bash
BASH(1) BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-2004 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes
commands read from the standard input or from a file. Bash also
incorporates useful features from the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the IEEE POSIX
Shell and Tools specification (IEEE Working Group 1003.2).
OPTIONS
In addition to the single-character shell options documented in the
description of the set builtin command, bash interprets the following
options when it is invoked:
-c string If the -c option is present, then commands are read from
string. If there are arguments after the string, they are
assigned to the positional parameters, starting with $0.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell
(see INVOCATION below).
-r If the -r option is present, the shell becomes restricted
(see RESTRICTED SHELL below).
-s If the -s option is present, or if no arguments remain after
option processing, then commands are read from the standard
input. This option allows the positional parameters to be
set when invoking an interactive shell.
-D A list of all double-quoted strings preceded by $ is printed
on the standard ouput. These are the strings that are sub-
ject to language translation when the current locale is not
C or POSIX. This implies the -n option; no commands will be
executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the
shopt builtin (see SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of that option;
+O unsets it. If shopt_option is not supplied, the names
and values of the shell options accepted by shopt are
printed on the standard output. If the invocation option is
+O, the output is displayed in a format that may be reused
as input.
-- A -- signals the end of options and disables further option
processing. Any arguments after the -- are treated as file-
names and arguments. An argument of - is equivalent to --.
Bash also interprets a number of multi-character options. These
options must appear on the command line before the single-character
options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the
shell starts. Turns on extended debugging mode (see the
description of the extdebug option to the shopt builtin below)
and shell function tracing (see the description of the -o func-
trace 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 success-
fully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal
initialization file ~/.bashrc if the shell is interactive (see
INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when
the shell is interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or
any of the personal initialization files ~/.bash_profile,
~/.bash_login, or ~/.profile. By default, bash reads these
files when it is invoked as a login shell (see INVOCATION
below).
--norc Do not read and execute the personal initialization file
~/.bashrc if the shell is interactive. This option is on by
default if the shell is invoked as sh.
--posix
Change the behavior of bash where the default operation differs
from the POSIX 1003.2 standard to match the standard (posix
mode).
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires
Produce the list of files that are required for the shell
script to run. This implies ’-n’ and is subject to the same
limitations as compile time error checking checking; Backticks,
[] tests, and evals are not parsed so some dependencies may be
missed. --verbose Equivalent to -v.
--version
Show version information for this instance of bash on the stan-
dard output and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor
the -s option has been supplied, the first argument is assumed to be
the name of a file containing shell commands. If bash is invoked in
this fashion, $0 is set to the name of the file, and the positional
parameters are set to the remaining arguments. Bash reads and exe-
cutes commands from this file, then exits. Bash’s exit status is the
exit status of the last command executed in the script. If no com-
mands are executed, the exit status is 0. An attempt is first made to
open the file in the current directory, and, if no file is found, then
the shell searches the directories in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or
one started with the --login option.
An interactive shell is one started without non-option arguments and
without the -c option whose standard input and error are both con-
nected to terminals (as determined by isatty(3)), or one started with
the -i option. PS1 is set and $- includes i if bash is interactive,
allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in file names as described below under Tilde
Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-inter-
active shell with the --login option, it first reads and executes com-
mands from the file /etc/profile, if that file exists. After reading
that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.pro-
file, in that order, and reads and executes commands from the first
one that exists and is readable. The --noprofile option may be used
when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands from the
file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash
reads and executes commands from ~/.bashrc, if that file exists. This
may be inhibited by using the --norc option. The --rcfile file option
will force bash to read and execute commands from file instead of
~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the fol-
lowing command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file
name.
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an interac-
tive login shell, or a non-interactive shell with the --login option,
it first attempts to read and execute commands from /etc/profile and
~/.profile, in that order. The --noprofile option may be used to
inhibit this behavior. When invoked as an interactive shell with the
name sh, bash looks for the variable ENV, expands its value if it is
defined, and uses the expanded value as the name of a file to read and
execute. Since a shell invoked as sh does not attempt to read and
execute commands from any other startup files, the --rcfile option has
no effect. A non-interactive shell invoked with the name sh does not
attempt to read any other startup files. When invoked as sh, bash
enters posix mode after the startup files are read.
When bash is started in posix mode, as with the --posix command line
option, it follows the POSIX standard for startup files. In this
mode, interactive shells expand the ENV variable and commands are read
and executed from the file whose name is the expanded value. No other
startup files are read.
Bash attempts to determine when it is being run by the remote shell
daemon, usually rshd. If bash determines it is being run by rshd, 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 variable, if it appears in the environment,
is ignored, and the effective user id is set to the real user id. If
the -p option is supplied at invocation, the startup behavior is the
same, but the effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this docu-
ment.
blank A space or tab.
word A sequence of characters considered as a single unit by the
shell. Also known as a token.
name A word consisting only of alphanumeric characters and under-
scores, and beginning with an alphabetic character or an under-
score. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the
following:
| & ; ( ) < > space tab
control operator
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and
either the first word of a simple command (see SHELL GRAMMAR below) or
the third word of a case or for command:
! case do done elif else esac fi for function if in select then until
while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments fol-
lowed by blank-separated words and redirections, and terminated by a
control operator. The first word specifies the command to be exe-
cuted, 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 the
character |. The format for a pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard
input of command2. This connection is performed before any redirec-
tions specified by the command (see REDIRECTION below).
The return status of a pipeline is the exit status of the last com-
mand, unless the pipefail option is enabled. If pipefail is enabled,
the pipeline’s return status is the value of the last (rightmost) com-
mand to exit with a non-zero status, or zero if all commands exit suc-
cessfully. If the reserved word ! precedes a pipeline, the exit sta-
tus 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 specifies 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 terminated by one of ;, &,
or <newline>.
Of these list operators, && and ││ have equal precedence, followed by
; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list instead of a
semicolon to delimit commands.
If a command is terminated by the control operator &, the shell exe-
cutes the command in the background in a subshell. The shell does not
wait for the command to finish, and the return status is 0. Commands
separated by a ; are executed sequentially; the shell waits for each
command to terminate in turn. The return status is the exit status of
the last command executed.
The control operators && and ││ denote AND lists and OR lists, respec-
tively. 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 EXECU-
TION ENVIRONMENT below). Variable assignments and builtin com-
mands that affect the shell’s environment do not remain in
effect after the command completes. The return status is the
exit status of list.
{ list; }
list is simply executed in the current shell environment. list
must be terminated with a newline or semicolon. This is known
as a group command. The return status is the exit status of
list. Note that unlike the metacharacters ( and ), { and } are
reserved words and must occur where a reserved word is permit-
ted to be recognized. Since they do not cause a word break,
they must be separated from list by whitespace.
((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION. If the value of the expres-
sion is non-zero, the return status is 0; otherwise the return
status is 1. This is exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression expression. Expressions are composed of
the primaries described below under CONDITIONAL EXPRESSIONS.
Word splitting and pathname expansion are not performed on the
words between the [[ and ]]; tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution,
process substitution, and quote removal are performed. Condi-
tional operators such as -f must be unquoted to be recognized
as primaries.
When 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. The
return value is 0 if the string matches or does not match the
pattern, respectively, and 1 otherwise. Any part of the pat-
tern 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 expres-
sion 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 con-
ditional expression’s return value is 2. If the shell option
nocaseglob is enabled, the match is performed without regard to
the case of alphabetic characters. Substrings matched by
parenthesized subexpressions within the regular expression are
saved in the array variable BASH_REMATCH. The element of
BASH_REMATCH with index 0 is the portion of the string matching
the entire regular expression. The element of BASH_REMATCH
with index n is the portion of the string matching the nth
parenthesized subexpression.
Expressions may be combined using the following operators,
listed in decreasing order of precedence:
( expression )
Returns the value of expression. This may be used to
override the normal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
for name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list
of items. The variable name is set to each element of this
list in turn, and list is executed each time. If the in word
is omitted, the for command executes list once for each posi-
tional parameter that is set (see PARAMETERS below). The
return status is the exit status of the last command that exe-
cutes. If the expansion of the items following in results in
an empty list, no commands are executed, and the return status
is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according
to the rules described below under ARITHMETIC EVALUATION. The
arithmetic expression expr2 is then evaluated repeatedly until
it evaluates to zero. Each time expr2 evaluates to a non-zero
value, list is executed and the arithmetic expression expr3 is
evaluated. If any expression is omitted, it behaves as if it
evaluates to 1. The return value is the exit status of the
last command in list that is executed, or false if any of the
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 stan-
dard input. If the line consists of a number corresponding to
one of the displayed words, then the value of name is set to
that word. If the line is empty, the words and prompt are dis-
played again. If EOF is read, the command completes. Any
other value read causes name to be set to null. The line read
is saved in the variable REPLY. The list is executed after
each selection until a break command is executed. The exit
status of select is the exit status of the last command exe-
cuted in list, or zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it
against each pattern in turn, using the same matching rules as
for pathname expansion (see Pathname Expansion below). When a
match is found, the corresponding list is executed. After the
first match, no subsequent matches are attempted. The exit
status is zero if no pattern matches. Otherwise, it is the
exit status of the last command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then
list is executed. Otherwise, each elif list is executed in
turn, and if its exit status is zero, the corresponding then
list is executed and the command completes. Otherwise, the
else list is executed, if present. The exit status is the exit
status of the last command executed, or zero if no condition
tested true.
while list; 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 exe-
cuted.
