequence of IFS characters not at the beginning or end serves to delimit
words. If IFS has a value other than the default, then sequences of the whitespace char‐
acters 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 whitespace character). Any character
in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters,
delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.

Explicit null arguments ("" or '') are retained. Unquoted implicit null arguments,
resulting from the expansion of parameters that have no values, are removed. If a parame‐
ter with no value is expanded within double quotes, a null argument results and is
retained.

Note that if no expansion occurs, no splitting is performed.

Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each word for the
characters *, ?, and [. If one of these characters appears, then the word is regarded as
a pattern, and replaced with an alphabetically sorted list of filenames matching the pat‐
tern (see Pattern Matching below). If no matching filenames are found, and the shell
option nullglob is not enabled, the word is left unchanged. If the nullglob option is
set, and no matches are found, the word is removed. If the failglob shell option is set,
and no matches are found, an error message is printed and the command is not executed. If
the shell option nocaseglob is enabled, the match is performed without regard to the case
of alphabetic characters. Note that when using range expressions like [a-z] (see below),
letters of the other case may be included, depending on the setting of LC_COLLATE. When a
pattern is used for pathname expansion, the character ``.'' at the start of a name or
immediately following a slash must be matched explicitly, unless the shell option dotglob
is set. 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 filenames matching a pat‐
tern. If GLOBIGNORE is set, each matching filename that also matches one of the patterns
in GLOBIGNORE is removed from the list of matches. The filenames ``.'' and ``..'' are
always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-
null value has the effect of enabling the dotglob shell option, so all other filenames
beginning with a ``.'' will match. To get the old behavior of ignoring filenames begin‐
ning 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 back‐
slash escapes the following character; the escaping backslash is discarded when matching.
The special pattern characters must be quoted if they are to be matched literally.

The special pattern characters have the following meanings:

* Matches any string, including the null string. When the globstar shell
option is enabled, and * is used in a pathname expansion context, two adja‐
cent *s used as a single pattern will match all files and zero or more
directories and subdirectories. If followed by a /, two adjacent *s will
match only directories and subdirectories.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated
by a hyphen denotes a range expression; any character that falls between
those two characters, inclusive, using the current locale's collating
sequence and character set, is matched. If the first character following
the [ is a ! or a ^ then any character not enclosed is matched. The sort‐
ing order of characters in range expressions is determined by the current
locale and the values of the LC_COLLATE or LC_ALL shell variables, if set.
To obtain the traditional interpretation of range expressions, where [a-d]
is equivalent to [abcd], set value of the LC_ALL shell variable to C, or
enable the globasciiranges shell option. A - may be matched by including it
as the first or last character in the set. A ] may be matched by including
it as the first character in the set.

Within [ and ], character classes can be specified using the syntax
[:class:], where class is one of the following classes defined in the POSIX
standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word
xdigit
A character class matches any character belonging to that class. The word
character class matches letters, digits, and the character _.

Within [ and ], an equivalence class can be specified using the syntax
[=c=], which matches all characters with the same collation weight (as
defined by the current locale) as the character c.

Within [ and ], the syntax [.symbol.] matches the collating symbol symbol.

If the extglob shell option is enabled using the shopt builtin, several extended pattern
matching operators are recognized. In the following description, a pattern-list is a list
of one or more patterns separated by a |. Composite patterns may be formed using one or
more of the following sub-patterns:

?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns

Quote Removal
After the preceding expansions, all unquoted occurrences of the characters \, ', and "
that did not result from one of the above expansions are removed.

REDIRECTION
Before a command is executed, its input and output may be redirected using a special nota‐
tion interpreted by the shell. Redirection allows commands' file handles to be dupli‐
cated, opened, closed, made to refer to different files, and can change the files the com‐
mand reads from and writes to. Redirection may also be used to modify file handles in the
current shell execution environment. The following redirection operators may precede or
appear anywhere within a simple command or may follow a command. Redirections are pro‐
cessed in the order they appear, from left to right.

Each redirection that may be preceded by a file descriptor number may instead be preceded
by a word of the form {varname}. In this case, for each redirection operator except >&-
and <&-, the shell will allocate a file descriptor greater than or equal to 10 and assign
it to varname. If >&- or <&- is preceded by {varname}, the value of varname defines the
file descriptor to close.

In the following descriptions, if the file descriptor number is omitted, and the first
character of the redirection operator is <, the redirection refers to the standard input
(file descriptor 0). If the first character of the redirection operator is >, the redi‐
rection refers to the standard output (file descriptor 1).

The word following the redirection operator in the following descriptions, unless other‐
wise noted, is subjected to brace expansion, tilde expansion, parameter and variable
expansion, command substitution, arithmetic expansion, quote removal, pathname expansion,
and word splitting. If it expands to more than one word, bash reports an error.

Note that the order of redirections is significant. For example, the command

ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was dupli‐
cated from the standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described
in the following table:

/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port
number or service name, bash attempts to open the corresponding TCP socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port
number or service name, bash attempts to open the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with care, as they may
conflict with file descriptors the shell uses internally.

Note that the exec builtin command can make redirections take effect in the current shell.

Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be
opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is
not specified.

The general format for redirecting input is:

[n]<word

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 trun‐
cated to zero size.

The general format for redirecting output is:

[n]>word

If the redirection operator is >, and the noclobber option to the set builtin has been
enabled, the redirection will fail if the file whose name results from the expansion of
word exists and is a regular file. If the redirection operator is >|, or the redirection
operator is > and the noclobber option to the set builtin command is not enabled, the re‐
direction 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 expan‐
sion of word to be opened for appending on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not exist it is created.

The general format for appending output is:

[n]>>word

Redirecting Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error
output (file descriptor 2) to be redirected to the file whose name is the expansion of
word.

There are two formats for redirecting standard output and standard error:

&>word
and
>&word

Of the two forms, the first is preferred. This is semantically equivalent to

>word 2>&1

When using the second form, word may not expand to a number or -. If it does, other redi‐
rection operators apply (see Duplicating File Descriptors below) for compatibility rea‐
sons.

Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error
output (file descriptor 2) to be appended to the file whose name is the expansion of word.

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

Here Documents
This type of redirection instructs the shell to read input from the current source until a
line containing only delimiter (with no trailing blanks) is seen. All of the lines read
up to that point are then used as the standard input for a command.

The format of here-documents is:

<<[-]word
here-document
delimiter

No parameter and variable expansion, command substitution, arithmetic expansion, or path‐
name expansion is performed on word. If any characters in word are quoted, the delimiter
is the result of quote removal on word, and the lines in the here-document are not
expanded. If word is unquoted, all lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion, the character sequence \<new‐
line> 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 undergoes brace expansion, tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal. Pathname expansion and
word splitting are not performed. The result is supplied as a single string to the com‐
mand 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

is used similarly to duplicate output file descriptors. If n is not specified, the stan‐
dard output (file descriptor 1) is used. If the digits in word do not specify a file
descriptor open for output, a redirection error occurs. If word evaluates to -, file
descriptor n is closed. As a special case, if n is omitted, and word does not expand to
one or more digits or -, the standard output and standard error are redirected as
described previously.

Moving File Descriptors
The redirection operator

[n]<&digit-

moves the file descriptor digit to file descriptor n, or the standard input (file descrip‐
tor 0) if n is not specified. digit is closed after being duplicated to n.

Similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n, or the standard output (file
descriptor 1) if n is not specified.

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

causes the file whose name is the expansion of word to be opened for both reading and
writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file
does not exist, it is created.

ALIASES
Aliases allow a string to be substituted for a word when it is used as the first word of a
simple command. The shell maintains a list of aliases that may be set and unset with the
alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below). The first word of
each simple command, if unquoted, is checked to see if it has an alias. If so, that word
is replaced by the text of the alias. The characters /, $, `, and = and any of the shell
metacharacters or quoting characters listed above may not appear in an alias name. The
replacement text may contain any valid shell input, including shell metacharacters. The
first word of the replacement text is tested for aliases, but a word that is identical to
an alias being expanded is not expanded a second time. This means that one may alias ls
to ls -F, for instance, and bash does not try to recursively expand the replacement text.
If the last character of the alias value is a blank, then the next command word following
the alias is also checked for alias expansion.

Aliases are created and listed with the alias command, and removed with the unalias com‐
mand.

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. There‐
fore, an alias definition appearing on the same line as another command does not take
effect until the next line of input is read. The commands following the alias definition
on that line are not affected by the new alias. This behavior is also an issue when func‐
tions 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 com‐
mands for later execution. When the name of a shell function is used as a simple command
name, the list of commands associated with that function name is executed. Functions are
executed in the context of the current shell; no new process is created to interpret them
(contrast this with the execution of a shell script). When a function is executed, the
arguments to the function become the positional parameters during its execution. The spe‐
cial parameter # is updated to reflect the change. Special parameter 0 is unchanged. The
first element of the FUNCNAME variable is set to the name of the function while the func‐
tion is executing.

All other aspects of the shell execution environment are identical between a function and
its caller with these exceptions: the DEBUG and RETURN traps (see the description of the
trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the function has
been given the trace attribute (see the description of the declare builtin below) or the
-o functrace shell option has been enabled with the set builtin (in which case all func‐
tions inherit the DEBUG and RETURN traps), and the ERR trap is not inherited unless the -o
errtrace shell option has been enabled.

Variables local to the function may be declared with the local builtin command. Ordinar‐
ily, variables and their values are shared between the function and its caller.

The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum func‐
tion 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 execu‐
tion resumes with the next command after the function call. Any command associated with
the RETURN trap is executed before execution resumes. When a function completes, the val‐
ues of the positional parameters and the special parameter # are restored to the values
they had prior to the function's execution.

Function names and definitions may be listed with the -f option to the declare or typeset
builtin commands. The -F option to declare or typeset will list the function names only
(and optionally the source file and line number, if the extdebug shell option is enabled).
Functions may be exported so that subshells automatically have them defined with the -f
option to the export builtin. A function definition may be deleted using the -f option to
the unset builtin. Note that shell functions and variables with the same name may result
in multiple identically-named entries in the environment passed to the shell's children.
Care should be taken in cases where this may cause a problem.

Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the
function call stack and restrict the number of function invocations. By default, no limit
is imposed on the number of recursive calls.

ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain circumstances (see
the let and declare builtin 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, associativity, and values are
the same as in the C language. The following list of operators is grouped into levels of
equal-precedence operators. The levels are listed in order of decreasing precedence.

id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma

Shell variables are allowed as operands; parameter expansion is performed before the
expression is evaluated. Within an expression, shell variables may also be referenced by
name without using the parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter expansion syntax. The
value of a variable is evaluated as an arithmetic expression when it is referenced, or
when a variable which has been given the integer attribute using declare -i is assigned a
value. A null value evaluates to 0. A shell variable need not have its integer attribute
turned on to be used in an expression.

