bash
(1)
Name
bash - Again SHell
Synopsis
bash [options] [file]
Description
User Commands BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2009 by the Free Software Founda-
tion, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that
executes commands read from the standard input or from a
file. Bash also incorporates useful features from the Korn
and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the
Shell and Utilities portion of the IEEE POSIX specification
(IEEE Standard 1003.1). Bash can be configured to be POSIX-
conformant by default.
OPTIONS
In addition to the single-character shell options documented
in the description of the set builtin command, bash inter-
prets the following options when it is invoked:
-c string If the -c option is present, then commands are
read from string. If there are arguments after
the string, they are assigned to the positional
parameters, starting with $0.
-i If the -i option is present, the shell is interac-
tive.
-l Make bash act as if it had been invoked as a login
shell (see INVOCATION below).
-r If the -r option is present, the shell becomes
restricted (see RESTRICTED SHELL below).
-s If the -s option is present, or if no arguments
remain after option processing, then commands are
read from the standard input. This option allows
the positional parameters to be set when invoking
an interactive shell.
-D A list of all double-quoted strings preceded by $
is printed on the standard output. These are the
strings that are subject to language translation
when the current locale is not C or POSIX. This
implies the -n option; no commands will be exe-
cuted.
[-+]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
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shopt_option is not supplied, the names and values
of the shell options accepted by shopt are printed
on the standard output. If the invocation option
is +O, the output is displayed in a format that
may be reused as input.
-- A -- signals the end of options and disables fur-
ther option processing. Any arguments after the
-- are treated as filenames and arguments. An
argument of - is equivalent to --.
Bash also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before
the shell starts. Turns on extended debugging mode
(see the description of the extdebug option to the
shopt builtin below) and shell function tracing (see
the description of the -o functrace option to the set
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 per-
sonal 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
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option is on by default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation
differs from the POSIX standard to match the standard
(posix mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL
below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on
the standard output and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the
-c nor the -s option has been supplied, the first argument
is assumed to be the name of a file containing shell com-
mands. If bash is invoked in this fashion, $0 is set to the
name of the file, and the positional parameters are set to
the remaining arguments. Bash reads and executes commands
from this file, then exits. Bash's exit status is the exit
status of the last command executed in the script. If no
commands are executed, the exit status is 0. An attempt is
first made to open the file in the current directory, and,
if no file is found, then the shell searches the directories
in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero
is a -, or one started with the --login option.
An interactive shell is one started without non-option argu-
ments and without the -c option whose standard input and
error are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. PS1 is set
and $- includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
The following paragraphs describe how bash executes its
startup files. If any of the files exist but cannot be
read, bash reports an error. Tildes are expanded in file
names as described below under Tilde Expansion in the EXPAN-
SION section.
When bash is invoked as an interactive login shell, or as a
non-interactive shell with the --login option, it first
reads and executes commands from the file /etc/profile, if
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that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and ~/.profile, in that
order, and reads and executes commands from the first one
that exists and is readable. The --noprofile option may be
used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands
from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is
started, bash reads and executes commands from ~/.bashrc, if
that file exists. This may be inhibited by using the --norc
option. The --rcfile file option will force bash to read
and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell
script, for example, it looks for the variable BASH_ENV in
the environment, expands its value if it appears there, and
uses the expanded value as the name of a file to read and
execute. Bash behaves as if the following command were exe-
cuted:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for
the file name.
If bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the POSIX standard as well.
When invoked as an interactive login shell, or a non-inter-
active shell with the --login option, it first attempts to
read and execute commands from /etc/profile and ~/.profile,
in that order. The --noprofile option may be used to
inhibit this behavior. When invoked as an interactive shell
with the name sh, bash looks for the variable ENV, expands
its value if it is defined, and uses the expanded value as
the name of a file to read and execute. Since a shell
invoked as sh does not attempt to read and execute commands
from any other startup files, the --rcfile option has no
effect. A non-interactive shell invoked with the name sh
does not attempt to read any other startup files. When
invoked as sh, bash enters posix mode after the startup
files are read.
When bash is started in posix mode, as with the --posix com-
mand line option, it follows the POSIX standard for startup
files. In this mode, interactive shells expand the ENV
variable and commands are read and executed from the file
whose name is the expanded value. No other startup files
are read.
Bash attempts to determine when it is being run with its
standard input connected to a a network connection, as if by
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the remote shell daemon, usually rshd, or the secure shell
daemon sshd. If bash determines it is being run in this
fashion, it reads and executes commands from ~/.bashrc, if
that file exists and is readable. It will not do this if
invoked as sh. The --norc option may be used to inhibit
this behavior, and the --rcfile option may be used to force
another file to be read, but rshd does not generally invoke
the shell with those options or allow them to be specified.
If the shell is started with the effective user (group) id
not equal to the real user (group) id, and the -p option is
not supplied, no startup files are read, shell functions are
not inherited from the environment, the SHELLOPTS, BASHOPTS,
CDPATH, and GLOBIGNORE variables, if they appear in the
environment, are ignored, and the effective user id is set
to the real user id. If the -p option is supplied at invo-
cation, the startup behavior is the same, but the effective
user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of
this document.
blank
A space or tab.
word A sequence of characters considered as a single unit by
the shell. Also known as a token.
name A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character
or an underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One
of the following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of
the following symbols:
|| & && ; ;; ( ) | |& <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the
shell. The following words are recognized as reserved when
unquoted and either the first word of a simple command (see
SHELL GRAMMAR below) or the third word of a case or for com-
mand:
! case do done elif else esac fi for function if in select
then until while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assign-
ments followed by blank-separated words and redirections,
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and terminated by a control operator. The first word speci-
fies the command to be executed, and is passed as argument
zero. The remaining words are passed as arguments to the
invoked command.
The return value of a simple command is its exit status, or
128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated
by one of the control operators | or |&. The format for a
pipeline is:
[time [-p]] [ ! ] command [ [|||&] command2 ... ]
The standard output of command is connected via a pipe to
the standard input of command2. This connection is per-
formed before any redirections specified by the command (see
REDIRECTION below). If |& is used, the standard error of
command is connected to command2's standard input through
the pipe; it is shorthand for 2>&1 |. This implicit redi-
rection of the standard error is performed after any redi-
rections specified by the command.
The return status of a pipeline is the exit status of the
last command, unless the pipefail option is enabled. If
pipefail is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-
zero status, or zero if all commands exit successfully. If
the reserved word ! precedes a pipeline, the exit status of
that pipeline is the logical negation of the exit status as
described above. The shell waits for all commands in the
pipeline to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed
as well as user and system time consumed by its execution
are reported when the pipeline terminates. The -p option
changes the output format to that specified by POSIX. The
TIMEFORMAT variable may be set to a format string that spec-
ifies how the timing information should be displayed; see
the description of TIMEFORMAT under Shell Variables below.
Each command in a pipeline is executed as a separate process
(i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by
one of the operators ;, &, &&, or ||, and optionally termi-
nated by one of ;, &, or <newline>.
Of these list operators, && and || have equal precedence,
followed by ; and &, which have equal precedence.
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A sequence of one or more newlines may appear in a list
instead of a semicolon to delimit commands.
If a command is terminated by the control operator &, the
shell executes the command in the background in a subshell.
The shell does not wait for the command to finish, and the
return status is 0. Commands separated by a ; are executed
sequentially; the shell waits for each command to terminate
in turn. The return status is the exit status of the last
command executed.
AND and OR lists are sequences of one of more pipelines sep-
arated by the && and || control operators, respectively.
AND and OR lists are executed with left associativity. An
AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an
exit status of zero.
An OR list has the form
command1 || command2
command2 is executed if and only if command1 returns a non-
zero exit status. The return status of AND and OR lists is
the exit status of the last command executed in the list.
Compound Commands
A compound command is one of the following:
(list)
list is executed in a subshell environment (see COMMAND
EXECUTION ENVIRONMENT below). 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 simply executed in the current shell environ-
ment. list must be terminated with a newline or semi-
colon. This is known as a group command. The return
status is the exit status of list. Note that unlike
the metacharacters ( and ), { and } are reserved words
and must occur where a reserved word is permitted to be
recognized. Since they do not cause a word break, they
must be separated from list by whitespace or another
shell metacharacter.
((expression))
The expression is evaluated according to the rules
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described below under ARITHMETIC EVALUATION. If the
value of the expression is non-zero, the return status
is 0; otherwise the return status is 1. This is
exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation
of the conditional expression expression. Expressions
are composed of the primaries described below under
CONDITIONAL EXPRESSIONS. Word splitting and pathname
expansion are not performed on the words between the [[
and ]]; tilde expansion, parameter and variable expan-
sion, arithmetic expansion, command substitution,
process substitution, and quote removal are performed.
Conditional operators such as -f must be unquoted to be
recognized as primaries.
When used with [[, The < and > operators sort lexico-
graphically using the current locale.
When the == and != operators are used, the string to
the right of the operator is considered a pattern and
matched according to the rules described below under
Pattern Matching. If the shell option nocasematch is
enabled, the match is performed without regard to the
case of alphabetic characters. The return value is 0
if the string matches (==) or does not match (!=) the
pattern, and 1 otherwise. Any part of the pattern may
be quoted to force it to be matched as a string.
An additional binary operator, =~, is available, with
the same precedence as == and !=. When it is used, the
string to the right of the operator is considered an
extended regular expression and matched accordingly (as
in regex(3)). The return value is 0 if the string
matches the pattern, and 1 otherwise. If the regular
expression is syntactically incorrect, the conditional
expression's return value is 2. If the shell option
nocasematch is enabled, the match is performed without
regard to the case of alphabetic characters. Any part
of the pattern may be quoted to force it to be matched
as a string. Substrings matched by parenthesized
subexpressions within the regular expression are saved
in the array variable BASH_REMATCH. The element of
BASH_REMATCH with index 0 is the portion of the string
matching the entire regular expression. The element of
BASH_REMATCH with index n is the portion of the string
matching the nth parenthesized subexpression.
Expressions may be combined using the following opera-
tors, listed in decreasing order of precedence:
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( expression )
Returns the value of expression. This may be used
to override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if
the value of expression1 is sufficient to determine the
return value of the entire conditional expression.
for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating
a list of items. The variable name is set to each ele-
ment of this list in turn, and list is executed each
time. If the in word is omitted, the for command exe-
cutes list once for each positional parameter that is
set (see PARAMETERS below). The return status is the
exit status of the last command that executes. If the
expansion of the items following in results in an empty
list, no commands are executed, and the return status
is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated
according to the rules described below under ARITHMETIC
EVALUATION. The arithmetic expression expr2 is then
evaluated repeatedly until it evaluates to zero. Each
time expr2 evaluates to a non-zero value, list is exe-
cuted and the arithmetic expression expr3 is evaluated.
If any expression is omitted, it behaves as if it eval-
uates to 1. The return value is the exit status of the
last command in list that is executed, or false if any
of the expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating
a list of items. The set of expanded words is printed
on the standard error, each preceded by a number. If
the in word is omitted, the positional parameters are
printed (see PARAMETERS below). The PS3 prompt is then
displayed and a line read from the standard input. If
the line consists of a number corresponding to one of
the displayed words, then the value of name is set to
that word. If the line is empty, the words and prompt
are displayed again. If EOF is read, the command com-
pletes. Any other value read causes name to be set to
null. The line read is saved in the variable REPLY.
The list is executed after each selection until a break
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command is executed. The exit status of select is the
exit status of the last command executed in list, or
zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ...
esac
A case command first expands word, and tries to match
it against each pattern in turn, using the same match-
ing rules as for pathname expansion (see Pathname
Expansion below). The word is expanded using tilde
expansion, parameter and variable expansion, arithmetic
substitution, command substitution, process substitu-
tion and quote removal. Each pattern examined is
expanded using tilde expansion, parameter and variable
expansion, arithmetic substitution, command substitu-
tion, and process substitution. If the shell option
nocasematch is enabled, the match is performed without
regard to the case of alphabetic characters. When a
match is found, the corresponding list is executed. If
the ;; operator is used, no subsequent matches are
attempted after the first pattern match. Using ;& in
place of ;; causes execution to continue with the list
associated with the next set of patterns. Using ;;& in
place of ;; causes the shell to test the next pattern
list in the statement, if any, and execute any associ-
ated list on a successful match. The exit status is
zero if no pattern matches. Otherwise, it is the exit
status of the last command executed in list.
fi
if list; then list; [ elif list; then list; ] ... [ else
list; ]
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; do list; done
until list; do list; done
The while command continuously executes the do list as
long as the last command in list returns an exit status
of zero. The until command is identical to the while
command, except that the test is negated; the do list
is executed as long as the last command in list returns
a non-zero exit status. The exit status of the while
and until commands is the exit status of the last do
list command executed, or zero if none was executed.
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Coprocesses
A coprocess is a shell command preceded by the coproc
reserved word. A coprocess is executed asynchronously in a
subshell, as if the command had been terminated with the &
control operator, with a two-way pipe established between
the executing shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not sup-
plied, the default name is COPROC. NAME must not be sup-
plied if command is a simple command (see above); otherwise,
it is interpreted as the first word of the simple command.
When the coproc is executed, the shell creates an array
variable (see Arrays below) named NAME in the context of the
executing shell. The standard output of command is con-
nected via a pipe to a file descriptor in the executing
shell, and that file descriptor is assigned to NAME[0]. The
standard input of command is connected via a pipe to a file
descriptor in the executing shell, and that file descriptor
is assigned to NAME[1]. This pipe is established before any
redirections specified by the command (see REDIRECTION
below). The file descriptors can be utilized as arguments
to shell commands and redirections using standard word
expansions. The process id of the shell spawned to execute
the coprocess is available as the value of the variable
NAME_PID. The wait builtin command may be used to wait for
the coprocess to terminate.
The return status of a coprocess is the exit status of com-
mand.
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 fol-
lows:
[ function ] name () compound-command [redirection]
This defines a function named name. The reserved word
function is optional. If the function reserved word is
supplied, the parentheses are optional. The body of
the function is the compound command compound-command
(see Compound Commands above). That command is usually
a list of commands between { and }, but may be any com-
mand listed under Compound Commands above. com-
pound-command is executed whenever name is specified as
the name of a simple command. Any redirections (see
REDIRECTION below) specified when a function is defined
are performed when the function is executed. The exit
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status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name
already exists. When executed, the exit status of a
function is the exit status of the last command exe-
cuted in the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which
the interactive_comments option to the shopt builtin is
enabled (see SHELL BUILTIN COMMANDS below), a word beginning
with # causes that word and all remaining characters on that
line to be ignored. An interactive shell without the inter-
active_comments option enabled does not allow comments. The
interactive_comments option is on by default in interactive
shells.
QUOTING
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to prevent
reserved words from being recognized as such, and to prevent
parameter expansion.
Each of the metacharacters listed above under DEFINITIONS
has special meaning to the shell and must be quoted if it is
to represent itself.
When the command history expansion facilities are being used
(see HISTORY EXPANSION below), the history expansion charac-
ter, usually !, must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character,
single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It pre-
serves the literal value of the next character that follows,
with the exception of <newline>. If a \<newline> pair
appears, and the backslash is not itself quoted, the \<new-
line> 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 excep-
tion of $, `, \, and, when history expansion is enabled, !.
The characters $ and ` retain their special meaning within
double quotes. The backslash retains its special meaning
only when followed by one of the following characters: $, `,
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", \, or <newline>. A double quote may be quoted within
double quotes by preceding it with a backslash. If enabled,
history expansion will be performed unless an ! appearing
in double quotes is escaped using a backslash. The back-
slash preceding the ! is not removed.
The special parameters * and @ have special meaning when in
double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word
expands to string, with backslash-escaped characters
replaced as specified by the ANSI C standard. Backslash
escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal
value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexa-
decimal value HH (one or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign
had not been present.
A double-quoted string preceded by a dollar sign ($"string")
will cause the string to be translated according to the cur-
rent locale. If the current locale is C or POSIX, the dol-
lar sign is ignored. If the string is translated and
replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a
name, a number, or one of the special characters listed
below under Special Parameters. A variable is a parameter
denoted by a name. A variable has a value and zero or more
attributes. Attributes are assigned using the declare
builtin command (see declare below in SHELL BUILTIN COM-
MANDS).
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
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SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null
string. All values undergo tilde expansion, parameter and
variable expansion, command substitution, arithmetic expan-
sion, and quote removal (see EXPANSION below). If the vari-
able has its integer attribute set, then value is evaluated
as an arithmetic expression even if the $((...)) expansion
is not used (see Arithmetic Expansion below). Word split-
ting is not performed, with the exception of "$@" as
explained below under Special Parameters. Pathname expan-
sion is not performed. Assignment statements may also
appear as arguments to the alias, declare, typeset, export,
readonly, and local builtin commands.
In the context where an assignment statement is assigning a
value to a shell variable or array index, the += operator
can be used to append to or add to the variable's previous
value. When += is applied to a variable for which the inte-
ger attribute has been set, value is evaluated as an arith-
metic expression and added to the variable's current value,
which is also evaluated. When += is applied to an array
variable using compound assignment (see Arrays below), the
variable's value is not unset (as it is when using =), and
new values are appended to the array beginning at one
greater than the array's maximum index (for indexed arrays)
or added as additional key-value pairs in an associative
array. When applied to a string-valued variable, value is
expanded and appended to the variable's value.
Positional Parameters
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parame-
ters are assigned from the shell's arguments when it is
invoked, and may be reassigned using the set builtin com-
mand. Positional parameters may not be assigned to with
assignment statements. The positional parameters are tempo-
rarily replaced when a shell function is executed (see FUNC-
TIONS below).
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see EXPAN-
SION below).
Special Parameters
The shell treats several parameters specially. These param-
eters may only be referenced; assignment to them is not
allowed.
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* Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes,
it expands to a single word with the value of each
parameter separated by the first character of the IFS
special variable. That is, "$*" is equivalent to
"$1c$2c...", where c is the first character of the
value of the IFS variable. If IFS is unset, the param-
eters are separated by spaces. If IFS is null, the
parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes,
each parameter expands to a separate word. That is,
"$@" is equivalent to "$1" "$2" ... If the double-
quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part
of the original word, and the expansion of the last
parameter is joined with the last part of the original
word. When there are no positional parameters, "$@"
and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in deci-
mal.