Shell Function Definitions
A shell function is an object that is called like a simple command and
executes a compound command with a new set of positional parameters.
Shell functions are declared as follows:
[ function ] name () compound-command [redirection]
This defines a function named name. The reserved word function
is optional. If the function reserved word is supplied, the
parentheses are optional. The body of the function is the com-
pound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above. com-
pound-command is executed whenever name is specified as the
name of a simple command. Any redirections (see REDIRECTION
below) specified when a function is defined are performed when
the function is executed. The exit status of a function defi-
nition is zero unless a syntax error occurs or a readonly func-
tion with the same name already exists. When executed, the
exit status of a function is the exit status of the last com-
mand executed in the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is enabled (see SHELL
BUILTIN COMMANDS below), a word beginning with # causes that word and
all remaining characters on that line to be ignored. An interactive
shell without the interactive_comments option enabled does not allow
comments. The interactive_comments option is on by default in inter-
active 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 recog-
nized 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, the his-
tory expansion character, usually !, must be quoted to prevent history
expansion.
There are three quoting mechanisms: the escape character, single
quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the
literal value of the next character that follows, with the exception
of <newline>. If a \<newline> pair appears, and the backslash is not
itself quoted, the \<newline> is treated as a line continuation (that
is, it is removed from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of
each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of
all characters within the quotes, with the exception of $, ‘, and \.
The characters $ and ‘ retain their special meaning within double
quotes. The backslash retains its special meaning only when followed
by one of the following characters: $, ‘, ", \, or <newline>. A dou-
ble quote may be quoted within double quotes by preceding it with a
backslash. When command history is being used, the double quote may
not be used to quote the history expansion character.
The special parameters * and @ have special meaning when in double
quotes (see PARAMETERS below).
Words of the form $’string’ are treated specially. The word expands
to string, with backslash-escaped characters replaced as specifed by
the ANSI C standard. Backslash escape sequences, if present, are
decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\’ single quote
\nnn the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not
been present.
A double-quoted string preceded by a dollar sign ($) will cause the
string to be translated according to the current locale. If the cur-
rent locale is C or POSIX, the dollar sign is ignored. If the string
is translated and replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a num-
ber, or one of the special characters listed below under Special
Parameters. A variable is a parameter denoted by a name. A variable
has a value and zero or more attributes. Attributes are assigned
using the declare builtin command (see declare below in SHELL BUILTIN
COMMANDS).
A parameter is set if it has been assigned a value. The null string
is a valid value. Once a variable is set, it may be unset only by
using the unset builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion, com-
mand substitution, arithmetic expansion, and quote removal (see EXPAN-
SION below). If the variable has its integer attribute set, then
value is evaluated as an arithmetic expression even if the $((...))
expansion is not used (see Arithmetic Expansion below). Word split-
ting is not performed, with the exception of "$@" as explained below
under Special Parameters. Pathname expansion is not performed.
Assignment statements may also appear as arguments to the alias,
declare, typeset, export, readonly, and local builtin commands.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits,
other than the single digit 0. Positional parameters are assigned
from the shell’s arguments when it is invoked, and may be reassigned
using the set builtin command. Positional parameters may not be
assigned to with assignment statements. The positional parameters are
temporarily replaced when a shell function is executed (see FUNCTIONS
below).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When
the expansion occurs within double quotes, it expands to a sin-
gle word with the value of each parameter separated by the
first character of the IFS special variable. That is, "$*" is
equivalent to "$1c$2c...", where c is the first character of
the value of the IFS variable. If IFS is unset, the parameters
are separated by spaces. If IFS is null, the parameters are
joined without intervening separators.
@ Expands to the positional parameters, starting from one. When
the expansion occurs within double quotes, each parameter
expands to a separate word. That is, "$@" is equivalent to
"$1" "$2" ... When there are no positional parameters, "$@"
and $@ expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground
pipeline.
- Expands to the current option flags as specified upon invoca-
tion, by the set builtin command, or those set by the shell
itself (such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the sub-
shell.
! Expands to the process ID of the most recently executed back-
ground (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 argu-
ment after the string to be executed, if one is present. Oth-
erwise, it is set to the file name used to invoke bash, as
given by argument zero.
_ At shell startup, set to the absolute file name of the shell or
shell script being executed as passed in the argument list.
Subsequently, expands to the last argument to the previous com-
mand, after expansion. Also set to the full file name of each
command executed and placed in the environment exported to that
command. When checking mail, this parameter holds the name of
the mail file currently being checked.
Shell Variables
The following variables are set by the shell:
BASH Expands to the full file name used to invoke this instance of
bash.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack. The num-
ber of parameters to the current subroutine (shell function or
script executed with . or source) is at the top of the stack.
When a subroutine is executed, the number of parameters passed
is pushed onto BASH_ARGC.
BASH_ARGV
An array variable containing all of the parameters in the cur-
rent bash execution call stack. The final parameter of the
last subroutine call is at the top of the stack; the first
parameter of the initial call is at the bottom. When a subrou-
tine is executed, the parameters supplied are pushed onto
BASH_ARGV.
BASH_COMMAND
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a
trap, in which case it is the command executing at the time of
the trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files corresponding to each member of @var{FUNCNAME}.
${BASH_LINENO[$i]} is the line number in the source file where
${FUNCNAME[$i + 1]} was called. The corresponding source file
name is ${BASH_SOURCE[$i + 1]}. Use LINENO to obtain the cur-
rent 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 filenames corre-
sponding 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 informa-
tion 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).
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the pro-
grammable completion facilities (see Programmable Completion
below).
COMP_POINT
The index of the current cursor position relative to the begin-
ning of the current command. If the current cursor position is
at the end of the current command, the value of this variable
is equal to ${#COMP_LINE}. This variable is available only in
shell functions and external commands invoked by the pro-
grammable completion facilities (see Programmable Completion
below).
COMP_WORDBREAKS
The set of characters that the Readline library treats as word
separators when performing word completion. If COMP_WORDBREAKS
is unset, it loses its special properties, even if it is subse-
quently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individ-
ual words in the current command line. This variable is avail-
able only in shell functions invoked by the programmable com-
pletion facilities (see Programmable Completion 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 mod-
ify directories already in the stack, but the pushd and popd
builtins must be used to add and remove directories. Assign-
ment 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, initial-
ized at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index
0 is the name of any currently-executing shell function. The
bottom-most element is "main". This variable exists only when
a shell function is executing. Assignments to FUNCNAME have no
effect and return an error status. If FUNCNAME is unset, it
loses its special properties, even if it is subsequently reset.
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 cur-
rent command. If HISTCMD is unset, it loses its special prop-
erties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type
of machine on which bash is executing. The default is system-
dependent.
LINENO Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to
be meaningful. If LINENO is unset, it loses its special prop-
erties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system
type on which bash is executing, in the standard GNU cpu-com-
pany-system format. The default is system-dependent.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating sys-
tem on which bash is executing. The default is system-depen-
dent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit
status values from the processes in the most-recently-executed
foreground pipeline (which may contain only a single command).
PPID The process ID of the shell’s parent. This variable is read-
only.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, a random integer
between 0 and 32767 is generated. The sequence of random num-
bers 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 command when
no arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds
since shell invocation is returned. If a value is assigned to
SECONDS, the value returned upon subsequent references is the
number of seconds since the assignment plus the value assigned.
If SECONDS is unset, it loses its special properties, even if
it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -o option to the set
builtin command (see SHELL BUILTIN COMMANDS below). The
options appearing in SHELLOPTS are those reported as on by set
-o. If this variable is in the environment when bash starts
up, each shell option in the list will be enabled before
reading any startup files. This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at
shell startup. This variable is readonly.
The following variables are used by the shell. In some cases, bash
assigns a default value to a variable; these cases are noted below.
BASH_ENV
If this parameter is set when bash is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in ~/.bashrc. The value of BASH_ENV
is subjected to parameter expansion, command substitution, and
arithmetic expansion before being interpreted as a file name.
PATH is not used to search for the resultant file name.
CDPATH The search path for the cd command. This is a colon-separated
list of directories in which the shell looks for destination
directories specified by the cd command. A sample value is
".:~:/usr".
COLUMNS
Used by the select builtin command to determine the terminal
width when printing selection lists. Automatically set upon
receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible comple-
tions 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 performing
filename completion (see READLINE below). A filename whose
suffix matches one of the entries in FIGNORE is excluded from
the list of matched filenames. A sample value is ".o:~".
GLOBIGNORE
A colon-separated list of patterns defining the set of file-
names to be ignored by pathname expansion. If a filename
matched by a pathname expansion pattern also matches one of the
patterns in GLOBIGNORE, it is removed from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are
saved on the history list. If the list of values includes
ignorespace, lines which begin with a space character are not
saved in the history list. A value of ignoredups causes lines
matching the previous history entry to not be saved. A value
of ignoreboth is shorthand for ignorespace and ignoredups. A
value of erasedups causes all previous lines matching the cur-
rent line to be removed from the history list before that line
is saved. Any value not in the above list is ignored. If
HISTCONTROL is unset, or does not include a valid value, all
lines read by the shell parser are saved on the history list,
subject to the value of HISTIGNORE. The second and subsequent
lines of a multi-line compound command are not tested, and are
added to the history regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below). The default value is ~/.bash_history. If
unset, the command history is not saved when an interactive
shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file.
