bash

BASH(1) General Commands Manual BASH(1)

NAME
bash - GNU Bourne-Again SHell

SYNOPSIS
bash [options] [command_string | file]

DESCRIPTION
Bash is a command language interpreter that executes commands read from
the standard input, from a string, or from a file. It is a
reimplementation and extension of the Bourne shell, the historical Unix
command language interpreter. Bash also incorporates useful features
from the Korn and C shells (ksh and csh).

POSIX is the name for a family of computing standards based on Unix.
Bash is intended to be a conformant implementation of the Shell and
Utilities portion of the IEEE POSIX specification (IEEE Standard 1003.1).
Bash POSIX mode (hereafter referred to as posix mode) changes the shell's
behavior where its default operation differs from the standard to
strictly conform to the standard. See SEE ALSO below for a reference to
a document that details how posix mode affects bash's behavior. Bash can
be configured to be POSIX-conformant by default.

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

-c If the -c option is present, then commands are read from the
first non-option argument command_string. If there are
arguments after the command_string, the first argument is
assigned to $0 and any remaining arguments are assigned to the
positional parameters. The assignment to $0 sets the name of
the shell, which is used in warning and error messages.

-i If the -i option is present, the shell is interactive.

-l Make bash act as if it had been invoked as a login shell (see
INVOCATION below).

-r If the -r option is present, the shell becomes restricted (see
RESTRICTED SHELL below).

-s If the -s option is present, or if no arguments remain after
option processing, the shell reads commands from the standard
input. This option allows the positional parameters to be set
when invoking an interactive shell or when reading input
through a pipe.

-D Print a list of all double-quoted strings preceded by $ on the
standard output. These are the strings that are subject to
language translation when the current locale is not C or POSIX.
This implies the -n option; no commands will be executed.

[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt
builtin (see SHELL BUILTIN COMMANDS below). If shopt_option is
present, -O sets the value of that option; +O unsets it. If
shopt_option is not supplied, bash prints the names and values
of the shell options accepted by shopt on the standard output.
If the invocation option is +O, the output is displayed in a
format that may be reused as input.

-- A -- signals the end of options and disables further option
processing. Any arguments after the -- are treated as a shell
script filename (see below) and arguments passed to that
script. An argument of - is equivalent to --.

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

--debugger
Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see the description of
the extdebug option to the shopt builtin below).

--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext “po”
(portable object) file format.

--dump-strings
Equivalent to -D.

--help Display a usage message on standard output and exit successfully.

--init-file file
--rcfile file
Execute commands from file instead of the standard personal
initialization file ~/.bashrc if the shell is interactive (see
INVOCATION below).

--login
Equivalent to -l.

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

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

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

--posix
Enable posix mode; change the behavior of bash where the default
operation differs from the POSIX standard to match the standard.

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

--verbose
Equivalent to -v.

--version
Show version information for this instance of bash on the standard
output and exit successfully.

ARGUMENTS
If arguments remain after option processing, and neither the -c nor the
-s option has been supplied, the first argument is treated as the name of
a file containing shell commands (a shell script). When bash is invoked
in this fashion, $0 is set to the name of the file, and the positional
parameters are set to the remaining arguments. Bash reads and executes
commands from this file, then exits. Bash's exit status is the exit
status of the last command executed in the script. If no commands are
executed, the exit status is 0. Bash first attempts to open the file in
the current directory, and, if no file is found, searches the directories
in PATH for the script.

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

An interactive shell is one started without non-option arguments (unless
-s is specified) and without the -c option, and whose standard input and
standard error are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. Bash sets PS1 and $-
includes i if the shell is interactive, so a shell script or a startup
file can test this state.

The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in filenames as described below under Tilde Expansion
in the EXPANSION section.

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

When an interactive login shell exits, or a non-interactive login shell
executes the exit builtin command, bash reads and executes commands from
the file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash
reads and executes commands from ~/.bashrc, if that file exists. The
--norc option inhibits this behavior. The --rcfile file option causes
bash to use file instead of ~/.bashrc.

When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment, expands
its value if it appears there, and uses the expanded value as the name of
a file to read and execute. Bash behaves as if the following command
were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but does not use the value of the PATH variable to search for the
filename.

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

When invoked as sh, bash enters posix mode after reading the startup
files.

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

Bash attempts to determine when it is being run with its standard input
connected to a network connection, as when executed by the historical and
rarely-seen remote shell daemon, usually rshd, or the secure shell daemon
sshd. If bash determines it is being run non-interactively in this
fashion, it reads and executes commands from ~/.bashrc, if that file
exists and is readable. Bash does not read this file if invoked as sh.
The --norc option inhibits this behavior, and the --rcfile option makes
bash use a different file instead of ~/.bashrc, but neither rshd nor sshd
generally invoke the shell with those options or allow them to be
specified.

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

DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
whitespace
A character belonging to the space character class in the current
locale, or for which isspace(3) returns true.
word A sequence of characters considered as a single unit by the shell.
Also known as a token.
name A word consisting only of alphanumeric characters and underscores,
and beginning with an alphabetic character or an underscore. Also
referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the
following:
| & ; ( ) < > space tab newline
control operator
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>

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

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

SHELL GRAMMAR
This section describes the syntax of the various forms of shell commands.

Simple Commands
A simple command is a sequence of optional variable assignments followed
by blank-separated words and redirections, and terminated by a control
operator. The first word specifies the command to be executed, and is
passed as argument zero. The remaining words are passed as arguments to
the invoked command.

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

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

[time [-p]] [ ! ] command1 [ [|⎪|&] command2 ... ]

The standard output of command1 is connected via a pipe to the standard
input of command2. This connection is performed before any redirections
specified by the command1(see REDIRECTION below). If |& is the pipeline
operator, command1's standard error, in addition to its standard output,
is connected to command2's standard input through the pipe; it is
shorthand for 2>&1 |. This implicit redirection of the standard error to
the standard output is performed after any redirections specified by
command1.

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

If the time reserved word precedes a pipeline, the shell reports the
elapsed as well as user and system time consumed by its execution when
the pipeline terminates. The -p option changes the output format to that
specified by POSIX. When the shell is in posix mode, it does not
recognize time as a reserved word if the next token begins with a “-”.
The value of the TIMEFORMAT variable is a format string that specifies
how the timing information should be displayed; see the description of
TIMEFORMAT below under Shell Variables.

When the shell is in posix mode, time may appear by itself as the only
word in a simple command. In this case, the shell displays the total
user and system time consumed by the shell and its children. The
TIMEFORMAT variable specifies the format of the time information.

Each command in a multi-command pipeline, where pipes are created, is
executed in a subshell, which is a separate process. See COMMAND
EXECUTION ENVIRONMENT for a description of subshells and a subshell
environment. If the lastpipe option is enabled using the shopt builtin
(see the description of shopt below), and job control is not active, the
last element of a pipeline may be run by the shell process.

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

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

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

If a command is terminated by the control operator &, the shell executes
the command in the background in a subshell. The shell does not wait for
the command to finish, and the return status is 0. These are referred to
as asynchronous commands. Commands separated by a ; are executed
sequentially; the shell waits for each command to terminate in turn. The
return status is the exit status of the last command executed.

AND and OR lists are sequences of one or more pipelines separated by the
&& and || control operators, respectively. AND and OR lists are executed
with left associativity. An AND list has the form

command1 && command2

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

An OR list has the form

command1 || command2

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

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

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

{ list; }
list is executed in the current shell environment. list must be
terminated with a newline or semicolon. This is known as a group
command. The return status is the exit status of list.

Note that unlike the metacharacters ( and ), { and } are reserved
words and must occur where a reserved word is permitted to be
recognized. Since they do not cause a word break, they must be
separated from list by whitespace or another shell metacharacter.

((expression))
The arithmetic expression is evaluated according to the rules
described below under ARITHMETIC EVALUATION. If the value of the
expression is non-zero, the return status is 0; otherwise the
return status is 1. The expression undergoes the same expansions
as if it were within double quotes, but unescaped double quote
characters in expression are not treated specially and are
removed. Since this can potentially result in empty strings, this
command treats those as expressions that evaluate to 0.

[[ expression ]]
Evaluate the conditional expression expression and return a status
of zero (true) or non-zero (false). Expressions are composed of
the primaries described below under CONDITIONAL EXPRESSIONS. The
words between the [[ and ]] do not undergo word splitting and
pathname expansion. The shell performs tilde expansion, parameter
and variable expansion, arithmetic expansion, command
substitution, process substitution, and quote removal on those
words. Conditional operators such as -f must be unquoted to be
recognized as primaries.

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

When the == and != operators are used, the string to the right of
the operator is considered a pattern and matched according to the
rules described below under Pattern Matching, as if the extglob
shell option were enabled. The = operator is equivalent to ==.
If the nocasematch shell option is enabled, the match is performed
without regard to the case of alphabetic characters. The return
value is 0 if the string matches (==) or does not match (!=) the
pattern, and 1 otherwise. If any part of the pattern is quoted,
the quoted portion is matched as a string: every character in the
quoted portion matches itself, instead of having any special
pattern matching meaning.

An additional binary operator, =~, is available, with the same
precedence as == and !=. When it is used, the string to the right
of the operator is considered a POSIX extended regular expression
and matched accordingly (using the POSIX regcomp and regexec
interfaces usually described in regex(3)). The return value is 0
if the string matches the pattern, and 1 otherwise. If the
regular expression is syntactically incorrect, the conditional
expression's return value is 2. If the nocasematch shell option
is enabled, the match is performed without regard to the case of
alphabetic characters.

If any part of the pattern is quoted, the quoted portion is
matched literally, as above. If the pattern is stored in a shell
variable, quoting the variable expansion forces the entire pattern
to be matched literally. Treat bracket expressions in regular
expressions carefully, since normal quoting and pattern characters
lose their meanings between brackets.

The match succeeds if the pattern matches any part of the string.
Anchor the pattern using the ^ and $ regular expression operators
to force it to match the entire string.

The array variable BASH_REMATCH records which parts of the string
matched the pattern. The element of BASH_REMATCH with index 0
contains the portion of the string matching the entire regular
expression. Substrings matched by parenthesized subexpressions
within the regular expression are saved in the remaining
BASH_REMATCH indices. The element of BASH_REMATCH with index n is
the portion of the string matching the nth parenthesized
subexpression. Bash sets BASH_REMATCH in the global scope;
declaring it as a local variable will lead to unexpected results.

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

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

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

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

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

Use the break and continue builtins (see SHELL BUILTIN COMMANDS
below) to control loop execution.

select name [ in word ] ; do list ; done
First, expand the list of words following in, generating a list of
items, and print the set of expanded words the standard error,
each preceded by a number. If the in word is omitted, print the
positional parameters (see PARAMETERS below). select then
displays the PS3 prompt and reads a line from the standard input.
If the line consists of a number corresponding to one of the
displayed words, then select sets the value of name to that word.
If the line is empty, select displays the words and prompt again.
If EOF is read, select completes and returns 1. Any other value
sets name to null. The line read is saved in the variable REPLY.
The list is executed after each selection until a break command is
executed. The exit status of select is the exit status of the
last command executed in list, or zero if no commands were
executed.

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against
each pattern in turn, proceeding from first to last, using the
matching rules described under Pattern Matching below. A pattern
list is a set of one or more patterns separated by , and the )
operator terminates the pattern list. The word is expanded using
tilde expansion, parameter and variable expansion, arithmetic
expansion, command substitution, process substitution and quote
removal. Each pattern examined is expanded using tilde expansion,
parameter and variable expansion, arithmetic expansion, command
substitution, process substitution, and quote removal. If the
nocasematch shell option is enabled, the match is performed
without regard to the case of alphabetic characters. A clause is
a pattern list and an associated list.

When a match is found, case executes the corresponding list. If
the ;; operator terminates the case clause, the case command
completes after the first match. Using ;& in place of ;; causes
execution to continue with the list associated with the next
pattern list. Using ;;& in place of ;; causes the shell to test
the next pattern list in the statement, if any, and execute any
associated list if the match succeeds, continuing the case
statement execution as if the pattern list had not matched. The
exit status is zero if no pattern matches.

Otherwise, it is the exit status of the last command executed in
the last list executed.

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

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

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

The syntax for a coprocess is:

coproc [NAME] command [redirections]

This creates a coprocess named NAME. command may be either a simple
command or a compound command (see above). NAME is a shell variable
name. If NAME is not supplied, the default name is COPROC.

The recommended form to use for a coprocess is

coproc NAME { command [redirections]; }

This form is preferred because simple commands result in the coprocess
always being named COPROC, and it is simpler to use and more complete
than the other compound commands.

If command is a compound command, NAME is optional. The word following
coproc determines whether that word is interpreted as a variable name: it
is interpreted as NAME if it is not a reserved word that introduces a
compound command. If command is a simple command, NAME is not allowed;
this is to avoid confusion between NAME and the first word of the simple
command.

When the coprocess is executed, the shell creates an array variable (see
Arrays below) named NAME in the context of the executing shell. The
standard output of command is connected via a pipe to a file descriptor
in the executing shell, and that file descriptor is assigned to NAME[0].
The standard input of command is connected via a pipe to a file
descriptor in the executing shell, and that file descriptor is assigned
to NAME[1]. This pipe is established before any redirections specified
by the command (see REDIRECTION below). The file descriptors can be
utilized as arguments to shell commands and redirections using standard
word expansions. Other than those created to execute command and process
substitutions, the file descriptors are not available in subshells.

The process ID of the shell spawned to execute the coprocess is available
as the value of the variable NAME_PID. The wait builtin may be used to
wait for the coprocess to terminate.

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

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

fname () compound-command [redirection]
function fname [()] compound-command [redirection]
This defines a function named fname. The reserved word function
is optional. If the function reserved word is supplied, the
parentheses are optional. The body of the function is the
compound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above. If the
function reserved word is used, but the parentheses are not
supplied, the braces are recommended. compound-command is
executed whenever fname is specified as the name of a simple
command. When in posix mode, fname must be a valid shell name and
may not be the name of one of the POSIX special builtins. In
default mode, a function name can be any unquoted shell word that
does not contain $.

Any redirections (see REDIRECTION below) specified when a function is
defined are performed when the function is executed.

The exit status of a function definition is zero unless a syntax error
occurs or a readonly function with the same name already exists. When
executed, the exit status of a function is the exit status of the last
command executed in the body. (See FUNCTIONS below.)

COMMENTS
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is enabled (see SHELL
BUILTIN COMMANDS below), a word beginning with # introduces a comment. A
word begins at the beginning of a line, after unquoted whitespace, or
after an operator. The comment causes that word and all remaining
characters on that line to be ignored. An interactive shell without the
interactive_comments option enabled does not allow comments. The
interactive_comments option is enabled by default in interactive shells.

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

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

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

There are four quoting mechanisms: the escape character, single quotes,
double quotes, and dollar-single quotes.

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

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

Enclosing characters in double quotes preserves the literal value of all
characters within the quotes, with the exception of $, `, \, and, when
history expansion is enabled, !. When the shell is in posix mode, the !
has no special meaning within double quotes, even when history expansion
is enabled. The characters $ and ` retain their special meaning within
double quotes. The backslash retains its special meaning only when
followed by one of the following characters: $, `, ", \, or <newline>.
Backslashes preceding characters without a special meaning are left
unmodified.

A double quote may be quoted within double quotes by preceding it with a
backslash. If enabled, history expansion will be performed unless an !
appearing in double quotes is escaped using a backslash. The backslash
preceding the ! is not removed.

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

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

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

Translating Strings
A double-quoted string preceded by a dollar sign ($"string") causes the
string to be translated according to the current locale. The gettext
infrastructure performs the lookup and translation, using the
LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN shell variables. If the
current locale is C or POSIX, if there are no translations available, or
if the string is not translated, the dollar sign is ignored, and the
string is treated as double-quoted as described above. This is a form of
double quoting, so the string remains double-quoted by default, whether
or not it is translated and replaced. If the noexpand_translation option
is enabled using the shopt builtin, translated strings are single-quoted
instead of double-quoted. See the description of shopt below under SHELL
BUILTIN COMMANDS.

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

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

A variable is assigned to using a statement of the form

name=[value]

If value is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, and quote removal (see EXPANSION
below). If the variable has its integer attribute set, then value is
evaluated as an arithmetic expression even if the $((...)) expansion is
not used (see Arithmetic Expansion below). Word splitting and pathname
expansion are not performed. Assignment statements may also appear as
arguments to the alias, declare, typeset, export, readonly, and local
builtin commands (declaration commands). When in posix mode, these
builtins may appear in a command after one or more instances of the
command builtin and retain these assignment statement properties.

In the context where an assignment statement is assigning a value to a
shell variable or array index, the “+=” operator appends to or adds to
the variable's previous value. This includes arguments to declaration
commands such as declare that accept assignment statements. When “+=” is
applied to a variable for which the integer attribute has been set, the
variable's current value and value are each evaluated as arithmetic
expressions, and the sum of the results is assigned as the variable's
value. The current value is usually an integer constant, but may be an
expression. When “+=” is applied to an array variable using compound
assignment (see Arrays below), the variable's value is not unset (as it
is when using “=”), and new values are appended to the array beginning at
one greater than the array's maximum index (for indexed arrays) or added
as additional key-value pairs in an associative array. When applied to a
string-valued variable, value is expanded and appended to the variable's
value.

A variable can be assigned the nameref attribute using the -n option to
the declare or local builtin commands (see the descriptions of declare
and local below) to create a nameref, or a reference to another variable.
This allows variables to be manipulated indirectly. Whenever the nameref
variable is referenced, assigned to, unset, or has its attributes
modified (other than using or changing the nameref attribute itself), the
operation is actually performed on the variable specified by the nameref
variable's value. A nameref is commonly used within shell functions to
refer to a variable whose name is passed as an argument to the function.
For instance, if a variable name is passed to a shell function as its
first argument, running

declare -n ref=$1

inside the function creates a local nameref variable ref whose value is
the variable name passed as the first argument. References and
assignments to ref, and changes to its attributes, are treated as
references, assignments, and attribute modifications to the variable
whose name was passed as $1. If the control variable in a for loop has
the nameref attribute, the list of words can be a list of shell
variables, and a name reference is established for each word in the list,
in turn, when the loop is executed. Array variables cannot be given the
nameref attribute. However, nameref variables can reference array
variables and subscripted array variables. Namerefs can be unset using
the -n option to the unset builtin. Otherwise, if unset is executed with
the name of a nameref variable as an argument, the variable referenced by
the nameref variable is unset.

When the shell starts, it reads its environment and creates a shell
variable from each environment variable that has a valid name, as
described below (see ENVIRONMENT).

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

When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces (see EXPANSION below). Without
braces, a digit following $ can only refer to one of the first nine
positional parameters ($1-$9) or the special parameter $0 (see the next
section).

Special Parameters
The shell treats several parameters specially. These parameters may only
be referenced; assignment to them is not allowed. Special parameters are
denoted by one of the following characters.

* ($*) Expands to the positional parameters, starting from one.
When the expansion is not within double quotes, each positional
parameter expands to a separate word. In contexts where word
expansions are performed, those words are subject to further word
splitting and pathname expansion. When the expansion occurs
within double quotes, it expands to a single word with the value
of each parameter separated by the first character of the IFS
variable. That is, "$*" is equivalent to "$1c$2c...", where c is
the first character of the value of the IFS variable. If IFS is
unset, the parameters are separated by spaces. If IFS is null,
the parameters are joined without intervening separators.
@ ($@) Expands to the positional parameters, starting from one. In
contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting. In contexts
where word splitting is not performed, such as the value portion
of an assignment statement, this expands to a single word with
each positional parameter separated by a space. When the
expansion occurs within double quotes, and word splitting is
performed, each parameter expands to a separate word. That is,
"$@" is equivalent to "$1" "$2" ... If the double-quoted
expansion occurs within a word, the expansion of the first
parameter is joined with the expansion of the beginning part of
the original word, and the expansion of the last parameter is
joined with the expansion of the last part of the original word.
When there are no positional parameters, "$@" and $@ expand to
nothing (i.e., they are removed).
# ($#) Expands to the number of positional parameters in decimal.
? ($?) Expands to the exit status of the most recently executed
command.
- ($-) Expands to the current option flags as specified upon
invocation, by the set builtin command, or those set by the shell
itself (such as the -i option).
$ ($$) Expands to the process ID of the shell. In a subshell, it
expands to the process ID of the parent shell, not the subshell.
! ($!)Expands to the process ID of the job most recently placed into
the background, whether executed as an asynchronous command or
using the bg builtin (see JOB CONTROL below).
0 ($0) Expands to the name of the shell or shell script. This is
set at shell initialization. If bash is invoked with a file of
commands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after the
string to be executed, if one is present. Otherwise, it is set to
the filename used to invoke bash, as given by argument zero.