? Expands to the exit status of the most recently exe-
cuted foreground pipeline.
- Expands to the current option flags as specified upon
invocation, by the set builtin command, or those set by
the shell itself (such as the -i option).
$ Expands to the process ID of the shell. In a () sub-
shell, it expands to the process ID of the current
shell, not the subshell.
! Expands to the process ID of the most recently executed
background (asynchronous) command.
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 file name used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to
invoke the shell or shell script being executed as
passed in the environment or argument list. Subse-
quently, expands to the last argument to the previous
command, after expansion. Also set to the full path-
name used to invoke each command executed and placed in
the environment exported to that command. When check-
ing mail, this parameter holds the name of the mail
file currently being checked.
Shell Variables
The following variables are set by the shell:
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BASH Expands to the full file name used to invoke this
instance of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each
word in the list is a valid argument for the -s option
to the shopt builtin command (see SHELL BUILTIN COM-
MANDS below). The options appearing in BASHOPTS are
those reported as on by shopt. If this variable is in
the environment when bash starts up, each shell option
in the list will be enabled before reading any startup
files. This variable is read-only.
BASHPID
Expands to the process id of the current bash process.
This differs from $$ under certain circumstances, such
as subshells that do not require bash to be re-initial-
ized.
BASH_ALIASES
An associative array variable whose members correspond
to the internal list of aliases as maintained by the
alias builtin Elements added to this array appear in
the alias list; unsetting array elements cause aliases
to be removed from the alias list.
BASH_ARGC
An array variable whose values are the number of param-
eters in each frame of the current bash execution call
stack. The number of parameters to the current subrou-
tine (shell function or script executed with . or
source) is at the top of the stack. When a subroutine
is executed, the number of parameters passed is pushed
onto BASH_ARGC. The shell sets BASH_ARGC only when in
extended debugging mode (see the description of the
extdebug option to the shopt builtin below)
BASH_ARGV
An array variable containing all of the parameters in
the current bash execution call stack. The final
parameter of the last subroutine call is at the top of
the stack; the first parameter of the initial call is
at the bottom. When a subroutine is executed, the
parameters supplied are pushed onto BASH_ARGV. The
shell sets BASH_ARGV only when in extended debugging
mode (see the description of the extdebug option to the
shopt builtin below)
BASH_CMDS
An associative array variable whose members correspond
to the internal hash table of commands as maintained by
the hash builtin. Elements added to this array appear
in the hash table; unsetting array elements cause com-
mands to be removed from the hash table.
BASH_COMMAND
The command currently being executed or about to be
executed, unless the shell is executing a command as
the result of a trap, in which case it is the command
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User Commands BASH(1)
executing at the time of the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in
source files corresponding to each member of FUNCNAME.
${BASH_LINENO[$i]} is the line number in the source
file where ${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within another shell
function). The corresponding source file name is
${BASH_SOURCE[$i]}. Use LINENO to obtain the current
line number.
BASH_REMATCH
An array variable whose members are assigned by the =~
binary operator to the [[ conditional command. The
element with index 0 is the portion of the string
matching the entire regular expression. The element
with index n is the portion of the string matching the
nth parenthesized subexpression. This variable is
read-only.
BASH_SOURCE
An array variable whose members are the source file-
names corresponding to the elements in the FUNCNAME
array variable.
BASH_SUBSHELL
Incremented by one each time a subshell or subshell
environment is spawned. The initial value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version
information for this instance of bash. The values
assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the
release).
BASH_VERSINFO[1] The minor version number (the
version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g.,
beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this
instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the
current cursor position. This variable is available
only in shell functions invoked by the programmable
completion facilities (see Programmable Completion
below).
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COMP_KEY
The key (or final key of a key sequence) used to invoke
the current completion function.
COMP_LINE
The current command line. This variable is available
only in shell functions and external commands invoked
by the programmable completion facilities (see Pro-
grammable 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 comple-
tion facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of
completion attempted that caused a completion function
to be called: TAB, for normal completion, ?, for list-
ing completions after successive tabs, !, for listing
alternatives on partial word completion, @, to list
completions if the word is not unmodified, or %, for
menu completion. This variable is available only in
shell functions and external 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 proper-
ties, 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 pro-
grammable completion facilities (see Programmable Com-
pletion below).
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
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the stack, but the pushd and popd builtins must be used
to add and remove directories. Assignment to this
variable will not change the current directory. If
DIRSTACK is unset, it loses its special properties,
even if it is subsequently reset.
EUID Expands to the effective user ID of the current user,
initialized at shell startup. This variable is read-
only.
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-exe-
cuting shell function. The bottom-most element is
"main". This variable exists only when a shell func-
tion is executing. Assignments to FUNCNAME have no
effect and return an error status. If FUNCNAME is
unset, it loses its special properties, even if it is
subsequently reset.
GROUPS
An array variable containing the list of groups of
which the current user is a member. Assignments to
GROUPS have no effect and return an error status. If
GROUPS is unset, it loses its special properties, even
if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of
the current command. If HISTCMD is unset, it loses its
special properties, even if it is subsequently reset.
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 sub-
stitutes 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 meaning-
ful. If LINENO is unset, it loses its special proper-
ties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the
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system type on which bash is executing, in the standard
GNU cpu-company-system format. The default is system-
dependent.
OLDPWD
The previous working directory as set by the cd com-
mand.
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 oper-
ating system on which bash is executing. The default
is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list
of exit status values from the processes in the most-
recently-executed foreground pipeline (which may con-
tain only a single command).
PPID The process ID of the shell's parent. This variable is
readonly.
PWD The current working directory as set by the cd command.
RANDOM
Each time this parameter is referenced, a random inte-
ger between 0 and 32767 is generated. The sequence of
random numbers may be initialized by assigning a value
to RANDOM. If RANDOM is unset, it loses its special
properties, even if it is subsequently reset.
REPLY
Set to the line of input read by the read builtin com-
mand when no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of
seconds since shell invocation is returned. If a value
is assigned to SECONDS, the value returned upon subse-
quent references is the number of seconds since the
assignment plus the value assigned. If SECONDS is
unset, it loses its special properties, even if it is
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User Commands BASH(1)
subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each
word in the list is a valid argument for the -o option
to the set builtin command (see SHELL BUILTIN COMMANDS
below). The options appearing in SHELLOPTS are those
reported as on by set -o. If this variable is in the
environment when bash starts up, each shell option in
the list will be enabled before reading any startup
files. This variable is read-only.
SHLVL
Incremented by one each time an instance of bash is
started.
UID Expands to the user ID of the current user, initialized
at shell startup. This variable is readonly.
The following variables are used by the shell. In some
cases, bash assigns a default value to a variable; these
cases are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell
script, its value is interpreted as a filename contain-
ing commands to initialize the shell, as in ~/.bashrc.
The value of BASH_ENV is subjected to parameter expan-
sion, command substitution, and arithmetic expansion
before being interpreted as a file name. PATH is not
used to search for the resultant file name.
CDPATH
The search path for the cd command. This is a colon-
separated list of directories in which the shell looks
for destination directories specified by the cd com-
mand. A sample value is ".:~:/usr".
BASH_XTRACEFD
If set to an integer corresponding to a valid file
descriptor, bash will write the trace output generated
when set -x is enabled to that file descriptor. The
file descriptor is closed when BASH_XTRACEFD is unset
or assigned a new value. Unsetting BASH_XTRACEFD or
assigning it the empty string causes the trace output
to be sent to the standard error. Note that setting
BASH_XTRACEFD to 2 (the standard error file descriptor)
and then unsetting it will result in the standard error
being closed.
COLUMNS
Used by the select builtin command to determine the
terminal width when printing selection lists. Automat-
ically set upon receipt of a SIGWINCH.
COMPREPLY
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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).
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.
FCEDIT
The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when per-
forming 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:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of
filenames to be ignored by pathname expansion. If a
filename matched by a pathname expansion pattern also
matches one of the patterns in GLOBIGNORE, it is
removed from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how com-
mands are saved on the history list. If the list of
values includes ignorespace, lines which begin with a
space character are not saved in the history list. A
value of ignoredups causes lines matching the previous
history entry to not be saved. A value of ignoreboth
is shorthand for ignorespace and ignoredups. A value
of erasedups causes all previous lines matching the
current line to be removed from the history list before
that line is saved. Any value not in the above list is
ignored. If HISTCONTROL is unset, or does not include
a valid value, all lines read by the shell parser are
saved on the history list, subject to the value of
HISTIGNORE. The second and subsequent lines of a
multi-line compound command are not tested, and are
added to the history regardless of the value of HIST-
CONTROL.
HISTFILE
The name of the file in which command history is saved
(see HISTORY below). The default value is ~/.bash_his-
tory. If unset, the command history is not saved when
an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the history
file. When this variable is assigned a value, the his-
tory file is truncated, if necessary, by removing the
oldest entries, to contain no more than that number of
lines. The default value is 500. The history file is
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User Commands BASH(1)
also truncated to this size after writing it when an
interactive shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which
command lines should be saved on the history list.
Each pattern is anchored at the beginning of the line
and must match the complete line (no implicit `*' is
appended). Each pattern is tested against the line
after the checks specified by HISTCONTROL are applied.
In addition to the normal shell pattern matching char-
acters, `&' matches the previous history line. `&' may
be escaped using a backslash; the backslash is removed
before attempting a match. The second and subsequent
lines of a multi-line compound command are not tested,
and are added to the history regardless of the value of
HISTIGNORE.
HISTSIZE
The number of commands to remember in the command his-
tory (see HISTORY below). The default value is 500.
HISTTIMEFORMAT
If this variable is set and not null, its value is used
as a format string for strftime(3) to print the time
stamp associated with each history entry displayed by
the history builtin. If this variable is set, time
stamps are written to the history file so they may be
preserved across shell sessions. This uses the history
comment character to distinguish timestamps from other
history lines.
HOME The home directory of the current user; the default
argument for the cd builtin command. The value of this
variable is also used when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as
/etc/hosts that should be read when the shell needs to
complete a hostname. The list of possible hostname
completions may be changed while the shell is running;
the next time hostname completion is attempted after
the value is changed, bash adds the 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 pos-
sible hostname completions. When HOSTFILE is unset,
the hostname list is cleared.
IFS The Internal Field Separator that is used for word
splitting after expansion and to split lines into words
with the read builtin command. The default value is
``<space><tab><newline>''.
IGNOREEOF
Controls the action of an interactive shell on receipt
of an EOF character as the sole input. If set, the
value is the number of consecutive EOF characters which
must be typed as the first characters on an input line
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User Commands BASH(1)
before bash exits. If the variable exists but does not
have a numeric value, or has no value, the default
value is 10. If it does not exist, EOF signifies the
end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding
the default of ~/.inputrc (see READLINE below).
LANG Used to determine the locale category for any category
not specifically selected with a variable starting with
LC_.
LC_ALL
This variable overrides the value of LANG and any other
LC_ variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when
sorting the results of pathname expansion, and deter-
mines the behavior of range expressions, equivalence
classes, and collating sequences within pathname expan-
sion and pattern matching.
LC_CTYPE
This variable determines the interpretation of charac-
ters and the behavior of character classes within path-
name expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate
double-quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for
number formatting.
LINES
Used by the select builtin command to determine the
column length for printing selection lists. Automati-
cally set upon receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAIL-
PATH variable is not set, bash informs the user of the
arrival of mail in the specified file.
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 pri-
mary prompt. If this variable is unset, or set to a
value that is not a number greater than or equal to
zero, the shell disables mail checking.
MAILPATH
A colon-separated list of file names to be checked for
mail. The message to be printed when mail arrives in a
particular file may be specified by separating the file
name from the message with a `?'. When used in the
text of the message, $_ expands to the name of the cur-
rent mailfile. Example:
MAILPATH='/var/mail/bfox?"You have
mail":~/shell-mail?"$_ has mail!"'
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User Commands BASH(1)
Bash supplies a default value for this variable, but
the location of the user mail files that it uses is
system dependent (e.g., /var/mail/$USER).
OPTERR
If set to the value 1, bash displays error messages
generated by the getopts builtin command (see SHELL
BUILTIN COMMANDS below). OPTERR is initialized to 1
each time the shell is invoked or a shell script is
executed.
PATH The search path for commands. It is a colon-separated
list of directories in which the shell looks for com-
mands (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/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin''.
POSIXLY_CORRECT
If this variable is in the environment when bash
starts, the shell enters posix mode before reading the
startup files, as if the --posix invocation option had
been supplied. If it is set while the shell is run-
ning, bash enables posix mode, as if the command set -o
posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to
issuing each primary prompt.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used
as the number of trailing directory components to
retain when expanding the \w and \W prompt string
escapes (see PROMPTING below). Characters removed are
replaced with an ellipsis.
PS1 The value of this parameter is expanded (see PROMPTING
below) and used as the primary prompt string. The
default value is ``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and
used as the secondary prompt string. The default is
``> ''.
PS3 The value of this parameter is used as the prompt for
the select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and
the value is printed before each command bash displays
during an execution trace. The first character of PS4
is replicated multiple times, as necessary, to indicate
multiple levels of indirection. The default is ``+ ''.
SHELL
The full pathname to the shell is kept in this environ-
ment variable. If it is not set when the shell starts,
bash assigns to it the full pathname of the current
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User Commands BASH(1)
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 dis-
played. The % character introduces an escape sequence
that is expanded to a time value or other information.
The escape sequences and their meanings are as follows;
the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system
mode.
%P The CPU percentage, computed as (%U + %S) /
%R.
The optional p is a digit specifying the precision, the
number of fractional digits after a decimal point. A
value of 0 causes no decimal point or fraction to be
output. At most three places after the decimal point
may be specified; values of p greater than 3 are
changed to 3. If p is not specified, the value 3 is
used.
The optional l specifies a longer format, including
minutes, of the form MMmSS.FFs. The value of p deter-
mines whether or not the fraction is included.
If this variable is not set, bash acts as if it had the
value $'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If the
value is null, no timing information is displayed. A
trailing newline is added when the format string is
displayed.
TMOUT
If set to a value greater than zero, TMOUT is treated
as the default timeout for the read builtin. The
select command terminates if input does not arrive
after TMOUT seconds when input is coming from a termi-
nal. In an interactive shell, the value is interpreted
as the number of seconds to wait for input after issu-
ing the primary prompt. Bash terminates after waiting
for that number of seconds if input does not arrive.
TMPDIR
If set, Bash uses its value as the name of a directory
in which Bash creates temporary files for the shell's
use.
auto_resume
This variable controls how the shell interacts with the
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User Commands BASH(1)
user and job control. If this variable is set, single
word simple commands without redirections are treated
as candidates for resumption of an existing stopped
job. There is no ambiguity allowed; if there is more
than one job beginning with the string typed, the job
most recently accessed is selected. The name of a
stopped job, in this context, is the command line used
to start it. If set to the value exact, the string
supplied must match the name of a stopped job exactly;
if set to substring, the string supplied needs to match
a substring of the name of a stopped job. The sub-
string value provides functionality analogous to the %?
job identifier (see JOB CONTROL below). If set to any
other value, the supplied string must be a prefix of a
stopped job's name; this provides functionality analo-
gous to the %string job identifier.
histchars
The two or three characters which control history
expansion and tokenization (see HISTORY EXPANSION
below). The first character is the history expansion
character, the character which signals the start of a
history expansion, normally `!'. The second character
is the quick substitution character, which is used as
shorthand for re-running the previous command entered,
substituting one string for another in the command.
The default is `^'. The optional third character is
the character which indicates that the remainder of the
line is a comment when found as the first character of
a word, normally `#'. The history comment character
causes history substitution to be skipped for the
remaining words on the line. It does not necessarily
cause the shell parser to treat the rest of the line as
a comment.
Arrays
Bash provides one-dimensional indexed and associative array
variables. Any variable may be used as an indexed array;
the declare builtin will explicitly declare an array. There
is no maximum limit on the size of an array, nor any
requirement that members be indexed or assigned contigu-
ously. Indexed arrays are referenced using integers
(including arithmetic expressions) and are zero-based;
associative arrays are referenced using arbitrary strings.
An indexed array is created automatically if any variable is
assigned to using the syntax name[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[sub-
script] is also accepted; the subscript is ignored.
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User Commands BASH(1)
Associative arrays are created using declare -A name.
Attributes may be specified for an array variable using the
declare and readonly builtins. Each attribute applies to
all members of an array.
Arrays are assigned to using compound assignments of the
form name=(value1 ... valuen), where each value is of the
form [subscript]=string. Indexed array assignments do not
require the bracket and subscript. When assigning to
indexed arrays, if the optional brackets and subscript are
supplied, that index is assigned to; otherwise the index of
the element assigned is the last index assigned to by the
statement plus one. Indexing starts at zero.
When assigning to an associative array, the subscript is
required.
This syntax is also accepted by the declare builtin. Indi-
vidual array elements may be assigned to using the name[sub-
script]=value syntax introduced above.
Any element of an array may be referenced using ${name[sub-
script]}. The braces are required to avoid conflicts with
pathname expansion. If subscript is @ or *, the word
expands to all members of name. These subscripts differ
only when the word appears within double quotes. If the
word is double-quoted, ${name[*]} expands to a single word
with the value of each array member separated by the first
character of the IFS special variable, and ${name[@]}
expands each element of name to a separate word. When there
are no array members, ${name[@]} expands to nothing. If the
double-quoted expansion occurs within a word, the expansion
of the first parameter is joined with the beginning part of
the original word, and the expansion of the last parameter
is joined with the last part of the original word. This is
analogous to the expansion of the special parameters * and @
(see Special Parameters above). ${#name[subscript]} expands
to the length of ${name[subscript]}. If subscript is * or
@, the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equiva-
lent to referencing the array with a subscript of 0.
An array variable is considered set if a subscript has been
assigned a value. The null string is a valid value.
The unset builtin is used to destroy arrays. unset
name[subscript] destroys the array element at index sub-
script. Care must be taken to avoid unwanted side effects
caused by pathname expansion. unset name, where name is an
array, or unset name[subscript], where subscript is * or @,
removes the entire array.
