bash Man page

BASH(1) General Commands Manual BASH(1)

NAME

bash – GNU Bourne-Again SHell

SYNOPSIS

bash [options] [command_string | file]

COPRYRIGHT

Bash is Copyright (C) 1989-2013 by the Free Software Foundation, Inc.

DESCRIPTION

Bash is an sh-compatible command language interpreter that executes
commands read from the standard input or from a file. Bash also incor‐
porates 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 descrip‐
tion 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 argu‐
ments 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 positional parameters to be
set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed
on the standard output. These are the strings that are sub‐
ject to language translation when the current locale is not C
or POSIX. This implies the -n option; no commands will be
executed.
[-+]O [shopt_option] shopt_option is one of the shell options accepted by the
shopt builtin (see SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of that option; +O
unsets it. If shopt_option is not supplied, the names and
values of the shell options accepted by shopt are printed on
the standard output. If the invocation option is +O, the
output is displayed in a format that may be reused as input.
— A — signals the end of options and disables further option
processing. Any arguments after the — are treated as file‐
names and arguments. An argument of – is equivalent to –.

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

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

–login
Equivalent to -l.

–noediting
Do not use the GNU readline library to read command lines when
the shell is interactive.

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

INVOCATION

below).

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

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

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

–verbose
Equivalent to -v.

–version
Show version information for this instance of bash on the stan‐
dard output and exit successfully.

ARGUMENTS

If arguments remain after option processing, and neither the -c nor the
-s option has been supplied, the first argument is assumed to be the
name of a file containing shell commands. If bash is invoked in this
fashion, $0 is set to the name of the file, and the positional parame‐
ters are set to the remaining arguments. Bash reads and executes com‐
mands from this file, then exits. Bash’s exit status is the exit sta‐
tus of the last command executed in the script. If no commands are
executed, the exit status is 0. An attempt is first made to open the
file in the current directory, and, if no file is found, then the shell
searches the directories in PATH for the script.

INVOCATION

A login shell is one whose first character of argument zero is a -, or
one started with the –login option.

An interactive shell is one started without non-option arguments and
without the -c option whose standard input and error are both connected
to terminals (as determined by isatty), 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 Expan‐
sion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-inter‐
active shell with the –login option, it first reads and executes com‐
mands 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 following com‐
mand were executed:
if [ -n “$BASH_ENV” ]; then . “$BASH_ENV”; fi
but the value of the PATH variable is not used to search for the file‐
name.

If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an interac‐
tive login shell, or a non-interactive shell with the –login option,
it first attempts to read and execute commands from /etc/profile and
~/.profile, in that order. The –noprofile option may be used to
inhibit this behavior. When invoked as an interactive shell with the
name sh, bash looks for the variable ENV, expands its value if it is
defined, and uses the expanded value as the name of a file to read and
execute. Since a shell invoked as sh does not attempt to read and exe‐
cute 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 deter‐
mines 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 neither rshd nor sshd generally invoke the
shell with those options or allow them to be specified.

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

DEFINITIONS

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

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 fol‐
lowed by blank-separated words and redirections, and terminated by a
control operator. The first word specifies the command to be executed,
and is passed as argument zero. The remaining words are passed as
arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if
the command is terminated by signal n.

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

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

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

The return status of a pipeline is the exit status of the last command,
unless the pipefail option is enabled. If pipefail is enabled, the
pipeline’s return status is the value of the last (rightmost) command
to exit with a non-zero status, or zero if all commands exit success‐
fully. 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 rec‐
ognize time as a reserved word if the next token begins with a `-‘.
The TIMEFORMAT variable may be set to a format string that specifies
how the timing information should be displayed; see the description of
TIMEFORMAT under Shell Variables below.

When the shell is in posix mode, time may be followed by a newline. In
this case, the shell displays the total user and system time consumed
by the shell and its children. The TIMEFORMAT variable may be used to
specify the format of the time information.

Each command in a pipeline is executed as a separate process (i.e., in
a subshell).

Lists
A list is a sequence of one or more pipelines separated by one of the
operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or
.

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 exe‐
cutes 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 description may be separated from the rest of the command by
one or more newlines, and may be followed by a newline in place of a
semicolon.

(list) list is executed in a subshell environment (see COMMAND EXECU‐
TION ENVIRONMENT below). Variable assignments and builtin com‐
mands that affect the shell’s environment do not remain in
effect after the command completes. The return status is the
exit status of list.

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

((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION. If the value of the expres‐
sion 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 CONDITIONAL 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. Condi‐
tional 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 parame‐
ters).

When the == and != operators are used, the string to the right of the
operator is considered a pattern and matched according to the rules
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 prece‐
dence 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). The return value is 0 if the string
matches the pattern, and 1 otherwise. If the regular expression is
syntactically incorrect, the conditional expression’s return value is
2. If the 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 subexpression.

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

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

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

for name [ [ in [ word … ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of
items. The variable name is set to each element of this list in
turn, and list is executed each time. If the in word is omit‐
ted, the for command executes list once for each positional
parameter that is set (see PARAMETERS below). The return status
is the exit status of the last command that executes. If the
expansion of the items following in results in an empty list, no
commands are executed, and the return status is 0.

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

select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of
items. 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 stan‐
dard 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 dis‐
played 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 path‐
name expansion (see Pathname Expansion below). The word is
expanded using tilde expansion, parameter and variable expan‐
sion, arithmetic substitution, command substitution, process
substitution and quote removal. Each pattern examined is
expanded using tilde expansion, parameter and variable expan‐
sion, 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 execution to continue with the list associated with
the next set of patterns. Using ;;& in place of ;; causes the
shell to test the next pattern list in the statement, if any,
and execute any associated list on a successful match. The exit
status is zero if no pattern matches. Otherwise, it is the exit
status of the last command executed in list.

if list; then list; [ elif list; then list; ] … [ else list; ] fi
The if list is executed. If its exit status is zero, the then
list is executed. Otherwise, each elif list is executed in
turn, and if its exit status is zero, the corresponding then
list is executed and the command completes. Otherwise, the else
list is executed, if present. The exit status is the exit sta‐
tus of the last command executed, or zero if no condition tested
true.

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

Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. A
coprocess is executed asynchronously in a subshell, as if the command
had been terminated with the & control operator, with a two-way pipe
established between the executing shell and the coprocess.

The format for a coprocess is:

coproc [NAME] command [redirections]

This creates a coprocess named NAME. If NAME is not supplied, the
default name is COPROC. NAME must not be supplied if command is a sim‐
ple command (see above); otherwise, it is interpreted as the first word
of the simple command. When the coprocess is executed, the shell cre‐
ates an array variable (see Arrays below) named NAME in the context of
the executing shell. The standard output of command is connected via a
pipe to a file descriptor in the executing shell, and that file
descriptor is assigned to NAME[0]. The standard input of command is
connected via a pipe to a file descriptor in the executing shell, and
that file descriptor is assigned to NAME[1]. This pipe is established
before any redirections specified by the command (see REDIRECTION
below). The file descriptors can be utilized as arguments to shell
commands and redirections using standard word expansions. 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 com‐
pound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above. com‐
pound-command is executed whenever name is specified as the name
of a simple command. When in posix mode, name may not be the
name of one of the POSIX special builtins. Any redirections
(see REDIRECTION below) specified when a function is defined are
performed when the function is executed. The exit status of a
function definition is zero unless a syntax error occurs or a
readonly function with the same name already exists. When exe‐
cuted, the exit status of a function is the exit status of the
last command executed in the body. (See FUNCTIONS below.)

COMMENTS

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

QUOTING

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

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

When the command history expansion facilities are being used (see HIS‐
TORY EXPANSION below), the history expansion character, usually !, must
be quoted to prevent history expansion.

There are three quoting mechanisms: the escape character, single
quotes, and double quotes.

A non-quoted backslash (\) is the escape character. It preserves the
literal value of the next character that follows, with the exception of
. If a \ pair appears, and the backslash is not
itself quoted, the \ is treated as a line continuation (that
is, it is removed from the input stream and effectively ignored).

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

Enclosing characters in double quotes preserves the literal value of
all characters within the quotes, with the exception of $, `, \, and,
when history expansion is enabled, !. 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 . A double quote may be quoted within double
quotes by preceding it with a backslash. If enabled, history expansion
will be performed unless an ! appearing in double quotes is escaped
using a backslash. The backslash preceding the ! is not removed.

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

Words of the form $’string’ are treated specially. The word expands to
string, with backslash-escaped characters replaced as specified by the
ANSI C standard. Backslash escape sequences, if present, are decoded
as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\’ single quote
\” double quote
\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 dollar 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 num‐
ber, or one of the special characters listed below under Special Param‐
eters. A variable is a parameter denoted by a name. A variable has a
value and zero or more attributes. Attributes are assigned using the
declare builtin command (see declare below in SHELL BUILTIN COMMANDS).

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

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

name=[value]

If value is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion, com‐
mand substitution, arithmetic expansion, and quote removal (see EXPAN‐
SION below). If the variable has its integer attribute set, then value
is evaluated as an arithmetic expression even if the $((…)) expansion
is not used (see Arithmetic Expansion below). Word splitting is not
performed, with the exception of “$@” as explained below under Special
Parameters. Pathname expansion is not performed. Assignment state‐
ments may also appear as arguments to the alias, declare, typeset,
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 vari‐
able for which the integer attribute has been set, value is evaluated
as an arithmetic expression and added to the variable’s current value,
which is also evaluated. When += is applied to an array variable using
compound assignment (see Arrays below), the variable’s value is not
unset (as it is when using =), and new values are appended to the array
beginning at one greater than the array’s maximum index (for indexed
arrays) or added as additional key-value pairs in an associative array.
When applied to a string-valued variable, value is expanded and
appended to the variable’s value.