When this variable is assigned a value, the history file is
truncated, if necessary, to contain no more than that number of
lines. The default value is 500. The history file is also
truncated to this size after writing it when an interactive
shell exits.
HISTIGNORE
A colon-separated list of patterns used to decide which command
lines should be saved on the history list. Each pattern is
anchored at the beginning of the line and must match the com-
plete line (no implicit ‘*’ is appended). Each pattern is
tested against the line after the checks specified by HISTCON-
TROL are applied. In addition to the normal shell pattern
matching characters, ‘&’ matches the previous history line.
‘&’ may be escaped using a backslash; the backslash is removed
before attempting a match. The second and subsequent lines of
a multi-line compound command are not tested, and are added to
the history regardless of the value of HISTIGNORE.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below). 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 associ-
ated 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.
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 host-
name. 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, bash attempts to read /etc/hosts to obtain the
list of possible hostname completions. When HOSTFILE is unset,
the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting
after expansion and to split lines into words with the read
builtin command. The default value is ‘‘<space><tab><new-
line>’’.
IGNOREEOF
Controls the action of an interactive shell on receipt of an
EOF character as the sole input. If set, the value is the num-
ber of consecutive EOF characters which must be typed as the
first characters on an input line before bash exits. If the
variable exists but does not have a numeric value, or has no
value, the default value is 10. If it does not exist, EOF sig-
nifies the end of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the
default of ~/.inputrc (see READLINE below).
LANG Used to determine the locale category for any category not
specifically selected with a variable starting with LC_.
LC_ALL This variable overrides the value of LANG and any other LC_
variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting
the results of pathname expansion, and determines the behavior
of range expressions, equivalence classes, and collating
sequences within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and
the behavior of character classes within pathname expansion and
pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
LINES Used by the select builtin command to determine the column
length for printing selection lists. Automatically set upon
receipt of a SIGWINCH.
MAIL If this parameter is set to a file name and the MAILPATH vari-
able 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 primary prompt. If this
variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail check-
ing.
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 mes-
sage with a ‘?’. When used in the text of the message, $_
expands to the name of the current mailfile. Example:
MAILPATH=’/var/mail/bfox?"You have mail":~/shell-mail?"$_ has
mail!"’
Bash supplies a default value for this variable, but the loca-
tion of the user mail files that it uses is system dependent
(e.g., /var/mail/$USER).
OPTERR If set to the value 1, bash displays error messages generated
by the getopts builtin command (see SHELL BUILTIN COMMANDS
below). OPTERR is initialized to 1 each time the shell is
invoked or a shell script is executed.
PATH The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null direc-
tory name may appear as two adjacent colons, or as an initial
or trailing colon. The default path is system-dependent, and
is set by the administrator who installs bash. A common value
is ‘‘/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin’’.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the
shell enters posix mode before reading the startup files, as if
the --posix invocation option had been supplied. If it is set
while the shell is running, bash enables posix mode, as if the
command set -o posix had been executed.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing
each primary prompt.
PS1 The value of this parameter is expanded (see PROMPTING below)
and used as the primary prompt string. The default value is
‘‘\s-\v\$ ’’.
PS2 The value of this parameter is expanded as with PS1 and used as
the secondary prompt string. The default is ‘‘> ’’.
PS3 The value of this parameter is used as the prompt for the
select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1 and the
value is printed before each command bash displays during an
execution trace. The first character of PS4 is replicated mul-
tiple times, as necessary, to indicate multiple levels of indi-
rection. The default is ‘‘+ ’’.
SHELL The full pathname to the shell is kept in this environment
variable. If it is not set when the shell starts, bash assigns
to it the full pathname of the current user’s login shell.
TIMEFORMAT
The value of this parameter is used as a format string specify-
ing how the timing information for pipelines prefixed with the
time reserved word should be displayed. The % character intro-
duces an escape sequence that is expanded to a time value or
other information. The escape sequences and their meanings are
as follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number
of fractional digits after a decimal point. A value of 0
causes no decimal point or fraction to be output. At most
three places after the decimal point may be specified; values
of p greater than 3 are changed to 3. If p is not specified,
the value 3 is used.
The optional l specifies a longer format, including minutes, of
the form MMmSS.FFs. The value of p determines whether or not
the fraction is included.
If this variable is not set, bash acts as if it had the value
$’\nreal\t%3lR\nuser\t%3lU\nsys%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 ter-
minates if input does not arrive after TMOUT seconds when input
is coming from a terminal. In an interactive shell, the value
is interpreted as the number of seconds to wait for input after
issuing the primary prompt. Bash terminates after waiting for
that number of seconds if input does not arrive.
auto_resume
This variable controls how the shell interacts with the user
and job control. If this variable is set, single word simple
commands without redirections are treated as candidates for
resumption of an existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning with the
string typed, the job most recently accessed is selected. The
name of a stopped job, in this context, is the command line
used to start it. If set to the value exact, the string sup-
plied must match the name of a stopped job exactly; if set to
substring, the string supplied needs to match a substring of
the name of a stopped job. The substring value provides func-
tionality analogous to the %? job identifier (see JOB CONTROL
below). If set to any other value, the supplied string must be
a prefix of a stopped job’s name; this provides functionality
analogous to the % job identifier.
histchars
The two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below). The first charac-
ter 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 nec-
essarily cause the shell parser to treat the rest of the line
as a comment.
Arrays
Bash provides one-dimensional array variables. Any variable may be
used as an array; the declare builtin will explicitly declare an
array. There is no maximum limit on the size of an array, nor any
requirement that members be indexed or assigned contiguously. Arrays
are indexed using integers and are zero-based.
An 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 array, use declare -a name
(see SHELL BUILTIN COMMANDS below). declare -a name[subscript] is
also accepted; the subscript is ignored. Attributes may be specified
for an array variable using the declare and readonly builtins. Each
attribute applies to all members of an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each value is of the form [sub-
script]=string. Only string is required. 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. This syntax is also
accepted by the declare builtin. Individual array elements may be
assigned to using the name[subscript]=value syntax introduced above.
Any element of an array may be referenced using ${name[subscript]}.
The braces are required to avoid conflicts with pathname expansion.
If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double
quotes. If the word is double-quoted, ${name[*]} expands to a single
word with the value of each array member separated by the first char-
acter 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. This is analogous to the expansion of
the special parameters * and @ (see Special Parameters above).
${#name[subscript]} expands to the length of ${name[subscript]}. If
subscript is * or @, the expansion is the number of elements in the
array. Referencing an array variable without a subscript is equiva-
lent to referencing element zero.
The unset builtin is used to destroy arrays. unset name[subscript]
destroys the array element at index subscript. unset name, where name
is an array, or unset name[subscript], where subscript is * or @,
removes the entire array.
The declare, local, and readonly builtins each accept a -a option to
specify an array. The read builtin accepts a -a option to assign a
list of words read from the standard input to an array. The set and
declare builtins display array values in a way that allows them to be
reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split
into words. There are seven kinds of expansion performed: brace
expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, and pathname
expansion.
The order of expansions is: brace expansion, tilde expansion, parame-
ter, variable and arithmetic expansion and command substitution (done
in a left-to-right fashion), word splitting, and pathname expansion.
On systems that can support it, there is an additional expansion
available: process substitution.
Only brace expansion, word splitting, and pathname expansion can
change the number of words of the expansion; other expansions expand a
single word to a single word. The only exceptions to this are the
expansions of "$@" and "${name[@]}" as explained above (see PARAME-
TERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be gen-
erated. This mechanism is similar to pathname expansion, but the
filenames generated need not exist. Patterns to be brace expanded
take the form of an optional preamble, followed by either a series of
comma-separated strings or a sequence expression between a pair of
braces, followed by an optional postscript. The preamble is prefixed
to each string contained within the braces, and the postscript is then
appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string
are not sorted; left to right order is preserved. For example,
a{d,c,b}e expands into ‘ade ace abe’.
A sequence expression takes the form {x..y}, where x and y are either
integers or single characters. When integers are supplied, the
expression expands to each number between x and y, inclusive. When
characters are supplied, the expression expands to each character lex-
icographically between x and y, inclusive. Note that both x and y
must be of the same type.
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 interpreta-
tion 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 param-
eter expansion, the string ${ is not considered eligible for brace
expansion.
This construct is typically used as shorthand when the common prefix
of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical
versions of sh. sh does not treat opening or closing braces specially
when they appear as part of a word, and preserves them in the output.
Bash removes braces from words as a consequence of brace expansion.
For example, a word entered to sh as file{1,2} appears identically in
the output. The same word is output as file1 file2 after expansion by
bash. If strict compatibility with sh is desired, start bash with the
+B option or disable brace expansion with the +B option to the set
command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (‘~’), all of the
characters preceding the first unquoted slash (or all characters, if
there is no unquoted slash) are considered a tilde-prefix. If none of
the characters in the tilde-prefix are quoted, the characters in the
tilde-prefix following the tilde are treated as a possible login name.
If this login name is the null string, the tilde is replaced with the
value of the shell parameter HOME. If HOME is unset, the home direc-
tory of the user executing the shell is substituted instead. Other-
wise, the tilde-prefix is replaced with the home directory associated
with the specified login name.