Constants with a leading 0 are interpreted as octal numbers. A leading 0x or 0X denotes
hexadecimal. Otherwise, numbers take the form [base#]n, where the optional base is a dec‐
imal number between 2 and 64 representing the arithmetic base, and n is a number in that
base. If base# is omitted, then base 10 is used. When specifying n, the digits greater<
than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that
order. If base is less than or equal to 36, lowercase and uppercase letters may be used
interchangeably to represent numbers between 10 and 35.

Operators are evaluated in order of precedence. Sub-expressions in parentheses are evalu‐
ated 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. Expres‐
sions are formed from the following unary or binary primaries. If any file argument to
one of the primaries is of the form /dev/fd/n, then file descriptor n is checked. If the
file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr,
file descriptor 0, 1, or 2, respectively, is checked.

Unless otherwise specified, primaries that operate on files follow symbolic links and
operate on the target of the link, rather than the link itself.

When used with [[, the < and > operators sort lexicographically using the current locale.
The test command sorts using ASCII ordering.

-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device and inode numbers.
file1 -nt file2
True if file1 is newer (according to modification date) than file2, or if file1
exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1 does not.
-o optname
True if the shell option optname is enabled. See the list of options under the
description of the -o option to the set builtin below.
-v varname
True if the shell variable varname is set (has been assigned a value).
-R varname
True if the shell variable varname is set and is a name reference.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.

string1 == string2
string1 = string2
True if the strings are equal. = should be used with the test command for POSIX
conformance. When used with the [[ command, this performs pattern matching as
described above (Compound Commands).

string1 != string2
True if the strings are not equal.

string1 < string2
True if string1 sorts before string2 lexicographically.

string1 > string2
True if string1 sorts after string2 lexicographically.

arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These 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, assign‐
ments, and redirections, from left to right.

1. The words that the parser has marked as variable assignments (those preceding the
command name) and redirections are saved for later processing.

2. The words that are not variable assignments or redirections are expanded. If any
words remain after expansion, the first word is taken to be the name of the command
and the remaining words are the arguments.

3. Redirections are performed as described above under REDIRECTION.

4. The text after the = in each variable assignment undergoes tilde expansion, parame‐
ter expansion, command substitution, arithmetic expansion, and quote removal before
being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment.
Otherwise, the variables are added to the environment of the executed command and do not
affect the current shell environment. If any of the assignments attempts to assign a
value to a readonly variable, an error occurs, and the command exits with a non-zero sta‐
tus.

If no command name results, redirections are performed, but do not affect the current
shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below.
Otherwise, the command exits. If one of the expansions contained a command substitution,
the exit status of the command is the exit status of the last command substitution per‐
formed. If there were no command substitutions, the command exits with a status of zero.

COMMAND EXECUTION
After a command has been split into words, if it results in a simple command and an
optional list of arguments, the following actions are taken.

If the command name contains no slashes, the shell attempts to locate it. If there exists
a shell function by that name, that function is invoked as described above in FUNCTIONS.
If the name does not match a function, the shell searches for it in the list of shell
builtins. If a match is found, that builtin is invoked.

If the name is neither a shell function nor a builtin, and contains no slashes, bash
searches each element of the PATH for a directory containing an executable file by that
name. Bash uses a hash table to remember the full pathnames of executable files (see hash
under SHELL BUILTIN COMMANDS below). A full search of the directories in PATH is per‐
formed only if the command is not found in the hash table. If the search is unsuccessful,
the shell searches for a defined shell function named command_not_found_handle. If that
function exists, it is invoked with the original command and the original command's argu‐
ments as its arguments, and the function's exit status becomes the exit status of the
shell. If that function is not defined, the shell prints an error message and returns an
exit status of 127.

If the search is successful, or if the command name contains one or more slashes, the
shell executes the named program in a separate execution environment. Argument 0 is set
to the name given, and the remaining arguments to the command are set to the arguments
given, if any.

If this execution fails because the file is not in executable format, and the file is not
a directory, it is assumed to be a shell script, a file containing shell commands. A sub‐
shell is spawned to execute it. This subshell reinitializes itself, so that the effect is
as if a new shell had been invoked to handle the script, with the exception that the loca‐
tions of commands remembered by the parent (see hash below under SHELL BUILTIN COMMANDS)
are retained by the child.

If the program is a file beginning with #!, the remainder of the first line specifies an
interpreter for the program. The shell executes the specified interpreter on operating
systems that do not handle this executable format themselves. The arguments to the inter‐
preter consist of a single optional argument following the interpreter name on the first
line of the program, followed by the name of the program, followed by the command argu‐
ments, 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 sup‐
plied 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 other‐
wise noted, the values are inherited from the shell.

· the shell's open files, plus any modifications and additions specified by redirec‐
tions to the command

· the current working directory

· the file creation mode mask

· shell variables and functions marked for export, along with variables exported for
the command, passed in the environment

· traps caught by the shell are reset to the values inherited from the shell's par‐
ent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the shell's execution envi‐
ronment.

Command substitution, commands grouped with parentheses, and asynchronous commands are
invoked in a subshell environment that is a duplicate of the shell environment, except
that traps caught by the shell are reset to the values that the shell inherited from its
parent at invocation. Builtin commands that are invoked as part of a pipeline are also
executed in a subshell environment. Changes made to the subshell environment cannot
affect the shell's execution environment.

Subshells spawned to execute command substitutions inherit the value of the -e option from
the parent shell. When not in posix mode, bash clears the -e option in such subshells.

If a command is followed by a & and job control is not active, the default standard input
for the command is the empty file /dev/null. Otherwise, the invoked command inherits the
file descriptors of the calling shell as modified by redirections.

ENVIRONMENT
When a program is invoked it is given an array of strings called the environment. This is
a list of name-value pairs, of the form name=value.

The shell provides several ways to manipulate the environment. On invocation, the shell
scans its own environment and creates a parameter for each name found, automatically mark‐
ing 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 mod‐
ified 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 prefix‐
ing 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 assign‐
ments are placed in the environment for a command, not just those that precede the command
name.

When bash invokes an external command, the variable _ is set to the full filename of the
command and passed to that command in its environment.

EXIT STATUS
The exit status of an executed command is the value returned by the waitpid system call or
equivalent function. Exit statuses fall between 0 and 255, though, as explained below,
the shell may use values above 125 specially. Exit statuses from shell builtins and com‐
pound commands are also limited to this range. Under certain circumstances, the shell will
use special values to indicate specific failure modes.

For the shell's purposes, a command which exits with a zero exit status has succeeded. An
exit status of zero indicates success. A non-zero exit status indicates failure. When a
command terminates on a fatal signal N, bash uses the value of 128+N as the exit status.

If a command is not found, the child process created to execute it returns a status of
127. If a command is found but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection, the exit status is
greater than zero.

Shell builtin commands return a status of 0 (true) if successful, and non-zero (false) if
an error occurs while they execute. All builtins return an exit status of 2 to indicate
incorrect usage.

Bash itself returns the exit status of the last command executed, unless a syntax error
occurs, in which case it exits with a non-zero value. See also the exit builtin command
below.

SIGNALS
When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill 0
does not kill an interactive shell), and SIGINT is caught and handled (so that the wait
builtin is interruptible). In all cases, bash ignores SIGQUIT. If job control is in
effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Non-builtin commands run by bash have signal handlers set to the values inherited by the
shell from its parent. When job control is not in effect, asynchronous commands ignore
SIGINT and SIGQUIT in addition to these inherited handlers. Commands run as a result of
command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU,
and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell
resends the SIGHUP to all jobs, running or stopped. 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 interface supplied jointly by the operating system kernel's
terminal driver and bash.

The shell associates a job with each pipeline. It keeps a table of currently executing
jobs, which may be listed with the jobs command. When bash starts a job asynchronously
(in the background), it prints a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last process in
the pipeline associated with this job is 25647. All of the processes in a single pipeline
are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the operating sys‐
tem 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 differs from the termi‐
nal's; such processes are immune to keyboard-generated signals. Only foreground processes
are allowed to read from or, if the user so specifies with stty tostop, write to the ter‐
minal. 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
background, the fg command to continue it in the foreground, or the kill command to kill
it. A ^Z takes effect immediately, and has the additional side effect of causing pending
output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The character % introduces a
job specification (jobspec). Job number n may be referred to as %n. A job may also be
referred to using a prefix of the name used to start it, or using a substring that appears
in its command line. For example, %ce refers to a stopped ce job. If a prefix matches
more than one job, bash reports an error. Using %?ce, on the other hand, refers to any
job containing the string ce in its command line. If the substring matches more than one
job, bash reports an error. The symbols %% and %+ refer to the shell's notion of the cur‐
rent job, which is the last job stopped while it was in the foreground or started in the
background. The previous job may be referenced using %-. If there is only a single job,
%+ and %- can both be used to refer to that job. In output pertaining to jobs (e.g., the
output of the jobs command), the current job is always flagged with a +, and the previous
job with a -. A single % (with no accompanying job specification) also refers to the cur‐
rent job.

Simply naming a job can be used to bring it into the foreground: %1 is a synonym for ``fg
%1'', bringing job 1 from the background into the foreground. Similarly, ``%1 &'' resumes
job 1 in the background, equivalent to ``bg %1''.

The shell learns immediately whenever a job changes state. Normally, bash waits until it
is about to print a prompt before reporting changes in a job's status so as to not inter‐
rupt any other output. If the -b option to the set builtin command is enabled, bash
reports such changes immediately. Any trap on SIGCHLD is executed for each child that
exits.

If an attempt to exit bash is made while jobs are stopped (or, if the checkjobs shell
option has been enabled using the shopt builtin, running), the shell prints a warning mes‐
sage, 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 com‐
mand. Bash allows these prompt strings to be customized by inserting a number of back‐
slash-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 representa‐
tion. The braces are required
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion following the final
slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated with a tilde (uses the
value of the PROMPT_DIRTRIM variable)
\W the basename of the current working directory, with $HOME abbreviated with a
tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could be used to embed a
terminal control sequence into the prompt
\] end a sequence of non-printing characters

The command number and the history number are usually different: the history number of a
command is its position in the history list, which may include commands restored from the
history file (see HISTORY below), while the command number is the position in the sequence
of commands executed during the current shell session. After the string is decoded, it is
expanded via parameter expansion, command substitution, arithmetic expansion, and quote
removal, subject to the value of the promptvars shell option (see the description of the
shopt command under SHELL BUILTIN COMMANDS below).

READLINE
This is the library that handles reading input when using an interactive shell, unless the
--noediting option is given at shell invocation. Line editing is also used when using the
-e option to the read builtin. By default, the line editing commands are similar to those
of Emacs. A vi-style line editing interface is also available. Line editing can be
enabled at any time using the -o emacs or -o vi options to the set builtin (see SHELL
BUILTIN COMMANDS below). To turn off line editing after the shell is running, use the +o
emacs or +o vi options to the set builtin.