A variable can be assigned the nameref attribute using the -n option to
the declare or local builtin commands (see the descriptions of declare
and local below) to create a nameref, or a reference to another vari‐
able. This allows variables to be manipulated indirectly. Whenever
the nameref variable is referenced 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”
“$2” … If the double-quoted expansion occurs within a word,
the expansion of the first parameter is joined with the begin‐
ning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word.
When there are no positional parameters, “$@” and $@ expand to
nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the exit status of the most recently executed fore‐
ground pipeline.
– Expands to the current option flags as specified upon invoca‐
tion, 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 sub‐
shell.
! Expands to the process ID of the job most recently placed into
the background, whether executed as an asynchronous command or
using the bg builtin (see JOB CONTROL below).
0 Expands to the name of the shell or shell script. This is set
at shell initialization. If bash is invoked with a file of com‐
mands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after
the string to be executed, if one is present. Otherwise, it is
set to the filename used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to invoke
the shell or shell script being executed as passed in the envi‐
ronment 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 environment exported to that command. When check‐
ing mail, this parameter holds the name of the mail file cur‐
rently being checked.

Shell Variables
The following variables are set by the shell:

BASH Expands to the full filename used to invoke this instance of
bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -s option to the shopt
builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in BASHOPTS are those reported as on by shopt. If
this variable is in the environment when bash starts up, each
shell option in the list will be enabled before reading any
startup files. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized.
BASH_ALIASES
An associative array variable whose members correspond to the
internal list of aliases as maintained by the alias builtin.
Elements added to this array appear in the alias list; unsetting
array elements cause aliases to be removed from the alias list.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack. The number
of parameters to the current subroutine (shell function or
script executed with . or source) is at the top of the stack.
When a subroutine is executed, the number of parameters passed
is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in
extended debugging mode (see the description of the extdebug
option to the shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in the cur‐
rent 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 exe‐
cuted, the parameters supplied are pushed onto BASH_ARGV. The
shell sets BASH_ARGV only when in extended debugging mode (see
the description of the extdebug option to the shopt builtin
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 executing at the time of the
trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files where each corresponding member of FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the source file
(${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within another shell func‐
tion). Use LINENO to obtain the current line number.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary
operator to the [[ conditional command. The element with index
0 is the portion of the string matching the entire regular
expression. The element with index n is the portion of the
string matching the nth parenthesized subexpression. This vari‐
able is read-only.
BASH_SOURCE
An array variable whose members are the source filenames where
the corresponding shell function names in the FUNCNAME array
variable are defined. The shell function ${FUNCNAME[$i]} is
defined in the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The ini‐
tial 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 func‐
tions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the cur‐
rent completion function.
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the program‐
mable completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the begin‐
ning of the current command. If the current cursor position is
at the end of the current command, the value of this variable is
equal to ${#COMP_LINE}. This variable is available only in
shell functions and external commands invoked by the program‐
mable completion facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called: TAB,
for normal completion, ?, for listing completions after succes‐
sive tabs, !, for listing alternatives on partial word comple‐
tion, @, to list completions if the word is not unmodified, or
%, for menu completion. This variable is available only in
shell functions and external commands invoked by the program‐
mable completion facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word
separators when performing word completion. If COMP_WORDBREAKS
is unset, it loses its special properties, even if it is subse‐
quently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individ‐
ual words in the current command line. The line is split into
words as readline would split it, using COMP_WORDBREAKS as
described above. This variable is available only in shell func‐
tions invoked by the programmable completion facilities (see
Programmable Completion below).
COPROC An array variable (see Arrays below) created to hold the file
descriptors for output from and input to an unnamed coprocess
(see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current con‐
tents of the directory stack. Directories appear in the stack
in the order they are displayed by the dirs builtin. Assigning
to members of this array variable may be used to modify directo‐
ries already in the stack, but the pushd and popd builtins must
be used to add and remove directories. Assignment to this vari‐
able will not change the current directory. If DIRSTACK is
unset, it loses its special properties, even if it is subse‐
quently reset.
EUID Expands to the effective user ID of the current user, initial‐
ized at shell startup. This variable is readonly.
FUNC

NAME

An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0
is the name of any currently-executing shell function. The bot‐
tom-most element (the one with the highest index) is “main”.
This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect and return an error sta‐
tus. If FUNCNAME is unset, it loses its special properties,
even if it is subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE.
Each element of FUNCNAME has corresponding elements in
BASH_LINENO and BASH_SOURCE to describe the call stack. For
instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The
caller builtin displays the current call stack using this infor‐
mation.
GROUPS An array variable containing the list of groups of which the
current user is a member. Assignments to GROUPS have no effect
and return an error status. If GROUPS is unset, it loses its
special properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current
command. If HISTCMD is unset, it loses its special properties,
even if it is subsequently reset.
HOST

NAME

Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type
of machine on which bash is executing. The default is system-
dependent.
LINENO Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to
be meaningful. If LINENO is unset, it loses its special proper‐
ties, 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-com‐
pany-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 sys‐
tem on which bash is executing. The default is system-depen‐
dent.
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 read‐
only.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer between
0 and 32767 is generated. The sequence of random numbers may be
initialized by assigning a value to RANDOM. If RANDOM is unset,
it loses its special properties, even if it is subsequently
reset.
READLINE_LINE
The contents of the readline line buffer, for use with “bind -x”
(see SHELL BUILTIN COMMANDS below).
READLINE_POINT
The position of the insertion point in the readline line buffer,
for use with “bind -x” (see SHELL BUILTIN COMMANDS below).
REPLY Set to the line of input read by the read builtin command when
no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds
since shell invocation is returned. If a value is assigned to
SECONDS, the value returned upon subsequent references is the
number of seconds since the assignment plus the value assigned.
If SECONDS is unset, it loses its special properties, even if it
is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -o option to the set
builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in SHELLOPTS are those reported as on by set -o. If
this variable is in the environment when bash starts up, each
shell option in the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell
startup. This variable is readonly.

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

BASH_COMPAT
The value is used to set the shell’s compatibility level. See
the description of the shopt builtin below under SHELL BUILTIN
COMMANDS for a description of the various 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 com‐
patibility level. If BASH_COMPAT is unset or set to the empty
string, the compatibility level is set to the default for the
current version. If BASH_COMPAT is set to a value that is not
one of the valid compatibility levels, the shell prints an error
message and sets the compatibility level to the default for the
current version. The valid compatibility levels correspond to
the compatibility options accepted by the shopt builtin
described below (for example, compat42 means that 4.2 and 42 are
valid values). The current version is also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in ~/.bashrc. The value of BASH_ENV is
subjected to parameter expansion, command substitution, and
arithmetic expansion before being interpreted as a filename.
PATH is not used to search for the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
bash will write the trace output generated when set -x is
enabled to that file descriptor. The file descriptor is closed
when BASH_XTRACEFD is unset or assigned a new value. Unsetting
BASH_XTRACEFD or assigning it the empty string causes the trace
output to be sent to the standard error. Note that setting
BASH_XTRACEFD to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.
CDPATH The search path for the cd command. This is a colon-separated
list of directories in which the shell looks for destination
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 (cur‐
rently 8192) that this may not exceed. The minimum value is
system-dependent.
COLUMNS
Used by the select compound command to determine the terminal
width when printing selection lists. Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions
generated by a shell function invoked by the programmable com‐
pletion 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 suf‐
fix matches one of the entries in FIGNORE is excluded from the
list of matched filenames. A sample value is “.o:~” (Quoting is
needed when assigning a value to this variable, which contains
tildes).
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
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 subsequent lines of a multi-line
compound command are not tested, and are added to the history
regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HIS‐
TORY below). The default value is ~/.bash_history. If unset,
the command history is not saved when a shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When
this variable is assigned a value, the history file is trun‐
cated, 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 com‐
plete line (no implicit `*’ is appended). Each pattern is
tested against the line after the checks specified by HISTCON‐
TROL 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 compound command are not tested, and are added to the
history regardless of the value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below). 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 to print the time stamp associated
with each history entry displayed by the history builtin. If
this variable is set, time stamps are written to the history
file so they may be preserved across shell sessions. This uses
the history comment character to distinguish timestamps from
other history lines.
HOME The home directory of the current user; the default argument for
the cd builtin command. The value of this variable is also used
when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts
that should be read when the shell needs to complete a hostname.
The list of possible hostname completions may be changed while
the shell is running; the next time hostname completion is
attempted after the value is changed, bash adds the contents of
the new file to the existing list. If HOSTFILE is set, but has
no value, or does not name a readable file, bash attempts to
read /etc/hosts to obtain the list of possible hostname comple‐
tions. 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 “”.
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 pathname expansion, and determines the behavior
of range expressions, equivalence classes, and collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and
the behavior of character classes within pathname expansion and
pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
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 direc‐
tory.
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 loca‐
tion of the user mail files that it uses is system dependent
(e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by
the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTERR is initialized to 1 each time the shell is invoked or a
shell script is executed.
PATH The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory
name may appear as two adjacent colons, or as an initial or
trailing colon. The default path is system-dependent, and is
set by the administrator who installs bash. A common value is
“/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:
/sbin”.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the
shell enters posix mode before reading the startup files, as if
the –posix invocation option had been supplied. If it is set
while the shell is running, bash enables posix mode, as if the
command set -o posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each
primary prompt.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the \w and \W prompt string escapes (see PROMPTING below).
Characters removed are replaced with an ellipsis.
PS1 The value of this parameter is expanded (see PROMPTING below)
and used as the primary prompt string. The default value is
“\s-\v\$ ”.
PS2 The value of this parameter is expanded as with PS1 and used as
the secondary prompt string. The default is “> ”.
PS3 The value of this parameter is used as the prompt for the select
command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the
value is printed before each command bash displays during an
execution trace. The first character of PS4 is replicated mul‐
tiple times, as necessary, to indicate multiple levels of indi‐
rection. The default is “+ ”.
SHELL The full pathname to the shell is kept in this environment vari‐
able. 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 specify‐
ing how the timing information for pipelines prefixed with the
time reserved word should be displayed. The % character intro‐
duces an escape sequence that is expanded to a time value or
other information. The escape sequences and their meanings are
as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.