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The declare, local, and readonly builtins each accept a -a
option to specify an indexed array and a -A option to spec-
ify an associative array. The read builtin accepts a -a
option to assign a list of words read from the standard
input to an array. The set and declare builtins display
array values in a way that allows them to be reused as
assignments.
EXPANSION
Expansion is performed on the command line after it has been
split into words. There are seven kinds of expansion per-
formed: brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic expan-
sion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expan-
sion, parameter, variable and arithmetic expansion and com-
mand substitution (done in a left-to-right fashion), word
splitting, and pathname expansion.
On systems that can support it, there is an additional
expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion
can change the number of words of the expansion; other
expansions expand a single word to a single word. The only
exceptions to this are the expansions of "$@" and
"${name[@]}" as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings
may be generated. This mechanism is similar to pathname
expansion, but the filenames generated need not exist. Pat-
terns to be brace expanded take the form of an optional pre-
amble, 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 pre-
fixed to each string contained within the braces, and the
postscript is then appended to each resulting string,
expanding left to right.
Brace expansions may be nested. The results of each
expanded string are not sorted; left to right order is pre-
served. For example, a{d,c,b}e expands into `ade ace abe'.
A sequence expression takes the form {x..y[..incr]}, where x
and y are either integers or single characters, and incr, an
optional increment, is an integer. When integers are sup-
plied, the expression expands to each number between x and
y, inclusive. Supplied integers may be prefixed with 0 to
force each term to have the same width. When either x or y
begins with a zero, the shell attempts to force all
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generated terms to contain the same number of digits, zero-
padding where necessary. When characters are supplied, the
expression expands to each character lexicographically
between x and y, inclusive. Note that both x and y must be
of the same type. When the increment is supplied, it is
used as the difference between each term. The default
increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions,
and any characters special to other expansions are preserved
in the result. It is strictly textual. Bash does not apply
any syntactic interpretation to the context of the expansion
or the text between the braces.
A correctly-formed brace expansion must contain unquoted
opening and closing braces, and at least one unquoted comma
or a valid sequence expression. Any incorrectly formed
brace expansion is left unchanged. A { or , may be quoted
with a backslash to prevent its being considered part of a
brace expression. To avoid conflicts with parameter expan-
sion, the string ${ is not considered eligible for brace
expansion.
This construct is typically used as shorthand when the com-
mon prefix of the strings to be generated is longer than in
the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with
historical versions of sh. sh does not treat opening or
closing braces specially when they appear as part of a word,
and preserves them in the output. Bash removes braces from
words as a consequence of brace expansion. For example, a
word entered to sh as file{1,2} appears identically in the
output. The same word is output as file1 file2 after expan-
sion by bash. If strict compatibility with sh is desired,
start bash with the +B option or disable brace expansion
with the +B option to the set command (see SHELL BUILTIN
COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'), all
of the characters preceding the first unquoted slash (or all
characters, if there is no unquoted slash) are considered a
tilde-prefix. If none of the 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
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home directory of the user executing the shell is substi-
tuted 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 vari-
able PWD replaces the tilde-prefix. If the tilde-prefix is
a `~-', the value of the shell variable OLDPWD, if it is
set, is substituted. If the characters following the tilde
in the tilde-prefix consist of a number N, optionally pre-
fixed by a `+' or a `-', the tilde-prefix is replaced with
the corresponding element from the directory stack, as it
would be displayed by the dirs builtin invoked with the
tilde-prefix as an argument. If the characters following
the tilde in the tilde-prefix consist of a number without a
leading `+' or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion fails,
the word is unchanged.
Each variable assignment is checked for unquoted tilde-pre-
fixes immediately following a : or the first =. In these
cases, tilde expansion is also performed. Consequently, one
may use file names with tildes in assignments to PATH, MAIL-
PATH, and CDPATH, and the shell assigns the expanded value.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be
expanded from characters immediately following it which
could be interpreted as part of the name.
When braces are used, the matching ending brace is the first
`}' not escaped by a backslash or within a quoted string,
and not within an embedded arithmetic expansion, command
substitution, or parameter expansion.
${parameter}
The value of parameter is substituted. The braces are
required when parameter is a positional parameter with
more than one digit, or when parameter is followed by a
character which is not to be interpreted as part of its
name.
If the first character of parameter is an exclamation point
(!), a level of variable indirection is introduced. Bash
uses the value of the variable formed from the rest of
parameter as the name of the variable; this variable is then
expanded and that value is used in the rest of the substitu-
tion, rather than the value of parameter itself. This is
known as indirect expansion. The exceptions to this are the
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User Commands BASH(1)
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 expan-
sion, parameter expansion, command substitution, and arith-
metic expansion.
When not performing substring expansion, using the forms
documented below, bash tests for a parameter that is unset
or null. Omitting the colon results in a test only for a
parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the
expansion of word is substituted. Otherwise, the value
of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null,
the expansion of word is assigned to parameter. The
value of parameter is then substituted. Positional
parameters and special parameters may not be assigned
to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null
or unset, the expansion of word (or a message to that
effect if word is not present) is written to the stan-
dard error and the shell, if it is not interactive,
exits. Otherwise, the value of parameter is substi-
tuted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset,
nothing is substituted, otherwise the expansion of word
is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length charac-
ters of parameter starting at the character specified
by offset. If length is omitted, expands to the sub-
string of parameter starting at the character specified
by offset. length and offset are arithmetic expres-
sions (see ARITHMETIC EVALUATION below). length must
evaluate to a number greater than or equal to zero. If
offset evaluates to a number less than zero, the value
is used as an offset from the end of the value of
parameter. If parameter is @, the result is length
positional parameters beginning at offset. If parame-
ter is an indexed array name subscripted by @ or *, the
result is the length members of the array beginning
with ${parameter[offset]}. A negative offset is taken
relative to one greater than the maximum index of the
specified array. Substring expansion applied to an
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User Commands BASH(1)
associative array produces undefined results. Note
that a negative offset must be separated from the colon
by at least one space to avoid being confused with the
:- expansion. Substring indexing is zero-based unless
the positional parameters are used, in which case the
indexing starts at 1 by default. If offset is 0, and
the positional parameters are used, $0 is prefixed to
the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of vari-
ables 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 expan-
sion appears within double quotes, each key expands to
a separate word.
${#parameter}
Parameter length. The length in characters of the
value of parameter is substituted. If parameter is *
or @, the value substituted is the number of positional
parameters. If parameter is an array name subscripted
by * or @, the value substituted is the number of ele-
ments in the array.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded
to produce a pattern just as in pathname expansion. If
the pattern matches the beginning of the value of
parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching
pattern (the ``#'' case) or the longest matching pat-
tern (the ``##'' case) deleted. If parameter is @ or
*, the pattern removal operation is applied to each
positional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable sub-
scripted 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}
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Remove matching suffix pattern. The word is expanded
to produce a pattern just as in pathname expansion. If
the pattern matches a trailing portion of the expanded
value of parameter, then the result of the expansion is
the expanded value of parameter with the shortest
matching pattern (the ``%'' case) or the longest match-
ing pattern (the ``%%'' case) deleted. If parameter is
@ or *, the pattern removal operation is applied to
each positional parameter in turn, and the expansion is
the resultant list. If parameter is an array variable
subscripted with @ or *, the pattern removal operation
is applied to each member of the array in turn, and the
expansion is the resultant list.
${parameter/pattern/string}
Pattern substitution. The pattern is expanded to pro-
duce a pattern just as in pathname expansion. Parame-
ter is expanded and the longest match of pattern
against its value is replaced with string. If pattern
begins with /, all matches of pattern are replaced with
string. Normally only the first match is replaced. If
pattern begins with #, it must match at the beginning
of the expanded value of parameter. If pattern begins
with %, it must match at the end of the expanded value
of parameter. If string is null, matches of pattern
are deleted and the / following pattern may be omitted.
If parameter is @ or *, the substitution operation is
applied to each positional parameter in turn, and the
expansion is the resultant list. If parameter is an
array variable subscripted with @ or *, the substitu-
tion operation is applied to each member of the array
in turn, and the expansion is the resultant list.
${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of
alphabetic characters in parameter. The pattern is
expanded to produce a pattern just as in pathname
expansion. The ^ operator converts lowercase letters
matching pattern to uppercase; the , operator converts
matching uppercase letters to lowercase. The ^^ and ,,
expansions convert each matched character in the
expanded value; the ^ and , expansions match and con-
vert only the first character in the expanded value..
If pattern is omitted, it is treated like a ?, which
matches every character. If parameter is @ or *, the
case modification operation is applied to each posi-
tional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable sub-
scripted with @ or *, the case modification operation
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User Commands BASH(1)
is applied to each member of the array in turn, and the
expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to
replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and replac-
ing the command substitution with the standard output of the
command, with any trailing newlines deleted. Embedded new-
lines are not deleted, but they may be removed during word
splitting. The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed
by $, `, or \. The first backquote not preceded by a back-
slash terminates the command substitution. When using the
$(command) form, all characters between the parentheses make
up the command; none are treated specially.
Command substitutions may be nested. To nest when using the
backquoted form, escape the inner backquotes with back-
slashes.
If the substitution appears within double quotes, word
splitting and pathname expansion are not performed on the
results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic
expression and the substitution of the result. The format
for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double
quotes, but a double quote inside the parentheses is not
treated specially. All tokens in the expression undergo
parameter expansion, string expansion, command substitution,
and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed
below under ARITHMETIC EVALUATION. If expression is
invalid, bash prints a message indicating failure and no
substitution occurs.
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Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open
files. It takes the form of <(list) or >(list). The
process list is run with its input or output connected to a
FIFO or some file in /dev/fd. The name of this file is
passed as an argument to the current command as the result
of the expansion. If the >(list) form is used, writing to
the file will provide input for list. If the <(list) form
is used, the file passed as an argument should be read to
obtain the output of list.
When available, process substitution is performed simultane-
ously with parameter and variable expansion, command substi-
tution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur
within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and
splits the results of the other expansions into words on
these characters. If IFS is unset, or its value is exactly
<space><tab><newline>, the default, then sequences of
<space>, <tab>, and <newline> at the beginning and end of
the results of the previous expansions are ignored, and any
sequence of IFS characters not at the beginning or end
serves to delimit words. If IFS has a value other than the
default, then sequences of the whitespace characters space
and tab are ignored at the beginning and end of the word, as
long as the whitespace character is in the value of IFS (an
IFS whitespace character). Any character in IFS that is not
IFS whitespace, along with any adjacent IFS whitespace char-
acters, delimits a field. A sequence of IFS whitespace
characters is also treated as a delimiter. If the value of
IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained. Unquoted
implicit null arguments, resulting from the expansion of
parameters that have no values, are removed. If a parameter
with no value is expanded within double quotes, a null argu-
ment results and is retained.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set,
bash scans each word for the characters *, ?, and [. If one
of these characters appears, then the word is regarded as a
pattern, and replaced with an alphabetically sorted list of
file names matching the pattern. If no matching file names
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User Commands BASH(1)
are found, and the shell option nullglob is not enabled, the
word is left unchanged. If the nullglob option is set, and
no matches are found, the word is removed. If the failglob
shell option is set, and no matches are found, an error mes-
sage is printed and the command is not executed. If the
shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters. When a
pattern is used for pathname expansion, the character ``.''
at the start of a name or immediately following a slash must
be matched explicitly, unless the shell option dotglob is
set. When matching a pathname, the slash character must
always be matched explicitly. In other cases, the ``.''
character is not treated specially. See the description of
shopt below under SHELL BUILTIN COMMANDS for a description
of the nocaseglob, nullglob, failglob, and dotglob shell
options.
The GLOBIGNORE shell variable may be used to restrict the
set of file names matching a pattern. If GLOBIGNORE is set,
each matching file name that also matches one of the pat-
terns in GLOBIGNORE is removed from the list of matches.
The file names ``.'' 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 file names beginning with a ``.''
will match. To get the old behavior of ignoring file names
beginning with a ``.'', make ``.*'' one of the patterns in
GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE
is unset.
Pattern Matching
Any character that appears in a pattern, other than the spe-
cial pattern characters described below, matches itself.
The NUL character may not occur in a pattern. A backslash
escapes the following character; the escaping backslash is
discarded when matching. The special pattern characters
must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When
the globstar shell option is enabled, and * is used in
a pathname expansion context, two adjacent *s used as a
single pattern will match all files and zero or more
directories and subdirectories. If followed by a /,
two adjacent *s will match only directories and subdi-
rectories.
? Matches any single character.
[...]
Matches any one of the enclosed characters. A pair of
characters separated by a hyphen denotes a range
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User Commands BASH(1)
expression; any character that sorts between those two
characters, inclusive, using the current locale's col-
lating sequence and character set, is matched. If the
first character following the [ is a ! or a ^ then any
character not enclosed is matched. The sorting order
of characters in range expressions is determined by the
current locale and the value of the LC_COLLATE shell
variable, if set. A - may be matched by including it
as the first or last character in the set. A ] may be
matched by including it as the first character in the
set.
Within [ and ], character classes can be specified
using the syntax [:class:], where class is one of the
following classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print
punct space upper word xdigit
A character class matches any character belonging to
that class. The word character class matches letters,
digits, and the character _.
Within [ and ], an equivalence class can be specified
using the syntax [=c=], which matches all characters
with the same collation weight (as defined by the cur-
rent locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the col-
lating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are
recognized. In the following description, a pattern-list is
a list of one or more patterns separated by a |. Composite
patterns may be formed using one or more of the following
sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given pat-
terns
*(pattern-list)
Matches zero or more occurrences of the given pat-
terns
+(pattern-list)
Matches one or more occurrences of the given pat-
terns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
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After the preceding expansions, all unquoted occurrences of
the characters \, ', and " that did not result from one of
the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be
redirected using a special notation interpreted by the
shell. Redirection may also be used to open and close files
for the current shell execution environment. The following
redirection operators may precede or appear anywhere within
a simple command or may follow a command. Redirections are
processed in the order they appear, from left to right.
Each redirection that may be preceded by a file descriptor
number may instead be preceded by a word of the form {var-
name}. In this case, for each redirection operator except
>&- and <&-, the shell will allocate a file descriptor
greater than 10 and assign it to varname. If >&- or <&- is
preceded by {varname}, the value of varname defines the file
descriptor to close.
In the following descriptions, if the file descriptor number
is omitted, and the first character of the redirection oper-
ator is <, the redirection refers to the standard input
(file descriptor 0). If the first character of the redi-
rection operator is >, the redirection refers to the stan-
dard 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 expansion, command
substitution, arithmetic expansion, quote removal, pathname
expansion, and word splitting. If it expands to more than
one word, bash reports an error.
Note that the order of redirections is significant. For
example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file
dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because
the standard error was duplicated from the standard output
before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used
in redirections, as described in the following table:
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User Commands BASH(1)
/dev/fd/fd
If fd is a valid integer, file descriptor fd is
duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address,
and port is an integer port number or service
name, bash attempts to open a TCP connection to
the corresponding socket.
/dev/udp/host/port
If host is a valid hostname or Internet address,
and port is an integer port number or service
name, bash attempts to open a UDP connection to
the corresponding socket.
A failure to open or create a file causes the redirection to
fail.
Redirections using file descriptors greater than 9 should be
used with care, as they may conflict with file descriptors
the shell uses internally.
Redirecting Input
Redirection of input causes the file whose name results from
the expansion of word to be opened for reading on file
descriptor n, or the standard input (file descriptor 0) if n
is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results
from the expansion of word to be opened for writing on file
descriptor n, or the standard output (file descriptor 1) if
n is not specified. If the file does not exist it is cre-
ated; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option
to the set builtin has been enabled, the redirection will
fail if the file whose name results from the expansion of
word exists and is a regular file. If the redirection
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User Commands BASH(1)
operator is >|, or the redirection operator is > and the
noclobber option to the set builtin command is not enabled,
the redirection is attempted even if the file named by word
exists.
Appending Redirected Output
Redirection of output in this fashion causes the file whose
name results from the expansion of word to be opened for
appending on file descriptor n, or the standard output (file
descriptor 1) if n is not specified. If the file does not
exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
This construct allows both the standard output (file
descriptor 1) and the standard error output (file descriptor
2) to be redirected to the file whose name is the expansion
of word.
There are two formats for redirecting standard output and
standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semanti-
cally equivalent to
>word 2>&1
Appending Standard Output and Standard Error
This construct allows both the standard output (file
descriptor 1) and the standard error output (file descriptor
2) to be appended to the file whose name is the expansion of
word.
The format for appending standard output and standard error
is:
&>>word
This is semantically equivalent to
>>word 2>&1
Here Documents
This type of redirection instructs the shell to read input
from the current source until a line containing only
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User Commands BASH(1)
delimiter (with no trailing blanks) is seen. All of the
lines read up to that point are then used as the standard
input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic
expansion, or pathname expansion is performed on word. If
any characters in word are quoted, the delimiter is the
result of quote removal on word, and the lines in the here-
document are not expanded. If word is unquoted, all lines
of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the lat-
ter case, the character sequence \<newline> is ignored, and
\ must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab
characters are stripped from input lines and the line con-
taining delimiter. This allows here-documents within shell
scripts to be indented in a natural fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its
standard input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word
expands to one or more digits, the file descriptor denoted
by n is made to be a copy of that file descriptor. If the
digits in word do not specify a file descriptor open for
input, a redirection error occurs. If word evaluates to -,
file descriptor n is closed. If n is not specified, the
standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If
n is not specified, the standard output (file descriptor 1)
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is used. If the digits in word do not specify a file
descriptor open for output, a redirection error occurs. As
a special case, if n is omitted, and word does not expand to
one or more digits, the standard output and standard error
are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the
standard input (file descriptor 0) if n is not specified.
digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the
standard output (file descriptor 1) if n is not specified.
Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be
opened for both reading and writing on file descriptor n, or
on file descriptor 0 if n is not specified. If the file
does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when it
is used as the first word of a simple command. The shell
maintains a list of aliases that may be set and unset with
the alias and unalias builtin commands (see SHELL BUILTIN
COMMANDS below). The first word of each simple command, if
unquoted, is checked to see if it has an alias. If so, that
word is replaced by the text of the alias. The characters
/, $, `, and = and any of the shell metacharacters or quot-
ing 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 sec-
ond time. This means that one may alias ls to ls -F, for
instance, and bash does not try to recursively expand the
replacement text. If the last character of the alias value
is a blank, then the next command word following the alias
is also checked for alias expansion.