If the tilde-prefix is a ‘~+’, the value of the shell variable PWD
replaces the tilde-prefix. If the tilde-prefix is a ‘~-’, the value
of the shell variable OLDPWD, if it is set, is substituted. If the
characters following the tilde in the tilde-prefix consist of a number
N, optionally prefixed by a ‘+’ or a ‘-’, the tilde-prefix is replaced
with the corresponding element from the directory stack, as it would
be displayed by the dirs builtin invoked with the tilde-prefix as an
argument. If the characters following the tilde in the tilde-prefix
consist of a number without a leading ‘+’ or ‘-’, ‘+’ is assumed.
If the login name is invalid, or the tilde expansion fails, the word
is unchanged.
Each variable assignment is checked for unquoted tilde-prefixes imme-
diately following a : or =. In these cases, tilde expansion is also
performed. Consequently, one may use file names with tildes in
assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the
expanded value.
Parameter Expansion
The ‘$’ character introduces parameter expansion, command substitu-
tion, 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 fol-
lowing 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 paramter
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 vari-
able; this variable is then expanded and that value is used in the
rest of the substitution, rather than the value of parameter itself.
This is known as indirect expansion. The exceptions to this are the
expansions of ${!prefix*} and ${!name[@]} described below. The excla-
mation point must immediately follow the left brace in order to intro-
duce indirection.
In each of the cases below, word is subject to tilde expansion, param-
eter expansion, command substitution, and arithmetic expansion. When
not performing substring expansion, bash tests for a parameter that is
unset or null; omitting the colon results in a test only for a parame-
ter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expan-
sion of word is substituted. Otherwise, the value of parameter
is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter. The value of
parameter is then substituted. Positional parameters and spe-
cial parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset,
the expansion of word (or a message to that effect if word is
not present) is written to the standard error and the shell, if
it is not interactive, exits. Otherwise, the value of parame-
ter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of
parameter starting at the character specified by offset. If
length is omitted, expands to the substring of parameter start-
ing at the character specified by offset. length and offset
are arithmetic expressions (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 parameter is an array name indexed by
@ or *, the result is the length members of the array beginning
with ${parameter[offset]}. Substring indexing is zero-based
unless the positional parameters are used, in which case the
indexing starts at 1.
${!prefix*}
${!prefix@}
Expands to the names of variables whose names begin with pre-
fix, separated by the first character of the IFS special vari-
able.
${!name[@]}
${!name[*]}
If name is an array variable, expands to the list of array
indices (keys) assigned in name. If name is not an array,
expands to 0 if name is set and null otherwise. When @ is used
and the expansion appears within double quotes, each key
expands to a separate word.
${#parameter}
The length in characters of the value of parameter is substi-
tuted. If parameter is * or @, the value substituted is the
number of positional parameters. If parameter is an array name
subscripted by * or @, the value substituted is the number of
elements in the array.
${parameter#word}
${parameter##word}
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 pattern (the ‘‘##’’ case)
deleted. If parameter is @ or *, the pattern removal operation
is applied to each positional parameter in turn, and the expan-
sion is the resultant list. If parameter is an array variable
subscripted with @ or *, the pattern removal operation is
applied to each member of the array in turn, and the expansion
is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as in pathname
expansion. If the pattern matches a trailing portion of the
expanded value of parameter, then the result of the expansion
is the expanded value of parameter with the shortest matching
pattern (the ‘‘%’’ case) or the longest matching pattern (the
‘‘%%’’ case) deleted. If parameter is @ or *, the pattern
removal operation is applied to each positional parameter in
turn, and the expansion is the resultant list. If parameter is
an array variable subscripted with @ or *, the pattern removal
operation is applied to each member of the array in turn, and
the expansion is the resultant list.
${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in path-
name expansion. Parameter is expanded and the longest match of
pattern against its value is replaced with string. In the
first form, only the first match is replaced. The second form
causes all matches of pattern to be replaced with string. 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 substitu-
tion operation is applied to each positional parameter in turn,
and the expansion is the resultant list. If parameter is an
array variable subscripted with @ or *, the substitution opera-
tion is applied to each member of the array in turn, and the
expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to replace the
command name. There are two forms:
$(command)
or
‘command‘
Bash performs the expansion by executing command and replacing the
command substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but
they may be removed during word splitting. The command substitution
$(cat file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, ‘, or \. The
first backquote not preceded by a backslash terminates the command
substitution. When using the $(command) form, all characters between
the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the back-
quoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic expan-
sion is:
$((expression))
The 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 expan-
sion, 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 mes-
sage indicating failure and no substitution occurs.
Process Substitution
Process substitution is supported on systems that support named pipes
(FIFOs) or the /dev/fd method of naming open files. It takes the form
of <(list) or >(list). The process list is run with its input or out-
put connected to a FIFO or some file in /dev/fd. The name of this
file is passed as an argument to the current command as the result of
the expansion. If the >(list) form is used, writing to the file will
provide input for list. If the <(list) form is used, the file passed
as an argument should be read to obtain the output of list.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
Word Splitting
The shell scans the results of parameter expansion, command substitu-
tion, and arithmetic expansion that did not occur within double quotes
for word splitting.
The shell treats each character of IFS as a delimiter, and splits the
results of the other expansions into words on these characters. If
IFS is unset, or its value is exactly <space><tab><newline>, the
default, then any sequence of IFS characters 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 characters,
delimits a field. A sequence of IFS whitespace characters is also
treated as a delimiter. If the value of IFS is null, no word split-
ting occurs.
Explicit null arguments ("" or ’’) are retained. Unquoted implicit
null arguments, resulting from the expansion of parameters that have
no values, are removed. If a parameter with no value is expanded
within double quotes, a null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set, bash scans
each word for the characters *, ?, and [. If one of these characters
appears, then the word is regarded as a pattern, and replaced with an
alphabetically sorted list of file names matching the pattern. If no
matching file names are found, and the shell option nullglob is dis-
abled, the word is left unchanged. If the nullglob option is set, and
no matches are found, the word is removed. If the failglob shell
option is set, and no matches are found, an error message is printed
and the command is not executed. If the shell option nocaseglob is
enabled, the match is performed without regard to the case of alpha-
betic 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, fail-
glob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file
names matching a pattern. If GLOBIGNORE is set, each matching file
name that also matches one of the patterns in GLOBIGNORE is removed
from the list of matches. The file names ‘‘.’’ and ‘‘..’’ are
always ignored when GLOBIGNORE is set and not null. 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 special pat-
tern characters described below, matches itself. The NUL character
may not occur in a pattern. A backslash escapes the following charac-
ter; 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.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of charac-
ters separated by a hyphen denotes a range expression; any
character that sorts between those two characters, inclusive,
using the current locale’s collating sequence and character
set, is matched. If the first character following the [ is a !
or a ^ then any character not enclosed is matched. The 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.2 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 colla-
tion weight (as defined by the current locale) as the character
c.
Within [ and ], the syntax [.symbol.] matches the collating
symbol symbol.
If the extglob shell option is enabled using the shopt builtin, sev-
eral extended pattern matching operators are recognized. In the fol-
lowing description, a pattern-list is a list of one or more patterns
separated by a |. Composite patterns may be formed using one or more
of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches exactly one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Quote Removal
After the preceding expansions, all unquoted occurrences of the char-
acters \, ’, and " that did not result from one of the above expan-
sions 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.
In the following descriptions, if the file descriptor number is omit-
ted, and the first character of the redirection operator is <, the
redirection refers to the standard input (file descriptor 0). If the
first character of the redirection operator is >, the redirection
refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descrip-
tions, 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 as standard output before the standard output was
redirected to dirlist.
Bash handles several filenames specially when they are used in redi-
rections, as described in the following table:
/dev/fd/fd
If fd is a valid integer, file descriptor fd is dupli-
cated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and
port is an integer port number or service name, bash
attempts to open 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.
Redirecting Input
Redirection of input causes the file whose name results from the
expansion of word to be opened for reading on file descriptor n, or
the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the
expansion of word to be opened for writing on file descriptor n, or
the standard output (file descriptor 1) if n is not specified. If the
file does not exist it is created; if it does exist it is truncated to
zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option to the set
builtin has been enabled, the redirection will fail if the file whose
name results from the expansion of word exists and is a regular file.
If the redirection operator is >|, or the redirection operator is >
and the noclobber option to the set builtin command is not enabled,
the 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
Bash allows both the standard output (file descriptor 1) and the stan-
dard error output (file descriptor 2) to be redirected to the file
whose name is the expansion of word with this construct.
There are two formats for redirecting standard output and standard
error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equiv-
alent 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 word (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 latter 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 containing delimiter. This
allows here-documents within shell scripts to be indented in a natural
fashion.
Here Strings
A variant of here documents, the format is:
<<<word
The word is expanded and supplied to the command on its standard
input.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one
or more digits, the file descriptor denoted by n is made to be a copy
of that file descriptor. If the digits in word do not specify a file
descriptor open for input, a redirection error occurs. If word evalu-
ates 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) 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 quoting characters
listed above may not appear in an alias name. The replacement text
may contain any valid shell input, including shell metacharacters.
The first word of the replacement text is tested for aliases, but a
word that is identical to an alias being expanded is not expanded a
second time. This means that one may alias ls to ls -F, for instance,
and bash does not try to recursively expand the replacement text. If
the last character of the alias value is a blank, then the next com-
mand word following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed
with the unalias command.
There is no mechanism for using arguments in the replacement text. If
arguments are needed, a shell function should be used (see FUNCTIONS
below).