Shell Variables
The shell sets following variables:

_ ($_, an underscore) This has a number of meanings depending on
context. At shell startup, _ is set to the pathname used to
invoke the shell or shell script being executed as passed in the
environment or argument list. Subsequently, it expands to the
last argument to the previous simple command executed in the
foreground, after expansion. It is also set to the full pathname
used to invoke each command executed and placed in the environment
exported to that command. When checking mail, $_ expands to the
name of the mail file currently being checked.
BASH Expands to the full filename used to invoke this instance of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the -s option to the shopt builtin
command (see SHELL BUILTIN COMMANDS below). The options appearing
in BASHOPTS are those reported as on by shopt. If this variable
is in the environment when bash starts up, the shell enables each
option in the list before reading any startup files. If this
variable is exported, child shells will enable each option in the
list. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized. Assignments to
BASHPID have no effect. If BASHPID is unset, it loses its special
properties, even if it is subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to the
internal list of aliases as maintained by the alias builtin.
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not remove aliases from
the alias list. If BASH_ALIASES is unset, it loses its special
properties, even if it is subsequently reset.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack. The number
of parameters to the current subroutine (shell function or script
executed with . or source) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed
onto BASH_ARGC. The shell sets BASH_ARGC only when in extended
debugging mode (see the description of the extdebug option to the
shopt builtin below). Setting extdebug after the shell has
started to execute a script, or referencing this variable when
extdebug is not set, may result in inconsistent values.
Assignments to BASH_ARGC have no effect, and it may not be unset.
BASH_ARGV
An array variable containing all of the parameters in the current
bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter of
the initial call is at the bottom. When a subroutine is executed,
the shell pushes the supplied parameters onto BASH_ARGV. The
shell sets BASH_ARGV only when in extended debugging mode (see the
description of the extdebug option to the shopt builtin below).
Setting extdebug after the shell has started to execute a script,
or referencing this variable when extdebug is not set, may result
in inconsistent values. Assignments to BASH_ARGV have no effect,
and it may not be unset.
BASH_ARGV0
When referenced, this variable expands to the name of the shell or
shell script (identical to $0; see the description of special
parameter 0 above). Assigning a value to BASH_ARGV0 sets $0 to
the same value. If BASH_ARGV0 is unset, it loses its special
properties, even if it is subsequently reset.
BASH_CMDS
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the hash builtin.
Adding elements to this array makes them appear in the hash table;
however, unsetting array elements currently does not remove
command names from the hash table. If BASH_CMDS is unset, it
loses its special properties, even if it is subsequently reset.
BASH_COMMAND
Expands to the command currently being executed or about to be
executed, unless the shell is executing a command as the result of
a trap, in which case it is the command executing at the time of
the trap. If BASH_COMMAND is unset, it loses its special
properties, even if it is subsequently reset.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files where each corresponding member of FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the source file
(${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within another shell function).
Use LINENO to obtain the current line number. Assignments to
BASH_LINENO have no effect, and it may not be unset.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the enable command
looks for dynamically loadable builtins.
BASH_MONOSECONDS
Each time this variable is referenced, it expands to the value
returned by the system's monotonic clock, if one is available. If
there is no monotonic clock, this is equivalent to EPOCHSECONDS.
If BASH_MONOSECONDS is unset, it loses its special properties,
even if it is subsequently reset.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary
operator to the [[ conditional command. The element with index 0
is the portion of the string matching the entire regular
expression. The element with index n is the portion of the string
matching the nth parenthesized subexpression.
BASH_SOURCE
An array variable whose members are the source filenames where the
corresponding shell function names in the FUNCNAME array variable
are defined. The shell function ${FUNCNAME[$i]} is defined in the
file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}.
Assignments to BASH_SOURCE have no effect, and it may not be
unset.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The initial
value is 0. If BASH_SUBSHELL is unset, it loses its special
properties, even if it is subsequently reset.
BASH_TRAPSIG
Set to the signal number corresponding to the trap action being
executed during its execution. See the description of trap under
SHELL BUILTIN COMMANDS below for information about signal numbers
and trap execution.
BASH_VERSINFO
A readonly array variable whose members hold version information
for this instance of bash. The values assigned to the array
members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of
bash (e.g., 5.2.37(3)-release).
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current
cursor position. This variable is available only in shell
functions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the
current completion function. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning
of the current command. If the current cursor position is at the
end of the current command, the value of this variable is equal to
${#COMP_LINE}. This variable is available only in shell functions
and external commands invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of attempted
completion that caused a completion function to be called: TAB,
for normal completion, ?, for listing completions after successive
tabs, !, for listing alternatives on partial word completion, @,
to list completions if the word is not unmodified, or %, for menu
completion. This variable is available only in shell functions
and external commands invoked by the programmable completion
facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word
separators when performing word completion. If COMP_WORDBREAKS is
unset, it loses its special properties, even if it is subsequently
reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual
words in the current command line. The line is split into words
as readline would split it, using COMP_WORDBREAKS as described
above. This variable is available only in shell functions invoked
by the programmable completion facilities (see Programmable
Completion below).
COPROC An array variable (see Arrays below) created to hold the file
descriptors for output from and input to an unnamed coprocess (see
Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current
contents of the directory stack. Directories appear in the stack
in the order they are displayed by the dirs builtin. Assigning to
members of this array variable may be used to modify directories
already in the stack, but the pushd and popd builtins must be used
to add and remove directories. Assigning to this variable does
not change the current directory. If DIRSTACK is unset, it loses
its special properties, even if it is subsequently reset.
EPOCHREALTIME
Each time this parameter is referenced, it expands to the number
of seconds since the Unix Epoch (see time(3)) as a floating-point
value with micro-second granularity. Assignments to EPOCHREALTIME
are ignored. If EPOCHREALTIME is unset, it loses its special
properties, even if it is subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the number
of seconds since the Unix Epoch (see time(3)). Assignments to
EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its
special properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized
at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0
is the name of any currently-executing shell function. The
bottom-most element (the one with the highest index) is “main”.
This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it
loses its special properties, even if it is subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE. Each
element of FUNCNAME has corresponding elements in BASH_LINENO and
BASH_SOURCE to describe the call stack. For instance,
${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at
line number ${BASH_LINENO[$i]}. The caller builtin displays the
current call stack using this information.
GROUPS An array variable containing the list of groups of which the
current user is a member. Assignments to GROUPS have no effect.
If GROUPS is unset, it loses its special properties, even if it is
subsequently reset.
HISTCMD
The history number, or index in the history list, of the current
command. Assignments to HISTCMD have no effect. If HISTCMD is
unset, it loses its special properties, even if it is subsequently
reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of
machine on which bash is executing. The default is system-
dependent.
LINENO Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to be
meaningful. If LINENO is unset, it loses its special properties,
even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type
on which bash is executing, in the standard GNU cpu-company-system
format. The default is system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text read
by the mapfile builtin when no variable name is supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system
on which bash is executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit
status values from the commands in the most-recently-executed
foreground pipeline, which may consist of only a simple command
(see SHELL GRAMMAR above). Bash sets PIPESTATUS after executing
multi-element pipelines, timed and negated pipelines, simple
commands, subshells created with the ( operator, the [[ and ((
compound commands, and after error conditions that result in the
shell aborting command execution.
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, it expands to a random
integer between 0 and 32767. Assigning a value to RANDOM
initializes (seeds) the sequence of random numbers. Seeding the
random number generator with the same constant value produces the
same sequence of values. If RANDOM is unset, it loses its special
properties, even if it is subsequently reset.
READLINE_ARGUMENT
Any numeric argument given to a readline command that was defined
using “bind -x” (see SHELL BUILTIN COMMANDS below) when it was
invoked.
READLINE_LINE
The contents of the readline line buffer, for use with “bind -x”
(see SHELL BUILTIN COMMANDS below).
READLINE_MARK
The position of the mark (saved insertion point) in the readline
line buffer, for use with “bind -x” (see SHELL BUILTIN COMMANDS
below). The characters between the insertion point and the mark
are often called the region.
READLINE_POINT
The position of the insertion point in the readline line buffer,
for use with “bind -x” (see SHELL BUILTIN COMMANDS below).
REPLY Set to the line of input read by the read builtin command when no
arguments are supplied.
SECONDS
Each time this parameter is referenced, it expands to the number
of seconds since shell invocation. If a value is assigned to
SECONDS, the value returned upon subsequent references is the
number of seconds since the assignment plus the value assigned.
The number of seconds at shell invocation and the current time are
always determined by querying the system clock at one-second
resolution. If SECONDS is unset, it loses its special properties,
even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the
list is a valid argument for the -o option to the set builtin
command (see SHELL BUILTIN COMMANDS below). The options appearing
in SHELLOPTS are those reported as on by set -o. If this variable
is in the environment when bash starts up, the shell enables each
option in the list before reading any startup files. If this
variable is exported, child shells will enable each option in the
list. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
SRANDOM
Each time it is referenced, this variable expands to a 32-bit
pseudo-random number. The random number generator is not linear
on systems that support /dev/urandom or arc4random(3), so each
returned number has no relationship to the numbers preceding it.
The random number generator cannot be seeded, so assignments to
this variable have no effect. If SRANDOM is unset, it loses its
special properties, even if it is subsequently reset.
UID Expands to the user ID of the current user, initialized at shell
startup. This variable is readonly.

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

BASH_COMPAT
The value is used to set the shell's compatibility level. See
SHELL COMPATIBILITY MODE below for a description of the various
compatibility levels and their effects. The value may be a
decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding
to the desired compatibility level. If BASH_COMPAT is unset or
set to the empty string, the compatibility level is set to the
default for the current version. If BASH_COMPAT is set to a value
that is not one of the valid compatibility levels, the shell
prints an error message and sets the compatibility level to the
default for the current version. A subset of the valid values
correspond to the compatibility levels described below under SHELL
COMPATIBILITY MODE. For example, 4.2 and 42 are valid values that
correspond to the compat42 shopt option and set the compatibility
level to 42. The current version is also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell script,
its expanded value is interpreted as a filename containing
commands to initialize the shell before it reads and executes
commands from the script. The value of BASH_ENV is subjected to
parameter expansion, command substitution, and arithmetic
expansion before being interpreted as a filename. PATH is not
used to search for the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
bash writes the trace output generated when “set -x” is enabled to
that file descriptor, instead of the standard error. The file
descriptor is closed when BASH_XTRACEFD is unset or assigned a new
value. Unsetting BASH_XTRACEFD or assigning it the empty string
causes the trace output to be sent to the standard error. Note
that setting BASH_XTRACEFD to 2 (the standard error file
descriptor) and then unsetting it will result in the standard
error being closed.
CDPATH The search path for the cd command. This is a colon-separated
list of directories where the shell looks for directories
specified as arguments to the cd command. A sample value is
“.:~:/usr”.
CHILD_MAX
Set the number of exited child status values for the shell to
remember. Bash will not allow this value to be decreased below a
POSIX-mandated minimum, and there is a maximum value (currently
8192) that this may not exceed. The minimum value is system-
dependent.
COLUMNS
Used by the select compound command to determine the terminal
width when printing selection lists. Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions
generated by a shell function invoked by the programmable
completion facility (see Programmable Completion below). Each
array element contains one possible completion.
EMACS If bash finds this variable in the environment when the shell
starts with value “t”, it assumes that the shell is running in an
Emacs shell buffer and disables line editing.
ENV Expanded and executed similarly to BASH_ENV (see INVOCATION above)
when an interactive shell is invoked in posix mode.
EXECIGNORE
A colon-separated list of shell patterns (see Pattern Matching)
defining the set of filenames to be ignored by command search
using PATH. Files whose full pathnames match one of these
patterns are not considered executable files for the purposes of
completion and command execution via PATH lookup. This does not
affect the behavior of the [, test, and [[ commands. Full
pathnames in the command hash table are not subject to EXECIGNORE.
Use this variable to ignore shared library files that have the
executable bit set, but are not executable files. The pattern
matching honors the setting of the extglob shell option.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion (see READLINE below). A filename whose suffix
matches one of the entries in FIGNORE is excluded from the list of
matched filenames. A sample value is “.o:~”.
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed this
nesting level cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of file names
to be ignored by pathname expansion. If a file name matched by a
pathname expansion pattern also matches one of the patterns in
GLOBIGNORE, it is removed from the list of matches. The pattern
matching honors the setting of the extglob shell option.
GLOBSORT
Controls how the results of pathname expansion are sorted. The
value of this variable specifies the sort criteria and sort order
for the results of pathname expansion. If this variable is unset
or set to the null string, pathname expansion uses the historical
behavior of sorting by name, in ascending lexicographic order as
determined by the LC_COLLATE shell variable.

If set, a valid value begins with an optional +, which is ignored,
or -, which reverses the sort order from ascending to descending,
followed by a sort specifier. The valid sort specifiers are name,
numeric, size, mtime, atime, ctime, and blocks, which sort the
files on name, names in numeric rather than lexicographic order,
file size, modification time, access time, inode change time, and
number of blocks, respectively. If any of the non-name keys
compare as equal (e.g., if two files are the same size), sorting
uses the name as a secondary sort key.

For example, a value of -mtime sorts the results in descending
order by modification time (newest first).

The numeric specifier treats names consisting solely of digits as
numbers and sorts them using their numeric value (so “2” sorts
before “10”, for example). When using numeric, names containing
non-digits sort after all the all-digit names and are sorted by
name using the traditional behavior.

A sort specifier of nosort disables sorting completely; bash
returns the results in the order they are read from the file
system, ignoring any leading -.

If the sort specifier is missing, it defaults to name, so a value
of + is equivalent to the null string, and a value of - sorts by
name in descending order. Any invalid value restores the
historical sorting behavior.
HISTCONTROL
A colon-separated list of values controlling how commands are
saved on the history list. If the list of values includes
ignorespace, lines which begin with a space character are not
saved in the history list. A value of ignoredups causes lines
matching the previous history entry not to be saved. A value of
ignoreboth is shorthand for ignorespace and ignoredups. A value
of erasedups causes all previous lines matching the current line
to be removed from the history list before that line is saved.
Any value not in the above list is ignored. If HISTCONTROL is
unset, or does not include a valid value, bash saves all lines
read by the shell parser on the history list, subject to the value
of HISTIGNORE. If the first line of a multi-line compound command
was saved, the second and subsequent lines are not tested, and are
added to the history regardless of the value of HISTCONTROL. If
the first line was not saved, the second and subsequent lines of
the command are not saved either.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below). Bash assigns a default value of ~/.bash_history.
If HISTFILE is unset or null, the shell does not save the command
history when it exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When
this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than the number of history
entries that total no more than that number of lines by removing
the oldest entries. If the history list contains multi-line
entries, the history file may contain more lines than this maximum
to avoid leaving partial history entries. The history file is
also truncated to this size after writing it when a shell exits or
by the history builtin. If the value is 0, the history file is
truncated to zero size. Non-numeric values and numeric values
less than zero inhibit truncation. The shell sets the default
value to the value of HISTSIZE after reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which command
lines should be saved on the history list. If a command line
matches one of the patterns in the value of HISTIGNORE, it is not
saved on the history list. Each pattern is anchored at the
beginning of the line and must match the complete line (bash does
not implicitly append a “*”). Each pattern is tested against the
line after the checks specified by HISTCONTROL are applied. In
addition to the normal shell pattern matching characters, “&”
matches the previous history line. A backslash escapes the “&”;
the backslash is removed before attempting a match. If the first
line of a multi-line compound command was saved, the second and
subsequent lines are not tested, and are added to the history
regardless of the value of HISTIGNORE. If the first line was not
saved, the second and subsequent lines of the command are not
saved either. The pattern matching honors the setting of the
extglob shell option.
HISTIGNORE subsumes some of the function of HISTCONTROL. A
pattern of “&” is identical to “ignoredups”, and a pattern of “[
]*” is identical to “ignorespace”. Combining these two patterns,
separating them with a colon, provides the functionality of “‐
ignoreboth”.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below). If the value is 0, commands are not saved in the
history list. Numeric values less than zero result in every
command being saved on the history list (there is no limit). The
shell sets the default value to 500 after reading any startup
files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a
format string for strftime(3) to print the time stamp associated
with each history entry displayed by the history builtin. If this
variable is set, the shell writes time stamps to the history file
so they may be preserved across shell sessions. This uses the
history comment character to distinguish timestamps from other
history lines.
HOME The home directory of the current user; the default argument for
the cd builtin command. The value of this variable is also used
when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that
should be read when the shell needs to complete a hostname. The
list of possible hostname completions may be changed while the
shell is running; the next time hostname completion is attempted
after the value is changed, bash adds the contents of the new file
to the existing list. If HOSTFILE is set, but has no value, or
does not name a readable file, bash attempts to read /etc/hosts to
obtain the list of possible hostname completions. When HOSTFILE
is unset, bash clears the hostname list.
IFS The Internal Field Separator that is used for word splitting after
expansion and to split lines into words with the read builtin
command. Word splitting is described below under EXPANSION. The
default value is “<space><tab><newline>”.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF
character as the sole input. If set, the value is the number of
consecutive EOF characters which must be typed as the first
characters on an input line before bash exits. If the variable is
set but does not have a numeric value, or the value is null, the
default value is 10. If it is unset, EOF signifies the end of
input to the shell.
INPUTRC
The filename for the readline startup file, overriding the default
of ~/.inputrc (see READLINE below).
INSIDE_EMACS
If this variable appears in the environment when the shell starts,
bash assumes that it is running inside an Emacs shell buffer and
may disable line editing, depending on the value of TERM.
LANG Used to determine the locale category for any category not
specifically selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_
variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting the
results of pathname expansion, and determines the behavior of
range expressions, equivalence classes, and collating sequences
within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within pathname expansion and
pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
LC_TIME
This variable determines the locale category used for data and
time formatting.
LINES Used by the select compound command to determine the column length
for printing selection lists. Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
MAIL If the value is set to a file or directory name and the MAILPATH
variable is not set, bash informs the user of the arrival of mail
in the specified file or Maildir-format directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The
default is 60 seconds. When it is time to check for mail, the
shell does so before displaying the primary prompt. If this
variable is unset, or set to a value that is not a number greater
than or equal to zero, the shell disables mail checking.
MAILPATH
A colon-separated list of filenames to be checked for mail. The
message to be printed when mail arrives in a particular file may
be specified by separating the filename from the message with a
“?”. When used in the text of the message, $_ expands to the name
of the current mailfile. For example:
MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'
Bash can be configured to supply a default value for this variable
(there is no value by default), but the location of the user mail
files that it uses is system dependent (e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated by
the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTERR is initialized to 1 each time the shell is invoked or a
shell script is executed.
PATH The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory
name may appear as two adjacent colons, or as an initial or
trailing colon. The default path is system-dependent, and is set
by the administrator who installs bash. A common value is
/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the shell
enters posix mode before reading the startup files, as if the
--posix invocation option had been supplied. If it is set while
the shell is running, bash enables posix mode, as if the command
“set -o posix” had been executed. When the shell enters posix
mode, it sets this variable if it was not already set.
PROMPT_COMMAND
If this variable is set, and is an array, the value of each set
element is executed as a command prior to issuing each primary
prompt. If this is set but not an array variable, its value is
used as a command to execute instead.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the \w and \W prompt string escapes (see PROMPTING below).
Characters removed are replaced with an ellipsis.
PS0 The value of this parameter is expanded (see PROMPTING below) and
displayed by interactive shells after reading a command and before
the command is executed.
PS1 The value of this parameter is expanded (see PROMPTING below) and
used as the primary prompt string. The default value is
“\s-\v\$ ”.
PS2 The value of this parameter is expanded as with PS1 and used as
the secondary prompt string. The default is “> ”.
PS3 The value of this parameter is used as the prompt for the select
command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the value
is printed before each command bash displays during an execution
trace. The first character of the expanded value of PS4 is
replicated multiple times, as necessary, to indicate multiple
levels of indirection. The default is “+ ”.
SHELL This variable expands to the full pathname to the shell. If it is
not set when the shell starts, bash assigns to it the full
pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the time
reserved word should be displayed. The % character introduces an
escape sequence that is expanded to a time value or other
information. The escape sequences and their meanings are as
follows; the brackets denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of
fractional digits after a decimal point. A value of 0 causes no
decimal point or fraction to be output. time prints at most six
digits after the decimal point; values of p greater than 6 are
changed to 6. If p is not specified, time prints three digits
after the decimal point.

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

If this variable is not set, bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'. If the value is null,
bash does not display any timing information. A trailing newline
is added when the format string is displayed.
TMOUT If set to a value greater than zero, the read builtin uses the
value as its default timeout. The select command terminates if
input does not arrive after TMOUT seconds when input is coming
from a terminal. In an interactive shell, the value is
interpreted as the number of seconds to wait for a line of input
after issuing the primary prompt. Bash terminates after waiting
for that number of seconds if a complete line of input does not
arrive.
TMPDIR If set, bash uses its value as the name of a directory in which
bash creates temporary files for the shell's use.
auto_resume
This variable controls how the shell interacts with the user and
job control. If this variable is set, simple commands consisting
of only a single word, without redirections, are treated as
candidates for resumption of an existing stopped job. There is no
ambiguity allowed; if there is more than one job beginning with or
containing the word, this selects the most recently accessed job.
The name of a stopped job, in this context, is the command line
used to start it, as displayed by jobs. If set to the value
exact, the word must match the name of a stopped job exactly; if
set to substring, the word needs to match a substring of the name
of a stopped job. The substring value provides functionality
analogous to the %? job identifier (see JOB CONTROL below). If
set to any other value (e.g., prefix), the word must be a prefix
of a stopped job's name; this provides functionality analogous to
the %string job identifier.
histchars
The two or three characters which control history expansion, quick
substitution, and tokenization (see HISTORY EXPANSION below). The
first character is the history expansion character, the character
which begins a history expansion, normally “!”. The second
character is the quick substitution character, normally “^”. When
it appears as the first character on the line, history
substitution repeats the previous command, replacing one string
with another. The optional third character is the history comment
character, normally “#”, which indicates that the remainder of
the line is a comment when it appears as the first character of a
word. The history comment character disables history substitution
for the remaining words on the line. It does not necessarily
cause the shell parser to treat the rest of the line as a comment.

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

The shell performs parameter and variable expansion, arithmetic
expansion, command substitution, and quote removal on indexed array
subscripts. Since this can potentially result in empty strings,
subscript indexing treats those as expressions that evaluate to 0.

The shell performs tilde expansion, parameter and variable expansion,
arithmetic expansion, command substitution, and quote removal on
associative array subscripts. Empty strings cannot be used as
associative array keys.

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

Associative arrays are created using
declare -A name
.

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

Arrays are assigned using compound assignments of the form name=(value1
... valuen), where each value may be of the form [subscript]=string.
Indexed array assignments do not require anything but string. Each value
in the list is expanded using the shell expansions described below under
EXPANSION, but values that are valid variable assignments including the
brackets and subscript do not undergo brace expansion and word splitting,
as with individual variable assignments.

When assigning to indexed arrays, if the optional brackets and subscript
are supplied, that index is assigned to; otherwise the index of the
element assigned is the last index assigned to by the statement plus one.
Indexing starts at zero.

When assigning to an associative array, the words in a compound
assignment may be either assignment statements, for which the subscript
is required, or a list of words that is interpreted as a sequence of
alternating keys and values: name=( key1 value1 key2 value2 ...). These
are treated identically to name=( [key1]=value1 [key2]=value2 ...). The
first word in the list determines how the remaining words are
interpreted; all assignments in a list must be of the same type. When
using key/value pairs, the keys may not be missing or empty; a final
missing value is treated like the empty string.

This syntax is also accepted by the declare builtin. Individual array
elements may be assigned to using the name[subscript]=value syntax
introduced above.

When assigning to an indexed array, if name is subscripted by a negative
number, that number is interpreted as relative to one greater than the
maximum index of name, so negative indices count back from the end of the
array, and an index of -1 references the last element.

The “+=” operator appends to an array variable when assigning using the
compound assignment syntax; see PARAMETERS above.

An array element is referenced using ${name[subscript]}. The braces are
required to avoid conflicts with pathname expansion. If subscript is @
or *, the word expands to all members of name, unless noted in the
description of a builtin or word expansion. These subscripts differ only
when the word appears within double quotes. If the word is double-
quoted, ${name[*]} expands to a single word with the value of each array
member separated by the first character of the IFS special variable, and
${name[@]} expands each element of name to a separate word. When there
are no array members, ${name[@]} expands to nothing. If the double-
quoted expansion occurs within a word, the expansion of the first
parameter is joined with the beginning part of the expansion of the
original word, and the expansion of the last parameter is joined with the
last part of the expansion of the original word. This is analogous to
the expansion of the special parameters * and @ (see Special Parameters
above).

${#name[subscript]} expands to the length of ${name[subscript]}. If
subscript is * or @, the expansion is the number of elements in the
array.

If the subscript used to reference an element of an indexed array
evaluates to a number less than zero, it is interpreted as relative to
one greater than the maximum index of the array, so negative indices
count back from the end of the array, and an index of -1 references the
last element.

Referencing an array variable without a subscript is equivalent to
referencing the array with a subscript of 0. Any reference to a variable
using a valid subscript is valid; bash creates an array if necessary.

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

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

The unset builtin is used to destroy arrays. unset name[subscript]
unsets the array element at index subscript, for both indexed and
associative arrays. Negative subscripts to indexed arrays are
interpreted as described above. Unsetting the last element of an array
variable does not unset the variable. unset name, where name is an
array, removes the entire array. unset name[subscript] behaves
differently depending on whether name is an indexed or associative array
when subscript is * or @. If name is an associative array, this unsets
the element with subscript * or @. If name is an indexed array, unset
removes all of the elements but does not remove the array itself.

When using a variable name with a subscript as an argument to a command,
such as with unset, without using the word expansion syntax described
above, (e.g., unset a[4]), the argument is subject to pathname expansion.
Quote the argument if pathname expansion is not desired (e.g., unset
'a[4]').

The declare, local, and readonly builtins each accept a -a option to
specify an indexed array and a -A option to specify an associative array.
If both options are supplied, -A takes precedence. The read builtin
accepts a -a option to assign a list of words read from the standard
input to an array. The set and declare builtins display array values in
a way that allows them to be reused as assignments. Other builtins
accept array name arguments as well (e.g., mapfile); see the descriptions
of individual builtins below for details. The shell provides a number of
builtin array variables.

EXPANSION
Expansion is performed on the command line after it has been split into
words. The shell performs these expansions: brace expansion, tilde
expansion, parameter and variable expansion, command substitution,
arithmetic expansion, word splitting, pathname expansion, and quote
removal.

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

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

Quote removal is always performed last. It removes quote characters
present in the original word, not ones resulting from one of the other
expansions, unless they have been quoted themselves.

Only brace expansion, word splitting, and pathname expansion can increase
the number of words of the expansion; other expansions expand a single
word to a single word. The only exceptions to this are the expansions of
"$@" and "${name[@]}", and, in most cases, $* and ${name[*]} as explained
above (see PARAMETERS).

Brace Expansion
Brace expansion is a mechanism to generate arbitrary strings sharing a
common prefix and suffix, either of which can be empty. This mechanism
is similar to pathname expansion, but the filenames generated need not
exist. Patterns to be brace expanded are formed from an optional
preamble, followed by either a series of comma-separated strings or a
sequence expression between a pair of braces, followed by an optional
postscript. The preamble is prefixed to each string contained within the
braces, and the postscript is then appended to each resulting string,
expanding left to right.

Brace expansions may be nested. The results of each expanded string are
not sorted; brace expansion preserves left to right order. For example,
a{d,c,b}e expands into “ade ace abe”.

A sequence expression takes the form x..y[..incr], where x and y are
either integers or single letters, and incr, an optional increment, is an
integer. When integers are supplied, the expression expands to each
number between x and y, inclusive. If either x or y begins with a zero,
each generated term will contain the same number of digits, zero-padding
where necessary. When letters are supplied, the expression expands to
each character lexicographically between x and y, inclusive, using the C
locale. Note that both x and y must be of the same type (integer or
letter). When the increment is supplied, it is used as the difference
between each term. The default increment is 1 or -1 as appropriate.