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Aliases are created and listed with the alias command, and
removed with the unalias command.
There is no mechanism for using arguments in the replacement
text. If arguments are needed, a shell function should be
used (see FUNCTIONS below).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using shopt
(see the description of shopt under SHELL BUILTIN COMMANDS
below).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one complete
line of input before executing any of the commands on that
line. Aliases are expanded when a command is read, not when
it is executed. Therefore, an alias definition appearing on
the same line as another command does not take effect until
the next line of input is read. The commands following the
alias definition on that line are not affected by the new
alias. This behavior is also an issue when functions are
executed. Aliases are expanded when a function definition
is read, not when the function is executed, because a func-
tion definition is itself a compound command. As a conse-
quence, aliases defined in a function are not available
until after that function is executed. To be safe, always
put alias definitions on a separate line, and do not use
alias in compound commands.
For almost every purpose, aliases are superseded by shell
functions.
FUNCTIONS
A shell function, defined as described above under SHELL
GRAMMAR, stores a series of commands for later execution.
When the name of a shell function is used as a simple com-
mand name, the list of commands associated with that func-
tion name is executed. Functions are executed in the con-
text of the current shell; no new process is created to
interpret them (contrast this with the execution of a shell
script). When a function is executed, the arguments to the
function become the positional parameters during its execu-
tion. The special parameter # is updated to reflect the
change. Special parameter 0 is unchanged. The first ele-
ment 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
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attribute (see the description of the declare builtin below)
or the -o functrace shell option has been enabled with the
set builtin (in which case all functions inherit the DEBUG
and RETURN traps), and the ERR trap is not inherited unless
the -o errtrace shell option has been enabled.
Variables local to the function may be declared with the
local builtin command. Ordinarily, variables and their val-
ues are shared between the function and its caller.
If the builtin command return is executed in a function, the
function completes and execution resumes with the next com-
mand after the function call. Any command associated with
the RETURN trap is executed before execution resumes. When
a function completes, the values of the positional parame-
ters and the special parameter # are restored to the values
they had prior to the function's execution.
Function names and definitions may be listed with the -f
option to the declare or typeset builtin commands. The -F
option to declare or typeset will list the function names
only (and optionally the source file and line number, if the
extdebug shell option is enabled). Functions may be
exported so that subshells automatically have them defined
with the -f option to the export builtin. A function defi-
nition may be deleted using the -f option to the unset
builtin. Note that shell functions and variables with the
same name may result in multiple identically-named entries
in the environment passed to the shell's children. Care
should be taken in cases where this may cause a problem.
Functions may be recursive. No limit is imposed on the num-
ber of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated,
under certain circumstances (see the let and declare builtin
commands and Arithmetic Expansion). Evaluation is done in
fixed-width integers with no check for overflow, though
division by 0 is trapped and flagged as an error. The oper-
ators and their precedence, associativity, and values are
the same as in the C language. The following list of opera-
tors 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
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* / %
multiplication, division, remainder
+ - addition, subtraction
<< >>
left and right bitwise shifts
<= >= < >
comparison
== !=
equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion
is performed before the expression is evaluated. Within an
expression, shell variables may also be referenced by name
without using the parameter expansion syntax. A shell vari-
able that is null or unset evaluates to 0 when referenced by
name without using the parameter expansion syntax. The
value of a variable is evaluated as an arithmetic expression
when it is referenced, or when a variable which has been
given the integer attribute using declare -i is assigned a
value. A null value evaluates to 0. A shell variable need
not have its integer attribute turned on to be used in an
expression.
Constants with a leading 0 are interpreted as octal numbers.
A leading 0x or 0X denotes hexadecimal. Otherwise, numbers
take the form [base#]n, where 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. The digits greater than 9 are represented by the low-
ercase letters, the uppercase letters, @, and _, in that
order. If base is less than or equal to 36, lowercase and
uppercase letters may be used interchangeably to represent
numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-expres-
sions 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
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and perform string and arithmetic comparisons. Expressions
are formed from the following unary or binary primaries. If
any file argument to one of the primaries is of the form
/dev/fd/n, then file descriptor n is checked. If the file
argument to one of the primaries is one of /dev/stdin,
/dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2,
respectively, is checked.
Unless otherwise specified, primaries that operate on files
follow symbolic links and operate on the target of the link,
rather than the link itself.
When used with [[, The < and > operators sort lexicographi-
cally using the current locale.
-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 ter-
minal.
-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.
-O file
True if file exists and is owned by the effective user
id.
-G file
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True if file exists and is owned by the effective group
id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it was
last read.
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.
file1 -ef file2
True if file1 and file2 refer to the same device and
inode numbers.
-o optname
True if shell option optname is enabled. See the list
of options under the description of the -o option to
the set builtin below.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with
the test command for POSIX conformance.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
string1 > string2
True if string1 sorts after string2 lexicographically.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These
arithmetic binary operators return true if arg1 is
equal to, not equal to, less than, less than or equal
to, greater than, or greater than or equal to arg2,
respectively. Arg1 and arg2 may be positive or nega-
tive integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the
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following expansions, assignments, and redirections, from
left to right.
1. The words that the parser has marked as variable
assignments (those preceding the command name) and
redirections are saved for later processing.
2. The words that are not variable assignments or redirec-
tions are expanded. If any words remain after expan-
sion, 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 RE-
DIRECTION.
4. The text after the = in each variable assignment under-
goes tilde expansion, parameter expansion, command sub-
stitution, arithmetic expansion, and quote removal
before being assigned to the variable.
If no command name results, the variable assignments affect
the current shell environment. Otherwise, the variables are
added to the environment of the executed command and do not
affect the current shell environment. If any of the assign-
ments 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 com-
mand substitutions, the command exits with a status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in
a simple command and an optional list of arguments, the fol-
lowing actions are taken.
If the command name contains no slashes, the shell attempts
to locate it. If there exists a shell function by that
name, that function is invoked as described above in FUNC-
TIONS. 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
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for a directory containing an executable file by that name.
Bash uses a hash table to remember the full pathnames of
executable files (see hash under SHELL BUILTIN COMMANDS
below). A full search of the directories in PATH is per-
formed only if the command is not found in the hash table.
If the search is unsuccessful, the shell searches for a
defined shell function named command_not_found_handle. If
that function exists, it is invoked with the original com-
mand and the original command's arguments as its arguments,
and the function's exit status becomes the exit status of
the shell. If that function is not defined, the shell
prints an error message and returns an exit status of 127.
If the search is successful, or if the command name contains
one or more slashes, the shell executes the named program in
a separate 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 exe-
cutable format, and the file is not a directory, it is
assumed to be a shell script, a file containing shell com-
mands. A subshell is spawned to execute it. This subshell
reinitializes itself, so that the effect is as if a new
shell had been invoked to handle the script, with the excep-
tion 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 them-
selves. The arguments to the interpreter consist of a sin-
gle optional argument following the interpreter name on the
first line of the program, followed by the name of the pro-
gram, followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of
the following:
o open files inherited by the shell at invocation, as
modified by redirections supplied to the exec builtin
o the current working directory as set by cd, pushd, or
popd, or inherited by the shell at invocation
o the file creation mode mask as set by umask or inher-
ited from the shell's parent
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o current traps set by trap
o shell parameters that are set by variable assignment or
with set or inherited from the shell's parent in the
environment
o shell functions defined during execution or inherited
from the shell's parent in the environment
o options enabled at invocation (either by default or
with command-line arguments) or by set
o options enabled by shopt
o shell aliases defined with alias
o various process IDs, including those of background
jobs, the value of $$, and the value of PPID
When a simple command other than a builtin or shell function
is to be executed, it is invoked in a separate execution
environment that consists of the following. Unless other-
wise noted, the values are inherited from the shell.
o the shell's open files, plus any modifications and
additions specified by redirections to the command
o the current working directory
o the file creation mode mask
o shell variables and functions marked for export, along
with variables exported for the command, passed in the
environment
o traps caught by the shell are reset to the values
inherited from the shell's parent, and traps ignored by
the shell are ignored
A command invoked in this separate environment cannot affect
the shell's execution environment.
Command substitution, commands grouped with parentheses, and
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 com-
mands that are invoked as part of a pipeline are also exe-
cuted in a subshell environment. Changes made to the sub-
shell environment cannot affect the shell's execution envi-
ronment.
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Subshells spawned to execute command substitutions inherit
the value of the -e option from the parent shell. When not
in posix mode, Bash clears the -e option in such subshells.
If a command is followed by a & and job control is not
active, the default standard input for the command is the
empty file /dev/null. Otherwise, the invoked command inher-
its 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 environ-
ment. 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 and declare -x commands
allow parameters and functions to be added to and deleted
from the environment. If the value of a parameter in the
environment is modified, the new value becomes part of the
environment, replacing the old. The environment inherited
by any executed command consists of the shell's initial
environment, whose values may be modified in the shell, less
any pairs removed by the unset command, plus any additions
via the export and declare -x commands.
The environment for any simple command or function may be
augmented temporarily by prefixing it with parameter assign-
ments, as described above in PARAMETERS. These assignment
statements affect only the environment seen by that command.
If the -k option is set (see the set builtin command below),
then all parameter assignments are placed in the environment
for a command, not just those that precede the command name.
When bash invokes an external command, the variable _ is set
to the full file name of the command and passed to that com-
mand 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.
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For the shell's purposes, a command which exits with a zero
exit status has succeeded. An exit status of zero indicates
success. A non-zero exit status indicates failure. When a
command terminates on a fatal signal N, bash uses the value
of 128+N as the exit status.
If a command is not found, the child process created to exe-
cute 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 suc-
cessful, and non-zero (false) if an error occurs while they
execute. All builtins return an exit status of 2 to indi-
cate incorrect usage.
Bash itself returns the exit status of the last command exe-
cuted, unless a syntax error occurs, in which case it exits
with a non-zero value. See also the exit builtin command
below.
SIGNALS
When bash is interactive, in the absence of any traps, it
ignores SIGTERM (so that kill 0 does not kill an interactive
shell), and SIGINT is caught and handled (so that the wait
builtin is interruptible). In all cases, bash ignores
SIGQUIT. If job control is in effect, bash ignores SIGTTIN,
SIGTTOU, and SIGTSTP.
Non-builtin commands run by bash have signal handlers set to
the values inherited by the shell from its parent. When job
control is not in effect, asynchronous commands ignore SIG-
INT and SIGQUIT in addition to these inherited handlers.
Commands run as a result of command substitution ignore the
keyboard-generated job control signals SIGTTIN, SIGTTOU, and
SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before
exiting, an interactive shell resends the SIGHUP to all
jobs, running or stopped. Stopped jobs are sent SIGCONT to
ensure that they receive the SIGHUP. To prevent the shell
from sending the signal to a particular job, it should be
removed from the jobs table with the disown builtin (see
SHELL BUILTIN COMMANDS below) or marked to not receive
SIGHUP using disown -h.
If the huponexit shell option has been set with shopt, bash
sends a SIGHUP to all jobs when an interactive login shell
exits.
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If bash is waiting for a command to complete and receives a
signal for which a trap has been set, the trap will not be
executed until the command completes. When bash is waiting
for an asynchronous command via the wait builtin, the recep-
tion of a signal for which a trap has been set will cause
the wait builtin to return immediately with an exit status
greater than 128, immediately after which the trap is exe-
cuted.
JOB CONTROL
Job control refers to the ability to selectively stop (sus-
pend) the execution of processes and continue (resume) their
execution at a later point. A user typically employs this
facility via an interactive interface supplied jointly by
the operating system kernel's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with
the jobs command. When bash starts a job asynchronously (in
the background), it prints a line that looks like:
[1] 25647
indicating that this job is job number 1 and that the
process ID of the last process in the pipeline associated
with this job is 25647. All of the processes in a single
pipeline are members of the same job. Bash uses the job
abstraction as the basis for job control.
To facilitate the implementation of the user interface to
job control, the operating system maintains the notion of a
current terminal process group ID. Members of this process
group (processes whose process group ID is equal to the cur-
rent terminal process group ID) receive keyboard-generated
signals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose
process group ID differs from the terminal's; such processes
are immune to keyboard-generated signals. Only foreground
processes are allowed to read from or, if the user so speci-
fies with stty tostop, write to the terminal. Background
processes which attempt to read from (write to when stty
tostop is in effect) the terminal are sent a SIGTTIN (SIGT-
TOU) signal by the kernel's terminal driver, which, unless
caught, suspends the process.
If the operating system on which bash is running supports
job control, bash contains facilities to use it. Typing the
suspend character (typically ^Z, Control-Z) while a process
is running causes that process to be stopped and returns
control to bash. Typing the delayed suspend character (typ-
ically ^Y, Control-Y) causes the process to be stopped when
it attempts to read input from the terminal, and control to
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User Commands BASH(1)
be returned to bash. The user may then manipulate the state
of this job, using the bg command to continue it in the
background, the fg command to continue it in the foreground,
or the kill command to kill it. A ^Z takes effect immedi-
ately, and has the additional side effect of causing pending
output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell.
The character % introduces a job specification (jobspec).
Job number n may be referred to as %n. A job may also be
referred to using a prefix of the name used to start it, or
using a substring that appears in its command line. For
example, %ce refers to a stopped ce job. If a prefix
matches more than one job, bash reports an error. Using
%?ce, on the other hand, refers to any job containing the
string ce in its command line. If the substring matches
more than one job, bash reports an error. The symbols %%
and %+ refer to the shell's notion of the current job, which
is the last job stopped while it was in the foreground or
started in the background. The previous job may be refer-
enced using %-. If there is only a single job, %+ and %-
can both be used to refer to that job. In output pertaining
to jobs (e.g., the output of the jobs command), the current
job is always flagged with a +, and the previous job with a
-. A single % (with no accompanying job specification) also
refers to the current job.
Simply naming a job can be used to bring it into the fore-
ground: %1 is a synonym for ``fg %1'', bringing job 1 from
the background into the foreground. Similarly, ``%1 &''
resumes job 1 in the background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not
interrupt any other output. If the -b option to the set
builtin command is enabled, bash reports such changes imme-
diately. Any trap on SIGCHLD is executed for each child
that exits.
If an attempt to exit bash is made while jobs are stopped
(or, if the checkjobs shell option has been enabled using
the shopt builtin, running), the shell prints a warning mes-
sage, and, if the checkjobs option is enabled, lists the
jobs and their statuses. The jobs command may then be used
to inspect their status. If a second attempt to exit is
made without an intervening command, the shell does not
print another warning, and any stopped jobs are terminated.
PROMPTING
When executing interactively, bash displays the primary
prompt PS1 when it is ready to read a command, and the
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secondary prompt PS2 when it needs more input to complete a
command. Bash allows these prompt strings to be customized
by inserting a number of backslash-escaped special charac-
ters 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 for-
mat results in a locale-specific time representa-
tion. The braces are required
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell's terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the
portion following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g.,
2.00.0)
\w the current working directory, with $HOME abbrevi-
ated with a tilde (uses the value of the
PROMPT_DIRTRIM variable)
\W the basename of the current working directory,
with $HOME abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number
nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which
could be used to embed a terminal control sequence
into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually dif-
ferent: 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 num-
ber 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
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substitution, arithmetic expansion, and quote removal, sub-
ject to the value of the promptvars shell option (see the
description of the shopt command under SHELL BUILTIN COM-
MANDS below).
READLINE
This is the library that handles reading input when using an
interactive shell, unless the --noediting option is given at
shell invocation. Line editing is also used when using the
-e option to the read builtin. By default, the line editing
commands are similar to those of emacs. A vi-style line
editing interface is also available. Line editing can be
enabled at any time using the -o emacs or -o vi options to
the set builtin (see SHELL BUILTIN COMMANDS below). To turn
off line editing after the shell is running, use the +o
emacs or +o vi options to the set builtin.
Readline Notation
In this section, the emacs-style notation is used to denote
keystrokes. Control keys are denoted by C-key, e.g., C-n
means Control-N. Similarly, meta keys are denoted by M-key,
so M-x means Meta-X. (On keyboards without a meta key, M-x
means ESC x, i.e., press the Escape key then the x key.
This makes ESC the meta prefix. The combination M-C-x means
ESC-Control-x, or press the Escape key then hold the Control
key while pressing the x key.)
Readline commands may be given numeric arguments, which nor-
mally act as a repeat count. Sometimes, however, it is the
sign of the argument that is significant. Passing a nega-
tive argument to a command that acts in the forward direc-
tion (e.g., kill-line) causes that command to act in a back-
ward direction. Commands whose behavior with arguments
deviates from this are noted below.
When a command is described as killing text, the text
deleted is saved for possible future retrieval (yanking).
The killed text is saved in a kill ring. Consecutive kills
cause the text to be accumulated into one unit, which can be
yanked all at once. Commands which do not kill text sepa-
rate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initializa-
tion file (the inputrc file). The name of this file is
taken from the value of the INPUTRC variable. If that vari-
able is unset, the default is ~/.inputrc. When a program
which uses the readline library starts up, the initializa-
tion file is read, and the key bindings and variables are
set. There are only a few basic constructs allowed in the
readline initialization file. Blank lines are ignored.
Lines beginning with a # are comments. Lines beginning with
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a $ indicate conditional constructs. Other lines denote key
bindings and variable settings.
The default key-bindings may be changed with an inputrc
file. Other programs that use this library may add their
own commands and bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline com-
mand universal-argument.
The following symbolic character names are recognized:
RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and
TAB.
In addition to command names, readline allows keys to be
bound to a string that is inserted when the key is pressed
(a macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc file
is simple. All that is required is the name of the command
or the text of a macro and a key sequence to which it should
be bound. The name may be specified in one of two ways: as a
symbolic key name, possibly with Meta- or Control- prefixes,
or as a key sequence.
When using the form keyname:function-name or macro, keyname
is the name of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function univer-
sal-argument, M-DEL is bound to the function back-
ward-kill-word, and C-o is bound to run the macro expressed
on the right hand side (that is, to insert the text ``> out-
put'' into the line).