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the description of
shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input
before executing any of the commands on that line. Aliases are
expanded when a command is read, not when it is executed. Therefore,
an alias definition appearing on the same line as another command does
not take effect until the next line of input is read. The commands
following the alias definition on that line are not affected by the
new alias. This behavior is also an issue when functions are exe-
cuted. Aliases are expanded when a function definition is read, not
when the function is executed, because a function definition is itself
a compound command. As a consequence, aliases defined in a function
are not available until after that function is executed. To be safe,
always put alias definitions on a separate line, and do not use alias
in compound commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR,
stores a series of commands for later execution. When the name of a
shell function is used as a simple command name, the list of commands
associated with that function name is executed. Functions are exe-
cuted in the context of the current shell; no new process is created
to interpret them (contrast this with the execution of a shell
script). When a function is executed, the arguments to the function
become the positional parameters during its execution. The special
parameter # is updated to reflect the change. Special parameter 0 is
unchanged. The first element of the FUNCNAME variable is set to the
name of the function while the function is executing. All other
aspects of the shell execution environment are identical between a
function and its caller with the exception that the DEBUG trap (see
the description of the trap builtin under SHELL BUILTIN COMMANDS
below) is not inherited unless the function has been given the trace
attribute (see the description of the declare builtin below) or the -o
functrace shell option has been enabled with the set builtin (in which
case all functions inherit the DEBUG trap).
Variables local to the function may be declared with the local builtin
command. Ordinarily, variables and their values are shared between
the function and its caller.
If the builtin command return is executed in a function, the function
completes and execution resumes with the next command after the func-
tion call. Any command associated with the RETURN trap is executed
before execution resumes. When a function completes, the values of
the positional parameters and the special parameter # are restored to
the values they had prior to the function’s execution.
Function names and definitions may be listed with the -f option to the
declare or typeset builtin commands. The -F option to declare or
typeset will list the function names only (and optionally the source
file and line number, if the extdebug shell option is enabled). Func-
tions may be exported so that subshells automatically have them
defined with the -f option to the export 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 prob-
lem.
Functions may be recursive. No limit is imposed on the number of
recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain
circumstances (see the let and declare builtin commands and Arithmetic
Expansion). Evaluation is done in fixed-width integers with no check
for overflow, though division by 0 is trapped and flagged as an error.
The operators and their precedence, associativity, and values are the
same as in the C language. The following list of operators is grouped
into levels of equal-precedence operators. The levels are listed in
order of decreasing precedence.
id++ id--
variable post-increment and post-decrement
++id --id
variable pre-increment and pre-decrement
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -= <<= >>= &= ^= |=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is per-
formed before the expression is evaluated. Within an expression,
shell variables may also be referenced by name without using the
parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter
expansion syntax. The value of a variable is evaluated as an arith-
metic 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 inte-
ger attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A lead-
ing 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 repre-
sented by the lowercase letters, the uppercase letters, @, and _, in
that order. If base is less than or equal to 36, lowercase and upper-
case letters may be used interchangably to represent numbers between
10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence rules
above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the
test and [ builtin commands to test file attributes and perform string
and arithmetic comparisons. Expressions are formed from the following
unary or binary primaries. If any file argument to one of the pri-
maries is of the form /dev/fd/n, then file descriptor n is checked.
If the file argument to one of the primaries is one of /dev/stdin,
/dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively,
is checked.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ‘‘sticky’’ bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective user id.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-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 num-
bers.
-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
True if the strings are equal. = may be used in place of ==
for strict POSIX compliance.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically in the
current locale.
string1 > string2
True if string1 sorts after string2 lexicographically in the
current locale.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic
binary operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than
or equal to arg2, respectively. Arg1 and arg2 may be positive
or negative integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
1. The words that the parser has marked as variable assignments
(those preceding the command name) and redirections are saved
for later processing.
2. The words that are not variable assignments or redirections are
expanded. If any words remain after expansion, the first word
is taken to be the name of the command and the remaining words
are the arguments.
3. Redirections are performed as described above under REDIREC-
TION.
4. The text after the = in each variable assignment undergoes
tilde expansion, parameter expansion, command substitution,
arithmetic expansion, and quote removal before being assigned
to the variable.
If no command name results, the variable assignments affect the cur-
rent shell environment. Otherwise, the variables are added to the
environment of the executed command and do not affect the current
shell environment. If any of the assignments attempts to assign a
value to a readonly variable, an error occurs, and the command exits
with a non-zero status.
If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the expan-
sions contained a command substitution, the exit status of the command
is the exit status of the last command substitution performed. If
there were no command substitutions, the command exits with a status
of zero.
COMMAND EXECUTION
After a command has been split into words, if it results in a simple
command and an optional list of arguments, the following actions are
taken.
If the command name contains no slashes, the shell attempts to locate
it. If there exists a shell function by that name, that function is
invoked as described above in FUNCTIONS. If the name does not match a
function, the shell searches for it in the list of shell builtins. If
a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no
slashes, bash searches each element of the PATH for a directory con-
taining an executable file by that name. Bash uses a hash table to
remember the full pathnames of executable files (see hash under SHELL
BUILTIN COMMANDS below). A full search of the directories in PATH is
performed only if the command is not found in the hash table. If the
search is unsuccessful, the shell 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 exe-
cution environment. Argument 0 is set to the name given, and the
remaining arguments to the command are set to the arguments given, if
any.
If this execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell script, a
file containing shell commands. A subshell is spawned to execute it.
This subshell reinitializes itself, so that the effect is as if a new
shell had been invoked to handle the script, with the exception that
the locations of commands remembered by the parent (see hash below
under SHELL BUILTIN COMMANDS) are retained by the child.
If the program is a file beginning with #!, the remainder of the first
line specifies an interpreter for the program. The shell executes the
specified interpreter on operating systems that do not handle this
executable format themselves. The arguments to the interpreter con-
sist of a single optional argument following the interpreter name on
the first line of the program, followed by the name of the program,
followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the follow-
ing:
· open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin
· the current working directory as set by cd, pushd, or popd, or
inherited by the shell at invocation
· the file creation mode mask as set by umask or inherited from
the shell’s parent
· current traps set by trap
· shell parameters that are set by variable assignment or with
set or inherited from the shell’s parent in the environment
· shell functions defined during execution or inherited from the
shell’s parent in the environment
· options enabled at invocation (either by default or with com-
mand-line arguments) or by set
· options enabled by shopt
· shell aliases defined with alias
· various process IDs, including those of background jobs, the
value of $$, and the value of $PPID
When a simple command other than a builtin or shell function is to be
executed, it is invoked in a separate execution environment that con-
sists of the following. Unless otherwise noted, the values are inher-
ited from the shell.
· the shell’s open files, plus any modifications and additions
specified by redirections to the command
· the current working directory
· the file creation mode mask
· shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
· traps caught by the shell are reset to the values inherited
from the shell’s parent, and traps ignored by the shell are
ignored
A command invoked in this separate environment cannot affect the
shell’s execution environment.
Command substitution, commands grouped with parentheses, and asyn-
chronous commands are invoked in a subshell environment that is a
duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a
pipeline are also executed in a subshell environment. Changes made to
the subshell environment cannot affect the shell’s execution environ-
ment.
If a command is followed by a & and job control is not active, the
default standard input for the command is the empty file /dev/null.
Otherwise, the invoked command inherits the file descriptors of the
calling shell as modified by redirections.
ENVIRONMENT
When a program is invoked it is given an array of strings called the
environment. This is a list of name-value pairs, of the form
name=value.
The shell provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a parame-
ter 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 environ-
ment, replacing the old. The environment inherited by any executed
command consists of the shell’s initial environment, whose values may
be modified in the shell, less any pairs removed by the unset command,
plus any additions via the export and declare -x commands.
The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter assignments, as described
above in PARAMETERS. These assignment statements affect only the
environment seen by that command.
If the -k option is set (see the set builtin command below), then all
parameter assignments are placed in the environment for a command, not
just those that precede the command name.
When bash invokes an external command, the variable _ is set to the
full file name of the command and passed to that command in its envi-
ronment.
EXIT STATUS
For the shell’s purposes, a command which exits with a zero exit sta-
tus has succeeded. An exit status of zero indicates success. A non-
zero exit status indicates failure. When a command terminates on a
fatal signal N, bash uses the value of 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not executable,
the return status is 126.
If a command fails because of an error during expansion or redirec-
tion, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful, and
non-zero (false) if an error occurs while they execute. All builtins
return an exit status of 2 to indicate incorrect usage.
Bash itself returns the exit status of the last command executed,
unless a syntax error occurs, in which case it exits with a non-zero
value. See also the exit builtin command below.
SIGNALS
When bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that kill 0 does not kill an interactive shell), and SIG-
INT 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 val-
ues inherited by the shell from its parent. When job control is not
in effect, asynchronous commands ignore SIGINT and SIGQUIT in addition
to these inherited handlers. Commands run as a result of command sub-
stitution 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 partic-
ular job, it should be removed from the jobs table with the disown
builtin (see SHELL BUILTIN COMMANDS below) or marked to not receive
SIGHUP using disown -h.
If the huponexit shell option has been set with shopt, bash sends a
SIGHUP to all jobs when an interactive login shell exits.
If 0for which a trap has been set, the trap will not be executed until
the command completes. When bash is waiting for an asynchronous com-
mand via the wait builtin, the reception of a signal for which a trap
has been set will cause the wait builtin to return immediately with an
exit status greater than 128, immediately after which the trap is exe-
cuted.
JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the
execution of processes and continue (resume) their execution at a
later point. A user typically employs this facility via an interac-
tive interface supplied jointly by the system’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 pro-
cess group ID. Members of this process group (processes whose process
group ID is equal to the current terminal process group ID) receive
keyboard-generated signals such as SIGINT. These processes are said
to be in the foreground. Background processes are those whose process
group ID differs from the terminal’s; such processes are immune to
keyboard-generated signals. Only foreground processes are allowed to
read from or write to the terminal. Background processes which
attempt to read from (write to) the terminal are sent a SIGTTIN (SIGT-
TOU) signal by the 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 pro-
cess to be stopped and returns control to bash. Typing the delayed
suspend character (typically ^Y, Control-Y) causes the process to be
stopped when it attempts to read input from the terminal, and control
to be returned to bash. The user may then manipulate the state of
this job, using the bg command to continue it in the background, the
fg command to continue it in the foreground, or the kill command to
kill it. A ^Z takes effect immediately, and has the additional side
effect of causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The char-
acter % introduces a job name. 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 exam-
ple, %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 sub-
string 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 referenced using %-. 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 -.
Simply naming a job can be used to bring it into the foreground: %1 is
a synonym for ‘‘fg %1’’, bringing job 1 from the background into the
foreground. Similarly, ‘‘%1 &’’ resumes job 1 in the background,
equivalent to ‘‘bg %1’’.
The shell learns immediately whenever a job changes state. Normally,
bash waits until it is about to print a prompt before reporting
changes in a job’s status so as to not interrupt any other output. If
the -b option to the set builtin command is enabled, bash reports such
changes immediately. Any trap on SIGCHLD is executed for each child
that exits.
If an attempt to exit bash is made while jobs are stopped, the shell
prints a warning message. 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 the
stopped jobs are terminated.
PROMPTING
When executing interactively, bash displays the primary prompt PS1
when it is ready to read a command, and the secondary prompt PS2 when
it needs more input to complete a command. Bash allows these prompt
strings to be customized by inserting a number of backslash-escaped
special characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format (e.g., "Tue May
26")
\D{format}
the format is passed to strftime(3) and the result is
inserted into the prompt string; an empty format results
in a locale-specific time representation. The braces
are required
\e an ASCII escape character (033)
\h the hostname up to the first ‘.’
\H the hostname
\j the number of jobs currently managed by the shell
\l the basename of the shell’s terminal device name
\n newline
\r carriage return
\s the name of the shell, the basename of $0 (the portion
following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patch level (e.g.,
2.00.0)
\w the current working directory, with $HOME abbreviated
with a tilde
\W the basename of the current working directory, with
$HOME abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could
be used to embed a terminal control sequence into the
prompt
\] end a sequence of non-printing characters
The command number and the history number are usually different: the
history number of a command is its position in the history list, which
may include commands restored from the history file (see HISTORY
below), while the command number is the position in the sequence of
commands executed during the current shell session. After the string
is decoded, it is expanded via parameter expansion, command substitu-
tion, arithmetic expansion, and quote removal, subject to the value of
the promptvars shell option (see the description of the shopt command
under SHELL BUILTIN COMMANDS below).
READLINE
This is the library that handles reading input when using an interac-
tive shell, unless the --noediting option is given at shell invoca-
tion. By default, the line editing commands are similar to those of
emacs. A vi-style line editing interface is also available. To turn
off line editing after the shell is running, use the +o emacs or +o vi
options to the set builtin (see SHELL BUILTIN COMMANDS below).
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 Con-
trol-N. Similarly, meta keys are denoted by M-key, so M-x means
Meta-X. (On keyboards without a meta key, M-x means ESC x, i.e.,
press the Escape key then the x key. This makes ESC the meta prefix.
The combination M-C-x means ESC-Control-x, or press the Escape key
then hold the Control key while pressing the x key.)
Readline commands may be given numeric arguments, which normally act
as a repeat count. Sometimes, however, it is the sign of the argument
that is significant. Passing a negative argument to a command that
acts in the forward direction (e.g., kill-line) causes that command to
act in a backward direction. Commands whose behavior with arguments
deviates from this are noted below.
When a command is described as killing text, the text deleted is saved
for possible future retrieval (yanking). The killed text is saved in
a kill ring. Consecutive kills cause the text to be accumulated into
one unit, which can be yanked all at once. Commands which do not kill
text separate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initialization file
(the inputrc file). The name of this file is taken from the value of
the INPUTRC variable. If that variable is unset, the default is
~/.inputrc. When a program which uses the readline library starts up,
the initialization file is read, and the key bindings and variables
are set. There are only a few basic constructs allowed in the read-
line initialization file. Blank lines are ignored. Lines beginning
with a # are comments. Lines beginning with a $ indicate conditional
constructs. Other lines denote key bindings and variable settings.
The default key-bindings may be changed with an inputrc file. Other
programs that use this library may add their own commands and bind-
ings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline command univer-
sal-argument.
The following symbolic character names are recognized: RUBOUT, DEL,
ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.
In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple.
All that is required is the name of the command or the text of a macro
and a key sequence to which it should be bound. The name may be speci-
fied in one of two ways: as a symbolic key name, possibly with Meta-
or Control- prefixes, or as a key sequence.
When using the form keyname:function-name or macro, keyname is the
name of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function universal-argument,
M-DEL is bound to the function backward-kill-word, and C-o is bound to
run the macro expressed on the right hand side (that is, to insert the
text ‘‘> output’’ into the line).
In the second form, "keyseq":function-name or macro, keyseq differs
from keyname above in that strings denoting an entire key sequence may
be specified by placing the sequence within double quotes. Some GNU
Emacs style key escapes can be used, as in the following example, but
the symbolic character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function universal-argu-
ment. C-x C-r is bound to the function re-read-init-file, and ESC [ 1
1 ~ is bound to insert the text ‘‘Function Key 1’’.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\’ literal ’
In addition to the GNU Emacs style escape sequences, a second set of
backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
When entering the text of a macro, single or double quotes must be
used to indicate a macro definition. Unquoted text is assumed to be a
function name. In the macro body, the backslash escapes described
above are expanded. Backslash will quote any other character in the
macro text, including " and ’.
Bash allows the current readline key bindings to be displayed or modi-
fied with the bind builtin command. The editing mode may be switched
during interactive use by using the -o option to the set builtin com-
mand (see SHELL BUILTIN COMMANDS below).
Readline Variables
Readline has variables that can be used to further customize its
behavior. A variable may be set in the inputrc file with a statement
of the form
set variable-name value
Except where noted, readline variables can take the values On or Off.
The variables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring the terminal
bell. If set to none, readline never rings the bell. If set
to visible, readline uses a visible bell if one is available.
If set to audible, readline attempts to ring the terminal’s
bell.
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 comple-
tion in a case-insensitive fashion.
completion-query-items (100)
This determines when the user is queried about viewing the num-
ber of possible completions generated by the possible-comple-
tions command. It may be set to any integer value greater than
or equal to zero. If the number of possible completions is
greater than or equal to the value of this variable, the user
is asked whether or not he wishes to view them; otherwise they
are simply listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth
bit set to an ASCII key sequence by stripping the eighth bit
and prefixing an escape character (in effect, using escape as
the meta prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion. Comple-
tion 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.
enable-keypad (Off)
When set to On, readline will try to enable the application
keypad when it is called. Some systems need this to enable the
arrow keys.
expand-tilde (Off)
If set to on, tilde expansion is performed when readline
attempts word completion.
history-preserve-point
If set to on, the history code attempts to place point at the
same location on each history line retrived with previous-his-
tory or next-history.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line for display,
scrolling the input horizontally on a single screen line when
it becomes longer than the screen width rather than wrapping to
a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input (that is, it
will not strip the high bit from the characters it reads),
regardless of what the terminal claims it can support. The
name meta-flag is a synonym for this variable.
isearch-terminators (‘‘C-[C-J’’)
The string of characters that should terminate an incremental
search without subsequently executing the character as a com-
mand. If this variable has not been given a value, the charac-
ters ESC and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names
is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-com-
mand, and vi-insert. vi is equivalent to vi-command; emacs is
equivalent to emacs-standard. The default value is emacs; the
value of editing-mode also affects the default keymap.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are dis-
played with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
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 completed.
output-meta (Off)
If set to On, readline will display characters with the eighth
bit set directly rather than as a meta-prefixed escape
sequence.
page-completions (On)
If set to On, readline uses an internal more-like pager to dis-
play a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the
screen.
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 comple-
tion cause the matches to be listed immediately instead of
ringing the bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in
a fashion similar to show-all-if-ambiguous. If set to on,
words which have more than one possible completion without any
possible partial completion (the possible completions don’t
share a common prefix) cause the matches to be listed immedi-
ately instead of ringing the bell.
visible-stats (Off)
If set to On, a character denoting a file’s type as reported by
stat(2) is appended to the filename when listing possible com-
pletions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings
and variable settings to be performed as the result of tests. There
are four parser directives used.
$if The $if construct allows bindings to be made based on the edit-
ing mode, the terminal being used, or the application using
readline. The text of the test extends to the end of the line;
no characters are required to isolate it.
mode The mode= form of the $if directive is used to test
whether readline is in emacs or vi mode. This may be
used in conjunction with the set keymap command, for
instance, to set bindings in the emacs-standard and
emacs-ctlx keymaps only if readline is starting out in
emacs mode.
term The term= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output
by the terminal’s function keys. The word on the right
side of the = is tested against the both full name of
the 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 applica-
tion-specific settings. Each program using the readline
library sets the application name, and an initialization
file can test for a particular value. This could be
used to bind key sequences to functions useful for a
specific program. For instance, the following command
adds a key sequence that quotes the current or previous
word in Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif This command, as seen in the previous example, terminates an
$if command.