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

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

If this variable is not set, bash acts as if it had the value
$’\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS’. If the value is null,
no timing information is displayed. A trailing newline is added
when the format string is displayed.
TMOUT If set to a value greater than zero, TMOUT is treated as the
default timeout for the read builtin. The select command termi‐
nates if input does not arrive after TMOUT seconds when input is
coming from a terminal. In an interactive shell, the value is
interpreted as the number of seconds to wait for a line of input
after issuing the primary prompt. Bash terminates after waiting
for that number of seconds if a complete line of input does not
arrive.
TMPDIR If set, bash uses its value as the name of a directory in which
bash creates temporary files for the shell’s use.
auto_resume
This variable controls how the shell interacts with the user and
job control. If this variable is set, single word simple com‐
mands without redirections are treated as candidates for resump‐
tion of an existing stopped job. There is no ambiguity allowed;
if there is more than one job beginning with the string typed,
the job most recently accessed is selected. The name of a
stopped job, in this context, is the command line used to start
it. If set to the value exact, the string supplied must match
the name of a stopped job exactly; if set to substring, the
string supplied needs to match a substring of the name of a
stopped job. The substring value provides functionality analo‐
gous to the %? job identifier (see JOB CONTROL below). If set
to any other value, the supplied string must be a prefix of a
stopped job’s name; this provides functionality analogous to the
%string job identifier.
histchars
The two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below). The first character
is the history expansion character, the character which signals
the start of a history expansion, normally `!’. The second
character is the quick substitution character, which is used as
shorthand for re-running the previous command entered, substi‐
tuting one string for another in the command. The default is
`^’. The optional third character is the character which indi‐
cates that the remainder of the line is a comment when found as
the first character of a word, normally `#’. The history com‐
ment character causes history substitution to be skipped for the
remaining words on the line. It does not necessarily cause the
shell parser to treat the rest of the line as a comment.

Arrays
Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the declare builtin will
explicitly declare an array. There is no maximum limit on the size of
an array, nor any requirement that members be indexed or assigned con‐
tiguously. Indexed arrays are referenced using integers (including
arithmetic expressions) and are zero-based; associative arrays are
referenced using arbitrary strings. Unless otherwise noted, indexed
array indices must be non-negative integers.

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

Associative arrays are created using declare -A name.

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

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

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

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

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 sep‐
arate word. When there are no array members, ${name[@]} expands to
nothing. If the double-quoted expansion occurs within a word, the
expansion of the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter is joined
with the last part of the original word. This is analogous to the
expansion of the special parameters * and @ (see Special Parameters
above). ${#name[subscript]} expands to the length of ${name[sub‐
script]}. If subscript is * or @, the expansion is the number of ele‐
ments 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 ele‐
ment.

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 variable expansion, command substitu‐
tion, arithmetic expansion, word splitting, and pathname expansion.

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

On systems that can support it, there is an additional expansion avail‐
able: process substitution. This is performed at the same time as
tilde, parameter, variable, and arithmetic expansion and command sub‐
stitution.

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 gener‐
ated. This mechanism is similar to pathname expansion, but the file‐
names generated need not exist. Patterns to be brace expanded take the
form of an optional preamble, followed by either a series of comma-sep‐
arated 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 supplied, the expression expands to
each number between x and y, inclusive. Supplied integers may be pre‐
fixed 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 neces‐
sary. When characters are supplied, the expression expands to each
character lexicographically between x and y, inclusive, using the
default C locale. Note that both x and y must be of the same type.
When the increment is supplied, it is used as the difference between
each term. The default increment is 1 or -1 as appropriate.

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

A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
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 expan‐
sion, 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 output. The same word is output as file1 file2 after expansion by
bash. If strict compatibility with sh is desired, start bash with the
+B option or disable brace expansion with the +B option to the set com‐
mand (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 direc‐
tory of the user executing the shell is substituted instead. Other‐
wise, the tilde-prefix is replaced with the home directory associated
with the specified login name.

If the tilde-prefix is a `~+’, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is a `~-‘, the value of
the shell variable OLDPWD, if it is set, is substituted. If the char‐
acters 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 argu‐
ment. If the characters following the tilde in the tilde-prefix con‐
sist of a number without a leading `+’ or `-‘, `+’ is assumed.

If the login name is invalid, or the tilde expansion fails, the word is
unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immedi‐
ately following a : or the first =. In these cases, tilde expansion is
also performed. Consequently, one may use filenames with tildes in
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 backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

${parameter}
The value of parameter is substituted. The braces are required
when parameter is a positional parameter with more than one
digit, or when parameter is followed by a 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 parameter as the name of the variable;
this variable is then expanded and that value is used in the rest of
the substitution, rather than the value of parameter itself. This is
known as indirect expansion. The exceptions to this are the expansions
of ${!prefix*} and ${!name[@]} described below. The exclamation point
must immediately follow the left brace in order to introduce indirec‐
tion.

In each of the cases below, word is subject to tilde expansion, parame‐
ter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the forms documented
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 expan‐
sion of word is substituted. Otherwise, the value of parameter
is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter. The value of param‐
eter is then substituted. Positional parameters and special
parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset,
the expansion of word (or a message to that effect if word is
not present) is written to the standard error and the shell, if
it is not interactive, exits. Otherwise, the value of parameter
is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of the
value of parameter starting at the character specified by off‐
set. 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 :- expan‐
sion.

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 expan‐
sion 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 parame‐
ters 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 substituted. If parameter is * or @, the value
substituted is the number of positional parameters. If parame‐
ter 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 maxi‐
mum index of parameter, so negative indices count back from the
end of the array, and an index of -1 references the last ele‐
ment.

${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 parameter, then the result of the
expansion is the expanded value of parameter with the shortest
matching pattern (the “#” case) or the longest matching pat‐
tern (the “##” case) deleted. If parameter is @ or *, the
pattern removal operation is applied to each positional parame‐
ter in turn, and the expansion is the resultant list. If param‐
eter 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
matching pattern (the “%%” case) deleted. If parameter is @
or *, the pattern removal operation is applied to each posi‐
tional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or
*, the 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 pat‐
tern 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 pattern are
replaced with string. Normally only the first match is
replaced. If pattern begins with #, it must match at the begin‐
ning of the expanded value of parameter. 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 / fol‐
lowing pattern may be omitted. If parameter is @ or *, the sub‐
stitution 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 alpha‐
betic characters in parameter. The pattern is expanded to pro‐
duce a pattern just as in pathname expansion. Each character in
the expanded value of parameter is tested against pattern, and,
if it matches the pattern, its case is converted. The pattern
should not attempt to match more than one character. The ^
operator converts lowercase letters matching pattern to upper‐
case; the , operator converts matching uppercase letters to low‐
ercase. 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 sub‐
scripted with @ or *, the case modification operation is applied
to each member of the array in turn, and the expansion is the
resultant list.

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

$(command)
or
`command`

Bash performs the expansion by executing command and replacing the com‐
mand substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(< file). When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the command sub‐ stitution. When using the $(command) form, all characters between the parentheses make up the command; none are treated specially. Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results. Arithmetic Expansion Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expan‐ sion 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, com‐ mand substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Arithmetic expansions may be nested. The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If expression is invalid, bash prints a message indicating failure and no substitution occurs. Process Substitution Process substitution 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 out‐
put 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 pro‐
vide 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 variable expansion, command substitution, and arithmetic expansion. Word Splitting The shell scans the results of parameter expansion, command substitu‐ tion, and arithmetic expansion that did not occur within double quotes for word splitting. The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators. If IFS is unset, or its value is exactly , the default, then sequences of , ,
and at the beginning and end of the results of the previous
expansions are ignored, and any sequence of IFS characters not at the
beginning or end serves to delimit words. If IFS has a value other
than the default, then sequences of the whitespace characters space and
tab are ignored at the beginning and end of the word, as long as the
whitespace character is in the value of IFS (an IFS whitespace charac‐
ter). Any character in IFS that is not IFS whitespace, along with any
adjacent IFS whitespace characters, delimits a field. A sequence of
IFS whitespace characters is also treated as a delimiter. If the value
of IFS is null, no word splitting occurs.

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

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

Pathname Expansion
After word splitting, unless the -f option has been set, bash scans
each word for the characters *, ?, and [. If one of these characters
appears, then the word is regarded as a pattern, and replaced with an
alphabetically sorted list of filenames matching the pattern (see Pat‐
tern 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 file‐
names matching a pattern. 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 beginning with a “.”, make “.*” one
of the patterns in GLOBIGNORE. The dotglob option is disabled when
GLOBIGNORE is unset.

Pattern Matching

Any character that appears in a pattern, other than the special pattern
characters described below, matches itself. The NUL character may not
occur in a pattern. A backslash escapes the following character; the
escaping backslash is discarded when matching. The special pattern
characters must be quoted if they are to be matched literally.

The special pattern characters have the following meanings:

* Matches any string, including the null string. When the
globstar shell option is enabled, and * is used in a
pathname expansion context, two adjacent *s used as a
single pattern will match all files and zero or more
directories and subdirectories. If followed by a /, two
adjacent *s will match only directories and subdirecto‐
ries.
? Matches any single character.
[…] Matches any one of the enclosed characters. A pair of
characters separated by a hyphen denotes a range expres‐
sion; any character that falls between those two charac‐
ters, inclusive, using the current locale’s collating
sequence and character set, is matched. If the first
character following the [ is a ! or a ^ then any charac‐
ter not enclosed is matched. The sorting order of char‐
acters 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 interpreta‐
tion 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 collat‐
ing 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 fol‐
lowing sub-patterns:

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

Quote Removal
After the preceding expansions, all unquoted occurrences of the charac‐
ters \, ‘, and ” that did not result from one of the above expansions
are removed.

REDIRECTION
Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell. Redirection allows
commands’ file handles to be duplicated, opened, closed, made to refer
to different files, and can change the files the command reads from and
writes to. Redirection may also be used to modify file handles in the
current shell execution environment. The following redirection opera‐
tors may precede or appear anywhere within a simple command or may fol‐
low a command. Redirections are processed in the order they appear,
from left to right.

Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {varname}. In this case, for
each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than or equal to 10 and assign it to varname. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close. In the following descriptions, if the file descriptor number is omit‐ ted, and the first character of the redirection operator is <, the re‐ direction refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection
refers to the standard output (file descriptor 1).

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

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

ls > dirlist 2>&1

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

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard
error was duplicated from the standard output before the standard out‐
put was redirected to dirlist.