In the second form, "keyseq":function-name or macro, keyseq
differs from keyname above in that strings denoting an
entire key sequence may be specified by placing the sequence
within double quotes. Some GNU Emacs style key escapes can
be used, as in the following example, but the symbolic char-
acter names are not recognized.
"\C-u": universal-argument
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"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function univer-
sal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the
text ``Function Key 1''.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
" \" literal "
\' literal '
In addition to the GNU Emacs style escape sequences, a sec-
ond 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 hexa-
decimal value HH (one or two hex digits)
When entering the text of a macro, single or double quotes
must be used to indicate a macro definition. Unquoted text
is assumed to be a function name. In the macro body, the
backslash escapes described above are expanded. Backslash
will quote any other character in the macro text, including
" and '.
Bash allows the current readline key bindings to be dis-
played or modified with the bind builtin command. The edit-
ing mode may be switched during interactive use by using the
-o option to the set builtin command (see SHELL BUILTIN COM-
MANDS below).
Readline Variables
Readline has variables that can be used to further customize
its behavior. A variable may be set in the inputrc file
with a statement of the form
set variable-name value
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Except where noted, readline variables can take the values
On or Off (without regard to case). Unrecognized variable
names are ignored. When a variable value is read, empty or
null values, "on" (case-insensitive), and "1" are equivalent
to On. All other values are equivalent to Off. The vari-
ables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring the
terminal bell. If set to none, readline never rings
the bell. If set to visible, readline uses a visible
bell if one is available. If set to audible, readline
attempts to ring the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control
characters treated specially by the kernel's terminal
driver to their readline equivalents.
comment-begin (``#'')
The string that is inserted when the readline
insert-comment command is executed. This command is
bound to M-# in emacs mode and to # in vi command mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching and
completion in a case-insensitive fashion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list
of possible completions that is displayed without modi-
fication. When set to a value greater than zero, com-
mon prefixes longer than this value are replaced with
an ellipsis when displaying possible completions.
completion-query-items (100)
This determines when the user is queried about viewing
the number of possible completions generated by 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, the user is asked
whether or not he wishes to view them; otherwise they
are simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the
eighth bit set to an ASCII key sequence by stripping
the eighth bit and prefixing an escape character (in
effect, using escape as the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion.
Completion characters will be inserted into the line as
if they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key
bindings similar to emacs or vi. editing-mode can be
set to either emacs or vi.
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echo-control-characters (On)
When set to On, on operating systems that indicate they
support it, readline echoes a character corresponding
to a signal generated from the keyboard.
enable-keypad (Off)
When set to On, readline will try to enable the appli-
cation keypad when it is called. Some systems need
this to enable the arrow keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta
modifier key the terminal claims to support when it is
called. On many terminals, the meta key is used to
send eight-bit characters.
expand-tilde (Off)
If set to on, tilde expansion is performed when read-
line attempts word completion.
history-preserve-point (Off)
If set to on, the history code attempts to place point
at the same location on each history line retrieved
with previous-history or next-history.
history-size (0)
Set the maximum number of history entries saved in the
history list. If set to zero, the number of entries in
the history list is not limited.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for
display, scrolling the input horizontally on a single
screen line when it becomes longer than the screen
width rather than wrapping to a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input
(that is, it will not strip the high bit from the char-
acters it reads), regardless of what the terminal
claims it can support. The name meta-flag is a synonym
for this variable.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an
incremental search without subsequently executing the
character as a command. If this variable has not been
given a value, the characters ESC and C-J will termi-
nate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid
keymap names is emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-command, and vi-insert. vi is
equivalent to vi-command; emacs is equivalent to
emacs-standard. The default value is emacs; the value
of editing-mode also affects the default keymap.
mark-directories (On)
If set to On, completed directory names have a slash
appended.
mark-modified-lines (Off)
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If set to On, history lines that have been modified are
displayed with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links
to directories have a slash appended (subject to the
value of mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match
files whose names begin with a `.' (hidden files) when
performing filename completion, unless the leading `.'
is supplied by the user in the filename to be com-
pleted.
output-meta (Off)
If set to On, readline will display characters with the
eighth bit set directly rather than as a meta-prefixed
escape sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager
to display a screenful of possible completions at a
time.
print-completions-horizontally (Off)
If set to On, readline will display completions with
matches sorted horizontally in alphabetical order,
rather than down the screen.
revert-all-at-newline (Off)
If set to on, readline will undo all changes to history
lines before returning when accept-line is executed.
By default, history lines may be modified and retain
individual undo lists across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion
functions. If set to on, words which have more than
one possible completion cause the matches to be listed
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-ambigu-
ous. If set to on, words which have more than one pos-
sible completion without any possible partial comple-
tion (the possible completions don't share a common
prefix) cause the matches to be listed immediately
instead of ringing the bell.
skip-completed-text (Off)
If set to On, this alters the default completion behav-
ior 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 charac-
ters from the completion that match characters after
point in the word being completed, so portions of the
word following the cursor are not duplicated.
visible-stats (Off)
If set to On, a character denoting a file's type as
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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 con-
ditional compilation features of the C preprocessor which
allows key bindings and variable settings to be performed as
the result of tests. There are four parser directives used.
$if The $if construct allows bindings to be made based on
the editing mode, the terminal being used, or the
application using readline. The text of the test
extends to the end of the line; no characters are
required to isolate it.
mode The mode= form of the $if directive is used to
test whether readline is in emacs or vi mode.
This may be used in conjunction with the set
keymap command, for instance, to set bindings in
the emacs-standard and emacs-ctlx keymaps only if
readline is starting out in emacs mode.
term The term= form may be used to include terminal-
specific key bindings, perhaps to bind the key
sequences output by the terminal's function keys.
The word on the right side of the = is tested
against the both full name of the terminal and the
portion of the terminal name before the first -.
This allows sun to match both sun and sun-cmd, for
instance.
application
The application construct is used to include
application-specific settings. Each program using
the readline library sets the application name,
and an initialization file can test for a particu-
lar value. This could be used to bind key
sequences to functions useful for a specific pro-
gram. For instance, the following command adds a
key sequence that quotes the current or previous
word in Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif
This command, as seen in the previous example, termi-
nates an $if command.
$else
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Commands in this branch of the $if directive are exe-
cuted if the test fails.
$include
This directive takes a single filename as an argument
and reads commands and bindings from that file. For
example, the following directive would read
/etc/bash/inputrc:
$include /etc/bash/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 typ-
ing the search string. As each character of the search
string is typed, readline displays the next entry from the
history matching the string typed so far. An incremental
search requires only as many characters as needed to find
the desired history entry. The characters present in the
value of the isearch-terminators variable are used to termi-
nate an incremental search. If that variable has not been
assigned a value the Escape and Control-J characters will
terminate an incremental search. Control-G will abort an
incremental search and restore the original line. When the
search is terminated, the history entry containing the
search string becomes the current line.
To find other matching entries in the history list, type
Control-S or Control-R as appropriate. This will search
backward or forward in the history for the next entry match-
ing the search string typed so far. Any other key sequence
bound to a readline command will terminate the search and
execute that command. For instance, a newline will termi-
nate the search and accept the line, thereby executing the
command from the history list.
Readline remembers the last incremental search string. If
two Control-Rs are typed without any intervening characters
defining a new search string, any remembered search string
is used.
Non-incremental searches read the entire search string
before starting to search for matching history lines. The
search string may be typed by the user or be part of the
contents of the current line.
Readline Command Names
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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 posi-
tion saved by the set-mark command. The text between the
point and mark is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are
composed of alphanumeric characters (letters and dig-
its).
backward-word (M-b)
Move back to the start of the current or previous word.
Words are composed of alphanumeric characters (letters
and digits).
shell-forward-word
Move forward to the end of the next word. Words are
delimited by non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word.
Words are delimited by non-quoted shell metacharacters.
clear-screen (C-l)
Clear the screen leaving the current line at the top of
the screen. With an argument, refresh the current line
without clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If
this line is non-empty, add it to the history list
according to the state of the HISTCONTROL variable. If
the line is a modified history line, then restore the
history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, mov-
ing back in the list.
next-history (C-n)
Fetch the next command from the history list, moving
forward in the list.
beginning-of-history (M-<)
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Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line
currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving
`up' through the history as necessary. This is an
incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving
`down' through the history as necessary. This is an
incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the
current line using a non-incremental search for a
string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incre-
mental search for a string supplied by the user.
history-search-forward
Search forward through the history for the string of
characters between the start of the current line and
the point. This is a non-incremental search.
history-search-backward
Search backward through the history for the string of
characters between the start of the current line and
the point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usu-
ally the second word on the previous line) at point.
With an argument n, insert the nth word from the previ-
ous 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 argu-
ment n is computed, the argument is extracted as if the
"!n" history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the
last word of the previous history entry). With an
argument, behave exactly like yank-nth-arg. Successive
calls to yank-last-arg move back through the history
list, inserting the last argument of each line in turn.
The history expansion facilities are used to extract
the last argument, as if the "!$" history expansion had
been specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias
and history expansion as well as all of the shell word
expansions. See HISTORY EXPANSION below for a descrip-
tion of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
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HISTORY EXPANSION below for a description of history
expansion.
magic-space
Perform history expansion on the current line and
insert a space. See HISTORY EXPANSION below for a
description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See
ALIASES above for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current
line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the
next line relative to the current line from the history
for editing. Any argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and exe-
cute the result as shell commands. Bash attempts to
invoke $VISUAL, $EDITOR, and emacs as the editor, in
that order.
Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the
beginning of the line, there are no characters in the
line, and the last character typed was not bound to
delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a
numeric argument, save the deleted text on the kill
ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cur-
sor is at the end of the line, in which case the char-
acter 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.
transpose-chars (C-t)
Drag the character before point forward over the char-
acter 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)
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Drag the word before point past the word after point,
moving point over that word as well. If point is at
the end of the line, this transposes the last two words
on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a neg-
ative argument, uppercase the previous word, but do not
move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a neg-
ative 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 read-
line() starts in insert mode. In overwrite mode, char-
acters bound to self-insert replace the text at point
rather than pushing the text to the right. Characters
bound to backward-delete-char replace the character
before point with a space. By default, this command is
unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line.
The killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter
where point is.
kill-word (M-d)
Kill from point to the end of the current word, or if
between words, to the end of the next word. Word
boundaries are the same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the
same as those used by backward-word.
shell-kill-word (M-d)
Kill from point to the end of the current word, or if
between words, to the end of the next word. Word
boundaries are the same as those used by shell-for-
ward-word.
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shell-backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the
same as those used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word
boundary. The killed text is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the
slash character as the word boundaries. The killed
text is saved on the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The
word boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The
word boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works
following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or
start a new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this
command is followed by one or more digits, optionally
with a leading minus sign, those digits define the
argument. If the command is followed by digits, exe-
cuting universal-argument again ends the numeric argu-
ment, but is otherwise ignored. As a special case, if
this command is immediately followed by a character
that is neither a digit or minus sign, the argument
count for the next command is multiplied by four. The
argument count is initially one, so executing this
function the first time makes the argument count four,
a second time makes the argument count sixteen, and so
on.
Completing
complete (TAB)
Attempt to perform completion on the text before point.
Bash attempts completion treating the text as a vari-
able (if the text begins with $), username (if the text
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begins with ~), hostname (if the text begins with @),
or command (including aliases and functions) in turn.
If none of these produces a match, filename completion
is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that
would have been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be com-
pleted with a single match from the list of possible
completions. Repeated execution of menu-complete steps
through the list of possible completions, inserting
each match in turn. At the end of the list of comple-
tions, the bell is rung (subject to the setting of
bell-style) and the original text is restored. An
argument of n moves n positions forward in the list of
matches; a negative argument may be used to move back-
ward through the list. This command is intended to be
bound to cAB, but is unbound by default.
menu-complete-krd
Identicalwto menu-complete, but moves backward through
the list of possible completions, as if menu-complete
had been given a negative argument. This command is
unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the
beginning or end of the line (like delete-char). If at
the end of the line, behaves identically to possi-
ble-completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating
it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating
it as a shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point,
treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating
it as a hostname.
possible-hostname-completions (C-x @)
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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 file-
names, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point,
treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing
the text against lines from the history list for possi-
ble completion matches.
dabbrev-expand
Attempt menu completion on the text before point, com-
paring the text against lines from the history list for
possible completion matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of pos-
sible 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 key-
board macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current key-
board 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.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorpo-
rate any bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the termi-
nal's bell (subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the com-
mand that is bound to the corresponding uppercase char-
acter.
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.
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revert-line (M-r)
Undo all changes made to this line. This is like exe-
cuting the undo command enough times to return the line
to its initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is
supplied, the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor posi-
tion is set to the saved position, and the old cursor
position is saved as the mark.
character-search (C-])
A character is read and point is moved to the next
occurrence of that character. A negative count
searches for previous occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous
occurrence of that character. A negative count
searches for subsequent occurrences.
skip-csi-sequence ()
Read enough characters to consume a multi-key sequence
such as those defined for keys like Home and End. Such
sequences begin with a Control Sequence Indicator
(CSI), usually ESC-[. If this sequence is bound to
"\[", keys producing such sequences will have no effect
unless explicitly bound to a readline command, instead
of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline
comment-begin variable is inserted at the beginning of
the current line. If a numeric argument is supplied,
this command acts as a toggle: if the characters at
the beginning of the line do not match the value of
comment-begin, the value is inserted, otherwise the
characters in comment-begin are deleted from the begin-
ning of the line. In either case, the line is accepted
as if a newline had been typed. The default value of
comment-begin causes this command to make the current
line a shell comment. If a numeric argument causes the
comment character to be removed, the line will be exe-
cuted by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for path-
name expansion, with an asterisk implicitly appended.
This pattern is used to generate a list of matching
file names for possible completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for path-
name expansion, and the list of matching file names is
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inserted, replacing the word. If a numeric argument is
supplied, an asterisk is appended before pathname
expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated
by glob-expand-word is displayed, and the line is
redrawn. If a numeric argument is supplied, an aster-
isk is appended before pathname expansion.
dump-functions
Print all of the functions and their key bindings to
the readline output stream. If a numeric argument is
supplied, the output is formatted in such a way that it
can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and their
values to the readline output stream. If a numeric
argument is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros
and the strings they output. If a numeric argument is
supplied, the output is formatted in such a way that it
can be made part of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance
of bash.
Programmable Completion
When word completion is attempted for an argument to a com-
mand for which a completion specification (a compspec) has
been defined using the complete builtin (see SHELL BUILTIN
COMMANDS below), the programmable completion facilities are
invoked.
First, the command name is identified. If the command word
is the empty string (completion attempted at the beginning
of an empty line), any compspec defined with the -E option
to complete is used. If a compspec has been defined for
that command, the compspec is used to generate the list of
possible completions for the word. If the command word is a
full pathname, a compspec for the full pathname is searched
for first. If no compspec is found for the full pathname,
an attempt is made to find a compspec for the portion fol-
lowing the final slash. If those searches to not result in
a compspec, any compspec defined with the -D option to com-
plete is used as the default.
Once a compspec has been found, it is used to generate the
list of matching words. If a compspec is not found, the
default bash completion as described above under Completing
is performed.
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First, the actions specified by the compspec are used. Only
matches which are prefixed by the word being completed are
returned. When the -f or -d option is used for filename or
directory name completion, the shell variable FIGNORE is
used to filter the matches.
Any completions specified by a pathname expansion pattern to
the -G option are generated next. The words generated by
the pattern need not match the word being completed. The
GLOBIGNORE shell variable is not used to filter the matches,
but the FIGNORE variable is used.
Next, the string specified as the argument to the -W option
is considered. The string is first split using the charac-
ters in the IFS special variable as delimiters. Shell quot-
ing is honored. Each word is then expanded using brace
expansion, tilde expansion, parameter and variable expan-
sion, command substitution, and arithmetic expansion, as
described above under EXPANSION. The results are split
using the rules described above under Word Splitting. The
results of the expansion are prefix-matched against the word
being completed, and the matching words become the possible
completions.
After these matches have been generated, any shell function
or command specified with the -F and -C options is invoked.
When the command or function is invoked, the COMP_LINE,
COMP_POINT, COMP_KEY, and COMP_TYPE variables are assigned
values as described above under Shell Variables. If a shell
function is being invoked, the COMP_WORDS and COMP_CWORD
variables are also set. When the function or command is
invoked, the first argument is the name of the command whose
arguments are being completed, the second argument is the
word being completed, and the third argument is the word
preceding the word being completed on the current command
line. No filtering of the generated completions against the
word being completed is performed; the function or command
has complete freedom in generating the matches.
Any function specified with -F is invoked first. The func-
tion may use any of the shell facilities, including the
compgen builtin described below, to generate the matches.
It must put the possible completions in the COMPREPLY array
variable.
Next, any command specified with the -C option is invoked in
an environment equivalent to command substitution. It
should print a list of completions, one per line, to the
standard output. Backslash may be used to escape a newline,
if necessary.
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After all of the possible completions are generated, any
filter specified with the -X option is applied to the list.
The filter is a pattern as used for pathname expansion; a &
in the pattern is replaced with the text of the word being
completed. A literal & may be escaped with a backslash; the
backslash is removed before attempting a match. Any comple-
tion that matches the pattern will be removed from the list.
A leading ! negates the pattern; in this case any completion
not matching the pattern will be removed.
Finally, any prefix and suffix specified with the -P and -S
options are added to each member of the completion list, and
the result is returned to the readline completion code as
the list of possible completions.
If the previously-applied actions do not generate any
matches, and the -o dirnames option was supplied to complete
when the compspec was defined, directory name completion is
attempted.
If the -o plusdirs option was supplied to complete when the
compspec was defined, directory name completion is attempted
and any matches are added to the results of the other
actions.
By default, if a compspec is found, whatever it generates is
returned to the completion code as the full set of possible
completions. The default bash completions are not
attempted, and the readline default of filename completion
is disabled. If the -o bashdefault option was supplied to
complete when the compspec was defined, the bash default
completions are attempted if the compspec generates no
matches. If the -o default option was supplied to complete
when the compspec was defined, readline's default completion
will be performed if the compspec (and, if attempted, the
default bash completions) generate no matches.