Readline Notation
In this section, the Emacs-style notation is used to denote keystrokes. Control keys are
denoted by C-key, e.g., C-n means Control-N. Similarly, meta keys are denoted by M-key,
so M-x means Meta-X. (On keyboards without a meta key, M-x means ESC x, i.e., press the
Escape key then the x key. This makes ESC the meta prefix. The combination M-C-x means
ESC-Control-x, or press the Escape key then hold the Control key while pressing the x
key.)

Readline commands may be given numeric arguments, which normally act as a repeat count.
Sometimes, however, it is the sign of the argument that is significant. Passing a nega‐
tive argument to a command that acts in the forward direction (e.g., kill-line) causes
that command to act in a backward direction. Commands whose behavior with arguments devi‐
ates from this are noted below.

When a command is described as killing text, the text deleted is saved for possible future
retrieval (yanking). The killed text is saved in a kill ring. Consecutive kills cause
the text to be accumulated into one unit, which can be yanked all at once. Commands which
do not kill text separate the chunks of text on the kill ring.

Readline Initialization
Readline is customized by putting commands in an initialization file (the inputrc file).
The name of this file is taken from the value of the INPUTRC variable. If that variable
is unset, the default is ~/.inputrc. When a program which uses the readline library
starts up, the initialization file is read, and the key bindings and variables are set.
There are only a few basic constructs allowed in the readline initialization file. Blank
lines are ignored. Lines beginning with a # are comments. Lines beginning with a $ indi‐
cate 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 backward-kill-word, and C-o is bound to run the macro expressed on the right
hand side (that is, to insert the text ``> output'' into the line).

In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in
that strings denoting an entire key sequence may be specified by placing the sequence
within double quotes. Some GNU Emacs style key escapes can be used, as in the following
example, but the symbolic character names are not recognized.

"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"

In this example, C-u is again bound to the function universal-argument. C-x C-r is bound
to the function re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text ``Function
Key 1''.

The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '

In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is
available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value nnn (one to three
digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two
hex digits)

When entering the text of a macro, single or double quotes must be used to indicate a
macro definition. Unquoted text is assumed to be a function name. In the macro body, the
backslash escapes described above are expanded. Backslash will quote any other character
in the macro text, including " and '.

Bash allows the current readline key bindings to be displayed or modified with the bind
builtin command. The editing mode may be switched during interactive use by using the -o
option to the set builtin command (see SHELL BUILTIN COMMANDS below).

Readline Variables
Readline has variables that can be used to further customize its behavior. A variable may
be set in the inputrc file with a statement of the form

set variable-name value

Except where noted, readline variables can take the values On or Off (without regard to
case). Unrecognized variable names are ignored. When a variable value is read, empty or
null values, "on" (case-insensitive), and "1" are equivalent to On. All other values are
equivalent to Off. The variables and their default values are:

bell-style (audible)
Controls what happens when readline wants to ring the terminal bell. If set to
none, readline never rings the bell. If set to visible, readline uses a visible
bell if one is available. If set to audible, readline attempts to ring the termi‐
nal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters treated specially by
the kernel's terminal driver to their readline equivalents.
colored-stats (Off)
If set to On, readline displays possible completions using different colors to
indicate their file type. The color definitions are taken from the value of the
LS_COLORS environment variable.
comment-begin (``#'')
The string that 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-insensi‐
tive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of possible completions
that is displayed without modification. When set to a value greater than zero,
common prefixes longer than this value are replaced with an ellipsis when display‐
ing possible completions.
completion-query-items (100)
This determines when the user is queried about viewing the number of possible com‐
pletions generated by the possible-completions command. It may be set to any inte‐
ger value greater than or equal to zero. If the number of possible completions is
greater than or equal to the value of this variable, the user is asked whether or
not he wishes to view them; otherwise they are simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth bit set to an ASCII
key sequence by stripping the eighth bit and prefixing an escape character (in
effect, using escape as the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion. Completion characters will be
inserted into the line as if they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key bindings similar to Emacs or vi.
editing-mode can be set to either emacs or vi.
echo-control-characters (On)
When set to On, on operating systems that indicate they support it, readline echoes
a character corresponding to a signal generated from the keyboard.
enable-keypad (Off)
When set to On, readline will try to enable the application keypad when it is
called. Some systems need this to enable the arrow keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta modifier key the terminal
claims to support when it is called. On many terminals, the meta key is used to
send eight-bit characters.
expand-tilde (Off)
If set to On, tilde expansion is performed when readline attempts word completion.
history-preserve-point (Off)
If set to On, the history code attempts to place point at the same location on each
history line retrieved with previous-history or next-history.
history-size (0)
Set the maximum number of history entries saved in the history list. If set to
zero, any existing history entries are deleted and no new entries are saved. If
set to a value less than zero, the number of history entries is not limited. By
default, the number of history entries is 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 subse‐
quently executing the character as a command. If this variable has not been given
a value, the characters ESC and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names is emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command, and vi-insert. vi is
equivalent to vi-command; emacs is equivalent to emacs-standard. The default value
is emacs; the value of editing-mode also affects the default keymap.
keyseq-timeout (500)
Specifies the duration readline will wait for a character when reading an ambiguous
key sequence (one that can form a complete key sequence using the input read so
far, or can take additional input to complete a longer key sequence). If no input
is received within the timeout, readline will use the shorter but complete key
sequence. The value is specified in milliseconds, so a value of 1000 means that
readline will wait one second for additional input. If this variable is set to a
value less than or equal to zero, or to a non-numeric value, readline will wait
until another key is pressed to decide which key sequence to complete.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are displayed with a preceding
asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to directories have a slash
appended (subject to the value of mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files whose names begin
with a `.' (hidden files) when 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)
If set to On, readline will display completions with matches sorted horizontally in
alphabetical order, rather than down the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines before returning when
accept-line is executed. By default, history lines may be modified and retain
individual undo lists across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If set to On, words
which have more than one possible completion cause the matches to be listed immedi‐
ately instead of ringing the bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a fashion similar
to show-all-if-ambiguous. If set to On, words which have more than one possible
completion without any possible partial completion (the possible completions don't
share a common prefix) cause the matches to be listed immediately instead of ring‐
ing the bell.
show-mode-in-prompt (Off)
If set to On, add a character to the beginning of the prompt indicating the editing
mode: emacs (@), vi command (:) or vi insertion (+).
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 termi‐
nal 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 com‐
mand, 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, per‐
haps 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 particular value. This could be used to
bind key sequences to functions useful for a specific program. For
instance, the following command adds a key sequence that quotes the current
or previous word in bash:

$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif

$endif This command, as seen in the previous example, terminates an $if command.

$else Commands in this branch of the $if directive are executed if the test fails.

$include
This directive takes a single filename as an argument and reads commands and bind‐
ings from that file. For example, the following directive would read /etc/inputrc:

$include /etc/inputrc

Searching
Readline provides commands for searching through the command history (see HISTORY below)
for lines containing a specified string. There are two search modes: incremental and non-
incremental.

Incremental searches begin before the user has finished typing the search string. As each
character of the search string is typed, readline displays the next entry from the history
matching the string typed so far. An incremental search requires only as many characters
as needed to find the desired history entry. The characters present in the value of the
isearch-terminators variable are used to terminate an incremental search. If that vari‐
able has not been assigned a value the Escape and Control-J characters will terminate an
incremental search. Control-G will abort an incremental search and restore the original
line. When the search is terminated, the history entry containing the search string
becomes the current line.

To find other matching entries in the history list, type Control-S or Control-R as appro‐
priate. This will search backward or forward in the history for the next entry matching
the search string typed so far. Any other key sequence bound to a readline command will
terminate the search and execute that command. For instance, a newline will terminate the
search and accept the line, thereby executing the command from the history list.

Readline remembers the last incremental search string. If two Control-Rs are typed with‐
out 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 con‐
tents of the current line.

Readline Command Names
The following is a list of the names of the commands and the default key sequences to
which they are bound. Command names without an accompanying key sequence are unbound by
default. In the following descriptions, point refers to the current cursor position, and
mark refers to a cursor position saved by the set-mark command. The text between the
point and mark is referred to as the region.

Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of alphanumeric char‐
acters (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 argu‐
ment, refresh the current line without clearing the screen.
redraw-current-line
Refresh the current line.

Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line is non-empty, add
it to the history list according to the state of the HISTCONTROL variable. If the
line is a modified history line, then restore the history line to its original
state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up' through the history as
necessary. This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down' through the history
as necessary. This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line using a non-incre‐
mental search for a string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search for a string sup‐
plied by the user.
history-search-forward
Search forward through the history for the string of characters between the start
of the current line and the point. This is a non-incremental search.
history-search-backward
Search backward through the history for the string of characters between the start
of the current line and the point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the second word on the
previous line) at point. With an argument n, insert the nth word from the previous
command (the words in the previous command begin with word 0). A negative argument
inserts the nth word from the end of the previous command. Once the argument n is
computed, the argument is extracted as if the "!n" history expansion had been spec‐
ified.
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. Suc‐
cessive calls to yank-last-arg move back through the history list, inserting the
last word (or the word specified by the argument to the first call) of each line in
turn. Any numeric argument supplied to these successive calls determines the
direction to move through the history. A negative argument switches the direction
through the history (back or forward). The history expansion facilities are used
to extract the last word, as if the "!$" history expansion had been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and history expansion as
well as all of the shell word expansions. See HISTORY EXPANSION below for a
description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY EXPANSION below for a
description of history expansion.
magic-space
Perform history expansion on the current line and insert a space. See HISTORY
EXPANSION below for a description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above for a description
of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line relative to the cur‐
rent 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 com‐
mands. Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that
order.

Commands for Changing Text
end-of-file (usually C-d)
The character indicating end-of-file as set, for example, by ``stty''. If this
character is read when there are no characters on the line, and point is at the
beginning of the line, Readline interprets it as the end of input and returns EOF.
delete-char (C-d)
Delete the character at point. If this function is bound to the same character as
the tty EOF character, as C-d commonly is, see above for the effects.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric argument, save the
deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at the end of the line,
in which case the character behind the cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to insert charac‐
ters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at point, moving point
forward as well. If point is at the end of the line, then this transposes the two
characters before point. Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving point over that word
as well. If point is at the end of the line, this transposes the last two words on
the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative argument, uppercase the
previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative argument, lowercase the
previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative argument, capitalize
the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argument, switches to
overwrite mode. With an explicit non-positive numeric argument, switches to insert
mode. This command affects only emacs mode; vi mode does 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 back‐
ward-word.
shell-kill-word (M-d)
Kill from point to the end of the current word, or if between words, to the end of
the next word. Word boundaries are the same as those used by shell-forward-word.
shell-backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those used by
shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary. The killed text
is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash character as the word
boundaries. The killed text is saved on the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word boundaries are the same as
backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word boundaries are the same
as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following yank or yank-pop.

Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a new argument. M--
starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is followed by one or
more digits, optionally with a leading minus sign, those digits define the argu‐
ment. If the command is followed by digits, executing universal-argument again
ends the numeric argument, but is otherwise ignored. As a special case, if this
command is immediately followed by a character that is neither a digit or minus
sign, the argument count for the next command is multiplied by four. The argument
count is initially one, so executing this function the first time makes the argu‐
ment count four, a second time makes the argument count sixteen, and so on.

Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash attempts completion
treating the text as a variable (if the text begins with $), username (if the text
begins with ~), hostname (if the text begins with @), or command (including aliases
and functions) in turn. If none of these produces a match, filename completion is
attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have been generated by
possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a single match from
the list of possible completions. Repeated execution of menu-complete steps
through the list of possible completions, inserting each match in turn. At the end
of the list of completions, the bell is rung (subject to the setting of bell-style)
and the original text is restored. An argument of n moves n positions forward in
the list of matches; a negative argument may be used to move backward through the
list. This command is intended to be bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list of possible comple‐
tions, as if menu-complete had been given a negative argument. This command is
unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or end of the line
(like delete-char). If at the end of the line, behaves identically to possi‐
ble-completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating it as a username.
complete-variable (M-$)
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. Com‐
mand completion attempts to match the text against aliases, reserved words, shell
functions, shell builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating it as a command
name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text against lines from
the history list for possible completion matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the text against lines
from the history list for possible completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible completions enclosed
within braces so the list is available to the shell (see Brace Expansion above).

Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro and store the def‐
inition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the characters in the macro
appear as if typed at the keyboard.
print-last-kbd-macro ()
Print the last keyboard macro defined in a format suitable for the inputrc file.

Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any bindings or variable
assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell (subject to the set‐
ting 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 cor‐
responding 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 charac‐
ter. A negative count searches for subsequent occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as those defined for
keys like Home and End. Such sequences begin with a Control Sequence Indicator
(CSI), usually ESC-[. If this sequence is bound to "\[", keys producing such
sequences will have no effect unless explicitly bound to a readline command,
instead of inserting stray characters into the editing buffer. This is unbound by
default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline comment-begin variable is
inserted at the beginning of the current line. If a numeric argument is supplied,
this command acts as a toggle: if the characters at the beginning of the line do
not match the value of comment-begin, the value is inserted, otherwise the charac‐
ters 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 com‐
ment-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 exe‐
cuted 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
filenames for possible completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expansion, and the list
of matching filenames is inserted, replacing the word. If a numeric argument is
supplied, an asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by glob-expand-word is dis‐
played, and the line is redrawn. If a numeric argument is supplied, an asterisk is
appended before pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline output stream.
If a numeric argument is supplied, the output is formatted in such a way that it
can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and their values to the readline out‐
put stream. If a numeric argument is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the strings they out‐
put. If a numeric argument is supplied, the output is formatted in such a way that
it can be made part of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.

Programmable Completion
When word completion is attempted for an argument to a command for which a completion
specification (a compspec) has been defined using the complete builtin (see SHELL BUILTIN
COMMANDS below), the programmable completion facilities are invoked.

First, the command name is identified. If the command word is the empty string (comple‐
tion attempted at the beginning of an empty line), any compspec defined with the -E option
to complete is used. If a compspec has been defined for that command, the compspec is
used to generate the list of possible completions for the word. If the command word is a
full pathname, a compspec for the full pathname is searched for first. If no compspec is
found for the full pathname, an attempt is made to find a compspec for the portion follow‐
ing the final slash. If those searches do not result in a compspec, any compspec defined
with the -D option to complete is used as the default.

Once a compspec has been found, it is used to generate the list of matching words. If a
compspec is not found, the default bash completion as described above under Completing is
performed.

First, the actions specified by the compspec are used. Only matches which are prefixed by
the word being completed are returned. When the -f or -d option is used for filename or
directory name completion, the shell variable FIGNORE is used to filter the matches.

Any completions specified by a pathname expansion pattern to the -G option are generated
next. The words generated by the pattern need not match the word being completed. The
GLOBIGNORE shell variable is not used to filter the matches, but the FIGNORE variable is
used.

Next, the string specified as the argument to the -W option is considered. The string is
first split using the characters in the IFS special variable as delimiters. Shell quoting
is honored. Each word is then expanded using brace expansion, tilde expansion, parameter
and variable expansion, command substitution, and arithmetic expansion, as described above
under EXPANSION. The results are split using the rules described above under Word Split‐
ting. The results of the expansion are prefix-matched against the word being completed,
and the matching words become the possible completions.

After these matches have been generated, any shell function or command specified with the
-F and -C options is invoked. When the command or function is invoked, the COMP_LINE,
COMP_POINT, COMP_KEY, and COMP_TYPE variables are assigned values as described above under
Shell Variables. If a shell function is being invoked, the COMP_WORDS and COMP_CWORD
variables are also set. When the function or command is invoked, the first argument ($1)
is the name of the command whose arguments are being completed, the second argument ($2)
is the word being completed, and the third argument ($3) is the word preceding the word
being completed on the current command line. No filtering of the generated completions
against the word being completed is performed; the function or command has complete free‐
dom 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, one per array element.

Next, any command specified with the -C option is invoked in an environment equivalent to
command substitution. It should print a list of completions, one per line, to the stan‐
dard 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 attempting a match. Any com‐
pletion that matches the pattern will be removed from the list. A leading ! negates the
pattern; in this case any completion not matching the pattern will be removed.

Finally, any prefix and suffix specified with the -P and -S options are added to each mem‐
ber of the completion list, and the result is returned to the readline completion code as
the list of possible completions.

If the previously-applied actions do not generate any matches, and the -o dirnames option
was supplied to complete when the compspec was defined, directory name completion is
attempted.

If the -o plusdirs option was supplied to complete when the compspec was defined, direc‐
tory name completion is attempted and any matches are added to the results of the other
actions.

By default, if a compspec is found, whatever it generates is returned to the completion
code as the full set of possible completions. The default bash completions are not
attempted, and the readline default of filename completion is disabled. If the -o bashde‐
fault option was supplied to complete when the compspec was defined, the bash default com‐
pletions are attempted if the compspec generates no matches. If the -o default option was
supplied to complete when the compspec was defined, readline's default completion will be
performed if the compspec (and, if attempted, the default bash completions) generate no
matches.

When a compspec indicates that directory name completion is desired, the programmable com‐
pletion functions force readline to append a slash to completed names which are symbolic
links to directories, subject to the value of the mark-directories readline variable,
regardless of the setting of the mark-symlinked-directories readline variable.

There is some support for dynamically modifying completions. This is most useful when
used in combination with a default completion specified with complete -D. It's possible
for shell functions executed as completion handlers to indicate that completion should be
retried by returning an exit status of 124. If a shell function returns 124, and changes
the compspec associated with the command on which completion is being attempted (supplied
as the first argument when the function is executed), programmable completion restarts
from the beginning, with an attempt to find a new compspec for that command. This allows
a set of completions to be built dynamically as completion is attempted, rather than being
loaded all at once.

For instance, assuming that there is a library of compspecs, each kept in a file corre‐
sponding to the name of the command, the following default completion function would load
completions dynamically:

_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default

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 vari‐
able 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 HIST‐
CONTROL.

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 nec‐
essary, to contain no more than the number of lines specified by the value of HISTFILE‐
SIZE. If HISTFILESIZE is unset, or set to null, a non-numeric value, or a numeric value
less than zero, the history file is not truncated. When the history file is read, lines
beginning with the history comment character followed immediately by a digit are inter‐
preted as timestamps for the preceding history line. These timestamps are optionally dis‐
played depending on the value of the HISTTIMEFORMAT variable. When a shell with history
enabled exits, the last $HISTSIZE lines are copied from the history list to $HISTFILE. If
the histappend shell option is enabled (see the description of shopt under SHELL BUILTIN
COMMANDS below), the lines are appended to the history file, otherwise the history file is
overwritten. If HISTFILE is unset, or if the history file is unwritable, the history is
not saved. If the HISTTIMEFORMAT 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 unset, or set to null, a non-numeric value,
or a numeric value less than zero, the history file is not truncated.

The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used to list or edit and
re-execute a portion of the history list. The history builtin may be used to display or
modify the history list and manipulate the history file. When using command-line editing,
search commands are available in each editing mode that provide access to the history
list.

The shell allows control over which commands are saved on the history list. The HISTCON‐
TROL and HISTIGNORE variables may be set to cause the shell to save only a subset of the
commands entered. The cmdhist shell option, if enabled, causes the shell to attempt to
save each line of a multi-line command in the same history entry, adding semicolons where
necessary to preserve syntactic correctness. The lithist shell option causes the shell to
save the command with embedded newlines instead of semicolons. See the description of the
shopt builtin below under SHELL BUILTIN COMMANDS for information on setting and unsetting
shell options.

HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the history expansion in
csh. This section describes what syntax features are available. This feature is enabled
by default for interactive shells, and can be disabled using the +H option to the set
builtin command (see SHELL BUILTIN COMMANDS below). Non-interactive shells do not perform
history expansion by default.

History expansions introduce words from the history list into the input stream, making it
easy to repeat commands, insert the arguments to a previous command into the current input
line, or fix errors in previous commands quickly.

History expansion is performed immediately after a complete line is read, before the shell
breaks it into words. It takes place in two parts. The first is to determine which line
from the history list to use during substitution. The second is to select portions of
that line for inclusion into the current one. The line selected from the history is the
event, and the portions of that line that are acted upon are words. Various modifiers are
available to manipulate the selected words. The line is broken into words in the same
fashion as when reading input, so that several metacharacter-separated words 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 imme‐
diately passed to the shell parser. Instead, the expanded line is reloaded into the read‐
line editing buffer for further modification. If readline is being used, and the
histreedit shell option is enabled, a failed history substitution will be reloaded into
the readline editing buffer for correction. The -p option to the history builtin command
may be used to see what a history expansion will do before using it. The -s option to the
history builtin may be used to add commands to the end of the history list without actu‐
ally executing them, so that they are available for subsequent recall.

The shell allows control of the various characters used by the history expansion mechanism
(see the description of histchars above under Shell Variables). The shell uses the his‐
tory comment character to mark history timestamps when writing the history file.

Event Designators
An event designator is a reference to a command line entry in the history list. Unless
the reference is absolute, events are relative to the current position in the history
list.

! Start a history substitution, except when followed by a blank, newline, carriage
return, = or ( (when the extglob shell option is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command preceding the current position in the history list
starting with string.
!?string[?]
Refer to the most recent command preceding the current position in the history list
containing string. The trailing ? may be omitted if string is followed immediately
by a newline.
^string1^string2^
Quick substitution. Repeat the previous command, replacing string1 with string2.
Equivalent to ``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.