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

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

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

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

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

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

The general format for redirecting input 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 expansion of word to be opened for appending on file
descriptor n, or the standard output (file descriptor 1) if n is not
specified. If the file does not exist it is created.

The general format for appending output is:

[n]>>word

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

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

&>word
and
>&word

Of the two forms, the first is preferred. This is semantically equiva‐
lent to

>word 2>&1

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

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

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

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

The format of here-documents is:

<<[-]word here-document delimiter No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any charac‐ ters 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 charac‐ ter sequence \ 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

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

Moving File Descriptors
The redirection operator

[n]<&digit- moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n. Similarly, the redirection operator [n]>&digit-

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

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

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

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

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

There is no mechanism for using arguments in the replacement text. If
arguments are needed, a shell function should be used (see FUNCTIONS
below).

Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the description of
shopt under SHELL BUILTIN COMMANDS below).

The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input
before executing any of the commands on that line. Aliases are
expanded when a command is read, not when it is executed. Therefore,
an alias definition appearing on the same line as another command does
not take effect until the next line of input is read. The commands
following the alias definition on that line are not affected by the new
alias. This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read, not when the
function is executed, because a function definition is itself a com‐
pound 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 com‐
pound commands.

For almost every purpose, aliases are superseded by shell functions.

FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR,
stores a series of commands for later execution. When the name of a
shell function is used as a simple command name, the list of commands
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 special parameter # is
updated to reflect the change. Special parameter 0 is unchanged. The
first element of the FUNCNAME variable is set to the name of the func‐
tion while the function is executing.

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

Variables local to the function may be declared with the local builtin
command. Ordinarily, 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 function nesting level. Function invocations that
exceed the limit cause the entire command to abort.

If the builtin command return is executed in a function, the function
completes and execution resumes with the next command after the func‐
tion call. Any command associated with the RETURN trap is executed
before execution resumes. When a function completes, the values of the
positional parameters and the special parameter # are restored to the
values they had prior to the function’s execution.

Function names and definitions may be listed with the -f option to the
declare or typeset builtin commands. The -F option to declare or type‐
set will list the function names only (and optionally the source file
and line number, if the extdebug shell option is enabled). Functions
may be exported so that 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 iden‐
tically-named entries in the environment passed to the shell’s chil‐
dren. 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 func‐
tion 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 per‐
formed 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 decimal number between 2 and 64
representing the arithmetic base, and n is a number in that base. If
base# is omitted, then base 10 is used. When specifying n, 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 repre‐ sent numbers between 10 and 35. Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above. CONDITIONAL EXPRESSIONS Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any file argument to one of the pri‐ maries is of the form /dev/fd/n, then file descriptor n is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked. Unless otherwise specified, primaries that operate on files follow sym‐ bolic links and operate on the target of the link, rather than the link itself. When used with [[, the < and > operators sort lexicographically using
the current locale. The test command sorts using ASCII ordering.

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

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

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

string1 < string2 True if string1 sorts before string2 lexicographically. string1 > string2
True if string1 sorts after string2 lexicographically.

arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic
binary operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than
or equal to arg2, respectively. Arg1 and arg2 may be positive
or negative integers.

SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.

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

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

3. Redirections are performed as described above under REDIRECTION.

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

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

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

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

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

If the command name contains no slashes, the shell attempts to locate
it. If there exists a shell function by that name, that function is
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 con‐
taining an executable file by that name. Bash uses a hash table to
remember the full pathnames of executable files (see hash under SHELL
BUILTIN COMMANDS below). A full search of the directories in PATH is
performed only if the command is not found in the hash table. If the
search is unsuccessful, the shell searches for a defined shell function
named command_not_found_handle. If that function exists, it is invoked
with the original command and the original command’s arguments 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 execu‐
tion environment. Argument 0 is set to the name given, and the remain‐
ing arguments to the command are set to the arguments given, if any.

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

If the program is a file beginning with #!, the remainder of the first
line specifies an interpreter for the program. The shell executes the
specified interpreter on operating systems that do not handle this exe‐
cutable format themselves. The arguments to the interpreter consist of
a single optional argument following the interpreter name on the first
line of the program, followed by the name of the program, followed by
the command arguments, if any.

COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the follow‐
ing:

· open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin

· the current working directory as set by cd, pushd, or popd, or
inherited by the shell at invocation

· the file creation mode mask as set by umask or inherited from
the shell’s parent

· current traps set by trap

· shell parameters that are set by variable assignment or with set
or inherited from the shell’s parent in the environment

· shell functions defined during execution or inherited from the
shell’s parent in the environment

· options enabled at invocation (either by default or with com‐
mand-line arguments) or by set

· options enabled by shopt

· shell aliases defined with alias

· various process IDs, including those of background jobs, the
value of $$, and the value of PPID

When a simple command other than a builtin or shell function is to be
executed, it is invoked in a separate execution environment that con‐
sists of the following. Unless otherwise noted, the values are inher‐
ited from the shell.

· the shell’s open files, plus any modifications and additions
specified by redirections to the command

· the current working directory

· the file creation mode mask

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

· traps caught by the shell are reset to the values inherited from
the shell’s parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the
shell’s execution environment.

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

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

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

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

The shell provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a parameter
for each name found, automatically marking it for export to child pro‐
cesses. 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 envi‐
ronment 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 modi‐
fied in the shell, less any pairs removed by the unset command, plus
any additions via the export and declare -x commands.

The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described
above in PARAMETERS. These assignment statements affect only the envi‐
ronment seen by that command.

If the -k option is set (see the set builtin command below), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.

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

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

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

If a command is not found, the child process created to execute it
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 substi‐
tution ignore the keyboard-generated job control signals SIGTTIN, SIGT‐
TOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting,
an interactive shell resends the SIGHUP to all jobs, running or
stopped. Stopped jobs are sent SIGCONT to ensure that they receive the
SIGHUP. To prevent the shell from sending the signal to a particular
job, it should be removed from the jobs table with the disown builtin
(see SHELL BUILTIN COMMANDS below) or marked to not receive SIGHUP
using disown -h.

If the huponexit shell option has been set with shopt, bash sends a
SIGHUP to all jobs when an interactive login shell exits.

If bash is waiting for a command to complete and receives a signal for
which a trap has been set, the trap will not be executed until the com‐
mand completes. When bash is waiting for an asynchronous command via
the wait builtin, the reception of a signal for which a trap has been
set will cause the wait builtin to return immediately with an exit sta‐
tus greater than 128, immediately after which the trap is executed.

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 system maintains the notion of a current terminal process
group ID. Members of this process group (processes whose process group
ID is equal to the current terminal process group ID) receive keyboard-
generated signals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose process group ID
differs from the terminal’s; such processes are immune to keyboard-gen‐
erated signals. Only foreground processes are allowed to read from or,
if the user so specifies with stty tostop, write to the terminal.
Background processes which attempt to read from (write to when stty
tostop is in effect) the terminal are sent a SIGTTIN (SIGTTOU) signal
by the kernel’s terminal driver, which, unless caught, suspends the
process.

If the operating system on which bash is running supports job control,
bash contains facilities to use it. Typing the suspend character (typ‐
ically ^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 charac‐
ter % 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 pre‐
vious job with a -. A single % (with no accompanying job specifica‐
tion) also refers to the current job.

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

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

If an attempt to exit bash is made while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin, run‐
ning), the shell prints a warning message, and, if the checkjobs option
is enabled, lists the jobs and their statuses. The jobs command may
then be used to inspect their status. If a second attempt to exit is
made without an intervening command, the shell does not print another
warning, and any stopped jobs are terminated.

PROMPTING
When executing interactively, bash displays the primary prompt PS1 when
it is ready to read a command, and the secondary prompt PS2 when it
needs more input to complete a command. Bash allows these prompt
strings to be customized by inserting a number of backslash-escaped
special characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in “Weekday Month Date” format (e.g., “Tue May
26”)
\D{format}
the format is passed to strftime and the result is
inserted into the prompt string; an empty format results
in a locale-specific time representation. The braces are
required
\e an ASCII escape character (033)
\h the hostname up to the first `.’
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell’s terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion
following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g., 2.00.0)
\w the current working directory, with $HOME abbreviated
with a tilde (uses the value of the PROMPT_DIRTRIM vari‐
able)
\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 substitu‐
tion, 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 interac‐
tive 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. Simi‐
larly, meta keys are denoted by M-key, so M-x means Meta-X. (On key‐
boards without a meta key, M-x means ESC x, i.e., press the Escape key
then the x key. This makes ESC the meta prefix. The combination M-C-x
means ESC-Control-x, or press the Escape key then hold the Control key
while pressing the x key.)

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

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

Readline Initialization
Readline is customized by putting commands in an initialization file
(the inputrc file). The name of this file is taken from the value of
the INPUTRC variable. If that variable is unset, the default is
~/.inputrc. When a program which uses the readline library starts up,
the initialization file is read, and the key bindings and variables are
set. There are only a few basic constructs allowed in the readline
initialization file. Blank lines are ignored. Lines beginning with a
# are comments. Lines beginning with a $ indicate conditional con‐
structs. Other lines denote key bindings and variable settings.

The default key-bindings may be changed with an inputrc file. Other
programs that use this library may add their own commands and bindings.

For example, placing

M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command univer‐
sal-argument.

The following symbolic character names are recognized: RUBOUT, DEL,
ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.

In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro).

Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple.
All that is required is the name of the command or the text of a macro
and a key sequence to which it should be bound. The name may be speci‐
fied in one of two ways: as a symbolic key name, possibly with Meta- or
Control- prefixes, or as a key sequence.

When using the form keyname:function-name or macro, keyname is the name
of a key spelled out in English. For example:

Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: “> output”

In the above example, C-u is bound to the function universal-argument,
M-DEL is bound to the function backward-kill-word, and C-o is bound to
run the macro expressed on the right hand side (that is, to insert the
text “> output” into the line).

In the second form, “keyseq”:function-name or macro, keyseq differs
from keyname above in that strings denoting an entire key sequence may
be specified by placing the sequence within double quotes. Some GNU
Emacs style key escapes can be used, as in the following example, but
the 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 func‐
tion name. In the macro body, the backslash escapes described above
are expanded. Backslash will quote any other character in the macro
text, including ” and ‘.