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

A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.

A “{” or Q , may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with parameter
expansion, the string “${” is not considered eligible for brace
expansion, and inhibits brace expansion until the closing “}”.

This construct is typically used as shorthand when the common prefix of
the strings to be generated is longer than in the above example:

mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Brace expansion introduces a slight incompatibility with historical
versions of sh. sh does not treat opening or closing braces specially
when they appear as part of a word, and preserves them in the output.
Bash removes braces from words as a consequence of brace expansion. For
example, a word entered to sh as “file{1,2}” appears identically in the
output. Bash outputs that word as “file1 file2” after brace expansion.
Start bash with the +B option or disable brace expansion with the +B
option to the set command (see SHELL BUILTIN COMMANDS below) for strict
sh compatibility.

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

If the tilde-prefix is a “~+”, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is a “~-”, the shell
substitutes the value of the shell variable OLDPWD, if it is set. If the
characters following the tilde in the tilde-prefix consist of a number N,
optionally prefixed by a “+” or a “-”, the tilde-prefix is replaced with
the corresponding element from the directory stack, as it would be
displayed by the dirs builtin invoked with the characters following the
tilde in the tilde-prefix as an argument. If the characters following
the tilde in the tilde-prefix consist of a number without a leading “+”
or “-”, tilde expansion assumes “+”.

The results of tilde expansion are treated as if they were quoted, so the
replacement is not subject to word splitting and pathname expansion.

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

Bash checks each variable assignment for unquoted tilde-prefixes
immediately following a : or the first =, and performs tilde expansion in
these cases. Consequently, one may use filenames with tildes in
assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the
expanded value.

Bash also performs tilde expansion on words satisfying the conditions of
variable assignments (as described above under PARAMETERS) when they
appear as arguments to simple commands. Bash does not do this, except
for the declaration commands listed above, when in posix mode.

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

When braces are used, the matching ending brace is the first “}” not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.

The basic form of parameter expansion is

${parameter}

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

If the first character of parameter is an exclamation point (!), and
parameter is not a nameref, it introduces a level of indirection. Bash
uses the value formed by expanding the rest of parameter as the new
parameter; this new parameter is then expanded and that value is used in
the rest of the expansion, rather than the expansion of the original
parameter. This is known as indirect expansion. The value is subject to
tilde expansion, parameter expansion, command substitution, and
arithmetic expansion. If parameter is a nameref, this expands to the
name of the parameter referenced by parameter instead of performing the
complete indirect expansion, for compatibility. The exceptions to this
are the expansions of ${!prefix*} and ${!name[@]} described below. The
exclamation point must immediately follow the left brace in order to
introduce indirection.

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

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

${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion
of word is substituted. Otherwise, the value of parameter is
substituted.

${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter, and the expansion is
the final value of parameter. Positional parameters and special
parameters may not be assigned in this way.

${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset,
the shell writes the expansion of word (or a message to that
effect if word is not present) to the standard error and, if it is
not interactive, exits with a non-zero status. An interactive
shell does not exit, but does not execute the command associated
with the expansion. Otherwise, the value of parameter is
substituted.

${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted. The
value of parameter is not used.

${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of the
value of parameter starting at the character specified by offset.
If parameter is @ or *, an indexed array subscripted by @ or *, or
an associative array name, the results differ as described below.
If :length is omitted (the first form above), this expands to the
substring of the value of parameter starting at the character
specified by offset and extending to the end of the value. If
offset is omitted, it is treated as 0. If length is omitted, but
the colon after offset is present, it is treated as 0. length and
offset are arithmetic expressions (see ARITHMETIC EVALUATION
below).

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

If parameter is @ or *, the result is length positional parameters
beginning at offset. A negative offset is taken relative to one
greater than the greatest positional parameter, so an offset of -1
evaluates to the last positional parameter (or 0 if there are no
positional parameters). It is an expansion error if length
evaluates to a number less than zero.

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

Substring expansion applied to an associative array produces
undefined results.

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

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

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

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

${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to produce a
pattern just as in pathname expansion, and matched against the
expanded value of parameter using the rules described under
Pattern Matching below. If the pattern matches the beginning of
the value of parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching pattern
(the “#” case) or the longest matching pattern (the “##” case)
deleted. If parameter is @ or *, the pattern removal operation is
applied to each positional parameter in turn, and the expansion is
the resultant list. If parameter is an array variable subscripted
with @ or *, the pattern removal operation is applied to each
member of the array in turn, and the expansion is the resultant
list.

${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to produce a
pattern just as in pathname expansion, and matched against the
expanded value of parameter using the rules described under
Pattern Matching below. If the pattern matches a trailing portion
of the expanded value of parameter, then the result of the
expansion is the expanded value of parameter with the shortest
matching pattern (the “%” case) or the longest matching pattern
(the “%%” case) deleted. If parameter is @ or *, the pattern
removal operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the pattern removal operation is
applied to each member of the array in turn, and the expansion is
the resultant list.

${parameter/pattern/string}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}
Pattern substitution. The pattern is expanded to produce a
pattern and matched against the expanded value of parameter as
described under Pattern Matching below. The longest match of
pattern in the expanded value is replaced with string. string
undergoes tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, and quote
removal.

In the first form above, only the first match is replaced. If
there are two slashes separating parameter and pattern (the second
form above), all matches of pattern are replaced with string. If
pattern is preceded by # (the third form above), it must match at
the beginning of the expanded value of parameter. If pattern is
preceded by % (the fourth form above), it must match at the end of
the expanded value of parameter.

If the expansion of string is null, matches of pattern are deleted
and the / following pattern may be omitted.

If the patsub_replacement shell option is enabled using shopt, any
unquoted instances of & in string are replaced with the matching
portion of pattern.

Quoting any part of string inhibits replacement in the expansion
of the quoted portion, including replacement strings stored in
shell variables. Backslash escapes & in string; the backslash is
removed in order to permit a literal & in the replacement string.
Backslash can also be used to escape a backslash; \\ results in a
literal backslash in the replacement. Users should take care if
string is double-quoted to avoid unwanted interactions between the
backslash and double-quoting, since backslash has special meaning
within double quotes. Pattern substitution performs the check for
unquoted & after expanding string; shell programmers should quote
any occurrences of & they want to be taken literally in the
replacement and ensure any instances of & they want to be replaced
are unquoted.

Like the pattern removal operators, double quotes surrounding the
replacement string quote the expanded characters, while double
quotes enclosing the entire parameter substitution do not, since
the expansion is performed in a context that doesn't take any
enclosing double quotes into account.

If the nocasematch shell option is enabled, the match is performed
without regard to the case of alphabetic characters.

If parameter is @ or *, the substitution operation is applied to
each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted
with @ or *, the substitution operation is applied to each member
of the array in turn, and the expansion is the resultant list.

${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of alphabetic
characters in parameter. First, the pattern is expanded to
produce a pattern as described below under Pattern Matching. Bash
then examines characters in the expanded value of parameter
against pattern as described below. If a character matches the
pattern, its case is converted. The pattern should not attempt to
match more than one character.

Using “^” converts lowercase letters matching pattern to
uppercase; “,” converts matching uppercase letters to lowercase.
The ^ and , variants examine the first character in the expanded
value and convert its case if it matches pattern; the ^^ and ,,
variants examine all characters in the expanded value and convert
each one that matches pattern. If pattern is omitted, it is
treated like a ?, which matches every character.

If parameter is @ or *, the case modification operation is applied
to each positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable subscripted
with @ or *, the case modification operation is applied to each
member of the array in turn, and the expansion is the resultant
list.

${parameter@operator}
Parameter transformation. The expansion is either a
transformation of the value of parameter or information about
parameter itself, depending on the value of operator. Each
operator is a single letter:
U The expansion is a string that is the value of parameter
with lowercase alphabetic characters converted to
uppercase.
u The expansion is a string that is the value of parameter
with the first character converted to uppercase, if it is
alphabetic.
L The expansion is a string that is the value of parameter
with uppercase alphabetic characters converted to
lowercase.
Q The expansion is a string that is the value of parameter
quoted in a format that can be reused as input.
E The expansion is a string that is the value of parameter
with backslash escape sequences expanded as with the $'...'
quoting mechanism.
P The expansion is a string that is the result of expanding
the value of parameter as if it were a prompt string (see
PROMPTING below).
A The expansion is a string in the form of an assignment
statement or declare command that, if evaluated, recreates
parameter with its attributes and value.
K Produces a possibly-quoted version of the value of
parameter, except that it prints the values of indexed and
associative arrays as a sequence of quoted key-value pairs
(see Arrays above). The keys and values are quoted in a
format that can be reused as input.
a The expansion is a string consisting of flag values
representing parameter's attributes.
k Like the K transformation, but expands the keys and values
of indexed and associative arrays to separate words after
word splitting.

If parameter is @ or *, the operation is applied to each
positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *,
the operation is applied to each member of the array in turn, and
the expansion is the resultant list.

The result of the expansion is subject to word splitting and
pathname expansion as described below.

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

$(command)
or (deprecated)
`command`.

Bash performs the expansion by executing command in a subshell
environment and replacing the command substitution with the standard
output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word splitting.
The command substitution $(cat file) can be replaced by the equivalent
but faster $(< file).

With the old-style backquote form of substitution, backslash retains its
literal meaning except when followed by $, `, or \. The first backquote
not preceded by a backslash terminates the command substitution. When
using the $(command) form, all characters between the parentheses make up
the command; none are treated specially.

There is an alternate form of command substitution:

${c command;}

which executes command in the current execution environment and captures
its output, again with trailing newlines removed.

The character c following the open brace must be a space, tab, newline,
or |, and the close brace must be in a position where a reserved word may
appear (i.e., preceded by a command terminator such as semicolon). Bash
allows the close brace to be joined to the remaining characters in the
word without being followed by a shell metacharacter as a reserved word
would usually require.

Any side effects of command take effect immediately in the current
execution environment and persist in the current environment after the
command completes (e.g., the exit builtin exits the shell).

This type of command substitution superficially resembles executing an
unnamed shell function: local variables are created as when a shell
function is executing, and the return builtin forces command to complete;
however, the rest of the execution environment, including the positional
parameters, is shared with the caller.

If the first character following the open brace is a |, the construct
expands to the value of the REPLY shell variable after command executes,
without removing any trailing newlines, and the standard output of
command remains the same as in the calling shell. Bash creates REPLY as
an initially-unset local variable when command executes, and restores
REPLY to the value it had before the command substitution after command
completes, as with any local variable.

Command substitutions may be nested. To nest when using the backquoted
form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, bash does not perform
word splitting and pathname expansion on the results.

Arithmetic Expansion
Arithmetic expansion evaluates an arithmetic expression and substitutes
the result. The format for arithmetic expansion is:

$((expression))

The expression undergoes the same expansions as if it were within double
quotes, but unescaped double quote characters in expression are not
treated specially and are removed. All tokens in the expression undergo
parameter and variable expansion, command substitution, and quote
removal. The result is treated as the arithmetic expression to be
evaluated. Since the way Bash handles double quotes can potentially
result in empty strings, arithmetic expansion treats those as expressions
that evaluate to 0. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below under
ARITHMETIC EVALUATION. If expression is invalid, bash prints a message
to standard error indicating failure, does not perform the substitution,
and does not execute the command associated with the expansion.

Process Substitution
Process substitution allows a process's input or output to be referred to
using a filename. It takes the form of <(list) or >(list). The process
list is run asynchronously, and its input or output appears as a
filename. This filename is passed as an argument to the current command
as the result of the expansion.

If the >(list) form is used, writing to the file provides input for list.
If the <(list) form is used, reading the file obtains the output of list.
No space may appear between the < or > and the left parenthesis,
otherwise the construct would be interpreted as a redirection.

Process substitution is supported on systems that support named pipes
(FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.

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

The shell treats each character of IFS as a delimiter, and splits the
results of the other expansions into words using these characters as
field terminators.

An IFS whitespace character is whitespace as defined above (see
Definitions) that appears in the value of IFS. Space, tab, and newline
are always considered IFS whitespace, even if they don't appear in the
locale's space category.

If IFS is unset, field splitting acts as if its value were
<space><tab><newline>, and treats these characters as IFS whitespace. If
the value of IFS is null, no word splitting occurs, but implicit null
arguments (see below) are still removed.

Word splitting begins by removing sequences of IFS whitespace characters
from the beginning and end of the results of the previous expansions,
then splits the remaining words.

If the value of IFS consists solely of IFS whitespace, any sequence of
IFS whitespace characters delimits a field, so a field consists of
characters that are not unquoted IFS whitespace, and null fields result
only from quoting.

If IFS contains a non-whitespace character, then any character in the
value of IFS that is not IFS whitespace, along with any adjacent IFS
whitespace characters, delimits a field. This means that adjacent non-
IFS-whitespace delimiters produce a null field. A sequence of IFS
whitespace characters also delimits a field.

Explicit null arguments ("" or '') are retained and passed to commands as
empty strings. Unquoted implicit null arguments, resulting from the
expansion of parameters that have no values, are removed. Expanding a
parameter with no value within double quotes produces a null field, which
is retained and passed to a command as an empty string.

When a quoted null argument appears as part of a word whose expansion is
non-null, word splitting removes the null argument portion, leaving the
non-null expansion. That is, the word “-d''” becomes “-d” after word
splitting and null argument removal.

Pathname Expansion
After word splitting, unless the -f option has been set, bash scans each
word for the characters *, ?, and [. If one of these characters appears,
and is not quoted, then the word is regarded as a pattern, and replaced
with a sorted list of filenames matching the pattern (see Pattern
Matching below) subject to the value of the GLOBSORT shell variable.

If no matching filenames are found, and the shell option nullglob is not
enabled, the word is left unchanged. If the nullglob option is set, and
no matches are found, the word is removed. If the failglob shell option
is set, and no matches are found, bash prints an error message and does
not execute the command. If the shell option nocaseglob is enabled, the
match is performed without regard to the case of alphabetic characters.

When a pattern is used for pathname expansion, the character “.” at the
start of a name or immediately following a slash must be matched
explicitly, unless the shell option dotglob is set. In order to match
the filenames . and .., the pattern must begin with “.” (for example,
“.?”), even if dotglob is set. If the globskipdots shell option is
enabled, the filenames . and .. never match, even if the pattern begins
with a “.”. When not matching pathnames, the “.” character is not
treated specially.

When matching a pathname, the slash character must always be matched
explicitly by a slash in the pattern, but in other matching contexts it
can be matched by a special pattern character as described below under
Pattern Matching.

See the description of shopt below under SHELL BUILTIN COMMANDS for a
description of the nocaseglob, nullglob, globskipdots, failglob, and
dotglob shell options.

The GLOBIGNORE shell variable may be used to restrict the set of file
names matching a pattern. If GLOBIGNORE is set, each matching file name
that also matches one of the patterns in GLOBIGNORE is removed from the
list of matches. If the nocaseglob option is set, the matching against
the patterns in GLOBIGNORE is performed without regard to case. The
filenames . and .. are always ignored when GLOBIGNORE is set and not
null. However, setting GLOBIGNORE to a non-null value has the effect of
enabling the dotglob shell option, so all other filenames beginning with
a “.” match. To get the old behavior of ignoring filenames beginning
with a “.”, make “.*” one of the patterns in GLOBIGNORE. The dotglob
option is disabled when GLOBIGNORE is unset. The GLOBIGNORE pattern
matching honors the setting of the extglob shell option.

The value of the GLOBSORT shell variable controls how the results of
pathname expansion are sorted, as described above under Shell Variables.

Pattern Matching

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

The special pattern characters have the following meanings:

* Matches any string, including the null string. When the
globstar shell option is enabled, and * is used in a
pathname expansion context, two adjacent *s used as a
single pattern match all files and zero or more directories
and subdirectories. If followed by a /, two adjacent *s
match only directories and subdirectories.
? Matches any single character.
[...] Matches any one of the characters enclosed between the
brackets. This is known as a bracket expression and
matches a single character. A pair of characters separated
by a hyphen denotes a range expression; any character that
falls between those two characters, inclusive, using the
current locale's collating sequence and character set,
matches. If the first character following the [ is a ! or
a ^ then any character not within the range matches. To
match a -, include it as the first or last character in the
set. To match a ], include it as the first character in
the set.

The sorting order of characters in range expressions, and
the characters included in the range, are determined by the
current locale and the values of the LC_COLLATE or LC_ALL
shell variables, if set. To obtain the traditional
interpretation of range expressions, where [a-d] is
equivalent to [abcd], set the value of the LC_COLLATE or
LC_ALL shell variables to C, or enable the globasciiranges
shell option.

Within a bracket expression, character classes can be
specified using the syntax [:class:], where class is one of
the following classes defined in the POSIX standard:

alnum alpha ascii blank cntrl digit graph lower print punct
space upper word xdigit

A character class matches any character belonging to that
class. The word character class matches letters, digits,
and the character _.

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

Within a bracket expression, the syntax [.symbol.] matches
the collating symbol symbol.

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

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

The extglob option changes the behavior of the parser, since the
parentheses are normally treated as operators with syntactic meaning. To
ensure that extended matching patterns are parsed correctly, make sure
that extglob is enabled before parsing constructs containing the
patterns, including shell functions and command substitutions.

When matching filenames, the dotglob shell option determines the set of
filenames that are tested: when dotglob is enabled, the set of filenames
includes all files beginning with “.”, but . and .. must be matched by a
pattern or sub-pattern that begins with a dot; when it is disabled, the
set does not include any filenames beginning with “.” unless the pattern
or sub-pattern begins with a “.”. If the globskipdots shell option is
enabled, the filenames . and .. never appear in the set. As above, “.”
only has a special meaning when matching filenames.

Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings contain
multiple matches. Using separate matches against shorter strings, or
using arrays of strings instead of a single long string, may be faster.

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

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. When used with the exec builtin, redirections modify file
handles in the current shell execution environment. The following
redirection operators may precede or appear anywhere within a simple
command or may follow a command. Redirections are processed in the order
they appear, from left to right.

Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {varname}. In this case, for
each redirection operator except >&- and <&-, the shell allocates a file
descriptor greater than or equal to 10 and assigns it to varname. If
{varname} precedes >&- or <&-, the value of varname defines the file
descriptor to close. If {varname} is supplied, the redirection persists
beyond the scope of the command, which allows the shell programmer to
manage the file descriptor's lifetime manually without using the exec
builtin. The varredir_close shell option manages this behavior.

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

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

The order of redirections is significant. For example, the command

ls > dirlist 2>&1

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

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard
error was directed to the standard output before the standard output was
redirected to dirlist.

Bash handles several filenames specially when they are used in
redirections, as described in the following table. If the operating
system on which bash is running provides these special files, bash uses
them; otherwise it emulates them internally with the behavior described
below.

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

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

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

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

The general format for redirecting input is:

[n]<word

Redirecting Output
Redirecting output opens the file whose name results from the expansion
of word for writing on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not exist it is
created; if it does exist it is truncated to zero size.

The general format for redirecting output is:

[n]>word

If the redirection operator is >, and the noclobber option to the set
builtin command has been enabled, the redirection fails if the file whose
name results from the expansion of word exists and is a regular file. If
the redirection operator is >|, or the redirection operator is > and the
noclobber option to the set builtin is not enabled, bash attempts the
redirection even if the file named by word exists.

Appending Redirected Output
Redirecting output in this fashion opens the file whose name results from
the expansion of word for appending on file descriptor n, or the standard
output (file descriptor 1) if n is not specified. If the file does not
exist it is created.

The general format for appending output is:

[n]>>word

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

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

&>word
and
>&word

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

>word 2>&1

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

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

The format for appending standard output and standard error is:

&>>word

This is semantically equivalent to

>>word 2>&1

(see Duplicating File Descriptors below).

Here Documents
This type of redirection instructs the shell to read input from the
current source until it reads a line containing only delimiter (with no
trailing blanks). All of the lines read up to that point then become the
standard input (or file descriptor n if n is specified) for a command.

The format of here-documents is:

[n]<<[-]word
here-document
delimiter

The shell does not perform parameter and variable expansion, command
substitution, arithmetic expansion, or pathname expansion on word.

If any part of word is quoted, the delimiter is the result of quote
removal on word, and the lines in the here-document are not expanded. If
word is unquoted, the delimiter is word itself, and the here-document
text is treated similarly to a double-quoted string: all lines of the
here-document are subjected to parameter expansion, command substitution,
and arithmetic expansion, the character sequence \<newline> is treated
literally, and \ must be used to quote the characters \, $, and `;
however, double quote characters have no special meaning.

If the redirection operator is <<-, then the shell strips all leading tab
characters from input lines and the line containing delimiter. This
allows here-documents within shell scripts to be indented in a natural
fashion.

If the delimiter is not quoted, the \<newline> sequence is treated as a
line continuation: the two lines are joined and the backslash-newline is
removed. This happens while reading the here-document, before the check
for the ending delimiter, so joined lines can form the end delimiter.

Here Strings
A variant of here documents, the format is:

[n]<<<word

The word undergoes tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal. Pathname
expansion and word splitting are not performed. The result is supplied
as a single string, with a newline appended, to the command on its
standard input (or file descriptor n if n is specified).

Duplicating File Descriptors
The redirection operator

[n]<&word

is used to duplicate input file descriptors. If word expands to one or
more digits, file descriptor n is made to be a copy of that file
descriptor. It is a redirection error if the digits in word do not
specify a file descriptor open for input. If word evaluates to -, file
descriptor n is closed. If n is not specified, this uses the standard
input (file descriptor 0).

The operator

[n]>&word

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

Moving File Descriptors
The redirection operator

[n]<&digit-

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

Similarly, the redirection operator

[n]>&digit-

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

Opening File Descriptors for Reading and Writing
The redirection operator

[n]<>word

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

ALIASES
Aliases allow a string to be substituted for a word that is in a position
in the input where it can be the first word of a simple command. Aliases
have names and corresponding values that are set and unset using the
alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below).

If the shell reads an unquoted word in the right position, it checks the
word to see if it matches an alias name. If it matches, the shell
replaces the word with the alias value, and reads that value as if it had
been read instead of the word. The shell doesn't look at any characters
following the word before attempting alias substitution.

The characters /, $, `, and = and any of the shell metacharacters or
quoting characters listed above may not appear in an alias name. The
replacement text may contain any valid shell input, including shell
metacharacters. The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded is not
expanded a second time. This means that one may alias ls to ls -F, for
instance, and bash does not try to recursively expand the replacement
text.

If the last character of the alias value is a blank, the shell checks the
next command word following the alias for alias expansion.

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

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

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

The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input, and
all lines that make up a compound command, before executing any of the
commands on that line or the compound command. Aliases are expanded when
a command is read, not when it is executed. Therefore, an alias
definition appearing on the same line as another command does not take
effect until the shell reads the next line of input, and an alias
definition in a compound command does not take effect until the shell
parses and executes the entire compound command. The commands following
the alias definition on that line, or in the rest of a compound command,
are not affected by the new alias. This behavior is also an issue when
functions are executed. Aliases are expanded when a function definition
is read, not when the function is executed, because a function definition
is itself a command. As a consequence, aliases defined in a function are
not available until after that function is executed. To be safe, always
put alias definitions on a separate line, and do not use alias in
compound commands.

For almost every purpose, shell functions are preferable to aliases.

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 shell executes the list of
commands associated with that function name. Functions are executed in
the context of the calling shell; there is no new process created to
interpret them (contrast this with the execution of a shell script).

When a function is executed, the arguments to the function become the
positional parameters during its execution. The special parameter # is
updated to reflect the new positional parameters. Special parameter 0 is
unchanged. The first element of the FUNCNAME variable is set to the name
of the function while the function is executing.

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

Variables local to the function are declared with the local builtin
command (local variables). Ordinarily, variables and their values are
shared between the function and its caller. If a variable is declared
local, the variable's visible scope is restricted to that function and
its children (including the functions it calls).

In the following description, the current scope is a currently- executing
function. Previous scopes consist of that function's caller and so on,
back to the “global” scope, where the shell is not executing any shell
function. A local variable at the current scope is a variable declared
using the local or declare builtins in the function that is currently
executing.

Local variables “shadow” variables with the same name declared at
previous scopes. For instance, a local variable declared in a function
hides variables with the same name declared at previous scopes, including
global variables: references and assignments refer to the local variable,
leaving the variables at previous scopes unmodified. When the function
returns, the global variable is once again visible.