Word Designators
Word designators are used to select desired words from the event. A : separates the event
specification from the word designator. It may be omitted if the word designator begins
with a ^, $, *, -, or %. Words are numbered from the beginning of the line, with the
first word being denoted by 0 (zero). Words are inserted into the current line separated
by single spaces.

0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last word. This is usually the last argument, but will expand to the zeroth
word if there is only one word in the line.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for `1-$'. It is not an error
to use * if there is just one word in the event; the empty string is returned in
that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.

If a word designator is supplied without an event specification, the previous command is
used as the event.

Modifiers
After the optional word designator, there may appear a sequence of one or more of the fol‐
lowing modifiers, each preceded by a `:'.

h Remove a trailing filename component, leaving only the head.
t Remove all leading filename components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further 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 char‐
acter 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 pre‐
vious 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 conjunc‐
tion with `:s' (e.g., `:gs/old/new/') or `:&'. If used with `:s', any delimiter
can be used in place of /, and the final delimiter is optional if it is the last
character of the event line. An a may be used as a synonym for g.
G Apply the following `s' modifier once to each word in the event line.

SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accepting
options preceded by - accepts -- to signify the end of the options. The :, true, false,
and test builtins do not accept options and do not treat -- specially. The exit, logout,
break, continue, let, and shift builtins accept and process arguments beginning with -
without requiring --. Other builtins that accept arguments but are not specified as
accepting options interpret arguments beginning with - as invalid options and require --
to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments and performing any
specified redirections. A zero exit code is returned.

. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and return
the exit status of the last command executed from filename. If filename does not
contain a slash, filenames in PATH are used to find the directory containing file‐
name. 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 arguments are supplied, they become the positional parameters
when filename is executed. Otherwise the positional parameters are unchanged. The
return status is the status of the last command exited within the script (0 if no
commands are executed), and false if filename is not found or cannot be read.

alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of aliases in the
form alias name=value on standard output. When arguments are supplied, an alias is
defined for each name whose value is given. A trailing space in value causes the
next word to be checked for alias substitution when the alias is expanded. For
each name in the argument list for which no value is supplied, the name and value
of the alias is printed. Alias returns true unless a name is given for which no
alias has been defined.

bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started with
&. If jobspec is not present, the shell's notion of the current job is used. bg
jobspec returns 0 unless run when job control is disabled or, when run with job
control enabled, any specified jobspec was not found or was started without job
control.

bind [-m keymap] [-lpsvPSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings, bind a key sequence to a read‐
line function or macro, or set a readline variable. Each non-option argument is a
command as it would appear in .inputrc, but each binding or command must be passed
as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'. Options, if sup‐
plied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent bindings. Accept‐
able 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.
-X List all key sequences bound to shell commands and the associated commands
in a format that can be reused as input.

The return value is 0 unless an unrecognized option is given or an error occurred.

break [n]
Exit from within a for, while, until, or select loop. If n is specified, break n
levels. n must be ≥ 1. If n is greater than the number of enclosing loops, all
enclosing loops are exited. The return value is 0 unless n is not greater than or
equal to 1.

builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and return its exit sta‐
tus. This is useful when defining a function whose name is the same as a shell
builtin, retaining the functionality of the builtin within the function. The cd
builtin is commonly redefined this way. The return status is false if
shell-builtin is not a shell builtin command.

caller [expr]
Returns the context of any active subroutine call (a shell function or a script
executed with the . or source builtins). Without expr, caller displays the line
number and source filename of the current subroutine call. If a non-negative inte‐
ger is supplied as expr, caller displays the line number, subroutine name, and
source file corresponding to that position in the current execution call stack.
This extra information may be used, for example, to print a stack trace. The cur‐
rent frame is frame 0. The return value is 0 unless the shell is not executing a
subroutine call or expr does not correspond to a valid position in the call stack.

cd [-L|[-P [-e]] [-@]] [dir]
Change the current directory to dir. if dir is not supplied, the value of the HOME
shell variable is the default. Any additional arguments following dir are ignored.
The variable CDPATH defines the search path for the directory containing dir: each
directory name in CDPATH is searched for dir. Alternative directory names in
CDPATH are separated by a colon (:). A null directory name in CDPATH is the same
as the current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH
is not used. The -P option causes cd to use the physical directory structure by
resolving symbolic links while traversing dir and before processing instances of ..
in dir (see also the -P option to the set builtin command); the -L option forces
symbolic links to be followed by resolving the link after processing instances of
.. in dir. If .. appears in dir, it is processed by removing the immediately pre‐
vious pathname component from dir, back to a slash or the beginning of dir. If the
-e option is supplied with -P, and the current working directory cannot be success‐
fully determined after a successful directory change, cd will return an unsuccess‐
ful status. On systems that support it, the -@ option presents the extended
attributes associated with a file as a directory. An argument of - is converted to
$OLDPWD before the directory change is attempted. If a non-empty directory name
from CDPATH is used, or if - is the first argument, and the directory change is
successful, the absolute 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 guaran‐
teed to find all of the standard utilities. If either the -V or -v option is sup‐
plied, a description of command is printed. The -v option causes a single word
indicating the command or filename used to invoke command to be displayed; the -V
option produces a more verbose description. If the -V or -v option is supplied,
the exit status is 0 if command was found, and 1 if not. If neither option is sup‐
plied and an error occurred or command cannot be found, the exit status is 127.
Otherwise, the exit status of the command builtin is the exit status of command.

compgen [option] [word]
Generate possible completion matches for word according to the options, which may
be any option accepted by the complete builtin with the exception of -p and -r, and
write the matches to the standard output. When using the -F or -C options, the
various shell variables set by the programmable completion facilities, while avail‐
able, will not have useful values.

The matches will be generated in the same way as if the programmable completion
code had generated them directly from a completion specification with the same
flags. If word is specified, only those completions matching word will be dis‐
played.

The return value is true unless an invalid option is supplied, or no matches were
generated.

complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G globpat] [-W wordlist] [-F
function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DE] [name ...]
Specify how arguments to each name should be completed. If the -p option is sup‐
plied, 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 comple‐
tion 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 generation of completions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions if the compspec
generates no matches.
default Use readline's default filename completion if the compspec gener‐
ates no matches.
dirnames
Perform directory name completion if the compspec generates no
matches.
filenames
Tell readline that the compspec generates filenames, so it can per‐
form any filename-specific processing (like adding a slash to
directory names, quoting special characters, or suppressing trail‐
ing spaces). Intended to be used with shell functions.
noquote Tell readline not to quote the completed words if they are file‐
names (quoting filenames is the default).
nospace Tell readline not to append a space (the default) to words com‐
pleted at the end of the line.
plusdirs
After any matches defined by the compspec are generated, directory
name completion is attempted and any matches are added to the
results of the other actions.
-A action
The action may be one of the following to generate a list of possible com‐
pletions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the HOSTFILE shell
variable.
job Job names, if job control is active. May also be specified as -j.
keyword Shell reserved words. May also be specified as -k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set builtin.
shopt Shell option names as accepted by the shopt builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be specified as -v.
-C command
command is executed in a subshell environment, and its output is used as
the possible completions.
-F function
The shell function function is executed in the current shell environment.
When the function is executed, the first argument ($1) is the name of the
command whose arguments are being completed, the second argument ($2) is
the word being completed, and the third argument ($3) is the word preceding
the word being completed on the current command line. When it finishes,
the possible completions are retrieved from the value of the COMPREPLY
array variable.
-G globpat
The pathname expansion pattern globpat is expanded to generate the possible
completions.
-P prefix
prefix is added at the beginning of each possible completion after all
other options have been applied.
-S suffix
suffix is appended to each possible completion after all other options have
been applied.
-W wordlist
The wordlist is split using the characters in the IFS special variable as
delimiters, and each resultant word is expanded. The possible completions
are the members of the resultant list which match the word being completed.
-X filterpat
filterpat is a pattern as used for pathname expansion. It is applied to
the list of possible completions generated by the preceding options and
arguments, and each completion matching filterpat is removed from the list.
A leading ! in filterpat negates the pattern; in this case, any completion
not matching filterpat is removed.

The return value is true unless an invalid option is supplied, an option other than
-p or -r is supplied without a name argument, an attempt is made to remove a com‐
pletion specification for a name for which no specification exists, or an error
occurs adding a completion specification.

compopt [-o option] [-DE] [+o option] [name]
Modify completion options for each name according to the options, or for the cur‐
rently-executing completion if no names are supplied. If no options are given,
display the completion options for each name or the current completion. The possi‐
ble values of option are those valid for the complete builtin described above. The
-D option indicates that the remaining options should apply to the ``default'' com‐
mand completion; that is, completion attempted on a command for which no completion
has previously been defined. The -E option indicates that the remaining options
should apply to ``empty'' command completion; that is, completion attempted on a
blank line.

The return value is true unless an invalid option is supplied, an attempt is made
to modify the options for a name for which no completion specification exists, or
an output error occurs.

continue [n]
Resume the next iteration of the enclosing for, while, until, or select loop. If n
is specified, resume at the nth enclosing loop. n must be ≥ 1. If n is greater
than the number of enclosing loops, the last enclosing loop (the ``top-level''
loop) is resumed. The return value is 0 unless n is not greater than or equal to
1.

declare [-aAfFgilnrtux] [-p] [name[=value] ...]
typeset [-aAfFgilnrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are given then display
the values of variables. The -p option will display the attributes and values of
each name. When -p is used with name arguments, additional options, other than -f
and -F, are ignored. When -p is supplied without name arguments, it will display
the attributes and values of all variables having the attributes specified by the
additional options. If no other options are supplied with -p, declare will display
the attributes and values of all shell variables. The -f option will restrict the
display to shell functions. The -F option inhibits the display of function defini‐
tions; only the function name and attributes are printed. If the extdebug shell
option is enabled using shopt, the source file name and line number where the func‐
tion is defined are displayed as well. The -F option implies -f. The -g option
forces variables to be created or modified at the global scope, even when declare
is executed in a shell function. It is ignored in all other cases. The following
options can be used to restrict output to variables with the specified attribute or
to give variables attributes:
-a Each name is an indexed array variable (see Arrays above).
-A Each name is an associative array variable (see Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC
EVALUATION above) is performed when the variable is assigned a value.
-l When the variable is assigned a value, all upper-case characters are con‐
verted to lower-case. The upper-case attribute is disabled.
-n Give each name the nameref attribute, making it a name reference to another
variable. That other variable is defined by the value of name. All refer‐
ences and assignments to name, except for changing the -n attribute itself,
are performed on the variable referenced by name's value. The -n attribute
cannot be applied to array variables.
-r Make names readonly. These names cannot then be assigned values by subse‐
quent assignment statements or unset.
-t Give each name the trace attribute. Traced functions inherit the DEBUG and
RETURN traps from the calling shell. The trace attribute has no special
meaning for variables.
-u When the variable is assigned a value, all lower-case characters are con‐
verted 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, declare and typeset make each name local, as
with the local command, unless the -g option is supplied. If a variable name is
followed by =value, the value of the variable is set to value. When using -a or -A
and the compound assignment syntax to create array variables, additional attributes
do not take effect until subsequent assignments. The return value is 0 unless an
invalid option is encountered, an attempt is made to define a function using ``-f
foo=bar'', an attempt is made to assign a value to a readonly variable, an attempt
is made to assign a value to an array variable without using the compound assign‐
ment syntax (see Arrays above), one of the names is not a valid shell variable
name, an attempt is made to turn off readonly status for a readonly variable, an
attempt is made to turn off array status for an array variable, or an attempt is
made to display a non-existent function with -f.

dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remembered directories. The
default display is on a single line with directory names separated by spaces.
Directories are added to the list with the pushd command; the popd command removes
entries from the list.
-c Clears the directory stack by deleting all of the entries.
-l Produces a listing using full pathnames; the default listing format uses a
tilde to denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, prefixing each entry with
its index in the stack.
+n Displays the nth entry counting from the left of the list shown by dirs when
invoked without options, starting with zero.
-n Displays the nth entry counting from the right of the list shown by dirs
when invoked without options, starting with zero.