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

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

set variable-name value

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-insen‐
sitive), 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 terminal’s bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters
treated specially by the kernel’s terminal driver to their read‐
line equivalents.
colored-stats (Off)
If set to On, readline displays possible completions using dif‐
ferent colors to indicate their file type. The color defini‐
tions 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-insensitive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of pos‐
sible completions that is displayed without modification. When
set to a value greater than zero, common prefixes longer than
this value are replaced with an ellipsis when displaying possi‐
ble completions.
completion-query-items (100)
This determines when the user is queried about viewing the num‐
ber of possible completions generated by the possible-comple‐
tions command. It may be set to any integer value greater than
or equal to zero. If the number of possible completions is
greater than or equal to the value of this variable, 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 sim‐
ilar 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 gener‐
ated from the keyboard.
enable-keypad (Off)
When set to On, readline will try to enable the application key‐
pad when it is called. Some systems need this to enable the
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-his‐
tory 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 subsequently executing the character as a com‐
mand. If this variable has not been given a value, the charac‐
ters 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-com‐
mand, 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 dis‐
played with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to direc‐
tories have a slash appended (subject to the value of
mark-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 dis‐
play a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the
screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines
before returning when accept-line is executed. By default, his‐
tory lines may be modified and retain individual undo lists
across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions.
If set to On, words which have more than one possible completion
cause the matches to be listed immediately instead of ringing
the bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in
a fashion similar to show-all-if-ambiguous. If set to On, words
which have more than one possible completion without any possi‐
ble partial completion (the possible completions don’t share a
common prefix) cause the matches to be listed immediately
instead of ringing the bell.
show-mode-in-prompt (Off)
If set to On, add a 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 is appended to the filename when listing possible com‐
pletions.

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 edit‐
ing mode, the terminal being used, or the application using
readline. The text of the test extends to the end of the line;
no characters are required to isolate it.

mode The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode. This may be
used in conjunction with the set keymap command, for
instance, to set bindings in the emacs-standard and
emacs-ctlx keymaps only if readline is starting out in
emacs mode.

term The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by
the terminal’s function keys. The word on the right side
of the = is tested against the both full name of the ter‐
minal 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 bindings from that file. For example, the follow‐
ing directive would read /etc/inputrc:

$include /etc/inputrc

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

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

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

Readline remembers the last incremental search string. If two Control-
Rs are typed without any intervening characters defining a new search
string, any remembered search string is used.

Non-incremental searches read the entire search string before starting
to search for matching history lines. The search string may be typed
by the user or be part of the contents of the current line.

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

Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of
alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words
are composed of alphanumeric characters (letters and digits).
shell-forward-word
Move forward to the end of the next word. Words are delimited
by non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word. Words
are delimited by non-quoted shell metacharacters.
clear-screen (C-l)
Clear the screen leaving the current line at the top of the
screen. With an argument, refresh the current line without
clearing the screen.
redraw-current-line
Refresh the current line.

Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line
is non-empty, add it to the history list according to the state
of the HISTCONTROL variable. If the line is a modified history
line, then restore the history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in
the list.
next-history (C-n)
Fetch the next command from the history list, moving 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-incremental search for a string supplied by the
user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental
search for a string supplied by the user.
history-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. This is a
non-incremental search.
history-search-backward
Search backward through the history for the string of characters
between the start of the current line and the point. This is a
non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument n,
insert the nth word from the previous command (the words in the
previous command begin with word 0). A negative argument
inserts the nth word from the end of the previous command. Once
the argument n is computed, the argument is extracted as if the
“!n” history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word
of the previous history entry). With a numeric argument, behave
exactly like yank-nth-arg. Successive calls to yank-last-arg
move back through the history list, inserting the last word (or
the word specified by the argument to the first call) of each
line in turn. Any numeric argument supplied to these successive
calls determines the direction to move through the history. A
negative argument switches the direction through the history
(back or forward). The history expansion facilities are used to
extract the last word, as if the “!$” history expansion had been
specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and his‐
tory expansion as well as all of the shell word expansions. See
HISTORY EXPANSION below for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a
space. See HISTORY EXPANSION below for a description of history
expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above
for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line
relative to the current line from the history for editing. Any
argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke $VISUAL,
$EDITOR, and emacs as the editor, in that order.

Commands for Changing Text
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 charac‐
ters 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 cur‐
sor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how
to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, …)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well. If point is at the end of
the line, then this transposes the two characters before point.
Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving
point over that word as well. If point is at the end of the
line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative
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 argu‐
ment, switches to overwrite mode. With an explicit non-positive
numeric argument, switches to insert mode. This command affects
only emacs mode; vi mode does overwrite differently. Each call
to readline() starts in insert mode. In overwrite mode, charac‐
ters bound to self-insert replace the text at point rather than
pushing the text to the right. Characters bound to back‐
ward-delete-char replace the character before point with a
space. By default, this command is unbound.

Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The
killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the
same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as
those used by backward-word.
shell-kill-word (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 bound‐
ary. 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 bound‐
aries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works follow‐
ing yank or yank-pop.

Numeric Arguments
digit-argument (M-0, M-1, …, M–)
Add this digit to the argument already accumulating, or start a
new argument. M– starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is fol‐
lowed by digits, executing universal-argument again ends the
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 $), username (if the text begins with ~), hostname
(if the text begins with @), or command (including aliases and
functions) in turn. If none of these produces a match, filename
completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with
a single match from the list of possible completions. Repeated
execution of menu-complete steps through the list of possible
completions, inserting each match in turn. At the end of the
list of completions, the bell is rung (subject to the setting of
bell-style) and the original text is restored. An argument of n
moves n positions forward in the list of matches; a negative
argument may be used to move backward through the list. This
command is intended to be bound to TAB, but is unbound by
default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list
of possible completions, as if menu-complete had been given a
negative argument. This command is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning
or end of the line (like delete-char). If at the end of the
line, behaves identically to possible-completions. This command
is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating
it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating
it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating
it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating
it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell
builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating
it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible com‐
pletions enclosed within braces so the list is available to the
shell (see Brace Expansion above).

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

Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any
bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal’s bell
(subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, …)
If the metafied character x is lowercase, run the command that
is bound to the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the
undo command enough times to return the line to its initial
state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-)
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 occur‐
rences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occur‐
rence of that character. A negative count searches for subse‐
quent 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 com‐
ment-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, other‐
wise the characters in comment-begin are deleted from the begin‐
ning of the line. In either case, the line is accepted as if a
newline had been typed. The default value of comment-begin
causes this command to make the current line a shell comment.
If a numeric argument causes the comment character to be
removed, the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended. This pattern
is used to generate a list of matching filenames for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname
expansion, and the list of matching filenames is inserted,
replacing the word. If a numeric argument is supplied, an
asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn. If a
numeric argument is supplied, an asterisk is appended before
pathname expansion.
dump-functions
Print all of the functions and their key bindings to the read‐
line output stream. If a numeric argument is supplied, the out‐
put is formatted in such a way that it can be made part of an
inputrc file.
dump-variables
Print all of the settable readline variables and their values to
the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part
of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied, the
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 program‐
mable completion facilities are invoked.

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

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 comple‐
tion 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 consid‐
ered. The string is first split using the characters in the IFS spe‐
cial variable as delimiters. Shell quoting is honored. Each word is
then expanded using brace expansion, tilde expansion, parameter and
variable expansion, command substitution, and arithmetic expansion, as
described above under EXPANSION. The results are split using the rules
described above under Word Splitting. The results of the expansion are
prefix-matched against the word being completed, and the matching words
become the possible completions.

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

Any function specified with -F is invoked first. The function may use
any of the shell facilities, including the compgen builtin described
below, to generate the matches. It must put the possible completions
in the COMPREPLY array variable, one per array element.

Next, any command specified with the -C option is invoked in an envi‐
ronment equivalent to command substitution. It should print a list of
completions, one per line, to the standard output. Backslash may be
used to escape a newline, if necessary.

After all of the possible completions are generated, any filter speci‐
fied with the -X option is applied to the list. The filter is a pat‐
tern as used for pathname expansion; a & in the pattern is replaced
with the text of the word being completed. A literal & may be escaped
with a backslash; the backslash is removed before attempting a match.
Any completion that matches the pattern will be removed from the list.
A leading ! negates the pattern; in this case any completion not match‐
ing the pattern will be removed.

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

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

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

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

When a compspec indicates that directory name completion is desired,
the programmable completion functions force readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the mark-directories readline variable, regardless of the
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 speci‐
fied 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 corresponding to the name of the command, the following
default completion function would load completions dynamically:

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

HISTORY
When the -o history option to the set builtin is enabled, the shell
provides access to the command history, the list of commands previously
typed. The value of the HISTSIZE variable is used as the number of
commands to save in a history list. The text of the last HISTSIZE com‐
mands (default 500) is saved. The shell stores each command in the
history list prior to parameter and variable expansion (see EXPANSION
above) but after history expansion is performed, subject to the values
of the shell variables HISTIGNORE and HISTCONTROL.

On startup, the history is initialized from the file named by the vari‐
able HISTFILE (default ~/.bash_history). The file named by the value
of HISTFILE is truncated, if necessary, to contain no more than the
number of lines specified by the value of HISTFILESIZE. If HISTFILE‐
SIZE 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 fol‐
lowed immediately by a digit are interpreted as timestamps for the pre‐
ceding history line. These timestamps are optionally displayed depend‐
ing 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 ses‐
sions. This uses the history comment character to distinguish time‐
stamps from other history lines. After saving the history, the history
file is truncated to contain no more than HISTFILESIZE lines. If HIST‐
FILESIZE 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 his‐
tory builtin may be used to display or modify the history list and
manipulate the history file. When using command-line editing, search
commands are available in each editing mode that provide access to the
history list.

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

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

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

History expansion is performed immediately after a complete line is
read, before the shell breaks it into words. 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 read‐
ing 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 fol‐
lowing the history expansion character, even if it is unquoted: space,
tab, newline, carriage return, and =. If the extglob shell option is
enabled, ( will also inhibit expansion.