The shell uses dynamic scoping to control a variable's visibility within
functions. With dynamic scoping, visible variables and their values are
a result of the sequence of function calls that caused execution to reach
the current function. The value of a variable that a function sees
depends on its value within its caller, if any, whether that caller is
the global scope or another shell function. This is also the value that
a local variable declaration shadows, and the value that is restored when
the function returns.

For example, if a variable var is declared as local in function func1,
and func1 calls another function func2, references to var made from
within func2 resolve to the local variable var from func1, shadowing any
global variable named var.

The unset builtin also acts using the same dynamic scope: if a variable
is local to the current scope, unset unsets it; otherwise the unset will
refer to the variable found in any calling scope as described above. If
a variable at the current local scope is unset, it remains so (appearing
as unset) until it is reset in that scope or until the function returns.
Once the function returns, any instance of the variable at a previous
scope becomes visible. If the unset acts on a variable at a previous
scope, any instance of a variable with that name that had been shadowed
becomes visible (see below how the localvar_unset shell option changes
this behavior).

The FUNCNEST variable, if set to a numeric value greater than 0, defines
a maximum function nesting level. Function invocations that exceed the
limit cause the entire command to abort.

If the builtin command return is executed in a function, the function
completes and execution resumes with the next command after the function
call. If return is supplied a numeric argument, that is the function's
return status; otherwise the function's return status is the exit status
of the last command executed before the return. Any command associated
with the RETURN trap is executed before execution resumes. When a
function completes, the values of the positional parameters and the
special parameter # are restored to the values they had prior to the
function's execution.

The -f option to the declare or typeset builtin commands lists function
names and definitions. The -F option to declare or typeset lists the
function names only (and optionally the source file and line number, if
the extdebug shell option is enabled). Functions may be exported so that
child shell processes (those created when executing a separate shell
invocation) automatically have them defined with the -f option to the
export builtin. The -f option to the unset builtin deletes a function
definition.

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

ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain
circumstances (see the let and declare builtin commands, the (( compound
command, the arithmetic for command, the [[ conditional command, and
Arithmetic Expansion).

Evaluation is done in the largest fixed-width integers available, with no
check for overflow, though division by 0 is trapped and flagged as an
error. The operators and their precedence, associativity, and values are
the same as in the C language. The following list of operators is
grouped into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.

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

Shell variables are allowed as operands; parameter expansion is performed
before the expression is evaluated. Within an expression, shell
variables may also be referenced by name without using the parameter
expansion syntax. This means you can use "x", where x is a shell
variable name, in an arithmetic expression, and the shell will evaluate
its value as an expression and use the result. A shell variable that is
null or unset evaluates to 0 when referenced by name in an expression.

The value of a variable is evaluated as an arithmetic expression when it
is referenced, or when a variable which has been given the integer
attribute using declare -i is assigned a value. A null value evaluates
to 0. A shell variable need not have its integer attribute turned on to
be used in an expression.

Integer constants follow the C language definition, without suffixes or
character constants. Constants with a leading 0 are interpreted as octal
numbers. A leading 0x or 0X denotes hexadecimal. Otherwise, numbers
take the form [base#]n, where the optional base is a decimal number
between 2 and 64 representing the arithmetic base, and n is a number in
that base. If base# is omitted, then base 10 is used. When specifying
n, if a non-digit is required, the digits greater than 9 are represented
by the lowercase letters, the uppercase letters, @, and _, in that order.
If base is less than or equal to 36, lowercase and uppercase letters may
be used interchangeably to represent numbers between 10 and 35.

Operators are evaluated in precedence order. Sub-expressions in
parentheses are evaluated first and may override the precedence rules
above.

CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test
and [ builtin commands to test file attributes and perform string and
arithmetic comparisons. The test and [ commands determine their behavior
based on the number of arguments; see the descriptions of those commands
for any other command-specific actions.

Expressions are formed from the unary or binary primaries listed below.
Unary expressions are often used to examine the status of a file or shell
variable. Binary operators are used for string, numeric, and file
attribute comparisons.

Bash handles several filenames specially when they are used in
expressions. If the operating system on which bash is running provides
these special files, bash will use them; otherwise it will emulate them
internally with this behavior: If any file argument to one of the
primaries is of the form /dev/fd/n, then bash checks file descriptor n.
If the file argument to one of the primaries is one of /dev/stdin,
/dev/stdout, or /dev/stderr, bash checks file descriptor 0, 1, or 2,
respectively.

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

When used with [[, or when the shell is in posix mode, the < and >
operators sort lexicographically using the current locale. When the
shell is not in posix mode, the test command sorts using ASCII ordering.

-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its “sticky” bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last
accessed.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
-o optname
True if the shell option optname is enabled. See the list of
options under the description of the -o option to the set builtin
below.
-v varname
True if the shell variable varname is set (has been assigned a
value). If varname is an indexed array variable name subscripted
by @ or *, this returns true if the array has any set elements.
If varname is an associative array variable name subscripted by @
or *, this returns true if an element with that key is set.
-R varname
True if the shell variable varname is set and is a name reference.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.

string1 == string2
string1 = string2
True if the strings are equal. = should be used with the test
command for POSIX conformance. When used with the [[ command,
this performs pattern matching as described above (Compound
Commands).
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
string1 > string2
True if string1 sorts after string2 lexicographically.

file1 -ef file2
True if file1 and file2 refer to the same device and inode
numbers.
file1 -nt file2
True if file1 is newer (according to modification date) than
file2, or if file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1
does not.

arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic
binary operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than or
equal to arg2, respectively. arg1 and arg2 may be positive or
negative integers. When used with the [[ command, arg1 and arg2
are evaluated as arithmetic expressions (see ARITHMETIC EVALUATION
above). Since the expansions the [[ command performs on arg1 and
arg2 can potentially result in empty strings, arithmetic
expression evaluation treats those as expressions that evaluate to
0.

SIMPLE COMMAND EXPANSION
When the shell executes a simple command, it performs the following
expansions, assignments, and redirections, from left to right, in the
following order.

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

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

3. Redirections are performed as described above under REDIRECTION.

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

If no command name results, the variable assignments affect the current
shell environment. In the case of such a command (one that consists only
of assignment statements and redirections), assignment statements are
performed before redirections. Otherwise, the variables are added to the
environment of the executed command and do not affect the current shell
environment. If any of the assignments attempts to assign a value to a
readonly variable, an error occurs, and the command exits with a non-zero
status.

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

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

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

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

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

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

If this execution fails because the file is not in executable format, and
the file is not a directory, it is assumed to be a shell script, a file
containing shell commands, and the shell creates a new instance of itself
to execute it. Bash tries to determine whether the file is a text file
or a binary, and will not execute files it determines to be binaries.
This subshell reinitializes itself, so that the effect is as if a new
shell had been invoked to handle the script, with the exception that the
locations of commands remembered by the parent (see hash below under
SHELL BUILTIN COMMANDS are retained by the child.

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

COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the following:

• Open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin.

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

• The file creation mode mask as set by umask or inherited from the
shell's parent.

• Current traps set by trap.

• Shell parameters that are set by variable assignment or with set
or inherited from the shell's parent in the environment.

• Shell functions defined during execution or inherited from the
shell's parent in the environment.

• Options enabled at invocation (either by default or with command-
line arguments) or by set.

• Options enabled by shopt.

• Shell aliases defined with alias.

• Various process IDs, including those of background jobs, the value
of $$, and the value of PPID.

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

• The shell's open files, plus any modifications and additions
specified by redirections to the command.

• The current working directory.

• The file creation mode mask.

• Shell variables and functions marked for export, along with
variables exported for the command, passed in the environment.

• Traps caught by the shell are reset to the values inherited from
the shell's parent, and traps ignored by the shell are ignored.

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

A subshell is a copy of the shell process.

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

When the shell is in posix mode, subshells spawned to execute command
substitutions inherit the value of the -e option from their parent shell.
When not in posix mode, bash clears the -e option in such subshells. See
the description of the inherit_errexit shell option below for how to
control this behavior when not in posix mode.

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

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
processes. Executed commands inherit the environment. The export,
declare -x, and unset commands modify the environment by adding and
deleting parameters and functions. If the value of a parameter in the
environment is modified, the new value automatically becomes part of the
environment, replacing the old. The environment inherited by any
executed command consists of the shell's initial environment, whose
values may be modified in the shell, less any pairs removed by the unset
or export -n commands, plus any additions via the export and declare -x
commands.

If any parameter assignments, as described above in PARAMETERS, appear
before a simple command, the variable assignments are part of that
command's environment for as long as it executes. These assignment
statements affect only the environment seen by that command. If these
assignments precede a call to a shell function, the variables are local
to the function and exported to that function's children.

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

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

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. So while an exit status of zero indicates success, a non-
zero exit status indicates failure.

When a command terminates on a fatal signal N, bash uses the value of
128+N as the exit status.

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

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

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

The exit status of the last command is available in the special parameter
$?.

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

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 catches and
handles SIGINT (so that the wait builtin is interruptible). When bash
receives SIGINT, it breaks out of any executing loops. In all cases,
bash ignores SIGQUIT. If job control is in effect, bash ignores SIGTTIN,
SIGTTOU, and SIGTSTP.

The trap builtin modifies the shell's signal handling, as described
below.

Non-builtin commands bash executes have signal handlers set to the values
inherited by the shell from its parent, unless trap sets them to be
ignored, in which case the child process will ignore them as well. When
job control is not in effect, asynchronous commands ignore SIGINT and
SIGQUIT in addition to these inherited handlers. Commands run as a
result of command substitution ignore the keyboard-generated job control
signals SIGTTIN, SIGTTOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting, an
interactive shell resends the SIGHUP to all jobs, running or stopped.
The shell sends SIGCONT to stopped jobs to ensure that they receive the
SIGHUP (see JOB CONTROL below for more information about running and
stopped jobs). To prevent the shell from sending the signal to a
particular job, remove it from the jobs table with the disown builtin
(see SHELL BUILTIN COMMANDS below) or mark it not to receive SIGHUP using
disown -h.

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

If bash is waiting for a command to complete and receives a signal for
which a trap has been set, it will not execute the trap until the command
completes. If bash is waiting for an asynchronous command via the wait
builtin, and it receives a signal for which a trap has been set, the wait
builtin will return immediately with an exit status greater than 128,
immediately after which the shell executes the trap.

When job control is not enabled, and bash is waiting for a foreground
command to complete, the shell receives keyboard-generated signals such
as SIGINT (usually generated by ^C) that users commonly intend to send to
that command. This happens because the shell and the command are in the
same process group as the terminal, and ^C sends SIGINT to all processes
in that process group. Since bash does not enable job control by default
when the shell is not interactive, this scenario is most common in non-
interactive shells.

When job control is enabled, and bash is waiting for a foreground command
to complete, the shell does not receive keyboard-generated signals,
because it is not in the same process group as the terminal. This
scenario is most common in interactive shells, where bash attempts to
enable job control by default. See JOB CONTROL below for more
information about process groups.

When job control is not enabled, and bash receives SIGINT while waiting
for a foreground command, it waits until that foreground command
terminates and then decides what to do about the SIGINT:

1. If the command terminates due to the SIGINT, bash concludes that
the user meant to send the SIGINT to the shell as well, and acts
on the SIGINT (e.g., by running a SIGINT trap, exiting a non-
interactive shell, or returning to the top level to read a new
command).

2. If the command does not terminate due to SIGINT, the program
handled the SIGINT itself and did not treat it as a fatal signal.
In that case, bash does not treat SIGINT as a fatal signal,
either, instead assuming that the SIGINT was used as part of the
program's normal operation (e.g., emacs uses it to abort editing
commands) or deliberately discarded. However, bash will run any
trap set on SIGINT, as it does with any other trapped signal it
receives while it is waiting for the foreground command to
complete, for compatibility.

When job control is enabled, bash does not receive keyboard-generated
signals such as SIGINT while it is waiting for a foreground command. An
interactive shell does not pay attention to the SIGINT, even if the
foreground command terminates as a result, other than noting its exit
status. If the shell is not interactive, and the foreground command
terminates due to the SIGINT, bash pretends it received the SIGINT itself
(scenario 1 above), for compatibility.

JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the
execution of processes and continue (resume) their execution at a later
point. A user typically employs this facility via an interactive
interface supplied jointly by the operating system kernel's terminal
driver and bash.

The shell associates a job with each pipeline. It keeps a table of
currently executing jobs, which the jobs command will display. Each job
has a job number, which jobs displays between brackets. Job numbers
start at 1. When bash starts a job asynchronously (in the background),
it prints a line that looks like:

[1] 25647

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

To facilitate the implementation of the user interface to job control,
each process has a process group ID, and the operating system maintains
the notion of a current terminal process group ID. This terminal process
group ID is associated with the controlling terminal.

Processes that have the same process group ID are said to be part of the
same process group. Members of the foreground process group (processes
whose process group ID is equal to the current terminal process group ID)
receive keyboard-generated signals such as SIGINT. Processes in the
foreground process group are said to be foreground processes. Background
processes are those whose process group ID differs from the controlling
terminal's; such processes are immune to keyboard-generated signals.
Only foreground processes are allowed to read from or, if the user so
specifies with “stty tostop”, write to the controlling terminal. The
system sends a SIGTTIN (SIGTTOU) signal to background processes which
attempt to read from (write to when “tostop” is in effect) the terminal,
which, unless caught, suspends the process.

If the operating system on which bash is running supports job control,
bash contains facilities to use it. Typing the suspend character
(typically ^Z, Control-Z) while a process is running stops that process
and returns control to bash. Typing the delayed suspend character
(typically ^Y, Control-Y) causes the process stop when it attempts to
read input from the terminal, and returns control to bash. The user then
manipulates the state of this job, using the bg command to continue it in
the background, the fg command to continue it in the foreground, or the
kill command to kill it. The suspend character takes effect immediately,
and has the additional side effect of discarding any pending output and
typeahead. To force a background process to stop, or stop a process
that's not associated with the current terminal session, send it the
SIGSTOP signal using kill.

There are a number of ways to refer to a job in the shell. The %
character introduces a job specification (jobspec).

Job number n may be referred to as %n. A job may also be referred to
using a prefix of the name used to start it, or using a substring that
appears in its command line. For example, %ce refers to a job whose
command name begins with ce. Using %?ce, on the other hand, refers to
any job containing the string ce in its command line. If the prefix or
substring matches more than one job, bash reports an error.

The symbols %% and %+ refer to the shell's notion of the current job. A
single % (with no accompanying job specification) also refers to the
current job. %- refers to the previous job. When a job starts in the
background, a job stops while in the foreground, or a job is resumed in
the background, it becomes the current job. The job that was the current
job becomes the previous job. When the current job terminates, the
previous job becomes the current job. If there is only a single job, %+
and %- can both be used to refer to that job. In output pertaining to
jobs (e.g., the output of the jobs command), the current job is always
marked with a +, and the previous job with a -.

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

The shell learns immediately whenever a job changes state. Normally,
bash waits until it is about to print a prompt before notifying the user
about changes in a job's status so as to not interrupt any other output,
though it will notify of changes in a job's status after a foreground
command in a list completes, before executing the next command in the
list. If the -b option to the set builtin command is enabled, bash
reports status changes immediately. Bash executes any trap on SIGCHLD
for each child that terminates.

When a job terminates and bash notifies the user about it, bash removes
the job from the table. It will not appear in jobs output, but wait will
report its exit status, as long as it's supplied the process ID
associated with the job as an argument. When the table is empty, job
numbers start over at 1.

If a user attempts to exit bash while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin,
running), the shell prints a warning message, and, if the checkjobs
option is enabled, lists the jobs and their statuses. The jobs command
may then be used to inspect their status. If the user immediately
attempts to exit again, without an intervening command, bash does not
print another warning, and terminates any stopped jobs.

When the shell is waiting for a job or process using the wait builtin,
and job control is enabled, wait will return when the job changes state.
The -f option causes wait to wait until the job or process terminates
before returning.

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 examines the value of the array variable PROMPT_COMMAND just before
printing each primary prompt. If any elements in PROMPT_COMMAND are set
and non-null, Bash executes each value, in numeric order, just as if it
had been typed on the command line. Bash displays PS0 after it reads a
command but before executing it.

Bash displays PS4 as described above before tracing each command when the
-x option is enabled.

Bash allows the prompt strings PS0, PS1, PS2, and PS4, to be customized
by inserting a number of backslash-escaped special characters that are
decoded as follows:

\a An ASCII bell character (07).
\d The date in “Weekday Month Date” format (e.g., “Tue May
26”).
\D{format}
The format is passed to strftime(3) and the result is
inserted into the prompt string; an empty format results in
a locale-specific time representation. The braces are
required.
\e An ASCII escape character (033).
\h The hostname up to the first “.”.
\H The hostname.
\j The number of jobs currently managed by the shell.
\l The basename of the shell's terminal device name (e.g.,
“ttys0”).
\n A newline.
\r A carriage return.
\s The name of the shell: the basename of $0 (the portion
following the final slash).
\t The current time in 24-hour HH:MM:SS format.
\T The current time in 12-hour HH:MM:SS format.
\@ The current time in 12-hour am/pm format.
\A The current time in 24-hour HH:MM format.
\u The username of the current user.
\v The bash version (e.g., 2.00).
\V The bash release, version + patch level (e.g., 2.00.0)
\w The value of the PWD shell variable ($PWD), with $HOME
abbreviated with a tilde (uses the value of the
PROMPT_DIRTRIM variable).
\W The basename of $PWD, with $HOME abbreviated with a tilde.
\! The history number of this command.
\# The command number of this command.
\$ If the effective UID is 0, a #, otherwise a $.
\nnn The character corresponding to the octal number nnn.
\\ A backslash.
\[ Begin a sequence of non-printing characters, which could be
used to embed a terminal control sequence into the prompt.
\] End a sequence of non-printing characters.

The command number and the history number are usually different: the
history number of a command is its position in the history list, which
may include commands restored from the history file (see HISTORY below),
while the command number is the position in the sequence of commands
executed during the current shell session. After the string is decoded,
it is expanded via parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the promptvars
shell option (see the description of the shopt command under SHELL
BUILTIN COMMANDS below). This can have unwanted side effects if escaped
portions of the string appear within command substitution or contain
characters special to word expansion.

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

Readline Notation
This section uses Emacs-style editing concepts and uses its notation for
keystrokes. Control keys are denoted by C-key, e.g., C-n means
Control-N. Similarly, meta keys are denoted by M-key, so M-x means
Meta-X. The Meta key is often labeled “Alt” or “Option”.

On keyboards without a Meta key, M-x means ESC x, i.e., press and release
the Escape key, then press and release the x key, in sequence. This
makes ESC the meta prefix. The combination M-C-x means ESC Control-x:
press and release the Escape key, then press and hold the Control key
while pressing the x key, then release both.

On some keyboards, the Meta key modifier produces characters with the
eighth bit (0200) set. You can use the enable-meta-key variable to
control whether or not it does this, if the keyboard allows it. On many
others, the terminal or terminal emulator converts the metafied key to a
key sequence beginning with ESC as described in the preceding paragraph.

If your Meta key produces a key sequence with the ESC meta prefix, you
can make M-key key bindings you specify (see Readline Key Bindings below)
do the same thing by setting the force-meta-prefix variable.

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

The point is the current cursor position, and mark refers to a saved
cursor position. The text between the point and mark is referred to as
the region. Readline has the concept of an active region: when the
region is active, readline redisplay highlights the region using the
value of the active-region-start-color variable. The
enable-active-region variable turns this on and off. Several commands
set the region to active; those are noted below.

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

Readline Initialization
Readline is customized by putting commands in an initialization file (the
inputrc file). The name of this file is taken from the value of the
INPUTRC shell variable. If that variable is unset, the default is
~/.inputrc. If that file does not exist or cannot be read, readline
looks for /etc/inputrc. When a program that uses the readline library
starts up, readline reads the initialization file and sets the key
bindings and variables found there, before reading any user input.

There are only a few basic constructs allowed in the inputrc file. Blank
lines are ignored. Lines beginning with a # are comments. Lines
beginning with a $ indicate conditional constructs. Other lines denote
key bindings and variable settings.

The default key-bindings in this section may be changed using key binding
commands in the inputrc file. Programs that use the readline library,
including bash, may add their own commands and bindings.

For example, placing

M-Control-u: universal-argument
or
C-Meta-u: universal-argument

into the inputrc would make M-C-u execute the readline command
universal-argument.

Key bindings may contain the following symbolic character names: DEL,
ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive backspace),
SPACE, SPC, and TAB.

In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro). The
difference between a macro and a command is that a macro is enclosed in
single or double quotes.

Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple.
All that is required is the name of the command or the text of a macro
and a key sequence to which it should be bound. The key sequence may be
specified in one of two ways: as a symbolic key name, possibly with Meta-
or Control- prefixes, or as a key sequence composed of one or more
characters enclosed in double quotes. The key sequence and name are
separated by a colon. There can be no whitespace between the name and
the colon.

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

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

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

In the second form, "keyseq":function-name or macro, keyseq differs from
keyname above in that strings denoting an entire key sequence may be
specified by placing the sequence within double quotes. Some GNU Emacs
style key escapes can be used, as in the following example, but none of
the symbolic character names are recognized.

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

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

The full set of GNU Emacs style escape sequences available when
specifying key sequences is
\C- A control prefix.
\M- Adding the meta prefix or converting the following
character to a meta character, as described below under
force-meta-prefix.
\e An escape character.
\\ Backslash.
\" Literal ", a double quote.
\' Literal ', a single quote.

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

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

Bash will display or modify the current readline key bindings with the
bind builtin command. The -o emacs or -o vi options to the set builtin
(see SHELL BUILTIN COMMANDS below) change the editing mode during
interactive use.

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

set variable-name value
or using the bind builtin command (see SHELL BUILTIN COMMANDS below).

Except where noted, readline variables can take the values On or Off
(without regard to case). Unrecognized variable names are ignored. When
readline reads a variable value, empty or null values, “on” (case-
insensitive), and “1” are equivalent to On. All other values are
equivalent to Off.

The bind -V command lists the current readline variable names and values
(see SHELL BUILTIN COMMANDS below).