$else Commands in this branch of the $if directive are executed if
the test fails.
$include
This directive takes a single filename as an argument and reads
commands and bindings from that file. For example, the follow-
ing directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command history
(see HISTORY below) for lines containing a specified string. There
are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the
search string. As each character of the search string is typed, read-
line displays the next entry from the history matching the string
typed so far. An incremental search requires only as many characters
as needed to find the desired history entry. The characters present
in the value of the isearch-terminators variable are used to terminate
an incremental search. If that variable has not been assigned a value
the Escape and Control-J characters will terminate an incremental
search. Control-G will abort an incremental search and restore the
original line. When the search is terminated, the history entry con-
taining the search string becomes the current line.
To find other matching entries in the history list, type Control-S or
Control-R as appropriate. This will search backward or forward in the
history for the next entry matching the search string typed so far.
Any other key sequence bound to a readline command will terminate the
search and execute that command. For instance, a newline will termi-
nate the search and accept the line, thereby executing the command
from the history list.
Readline remembers the last incremental search string. If two Con-
trol-Rs are typed without any intervening characters defining a new
search string, any remembered search string is used.
Non-incremental searches read the entire search string before starting
to search for matching history lines. The search string may be typed
by the user or be part of the contents of the current line.
Readline Command Names
The following is a list of the names of the commands and the default
key sequences to which they are bound. Command names without an
accompanying key sequence are unbound by default. In the following
descriptions, point refers to the current cursor position, and mark
refers to a cursor position saved by the set-mark command. The text
between the point and mark is referred to as the region.
Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words are composed
of alphanumeric characters (letters and digits).
backward-word (M-b)
Move back to the start of the current or previous word. Words
are composed of alphanumeric characters (letters and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the top of the
screen. With an argument, refresh the current line without
clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this
line is non-empty, add it to the history list according to the
state of the HISTCONTROL variable. If the line is a modified
history line, then restore the history line to its original
state.
previous-history (C-p)
Fetch the previous command from the history list, moving back
in the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in
the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently
being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving ‘up’
through the history as necessary. This is an incremental
search.
forward-search-history (C-s)
Search forward starting at the current line and moving ‘down’
through the history as necessary. This is an incremental
search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current
line using a non-incremental search for a string supplied by
the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental
search for a string supplied by the user.
history-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. This is a
non-incremental search.
history-search-backward
Search backward through the history for the string of charac-
ters between the start of the current line and the point. This
is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument
n, insert the nth word from the previous command (the words in
the previous command begin with word 0). A negative argument
inserts the nth word from the end of the previous command.
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.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and
history expansion as well as all of the shell word expansions.
See HISTORY EXPANSION below for a description of history expan-
sion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a
space. See HISTORY EXPANSION below for a description of his-
tory expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above
for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the next line
relative to the current line from the history for editing. Any
argument is ignored.
edit-and-execute-command (C-xC-e)
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke $FCEDIT,
$EDITOR, and emacs as the editor, in that order.
Commands for Changing Text
delete-char (C-d)
Delete the character at point. If point is at the beginning of
the line, there are no characters in the line, and the last
character typed was not bound to delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric
argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at
the end of the line, in which case the character behind the
cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how
to insert characters like C-q, for example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the character at
point, moving point forward as well. If point is at the end of
the line, then this transposes the two characters before point.
Negative arguments have no effect.
transpose-words (M-t)
Drag the word before point past the word after point, moving
point over that word as well. If point is at the end of the
line, this transposes the last two words on the line.
upcase-word (M-u)
Uppercase the current (or following) word. With a negative
argument, uppercase the previous word, but do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a negative
argument, lowercase the previous word, but do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a negative
argument, capitalize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric argu-
ment, switches to overwrite mode. With an explicit non-posi-
tive numeric argument, switches to insert mode. This command
affects only emacs mode; vi mode does overwrite differently.
Each call to readline() starts in insert mode. In overwrite
mode, characters bound to self-insert replace the text at point
rather than pushing the text to the right. Characters bound to
backward-delete-char replace the character before point with a
space. By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The
killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the
same as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as
those used by backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word bound-
ary. The killed text is saved on the kill-ring.
unix-filename-rubout
Kill the word behind point, using white space and the slash
character as the word boundaries. The killed text is saved on
the kill-ring.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text in the current region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The word bound-
aries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works follow-
ing yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a
new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is fol-
lowed by digits, executing universal-argument again ends the
numeric argument, but is otherwise ignored. As a special case,
if this command is immediately followed by a character that is
neither a digit or minus sign, the argument count for the next
command is multiplied by four. The argument count is initially
one, so executing this function the first time makes the argu-
ment count four, a second time makes the argument count six-
teen, and so on.
Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash
attempts completion treating the text as a variable (if the
text begins with $), username (if the text begins with ~),
hostname (if the text begins with @), or command (including
aliases and functions) in turn. If none of these produces a
match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with
a single match from the list of possible completions. Repeated
execution of menu-complete steps through the list of possible
completions, inserting each match in turn. At the end of the
list of completions, the bell is rung (subject to the setting
of bell-style) and the original text is restored. An argument
of n moves n positions forward in the list of matches; a nega-
tive argument may be used to move backward through the list.
This command is intended to be bound to TAB, but is unbound by
default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning
or end of the line (like delete-char). If at the end of the
line, behaves identically to possible-completions. This com-
mand is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treat-
ing it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treat-
ing it as a username.
complete-variable (M-$)
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, treat-
ing it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treat-
ing it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell
builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treat-
ing it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.
complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to
the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard
macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard
macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the char-
acters 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 incorporate any
bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal’s bell
(subject to the setting of bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the command that
is bound to the corresponding uppercase character.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to
Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the
undo command enough times to return the line to its initial
state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied,
the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is
set to the saved position, and the old cursor position is saved
as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence
of that character. A negative count searches for previous
occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occur-
rence of that character. A negative count searches for subse-
quent occurrences.
insert-comment (M-#)
Without a numeric argument, the value of the readline com-
ment-begin variable is inserted at the beginning of the current
line. If a numeric argument is supplied, this command acts as
a toggle: if the characters at the beginning of the line do
not match the value of comment-begin, the value is inserted,
otherwise the characters in comment-begin are deleted from the
beginning of the line. In either case, the line is accepted as
if a newline had been typed. The default value of com-
ment-begin causes this command to make the current line a shell
comment. If a numeric argument causes the comment character to
be removed, the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended. This pattern
is used to generate a list of matching file names for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname
expansion, and the list of matching file names is inserted,
replacing the word. If a numeric argument is supplied, an
asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn. If a
numeric argument is supplied, an asterisk is appended before
pathname expansion.
dump-functions
Print all of the functions and their key bindings to the read-
line output stream. If a numeric argument is supplied, the
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 sup-
plied, 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 ouput. If a numeric argument is supplied, the
output is formatted in such a way that it can be made part of
an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
Programmable Completion
When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined using
the complete builtin (see SHELL BUILTIN COMMANDS below), the pro-
grammable completion facilities are invoked.
First, the command name is identified. If a compspec has been defined
for that command, the compspec is used to generate the list of possi-
ble completions for the word. If the command word is a full pathname,
a compspec for the full pathname is searched for first. If no comp-
spec is found for the full pathname, an attempt is made to find a
compspec for the portion following the final slash.
Once a compspec has been found, it is used to generate the list of
matching words. If a compspec is not found, the default bash comple-
tion as described above under Completing is performed.
First, the actions specified by the compspec are used. Only matches
which are prefixed by the word being completed are returned. When the
-f or -d option is used for filename or directory name completion, the
shell variable FIGNORE is used to filter the matches.
Any completions specified by a filename expansion pattern to the -G
option are generated next. The words generated by the pattern need
not match the word being completed. The GLOBIGNORE shell variable is
not used to filter the matches, but the FIGNORE variable is used.
Next, the string specified as the argument to the -W option is consid-
ered. The string is first split using the characters in the IFS spe-
cial variable as delimiters. Shell quoting is honored. Each word is
then expanded using brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic expansion, and
pathname 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 and COMP_POINT 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 function may use
any of the shell facilities, including the compgen builtin described
below, to generate the matches. It must put the possible completions
in the COMPREPLY array variable.
Next, any command specified with the -C option is invoked in an envi-
ronment equivalent to command substitution. It should print a list of
completions, one per line, to the standard output. Backslash may be
used to escape a newline, if necessary.
After all of the possible completions are generated, any filter speci-
fied with the -X option is applied to the list. The filter is a pat-
tern as used for pathname expansion; a & in the pattern is replaced
with the text of the word being completed. A literal & may be escaped
with a backslash; the backslash is removed before attempting a match.
Any completion that matches the pattern will be removed from the list.
A leading ! negates the pattern; in this case any completion not
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 com-
pletions.
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 comp-
spec (and, if attempted, the default bash completions) generate no
matches.
When a compspec indicates that directory name completion is desired,
the programmable completion functions force readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the mark-directories readline variable, regardless of the
setting of the mark-symlinked-directories readline variable.