Several shell options settable with the shopt builtin may be used to
tailor the behavior of history expansion. If the histverify shell
option is enabled (see the description of the shopt builtin below), and
readline is being used, history substitutions are not immediately
passed to the shell parser. Instead, the expanded line is reloaded
into the readline editing buffer for further modification. If readline
is being used, and the histreedit shell option is enabled, a failed
history substitution will be reloaded into the readline editing buffer
for correction. The -p option to the history builtin command may be
used to see what a history expansion will do before using it. The -s
option to the history builtin may be used to add commands to the end of
the history list without actually executing them, so that they are
available for subsequent recall.

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

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

! Start a history substitution, except when followed by a blank,
newline, carriage return, = or ( (when the extglob shell option
is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1′.
!string
Refer to the most recent command preceding the current position
in the history list starting with string.
!?string[?] Refer to the most recent command preceding the current position
in the history list containing string. The trailing ? may be
omitted if string is followed immediately by a newline.
^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 sepa‐
rated by single spaces.

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

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

Modifiers
After the optional word designator, there may appear a sequence of one
or more of the following modifiers, each preceded by a `:’.

h Remove a trailing filename component, leaving only the head.
t Remove all leading filename components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at
blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the event
line. Any delimiter can be used in place of /. The final
delimiter is optional if it is the last character of the event
line. The delimiter may be quoted in old and new with a single
backslash. If & appears in new, it is replaced by old. A sin‐
gle backslash will quote the &. If old is null, it is set to
the last old substituted, or, if no previous history substitu‐
tions took place, the last string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is
used in conjunction with `:s’ (e.g., `:gs/old/new/’) or `:&’.
If used with `:s’, any delimiter can be used in place of /, and
the final delimiter is optional if it is the last character of
the event line. An a may be used as a synonym for g.
G Apply the following `s’ modifier once to each word in the event
line.

SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section
as accepting options preceded by – accepts — to signify the end of the
options. The :, true, false, and test builtins do not accept options
and do not treat — specially. The exit, logout, break, continue, let,
and shift builtins accept and process arguments beginning with – with‐
out 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 exe‐
cuted from filename. If filename does not contain a slash,
filenames in PATH are used to find the directory containing
filename. The file searched for in PATH need not be executable.
When bash is not in posix mode, the current directory is
searched if no file is found in PATH. If the sourcepath option
to the shopt builtin command is turned off, the PATH is not
searched. If any arguments are supplied, they become the posi‐
tional 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 sup‐
plied, the name and value of the alias is printed. Alias
returns true unless a name is given for which no alias has been
defined.

bg [jobspec …] Resume each suspended job jobspec in the background, as if it
had been started with &. If jobspec is not present, the shell’s
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 readline function or macro, or set a readline
variable. Each non-option argument is a command as it would
appear in .inputrc, but each binding or command must be passed
as a separate argument; e.g., ‘”\C-x\C-r”: re-read-init-file’.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent
bindings. Acceptable keymap names are emacs, emacs-stan‐
dard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command,
and vi-insert. vi is equivalent to vi-command; 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 read‐
line 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 READ‐
LINE_POINT, those new values will be reflected in the
editing state.
-X List all key sequences bound to shell commands and the
associated commands in a format that can be reused as
input.

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

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

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

caller [expr] Returns the context of any active subroutine call (a shell func‐
tion or a script executed with the . or source builtins). With‐
out expr, caller displays the line number and source filename of
the current subroutine call. If a non-negative integer is sup‐
plied as expr, caller displays the line number, subroutine name,
and source file corresponding to that position in the current
execution call stack. This extra information may be used, for
example, to print a stack trace. The current frame is frame 0.
The return value is 0 unless the shell is not executing a sub‐
routine call or expr does not correspond to a valid 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 addi‐
tional 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 previous pathname component from dir, back to a
slash or the beginning of dir. If the -e option is supplied
with -P, and the current working directory cannot be success‐
fully determined after a successful directory change, cd will
return an unsuccessful status. On systems that support it, the
-@ option presents the extended attributes associated with a
file as a directory. An argument of – is converted to $OLDPWD
before the directory change is attempted. If a non-empty direc‐
tory name from CDPATH is used, or if – is the first argument,
and the directory change is successful, the absolute pathname of
the new working directory is written to the standard output.
The return value is true if the directory was successfully
changed; false otherwise.

command [-pVv] command [arg …] Run command with args suppressing the normal shell function
lookup. Only builtin commands or commands found in the PATH are
executed. If the -p option is given, the search for command is
performed using a default value for PATH that is guaranteed to
find all of the standard utilities. If either the -V or -v
option is supplied, a description of command is printed. The -v
option causes a single word indicating the command or filename
used to invoke command to be displayed; the -V option produces a
more verbose description. If the -V or -v option is supplied,
the exit status is 0 if command was found, and 1 if not. If
neither option is supplied and an error occurred or command can‐
not be found, the exit status is 127. Otherwise, the exit sta‐
tus of the command builtin is the exit status of command.

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

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

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

complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G glob‐
pat] [-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 supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them
to be reused as input. The -r option removes a completion spec‐
ification for each name, or, if no names are supplied, all com‐
pletion 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 Program‐
mable Completion.

Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options (and, if necessary, the
-P and -S options) should be quoted to protect them from expan‐
sion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the comp‐
spec’s behavior beyond the simple generation of comple‐
tions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions
if the compspec generates no matches.
default Use readline’s default filename completion if
the compspec generates no matches.
dirnames
Perform directory name completion if the comp‐
spec generates no matches.
filenames
Tell readline that the compspec generates file‐
names, so it can perform any filename-specific
processing (like adding a slash to directory
names, quoting special characters, or suppress‐
ing trailing spaces). Intended to be used with
shell functions.
noquote Tell readline not to quote the completed words
if they are filenames (quoting filenames is the
default).
nospace Tell readline not to append a space (the
default) to words completed at the end of the
line.
plusdirs
After any matches defined by the compspec are
generated, directory name completion is
attempted and any matches are added to the
results of the other actions.
-A action
The action may be one of the following to generate a
list of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be
specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be
specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by
the HOSTFILE shell variable.
job Job names, if job control is active. May also
be specified as -j.
keyword Shell reserved words. May also be specified as
-k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be spec‐
ified 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 COMPRE‐
PLY 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 com‐
pletion 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 com‐
pleted.
-X filterpat
filterpat is a pattern as used for pathname expansion.
It is applied to the list of possible completions gener‐
ated by the preceding options and arguments, and each
completion matching filterpat is removed from the list.
A leading ! in filterpat negates the pattern; in this
case, any completion not matching filterpat is removed.

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

compopt [-o option] [-DE] [+o option] [name] Modify completion options for each name according to the
options, or for the currently-executing completion if no names
are supplied. If no options are given, display the completion
options for each name or the current completion. The possible
values of option are those valid for the complete builtin
described above. The -D option indicates that the remaining
options 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 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 definitions; only the function
name and attributes are printed. If the extdebug shell option
is enabled using shopt, the source file name and line number
where the function is defined are displayed as well. The -F
option implies -f. The -g option forces variables to be created
or 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 evalua‐
tion (see ARITHMETIC EVALUATION above) is performed when
the variable is assigned a value.
-l When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
-n Give each name the nameref attribute, making it a name
reference to another variable. That other variable is
defined by the value of name. All references and assign‐
ments 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 subsequent assignment statements or unset.
-t Give each name the trace attribute. Traced functions
inherit the DEBUG and RETURN traps from the calling
shell. The trace attribute has no special meaning for
variables.
-u When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.
-x Mark 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 vari‐
able 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 vari‐
able name is followed by =value, the value of the variable is
set to value. When using -a or -A and the compound assignment
syntax to create array variables, additional attributes do not
take effect until subsequent assignments. The return value is 0
unless an invalid option is encountered, an attempt is made to
define a function using “-f foo=bar”, an attempt is made to
assign a value to a readonly variable, an attempt is made to
assign a value to an array variable without using the compound
assignment syntax (see Arrays above), one of the names is not a
valid shell variable name, an attempt is made to turn off read‐
only 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, pre‐
fixing 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 job‐
spec 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 suppressed. If the -e option
is given, interpretation of the following backslash-escaped
characters is enabled. The -E option disables the interpreta‐
tion of these escape characters, even on systems where they are
interpreted by default. The xpg_echo shell option may be used
to dynamically determine whether or not echo expands these
escape characters by default. echo does not interpret — to
mean the end of options. echo interprets the following escape
sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value
nnn (zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)

enable [-a] [-dnps] [-f filename] [name …] Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though
the shell normally searches for builtins before disk commands.
If -n is used, each name is disabled; otherwise, names are
enabled. For example, to use the test binary found via the PATH
instead of the shell builtin version, run “enable -n test”.
The -f option means to load the new builtin command name from
shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin previously loaded with -f.
If no name arguments are given, or if the -p option is supplied,
a list of shell builtins is printed. With no other option argu‐
ments, 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 indica‐
tion 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 com‐
mand. This command is then read and executed by the shell, and
its exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.

exec [-cl] [-a name] [command [arguments]] If command is specified, it replaces the shell. No new process
is created. The arguments become the arguments to command. If
the -l option is supplied, the shell places a dash at the begin‐
ning of the zeroth argument passed to command. This is what
login 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 can‐
not be executed for some reason, a non-interactive shell exits,
unless the execfail shell option is enabled. In that case, it
returns failure. An interactive shell returns failure if the
file cannot be executed. If command is not specified, any redi‐
rections take effect in the current shell, and the return status
is 0. If there is a redirection error, the return status is 1.