The variables and their default values are:

active-region-start-color
A string variable that controls the text color and background when
displaying the text in the active region (see the description of
enable-active-region below). This string must not take up any
physical character positions on the display, so it should consist
only of terminal escape sequences. It is output to the terminal
before displaying the text in the active region. This variable is
reset to the default value whenever the terminal type changes.
The default value is the string that puts the terminal in standout
mode, as obtained from the terminal's terminfo description. A
sample value might be “\e[01;33m”.
active-region-end-color
A string variable that “undoes” the effects of
active-region-start-color and restores “normal” terminal display
appearance after displaying text in the active region. This
string must not take up any physical character positions on the
display, so it should consist only of terminal escape sequences.
It is output to the terminal after displaying the text in the
active region. This variable is reset to the default value
whenever the terminal type changes. The default value is the
string that restores the terminal from standout mode, as obtained
from the terminal's terminfo description. A sample value might be
“\e[0m”.
bell-style (audible)
Controls what happens when readline wants to ring the terminal
bell. If set to none, readline never rings the bell. If set to
visible, readline uses a visible bell if one is available. If set
to audible, readline attempts to ring the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters
that are treated specially by the kernel's terminal driver to
their readline equivalents. These override the default readline
bindings described here. Type “stty -a” at a bash prompt to see
your current terminal settings, including the special control
characters (usually cchars).
blink-matching-paren (Off)
If set to On, readline attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted.
colored-completion-prefix (Off)
If set to On, when listing completions, readline displays the
common prefix of the set of possible completions using a different
color. The color definitions are taken from the value of the
LS_COLORS environment variable. If there is a color definition in
$LS_COLORS for the custom suffix “.readline-colored-completion-
prefix”, readline uses this color for the common prefix instead of
its default.
colored-stats (Off)
If set to On, readline displays possible completions using
different colors to indicate their file type. The color
definitions are taken from the value of the LS_COLORS environment
variable.
comment-begin (“#”)
The string that the readline insert-comment command inserts. This
command is bound to M-# in emacs mode and to # in vi command mode.
completion-display-width (-1)
The number of screen columns used to display possible matches when
performing completion. The value is ignored if it is less than 0
or greater than the terminal screen width. A value of 0 causes
matches to be displayed one per line. The default value is -1.
completion-ignore-case (Off)
If set to On, readline performs filename matching and completion
in a case-insensitive fashion.
completion-map-case (Off)
If set to On, and completion-ignore-case is enabled, readline
treats hyphens (-) and underscores (_) as equivalent when
performing case-insensitive filename matching and completion.
completion-prefix-display-length (0)
The maximum length in characters of the common prefix of a list of
possible completions that is displayed without modification. When
set to a value greater than zero, readline replaces common
prefixes longer than this value with an ellipsis when displaying
possible completions. If a completion begins with a period, and
eadline is completing filenames, it uses three underscores instead
of an ellipsis.
completion-query-items (100)
This determines when the user is queried about viewing the number
of possible completions generated by the possible-completions
command. It may be set to any integer value greater than or equal
to zero. If the number of possible completions is greater than or
equal to the value of this variable, readline asks whether or not
the user wishes to view them; otherwise readline simply lists them
on the terminal. A zero value means readline should never ask;
negative values are treated as zero.
convert-meta (On)
If set to On, readline converts characters it reads that have the
eighth bit set to an ASCII key sequence by clearing the eighth bit
and prefixing it with an escape character (converting the
character to have the meta prefix). The default is On, but
readline sets it to Off if the locale contains characters whose
encodings may include bytes with the eighth bit set. This
variable is dependent on the LC_CTYPE locale category, and may
change if the locale changes. This variable also affects key
bindings; see the description of force-meta-prefix below.
disable-completion (Off)
If set to On, readline inhibits word completion. Completion
characters are inserted into the line as if they had been mapped
to self-insert.
echo-control-characters (On)
When set to On, on operating systems that indicate they support
it, readline echoes a character corresponding to a signal
generated from the keyboard.
editing-mode (emacs)
Controls whether readline uses a set of key bindings similar to
Emacs or vi. editing-mode can be set to either emacs or vi.
emacs-mode-string (@)
If the show-mode-in-prompt variable is enabled, this string is
displayed immediately before the last line of the primary prompt
when emacs editing mode is active. The value is expanded like a
key binding, so the standard set of meta- and control- prefixes
and backslash escape sequences is available. The \1 and \2
escapes begin and end sequences of non-printing characters, which
can be used to embed a terminal control sequence into the mode
string.
enable-active-region (On)
When this variable is set to On, readline allows certain commands
to designate the region as active. When the region is active,
readline highlights the text in the region using the value of the
active-region-start-color variable, which defaults to the string
that enables the terminal's standout mode. The active region
shows the text inserted by bracketed-paste and any matching text
found by incremental and non-incremental history searches.
enable-bracketed-paste (On)
When set to On, readline configures the terminal to insert each
paste into the editing buffer as a single string of characters,
instead of treating each character as if it had been read from the
keyboard. This is called bracketed-paste mode; it prevents
readline from executing any editing commands bound to key
sequences appearing in the pasted text.
enable-keypad (Off)
When set to On, readline tries to enable the application keypad
when it is called. Some systems need this to enable the arrow
keys.
enable-meta-key (On)
When set to On, readline tries to enable any meta modifier key the
terminal claims to support. On many terminals, the Meta key is
used to send eight-bit characters; this variable checks for the
terminal capability that indicates the terminal can enable and
disable a mode that sets the eighth bit of a character (0200) if
the Meta key is held down when the character is typed (a meta
character).
expand-tilde (Off)
If set to On, readline performs tilde expansion when it attempts
word completion.
force-meta-prefix (Off)
If set to On, readline modifies its behavior when binding key
sequences containing \M- or Meta- (see Key Bindings above) by
converting a key sequence of the form \M-C or Meta-C to the two-
character sequence ESC C (adding the meta prefix). If
force-meta-prefix is set to Off (the default), readline uses the
value of the convert-meta variable to determine whether to perform
this conversion: if convert-meta is On, readline performs the
conversion described above; if it is Off, readline converts C to a
meta character by setting the eighth bit (0200).
history-preserve-point (Off)
If set to On, the history code attempts to place point at the same
location on each history line retrieved with previous-history or
next-history.
history-size (unset)
Set the maximum number of history entries saved in the history
list. If set to zero, any existing history entries are deleted
and no new entries are saved. If set to a value less than zero,
the number of history entries is not limited. By default, bash
sets the maximum number of history entries to the value of the
HISTSIZE shell variable. Setting history-size to a non-numeric
value will set the maximum number of history entries to 500.
horizontal-scroll-mode (Off)
Setting this variable to On makes readline use a single line for
display, scrolling the input horizontally on a single screen line
when it becomes longer than the screen width rather than wrapping
to a new line. This setting is automatically enabled for
terminals of height 1.
input-meta (Off)
If set to On, readline enables eight-bit input (that is, it does
not clear the eighth bit in the characters it reads), regardless
of what the terminal claims it can support. The default is Off,
but readline sets it to On if the locale contains characters whose
encodings may include bytes with the eighth bit set. This
variable is dependent on the LC_CTYPE locale category, and its
value may change if the locale changes. The name meta-flag is a
synonym for input-meta.
isearch-terminators (“C-[C-j”)
The string of characters that should terminate an incremental
search without subsequently executing the character as a command.
If this variable has not been given a value, the characters ESC
and C-j terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names is
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command, and
vi-insert. vi is equivalent to vi-command; emacs is equivalent to
emacs-standard. The default value is emacs; the value of
editing-mode also affects the default keymap.
keyseq-timeout (500)
Specifies the duration readline will wait for a character when
reading an ambiguous key sequence (one that can form a complete
key sequence using the input read so far, or can take additional
input to complete a longer key sequence). If readline does not
receive any input within the timeout, it uses the shorter but
complete key sequence. The value is specified in milliseconds, so
a value of 1000 means that readline will wait one second for
additional input. If this variable is set to a value less than or
equal to zero, or to a non-numeric value, readline waits until
another key is pressed to decide which key sequence to complete.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, readline displays history lines that have been
modified with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
directories have a slash appended, subject to the value of
mark-directories.
match-hidden-files (On)
This variable, when set to On, forces readline to match files
whose names begin with a “.” (hidden files) when performing
filename completion. If set to Off, the user must include the
leading “.” in the filename to be completed.
menu-complete-display-prefix (Off)
If set to On, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling
through the list.
output-meta (Off)
If set to On, readline displays characters with the eighth bit set
directly rather than as a meta-prefixed escape sequence. The
default is Off, but readline sets it to On if the locale contains
characters whose encodings may include bytes with the eighth bit
set. This variable is dependent on the LC_CTYPE locale category,
and its value may change if the locale changes.
page-completions (On)
If set to On, readline uses an internal pager resembling more(1)
to display a screenful of possible completions at a time.
prefer-visible-bell
See bell-style.
print-completions-horizontally (Off)
If set to On, readline displays completions with matches sorted
horizontally in alphabetical order, rather than down the screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines
before returning when executing accept-line. By default, history
lines may be modified and retain individual undo lists across
calls to readline.
search-ignore-case (Off)
If set to On, readline performs incremental and non-incremental
history list searches in a case-insensitive fashion.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If
set to On, words which have more than one possible completion
cause the matches to be listed immediately instead of ringing the
bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a
fashion similar to show-all-if-ambiguous. If set to On, words
which have more than one possible completion without any possible
partial completion (the possible completions don't share a common
prefix) cause the matches to be listed immediately instead of
ringing the bell.
show-mode-in-prompt (Off)
If set to On, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi insertion.
The mode strings are user-settable (e.g., emacs-mode-string).
skip-completed-text (Off)
If set to On, this alters the default completion behavior when
inserting a single match into the line. It's only active when
performing completion in the middle of a word. If enabled,
readline does not insert characters from the completion that match
characters after point in the word being completed, so portions of
the word following the cursor are not duplicated.
vi-cmd-mode-string ((cmd))
If the show-mode-in-prompt variable is enabled, this string is
displayed immediately before the last line of the primary prompt
when vi editing mode is active and in command mode. The value is
expanded like a key binding, so the standard set of meta- and
control- prefixes and backslash escape sequences is available.
The \1 and \2 escapes begin and end sequences of non-printing
characters, which can be used to embed a terminal control sequence
into the mode string.
vi-ins-mode-string ((ins))
If the show-mode-in-prompt variable is enabled, this string is
displayed immediately before the last line of the primary prompt
when vi editing mode is active and in insertion mode. The value
is expanded like a key binding, so the standard set of meta- and
control- prefixes and backslash escape sequences is available.
The \1 and \2 escapes begin and end sequences of non-printing
characters, which can be used to embed a terminal control sequence
into the mode string.
visible-stats (Off)
If set to On, a character denoting a file's type as reported by
stat(2) is appended to the filename when listing possible
completions.

Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings and
variable settings to be performed as the result of tests. There are four
parser directives available.

$if The $if construct allows bindings to be made based on the editing
mode, the terminal being used, or the application using readline.
The text of the test, after any comparison operator, extends to
the end of the line; unless otherwise noted, no characters are
required to isolate it.

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

term The term= form may be used to include terminal-specific key
bindings, perhaps to bind the key sequences output by the
terminal's function keys. The word on the right side of
the = is tested against both the full name of the terminal
and the portion of the terminal name before the first -.
This allows xterm to match both xterm and xterm-256color,
for instance.

version
The version test may be used to perform comparisons against
specific readline versions. The version expands to the
current readline version. The set of comparison operators
includes =, (and ==), !=, <=, >=, <, and >. The version
number supplied on the right side of the operator consists
of a major version number, an optional decimal point, and
an optional minor version (e.g., 7.1). If the minor
version is omitted, it defaults to 0. The operator may be
separated from the string version and from the version
number argument by whitespace.

application
The application construct is used to include application-
specific settings. Each program using the readline library
sets the application name, and an initialization file can
test for a particular value. This could be used to bind
key sequences to functions useful for a specific program.
For instance, the following command adds a key sequence
that quotes the current or previous word in bash:

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

variable
The variable construct provides simple equality tests for
readline variables and values. The permitted comparison
operators are =, ==, and !=. The variable name must be
separated from the comparison operator by whitespace; the
operator may be separated from the value on the right hand
side by whitespace. String and boolean variables may be
tested. Boolean variables must be tested against the
values on and off.

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

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

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

$include /etc/inputrc

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

Incremental searches begin before the user has finished typing the search
string. As each character of the search string is typed, readline
displays the next entry from the history matching the string typed so
far. An incremental search requires only as many characters as needed to
find the desired history entry. When using emacs editing mode, type C-r
to search backward in the history for a particular string. Typing C-s
searches forward through the history. The characters present in the
value of the isearch-terminators variable are used to terminate an
incremental search. If that variable has not been assigned a value, ESC
and C-j terminate an incremental search. C-g aborts an incremental
search and restores the original line. When the search is terminated,
the history entry containing the search string becomes the current line.

To find other matching entries in the history list, type C-r or C-s as
appropriate. This searches backward or forward in the history for the
next entry matching the search string typed so far. Any other key
sequence bound to a readline command terminates the search and executes
that command. For instance, a newline terminates the search and accepts
the line, thereby executing the command from the history list. A
movement command will terminate the search, make the last line found the
current line, and begin editing.

Readline remembers the last incremental search string. If two C-rs are
typed without any intervening characters defining a new search string,
readline uses any remembered search string.

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

Readline Command Names
The following is a list of the names of the commands and the default key
sequences to which they are bound. Command names without an accompanying
key sequence are unbound by default.

In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the set-mark
command. The text between the point and mark is referred to as the
region. Readline has the concept of an active region: when the region is
active, readline redisplay highlights the region using the value of the
active-region-start-color variable. The enable-active-region readline
variable turns this on and off. Several commands set the region to
active; those are noted below.

Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line. This may also be bound to
the Home key on some keyboards.
end-of-line (C-e)
Move to the end of the line. This may also be bound to the End
key on some keyboards.
forward-char (C-f)
Move forward a character. This may also be bound to the right
arrow key on some keyboards.
backward-char (C-b)
Move back a character. This may also be bound to the left arrow
key on some keyboards.
forward-word (M-f)
Move forward to the end of the next word. Words are composed of
alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words are
composed of alphanumeric characters (letters and digits).
shell-forward-word (M-C-f)
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.
shell-backward-word (M-C-b)
Move back to the start of the current or previous word. Words are
delimited by non-quoted shell metacharacters.
previous-screen-line
Attempt to move point to the same physical screen column on the
previous physical screen line. This will not have the desired
effect if the current readline line does not take up more than one
physical line or if point is not greater than the length of the
prompt plus the screen width.
next-screen-line
Attempt to move point to the same physical screen column on the
next physical screen line. This will not have the desired effect
if the current readline line does not take up more than one
physical line or if the length of the current readline line is not
greater than the length of the prompt plus the screen width.
clear-display (M-C-l)
Clear the screen and, if possible, the terminal's scrollback
buffer, then redraw the current line, leaving the current line at
the top of the screen.
clear-screen (C-l)
Clear the screen, then redraw the current line, leaving the
current line at the top of the screen. With a numeric argument,
refresh the current line without clearing the screen.
redraw-current-line
Refresh the current line.

Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line
is non-empty, add it to the history list according to the state of
the HISTCONTROL and HISTIGNORE variables. If the line is a
modified history line, restore the history line to its original
state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in
the list. This may also be bound to the up arrow key on some
keyboards.
next-history (C-n)
Fetch the next command from the history list, moving forward in
the list. This may also be bound to the down arrow key on some
keyboards.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently
being entered.
operate-and-get-next (C-o)
Accept the current line for execution as if a newline had been
entered, and fetch the next line relative to the current line from
the history for editing. A numeric argument, if supplied,
specifies the history entry to use instead of the current line.
fetch-history
With a numeric argument, fetch that entry from the history list
and make it the current line. Without an argument, move back to
the first entry in the history list.
reverse-search-history (C-r)
Search backward starting at the current line and moving “up”
through the history as necessary. This is an incremental search.
This command sets the region to the matched text and activates the
region.
forward-search-history (C-s)
Search forward starting at the current line and moving “down”
through the history as necessary. This is an incremental search.
This command sets the region to the matched text and activates the
region.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line
using a non-incremental search for a string supplied by the user.
The search string may match anywhere in a history line.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search
for a string supplied by the user. The search string may match
anywhere in a history line.
history-search-backward
Search backward through the history for the string of characters
between the start of the current line and the point. The search
string must match at the beginning of a history line. This is a
non-incremental search. This may be bound to the Page Up key on
some keyboards.
history-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string must match at the beginning of a history line. This is a
non-incremental search. This may be bound to the Page Down key on
some keyboards.
history-substring-search-backward
Search backward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a non-
incremental search.
history-substring-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a non-
incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument n,
insert the nth word from the previous command (the words in the
previous command begin with word 0). A negative argument inserts
the nth word from the end of the previous command. Once the
argument n is computed, this uses the history expansion facilities
to extract the nth word, as if the “!n” history expansion had been
specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word of
the previous history entry). With a numeric argument, behave
exactly like yank-nth-arg. Successive calls to yank-last-arg move
back through the history list, inserting the last word (or the
word specified by the argument to the first call) of each line in
turn. Any numeric argument supplied to these successive calls
determines the direction to move through the history. A negative
argument switches the direction through the history (back or
forward). This uses the history expansion facilities to extract
the last word, as if the “!$” history expansion had been
specified.
shell-expand-line (M-C-e)
Expand the line by performing shell word expansions. This
performs alias and history expansion, $'string' and $"string"
quoting, tilde expansion, parameter and variable expansion,
arithmetic expansion, command and process substitution, word
splitting, and quote removal. An explicit argument suppresses
command and process substitution. See HISTORY EXPANSION below for
a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a space.
See HISTORY EXPANSION below for a description of history
expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above
for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
edit-and-execute-command (C-x C-e)
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke $VISUAL,
$EDITOR, and emacs as the editor, in that order.

Commands for Changing Text
end-of-file (usually C-d)
The character indicating end-of-file as set, for example, by
stty(1). If this character is read when there are no characters
on the line, and point is at the beginning of the line, readline
interprets it as the end of input and returns EOF.
delete-char (C-d)
Delete the character at point. If this function is bound to the
same character as the tty EOF character, as C-d commonly is, see
above for the effects. This may also be bound to the Delete key
on some keyboards.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric
argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at the
end of the line, in which case the character behind the cursor is
deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how to
insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
bracketed-paste-begin
This function is intended to be bound to the “bracketed paste”
escape sequence sent by some terminals, and such a binding is
assigned by default. It allows readline to insert the pasted text
as a single unit without treating each character as if it had been
read from the keyboard. The pasted characters are inserted as if
each one was bound to self-insert instead of executing any editing
commands.
Bracketed paste sets the region to the inserted text and activates
the region.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well. If point is at the end of
the line, then this transposes the two characters before point.
Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving point
past that word as well. If point is at the end of the line, this
transposes the last two words on the line.
shell-transpose-words (M-C-t)
Drag the word before point past the word after point, moving point
past that word as well. If the insertion point is at the end of
the line, this transposes the last two words on the line. Word
boundaries are the same as shell-forward-word and
shell-backward-word.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative
argument, uppercase the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative
argument, lowercase the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative
argument, capitalize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric
argument, switches to overwrite mode. With an explicit non-
positive numeric argument, switches to insert mode. This command
affects only emacs mode; vi mode does overwrite differently. Each
call to readline() starts in insert mode.
In overwrite mode, characters bound to self-insert replace the
text at point rather than pushing the text to the right.
Characters bound to backward-delete-char replace the character
before point with a space. By default, this command is unbound,
but may be bound to the Insert key on some keyboards.

Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the current line. With a
negative numeric argument, kill backward from the cursor to the
beginning of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the current line. With a
negative numeric argument, kill forward from the cursor to the end
of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line, saving the
killed text on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as those
used by backward-word.
shell-kill-word (M-C-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by shell-forward-word.
shell-backward-kill-word
Kill the word behind point. Word boundaries are the same as those
used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary,
saving the killed text on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash
character as the word boundaries, saving the killed text on the
kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer, so it can be
yanked immediately.
copy-backward-word
Copy the word before point to the kill buffer. The word
boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following
yank or yank-pop.

Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a
new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is
followed by digits, executing universal-argument again ends the
numeric argument, but is otherwise ignored. As a special case, if
this command is immediately followed by a character that is
neither a digit nor minus sign, the argument count for the next
command is multiplied by four. The argument count is initially
one, so executing this function the first time makes the argument
count four, a second time makes the argument count sixteen, and so
on.

Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash
attempts completion by first checking for any programmable
completions for the command word (see Programmable Completion
below), otherwise treating the text as a variable (if the text
begins with $), username (if the text begins with ~), hostname (if
the text begins with @), or command (including aliases, functions,
and builtins) in turn. If none of these produces a match, it
falls back to filename completion.
possible-completions (M-?)
List the possible completions of the text before point. When
displaying completions, readline sets the number of columns used
for display to the value of completion-display-width, the value of
the shell variable COLUMNS, or the screen width, in that order.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions, separated by a space.
menu-complete
Similar to complete, but replaces the word to be completed with a
single match from the list of possible completions. Repeatedly
executing menu-complete steps through the list of possible
completions, inserting each match in turn. At the end of the list
of completions, menu-complete rings the bell (subject to the
setting of bell-style) and restores the original text. An
argument of n moves n positions forward in the list of matches; a
negative argument moves backward through the list. This command
is intended to be bound to TAB, but is unbound by default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list of
possible completions, as if menu-complete had been given a
negative argument. This command is unbound by default.
export-completions
Perform completion on the word before point as described above and
write the list of possible completions to readline's output stream
using the following format, writing information on separate lines:

• the number of matches N;
• the word being completed;
• S:E, where S and E are the start and end offsets of the
word in the readline line buffer; then
• each match, one per line

If there are no matches, the first line will be “0”, and this
command does not print any output after the S:E. If there is only
a single match, this prints a single line containing it. If there
is more than one match, this prints the common prefix of the
matches, which may be empty, on the first line after the S:E, then
the matches on subsequent lines. In this case, N will include the
first line with the common prefix.

The user or application should be able to accommodate the
possibility of a blank line. The intent is that the user or
application reads N lines after the line containing S:E to obtain
the match list. This command is unbound by default.

delete-char-or-list
Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char). At the end of the line, it
behaves identically to possible-completions. This command is
unbound by default.

complete-filename (M-/)
Attempt filename completion on the text before point.

possible-filename-completions (C-x /)
List the possible completions of the text before point, treating
it as a filename.

complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.

possible-username-completions (C-x ~)
List the possible completions of the text before point, treating
it as a username.

complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.

possible-variable-completions (C-x $)
List the possible completions of the text before point, treating
it as a shell variable.

complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.

possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating
it as a hostname.

complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.

possible-command-completions (C-x !)
List the possible completions of the text before point, treating
it as a command name.

dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against history list entries for possible completion matches.

dabbrev-expand
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.

complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to the
shell (see Brace Expansion above).

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

Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any
bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell
(subject to the setting of bell-style).
do-lowercase-version (M-A, M-B, M-x, ...)
If the metafied character x is uppercase, run the command that is
bound to the corresponding metafied lowercase character. The
behavior is undefined if x is already lowercase.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the
undo command enough times to return the line to its initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied, set
the mark to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. Set the current cursor position to
the saved position, then set the mark to the old cursor position.
character-search (C-])
Read a character and move point to the next occurrence of that
character. A negative argument searches for previous occurrences.
character-search-backward (M-C-])
Read a character and move point to the previous occurrence of that
character. A negative argument searches for subsequent
occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. CSI sequences begin
with a Control Sequence Indicator (CSI), usually ESC [. If this
sequence is bound to “\e[”, keys producing CSI sequences have no
effect unless explicitly bound to a readline command, instead of
inserting stray characters into the editing buffer. This is
unbound by default, but usually bound to ESC [.
insert-comment (M-#)
Without a numeric argument, insert the value of the readline
comment-begin variable at the beginning of the current line. If a
numeric argument is supplied, this command acts as a toggle: if
the characters at the beginning of the line do not match the value
of comment-begin, insert the value; otherwise delete the
characters in comment-begin from the beginning of the line. In
either case, the line is accepted as if a newline had been typed.
The default value of comment-begin causes this command to make the
current line a shell comment. If a numeric argument causes the
comment character to be removed, the line will be executed by the
shell.
spell-correct-word (C-x s)
Perform spelling correction on the current word, treating it as a
directory or filename, in the same way as the cdspell shell
option. Word boundaries are the same as those used by
shell-forward-word.
glob-complete-word (M-g)
Treat the word before point as a pattern for pathname expansion,
with an asterisk implicitly appended, then use the pattern to
generate a list of matching file names for possible completions.
glob-expand-word (C-x *)
Treat the word before point as a pattern for pathname expansion,
and insert the list of matching file names, replacing the word.
If a numeric argument is supplied, append a * before pathname
expansion.
glob-list-expansions (C-x g)
Display the list of expansions that would have been generated by
glob-expand-word and redisplay the line. If a numeric argument is
supplied, append a * before pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline
output stream. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an inputrc
file.
dump-variables
Print all of the settable readline variables and their values to
the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part of
an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the
strings they output to the readline output stream. If a numeric
argument is supplied, the output is formatted in such a way that
it can be made part of an inputrc file.
execute-named-command (M-x)
Read a bindable readline command name from the input and execute
the function to which it's bound, as if the key sequence to which
it was bound appeared in the input. If this function is supplied
with a numeric argument, it passes that argument to the function
it executes.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.

Programmable Completion
When a user attempts word completion for a command or an argument to a
command for which a completion specification (a compspec) has been
defined using the complete builtin (see SHELL BUILTIN COMMANDS below),
readline invokes the programmable completion facilities.

First, bash identifies the command name. If a compspec has been defined
for that command, the compspec is used to generate the list of possible
completions for the word. If the command word is the empty string
(completion attempted at the beginning of an empty line), bash uses any
compspec defined with the -E option to complete. The -I option to
complete indicates that the command word is the first non-assignment word
on the line, or after a command delimiter such as ; or |. This usually
indicates command name completion.

If the command word is a full pathname, bash searches for a compspec for
the full pathname first. If there is no compspec for the full pathname,
bash attempts to find a compspec for the portion following the final
slash. If those searches do not result in a compspec, or if there is no
compspec for the command word, bash uses any compspec defined with the -D
option to complete as the default. If there is no default compspec, bash
performs alias expansion on the command word as a final resort, and
attempts to find a compspec for the command word resulting from any
successful expansion.