When a compspec indicates that directory name completion is
desired, the programmable completion functions force read-
line 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 set-
ting of the mark-symlinked-directories readline variable.
There is some support for dynamically modifying completions.
This is most useful when used in combination with a default
completion specified with complete -D. It's possible for
shell functions executed as completion handlers to indicate
that completion should be retried by returning an exit sta-
tus 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
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function is executed), programmable completion restarts from
the beginning, with an attempt to find a compspec for that
command. This allows a set of completions to be built
dynamically as completion is attempted, rather than being
loaded all at once.
For instance, assuming that there is a library of compspecs,
each kept in a file corresponding to the name of the com-
mand, the following default completion function would load
completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 &&
return 124
}
complete -D -F _completion_loader
HISTORY
When the -o history option to the set builtin is enabled,
the shell provides access to the command history, the list
of commands previously typed. The value of the HISTSIZE
variable is used as the number of commands to save in a his-
tory list. The text of the last HISTSIZE commands (default
500) is saved. The shell stores each command in the history
list prior to parameter and variable expansion (see EXPAN-
SION above) but after history expansion is performed, sub-
ject to the values of the shell variables HISTIGNORE and
HISTCONTROL.
On startup, the history is initialized from the file named
by the variable HISTFILE (default ~/.bash_history). The
file named by the value of HISTFILE is truncated, if neces-
sary, to contain no more than the number of lines specified
by the value of HISTFILESIZE. When the history file is
read, lines beginning with the history comment character
followed immediately by a digit are interpreted as time-
stamps for the preceding history line. These timestamps are
optionally displayed depending on the value of the HISTTIME-
FORMAT variable. When an interactive shell exits, the last
$HISTSIZE lines are copied from the history list to $HIST-
FILE. If the histappend shell option is enabled (see the
description of shopt under SHELL BUILTIN COMMANDS below),
the lines are appended to the history file, otherwise the
history file is overwritten. If HISTFILE is unset, or if
the history file is unwritable, the history is not saved.
If the HISTTIMEFORMAT variable is set, time stamps are writ-
ten to the history file, marked with the history comment
character, so they may be preserved across shell sessions.
This uses the history comment character to distinguish time-
stamps from other history lines. After saving the history,
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the history file is truncated to contain no more than HIST-
FILESIZE lines. If HISTFILESIZE is not set, no truncation
is performed.
The builtin command fc (see SHELL BUILTIN COMMANDS below)
may be used to list or edit and re-execute a portion of the
history list. The history builtin may be used to display or
modify the history list and manipulate the history file.
When using command-line editing, search commands are avail-
able in each editing mode that provide access to the history
list.
The shell allows control over which commands are saved on
the history list. The HISTCONTROL and HISTIGNORE variables
may be set to cause the shell to save only a subset of the
commands entered. The cmdhist shell option, if enabled,
causes the shell to attempt to save each line of a multi-
line command in the same history entry, adding semicolons
where necessary to preserve syntactic correctness. The
lithist shell option causes the shell to save the command
with embedded newlines instead of semicolons. See the
description of the shopt builtin below under SHELL BUILTIN
COMMANDS for information on setting and unsetting shell
options.
HISTORY EXPANSION
The shell supports a history expansion feature that is simi-
lar to the history expansion in csh. This section describes
what syntax features are available. This feature is enabled
by default for interactive shells, and can be disabled using
the +H option to the set builtin command (see SHELL BUILTIN
COMMANDS below). Non-interactive shells do not perform his-
tory expansion by default.
History expansions introduce words from the history list
into the input stream, making it easy to repeat commands,
insert the arguments to a previous command into the current
input line, or fix errors in previous commands quickly.
History expansion is performed immediately after a complete
line is read, before the shell breaks it into words. It
takes place in two parts. The first is to determine which
line from the history list to use during substitution. The
second is to select portions of that line for inclusion into
the current one. The line selected from the history is the
event, and the portions of that line that are acted upon are
words. Various modifiers are available to manipulate the
selected words. The line is broken into words in the same
fashion as when reading input, so that several metacharac-
ter-separated words surrounded by quotes are considered one
word. History expansions are introduced by the appearance
of the history expansion character, which is ! by default.
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Only backslash (\) and single quotes can quote the history
expansion character.
Several characters inhibit history expansion if found imme-
diately following the history expansion character, even if
it is unquoted: space, tab, newline, carriage return, and =.
If the extglob shell option is enabled, ( will also inhibit
expansion.
Several shell options settable with the shopt builtin may be
used to tailor the behavior of history expansion. If the
histverify shell option is enabled (see the description of
the shopt builtin below), and readline is being used, his-
tory substitutions are not immediately passed to the shell
parser. Instead, the expanded line is reloaded into the
readline editing buffer for further modification. If read-
line is being used, and the histreedit shell option is
enabled, a failed history substitution will be reloaded into
the readline editing buffer for correction. The -p option
to the history builtin command may be used to see what a
history expansion will do before using it. The -s option to
the history builtin may be used to add commands to the end
of the history list without actually executing them, so that
they are available for subsequent recall.
The shell allows control of the various characters used by
the history expansion mechanism (see the description of
histchars above under Shell Variables). The shell uses the
history comment character to mark history timestamps when
writing the history file.
Event Designators
An event designator is a reference to a command line entry
in the history list.
! Start a history substitution, except when followed by a
blank, newline, carriage return, = or ( (when the ext-
glob shell option is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command line minus n.
!! Refer to the previous command. This is a synonym for
`!-1'.
!string
Refer to the most recent command starting with string.
!?string[?]
Refer to the most recent command containing string.
The trailing ? may be omitted if string is followed
immediately by a newline.
^string1^string2^
Quick substitution. Repeat the last command, replacing
string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
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!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the
event. A : separates the event specification from the word
designator. It may be omitted if the word designator begins
with a ^, $, *, -, or %. Words are numbered from the begin-
ning of the line, with the first word being denoted by 0
(zero). Words are inserted into the current line separated
by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command
word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for
`1-$'. It is not an error to use * if there is just
one word in the event; the empty string is returned in
that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specifica-
tion, the previous command is used as the event.
Modifiers
After the optional word designator, there may appear a
sequence of one or more of the following modifiers, each
preceded by a `:'.
h Remove a trailing file name component, leaving only the
head.
t Remove all leading file name components, leaving the
tail.
r Remove a trailing suffix of the form .xxx, leaving the
basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitu-
tions.
x Quote the substituted words as with q, but break into
words at blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the
event line. Any delimiter can be used in place of /.
The final delimiter is optional if it is the last char-
acter of the event line. The delimiter may be quoted
in old and new with a single backslash. If & appears
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in new, it is replaced by old. A single backslash will
quote the &. If old is null, it is set to the last old
substituted, or, if no previous history substitutions
took place, the last string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line.
This is used in conjunction with `:s' (e.g.,
`:gs/old/new/') or `:&'. If used with `:s', any delim-
iter can be used in place of /, and the final delimiter
is optional if it is the last character of the event
line. An a may be used as a synonym for g.
G Apply the following `s' modifier once to each word in
the event line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in
this section as accepting options preceded by - accepts --
to signify the end of the options. The :, true, false, and
test builtins do not accept options and do not treat -- spe-
cially. The exit, logout, break, continue, let, and shift
builtins accept and process arguments beginning with - with-
out requiring --. Other builtins that accept arguments but
are not specified as accepting options interpret arguments
beginning with - as invalid options and require -- to pre-
vent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding
arguments and performing any specified redirections. A
zero exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current
shell environment and return the exit status of the
last command executed from filename. If filename does
not contain a slash, file names in PATH are used to
find the directory containing filename. The file
searched for in PATH need not be executable. When bash
is not in posix mode, the current directory is searched
if no file is found in PATH. If the sourcepath option
to the shopt builtin command is turned off, the PATH is
not searched. If any arguments are supplied, they
become the positional parameters when filename is exe-
cuted. Otherwise the positional parameters are
unchanged. The return status is the status of the last
command exited within the script (0 if no commands are
executed), and false if filename is not found or cannot
be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints
the list of aliases in the form alias name=value on
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standard output. When arguments are supplied, an alias
is defined for each name whose value is given. A
trailing space in value causes the next word to be
checked for alias substitution when the alias is
expanded. For each name in the argument list for which
no value is supplied, the name and value of the alias
is printed. Alias returns true unless a name is given
for which no alias has been defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as
if it had been started with &. If jobspec is not
present, the shell's notion of the current job is used.
bg jobspec returns 0 unless run when job control is
disabled or, when run with job control enabled, any
specified jobspec was not found or was started without
job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current readline key and function bindings,
bind a key sequence to a readline function or macro, or
set a readline variable. Each non-option argument is a
command as it would appear in .inputrc, but each bind-
ing or command must be passed as a separate argument;
e.g., '"\C-x\C-r": re-read-init-file'. Options, if
supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the
subsequent bindings. Acceptable keymap names are
emacs, emacs-standard, emacs-meta, emacs-ctlx, vi,
vi-move, vi-command, and vi-insert. vi is equiva-
lent to vi-command; emacs is equivalent to
emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in
such a way that they can be re-read.
-P List current readline function names and bindings.
-s Display readline key sequences bound to macros and
the strings they output in such a way that they
can be re-read.
-S Display readline key sequences bound to macros and
the strings they output.
-v Display readline variable names and values in such
a way that they can be re-read.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
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-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq
is entered. When shell-command is executed, the
shell sets the READLINE_LINE variable to the con-
tents of the readline line buffer and the READ-
LINE_POINT variable to the current location of the
insertion point. If the executed command changes
the value of READLINE_LINE or READLINE_POINT,
those new values will be reflected in the editing
state.
The return value is 0 unless an unrecognized option is
given or an error occurred.
break [n]
Exit from within a for, while, until, or select loop.
If n is specified, break n levels. n must be >= 1. If
n is greater than the number of enclosing loops, all
enclosing loops are exited. The return value is 0
unless n is not greater than or equal to 1.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it argu-
ments, 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 rede-
fined 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 subrou-
tine 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|-P] [dir]
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Change the current directory to dir. The variable HOME
is the default dir. The variable CDPATH defines the
search path for the directory containing dir. Alterna-
tive directory names in CDPATH are separated by a colon
(:). A null directory name in CDPATH is the same as
the current directory, i.e., ``.''. If dir begins with
a slash (/), then CDPATH is not used. The -P option
says to use the physical directory structure instead of
following symbolic links (see also the -P option to the
set builtin command); the -L option forces symbolic
links to be followed. An argument of - is equivalent
to $OLDPWD. If a non-empty directory name from CDPATH
is used, or if - is the first argument, and the direc-
tory change is successful, the absolute pathname of the
new working directory is written to the standard out-
put. The return value is true if the directory was
successfully changed; false otherwise.
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell
function lookup. Only builtin commands or commands
found in the PATH are executed. If the -p option is
given, the search for command is performed using a
default value for PATH that is guaranteed to find all
of the standard utilities. If either the -V or -v
option is supplied, a description of command is
printed. The -v option causes a single word indicating
the command or file name used to invoke command to be
displayed; the -V option produces a more verbose
description. If the -V or -v option is supplied, the
exit status is 0 if command was found, and 1 if not.
If neither option is supplied and an error occurred or
command cannot be found, the exit status is 127. Oth-
erwise, the exit status of the command builtin is the
exit status of command.
compgen [option] [word]
Generate possible completion matches for word according
to the options, which may be any option accepted by the
complete builtin with the exception of -p and -r, and
write the matches to the standard output. When using
the -F or -C options, the various shell variables set
by the programmable completion facilities, while avail-
able, will not have useful values.
The matches will be generated in the same way as if the
programmable completion code had generated them
directly from a completion specification with the same
flags. If word is specified, only those completions
matching word will be displayed.
The return value is true unless an invalid option is
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supplied, or no matches were generated.
globpat] [-W wordlist] [-F function] [-C command]
complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action]
[-G
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DE] [name ...]
Specify how arguments to each name should be completed.
If the -p option is supplied, or if no options are sup-
plied, existing completion specifications are printed
in a way that allows them to be reused as input. The
-r option removes a completion specification for each
name, or, if no names are supplied, all completion
specifications. The -D option indicates that the
remaining options and actions should apply to the
``default'' command completion; that is, completion
attempted on a command for which no completion has pre-
viously been defined. The -E option indicates that the
remaining options and actions should apply to ``empty''
command completion; that is, completion attempted on a
blank line.
The process of applying these completion specifications
when word completion is attempted is described above
under Programmable Completion.
Other options, if specified, have the following mean-
ings. 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 com-
plete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the
compspec's behavior beyond the simple genera-
tion of completions. comp-option may be one
of:
bashdefault
Perform the rest of the default bash
completions if the compspec generates
no matches.
default Use readline's default filename comple-
tion if the compspec generates no
matches.
dirnames
Perform directory name completion if
the compspec generates no matches.
filenames
Tell readline that the compspec gener-
ates filenames, so it can perform any
filename-specific processing (like
adding a slash to directory names,
quoting special characters, or
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suppressing trailing spaces). Intended
to be used with shell functions.
nospace Tell readline not to append a space
(the default) to words completed at the
end of the line.
plusdirs
After any matches defined by the comp-
spec are generated, directory name com-
pletion is attempted and any matches
are added to the results of the other
actions.
-A action
The action may be one of the following to gen-
erate a list of possible completions:
alias Alias names. May also be specified as
-a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May
also be specified as -b.
command Command names. May also be specified
as -c.
directory
Directory names. May also be specified
as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May
also be specified as -e.
file File names. May also be specified as
-f.
function
Names of shell functions.
group Group names. May also be specified as
-g.
helptopic
Help topics as accepted by the help
builtin.
hostname
Hostnames, as taken from the file spec-
ified 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
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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.
-G globpat
The pathname expansion pattern globpat is
expanded to generate the possible completions.
-W wordlist
The wordlist is split using the characters in
the IFS special variable as delimiters, and
each resultant word is expanded. The possible
completions are the members of the resultant
list which match the word being completed.
-C command
command is executed in a subshell environment,
and its output is used as the possible comple-
tions.
-F function
The shell function function is executed in the
current shell environment. When it finishes,
the possible completions are retrieved from the
value of the COMPREPLY array variable.
-X filterpat
filterpat is a pattern as used for pathname
expansion. It is applied to the list of possi-
ble 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 filter-
pat is removed.
-P prefix
prefix is added at the beginning of each possi-
ble completion after all other options have
been applied.
-S suffix
suffix is appended to each possible completion
after all other options have been applied.
The return value is true unless an invalid option is
supplied, an option other than -p or -r is supplied
without a name argument, an attempt is made to remove a
completion specification for a name for which no speci-
fication exists, or an error occurs adding a completion
specification.
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compopt [-o option] [-DE] [+o option] [name]
Modify completion options for each name according to
the options, or for the currently-execution completion
if no names are supplied. If no options are given,
display the completion options for each name or the
current completion. The possible values of option are
those valid for the complete builtin described above.
The -D option indicates that the remaining options
should apply to the ``default'' command completion;
that is, completion attempted on a command for which no
completion has previously been defined. The -E option
indicates that the remaining options should apply to
``empty'' command completion; that is, completion
attempted on a blank line.
The return value is true unless an invalid option is sup-
plied, an attempt is made to modify the options for a name
for which no completion specification exists, or an output
error occurs.
continue [n]
Resume the next iteration of the enclosing for, while,
until, or select loop. If n is specified, resume at
the nth enclosing loop. n must be >= 1. If n is
greater than the number of enclosing loops, the last
enclosing loop (the ``top-level'' loop) is resumed.
The return value is 0 unless n is not greater than or
equal to 1.
declare [-aAfFilrtux] [-p] [name[=value] ...]
typeset [-aAfFilrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no
names are given then display the values of variables.
The -p option will display the attributes and values of
each name. When -p is used with name arguments, addi-
tional options are ignored. When -p is supplied with-
out name arguments, it will display the attributes and
values of all variables having the attributes specified
by the additional options. If no other options are
supplied with -p, declare will display the attributes
and values of all shell variables. The -f option will
restrict the display to shell functions. The -F option
inhibits the display of function definitions; only the
function name and attributes are printed. If the
extdebug shell option is enabled using shopt, the
source file name and line number where the function is
defined are displayed as well. The -F option implies
-f. The following options can be used to restrict out-
put to variables with the specified attribute or to
give variables attributes:
-a Each name is an indexed array variable (see Arrays
above).
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-A Each name is an associative array variable (see
Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic
evaluation (see ARITHMETIC EVALUATION above) is
performed when the variable is assigned a value.
-l When the variable is assigned a value, all upper-
case characters are converted to lower-case. The
upper-case attribute is disabled.
-r Make names readonly. These names cannot then be
assigned values by subsequent assignment state-
ments or unset.
-t Give each name the trace attribute. Traced func-
tions inherit the DEBUG and RETURN traps from the
calling shell. The trace attribute has no special
meaning for variables.
-u When the variable is assigned a value, all lower-
case characters are converted to upper-case. The
lower-case attribute is disabled.
-x Mark names for export to subsequent commands via
the environment.
Using `+' instead of `-' turns off the attribute
instead, with the exceptions that +a may not be used to
destroy an array variable and +r will not remove the
readonly attribute. When used in a function, makes
each name local, as with the local command. If a vari-
able name is followed by =value, the value of the vari-
able is set to value. The return value is 0 unless an
invalid option is encountered, an attempt is made to
define a function using ``-f foo=bar'', an attempt is
made to assign a value to a readonly variable, an
attempt is made to assign a value to an array variable
without using the compound assignment syntax (see
Arrays above), one of the names is not a valid shell
variable name, an attempt is made to turn off readonly
status for a readonly variable, an attempt is made to
turn off array status for an array variable, or an
attempt is made to display a non-existent function with
-f.
dirs [+n] [-n] [-cplv]
Without options, displays the list of currently remem-
bered directories. The default display is on a single
line with directory names separated by spaces. Direc-
tories are added to the list with the pushd command;
the popd command removes entries from the list.
+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
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options, starting with zero.
-c Clears the directory stack by deleting all of the
entries.
-l Produces a longer listing; 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.