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

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

fc [-e ename] [-lnr] [first] [last] fc -s [pat=rep] [cmd] The first form selects a range of commands from first to last
from the history list and displays or edits and re-executes
them. First and last may be specified as a string (to locate
the last command beginning with that string) or as a number (an
index into the history list, where a negative number is used as
an offset from the current command number). 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 com‐
mand 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. Other‐
wise, the editor given by ename is invoked on a file containing
those commands. If ename is not given, the value of the FCEDIT
variable is used, and the value of EDITOR if FCEDIT is not set.
If neither variable is set, vi is used. When editing is com‐
plete, 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-exe‐
cuted, unless cmd does not specify a valid history line, in
which case fc returns failure.

fg [jobspec] Resume jobspec in the foreground, and make it the current job.
If jobspec is not present, the shell’s notion of the current job
is used. The return value is that of the command placed into
the foreground, or failure if run when job control is disabled
or, when run with job control enabled, if jobspec does not spec‐
ify 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 parame‐
ters. optstring contains the option characters to be recog‐
nized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it
by white space. The colon and question mark characters may not
be used as option characters. Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist, and the index of the next argument to
be processed into the variable OPTIND. OPTIND is initialized to
1 each time the shell or a shell script is invoked. When an
option requires an argument, getopts places that argument into
the variable OPTARG. The shell does not reset OPTIND automati‐
cally; it must be manually reset between multiple calls to
getopts within the same shell invocation if a new set of parame‐
ters 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 dis‐
played, even if the first character of optstring is not a colon.

If an invalid option is seen, getopts places ? into name and, if
not silent, prints an error message and unsets OPTARG. If
getopts is silent, the option character found is placed in
OPTARG and no 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 remem‐
bered. Any previously-remembered pathname is discarded. If the
-p option is supplied, no path search is performed, and filename
is used as the full filename of the command. The -r option
causes the shell to forget all remembered locations. The -d
option causes the shell to forget the remembered location of
each name. If the -t option is supplied, the full pathname to
which each name corresponds is printed. If multiple name argu‐
ments are supplied with -t, the name is printed before the
hashed full pathname. The -l option causes output to be dis‐
played in a format that may be reused as input. If no arguments
are given, or if only -l is supplied, information about remem‐
bered 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 num‐
bers. Lines listed with a * have been modified. An argument of
n lists only the last n lines. If the shell variable HISTTIME‐
FORMAT is set and not null, it is used as a format string for
strftime to display the time stamp associated with each dis‐
played history entry. No intervening blank is printed between
the formatted time stamp and the history line. If filename is
supplied, it is used as the name of the history file; if not,
the value of HISTFILE is used. Options, if supplied, have the
following meanings:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset.
-a Append the “new” history lines (history lines entered
since the beginning of the current bash session) to the
history file.
-n Read the history lines not already read from the history
file into the current history list. These are lines
appended to the history file since the beginning of the
current bash session.
-r Read the contents of the history file and append them to
the current history list.
-w Write the current history list to the history file, over‐
writing the history file’s contents.
-p Perform history substitution on the following args and
display the result on the standard output. Does not
store the results in the history list. Each arg must be
quoted to disable normal history expansion.
-s Store the args in the history list as a single entry.
The last command in the history list is removed before
the args are added.

If the HISTTIMEFORMAT variable is set, the time stamp informa‐
tion associated with each history entry is written to the his‐
tory 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 encountered, an error occurs while
reading or writing the history file, an invalid offset is sup‐
plied 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 fol‐
lowing 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 jobspec. sigspec is either a case-insensitive
signal name such as SIGKILL (with or without the SIG prefix) or
a signal number; signum is a signal number. If sigspec is not
present, then SIGTERM is assumed. An argument of -l lists the
signal names. If any arguments are supplied when -l is given,
the names of the signals corresponding to the arguments are
listed, and the return status is 0. The exit_status argument to
-l is a number specifying either a signal number or the exit
status of a process terminated by a signal. kill returns true
if at least one signal was successfully sent, or false if an
error occurs or an invalid option is encountered.

let arg [arg …] Each arg is an arithmetic expression to be evaluated (see ARITH‐
METIC EVALUATION above). If the last arg evaluates to 0, let
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 func‐
tion and its children. With no operands, local writes a list of
local variables to the standard output. It is an error to use
local when not within a function. The return status is 0 unless
local is used outside a function, an invalid name is supplied,
or name is a readonly variable.

logout Exit a login shell.

mapfile [-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 vari‐
able array, or from file descriptor fd if the -u option is sup‐
plied. The variable MAPFILE is the default array. Options, if
supplied, have the following meanings:
-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 stan‐
dard input.
-C Evaluate callback each time quantum lines are read. The
-c option specifies quantum.
-c Specify the number of lines read between each call to
callback.

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

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

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

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

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

printf [-v var] format [arguments] Write the formatted arguments to the standard output under the
control of the format. The -v option causes the output to be
assigned to the variable var rather than being printed to the
standard output.

The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are 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 format specifications, printf
interprets the following extensions:
%b causes printf to expand backslash escape sequences in the
corresponding argument (except that \c terminates output,
backslashes in \’, \”, and \? are not removed, and octal
escapes beginning with \0 may contain up to four digits).
%q causes printf to output the corresponding argument in a
format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting
from using datefmt as a format string for strftime.
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 con‐
stants, except that a leading plus or minus sign is allowed, and
if the leading character is a single or double quote, the value
is the ASCII value of the following character.

The format is reused as necessary to consume all of the argu‐
ments. If the format requires more arguments than are supplied,
the extra format specifications behave as if a zero value or
null string, as appropriate, had been supplied. The return
value is zero on success, non-zero on failure.

pushd [-n] [+n] [-n] pushd [-n] [dir] Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory. With no arguments, 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 working directory as if it had been supplied
as the argument to the cd builtin.

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

pwd [-LP] Print the absolute pathname of the current working 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 timeout] [-u fd] [name …] One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u option, and the
first word is assigned to the first name, the second word to the
second name, and so on, with leftover words and their interven‐
ing 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 back‐
slash 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. Read‐
line 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 complete line of input, but honor a delim‐
iter if fewer than nchars characters are read before the
delimiter.
-N nchars
read returns after reading exactly nchars characters
rather than waiting for a complete line of input, unless
EOF is encountered or read times out. Delimiter charac‐
ters encountered in the input are not treated specially
and do not cause read to return until nchars characters
are read.
-p prompt
Display prompt on standard error, without a trailing new‐
line, before attempting to read any input. The prompt is
displayed only if input is coming from a terminal.
-r Backslash does not act as an escape character. The back‐
slash is considered to be part of the line. In particu‐
lar, a backslash-newline pair may not be used as a line
continuation.
-s Silent mode. If input is coming from a terminal, charac‐
ters are not echoed.
-t timeout
Cause read to time out and return failure if a complete
line of input (or a specified number of characters) is
not read within timeout seconds. timeout may be a deci‐
mal number with a fractional portion following the deci‐
mal point. This option is only effective if read is
reading input from a terminal, pipe, or other special
file; it has no effect when reading from regular files.
If read times out, read saves any partial input read into
the specified variable name. If timeout is 0, read
returns immediately, without trying to read any data.
The exit status is 0 if input is available on the speci‐
fied file descriptor, non-zero otherwise. The exit sta‐
tus is greater than 128 if the timeout is exceeded.
-u fd Read input from file descriptor fd.

If no names are supplied, the line read is assigned to the vari‐
able 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 assign‐
ing to a readonly variable) occurs, or an invalid file descrip‐
tor is supplied as the argument to -u.

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

return [n] Causes a function to stop executing and return the value speci‐
fied by n to its caller. If n is omitted, the return status is
that of the last command executed in the function body. If
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 com‐
mand associated with the RETURN trap is executed before execu‐
tion resumes after the function or script.

set [–abefhkmnptuvxBCEHPT] [-o option-name] [arg …] set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg …] Without options, the name and value of each shell variable are
displayed in a format that can be reused as input for setting or
resetting the currently-set variables. Read-only variables can‐
not 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 val‐
ues 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 immedi‐
ately, rather than before the next primary prompt. This
is effective only when job control is enabled.
-e Exit immediately if a pipeline (which may consist of a
single simple command), a list, or a compound command
(see SHELL GRAMMAR above), exits with a non-zero sta‐
tus. The shell does not exit if the command that fails
is part of the command list immediately following a
while or until keyword, part of the test following the
if or elif reserved words, part of any command executed
in a && or || list except the command following the
final && or ||, any command in a pipeline but the last,
or if the command’s return value is being inverted with
!. If a compound command other than a subshell returns
a non-zero status because a command failed while -e was
being ignored, the shell does not exit. A trap on ERR,
if set, is executed before the shell exits. This option
applies to the shell environment and each subshell envi‐
ronment separately (see COMMAND EXECUTION ENVIRONMENT
above), and may cause subshells to exit before executing
all the commands in the subshell.

If a compound command or shell function executes in a
context where -e is being ignored, none of the commands
executed within the compound command or function body
will be affected by the -e setting, even if -e is set
and a command returns a failure status. If a compound
command or shell function sets -e while executing in a
context where -e is ignored, that setting will not have
any effect until the compound command or the command
containing the function call completes.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up
for execution. This is enabled by default.
-k All arguments in the form of assignment statements are
placed in the environment for a command, not just those
that precede the command name.
-m Monitor mode. Job control is enabled. This option is
on by default for interactive shells on systems that
support it (see JOB CONTROL above). All processes run
in a separate process group. When a background job com‐
pletes, the shell prints a line containing its exit sta‐
tus.
-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 inter‐
face. This is enabled by default when the shell
is interactive, unless the shell is started with
the –noediting option. This also affects the
editing interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under
HISTORY. This option is on by default in inter‐
active shells.
ignoreeof
The effect is as if the shell command
“IGNOREEOF=10” had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all commands
in the pipeline exit successfully. This option
is disabled by default.
posix 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 commands to recreate the
current option settings is displayed on the standard
output.
-p Turn on privileged mode. In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to
the real user (group) id, and the -p option is not sup‐
plied, these actions are taken and the effective user id
is set to the real user id. If the -p option is sup‐
plied at startup, the effective user id is not reset.
Turning this option off causes the effective user and
group ids to be set to the real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the spe‐
cial parameters “@” and “*” as an error when performing
parameter expansion. If expansion is attempted on an
unset variable or parameter, the shell prints an error
message, and, if not interactive, exits with a non-zero
status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command, case
command, select command, or arithmetic for command, dis‐
play the expanded value of PS4, followed by the command
and its expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace Expansion
above). This is on by default.
-C If set, bash does not overwrite an existing file with
the >, >&, and <> redirection operators. This may be
overridden when creating output files by using the redi‐
rection operator >| instead of >.
-E If set, any trap on ERR is inherited by shell functions,
command substitutions, and commands executed in a sub‐
shell environment. The ERR trap is normally not inher‐
ited in such cases.
-H Enable ! style history substitution. This option is on
by default when the shell is interactive.
-P If set, the shell does not resolve symbolic links when
executing commands such as cd that change the current
working directory. It uses the physical directory
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 parame‐
ters are set to the args, even if some of them begin
with a -.
– Signal the end of options, cause all remaining args to
be assigned to the positional parameters. The -x and -v
options are turned off. If there are no args, the posi‐
tional parameters remain unchanged.