If a compspec is not found, bash performs its default completion as
described above under Completing. Otherwise, once a compspec has been
found, bash uses it to generate the list of matching words.

First, bash performs the actions specified by the compspec. This only
returns matches which are prefixes of the word being completed. When the
-f or -d option is used for filename or directory name completion, bash
uses the shell variable FIGNORE to filter the matches.

Next, programmable completion generates matches specified by a pathname
expansion pattern supplied as an argument to the -G option. The words
generated by the pattern need not match the word being completed. Bash
uses the FIGNORE variable to filter the matches, but does not use the
GLOBIGNORE shell variable.

Next, completion considers the string specified as the argument to the -W
option. The string is first split using the characters in the IFS
special variable as delimiters. This honors shell quoting within the
string, in order to provide a mechanism for the words to contain shell
metacharacters or characters in the value of IFS. Each word is then
expanded using brace expansion, tilde expansion, parameter and variable
expansion, command substitution, and arithmetic expansion, as described
above under EXPANSION. The results are split using the rules described
above under Word Splitting. The results of the expansion are prefix-
matched against the word being completed, and the matching words become
possible completions.

After these matches have been generated, bash executes any shell function
or command specified with the -F and -C options. When the command or
function is invoked, bash assigns values to the COMP_LINE, COMP_POINT,
COMP_KEY, and COMP_TYPE variables as described above under Shell
Variables. If a shell function is being invoked, bash also sets the
COMP_WORDS and COMP_CWORD variables. When the function or command is
invoked, the first argument ($1) is the name of the command whose
arguments are being completed, the second argument ($2) is the word being
completed, and the third argument ($3) is the word preceding the word
being completed on the current command line. There is no filtering of
the generated completions against the word being completed; the function
or command has complete freedom in generating the matches and they do not
need to match a prefix of the word.

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

Next, any command specified with the -C option is invoked in an
environment equivalent to command substitution. It should print a list
of completions, one per line, to the standard output. Backslash will
escape a newline, if necessary. These are added to the set of possible
completions.

After generating all of the possible completions, bash applies any filter
specified with the -X option to the completions in the list. The filter
is a pattern as used for pathname expansion; a & in the pattern is
replaced with the text of the word being completed. A literal & may be
escaped with a backslash; the backslash is removed before attempting a
match. Any completion that matches the pattern is removed from the list.
A leading ! negates the pattern; in this case bash removes any completion
that does not match the pattern. If the nocasematch shell option is
enabled, bash performs the match without regard to the case of alphabetic
characters.

Finally, programmable completion adds any prefix and suffix specified
with the -P and -S options, respectively, to each completion, and returns
the result to readline as the list of possible completions.

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

If the -o plusdirs option was supplied to complete when the compspec was
defined, bash attempts directory name completion and adds any matches to
the set of possible completions.

By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions. The default
bash completions and the readline default of filename completion are
disabled. If the -o bashdefault option was supplied to complete when the
compspec was defined, and the compspec generates no matches, bash
attempts its default completions. If the compspec and, if attempted, the
default bash completions generate no matches, and the -o default option
was supplied to complete when the compspec was defined, programmable
completion performs readline's default completion.

The options supplied to complete and compopt can control how readline
treats the completions. For instance, the -o fullquote option tells
readline to quote the matches as if they were filenames. See the
description of complete below for details.

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

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

For instance, assuming that there is a library of compspecs, each kept in
a file corresponding to the name of the command, the following default
completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default

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 shell saves the text of the last
HISTSIZE commands (default 500). The shell stores each command in the
history list prior to parameter and variable expansion (see EXPANSION
above) but after history expansion is performed, subject to the values of
the shell variables HISTIGNORE and HISTCONTROL.

On startup, bash initializes the history list by reading history entries
from the file named by the HISTFILE variable (default ~/.bash_history).
That file is referred to as the history file. The history file is
truncated, if necessary, to contain no more than the number of history
entries specified by the value of the HISTFILESIZE variable. If
HISTFILESIZE is unset, or set to null, a non-numeric value, or a numeric
value less than zero, the history file is not truncated.

When the history file is read, lines beginning with the history comment
character followed immediately by a digit are interpreted as timestamps
for the following history line. These timestamps are optionally
displayed depending on the value of the HISTTIMEFORMAT variable. When
present, history timestamps delimit history entries, making multi-line
entries possible.

When a shell with history enabled exits, bash copies the last $HISTSIZE
entries from the history list to $HISTFILE. If the histappend shell
option is enabled (see the description of shopt under SHELL BUILTIN
COMMANDS below), bash appends the entries to the history file, otherwise
it overwrites the history file. If HISTFILE is unset or null, or if the
history file is unwritable, the history is not saved. After saving the
history, bash truncates the history file to contain no more than
HISTFILESIZE lines as described above.

If the HISTTIMEFORMAT variable is set, the shell writes the timestamp
information associated with each history entry to the history file,
marked with the history comment character, so timestamps are preserved
across shell sessions. This uses the history comment character to
distinguish timestamps from other history lines. As above, when using
HISTTIMEFORMAT, the timestamps delimit multi-line history entries.

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

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

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.

History expansion is enabled by default for interactive shells, and can
be disabled using the +H option to the set builtin command (see SHELL
BUILTIN COMMANDS below). Non-interactive shells do not perform history
expansion by default, but it can be enabled with “set -H”.

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

History expansion is performed immediately after a complete line is read,
before the shell breaks it into words, and is performed on each line
individually. The shell attempts to inform the history expansion
functions about quoting still in effect from previous lines.

It takes place in two parts. The first is to determine which history
list entry to use during substitution. The second is to select portions
of that entry to include into the current one.

The entry selected from the history is the event, and the portions of
that entry that are acted upon are words. Various modifiers are
available to manipulate the selected words. The entry is split into
words in the same fashion as when reading input, so that several
metacharacter-separated words surrounded by quotes are considered one
word. The event designator selects the event, the optional word
designator selects words from the event, and various optional modifiers
are available to manipulate the selected words.

History expansions are introduced by the appearance of the history
expansion character, which is ! by default. History expansions may
appear anywhere in the input, but do not nest.

Only backslash (\) and single quotes can quote the history expansion
character, but the history expansion character is also treated as quoted
if it immediately precedes the closing double quote in a double-quoted
string.

Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted: space,
tab, newline, carriage return, =, and the other shell metacharacters
defined above.

There is a special abbreviation for substitution, active when the quick
substitution character (described above under histchars) is the first
character on the line. It selects the previous history list entry, using
an event designator equivalent to !!, and substitutes one string for
another in that entry. It is described below under Event Designators.
This is the only history expansion that does not begin with the history
expansion character.

Several shell options settable with the shopt builtin will modify history
expansion behavior (see the description of the shopt builtin below).and
If the histverify shell option is enabled, and readline is being used,
history substitutions are not immediately passed to the shell parser.
Instead, the expanded line is reloaded into the readline editing buffer
for further modification. If readline is being used, and the histreedit
shell option is enabled, a failed history substitution is reloaded into
the readline editing buffer for correction.

The -p option to the history builtin command shows what a history
expansion will do before using it. The -s option to the history builtin
will add commands to the end of the history list without actually
executing them, so that they are available for subsequent recall.

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

Event Designators
An event designator is a reference to an entry in the history list. The
event designator consists of the portion of the word beginning with the
history expansion character and ending with the word designator if
present, or the end of the word. Unless the reference is absolute,
events are relative to the current position in the history list.

! Start a history substitution, except when followed by a blank,
newline, carriage return, =, or, when the extglob shell option is
enabled using the shopt builtin, (.
!n Refer to history list entry n.
!-n Refer to the current entry minus n.
!! Refer to the previous entry. This is a synonym for “!-1”.
!string
Refer to the most recent command preceding the current position in
the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current position in
the history list containing string. The trailing ? may be omitted
if string is followed immediately by a newline. If string is
missing, this uses the string from the most recent search; it is
an error if there is no previous search string.
^string1^string2^
Quick substitution. Repeat the previous command, replacing
string1 with string2. Equivalent to “!!:s^string1^string2^” (see
Modifiers below).
!# The entire command line typed so far.

Word Designators
Word designators are used to select desired words from the event. They
are optional; if the word designator isn't supplied, the history
expansion uses the entire event. A : separates the event specification
from the word designator. It may be omitted if the word designator
begins with a ^, $, *, -, or %. Words are numbered from the beginning of
the line, with the first word being denoted by 0 (zero). Words are
inserted into the current line separated by single spaces.

0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument: word 1.
$ The last word. This is usually the last argument, but will expand
to the zeroth word if there is only one word in the line.
% The first word matched by the most recent “?string?” search, if
the search string begins with a character that is part of a word.
By default, searches begin at the end of each line and proceed to
the beginning, so the first word matched is the one closest to the
end of the line.
x-y A range of words; “-y” abbreviates “0-y”.
* All of the words but the zeroth. This is a synonym for “1-$”. It
is not an error to use * if there is just one word in the event;
it expands to the empty string in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word. If x is
missing, it defaults to 0.

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

Modifiers
After the optional word designator, the expansion may include a sequence
of one or more of the following modifiers, each preceded by a “:”. These
modify, or edit, the word or words selected from the history event.

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

SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section
as accepting options preceded by - accepts -- to signify the end of the
options. The :, true, false, and test/[ builtins do not accept options
and do not treat -- specially. The exit, logout, return, break,
continue, let, and shift builtins accept and process arguments beginning
with - without requiring --. Other builtins that accept arguments but
are not specified as accepting options interpret arguments beginning with
- as invalid options and require -- to prevent this interpretation.

: [arguments]
No effect; the command does nothing beyond expanding arguments and
performing any specified redirections. The return status is zero.

. [-p path] filename [arguments]
source [-p path] filename [arguments]
The . command (source) reads and execute commands from filename in
the current shell environment and returns the exit status of the
last command executed from filename.

If filename does not contain a slash, . searches for it. If the
-p option is supplied, . treats path as a colon-separated list of
directories in which to find filename; otherwise, . uses the
entries in PATH to find the directory containing filename.
filename does not need to be executable. When bash is not in
posix mode, it searches the current directory if filename is not
found in PATH, but does not search the current directory if -p is
supplied. If the sourcepath option to the shopt builtin command
is turned off, . does not search PATH.

If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the positional
parameters are unchanged.

If the -T option is enabled, . inherits any trap on DEBUG; if it
is not, any DEBUG trap string is saved and restored around the
call to ., and . unsets the DEBUG trap while it executes. If -T
is not set, and the sourced file changes the DEBUG trap, the new
value persists after . completes. The return status is the status
of the last command executed from filename (0 if no commands are
executed), and non-zero if filename is not found or cannot be
read.

alias [-p] [name[=value] ...]
With no arguments or with the -p option, alias prints the list of
aliases in the form alias name=value on standard output. When
arguments are supplied, define an alias for each name whose value
is given. A trailing space in value causes the next word to be
checked for alias substitution when the alias is expanded during
command parsing. For each name in the argument list for which no
value is supplied, print the name and value of the alias name.
alias returns true unless a name is given (without a corresponding
=value) for which no alias has been defined.

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

bind [-m keymap] [-lsvSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq[:] shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] -p|-P [readline-command]
bind [-m keymap] keyseq:readline-command
bind readline-command-line
Display current readline key and function bindings, bind a key
sequence to a readline function or macro or to a shell command, or
set a readline variable. Each non-option argument is a key
binding or command as it would appear in a readline initialization
file such as .inputrc, but each binding or command must be passed
as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'. In
the following descriptions, output available to be re-read is
formatted as commands that would appear in a readline
initialization file or that would be supplied as individual
arguments to a bind command. Options, if supplied, have the
following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent
bindings. Acceptable keymap names are emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move,
vi-command, and vi-insert. vi is equivalent to vi-command
(vi-move is also a synonym); emacs is equivalent to
emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way
that they can be used as an argument to a subsequent bind
command or in a readline initialization file. If arguments
remain after option processing, bind treats them as
readline command names and restricts output to those names.
-P List current readline function names and bindings. If
arguments remain after option processing, bind treats them
as readline command names and restricts output to those
names.
-s Display readline key sequences bound to macros and the
strings they output in such a way that they can be used as
an argument to a subsequent bind command or in a readline
initialization file.
-S Display readline key sequences bound to macros and the
strings they output.
-v Display readline variable names and values in such a way
that they can be used as an argument to a subsequent bind
command or in a readline initialization file.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Display key sequences that invoke the named readline
function.
-u function
Unbind all key sequences bound to the named readline
function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq[: ]shell-command
Cause shell-command to be executed whenever keyseq is
entered. The separator between keyseq and shell-command is
either whitespace or a colon optionally followed by
whitespace. If the separator is whitespace, shell-command
must be enclosed in double quotes and readline expands any
of its special backslash-escapes in shell-command before
saving it. If the separator is a colon, any enclosing
double quotes are optional, and readline does not expand
the command string before saving it. Since the entire key
binding expression must be a single argument, it should be
enclosed in single quotes. When shell-command is executed,
the shell sets the READLINE_LINE variable to the contents
of the readline line buffer and the READLINE_POINT and
READLINE_MARK variables to the current location of the
insertion point and the saved insertion point (the mark),
respectively. The shell assigns any numeric argument the
user supplied to the READLINE_ARGUMENT variable. If there
was no argument, that variable is not set. If the executed
command changes the value of any of READLINE_LINE,
READLINE_POINT, or READLINE_MARK, those new values will be
reflected in the editing state.
-X List all key sequences bound to shell commands and the
associated commands in a format that can be reused as an
argument to a subsequent bind command.

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

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

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

caller [expr]
Returns the context of any active subroutine call (a shell
function or a script executed with the . or source builtins).

Without expr, caller displays the line number and source filename
of the current subroutine call. If a non-negative integer is
supplied as expr, caller displays the line number, subroutine
name, and source file corresponding to that position in the
current execution call stack. This extra information may be used,
for example, to print a stack trace. The current frame is frame
0.

The return value is 0 unless the shell is not executing a
subroutine call or expr does not correspond to a valid position in
the call stack.

cd [-L] [-@] [dir]
cd -P [-e] [-@] [dir]
Change the current directory to dir. if dir is not supplied, the
value of the HOME shell variable is used as dir. The variable
CDPATH exists, and dir does not begin with a slash (/), cd uses it
as a search path: the shell searches each directory name in CDPATH
for dir. Alternative directory names in CDPATH are separated by a
colon (:). A null directory name in CDPATH is the same as the
current directory, i.e., “.”.

The -P option causes cd to use the physical directory structure by
resolving symbolic links while traversing dir and before
processing instances of .. in dir (see also the -P option to the
set builtin command).

The -L option forces cd to follow symbolic links by resolving the
link after processing instances of .. in dir. If .. appears in
dir, cd processes it by removing the immediately previous pathname
component from dir, back to a slash or the beginning of dir, and
verifying that the portion of dir it has processed to that point
is still a valid directory name after removing the pathname
component. If it is not a valid directory name, cd returns a non-
zero status. If neither -L nor -P is supplied, cd behaves as if
-L had been supplied.

If the -e option is supplied with -P, and cd cannot successfully
determine the current working directory after a successful
directory change, it returns a non-zero status.

On systems that support it, the -@ option presents the extended
attributes associated with a file as a directory.

An argument of - is converted to $OLDPWD before attempting the
directory change.

If cd uses a non-empty directory name from CDPATH, or if - is the
first argument, and the directory change is successful, cd writes
the absolute pathname of the new working directory to the standard
output.

If the directory change is successful, cd sets the value of the
PWD environment variable to the new directory name, and sets the
OLDPWD environment variable to the value of the current working
directory before the change.

The return value is true if the directory was successfully
changed; false otherwise.

command [-pVv] command [arg ...]
The command builtin runs command with args suppressing the normal
shell function lookup for command. Only builtin commands or
commands found in the PATH named command are executed. If the -p
option is supplied, the search for command is performed using a
default value for PATH that is guaranteed to find all of the
standard utilities.

If either the -V or -v option is supplied, command prints a
description of command. The -v option displays a single word
indicating the command or filename used to invoke command; the -V
option produces a more verbose description.

If the -V or -v option is supplied, the exit status is zero if
command was found, and non-zero if not. If neither option is
supplied and an error occurred or command cannot be found, the
exit status is 127. Otherwise, the exit status of the command
builtin is the exit status of command.

compgen [-V varname] [option] [word]
Generate possible completion matches for word according to the
options, which may be any option accepted by the complete builtin
with the exceptions of -p, -r, -D, -E, and -I, and write the
matches to the standard output.

If the -V option is supplied, compgen stores the generated
completions into the indexed array variable varname instead of
writing them to the standard output.

When using the -F or -C options, the various shell variables set
by the programmable completion facilities, while available, will
not have useful values.

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

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

complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action]
[-G globpat] [-W wordlist] [-F function] [-C command]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...]
Specify how arguments to each name should be completed.

If the -p option is supplied, or if no options or names are
supplied, print existing completion specifications in a way that
allows them to be reused as input. The -r option removes a
completion specification for each name, or, if no names are
supplied, all completion specifications.

The -D option indicates that other supplied options and actions
should apply to the “default” command completion; that is,
completion attempted on a command for which no completion has
previously been defined. The -E option indicates that other
supplied options and actions should apply to “empty” command
completion; that is, completion attempted on a blank line. The -I
option indicates that other supplied options and actions should
apply to completion on the initial non-assignment word on the
line, or after a command delimiter such as ; or |, which is
usually command name completion. If multiple options are
supplied, the -D option takes precedence over -E, and both take
precedence over -I. If any of -D, -E, or -I are supplied, any
other name arguments are ignored; these completions only apply to
the case specified by the option.

The process of applying these completion specifications when
attempting word completion is described above under Programmable
Completion.

Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options (and, if necessary, the -P
and -S options) should be quoted to protect them from expansion
before the complete builtin is invoked.

-o comp-option
The comp-option controls several aspects of the compspec's
behavior beyond the simple generation of completions.
comp-option may be one of:
bashdefault
Perform the rest of the default bash completions
if the compspec generates no matches.
default Use readline's default filename completion if the
compspec generates no matches.
dirnames
Perform directory name completion if the compspec
generates no matches.
filenames
Tell readline that the compspec generates
filenames, so it can perform any filename-specific
processing (such as adding a slash to directory
names, quoting special characters, or suppressing
trailing spaces). This is intended to be used
with shell functions.
fullquote
Tell readline to quote all the completed words
even if they are not filenames.
noquote Tell readline not to quote the completed words if
they are filenames (quoting filenames is the
default).
nosort Tell readline not to sort the list of possible
completions alphabetically.
nospace Tell readline not to append a space (the default)
to words completed at the end of the line.
plusdirs
After generating any matches defined by the
compspec, attempt directory name completion and
add any matches to the results of the other
actions.
-A action
The action may be one of the following to generate a list
of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be
specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be
specified as -e.
file File and directory names, similar to readline's
filename completion. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by the
HOSTFILE shell variable.
job Job names, if job control is active. May also be
specified as -j.
keyword Shell reserved words. May also be specified as
-k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be
specified as -v.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions. Arguments are
passed as with the -F option.
-F function
The shell function function is executed in the current
shell environment. When the function is executed, the
first argument ($1) is the name of the command whose
arguments are being completed, the second argument ($2) is
the word being completed, and the third argument ($3) is
the word preceding the word being completed on the current
command line. When function finishes, programmable
completion retrieves the possible completions from the
value of the COMPREPLY array variable.
-G globpat
Expand the pathname expansion pattern globpat to generate
the possible completions.
-P prefix
Add prefix to the beginning of each possible completion
after all other options have been applied.
-S suffix
Append suffix to each possible completion after all other
options have been applied.
-W wordlist
Split the wordlist using the characters in the IFS special
variable as delimiters, and expand each resulting word.
Shell quoting is honored within wordlist, in order to
provide a mechanism for the words to contain shell
metacharacters or characters in the value of IFS. The
possible completions are the members of the resultant list
which match a prefix of the word being completed.
-X filterpat
filterpat is a pattern as used for pathname expansion. It
is applied to the list of possible completions generated
by the preceding options and arguments, and each
completion matching filterpat is removed from the list. A
leading ! in filterpat negates the pattern; in this case,
any completion not matching filterpat is removed.

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

compopt [-o option] [-DEI] [+o option] [name]
Modify completion options for each name according to the options,
or for the currently-executing completion if no names are
supplied. If no options are supplied, display the completion
options for each name or the current completion. The possible
values of option are those valid for the complete builtin
described above.

The -D option indicates that other supplied options should apply
to the “default” command completion; the -E option indicates that
other supplied options should apply to “empty” command completion;
and the -I option indicates that other supplied options should
apply to completion on the initial word on the line. These are
determined in the same way as the complete builtin.

If multiple options are supplied, the -D option takes precedence
over -E, and both take precedence over -I.

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

continue [n]
continue resumes the next iteration of the enclosing for, while,
until, or select loop. If n is specified, bash resumes the nth
enclosing loop. n must be ≥ 1. If n is greater than the number
of enclosing loops, the shell resumes the last enclosing loop (the
“top-level” loop). The return value is 0 unless n is not greater
than or equal to 1.

declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]
typeset [-aAfFgiIlnrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are
given then display the values of variables or functions. The -p
option will display the attributes and values of each name. When
-p is used with name arguments, additional options, other than -f
and -F, are ignored.

When -p is supplied without name arguments, declare will display
the attributes and values of all variables having the attributes
specified by the additional options. If no other options are
supplied with -p, declare will display the attributes and values
of all shell variables. The -f option restricts the display to
shell functions.

The -F option inhibits the display of function definitions; only
the function name and attributes are printed. If the extdebug
shell option is enabled using shopt, the source file name and line
number where each name is defined are displayed as well. The -F
option implies -f.

The -g option forces variables to be created or modified at the
global scope, even when declare is executed in a shell function.
It is ignored when declare is not executed in a shell function.

The -I option causes local variables to inherit the attributes
(except the nameref attribute) and value of any existing variable
with the same name at a surrounding scope. If there is no
existing variable, the local variable is initially unset.

The following options can be used to restrict output to variables
with the specified attribute or to give variables attributes:
-a Each name is an indexed array variable (see Arrays above).
-A Each name is an associative array variable (see Arrays
above).
-f Each name refers to a shell function.
-i The variable is treated as an integer; arithmetic
evaluation (see ARITHMETIC EVALUATION above) is performed
when the variable is assigned a value.
-l When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
-n Give each name the nameref attribute, making it a name
reference to another variable. That other variable is
defined by the value of name. All references, assignments,
and attribute modifications to name, except those using or
changing the -n attribute itself, are performed on the
variable referenced by name's value. The nameref attribute
cannot be applied to array variables.
-r Make names readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
-t Give each name the trace attribute. Traced functions
inherit the DEBUG and RETURN traps from the calling shell.
The trace attribute has no special meaning for variables.
-u When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.
-x Mark each name for export to subsequent commands via the
environment.

Using “+” instead of “-” turns off the specified attribute
instead, with the exceptions that +a and +A may not be used to
destroy array variables and +r will not remove the readonly
attribute.

When used in a function, declare and typeset make each name local,
as with the local command, unless the -g option is supplied. If a
variable name is followed by =value, the value of the variable is
set to value. When using -a or -A and the compound assignment
syntax to create array variables, additional attributes do not
take effect until subsequent assignments.

The return value is 0 unless an invalid option is encountered, an
attempt is made to define a function using “-f foo=bar”, an
attempt is made to assign a value to a readonly variable, an
attempt is made to assign a value to an array variable without
using the compound assignment syntax (see Arrays above), one of
the names is not a valid shell variable name, an attempt is made
to turn off readonly status for a readonly variable, an attempt is
made to turn off array status for an array variable, or an attempt
is made to display a non-existent function with -f.

dirs [-clpv] [+n] [-n]
Without options, display the list of currently remembered
directories. The default display is on a single line with
directory names separated by spaces. Directories are added to the
list with the pushd command; the popd command removes entries from
the list. The current directory is always the first directory in
the stack.

Options, if supplied, have the following meanings:
-c Clears the directory stack by deleting all of the entries.
-l Produces a listing using full pathnames; the default
listing format uses a tilde to denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line,
prefixing each entry with its index in the stack.
+n Displays the nth entry counting from the left of the list
shown by dirs when invoked without options, starting with
zero.
-n Displays the nth entry counting from the right of the list
shown by dirs when invoked without options, starting with
zero.