The return value is 0 unless an invalid option is supplied or n indexes beyond the
end of the directory stack.

disown [-ar] [-h] [jobspec ...]
Without options, remove each jobspec from the table of active jobs. If jobspec is
not present, and neither the -a nor the -r option is supplied, the current job is
used. If the -h option is given, each jobspec is not removed from the table, but
is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If
no jobspec is supplied, the -a option means to remove or mark all jobs; the -r
option without a jobspec argument 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
0 unless a write error occurs. If -n is specified, the trailing newline is sup‐
pressed. If the -e option is given, interpretation of the following backslash-
escaped characters is enabled. The -E option disables the interpretation of these
escape characters, even on systems where they are interpreted by default. The
xpg_echo shell option may be used to dynamically determine whether or not echo
expands these escape characters by default. echo does not interpret -- to mean the
end of options. echo interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value nnn (zero to three
octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two
hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH (one to eight hex digits)

enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin allows a disk com‐
mand 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 com‐
mand name from shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin previously loaded with -f. If no name argu‐
ments 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 sup‐
plied, the list printed includes all builtins, with an indication of whether or not
each is enabled. If -s is supplied, the output is restricted to the POSIX special
builtins. The return value is 0 unless a name is not a shell builtin or there is
an error loading a new builtin from a shared object.

eval [arg ...]
The args are read and concatenated together into a single command. This command is
then read and executed by the shell, and its exit status is returned as the value
of eval. If there are no args, or only null arguments, eval returns 0.

exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process is created. The
arguments become the arguments to command. If the -l option is supplied, the shell
places a dash at the beginning of the zeroth argument passed to command. This is
what login(1) does. The -c option causes command to be executed with an empty
environment. If -a is supplied, the shell passes name as the zeroth argument to
the executed command. If command cannot be executed for some reason, a non-inter‐
active shell exits, unless the execfail shell option is enabled. In that case, it
returns failure. An interactive shell returns failure if the file cannot be exe‐
cuted. If command is not specified, any redirections take effect in the current
shell, and the return status is 0. If there is a redirection error, the return
status is 1.

exit [n]
Cause the shell to exit with a status of n. If n is omitted, the exit status is
that of the last command executed. A trap on EXIT is executed before the shell
terminates.

export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the environment of subse‐
quently executed commands. If the -f option is given, the names refer to func‐
tions. If no names are given, or if the -p option is supplied, a list of names of
all exported variables is printed. The -n option causes the export property to be
removed from each name. If a variable name is followed by =word, the value of the
variable is set to word. export returns an exit status of 0 unless an invalid
option is encountered, one of the names is not a valid shell variable name, or -f
is supplied with a name that is not a function.

fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
The first form selects a range of commands from first to last from the history list
and displays or edits and re-executes them. First and last may be specified as a
string (to locate the last command beginning with that string) or as a number (an
index into the history list, where a negative number is used as an offset from the
current command number). If last is not specified it is set to the current command
for listing (so that ``fc -l -10'' prints the last 10 commands) and to first other‐
wise. If first is not specified it is set to the previous command for editing and
-16 for listing.

The -n option suppresses the command numbers when listing. The -r option reverses
the order of the commands. If the -l option is given, the commands are listed on
standard output. Otherwise, the editor given by ename is invoked on a file con‐
taining those commands. If ename is not given, the value of the FCEDIT variable is
used, and the value of EDITOR if FCEDIT is not set. If neither variable is set, vi
is used. When editing is complete, the edited commands are echoed and executed.

In the second form, command is re-executed after each instance of pat is replaced
by rep. Command is intepreted the same as first above. A useful alias to use with
this is ``r="fc -s"'', so that typing ``r cc'' runs the last command beginning with
``cc'' and typing ``r'' re-executes the last command.

If the first form is used, the return value is 0 unless an invalid option is
encountered or first or last specify 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 con‐
tains the option characters to be recognized; if a character is followed by a
colon, the option is expected to have an argument, which should be separated from
it by white space. The colon and question mark characters may not be used as
option characters. Each time it is invoked, getopts places the next option in the
shell variable name, initializing name if it does not exist, and the index of the
next argument to be processed into the variable OPTIND. OPTIND is initialized to 1
each time the shell or a shell script is invoked. When an option requires an argu‐
ment, getopts places that argument into the variable OPTARG. The shell does not
reset OPTIND automatically; it must be manually reset between multiple calls to
getopts within the same shell invocation if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a return value greater
than zero. OPTIND is set to the index of the first non-option argument, and name
is set to ?.

getopts normally parses the positional parameters, but if more arguments are given
in args, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a
colon, silent error reporting is used. In normal operation, diagnostic messages
are printed when invalid options or missing option arguments are encountered. If
the variable OPTERR is set to 0, no error messages will be displayed, even if the
first character of optstring is not a colon.

If an invalid option is seen, getopts places ? into name and, if not silent, prints
an error message and unsets OPTARG. If getopts is silent, the option character
found is placed in OPTARG and no diagnostic message is printed.

If a required argument is not found, and getopts is not silent, a question mark (?)
is placed in name, OPTARG is unset, and a diagnostic message is printed. If
getopts is silent, then a colon (:) is placed in name and OPTARG is set to the
option character found.

getopts returns true if an option, specified or unspecified, is found. It returns
false if the end of options is encountered or an error occurs.

hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, the full pathname of the command name is determined by
searching the directories in $PATH and remembered. Any previously-remembered path‐
name is discarded. If the -p option is supplied, no path search is performed, and
filename is used as the full filename of the command. The -r option causes the
shell to forget all remembered locations. The -d option causes the shell to forget
the remembered location of each name. If the -t option is supplied, the full path‐
name 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 supplied.

help [-dms] [pattern]
Display helpful information about builtin commands. If pattern is specified, help
gives detailed help on all commands matching pattern; otherwise help for all the
builtins and shell control structures is printed.
-d Display a short description of each pattern
-m Display the description of each pattern in a manpage-like format
-s Display only a short usage synopsis for each pattern

The return status is 0 unless no command matches pattern.

history [n]
history -c
history -d offset
history -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 cur‐
rent history list. These are lines appended to the history file since the
beginning of the current bash session.
-r Read the contents of the history file and append them to the current history
list.
-w Write the current history list to the history file, overwriting the history
file's contents.
-p Perform history substitution on the following args and display the result on
the standard output. Does not store the results in the history list. Each
arg must be quoted to disable normal history expansion.
-s Store the args in the history list as a single entry. The last command in
the history list is removed before the args are added.

If the HISTTIMEFORMAT variable is set, the time stamp information associated with
each history entry is written to the history file, marked with the history comment
character. When the history file is read, lines beginning with the history comment
character followed immediately by a digit are interpreted as timestamps for the
previous history line. The return value is 0 unless an invalid option is encoun‐
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 Display only running jobs.
-s Display only stopped jobs.

If jobspec is given, output is restricted to information about that job. The
return status is 0 unless an invalid option is encountered or an invalid jobspec is
supplied.

If the -x option is supplied, jobs replaces any jobspec found in command or args
with the corresponding process group ID, and executes command passing it args,
returning its exit status.

kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes named by pid or job‐
spec. 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 corre‐
sponding 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 vari‐
ables 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.
-C Evaluate callback each time quantum lines are read. The -c option specifies
quantum.
-c Specify the number of lines read between each call to callback.

If -C is specified without -c, the default quantum is 5000. When callback is eval‐
uated, it is supplied the index of the next array element to be assigned and the
line to be assigned to that element as additional arguments. callback is evaluated
after the line is read but before the array element is assigned.

If not supplied with an explicit origin, mapfile will clear array before assigning
to it.

mapfile returns successfully unless an invalid option or option argument is sup‐
plied, array is invalid or unassignable, or if array is not an indexed array.

popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments, removes the top
directory from the stack, and performs a cd to the new top directory. Arguments,
if supplied, have the following meanings:
-n Suppresses the normal change of directory when removing directories from the
stack, so that only the stack is manipulated.
+n Removes the nth entry counting from the left of the list shown by dirs,
starting with zero. For example: ``popd +0'' removes the first directory,
``popd +1'' the second.
-n Removes the nth entry counting from the right of the list shown by dirs,
starting with zero. For example: ``popd -0'' removes the last directory,
``popd -1'' the next to last.

If the popd command is successful, a dirs is performed as well, and the return sta‐
tus is 0. popd returns false if an invalid option is encountered, the directory
stack is empty, a non-existent directory stack entry is specified, or the directory
change fails.

printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the control of the for‐
mat. 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 char‐
acters, which are simply copied to standard output, character escape sequences,
which are converted and copied to the standard output, and format specifications,
each of which causes printing of the next successive argument. In addition to the
standard printf(1) format specifications, printf interprets the following exten‐
sions:
%b causes printf to expand backslash escape sequences in the corresponding
argument (except that \c terminates output, backslashes in \', \", and \?
are not removed, and octal escapes beginning with \0 may contain up to four
digits).
%q causes printf to output the corresponding argument in a format that can be
reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting from using datefmt as
a format string for strftime(3). The corresponding argument is an integer
representing the number of seconds since the epoch. Two special argument
values may be used: -1 represents the current time, and -2 represents the
time the shell was invoked. If no argument is specified, conversion behaves
as if -1 had been given. This is an exception to the usual printf behavior.

Arguments to non-string format specifiers are treated as C constants, except that a
leading plus or minus sign is allowed, and if the leading character is a single or
double quote, the value is the ASCII value of the following character.