The return value is 0 unless an invalid option is sup-
plied or n indexes beyond the end of the directory
stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table
of active jobs. If jobspec is not present, and neither
-a nor -r is supplied, the shell's notion of the cur-
rent job is used. If the -h option is given, each job-
spec is not removed from the table, but is marked so
that SIGHUP is not sent to the job if the shell
receives a SIGHUP. If no jobspec is present, and nei-
ther the -a nor the -r option is supplied, the current
job is used. If no jobspec is supplied, the -a option
means to remove or mark all jobs; the -r option without
a jobspec argument restricts operation to running jobs.
The return value is 0 unless a jobspec does not specify
a valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a
newline. The return status is always 0. If -n is
specified, the trailing newline is suppressed. If the
-e option is given, interpretation of the following
backslash-escaped characters is enabled. The -E option
disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The xpg_echo shell option may be used to dynamically
determine whether or not echo expands these escape
characters by default. echo does not interpret -- to
mean the end of options. echo interprets the following
escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn
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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 hexa-
decimal value HH (one or two hex digits)
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a
builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a
full pathname, even though the shell normally searches
for builtins before disk commands. If -n is used, each
name is disabled; otherwise, names are enabled. For
example, to use the test binary found via the PATH
instead of the shell builtin version, run ``enable -n
test''. The -f option means to load the new builtin
command name from shared object filename, on systems
that support dynamic loading. The -d option will
delete a builtin previously loaded with -f. If no name
arguments are given, or if the -p option is supplied, a
list of shell builtins is printed. With no other
option arguments, the list consists of all enabled
shell builtins. If -n is supplied, only disabled
builtins are printed. If -a is supplied, the list
printed includes all builtins, with an indication of
whether or not each is enabled. If -s is supplied, the
output is restricted to the POSIX special builtins.
The return value is 0 unless a name is not a shell
builtin or there is an error loading a new builtin from
a shared object.
eval [arg ...]
The args are read and concatenated together into a sin-
gle command. This command is then read and executed by
the shell, and its exit status is returned as the value
of eval. If there are no args, or only null arguments,
eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new
process is created. The arguments become the arguments
to command. If the -l option is supplied, the shell
places a dash at the beginning of the zeroth argument
passed to command. This is what login(1) does. The -c
option causes command to be executed with an empty
environment. If -a is supplied, the shell passes name
as the zeroth argument to the executed command. If
command cannot be executed for some reason, a non-
interactive shell exits, unless the shell option exec-
fail is enabled, in which case it returns failure. An
interactive shell returns failure if the file cannot be
executed. If command is not specified, any redirec-
tions take effect in the current shell, and the return
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status is 0. If there is a redirection error, the
return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is
omitted, the exit status is that of the last command
executed. A trap on EXIT is executed before the shell
terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to
the environment of subsequently executed commands. If
the -f option is given, the names refer to functions.
If no names are given, or if the -p option is supplied,
a list of all names that are exported in this shell is
printed. The -n option causes the export property to
be removed from each name. If a variable name is fol-
lowed by =word, the value of the variable is set to
word. export returns an exit status of 0 unless an
invalid option is encountered, one of the names is not
a valid shell variable name, or -f is supplied with a
name that is not a function.
fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands
from first to last is selected from the history list.
First and last may be specified as a string (to locate
the last command beginning with that string) or as a
number (an index into the history list, where a nega-
tive number is used as an offset from the current com-
mand number). If last is not specified it is set to
the current command for listing (so that ``fc -l -10''
prints the last 10 commands) and to first otherwise.
If first is not specified it is set to the previous
command for editing and -16 for listing.
The -n option suppresses the command numbers when list-
ing. The -r option reverses the order of the commands.
If the -l option is given, the commands are listed on
standard output. Otherwise, the editor given by ename
is invoked on a file containing those commands. If
ename is not given, the value of the FCEDIT variable is
used, and the value of EDITOR if FCEDIT is not set. If
neither variable is set, vi is used. When editing is
complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each
instance of pat is replaced by rep. A useful alias to
use with this is ``r="fc -s"'', so that typing ``r cc''
runs the last command beginning with ``cc'' and typing
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``r'' re-executes the last command.
If the first form is used, the return value is 0 unless
an invalid option is encountered or first or last spec-
ify history lines out of range. If the -e option is
supplied, the return value is the value of the last
command executed or failure if an error occurs with the
temporary file of commands. If the second form is
used, the return status is that of the command re-exe-
cuted, unless cmd does not specify a valid history
line, in which case fc returns failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the cur-
rent job. If jobspec is not present, the shell's
notion of the current job is used. The return value is
that of the command placed into the foreground, or
failure if run when job control is disabled or, when
run with job control enabled, if jobspec does not spec-
ify a valid job or jobspec specifies a job that was
started without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional
parameters. optstring contains the option characters
to be recognized; if a character is followed by a
colon, the option is expected to have an argument,
which should be separated from it by white space. The
colon and question mark characters may not be used as
option characters. Each time it is invoked, getopts
places the next option in the shell variable name, ini-
tializing 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 auto-
matically; it must be manually reset between multiple
calls to getopts within the same shell invocation if a
new set of parameters is to be used.
When the end of options is encountered, getopts exits
with a return value greater than zero. OPTIND is set
to the index of the first non-option argument, and name
is set to ?.
getopts normally parses the positional parameters, but
if more arguments are given in args, getopts parses
those instead.
getopts can report errors in two ways. If the first
character of optstring is a colon, silent error
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reporting is used. In normal operation diagnostic mes-
sages are printed when invalid options or missing
option arguments are encountered. If the variable
OPTERR is set to 0, no error messages will be dis-
played, even if the first character of optstring is not
a colon.
If an invalid option is seen, getopts places ? into
name and, if not silent, prints an error message and
unsets OPTARG. If getopts is silent, the option char-
acter found is placed in OPTARG and no diagnostic mes-
sage is printed.
If a required argument is not found, and getopts is not
silent, a question mark (?) is placed in name, OPTARG
is unset, and a diagnostic message is printed. If
getopts is silent, then a colon (:) is placed in name
and OPTARG is set to the option character found.
getopts returns true if an option, specified or unspec-
ified, is found. It returns false if the end of
options is encountered or an error occurs.
hash [-lr] [-p filename] [-dt] [name]
For each name, the full file name of the command is
determined by searching the directories in $PATH and
remembered. If the -p option is supplied, no path
search is performed, and filename is used as the full
file name of the command. The -r option causes the
shell to forget all remembered locations. The -d
option causes the shell to forget the remembered loca-
tion of each name. If the -t option is supplied, the
full pathname to which each name corresponds is
printed. If multiple name arguments are supplied with
-t, the name is printed before the hashed full path-
name. The -l option causes output to be displayed in a
format that may be reused as input. If no arguments
are given, or if only -l is supplied, information about
remembered commands is printed. The return status is
true unless a name is not found or an invalid option is
supplied.
help [-dms] [pattern]
Display helpful information about builtin commands. If
pattern is specified, help gives detailed help on all
commands matching pattern; otherwise help for all the
builtins and shell control structures is printed.
-d Display a short description of each pattern
-m Display the description of each pattern in a man-
page-like format
-s Display only a short usage synopsis for each pat-
tern
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The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with
line numbers. Lines listed with a * have been modi-
fied. An argument of n lists only the last n lines.
If the shell variable HISTTIMEFORMAT is set and not
null, it is used as a format string for strftime(3) to
display the time stamp associated with each displayed
history entry. No intervening blank is printed between
the formatted time stamp and the history line. If
filename is supplied, it is used as the name of the
history file; if not, the value of HISTFILE is used.
Options, if supplied, have the following meanings:
-c Clear the history list by deleting all the
entries.
-d offset
Delete the history entry at position offset.
-a Append the ``new'' history lines (history lines
entered since the beginning of the current bash
session) to the history file.
-n Read the history lines not already read from the
history file into the current history list. These
are lines appended to the history file since the
beginning of the current bash session.
-r Read the contents of the history file and use them
as the current history.
-w Write the current history to the history file,
overwriting the history file's contents.
-p Perform history substitution on the following args
and display the result on the standard output.
Does not store the results in the history list.
Each arg must be quoted to disable normal history
expansion.
-s Store the args in the history list as a single
entry. The last command in the history list is
removed before the args are added.
If the HISTTIMEFORMAT variable is set, the time stamp
information associated with each history entry is writ-
ten to the history file, marked with the history com-
ment character. When the history file is read, lines
beginning with the history comment character followed
immediately by a digit are interpreted as timestamps
for the previous history line. The return value is 0
unless an invalid option is encountered, an error
occurs while reading or writing the history file, an
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invalid offset is supplied as an argument to -d, or the
history expansion supplied as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have
the following meanings:
-l List process IDs in addition to the normal infor-
mation.
-p List only the process ID of the job's process
group leader.
-n Display information only about jobs that have
changed status since the user was last notified of
their status.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.
If jobspec is given, output is restricted to informa-
tion 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, return-
ing its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the pro-
cesses named by pid or jobspec. sigspec is either a
case-insensitive signal name such as SIGKILL (with or
without the SIG prefix) or a signal number; signum is a
signal number. If sigspec is not present, then SIGTERM
is assumed. An argument of -l lists the signal names.
If any arguments are supplied when -l is given, the
names of the signals corresponding to the arguments are
listed, and the return status is 0. The exit_status
argument to -l is a number specifying either a signal
number or the exit status of a process terminated by a
signal. kill returns true if at least one signal was
successfully sent, or false if an error occurs or an
invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated
(see ARITHMETIC EVALUATION above). If the last arg
evaluates to 0, let returns 1; 0 is returned otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is cre-
ated, and assigned value. The option can be any of the
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options accepted by declare. When local is used within
a function, it causes the variable name to have a visi-
ble scope restricted to that function and its children.
With no operands, local writes a list of local vari-
ables to the standard output. It is an error to use
local when not within a function. The return status is
0 unless local is used outside a function, an invalid
name is supplied, or name is a readonly variable.
logout
Exit a login shell.
back] [-c quantum] [array]
mapfile [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
call-
callback] [-c quantum] [array]
readarray [-n count] [-O origin] [-s count] [-t] [-u fd]
[-C
Read lines from the standard input into the indexed
array variable array, or from file descriptor fd if the
-u option is supplied. The variable MAPFILE is the
default array. Options, if supplied, have the follow-
ing meanings:
-n Copy at most count lines. If count is 0, all
lines are copied.
-O Begin assigning to array at index origin. The
default index is 0.
-s Discard the first count lines read.
-t Remove a trailing newline from each line read.
-u Read lines from file descriptor fd instead of the
standard input.
-C Evaluate callback each time quantum lines are
read. The -c option specifies quantum.
-c Specify the number of lines read between each call
to callback.
If -C is specified without -c, the default quantum is
5000. When callback is evaluated, it is supplied the
index of the next array element to be assigned as an
additional argument. callback is evaluated after the
line is read but before the array element is assigned.
If not supplied with an explicit origin, mapfile will
clear array before assigning to it.
mapfile returns successfully unless an invalid option
or option argument is supplied, array is invalid or
unassignable, or if array is not an indexed array.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no
arguments, removes the top directory from the stack,
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and performs a cd to the new top directory. Arguments,
if supplied, have the following meanings:
-n Suppresses the normal change of directory when
removing directories from the stack, so that only
the stack is manipulated.
+n Removes the nth entry counting from the left of
the list shown by dirs, starting with zero. For
example: ``popd +0'' removes the first directory,
``popd +1'' the second.
-n Removes the nth entry counting from the right of
the list shown by dirs, starting with zero. For
example: ``popd -0'' removes the last directory,
``popd -1'' the next to last.
If the popd command is successful, a dirs is performed
as well, and the return status is 0. popd returns
false if an invalid option is encountered, the direc-
tory stack is empty, a non-existent directory stack
entry is specified, or the directory change fails.
printf [-v var] format [arguments]
Write the formatted arguments to the standard output
under the control of the format. The format is a char-
acter 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 specifi-
cations, each of which causes printing of the next suc-
cessive argument. In addition to the standard
printf(1) formats, %b causes printf to expand backslash
escape sequences in the corresponding argument (except
that \c terminates output, backslashes in \', \", and
\? are not removed, and octal escapes beginning with \0
may contain up to four digits), and %q causes printf to
output the corresponding argument in a format that can
be reused as shell input.
The -v option causes the output to be assigned to the
variable var rather than being printed to the standard
output.
The format is reused as necessary to consume all of the
arguments. If the format requires more arguments than
are supplied, the extra format specifications behave as
if a zero value or null string, as appropriate, had
been supplied. The return value is zero on success,
non-zero on failure.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
Adds a directory to the top of the directory stack, or
rotates the stack, making the new top of the stack the
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current working directory. With no arguments,
exchanges the top two directories and returns 0, unless
the directory stack is empty. Arguments, if supplied,
have the following meanings:
-n Suppresses the normal change of directory when
adding directories to the stack, so that only the
stack is manipulated.
+n Rotates the stack so that the nth directory
(counting from the left of the list shown by dirs,
starting with zero) is at the top.
-n Rotates the stack so that the nth directory
(counting from the right of the list shown by
dirs, starting with zero) is at the top.
dir Adds dir to the directory stack at the top, making
it the new current working directory.
If the pushd command is successful, a dirs is performed
as well. If the first form is used, pushd returns 0
unless the cd to dir fails. With the second form,
pushd returns 0 unless the directory stack is empty, a
non-existent directory stack element is specified, or
the directory change to the specified new current
directory fails.
pwd [-LP]
Print the absolute pathname of the current working
directory. The pathname printed contains no symbolic
links if the -P option is supplied or the -o physical
option to the set builtin command is enabled. If the
-L option is used, the pathname printed may contain
symbolic links. The return status is 0 unless an error
occurs while reading the name of the current directory
or an invalid option is supplied.
nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars]
[-N
One line is read from the standard input, or from the
file descriptor fd supplied as an argument to the -u
option, and the first word is assigned to the first
name, the second word to the second name, and so on,
with leftover words and their intervening separators
assigned to the last name. If there are fewer words
read from the input stream than names, the remaining
names are assigned empty values. The characters in IFS
are used to split the line into words. The backslash
character (\) may be used to remove any special meaning
for the next character read and for line continuation.
Options, if supplied, have the following meanings:
-a aname
The words are assigned to sequential indices of
the array variable aname, starting at 0. aname is
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unset before any new values are assigned. Other
name arguments are ignored.
-d delim
The first character of delim is used to terminate
the input line, rather than newline.
-e If the standard input is coming from a terminal,
readline (see READLINE above) is used to obtain
the line. Readline uses the current (or default,
if line editing was not previously active) editing
settings.
-i text
If readline is being used to read the line, text
is placed into the editing buffer before editing
begins.
-n nchars
read returns after reading nchars characters
rather than waiting for a complete line of input,
but honor a delimiter if fewer than nchars charac-
ters are read before the delimiter.
-N nchars
read returns after reading exactly nchars charac-
ters rather than waiting for a complete line of
input, unless EOF is encountered or read times
out. Delimiter characters encountered in the
input are not treated specially and do not cause
read to return until nchars characters are read.
-p prompt
Display prompt on standard error, without a trail-
ing newline, before attempting to read any input.
The prompt is displayed only if input is coming
from a terminal.
-r Backslash does not act as an escape character.
The backslash is considered to be part of the
line. In particular, a backslash-newline pair may
not be used as a line continuation.
-s Silent mode. If input is coming from a terminal,
characters are not echoed.
-t timeout
Cause read to time out and return failure if a
complete line of input is not read within timeout
seconds. timeout may be a decimal number with a
fractional portion following the decimal point.
This option is only effective if read is reading
input from a terminal, pipe, or other special
file; it has no effect when reading from regular
files. If timeout is 0, read returns success if
input is available on the specified file descrip-
tor, failure otherwise. The exit status is
greater than 128 if the timeout is exceeded.
-u fd
Read input from file descriptor fd.
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If no names are supplied, the line read is assigned to
the variable REPLY. The return code is zero, unless
end-of-file is encountered, read times out (in which
case the return code is greater than 128), or an
invalid file descriptor is supplied as the argument to
-u.
readonly [-aApf] [name[=word] ...]
The given names are marked readonly; the values of
these names may not be changed by subsequent assign-
ment. If the -f option is supplied, the functions cor-
responding to the names are so marked. The -a option
restricts the variables to indexed arrays; the -A
option restricts the variables to associative arrays.
If no name arguments are given, or if the -p option is
supplied, a list of all readonly names is printed. The
-p option causes output to be displayed in a format
that may be reused as input. If a variable name is
followed by =word, the value of the variable is set to
word. The return status is 0 unless an invalid option
is encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that is
not a function.
return [n]
Causes a function to exit with the return value speci-
fied by n. If n is omitted, the return status is that
of the last command executed in the function body. If
used outside a function, but during execution of a
script by the . (source) command, it causes the shell
to stop executing that script and return either n or
the exit status of the last command executed within the
script as the exit status of the script. If used out-
side a function and not during execution of a script by
., the return status is false. Any command associated
with the RETURN trap is executed before execution
resumes after the function or script.
set [--abefhkmnptuvxBCEHPT] [-o option] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option] [arg ...]
Without options, the name and value of each shell vari-
able are displayed in a format that can be reused as
input for setting or resetting the currently-set vari-
ables. Read-only variables cannot be reset. In posix
mode, only shell variables are listed. The output is
sorted according to the current locale. When options
are specified, they set or unset shell attributes. Any
arguments remaining after option processing are treated
as values for the positional parameters and are
assigned, in order, to $1, $2, ... $n. Options, if
specified, have the following meanings:
-a Automatically mark variables and functions
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which are modified or created for export to the
environment of subsequent commands.
-b Report the status of terminated background jobs
immediately, rather than before the next pri-
mary prompt. This is effective only when job
control is enabled.
-e Exit immediately if a pipeline (which may con-
sist of a single simple command), a subshell
command enclosed in parentheses, or one of the
commands executed as part of a command list
enclosed by braces (see SHELL GRAMMAR above)
exits with a non-zero status. The shell does
not exit if the command that fails is part of
the command list immediately following a while
or until keyword, part of the test following
the if or elif reserved words, part of any com-
mand executed in a && or || list except the
command following the final && or ||, any com-
mand in a pipeline but the last, or if the com-
mand's return value is being inverted with !.