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

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

shopt [-pqsu] [-o] [optname …] Toggle the values of settings controlling optional shell behav‐
ior. The settings can be either those listed below, or, if the
-o option is used, those available with the -o option to the set
builtin command. With no options, or with the -p option, a list
of all settable options is displayed, with an indication of
whether or not each is set. 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 multi‐
ple optname arguments are given with -q, the return sta‐
tus 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 com‐
mand. This option is only used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command that is
not a directory is assumed to be the name of a variable
whose value is the directory to change to.
cdspell If set, minor errors in the spelling of a directory com‐
ponent in a cd command will be corrected. The errors
checked for are transposed characters, a missing charac‐
ter, and one character too many. If a correction is
found, the corrected filename is printed, and the com‐
mand proceeds. This option is only used by interactive
shells.
checkhash
If set, bash checks that a command found in the hash ta‐
ble exists before trying to execute it. If a hashed
command no longer exists, a normal path search is per‐
formed.
checkjobs
If set, bash lists the status of any stopped and running
jobs before exiting an interactive shell. If any jobs
are running, this causes the exit to be deferred until a
second exit is attempted without an intervening command
(see JOB CONTROL above). The shell always postpones
exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each command
and, if necessary, updates the values of LINES and COL‐
UMNS.
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 compar‐
ison when using the [[ conditional command’s < and >
operators. Bash versions prior to bash-4.1 use ASCII
collation and strcmp; bash-4.1 and later use the cur‐
rent 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 inter‐
rupting a command list. Bash versions 4.0 and later
interrupt the list as if the shell received the inter‐
rupt; 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 num‐
ber) and the characters between the single quotes are
considered quoted. This is the behavior of posix mode
through version 4.1. The default bash behavior remains
as in previous versions.
compat42
If set, bash does not process the replacement string in
the pattern substitution word expansion using quote
removal.
complete_fullquote
If set, bash quotes all shell metacharacters in file‐
names and directory names when performing completion.
If not set, bash removes metacharacters such as the dol‐
lar sign from the set of characters that will be quoted
in completed filenames when these metacharacters appear
in shell variable references in words to be completed.
This means that dollar signs in variable names that
expand to directories will not be quoted; however, any
dollar signs appearing in filenames will not be quoted,
either. This is active only when bash is using back‐
slashes 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 buf‐
fer. If not set, bash attempts to preserve what the
user typed.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name ini‐
tially supplied does not exist.
dotglob If set, bash includes filenames beginning with a `.’ in
the results of pathname expansion.
execfail
If set, a non-interactive shell will not exit if it can‐
not execute the file specified as an argument to the
exec builtin command. An interactive shell does not
exit if exec fails.
expand_aliases
If set, aliases are expanded as described above under
ALIASES. This option is enabled by default for interac‐
tive shells.
extdebug
If set, 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 sub‐
routine (a shell function or a shell script exe‐
cuted by the . or source builtins), a call to
return is simulated.
4. BASH_ARGC and BASH_ARGV are updated as described
in their descriptions above.
5. Function tracing is enabled: command substitu‐
tion, shell functions, and subshells invoked with
( command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the ERR trap.
extglob If set, the extended pattern matching features described
above under Pathname Expansion are enabled.
extquote
If set, $’string’ and $”string” quoting is performed
within ${parameter} expansions enclosed in double
quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell
variable cause words to be ignored when performing word
completion even if the ignored words are the only possi‐
ble completions. See SHELL VARIABLES above for a
description of FIGNORE. This option is enabled by
default.
globasciiranges
If set, range expressions used in pattern matching
bracket expressions (see Pattern Matching above) behave
as if in the traditional C locale when performing com‐
parisons. That is, the current locale’s collating
sequence is not taken into account, so b will not col‐
late 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 con‐
text will match all files and zero or more directories
and subdirectories. If the pattern is followed by a /,
only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU error message format.
histappend
If set, the history list is appended to the file named
by the value of the HISTFILE variable when the shell
exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of his‐
tory substitution are not immediately passed to the
shell parser. Instead, the resulting line is loaded
into the readline editing buffer, allowing further modi‐
fication.
hostcomplete
If set, and readline is being used, bash will attempt to
perform hostname completion when a word containing a @
is being completed (see Completing under READLINE
above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an inter‐
active login shell exits.
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 back‐
ground 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 INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has
been accessed since the last time it was checked, the
message “The mail in mailfile has been read” is dis‐
played.
no_empty_cmd_completion
If set, and readline is being used, bash will not
attempt to search the PATH for possible completions when
completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case or
[[ conditional commands.
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 Pro‐
grammable Completion above) are enabled. This option is
enabled by default.
promptvars
If set, prompt strings undergo parameter expansion, com‐
mand 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 dis‐
cover whether or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when
the shift count exceeds the number of positional parame‐
ters.
sourcepath
If set, the source (.) builtin uses the value of PATH to
find the directory containing the file supplied as an
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 sta‐
tus is 0 unless the shell is a login shell and -f is not sup‐
plied, or if job control is not enabled.

test expr
[ expr ] Return a status of 0 (true) or 1 (false) depending on the evalu‐
ation of the conditional expression expr. Each operator and op‐
erand must be a separate argument. Expressions are composed of
the primaries described above under CONDITIONAL EXPRESSIONS.
test does not accept any options, nor does it accept and ignore
an argument of — as signifying the end of options.

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

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

0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not
null.
2 arguments
If the first argument is !, the expression is true if and
only if the second argument is null. If the first argu‐
ment is one of the unary conditional operators listed
above under CONDITIONAL EXPRESSIONS, the expression is
true if the unary test is true. If the first argument is
not a valid unary conditional operator, the expression is
false.
3 arguments
The following conditions are applied in the order listed.
If the second argument is one of the binary conditional
operators listed above under CONDITIONAL EXPRESSIONS, the
result of the expression is the result of the binary test
using the first and third arguments as operands. The -a
and -o operators are considered binary operators when
there are three arguments. If the first argument is !,
the value is the negation of the two-argument test using
the second and third arguments. If the first argument is
exactly ( and the third argument is exactly ), the result
is the one-argument test of the second argument. Other‐
wise, the expression is false.
4 arguments
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 eval‐
uated 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 lexico‐
graphically 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 cor‐
responding numbers. Each sigspec is either a signal name
defined in , 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 exe‐
cuted before every simple command, for command, case command,
select command, every arithmetic for command, and before the
first command executes in a shell function (see SHELL GRAMMAR
above). Refer to the description of the extdebug option to the
shopt builtin for details of its effect on the DEBUG trap. If a
sigspec is RETURN, the command arg is executed each time a shell
function or a script executed with the . or source builtins fin‐
ishes executing.

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 executed in a && or || list except the command
following the final && or ||, any command in a pipeline but the
last, or if the command’s return value is being inverted using
!. These are the same conditions obeyed by the errexit (-e)
option.

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

type [-aftpP] name [name …] With no options, indicate how each name would be interpreted if
used as a command name. If the -t option is used, type prints a
string which is one of alias, keyword, function, builtin, or
file if name is an alias, shell reserved word, function,
builtin, or disk file, respectively. If the name is not found,
then nothing is printed, and an exit status of false is
returned. If the -p option is used, type either returns the
name of the disk file that would be executed if name were speci‐
fied 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 necessar‐
ily 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 speci‐
fied, both the soft and hard limits are set. The value of limit
can be a number in the unit specified for the resource or one of
the special values hard, soft, or unlimited, which stand for the
current hard limit, the current soft limit, and no limit,
respectively. If limit is omitted, the current value of the
soft limit of the resource is printed, unless the -H option is
given. When more than one resource is specified, the limit name
and unit are printed before the value. Other options are inter‐
preted 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 sup‐
plied, 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. If mode is omitted, the current value of the mask is
printed. The -S option causes the mask to be printed in sym‐
bolic form; the default output is an octal number. If the -p
option is supplied, and mode is omitted, the output is in a form
that may be reused as input. The return status is 0 if the mode
was successfully changed or if no mode argument was supplied,
and false otherwise.

unalias [-a] [name …] Remove each name from the list of defined aliases. If -a is
supplied, all alias definitions are removed. The return value
is true unless a supplied name is not a defined alias.

unset [-fv] [-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 func‐
tion, 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 vari‐
able 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, RAN‐
DOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are
unset, they lose their special properties, even if they are sub‐
sequently reset. The exit status is true unless a name is read‐
only.

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 pro‐
cesses 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 controlled than the standard shell. It
behaves identically to bash with the exception that the following are
disallowed or not performed:

· changing directories with cd

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

· specifying command names containing /

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

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

· importing function definitions from the shell environment at
startup

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

· redirecting output using the >, >|, <>, >&, &>, and >> redirect‐
ion 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 COM‐
MAND EXECUTION above), rbash turns off any restrictions in the shell
spawned to execute the script.

SEE ALSO

Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utili‐
ties, IEEE —
http://pubs.opengroup.org/onlinepubs/9699919799/
http://tiswww.case.edu/~chet/bash/POSIX — a description of posix mode
sh, ksh, csh
emacs(1), vi
readline

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 filing a bug report.

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

BUGS

It’s too big and too slow.

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

Aliases are confusing in some uses.

Shell builtin commands and functions are not stoppable/restartable.

Compound commands and command sequences of the form `a ; b ; c’ are not
handled gracefully when process suspension is attempted. When a
process is stopped, the shell immediately executes the next command in
the 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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