The format is reused as necessary to consume all of the arguments. If the format
requires more arguments than are supplied, the extra format specifications behave
as if a zero value or null string, as appropriate, had been 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.
dir Adds dir to the directory stack at the top, making it the new current work‐
ing directory as if it had been supplied as the argument to the cd builtin.

If the pushd command is successful, a dirs is performed as well. If the first form
is used, pushd returns 0 unless the cd to dir fails. With the second form, pushd
returns 0 unless the directory stack is empty, a non-existent directory stack ele‐
ment is specified, or the directory change to the specified new current directory
fails.

pwd [-LP]
Print the absolute pathname of the current working directory. The pathname printed
contains no symbolic links if the -P option is supplied or the -o physical option
to the set builtin command is enabled. If the -L option is used, the pathname
printed may contain symbolic links. The return status is 0 unless an error occurs
while reading the name of the current directory or an invalid option is supplied.

read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t time‐
out] [-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 inter‐
vening separators assigned to the last name. If there are fewer words read from
the input stream than names, the remaining names are assigned empty values. The
characters in IFS are used to split the line into words using the same rules the
shell uses for expansion (described above under Word Splitting). The backslash
character (\) may be used to remove any special meaning for the next character read
and for line continuation. Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array variable aname,
starting at 0. aname is unset before any new values are assigned. Other
name arguments are ignored.
-d delim
The first character of delim is used to terminate the input line, rather
than newline.
-e If the standard input is coming from a terminal, readline (see READLINE
above) is used to obtain the line. Readline uses the current (or default,
if line editing was not previously active) editing settings.
-i text
If readline is being used to read the line, text is placed into the editing
buffer before editing begins.
-n nchars
read returns after reading nchars characters rather than waiting for a com‐
plete 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 com‐
ing 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 (or a
specified number of characters) is not read within timeout seconds. timeout
may be a decimal number with a fractional portion following the decimal
point. This option is only effective if read is reading input from a termi‐
nal, pipe, or other special file; it has no effect when reading from regular
files. If read times out, read saves any partial input read into the speci‐
fied variable name. If timeout is 0, read returns immediately, without try‐
ing to read any data. The exit status is 0 if input is available on the
specified file descriptor, non-zero otherwise. The exit status is greater
than 128 if the timeout is exceeded.
-u fd Read input from file descriptor fd.

If no names are supplied, the line read 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), a variable assignment error (such as
assigning to a readonly variable) occurs, or an invalid file descriptor is supplied
as the argument to -u.

readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these names may not be changed
by subsequent assignment. If the -f option is supplied, the functions correspond‐
ing to the names are so marked. The -a option restricts the variables to indexed
arrays; the -A option restricts the variables to associative arrays. If both
options are supplied, -A takes precedence. If no name arguments are given, or if
the -p option is supplied, a list of all readonly names is printed. The other
options may be used to restrict the output to a subset of the set of readonly
names. The -p option causes output to be displayed in a format that may be reused
as input. If a variable name is followed by =word, the value of the variable is
set to word. The return status is 0 unless an invalid option is encountered, one
of the names is not a valid shell variable name, or -f is supplied with a name that
is not a function.

return [n]
Causes a function to stop executing and return the value specified by n to its
caller. If n is omitted, the return status is that of the last command executed in
the function body. If return is used outside a function, but during execution of a
script by the . (source) command, it causes the shell to stop executing that
script and return either n or the exit status of the last command executed within
the script as the exit status of the script. If n is supplied, the return value is
its least significant 8 bits. The return status is non-zero if return is supplied
a non-numeric argument, or is used outside a function and not during execution of a
script by . or source. Any command associated with the 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 for‐
mat that can be reused as input for setting or resetting the currently-set vari‐
ables. Read-only variables cannot be reset. In posix mode, only shell variables
are listed. The output is sorted according to the current locale. When options
are specified, they set or unset shell attributes. Any arguments remaining after
option processing are treated as values for the positional parameters and are
assigned, in order, to $1, $2, ... $n. Options, if specified, have the following
meanings:
-a Automatically mark variables and functions which are modified or created
for export to the environment of subsequent commands.
-b Report the status of terminated background jobs immediately, rather than
before the next primary prompt. This is effective only when job control is
enabled.
-e Exit immediately if a pipeline (which may consist of a single simple com‐
mand), a list, or a compound command (see SHELL GRAMMAR above), exits with
a non-zero status. The shell does not exit if the command that fails is
part of the command list immediately following a while or until keyword,
part of the test following the if or elif reserved words, part of any com‐
mand executed in a && or || list except the command following the final &&
or ||, any command in a pipeline but the last, or if the command's return
value is being inverted with !. If a compound command other than a sub‐
shell returns a non-zero status because a command failed while -e was being
ignored, the shell does not exit. A trap on ERR, if set, is executed
before the shell exits. This option applies to the shell environment and
each subshell environment separately (see COMMAND EXECUTION ENVIRONMENT
above), and may cause subshells to exit before executing all the commands
in the subshell.

If a compound command or shell function executes in a context where -e is
being ignored, none of the commands executed within the compound command or
function body will be affected by the -e setting, even if -e is set and a
command returns a failure status. If a compound command or shell function
sets -e while executing in a context where -e is ignored, that setting will
not have any effect until the compound command or the command containing
the function call completes.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up for execution.
This is enabled by default.
-k All arguments in the form of assignment statements are placed in the envi‐
ronment for a command, not just those that precede the command name.
-m Monitor mode. Job control is enabled. This option is on by default for
interactive shells on systems that support it (see JOB CONTROL above). All
processes run in a separate process group. When a background job com‐
pletes, the shell prints a line containing its exit status.
-n Read commands but do not execute them. This may be used to check a shell
script for syntax errors. This is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing interface. This is enabled
by default when the shell is interactive, unless the shell is
started with the --noediting option. This also affects the editing
interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under HISTORY. This
option is on by default in interactive shells.
ignoreeof
The effect is as if the shell command ``IGNOREEOF=10'' had been
executed (see Shell Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the value of the last
(rightmost) command to exit with a non-zero status, or zero if all
commands in the pipeline exit successfully. This option is dis‐
abled by default.
posix Change the behavior of bash where the default operation differs
from the POSIX standard to match the standard (posix mode). See
SEE ALSO below for a reference to a document that details how posix
mode affects bash's behavior.
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing 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 com‐
mands to recreate the current option settings is displayed on the standard
output.
-p Turn on privileged mode. In this mode, the $ENV and $BASH_ENV files are
not processed, shell functions are not inherited from the environment, and
the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear
in the environment, are ignored. If the shell is started with the effec‐
tive user (group) id not equal to the real user (group) id, and the -p
option is not supplied, these actions are taken and the effective user id
is set to the real user id. If the -p option is supplied at startup, the
effective user id is not reset. Turning this option off causes the effec‐
tive user and group ids to be set to the real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the special parameters "@"
and "*" as an error when performing parameter expansion. If expansion is
attempted on an unset variable or parameter, the shell prints an error mes‐
sage, 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 com‐
mand, or arithmetic for command, display the expanded value of PS4, fol‐
lowed by the command and its expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace Expansion above). This is on
by default.
-C If set, bash does not overwrite an existing file with the >, >&, and <> re‐
direction 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 substitu‐
tions, and commands executed in a subshell environment. The ERR trap is
normally not inherited in such cases.
-H Enable ! style history substitution. This option is on by default when
the shell is interactive.
-P If set, the shell does not resolve symbolic links when executing commands
such as cd that change the current working directory. It uses the physical
directory structure instead. By default, bash follows the logical chain of
directories when performing commands which change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited by shell functions,
command substitutions, and commands executed in a subshell environment.
The DEBUG and RETURN traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional parameters are
unset. Otherwise, the positional parameters are set to the args, even if
some of them begin with a -.
- Signal the end of options, cause all remaining args to be assigned to the
positional parameters. The -x and -v options are turned off. If there are
no args, the positional parameters remain unchanged.

The options are off by default unless otherwise noted. Using + rather than -
causes these options to be turned off. The options can also be specified as argu‐
ments to an invocation of the shell. The current set of options may be found in
$-. The return status is always true unless an invalid option is encountered.

shift [n]
The positional parameters from n+1 ... are renamed to $1 .... Parameters repre‐
sented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number
less than or equal to $#. If n is 0, no parameters are changed. If n is not
given, it is assumed to be 1. If n is greater than $#, the positional parameters
are not changed. The return status is greater than zero if n is greater than $# or
less than zero; otherwise 0.

shopt [-pqsu] [-o] [optname ...]
Toggle the values of settings controlling optional shell behavior. The settings
can be either those listed below, or, if the -o option is used, those available
with the -o option to the set builtin command. With no options, or with the -p
option, a list of all settable options is displayed, with an indication of whether
or not each is set. The -p option causes output to be displayed in a form that may
be reused as input. Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status indicates whether
the optname is set or unset. If multiple optname arguments are given with
-q, the return status is zero if all optnames are enabled; non-zero other‐
wise.
-o Restricts the values of optname to be those defined for the -o option to the
set builtin.

If either -s or -u is used with no optname arguments, shopt shows only those
options which are set or unset, respectively. Unless otherwise noted, the shopt
options are disabled (unset) by default.

The return status when listing options is zero if all optnames are enabled, non-
zero otherwise. When setting or unsetting options, the return status is zero
unless an optname is not a valid shell option.