A trap on ERR, if set, is executed before the
shell exits. This option applies to the shell
environment and each subshell environment sepa-
rately (see COMMAND EXECUTION ENVIRONMENT
above), and may cause subshells to exit before
executing all the commands in the subshell.
-f Disable pathname expansion.
-h Remember the location of commands as they are
looked up for execution. This is enabled by
default.
-k All arguments in the form of assignment state-
ments are placed in the environment for a com-
mand, not just those that precede the command
name.
-m Monitor mode. Job control is enabled. This
option is on by default for interactive shells
on systems that support it (see JOB CONTROL
above). Background processes run in a separate
process group and a line containing their exit
status is printed upon their completion.
-n Read commands but do not execute them. This
may be used to check a shell script for syntax
errors. This is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing
interface. This is enabled by default
when the shell is interactive, unless
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the shell is started with the --noedit-
ing option. This also affects the
editing interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described
above under HISTORY. This option is on
by default in interactive shells.
ignoreeof
The effect is as if the shell command
``IGNOREEOF=10'' had been executed (see
Shell Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline
is the value of the last (rightmost)
command to exit with a non-zero status,
or zero if all commands in the pipeline
exit successfully. This option is dis-
abled by default.
posix Change the behavior of bash where the
default operation differs from the
POSIX standard to match the standard
(posix mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing
interface. This also affects the edit-
ing interface used for read -e.
xtrace Same as -x.
If -o is supplied with no option-name, the val-
ues of the current options are printed. If +o
is supplied with no option-name, a series of
set commands to recreate the current option
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settings is displayed on the standard output.
-p Turn on privileged mode. In this mode, the
$ENV and $BASH_ENV files are not processed,
shell functions are not inherited from the
environment, and the SHELLOPTS, BASHOPTS,
CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored. If the
shell is started with the effective user
(group) id not equal to the real user (group)
id, and the -p option is not supplied, these
actions are taken and the effective user id is
set to the real user id. If the -p option is
supplied at startup, the effective user id is
not reset. Turning this option off causes the
effective user and group ids to be set to the
real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than
the special parameters "@" and "*" as an error
when performing parameter expansion. If expan-
sion 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 com-
mand, case command, select command, or arith-
metic for command, display the expanded value
of PS4, followed by the command and its
expanded arguments or associated word list.
-B The shell performs brace expansion (see Brace
Expansion above). This is on by default.
-C If set, bash does not overwrite an existing
file with the >, >&, and <> redirection opera-
tors. This may be overridden when creating
output files by using the redirection operator
>| instead of >.
-E If set, any trap on ERR is inherited by shell
functions, command substitutions, and commands
executed in a subshell environment. The ERR
trap is normally not inherited in such cases.
-H Enable ! style history substitution. This
option is on by default when the shell is
interactive.
-P If set, the shell does not follow symbolic
links when executing commands such as cd that
change the current working directory. It uses
the physical directory structure instead. By
default, bash follows the logical chain of
directories when performing commands which
change the current directory.
-T If set, any traps on DEBUG and RETURN are
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inherited by shell functions, command substitu-
tions, and commands executed in a subshell
environment. The DEBUG and RETURN traps are
normally not inherited in such cases.
-- If no arguments follow this option, then the
positional parameters are unset. Otherwise,
the positional parameters are set to the args,
even if some of them begin with a -.
- Signal the end of options, cause all remaining
args to be assigned to the positional parame-
ters. The -x and -v options are turned off.
If there are no args, the positional parameters
remain unchanged.
The options are off by default unless otherwise noted.
Using + rather than - causes these options to be turned
off. The options can also be specified as arguments to
an invocation of the shell. The current set of options
may be found in $-. The return status is always true
unless an invalid option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed to
$1 .... Parameters represented by the numbers $# down
to $#-n+1 are unset. n must be a non-negative number
less than or equal to $#. If n is 0, no parameters are
changed. If n is not given, it is assumed to be 1. If
n is greater than $#, the positional parameters are not
changed. The return status is greater than zero if n
is greater than $# or less than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional
shell behavior. With no options, or with the -p
option, a list of all settable options is displayed,
with an indication of whether or not each is set. The
-p option causes output to be displayed in a form that
may be reused as input. Other options have the follow-
ing meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return
status indicates whether the optname is set or
unset. If multiple optname arguments are given
with -q, the return status is zero if all optnames
are enabled; non-zero otherwise.
-o Restricts the values of optname to be those
defined for the -o option to the set builtin.
If either -s or -u is used with no optname arguments,
the display is limited to those options which are set
or unset, respectively. Unless otherwise noted, the
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shopt options are disabled (unset) by default.
The return status when listing options is zero if all
optnames are enabled, non-zero otherwise. When setting
or unsetting options, the return status is zero unless
an optname is not a valid shell option.
The list of shopt options is:
autocd If set, a command name that is the name of a
directory is executed as if it were the argu-
ment to the cd command. This option is only
used by interactive shells.
cdable_vars
If set, an argument to the cd builtin command
that is not a directory is assumed to be the
name of a variable whose value is the directory
to change to.
cdspell If set, minor errors in the spelling of a
directory component in a cd command will be
corrected. The errors checked for are trans-
posed characters, a missing character, and one
character too many. If a correction is found,
the corrected file name is printed, and the
command proceeds. This option is only used by
interactive shells.
checkhash
If set, bash checks that a command found in the
hash table exists before trying to execute it.
If a hashed command no longer exists, a normal
path search is performed.
checkjobs
If set, bash lists the status of any stopped
and running jobs before exiting an interactive
shell. If any jobs are running, this causes
the exit to be deferred until a second exit is
attempted without an intervening command (see
JOB CONTROL above). The shell always postpones
exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each
command and, if necessary, updates the values
of LINES and COLUMNS.
cmdhist If set, bash attempts to save all lines of a
multiple-line command in the same history
entry. This allows easy re-editing of multi-
line commands.
compat31
If set, bash changes its behavior to that of
version 3.1 with respect to quoted arguments to
the conditional command's =~ operator.
compat32
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If set, bash changes its behavior to that of
version 3.2 with respect to locale-specific
string comparison when using the conditional
command's < and > operators.
compat40
If set, bash changes its behavior to that of
version 4.0 with respect to locale-specific
string comparison when using the conditional
command's < and > operators and the effect of
interrupting a command list.
dirspell
If set, bash attempts spelling correction on
directory names during word completion if the
directory name initially supplied does not
exist.
dotglob If set, bash includes filenames beginning with
a `.' in the results of pathname expansion.
execfail
If set, a non-interactive shell will not exit
if it cannot execute the file specified as an
argument to the exec builtin command. An
interactive shell does not exit if exec fails.
expand_aliases
If set, aliases are expanded as described above
under ALIASES. This option is enabled by
default for interactive shells.
extdebug
If set, behavior intended for use by debuggers
is enabled:
1. The -F option to the declare builtin dis-
plays the source file name and line number
corresponding to each function name sup-
plied 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 func-
tion or a shell script executed by the .
or source builtins), a call to return is
simulated.
4. BASH_ARGC and BASH_ARGV are updated as
described in their descriptions above.
5. Function tracing is enabled: command sub-
stitution, shell functions, and subshells
invoked with ( command ) inherit the DEBUG
and RETURN traps.
6. Error tracing is enabled: command substi-
tution, shell functions, and subshells
invoked with ( command ) inherit the ERROR
trap.
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extglob If set, the extended pattern matching features
described above under Pathname Expansion are
enabled.
extquote
If set, $'string' and $"string" quoting is per-
formed 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 expan-
sion 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 FIG-
NORE. This option is enabled by default.
globstar
If set, the pattern ** used in a pathname
expansion context will match a 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 vari-
able when the shell exits, rather than over-
writing the file.
histreedit
If set, and readline is being used, a user is
given the opportunity to re-edit a failed his-
tory 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.
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interactive_comments
If set, allow a word beginning with # to cause
that word and all remaining characters on that
line to be ignored in an interactive shell (see
COMMENTS above). This option is enabled by
default.
lithist If set, and the cmdhist option is enabled,
multi-line commands are saved to the history
with embedded newlines rather than using semi-
colon separators where possible.
login_shell
The shell sets this option if it is started as
a login shell (see INVOCATION above). The
value may not be changed.
mailwarn
If set, and a file that bash is checking for
mail has been accessed since the last time it
was checked, the message ``The mail in mailfile
has been read'' is displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will
not attempt to search the PATH for possible
completions when completion is attempted on an
empty line.
nocaseglob
If set, bash matches filenames in a case-insen-
sitive fashion when performing pathname expan-
sion (see Pathname Expansion above).
nocasematch
If set, bash matches patterns in a case-insen-
sitive fashion when performing matching while
executing case or [[ conditional commands.
nullglob
If set, bash allows patterns which match no
files (see Pathname Expansion above) to expand
to a null string, rather than themselves.
progcomp
If set, the programmable completion facilities
(see Programmable Completion above) are
enabled. This option is enabled by default.
promptvars
If set, prompt strings undergo parameter expan-
sion, command substitution, arithmetic expan-
sion, 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
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or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error mes-
sage when the shift count exceeds the number of
positional parameters.
sourcepath
If set, the source (.) builtin uses the value
of PATH to find the directory containing the
file supplied as an argument. This option is
enabled by default.
xpg_echo
If set, the echo builtin expands backslash-
escape sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT signal. A login shell cannot be suspended; the
-f option can be used to override this and force the
suspension. The return status is 0 unless the shell is
a login shell and -f is not supplied, or if job control
is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 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 CONDI-
TIONAL EXPRESSIONS. test does not accept any options,
nor does it accept and ignore an argument of -- as sig-
nifying the end of options.
Expressions may be combined using the following opera-
tors, listed in decreasing order of precedence. The
evaluation depends on the number of arguments; see
below.
! expr
True if expr is false.
( expr )
Returns the value of expr. This may be used to
override the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set
of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument
is not null.
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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 EXPRES-
SIONS, 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
If the second argument is one of the binary condi-
tional 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 opera-
tors are considered binary operators when there
are three arguments. If the first argument is !,
the value is the negation of the two-argument test
using the second and third arguments. If the
first argument is exactly ( and the third argument
is exactly ), the result is the one-argument test
of the second argument. Otherwise, the expression
is false.
4 arguments
If the first argument is !, the result is the
negation of the three-argument expression composed
of the remaining arguments. Otherwise, the
expression is parsed and evaluated according to
precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according
to precedence using the rules listed above.
times
Print the accumulated user and system times for the
shell and for processes run from the shell. The return
status is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the
shell receives signal(s) sigspec. If arg is absent
(and there is a single sigspec) or -, each specified
signal is reset to its original disposition (the value
it had upon entrance to the shell). If arg is the null
string the signal specified by each sigspec is ignored
by the shell and by the commands it invokes. If arg is
not present and -p has been supplied, then the trap
commands associated with each sigspec are displayed.
If no arguments are supplied or if only -p is given,
trap prints the list of commands associated with each
signal. The -l option causes the shell to print a list
of signal names and their corresponding numbers. Each
sigspec is either a signal name defined in <signal.h>,
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or a signal number. Signal names are case insensitive
and the SIG prefix is optional.
If a sigspec is EXIT (0) the command arg is executed on
exit from the shell. If a sigspec is DEBUG, the com-
mand arg is executed before every simple command, for
command, case command, select command, every arithmetic
for command, and before the first command executes in a
shell function (see SHELL GRAMMAR above). Refer to the
description of the extdebug option to the shopt builtin
for details of its effect on the DEBUG trap. If a
sigspec is RETURN, the command arg is executed each
time a shell function or a script executed with the
If a sigspec is ERR, the command arg is executed when-
ever a simple command has a non-zero exit status, sub-
ject to the following conditions. The ERR trap is not
executed if the failed command is part of the command
list immediately following a while or until keyword,
part of the test in an if statement, part of a command
executed in a && or || list, or if the command's return
value is being inverted via !. These are the same con-
ditions obeyed by the errexit option.
Signals ignored upon entry to the shell cannot be
trapped or reset. Trapped signals that are not being
ignored are reset to their original values in a sub-
shell or subshell environment when one is created. The
return status is false if any sigspec is invalid; oth-
erwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be inter-
preted if used as a command name. If the -t option is
used, type prints a string which is one of alias, key-
word, function, builtin, or file if name is an alias,
shell reserved word, function, builtin, or disk file,
respectively. If the name is not found, then nothing
is printed, and an exit status of false is returned.
If the -p option is used, type either returns the name
of the disk file that would be executed if name were
specified as a command name, or nothing if ``type -t
name'' would not return file. The -P option forces a
PATH search for each name, even if ``type -t name''
would not return file. If a command is hashed, -p and
-P print the hashed value, not necessarily the file
that appears first in PATH. If the -a option is used,
type prints all of the places that contain an exe-
cutable named name. This includes aliases and func-
tions, if and only if the -p option is not also used.
The table of hashed commands is not consulted when
using -a. The -f option suppresses shell function
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User Commands BASH(1)
lookup, as with the command builtin. type returns true
if all of the arguments are found, false if any are not
found.
ulimit [-HSTabcdefilmnpqrstuvx [limit]]
Provides control over the resources available to the
shell and to processes started by it, on systems that
allow such control. The -H and -S options specify that
the hard or soft limit is set for the given resource.
A hard limit cannot be increased by a non-root user
once it is set; a soft limit may be increased up to the
value of the hard limit. If neither -H nor -S is spec-
ified, both the soft and hard limits are set. The
value of limit can be a number in the unit specified
for the resource or one of the special values hard,
soft, or unlimited, which stand for the current hard
limit, the current soft limit, and no limit, respec-
tively. If limit is omitted, the current value of the
soft limit of the resource is printed, unless the -H
option is given. When more than one resource is speci-
fied, the limit name and unit are printed before the
value. Other options are interpreted as follows:
-a All current limits are reported
-b The maximum socket buffer size
-c The maximum size of core files created
-d The maximum size of a process's data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and
its children
-i The maximum number of pending signals
-l The maximum size that may be locked into memory
-m The maximum resident set size (many systems do not
honor this limit)
-n The maximum number of open file descriptors (most
systems do not allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be
set)
-q The maximum number of bytes in POSIX message
queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a
single user
-v The maximum amount of virtual memory available to
the shell
-x The maximum number of file locks
-T The maximum number of threads
If limit is given, it is the new value of the specified
resource (the -a option is display only). If no option
is given, then -f is assumed. Values are in 1024-byte
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User Commands BASH(1)
increments, except for -t, which is in seconds, -p,
which is in units of 512-byte blocks, and -T, -b, -n,
and -u, which are unscaled values. The return status
is 0 unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode
begins with a digit, it is interpreted as an octal num-
ber; otherwise it is interpreted as a symbolic mode
mask similar to that accepted by chmod(1). If mode is
omitted, the current value of the mask is printed. The
-S option causes the mask to be printed in symbolic
form; the default output is an octal number. If the -p
option is supplied, and mode is omitted, the output is
in a form that may be reused as input. The return sta-
tus is 0 if the mode was successfully changed or if no
mode argument was supplied, and false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If
-a is supplied, all alias definitions are removed. The
return value is true unless a supplied name is not a
defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or
function. If no options are supplied, or the -v option
is given, each name refers to a shell variable. Read-
only variables may not be unset. If -f is specified,
each name refers to a shell function, and the function
definition is removed. Each unset variable or function
is removed from the environment passed to subsequent
commands. If any of COMP_WORDBREAKS, RANDOM, SECONDS,
LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are
unset, they lose their special properties, even if they
are subsequently reset. The exit status is true unless
a name is readonly.
wait [n ...]
Wait for each specified process and return its termina-
tion status. Each n may be a process ID or a job spec-
ification; if a job spec is given, all processes in
that job's pipeline are waited for. If n is not given,
all currently active child processes are waited for,
and the return status is zero. If n specifies a non-
existent process or job, the return status is 127.
Otherwise, the return status is the exit status of the
last process or job waited for.
RESTRICTED SHELL
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If bash is started with the name rbash, or the -r option is
supplied at invocation, the shell becomes restricted. A
restricted shell is used to set up an environment more con-
trolled than the standard shell. It behaves identically to
bash with the exception that the following are disallowed or
not performed:
o changing directories with cd
o setting or unsetting the values of SHELL, PATH, ENV, or
BASH_ENV
o specifying command names containing /
o specifying a file name containing a / as an argument to
the . builtin command
o Specifying a filename containing a slash as an argument
to the -p option to the hash builtin command
o importing function definitions from the shell environ-
ment at startup
o parsing the value of SHELLOPTS from the shell environ-
ment at startup
o redirecting output using the >, >|, <>, >&, &>, and >>
redirection operators
o using the exec builtin command to replace the shell
with another command
o adding or deleting builtin commands with the -f and -d
options to the enable builtin command
o Using the enable builtin command to enable disabled
shell builtins
o specifying the -p option to the command builtin command
o turning off restricted mode with set +r or set +o
restricted.
These restrictions are enforced after any startup files are
read.
When a command that is found to be a shell script is exe-
cuted (see COMMAND EXECUTION above), rbash turns off any
restrictions in the shell spawned to execute the script.
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ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | shell/bash |
+---------------+------------------+
|Stability | Volatile |
+---------------+------------------+
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
Utilities, IEEE
Portable Operating System Interface (POSIX) Part 2:
Shell and
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
~/.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/bash/.
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User Commands BASH(1)
Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report. If you have a fix,
you are encouraged to mail that as well! Suggestions and
`philosophical' bug reports may be mailed to bug-
bash@gnu.org or posted to the Usenet newsgroup gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into the
template it provides for filing a bug report.
Comments and bug reports concerning this manual page should
be directed to chet@po.cwru.edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and tradi-
tional versions of sh, mostly because of the POSIX specifi-
cation.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stop-
pable/restartable.
Compound commands and command sequences of the form `a ; b ;
c' are not handled gracefully when process suspension is
attempted. When a process is stopped, the shell immediately
executes the next command in the sequence. It suffices to
place the sequence of commands between parentheses to force
it into a subshell, which may be stopped as a unit.
Array variables may not (yet) be exported.
There may be only one active coprocess at a time.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://ftp.gnu.org/gnu/bash/bash-4.1.tar.gz
Further information about this software can be found on the
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User Commands BASH(1)
open source community website at http://www.gnu.org/soft-
ware/bash/.
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