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

disown [-ar] [-h] [id ...]
Without options, remove each id from the table of active jobs.
Each id may be a job specification jobspec or a process ID pid; if
id is a pid, disown uses the job containing pid as jobspec.

If the -h option is supplied, disown does not remove the jobs
corresponding to each id from the jobs table, but rather marks
them so the shell does not send SIGHUP to the job if the shell
receives a SIGHUP.

If no id is supplied, the -a option means to remove or mark all
jobs; the -r option without an id argument removes or marks
running jobs. If no id is supplied, and neither the -a nor the -r
option is supplied, disown removes or marks the current job.

The return value is 0 unless an id does not specify a valid job.

echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The
return status is 0 unless a write error occurs. If -n is
specified, the trailing newline is not printed.

If the -e option is given, echo interprets the following
backslash-escaped characters. The -E option disables
interpretation of these escape characters, even on systems where
they are interpreted by default. The xpg_echo shell option
determines whether or not echo interprets any options and expands
these escape characters. echo does not interpret -- to mean the
end of options.

echo interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn The eight-bit character whose value is the octal value nnn
(zero to three octal digits).
\xHH The eight-bit character whose value is the hexadecimal
value HH (one or two hex digits).
\uHHHH The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits).
\UHHHHHHHH
The Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits).

echo writes any unrecognized backslash-escaped characters
unchanged.

enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows an executable file which has the same name as a shell
builtin to be executed without specifying a full pathname, even
though the shell normally searches for builtins before files.

If -n is supplied, each name is disabled; otherwise, names are
enabled. For example, to use the test binary found using PATH
instead of the shell builtin version, run “enable -n test”.

If no name arguments are supplied, or if the -p option is
supplied, print a list of shell builtins. With no other option
arguments, the list consists of all enabled shell builtins. If -n
is supplied, print only disabled builtins. If -a is supplied, the
list printed includes all builtins, with an indication of whether
or not each is enabled. The -s option means to restrict the
output to the POSIX special builtins.

The -f option means to load the new builtin command name from
shared object filename, on systems that support dynamic loading.
If filename does not contain a slash, Bash will use the value of
the BASH_LOADABLES_PATH variable as a colon-separated list of
directories in which to search for filename. The default for
BASH_LOADABLES_PATH is system-dependent, and may include “.” to
force a search of the current directory. The -d option will
delete a builtin previously loaded with -f. If -s is used with
-f, the new builtin becomes a POSIX special builtin.

If no options are supplied and a name is not a shell builtin,
enable will attempt to load name from a shared object named name,
as if the command were “enable -f name name”.

The return value is 0 unless a name is not a shell builtin or
there is an error loading a new builtin from a shared object.

eval [arg ...]
Concatenate the args together into a single command, separating
them with spaces. Bash then reads and execute this command, and
returns its exit status as the return status of eval. If there
are no args, or only null arguments, eval returns 0.

exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell without creating a
new process. command cannot be a shell builtin or function. The
arguments become the arguments to command. If the -l option is
supplied, the shell places a dash at the beginning of the zeroth
argument passed to command. This is what login(1) does. The -c
option causes command to be executed with an empty environment.
If -a is supplied, the shell passes name as the zeroth argument to
the executed command.

If command cannot be executed for some reason, a non-interactive
shell exits, unless the execfail shell option is enabled. In that
case, it returns a non-zero status. An interactive shell returns
a non-zero status if the file cannot be executed. A subshell
exits unconditionally if exec fails.

If command is not specified, any redirections take effect in the
current shell, and the return status is 0. If there is a
redirection error, the return status is 1.

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

export [-fn] [name[=value]] ...
export -p [-f]
The supplied names are marked for automatic export to the
environment of subsequently executed commands. If the -f option
is given, the names refer to functions.

The -n option unexports, or removes the export attribute, from
each name. If no names are given, or if only the -p option is
supplied, export displays a list of names of all exported
variables on the standard output. Using -p and -f together
displays exported functions. The -p option displays output in a
form that may be reused as input.

export allows the value of a variable to be set when it is
exported or unexported by following the variable name with =value.
This sets the value of the variable to value while modifying the
export attribute. export returns an exit status of 0 unless an
invalid option is encountered, one of the names is not a valid
shell variable name, or -f is supplied with a name that is not a
function.

false Does nothing; returns a non-zero status.

fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
The first form selects a range of commands from first to last from
the history list and displays or edits and re-executes them.
First and last may be specified as a string (to locate the last
command beginning with that string) or as a number (an index into
the history list, where a negative number is used as an offset
from the current command number).

When listing, a first or last of 0 is equivalent to -1 and -0 is
equivalent to the current command (usually the fc command);
otherwise 0 is equivalent to -1 and -0 is invalid. If last is not
specified, it is set to the current command for listing (so that
“fc -l -10” prints the last 10 commands) and to first otherwise.
If first is not specified, it is set to the previous command for
editing and -16 for listing.

If the -l option is supplied, the commands are listed on the
standard output. The -n option suppresses the command numbers
when listing. The -r option reverses the order of the commands.

Otherwise, fc invokes the editor named by ename on a file
containing those commands. If ename is not supplied, fc uses the
value of the FCEDIT variable, and the value of EDITOR if FCEDIT is
not set. If neither variable is set, fc uses vi. When editing is
complete, fc reads the file containing the edited commands and
echoes and executes them.

In the second form, fc re-executes command after replacing each
instance of pat with rep. Command is interpreted the same as
first above.

A useful alias to use with fc is “r="fc -s"”, so that typing “r
cc” runs the last command beginning with “cc” and typing “r” re-
executes the last command.

If the first form is used, the return value is zero unless an
invalid option is encountered or first or last specify history
lines out of range. When editing and re-executing a file of
commands, the return value is the value of the last command
executed or failure if an error occurs with the temporary file.
If the second form is used, the return status is that of the re-
executed command, unless cmd does not specify a valid history
entry, in which case fc returns a non-zero status.

fg [jobspec]
Resume jobspec in the foreground, and make it the current job. If
jobspec is not present, fg uses the shell's notion of the current
job. The return value is that of the command placed into the
foreground, or failure if run when job control is disabled or,
when run with job control enabled, if jobspec does not specify a
valid job or jobspec specifies a job that was started without job
control.

getopts optstring name [arg ...]
getopts is used by shell scripts and functions to parse positional
parameters and obtain options and their arguments. optstring
contains the option characters to be recognized; if a character is
followed by a colon, the option is expected to have an argument,
which should be separated from it by white space. The colon and
question mark characters may not be used as option characters.

Each time it is invoked, getopts places the next option in the
shell variable name, initializing name if it does not exist, and
the index of the next argument to be processed into the variable
OPTIND. OPTIND is initialized to 1 each time the shell or a shell
script is invoked. When an option requires an argument, getopts
places that argument into the variable OPTARG.

The shell does not reset OPTIND automatically; it must be manually
reset between multiple calls to getopts within the same shell
invocation to use a new set of parameters.

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

getopts normally parses the positional parameters, but if more
arguments are supplied as arg values, getopts parses those
instead.

getopts can report errors in two ways. If the first character of
optstring is a colon, getopts uses silent error reporting. In
normal operation, getopts prints diagnostic messages when it
encounters invalid options or missing option arguments. If the
variable OPTERR is set to 0, getopts does not display any error
messages, even if the first character of optstring is not a colon.

If getopts detects an invalid option, it places ? into name and,
if not silent, prints an error message and unsets OPTARG. If
getopts is silent, it assigns the option character found to OPTARG
and does not print a diagnostic message.

If a required argument is not found, and getopts is not silent, it
sets the value of name to a question mark (?), unsets OPTARG, and
prints a diagnostic message. If getopts is silent, it sets the
value of name to a colon (:) and sets OPTARG to the option
character found.

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

hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, it remembers the full pathname of the
command name as determined by searching the directories in $PATH.
Any previously-remembered pathname associated with name is
discarded. If the -p option is supplied, hash uses filename as
the full pathname of the command.

The -r option causes the shell to forget all remembered locations.
Assigning to the PATH variable also clears all hashed filenames.
The -d option causes the shell to forget the remembered location
of each name.

If the -t option is supplied, hash prints the full pathname
corresponding to each name. If multiple name arguments are
supplied with -t, hash prints the name before the corresponding
hashed full pathname. The -l option displays output in a format
that may be reused as input.

If no arguments are given, or if only -l is supplied, hash prints
information about remembered commands. The -t, -d, and -p options
(the options that act on the name arguments) are mutually
exclusive. Only one will be active. If more than one is
supplied, -t has higher priority than -p, and both have higher
priority than -d.

The return status is zero unless a name is not found or an invalid
option is supplied.

help [-dms] [pattern]
Display helpful information about builtin commands. If pattern is
specified, help gives detailed help on all commands matching
pattern as described below; otherwise it displays a list of all
the builtins and shell compound commands.

Options, if supplied, have the follow meanings:

-d Display a short description of each pattern
-m Display the description of each pattern in a manpage-like
format
-s Display only a short usage synopsis for each pattern

If pattern contains pattern matching characters (see Pattern
Matching above) it's treated as a shell pattern and help prints
the description of each help topic matching pattern.

If not, and pattern exactly matches the name of a help topic, help
prints the description associated with that topic. Otherwise,
help performs prefix matching and prints the descriptions of all
matching help topics.

The return status is 0 unless no command matches pattern.

history [n]
history -c
history -d offset
history -d start-end
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with numbers.
Entries prefixed with a * have been modified. An argument of n
lists only the last n entries. If the shell variable
HISTTIMEFORMAT is set and not null, it is used as a format string
for strftime(3) to display the time stamp associated with each
displayed history entry. If history uses HISTTIMEFORMAT, it does
not print an intervening space between the formatted time stamp
and the history entry.

If filename is supplied, history uses it as the name of the
history file; if not, it uses the value of HISTFILE. If filename
is not supplied and HISTFILE is unset or null, the -a, -n, -r, and
-w options have no effect.

Options, if supplied, have the following meanings:
-c Clear the history list by deleting all the entries. This
can be used with the other options to replace the history
list.
-d offset
Delete the history entry at position offset. If offset is
negative, it is interpreted as relative to one greater than
the last history position, so negative indices count back
from the end of the history, and an index of -1 refers to
the current history -d command.
-d start-end
Delete the range of history entries between positions start
and end, inclusive. Positive and negative values for start
and end are interpreted as described above.
-a Append the “new” history lines to the history file. These
are history lines entered since the beginning of the
current bash session, but not already appended to the
history file.
-n Read the history lines not already read from the history
file and add them to the current history list. These are
lines appended to the history file since the beginning of
the current bash session.
-r Read the history file and append its contents to the
current history list.
-w Write the current history list to the history file,
overwriting the history file.
-p Perform history substitution on the following args and
display the result on the standard output, without storing
the results in the history list. Each arg must be quoted
to disable normal history expansion.
-s Store the args in the history list as a single entry. The
last command in the history list is removed before adding
the args.

If the HISTTIMEFORMAT variable is set, history writes the time
stamp information associated with each history entry to the
history file, marked with the history comment character as
described above. When the history file is read, lines beginning
with the history comment character followed immediately by a digit
are interpreted as timestamps for the following history entry.

The return value is 0 unless an invalid option is encountered, an
error occurs while reading or writing the history file, an invalid
offset or range is supplied as an argument to -d, or the history
expansion supplied as an argument to -p fails.

jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the
following meanings:
-l List process IDs in addition to the normal information.
-n Display information only about jobs that have changed
status since the user was last notified of their status.
-p List only the process ID of the job's process group leader.
-r Display only running jobs.
-s Display only stopped jobs.

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

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

kill [-s sigspec | -n signum | -sigspec] id [ ... ]
kill -l|-L [sigspec | exit_status]
Send the signal specified by sigspec or signum to the processes
named by each id. Each id may be a job specification jobspec or a
process ID pid. sigspec is either a case-insensitive signal name
such as SIGKILL (with or without the SIG prefix) or a signal
number; signum is a signal number. If sigspec is not supplied,
then kill sends SIGTERM.

The -l option lists the signal names. If any arguments are
supplied when -l is given, kill lists the names of the signals
corresponding to the arguments, and the return status is 0. The
exit_status argument to -l is a number specifying either a signal
number or the exit status of a process terminated by a signal; if
it is supplied, kill prints the name of the signal that caused the
process to terminate. kill assumes that process exit statuses are
greater than 128; anything less than that is a signal number. The
-L option is equivalent to -l.

kill returns true if at least one signal was successfully sent, or
false if an error occurs or an invalid option is encountered.

let arg [arg ...]
Each arg is evaluated as an arithmetic expression (see ARITHMETIC
EVALUATION above). If the last arg evaluates to 0, let returns 1;
otherwise let returns 0.

local [option] [name[=value] ... | - ]
For each argument, create a local variable named name and assign
it value. The option can be any of the options accepted by
declare. When local is used within a function, it causes the
variable name to have a visible scope restricted to that function
and its children. It is an error to use local when not within a
function.

If name is -, it makes the set of shell options local to the
function in which local is invoked: any shell options changed
using the set builtin inside the function after the call to local
are restored to their original values when the function returns.
The restore is performed as if a series of set commands were
executed to restore the values that were in place before the
function.

With no operands, local writes a list of local variables to the
standard output.

The return status is 0 unless local is used outside a function, an
invalid name is supplied, or name is a readonly variable.

logout [n]
Exit a login shell, returning a status of n to the shell's parent.

mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]
readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]
Read lines from the standard input, or from file descriptor fd if
the -u option is supplied, into the indexed array variable array.
The variable MAPFILE is the default array. Options, if supplied,
have the following meanings:
-d Use the first character of delim to terminate each input
line, rather than newline. If delim is the empty string,
mapfile will terminate a line when it reads a NUL
character.
-n Copy at most count lines. If count is 0, copy all lines.
-O Begin assigning to array at index origin. The default
index is 0.
-s Discard the first count lines read.
-t Remove a trailing delim (default newline) from each line
read.
-u Read lines from file descriptor fd instead of the standard
input.
-C Evaluate callback each time quantum lines are read. The -c
option specifies quantum.
-c Specify the number of lines read between each call to
callback.

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

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

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

popd [-n] [+n] [-n]
Remove entries from the directory stack. The elements are
numbered from 0 starting at the first directory listed by dirs, so
popd is equivalent to “popd +0.” With no arguments, popd removes
the top directory from the stack, and changes to the new top
directory. Arguments, if supplied, have the following meanings:
-n Suppress the normal change of directory when removing
directories from the stack, only manipulate the stack.
+n Remove the nth entry counting from the left of the list
shown by dirs, starting with zero, from the stack. For
example: “popd +0” removes the first directory, “popd +1”
the second.
-n Remove the nth entry counting from the right of the list
shown by dirs, starting with zero. For example: “popd -0”
removes the last directory, “popd -1” the next to last.

If the top element of the directory stack is modified, and the -n
option was not supplied, popd uses the cd builtin to change to the
directory at the top of the stack. If the cd fails, popd returns
a non-zero value.

Otherwise, popd returns false if an invalid option is supplied,
the directory stack is empty, or n specifies a non-existent
directory stack entry.

If the popd command is successful, bash runs dirs to show the
final contents of the directory stack, and the return status is 0.

printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format. The -v option assigns the output to the
variable var rather than printing it to the standard output.

The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and copied
to the standard output, and format specifications, each of which
causes printing of the next successive argument. In addition to
the standard printf(3) format characters cCsSndiouxXeEfFgGaA,
printf interprets the following additional format specifiers:
%b causes printf to expand backslash escape sequences in the
corresponding argument in the same way as echo -e.
%q causes printf to output the corresponding argument in a
format that can be reused as shell input. %q and %Q use
the $'' quoting style if any characters in the argument
string require it, and backslash quoting otherwise. If the
format string uses the printf alternate form, these two
formats quote the argument string using single quotes.
%Q like %q, but applies any supplied precision to the argument
before quoting it.
%(datefmt)T
causes printf to output the date-time string resulting from
using datefmt as a format string for strftime(3). The
corresponding argument is an integer representing the
number of seconds since the epoch. This format specifier
recognizes two special argument values: -1 represents the
current time, and -2 represents the time the shell was
invoked. If no argument is specified, conversion behaves
as if -1 had been supplied. This is an exception to the
usual printf behavior.

The %b, %q, and %T format specifiers all use the field width and
precision arguments from the format specification and write that
many bytes from (or use that wide a field for) the expanded
argument, which usually contains more characters than the
original.

The %n format specifier accepts a corresponding argument that is
treated as a shell variable name.

The %s and %c format specifiers accept an l (long) modifier, which
forces them to convert the argument string to a wide-character
string and apply any supplied field width and precision in terms
of characters, not bytes. The %S and %C format specifiers are
equivalent to %ls and %lc, respectively.

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

The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the extra
format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on
success, non-zero if an invalid option is supplied or a write or
assignment error occurs.

pushd [-n] [+n] [-n]
pushd [-n] [dir]
Add a directory to the top of the directory stack, or rotate the
stack, making the new top of the stack the current working
directory. With no arguments, pushd exchanges the top two
elements of the directory stack. Arguments, if supplied, have the
following meanings:
-n Suppress the normal change of directory when rotating or
adding directories to the stack, only manipulate the stack.
+n Rotate the stack so that the nth directory (counting from
the left of the list shown by dirs, starting with zero) is
at the top.
-n Rotates the stack so that the nth directory (counting from
the right of the list shown by dirs, starting with zero) is
at the top.
dir Adds dir to the directory stack at the top.

After the stack has been modified, if the -n option was not
supplied, pushd uses the cd builtin to change to the directory at
the top of the stack. If the cd fails, pushd returns a non-zero
value.

Otherwise, if no arguments are supplied, pushd returns zero unless
the directory stack is empty. When rotating the directory stack,
pushd returns zero unless the directory stack is empty or n
specifies a non-existent directory stack element.

If the pushd command is successful, bash runs dirs to show the
final contents of the directory stack.

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

read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p
prompt] [-t timeout] [-u fd] [name ...]
Read one line from the standard input, or from the file descriptor
fd supplied as an argument to the -u option, split it into words
as described above under Word Splitting, and assign the first word
to the first name, the second word to the second name, and so on.
If there are more words than names, the remaining words and their
intervening delimiters are assigned to the last name. If there
are fewer words read from the input stream than names, the
remaining names are assigned empty values. The characters in the
value of the IFS variable are used to split the line into words
using the same rules the shell uses for expansion (described above
under Word Splitting). The backslash character (\) removes any
special meaning for the next character read and is used for line
continuation.

Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of the array
variable aname, starting at 0. aname is unset before any
new values are assigned. Other name arguments are ignored.
-d delim
The first character of delim terminates the input line,
rather than newline. If delim is the empty string, read
will terminate a line when it reads a NUL character.
-e If the standard input is coming from a terminal, read uses
readline (see READLINE above) to obtain the line. Readline
uses the current (or default, if line editing was not
previously active) editing settings, but uses readline's
default filename completion.
-E If the standard input is coming from a terminal, read uses
readline (see READLINE above) to obtain the line. Readline
uses the current (or default, if line editing was not
previously active) editing settings, but uses bash's
default completion, including programmable completion.
-i text
If readline is being used to read the line, read places
text into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather than
waiting for a complete line of input, unless it encounters
EOF or read times out, but honors a delimiter if it reads
fewer than nchars characters before the delimiter.
-N nchars
read returns after reading exactly nchars characters rather
than waiting for a complete line of input, unless it
encounters EOF or read times out. Any delimiter characters
in the input are not treated specially and do not cause
read to return until it has read nchars characters. The
result is not split on the characters in IFS; the intent is
that the variable is assigned exactly the characters read
(with the exception of backslash; see the -r option below).
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input, but only if
input is coming from a terminal.
-r Backslash does not act as an escape character. The
backslash is considered to be part of the line. In
particular, a backslash-newline pair may not then be used
as a line continuation.
-s Silent mode. If input is coming from a terminal,
characters are not echoed.
-t timeout
Cause read to time out and return failure if it does not
read a complete line of input (or a specified number of
characters) within timeout seconds. timeout may be a
decimal number with a fractional portion following the
decimal point. This option is only effective if read is
reading input from a terminal, pipe, or other special file;
it has no effect when reading from regular files. If read
times out, it saves any partial input read into the
specified variable name, and the exit status is greater
than 128. If timeout is 0, read returns immediately,
without trying to read any data. In this case, the exit
status is 0 if input is available on the specified file
descriptor, or the read will return EOF, non-zero
otherwise.
-u fd Read input from file descriptor fd instead of the standard
input.

Other than the case where delim is the empty string, read ignores
any NUL characters in the input.

If no names are supplied, read assigns the line read, without the
ending delimiter but otherwise unmodified, to the variable REPLY.

The exit status is zero, unless end-of-file is encountered, read
times out (in which case the status is greater than 128), a
variable assignment error (such as assigning to a readonly
variable) occurs, or an invalid file descriptor is supplied as the
argument to -u.

readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these names may
not be changed by subsequent assignment or unset. If the -f
option is supplied, each name refers to a shell function. The -a
option restricts the variables to indexed arrays; the -A option
restricts the variables to associative arrays. If both options
are supplied, -A takes precedence. If no name arguments are
supplied, or if the -p option is supplied, print a list of all
readonly names. The other options may be used to restrict the
output to a subset of the set of readonly names. The -p option
displays output in a format that may be reused as input.

readonly allows the value of a variable to be set at the same time
the readonly attribute is changed by following the variable name
with =value. This sets the value of the variable is to value
while modifying the readonly attribute.

The return status is 0 unless an invalid option is encountered,
one of the names is not a valid shell variable name, or -f is
supplied with a name that is not a function.

return [n]
Stop executing a shell function or sourced file and return the
value specified by n to its caller. If n is omitted, the return
status is that of the last command executed. If return is
executed by a trap handler, the last command used to determine the
status is the last command executed before the trap handler. If
return is executed during a DEBUG trap, the last command used to
determine the status is the last command executed by the trap
handler before return was invoked.

When return is used to terminate execution of a script being
executed by the . (source) command, it causes the shell to stop
executing that script and return either n or the exit status of
the last command executed within the script as the exit status of
the script. If n is supplied, the return value is its least
significant 8 bits.

Any command associated with the RETURN trap is executed before
execution resumes after the function or script.

The return status is non-zero if return is supplied a non-numeric
argument, or is used outside a function and not during execution
of a script by . or source.

set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]
set -o
set +o Without options, display the name and value of each shell variable
in a format that can be reused as input for setting or resetting
the currently-set variables. Read-only variables cannot be reset.
In posix mode, only shell variables are listed. The output is
sorted according to the current locale. When options are
specified, they set or unset shell attributes. Any arguments
remaining after option processing are treated as values for the
positional parameters and are assigned, in order, to $1, $2, ...,
$n. Options, if specified, have the following meanings:
-a Each variable or function that is created or modified is
given the export attribute and marked for export to the
environment of subsequent commands.
-b Report the status of terminated background jobs
immediately, rather than before the next primary prompt or
after a foreground command terminates. This is effective
only when job control is enabled.
-e Exit immediately if a pipeline (which may consist of a
single simple command), a list, or a compound command (see
SHELL GRAMMAR above), exits with a non-zero status. The
shell does not exit if the command that fails is part of
the command list immediately following a while or until
reserved word, part of the test following the if or elif
reserved words, part of any command executed in a && or ||
list except the command following the final && or ||, any
command in a pipeline but the last (subject to the state
of the pipefail shell option), or if the command's return
value is being inverted with !. If a compound command
other than a subshell returns a non-zero status because a
command failed while -e was being ignored, the shell does
not exit. A trap on ERR, if set, is executed before the
shell exits. This option applies to the shell environment
and each subshell environment separately (see COMMAND
EXECUTION ENVIRONMENT above), and may cause subshells to
exit before executing all the commands in the subshell.

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

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

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

shopt [-pqsu] [-o] [optname ...]
Toggle the values of settings controlling optional shell behavior.
The settings can be either those listed below, or, if the -o
option is used, those available with the -o option to the set
builtin command.

With no options, or with the -p option, display a list of all
settable options, with an indication of whether or not each is
set; if any optnames are supplied, the output is restricted to
those options. The -p option displays output in a form that may
be reused as input.

Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status
indicates whether the optname is set or unset. If multiple
optname arguments are supplied with -q, the return status
is zero if all optnames are enabled; non-zero otherwise.
-o Restricts the values of optname to be those defined for the
-o option to the set builtin.

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

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

The list of shopt options is:

array_expand_once
If set, the shell suppresses multiple evaluation of
associative and indexed array subscripts during arithmetic
expression evaluation, while executing builtins that can
perform variable assignments, and while executing builtins
that perform array dereferencing.
assoc_expand_once
Deprecated; a synonym for array_expand_once.
autocd If set, a command name that is the name of a directory is
executed as if it were the argument to the cd command.
This option is only used by interactive shells.
bash_source_fullpath
If set, filenames added to the BASH_SOURCE array variable
are converted to full pathnames (see Shell Variables
above).
cdable_vars
If set, an argument to the cd builtin command that is not
a directory is assumed to be the name of a variable whose
value is the directory to change to.
cdspell If set, the cd command attempts to correct minor errors in
the spelling of a directory component. Minor errors
include transposed characters, a missing character, and
one extra character. If cd corrects the directory name,
it prints the corrected filename, and the command
proceeds. This option is only used by interactive shells.
checkhash
If set, bash checks that a command found in the hash table
exists before trying to execute it. If a hashed command
no longer exists, bash performs a normal path search.
checkjobs
If set, bash lists the status of any stopped and running
jobs before exiting an interactive shell. If any jobs are
running, bash defers the exit until a second exit is
attempted without an intervening command (see JOB CONTROL
above). The shell always postpones exiting if any jobs
are stopped.
checkwinsize
If set, bash checks the window size after each external
(non-builtin) command and, if necessary, updates the
values of LINES and COLUMNS, using the file descriptor
associated with the standard error if it is a terminal.
This option is enabled by default.
cmdhist If set, bash attempts to save all lines of a multiple-line
command in the same history entry. This allows easy re-
editing of multi-line commands. This option is enabled by
default, but only has an effect if command history is
enabled, as described above under HISTORY.
compat31
compat32
compat40
compat41
compat42
compat43
compat44
These control aspects of the shell's compatibility mode
(see SHELL COMPATIBILITY MODE below).
complete_fullquote
If set, bash quotes all shell metacharacters in filenames
and directory names when performing completion. If not
set, bash removes metacharacters such as the dollar sign
from the set of characters that will be quoted in
completed filenames when these metacharacters appear in
shell variable references in words to be completed. This
means that dollar signs in variable names that expand to
directories will not be quoted; however, any dollar signs
appearing in filenames will not be quoted, either. This
is active only when bash is using backslashes to quote
completed filenames. This variable is set by default,
which is the default bash behavior in versions through
4.2.
direxpand
If set, bash replaces directory names with the results of
word expansion when performing filename completion. This
changes the contents of the readline editing buffer. If
not set, bash attempts to preserve what the user typed.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name
initially supplied does not exist.
dotglob If set, bash includes filenames beginning with a “.” in
the results of pathname expansion. The filenames . and ..
must always be matched explicitly, even if dotglob is set.
execfail
If set, a non-interactive shell will not exit if it cannot
execute the file specified as an argument to the exec
builtin. An interactive shell does not exit if exec
fails.
expand_aliases
If set, aliases are expanded as described above under
ALIASES. This option is enabled by default for
interactive shells.
extdebug
If set at shell invocation, or in a shell startup file,
arrange to execute the debugger profile before the shell
starts, identical to the --debugger option. If set after
invocation, behavior intended for use by debuggers is
enabled:
1. The -F option to the declare builtin displays the
source file name and line number corresponding to
each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a non-
zero value, the next command is skipped and not
executed.
3. If the command run by the DEBUG trap returns a
value of 2, and the shell is executing in a
subroutine (a shell function or a shell script
executed by the . or source builtins), the shell
simulates a call to return.
4. BASH_ARGC and BASH_ARGV are updated as described in
their descriptions above).
5. Function tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the ERR trap.
extglob If set, enable the extended pattern matching features
described above under Pathname Expansion.
extquote
If set, $'string' and $"string" quoting is performed
within ${parameter} expansions enclosed in double quotes.
This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell
variable cause words to be ignored when performing word
completion even if the ignored words are the only possible
completions. See Shell Variables above for a description
of FIGNORE. This option is enabled by default.
globasciiranges
If set, range expressions used in pattern matching bracket
expressions (see Pattern Matching above) behave as if in
the traditional C locale when performing comparisons.
That is, pattern matching does not take the current
locale's collating sequence into account, so b will not
collate between A and B, and upper-case and lower-case
ASCII characters will collate together.
globskipdots
If set, pathname expansion will never match the filenames
. and .., even if the pattern begins with a “.”. This
option is enabled by default.
globstar
If set, the pattern ** used in a pathname expansion
context will match all files and zero or more directories
and subdirectories. If the pattern is followed by a /,
only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU error message format.
histappend
If set, the history list is appended to the file named by
the value of the HISTFILE variable when the shell exits,
rather than overwriting the file.
histreedit
If set, and readline is being used, the user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of history
substitution are not immediately passed to the shell
parser. Instead, the resulting line is loaded into the
readline editing buffer, allowing further modification.
hostcomplete
If set, and readline is being used, bash will attempt to
perform hostname completion when a word containing a @ is
being completed (see Completing under READLINE above).
This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an
interactive login shell exits.
inherit_errexit
If set, command substitution inherits the value of the
errexit option, instead of unsetting it in the subshell
environment. This option is enabled when posix mode is
enabled.
interactive_comments
In an interactive shell, a word beginning with # causes
that word and all remaining characters on that line to be
ignored, as in a non-interactive shell (see COMMENTS
above). This option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the
last command of a pipeline not executed in the background
in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded newlines
rather than using semicolon separators where possible.
localvar_inherit
If set, local variables inherit the value and attributes
of a variable of the same name that exists at a previous
scope before any new value is assigned. The nameref
attribute is not inherited.
localvar_unset
If set, calling unset on local variables in previous
function scopes marks them so subsequent lookups find them
unset until that function returns. This is identical to
the behavior of unsetting local variables at the current
function scope.
login_shell
The shell sets this option if it is started as a login
shell (see INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has been
accessed since the last time it was checked, bash displays
the message “The mail in mailfile has been read”.
no_empty_cmd_completion
If set, and readline is being used, bash does not search
PATH for possible completions when completion is attempted
on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case or
[[ conditional commands, when performing pattern
substitution word expansions, or when filtering possible
completions as part of programmable completion.
noexpand_translation
If set, bash encloses the translated results of $"..."
quoting in single quotes instead of double quotes. If the
string is not translated, this has no effect.
nullglob
If set, pathname expansion patterns which match no files
(see Pathname Expansion above) expand to nothing and are
removed, rather than expanding to themselves.
patsub_replacement
If set, bash expands occurrences of & in the replacement
string of pattern substitution to the text matched by the
pattern, as described under Parameter Expansion above.
This option is enabled by default.
progcomp
If set, enable the programmable completion facilities (see
Programmable Completion above). This option is enabled by
default.
progcomp_alias
If set, and programmable completion is enabled, bash
treats a command name that doesn't have any completions as
a possible alias and attempts alias expansion. If it has
an alias, bash attempts programmable completion using the
command word resulting from the expanded alias.
promptvars
If set, prompt strings undergo parameter expansion,
command substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING
above. This option is enabled by default.
restricted_shell
The shell sets this option if it is started in restricted
mode (see RESTRICTED SHELL below). The value may not be
changed. This is not reset when the startup files are
executed, allowing the startup files to discover whether
or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when the
shift count exceeds the number of positional parameters.
sourcepath
If set, the . (source) builtin uses the value of PATH to
find the directory containing the file supplied as an
argument when the -p option is not supplied. This option
is enabled by default.
varredir_close
If set, the shell automatically closes file descriptors
assigned using the {varname} redirection syntax (see
REDIRECTION above) instead of leaving them open when the
command completes.
xpg_echo
If set, the echo builtin expands backslash-escape
sequences by default. If the posix shell option is also
enabled, echo does not interpret any options.

suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT
signal. A login shell, or a shell without job control enabled,
cannot be suspended; the -f option will override this and force
the suspension. The return status is 0 unless the shell is a
login shell or job control is not enabled and -f is not supplied.

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

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

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

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

When the shell is in posix mode, or if the expression is part of
the [[ command, the < and > operators sort using the current
locale. If the shell is not in posix mode, the test and [
commands sort lexicographically using ASCII ordering.

The historical operator-precedence parsing with 4 or more
arguments can lead to ambiguities when it encounters strings that
look like primaries. The POSIX standard has deprecated the -a and
-o primaries and enclosing expressions within parentheses.
Scripts should no longer use them. It's much more reliable to
restrict test invocations to a single primary, and to replace uses
of -a and -o with the shell's && and || list operators.

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

trap [-lpP] [[action] sigspec ...]
The action is a command that is read and executed when the shell
receives any of the signals sigspec. If action is absent (and
there is a single sigspec) or -, each specified sigspec is reset
to the value it had when the shell was started. If action is the
null string the signal specified by each sigspec is ignored by the
shell and by the commands it invokes.

If no arguments are supplied, trap displays the actions associated
with each trapped signal as a set of trap commands that can be
reused as shell input to restore the current signal dispositions.
If -p is given, and action is not present, then trap displays the
actions associated with each sigspec or, if none are supplied, for
all trapped signals, as a set of trap commands that can be reused
as shell input to restore the current signal dispositions. The -P
option behaves similarly, but displays only the actions associated
with each sigspec argument. -P requires at least one sigspec
argument. The -P or -p options may be used in a subshell
environment (e.g., command substitution) and, as long as they are
used before trap is used to change a signal's handling, will
display the state of its parent's traps.

The -l option prints a list of signal names and their
corresponding numbers. Each sigspec is either a signal name
defined in <signal.h>, or a signal number. Signal names are case
insensitive and the SIG prefix is optional. If -l is supplied
with no sigspec arguments, it prints a list of valid signal names.

If a sigspec is EXIT (0), action is executed on exit from the
shell. If a sigspec is DEBUG, action is executed before every
simple command, for command, case command, select command, ((
arithmetic command, [[ conditional command, arithmetic for
command, and before the first command executes in a shell function
(see SHELL GRAMMAR above). Refer to the description of the
extdebug shell option (see shopt above) for details of its effect
on the DEBUG trap. If a sigspec is RETURN, action is executed
each time a shell function or a script executed with the . or
source builtins finishes executing.

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

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

true Does nothing, returns a 0 status.

type [-aftpP] name [name ...]
Indicate how each name would be interpreted if used as a command
name.

If the -t option is used, type prints a string which is one of
alias, keyword, function, builtin, or file if name is an alias,
shell reserved word, function, builtin, or executable file,
respectively. If the name is not found, type prints nothing and
returns a non-zero exit status.

If the -p option is used, type either returns the pathname of the
executable file that would be found by searching $PATH for name or
nothing if “type -t name” would not return file. The -P option
forces a PATH search for each name, even if “type -t name” would
not return file. If name is present in the table of hashed
commands, -p and -P print the hashed value, which is not
necessarily the file that appears first in PATH.

If the -a option is used, type prints all of the places that
contain a command named name. This includes aliases, reserved
words, functions, and builtins, but the path search options (-p
and -P) can be supplied to restrict the output to executable
files. type does not consult the table of hashed commands when
using -a with -p, and only performs a PATH search for name.

The -f option suppresses shell function lookup, as with the
command builtin. type returns true if all of the arguments are
found, false if any are not found.

ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]
Provides control over the resources available to the shell and to
processes it starts, on systems that allow such control.

The -H and -S options specify whether the hard or soft limit is
set for the given resource. A hard limit cannot be increased by a
non-root user once it is set; a soft limit may be increased up to
the value of the hard limit. If neither -H nor -S is specified,
ulimit sets both the soft and hard limits.

The value of limit can be a number in the unit specified for the
resource or one of the special values hard, soft, or unlimited,
which stand for the current hard limit, the current soft limit,
and no limit, respectively. If limit is omitted, ulimit prints
the current value of the soft limit of the resource, unless the -H
option is given. When more than one resource is specified, the
limit name and unit, if appropriate, are printed before the value.
Other options are interpreted as follows:
-a Report all current limits; no limits are set.
-b The maximum socket buffer size.
-c The maximum size of core files created.
-d The maximum size of a process's data segment.
-e The maximum scheduling priority (“nice”).
-f The maximum size of files written by the shell and its
children.
-i The maximum number of pending signals.
-k The maximum number of kqueues that may be allocated.
-l The maximum size that may be locked into memory.
-m The maximum resident set size (many systems do not honor
this limit).
-n The maximum number of open file descriptors (most systems
do not allow this value to be set).
-p The pipe size in 512-byte blocks (this may not be set).
-q The maximum number of bytes in POSIX message queues.
-r The maximum real-time scheduling priority.
-s The maximum stack size.
-t The maximum amount of cpu time in seconds.
-u The maximum number of processes available to a single user.
-v The maximum amount of virtual memory available to the shell
and, on some systems, to its children.
-x The maximum number of file locks.
-P The maximum number of pseudoterminals.
-R The maximum time a real-time process can run before
blocking, in microseconds.
-T The maximum number of threads.

If limit is supplied, and the -a option is not used, limit is the
new value of the specified resource. If no option is supplied,
then -f is assumed.

Values are in 1024-byte increments, except for -t, which is in
seconds; -R, which is in microseconds; -p, which is in units of
512-byte blocks; -P, -T, -b, -k, -n, and -u, which are unscaled
values; and, when in posix mode, -c and -f, which are in 512-byte
increments. The return status is 0 unless an invalid option or
argument is supplied, or an error occurs while setting a new
limit.

umask [-p] [-S] [mode]
Set the user file-creation mask to mode. If mode begins with a
digit, it is interpreted as an octal number; otherwise it is
interpreted as a symbolic mode mask similar to that accepted by
chmod(1). If mode is omitted, umask prints the current value of
the mask. The -S option without a mode argument prints the mask
in a symbolic format; the default output is an octal number. If
the -p option is supplied, and mode is omitted, the output is in a
form that may be reused as input. The return status is zero if
the mode was successfully changed or if no mode argument was
supplied, and non-zero otherwise.

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

unset [-fv] [-n] [name ...]
For each name, remove the corresponding variable or function. If
the -v option is given, each name refers to a shell variable, and
that variable is removed. If -f is specified, each name refers to
a shell function, and the function definition is removed. If the
-n option is supplied, and name is a variable with the nameref
attribute, name will be unset rather than the variable it
references. -n has no effect if the -f option is supplied. Read-
only variables and functions may not be unset. When variables or
functions are removed, they are also removed from the environment
passed to subsequent commands. If no options are supplied, each
name refers to a variable; if there is no variable by that name, a
function with that name, if any, is unset. Some shell variables
may not be unset. If any of BASH_ALIASES, BASH_ARGV0, BASH_CMDS,
BASH_COMMAND, BASH_SUBSHELL, BASHPID, COMP_WORDBREAKS, DIRSTACK,
EPOCHREALTIME, EPOCHSECONDS, FUNCNAME, GROUPS, HISTCMD, LINENO,
RANDOM, SECONDS, or SRANDOM are unset, they lose their special
properties, even if they are subsequently reset. The exit status
is true unless a name is readonly or may not be unset.

wait [-fn] [-p varname] [id ...]
Wait for each specified child process id and return the
termination status of the last id. Each id may be a process ID
pid or a job specification jobspec; if a jobspec is supplied, wait
waits for all processes in the job.

If no options or ids are supplied, wait waits for all running
background jobs and the last-executed process substitution, if its
process id is the same as $!, and the return status is zero.

If the -n option is supplied, wait waits for any one of the given
ids or, if no ids are supplied, any job or process substitution,
to complete and returns its exit status. If none of the supplied
ids is a child of the shell, or if no ids are supplied and the
shell has no unwaited-for children, the exit status is 127.

If the -p option is supplied, wait assigns the process or job
identifier of the job for which the exit status is returned to the
variable varname named by the option argument. The variable,
which cannot be readonly, will be unset initially, before any
assignment. This is useful only when used with the -n option.

Supplying the -f option, when job control is enabled, forces wait
to wait for each id to terminate before returning its status,
instead of returning when it changes status.

If none of the ids specify one of the shell's active child
processes, the return status is 127. If wait is interrupted by a
signal, any varname will remain unset, and the return status will
be greater than 128, as described under SIGNALS above. Otherwise,
the return status is the exit status of the last id.

SHELL COMPATIBILITY MODE
Bash-4.0 introduced the concept of a shell compatibility level, specified
as a set of options to the shopt builtin (compat31, compat32, compat40,
compat41, and so on). There is only one current compatibility level —
each option is mutually exclusive. The compatibility level is intended
to allow users to select behavior from previous versions that is
incompatible with newer versions while they migrate scripts to use
current features and behavior. It's intended to be a temporary solution.

This section does not mention behavior that is standard for a particular
version (e.g., setting compat32 means that quoting the right hand side of
the regexp matching operator quotes special regexp characters in the
word, which is default behavior in bash-3.2 and subsequent versions).

If a user enables, say, compat32, it may affect the behavior of other
compatibility levels up to and including the current compatibility level.
The idea is that each compatibility level controls behavior that changed
in that version of bash, but that behavior may have been present in
earlier versions. For instance, the change to use locale-based
comparisons with the [[ command came in bash-4.1, and earlier versions
used ASCII-based comparisons, so enabling compat32 will enable ASCII-
based comparisons as well. That granularity may not be sufficient for
all uses, and as a result users should employ compatibility levels
carefully. Read the documentation for a particular feature to find out
the current behavior.

Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value
assigned to this variable (a decimal version number like 4.2, or an
integer corresponding to the compatNN option, like 42) determines the
compatibility level.

Starting with bash-4.4, bash began deprecating older compatibility
levels. Eventually, the options will be removed in favor of BASH_COMPAT.

Bash-5.0 was the final version for which there was an individual shopt
option for the previous version. BASH_COMPAT is the only mechanism to
control the compatibility level in versions newer than bash-5.0.

The following table describes the behavior changes controlled by each
compatibility level setting. The compatNN tag is used as shorthand for
setting the compatibility level to NN using one of the following
mechanisms. For versions prior to bash-5.0, the compatibility level may
be set using the corresponding compatNN shopt option. For bash-4.3 and
later versions, the BASH_COMPAT variable is preferred, and it is required
for bash-5.1 and later versions.

compat31
• Quoting the rhs of the [[ command's regexp matching
operator (=~) has no special effect.

compat32
• The < and > operators to the [[ command do not consider the
current locale when comparing strings; they use ASCII
ordering.

compat40
• The < and > operators to the [[ command do not consider the
current locale when comparing strings; they use ASCII
ordering. Bash versions prior to bash-4.1 use ASCII
collation and strcmp(3); bash-4.1 and later use the current
locale's collation sequence and strcoll(3).

compat41
• In posix mode, time may be followed by options and still be
recognized as a reserved word (this is POSIX interpretation
267).
• In posix mode, the parser requires that an even number of
single quotes occur in the word portion of a double-quoted
parameter expansion and treats them specially, so that
characters within the single quotes are considered quoted
(this is POSIX interpretation 221).

compat42
• The replacement string in double-quoted pattern
substitution does not undergo quote removal, as it does in
versions after bash-4.2.
• In posix mode, single quotes are considered special when
expanding the word portion of a double-quoted parameter
expansion and can be used to quote a closing brace or other
special character (this is part of POSIX interpretation
221); in later versions, single quotes are not special
within double-quoted word expansions.

compat43
• Word expansion errors are considered non-fatal errors that
cause the current command to fail, even in posix mode (the
default behavior is to make them fatal errors that cause
the shell to exit).
• When executing a shell function, the loop state
(while/until/etc.) is not reset, so break or continue in
that function will break or continue loops in the calling
context. Bash-4.4 and later reset the loop state to
prevent this.

compat44
• The shell sets up the values used by BASH_ARGV and
BASH_ARGC so they can expand to the shell's positional
parameters even if extended debugging mode is not enabled.
• A subshell inherits loops from its parent context, so break
or continue will cause the subshell to exit. Bash-5.0 and
later reset the loop state to prevent the exit
• Variable assignments preceding builtins like export and
readonly that set attributes continue to affect variables
with the same name in the calling environment even if the
shell is not in posix mode.

compat50
• Bash-5.1 changed the way $RANDOM is generated to introduce
slightly more randomness. If the shell compatibility level
is set to 50 or lower, it reverts to the method from
bash-5.0 and previous versions, so seeding the random
number generator by assigning a value to RANDOM will
produce the same sequence as in bash-5.0.
• If the command hash table is empty, bash versions prior to
bash-5.1 printed an informational message to that effect,
even when producing output that can be reused as input.
Bash-5.1 suppresses that message when the -l option is
supplied.

compat51
• The unset builtin treats attempts to unset array subscripts
@ and * differently depending on whether the array is
indexed or associative, and differently than in previous
versions.
• Arithmetic commands ( ((...)) ) and the expressions in an
arithmetic for statement can be expanded more than once.
• Expressions used as arguments to arithmetic operators in
the [[ conditional command can be expanded more than once.
• The expressions in substring parameter brace expansion can
be expanded more than once.
• The expressions in the $((...)) word expansion can be
expanded more than once.
• Arithmetic expressions used as indexed array subscripts can
be expanded more than once.
• test -v, when given an argument of A[@], where A is an
existing associative array, will return true if the array
has any set elements. Bash-5.2 will look for and report on
a key named @.
• The ${parameter[:]=value} word expansion will return value,
before any variable-specific transformations have been
performed (e.g., converting to lowercase). Bash-5.2 will
return the final value assigned to the variable.
• Parsing command substitutions will behave as if extended
globbing (see the description of the shopt builtin above)
is enabled, so that parsing a command substitution
containing an extglob pattern (say, as part of a shell
function) will not fail. This assumes the intent is to
enable extglob before the command is executed and word
expansions are performed. It will fail at word expansion
time if extglob hasn't been enabled by the time the command
is executed.

compat52
• The test builtin uses its historical algorithm to parse
parenthesized subexpressions when given five or more
arguments.
• If the -p or -P option is supplied to the bind builtin,
bind treats any arguments remaining after option processing
as bindable command names, and displays any key sequences
bound to those commands, instead of treating the arguments
as key sequences to bind.

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, HISTFILE, ENV, or
BASH_ENV.

• Specifying command names containing /.

• Specifying a filename containing a / as an argument to the .
builtin command.

• Using the -p option to the . builtin command to specify a search
path.

• Specifying a filename containing a slash as an argument to the
history builtin command.

• Specifying a filename containing a slash as an argument to the -p
option to the hash builtin command.

• Importing function definitions from the shell environment at
startup.

• Parsing the values of BASHOPTS and SHELLOPTS from the shell
environment at startup.

• Redirecting output using the >, >|, <>, >&, &>, and >> redirection
operators.

• Using the exec builtin command to replace the shell with another
command.

• Adding or deleting builtin commands with the -f and -d options to
the enable builtin command.

• Using the enable builtin command to enable disabled shell
builtins.

• Specifying the -p option to the command builtin command.

• Turning off restricted mode with set +r or shopt -u
restricted_shell.

These restrictions are enforced after any startup files are read.

When a command that is found to be a shell script is executed (see
COMMAND EXECUTION above), rbash turns off any restrictions in the shell
spawned to execute the script.

SEE ALSO
Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities,
IEEE —
http://pubs.opengroup.org/onlinepubs/9799919799/
http://tiswww.case.edu/~chet/bash/POSIX — a description of posix mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)

FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login
shell exits
~/.bash_history
The default value of HISTFILE, the file in which bash saves the
command history
~/.inputrc
Individual readline initialization file

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

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

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

Once you have determined that a bug actually exists, use the bashbug
command to submit a bug report. If you have a fix, you are encouraged to
mail that as well! You may send suggestions and “philosophical” bug
reports to bug-bash@gnu.org or post them to the Usenet newsgroup
gnu.bash.bug.

ALL bug reports should include:

The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behavior
A short script or “recipe” which exercises the bug

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

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

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 lists of the form “a ; b ; c” are not
handled gracefully when combined with process suspension. When a process
is stopped, the shell immediately executes the next command in the list
or breaks out of any existing loops. It suffices to enclose the command
in parentheses to force it into a subshell, which may be stopped as a
unit, or to start the command in the background and immediately bring it
into the foreground.

Array variables may not (yet) be exported.

GNU Bash 5.3 2025 April 7 BASH(1)