The list of shopt options is:

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 interac‐
tive shells.
cdable_vars
If set, an argument to the cd builtin command that is not a directory is
assumed to be the name of a variable whose value is the directory to change
to.
cdspell If set, minor errors in the spelling of a directory component in a cd com‐
mand will be corrected. The errors checked for are transposed characters,
a missing character, and one character too many. If a correction is found,
the corrected filename is printed, and the command proceeds. This option
is only used by interactive shells.
checkhash
If set, bash checks that a command found in the hash table exists before
trying to execute it. If a hashed command no longer exists, a normal path
search is performed.
checkjobs
If set, bash lists the status of any stopped and running jobs before exit‐
ing 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 and locale-
specific string comparison when using the [[ conditional command's < and >
operators. Bash versions prior to bash-4.1 use ASCII collation and str‐
cmp(3); bash-4.1 and later use the current locale's collation sequence and
strcoll(3).
compat32
If set, bash changes its behavior to that of version 3.2 with respect to
locale-specific string comparison when using the [[ conditional command's <
and > operators (see previous item).
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 description of compat31) and the effect of interrupt‐
ing a command list. Bash versions 4.0 and later interrupt the list as if
the shell received the interrupt; previous versions continue with the next
command in the list.
compat41
If set, bash, when in posix mode, treats a single quote in a double-quoted
parameter expansion as a special character. The single quotes must match
(an even number) and the characters between the single quotes are consid‐
ered quoted. This is the behavior of posix mode through version 4.1. The
default bash behavior remains as in previous versions.
compat42
If set, bash does not process the replacement string in the pattern substi‐
tution word expansion using quote removal.
complete_fullquote
If set, bash quotes all shell metacharacters in filenames and directory
names when performing completion. If not set, bash removes metacharacters
such as the dollar sign from the set of characters that will be quoted in
completed filenames when these metacharacters appear in shell variable ref‐
erences in words to be completed. This means that dollar signs in variable
names that expand to directories will not be quoted; however, any dollar
signs appearing in filenames will not be quoted, either. This is active
only when bash is using backslashes to quote completed filenames. This
variable is set by default, which is the default bash behavior in versions
through 4.2.
direxpand
If set, bash replaces directory names with the results of word expansion
when performing filename completion. This changes the contents of the
readline editing buffer. If not set, bash attempts to preserve what the
user typed.
dirspell
If set, bash attempts spelling correction on directory names during word
completion if the directory name initially supplied does not exist.
dotglob If set, bash includes filenames beginning with a `.' in the results of
pathname expansion.
execfail
If set, a non-interactive shell will not exit if it cannot execute the file
specified as an argument to the exec builtin command. An interactive shell
does not exit if exec fails.
expand_aliases
If set, aliases are expanded as described above under ALIASES. This option
is enabled by default for 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 corresponding to each function name supplied as an
argument.
2. If the command run by the DEBUG trap returns a non-zero value, the
next command is skipped and not executed.
3. If the command run by the DEBUG trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell
script executed by the . or source builtins), a call to return is
simulated.
4. BASH_ARGC and BASH_ARGV are updated as described in their descrip‐
tions 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 Path‐
name 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.
globasciiranges
If set, range expressions used in pattern matching bracket expressions (see
Pattern Matching above) behave as if in the traditional C locale when per‐
forming comparisons. That is, the current locale's collating sequence is
not taken into account, so b will not collate between A and B, and upper-
case and lower-case ASCII characters will collate together.
globstar
If set, the pattern ** used in a pathname expansion context will match all
files and zero or more directories and subdirectories. If the pattern is
followed by a /, only directories and 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
If set, bash will send SIGHUP to all jobs when an interactive login shell
exits.
interactive_comments
If set, allow a word beginning with # to cause that word and all remaining
characters on that line to be ignored in an interactive shell (see COMMENTS
above). This option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the last command of a
pipeline not executed in the background in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-line commands are saved to
the history with embedded newlines rather than using semicolon separators
where possible.
login_shell
The shell sets this option if it is started as a login shell (see INVOCA‐
TION above). The value may not be changed.
mailwarn
If set, and a file that bash is checking for mail has been accessed since
the last time it was checked, the message ``The mail in mailfile has been
read'' is displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not attempt to search the
PATH for possible completions when completion is attempted on an empty
line.
nocaseglob
If set, bash matches filenames in a case-insensitive fashion when perform‐
ing pathname expansion (see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive fashion when performing
matching while executing case or [[ conditional commands.
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. A login
shell cannot be suspended; the -f option can be used to override this and force the
suspension. The return status is 0 unless the shell is a login shell and -f is not
supplied, or if job control is not enabled.

test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending on the evaluation of the condi‐
tional expression expr. Each operator and operand must be a separate argument.
Expressions are composed of the primaries described above under CONDITIONAL EXPRES‐
SIONS. test does not accept any options, nor does it accept and ignore an argument
of -- as signifying the end of options.

Expressions may be combined using the following operators, listed in decreasing
order of precedence. The evaluation depends on the number of arguments; see below.
Operator precedence is used when there are five or more arguments.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override the normal prece‐
dence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.

test and [ evaluate conditional expressions using a set of rules based on the num‐
ber of arguments.

0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not null.
2 arguments
If the first argument is !, the expression is true if and only if the second
argument is null. If the first argument is one of the unary conditional
operators listed above under CONDITIONAL EXPRESSIONS, the expression is true
if the unary test is true. If the first argument is not a valid unary con‐
ditional 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 CON‐
DITIONAL EXPRESSIONS, the result of the expression is the result of the
binary test using the first and third arguments as operands. The -a and -o
operators are considered binary operators when there are three arguments.
If the first argument is !, the value is the negation of the two-argument
test using the second and third arguments. If the first argument is exactly
( and the third argument is exactly ), the result is the one-argument test
of the second argument. Otherwise, the expression is false.
4 arguments
If the first argument is !, the result is the negation of the three-argument
expression composed of the remaining arguments. Otherwise, the expression
is parsed and evaluated according to precedence using the rules listed
above.
5 or more arguments
The expression is parsed and evaluated according to precedence using the
rules listed above.

When used with test or [, the < and > operators sort lexicographically using ASCII
ordering.

times Print the accumulated user and system times for the shell and for processes run
from the shell. The return status is 0.

trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell receives signal(s)
sigspec. If arg is absent (and there is a single sigspec) or -, each specified
signal is reset to its original disposition (the value it had upon entrance to the
shell). If arg is the null string the signal specified by each sigspec is ignored
by the shell and by the commands it invokes. If arg is not present and -p has been
supplied, then the trap commands associated with each sigspec are displayed. If no
arguments are supplied or if only -p is given, trap prints the list of commands
associated with each signal. The -l option causes the shell to print a list of
signal names and their corresponding numbers. Each sigspec is either a signal name
defined in <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 com‐
mand, 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 execut‐
ing.

If a sigspec is ERR, the command arg is executed whenever a a pipeline (which may
consist of a single simple command), a list, or a compound command returns a
non-zero exit status, subject to the following conditions. The ERR trap is not
executed if the failed command is part of the command list immediately following a
while or until keyword, part of the test in an if statement, part of a command exe‐
cuted in a && or || list except the command following the final && or ||, any com‐
mand in a pipeline but the last, or if the command's return value is being inverted
using !. These are the same conditions obeyed by the errexit (-e) option.

Signals ignored upon entry to the shell cannot be trapped or reset. Trapped sig‐
nals that are not being ignored are reset to their original values in a subshell or
subshell environment when one is created. The return status is false if any
sigspec is invalid; otherwise trap returns true.

type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if used as a command
name. If the -t option is used, type prints a string which is one of alias, key‐
word, function, builtin, or file if name is an alias, shell reserved word, func‐
tion, builtin, or disk file, respectively. If the name is not found, then nothing
is printed, and an exit status of false is returned. If the -p option is used,
type either returns the name of the disk file that would be executed if name were
specified as a command name, or nothing if ``type -t name'' would not return file.
The -P option forces a PATH search for each name, even if ``type -t name'' would
not return file. If a command is hashed, -p and -P print the hashed value, which
is not necessarily the file that appears first in PATH. If the -a option is used,
type prints all of the places that contain an executable named name. This includes
aliases and functions, if and only if the -p option is not also used. The table of
hashed commands is not consulted when using -a. The -f option suppresses shell
function lookup, as with the command builtin. type returns true if all of the
arguments are found, false if any are not found.

ulimit [-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 omit‐
ted, 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 children
-x The maximum number of file locks
-T The maximum number of threads

If limit is given, and the -a option is not used, limit is the new value of the
specified resource. If no option is given, then -f is assumed. Values are in
1024-byte increments, except for -t, which is in seconds; -p, which is in units of
512-byte blocks; and -T, -b, -n, and -u, which are unscaled values. The return
status is 0 unless an invalid option or argument is supplied, or an error occurs
while setting a new limit.

umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins with a digit, it is
interpreted as an octal number; 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 omit‐
ted, 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] [-n] [name ...]
For each name, remove the corresponding variable or function. If the -v option is
given, each name refers to a shell variable, and that variable is removed. Read-
only variables may not be unset. If -f is specified, each name refers to a shell
function, and the function definition is removed. If the -n option is supplied,
and name is a variable with the nameref attribute, name will be unset rather than
the variable it references. -n has no effect if the -f option is supplied. If no
options are supplied, each name refers to a variable; if there is no variable by
that name, any function with that name is unset. Each unset variable or function
is removed from the environment passed to subsequent commands. If any of
COMP_WORDBREAKS, RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK
are unset, they lose their special properties, even if they are subsequently reset.
The exit status is true unless a name is readonly.

wait [-n] [n ...]
Wait for each specified child process and return its termination status. Each n
may be a process ID or a job specification; if a job spec is given, all processes
in that job's pipeline are waited for. If n is not given, all currently active
child processes are waited for, and the return status is zero. If the -n option is
supplied, wait waits for any job to terminate and returns its exit status. If n
specifies a non-existent process or job, the return status is 127. Otherwise, the
return status is the exit status of the last process or job waited for.

RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied at invocation, the
shell becomes restricted. A restricted shell is used to set up an environment more con‐
trolled than the standard shell. It behaves identically to bash with the exception that
the following are disallowed or not performed:

· changing directories with cd

· setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

· specifying command names containing /

· specifying a filename containing a / as an argument to the . builtin command

· specifying a filename containing a slash as an argument to the -p option to the
hash builtin command

· importing function definitions from the shell environment at startup

· parsing the value of SHELLOPTS from the shell environment at startup

· redirecting output using the >, >|, <>, >&, &>, and >> 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 --
http://pubs.opengroup.org/onlinepubs/9699919799/
http://tiswww.case.edu/~chet/bash/POSIX -- a description of posix mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)

FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
/etc/bash.bashrc
The systemwide per-interactive-shell startup file
/etc/bash.bash.logout
The systemwide login shell cleanup file, executed when a login shell exits
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login shell exits
~/.inputrc
Individual readline initialization file

AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org

Chet Ramey, Case Western Reserve University
chet.ramey@case.edu

BUG REPORTS
If you find a bug in bash, you should report it. But first, you should make sure that it
really is a bug, and that it appears in the latest version of bash. The latest version is
always available from ftp://ftp.gnu.org/pub/gnu/bash/.

Once you have determined that a bug actually exists, use the bashbug command to submit a
bug report. If you have a fix, you are encouraged to mail that as well! Suggestions and
`philosophical' bug reports may be mailed to bug-bash@gnu.org or posted to the Usenet
newsgroup gnu.bash.bug.

ALL bug reports should include:

The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug

bashbug inserts the first three items automatically into the template it provides for fil‐
ing a bug report.

Comments and bug reports concerning this manual page should be directed to
chet.ramey@case.edu.

BUGS
It's too big and too slow.

There are some subtle differences between bash and traditional versions of sh, mostly
because of the POSIX specification.

Aliases are confusing in some uses.

Shell builtin commands and functions are not stoppable/restartable.

Compound commands and command sequences of the form `a ; b ; c' are not handled gracefully
when process suspension is attempted. When a process is stopped, the shell immediately
executes the next command in the sequence. It suffices to place the sequence of commands
between parentheses to force it into a subshell, which may be stopped as a unit.

Array variables may not (yet) be exported.

There may be only one active coprocess at a time.

GNU Bash 4.3 2014 February 2 BASH(1)

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