HISTORY
When the -o history option to the set builtin is enabled, the shell
provides access to the command history, the list of commands previ-
ously typed. The value of the HISTSIZE variable is used as the number
of commands to save in a history list. The text of the last HISTSIZE
commands (default 500) is saved. The shell stores each command in the
history list prior to parameter and variable expansion (see EXPANSION
above) but after history expansion is performed, subject to the values
of the shell variables HISTIGNORE and HISTCONTROL.
On startup, the history is initialized from the file named by the
variable HISTFILE (default ~/.bash_history). The file named by the
value of HISTFILE is truncated, if necessary, to contain no more than
the number of lines specified by the value of HISTFILESIZE. When an
interactive shell exits, the last $HISTSIZE lines are copied from the
history list to $HISTFILE. If the histappend shell option is enabled
(see the description of shopt under SHELL BUILTIN COMMANDS below), the
lines are appended to the history file, otherwise the history file is
overwritten. If HISTFILE is unset, or if the history file is
unwritable, the history is not saved. After saving the history, the
history file is truncated to contain no more than HISTFILESIZE lines.
If HISTFILESIZE is not set, no truncation is performed.
The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used
to list or edit and re-execute a portion of the history list. The
history builtin may be used to display or modify the history list and
manipulate the history file. When using command-line editing, search
commands are available in each editing mode that 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 semi-
colons where necessary to preserve syntactic correctness. The lithist
shell option causes the shell to save the command with embedded new-
lines instead of semicolons. See the description of the shopt builtin
below under SHELL BUILTIN COMMANDS for information on setting and
unsetting shell options.
HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the
history expansion in csh. This section describes what syntax features
are available. This feature is enabled by default for interactive
shells, and can be disabled using the +H option to the set builtin
command (see SHELL BUILTIN COMMANDS below). Non-interactive shells do
not perform history expansion by default.
History expansions introduce words from the history list into the
input stream, making it easy to repeat commands, insert the arguments
to a previous command into the current input line, or fix errors in
previous commands quickly.
History expansion is performed immediately after a complete line is
read, before the shell breaks it into words. It takes place in two
parts. The first is to determine which line from the history list to
use during substitution. The second is to select portions of that
line for inclusion into the current one. The line selected from the
history is the event, and the portions of that line that are acted
upon are words. Various modifiers are available to manipulate the
selected words. The line is broken into words in the same fashion as
when reading input, so that several metacharacter-separated words sur-
rounded by quotes are considered one word. History expansions are
introduced by the appearance of the history expansion character, which
is ! by default. Only backslash (\) and single quotes can quote the
history expansion character.
Several characters inhibit history expansion if found immediately fol-
lowing the history expansion character, even if it is unquoted: space,
tab, newline, carriage return, and =. If the extglob shell option is
enabled, ( will also inhibit expansion.
Several shell options settable with the shopt builtin may be used to
tailor the behavior of history expansion. If the histverify shell
option is enabled (see the description of the shopt builtin), and
readline is being used, history substitutions are not immediately
passed to the shell parser. Instead, the expanded line is reloaded
into the readline editing buffer for further modification. If 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).
Event Designators
An event designator is a reference to a command line entry in the his-
tory list.
! Start a history substitution, except when followed by a blank,
newline, carriage return, = or ( (when the extglob shell option
is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command 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 trail-
ing ? 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).
!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the event. A :
separates the event specification from the word designator. It may be
omitted if the word designator begins with a ^, $, *, -, or %. Words
are numbered from the beginning of the line, with the first word being
denoted by 0 (zero). Words are inserted into the current line sepa-
rated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last 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 specification, the
previous command is used as the event.
Modifiers
After the optional word designator, there may appear a sequence of one
or more of the following modifiers, each preceded by a ‘:’.
h Remove a trailing file name component, leaving only the head.
t Remove all leading file name components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the base-
name.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at
blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in the event
line. Any delimiter can be used in place of /. The final
delimiter is optional if it is the last character of the event
line. The delimiter may be quoted in old and new with a single
backslash. If & appears in new, it is replaced by old. A sin-
gle backslash will quote the &. If old is null, it is set to
the last old substituted, or, if no previous history substitu-
tions took place, the last string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This
is used in conjunction with ‘:s’ (e.g., ‘:gs/old/new/’) or
‘:&’. If used with ‘:s’, any delimiter can be used in place of
/, and the final delimiter is optional if it is the last char-
acter 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 sec-
tion as accepting options preceded by - accepts -- to signify the end
of the options.
: [arguments]
No effect; the command does nothing beyond expanding arguments
and performing any specified redirections. A zero exit code is
returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell
environment and return the exit status of the last command exe-
cuted from filename. If filename does not contain a slash,
file names in PATH are used to find the directory containing
filename. The file searched for in PATH need not be exe-
cutable. When bash is not in posix mode, the current directory
is searched if no file is found in PATH. If the sourcepath
option to the shopt builtin command is turned off, the PATH is
not searched. If any arguments are supplied, they become the
positional parameters when filename is executed. Otherwise the
positional parameters are unchanged. The return status is the
status of the last command exited within the script (0 if no
commands are executed), and false if filename is not found or
cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list
of aliases in the form alias name=value on standard output.
When arguments are supplied, an alias is defined for each name
whose value is given. A trailing space in value causes the
next word to be checked for alias substitution when the alias
is expanded. For each name in the argument list for which no
value is supplied, the name and value of the alias is printed.
Alias returns true unless a name is given for which no alias
has been defined.
bg [jobspec]
Resume the 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, if jobspec was not found or 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 binding or command must be passed
as a separate argument; e.g., ’"\C-x\C-r": re-read-init-file’.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subse-
quent bindings. Acceptable keymap names are emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move,
vi-command, and vi-insert. vi is equivalent to vi-com-
mand; 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.
-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.
-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.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is
entered.
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 the shell is not execut-
ing a loop when break is executed.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and
return its exit status. This is useful when defining a func-
tion whose name is the same as a shell builtin, retaining the
functionality of the builtin within the function. The cd
builtin is commonly redefined this way. The return status is
false if shell-builtin is not a shell builtin command.
cd [-L|-P] [dir]
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. Alternative directory names in
CDPATH are separated by a colon (:). A null directory name in
CDPATH is the same as the current directory, i.e., ‘‘.’’. If
dir begins with a slash (/), then CDPATH is not used. The -P
option 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 directory change is successful, the
absolute pathname of the new working directory is written to
the standard output. The return value is true if the directory
was successfully changed; false otherwise.
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 file-
name of the current subroutine call. If a non-negative integer
is supplied as expr, caller displays the line number, subrou-
tine 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.
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 pro-
duces a more verbose description. If the -V or -v option is
supplied, the exit status is 0 if command was found, and 1 if
not. If neither option is supplied and an error occurred or
command cannot be found, the exit status is 127. Otherwise,
the exit status of the command builtin is the exit status of
command.
compgen [option] [word]
Generate possible completion matches for word according to the
options, which may be any option accepted by the complete
builtin with the exception of -p and -r, and write the matches
to the standard output. When using the -F or -C options, the
various shell variables set by the programmable completion
facilities, while available, will not have useful values.
The matches will be generated in the same way as if the pro-
grammable completion code had generated them directly from a
completion specification with the same flags. If word is spec-
ified, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied,
or no matches were generated.
complete [-abcdefgjksuv] [-o comp-option] [-A action] [-G globpat] [-W
wordlist] [-P prefix] [-S suffix]
[-X filterpat] [-F function] [-C command] name [name ...]
complete -pr [name ...]
Specify how arguments to each name should be completed. If the
-p option is supplied, or if no options are supplied, existing
completion specifications are printed in a way that allows them
to be reused as input. The -r option removes a completion
specification for each name, or, if no names are supplied, all
completion specifications.
The process of applying these completion specifications when
word completion is attempted is described above under Pro-
grammable Completion.
Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options (and, if necessary, the
-P and -S options) should be quoted to protect them from expan-
sion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the comp-
spec’s behavior beyond the simple generation of comple-
tions. comp-option may be one of:
bashdefault
Perform the rest of the default bash comple-
tions if the compspec generates no matches.
default Use readline’s default filename completion if
the compspec generates no matches.
dirnames
Perform directory name completion if the comp-
spec generates no matches.
filenames
Tell readline that the compspec generates file-
names, so it can perform any filename-specific
processing (like adding a slash to directory
names or 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.
-A action
The action may be one of the following to generate a
list of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be
specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be
specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by
the HOSTFILE shell variable.
job Job names, if job control is active. May also
be specified as -j.
keyword Shell reserved words. May also be specified as
-k.
running Names of running jobs, if job control is
active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is
active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be
specified as -v.
-G globpat
The filename 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 com-
pleted.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions.
-F function
The shell function function is executed in the current
shell environment. When it finishes, the possible com-
pletions are retrieved from the value of the COMPREPLY
array variable.
-X filterpat
filterpat is a pattern as used for filename expansion.
It is applied to the list of possible completions
generated by the preceding options and arguments, and
each completion matching filterpat is removed from the
list. A leading ! in filterpat negates the pattern; in
this case, any completion not matching filterpat is
removed.
-P prefix
prefix is added at the beginning of each possible com-
pletion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after
all other options have been applied.
The return value is true unless an invalid option is supplied,
an option other than -p or -r is supplied without a name argu-
ment, an attempt is made to remove a completion specification
for a name for which no specification exists, or an error
occurs adding a completion specification.
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 the shell is
not executing a loop when continue is executed.
declare [-afFirtx] [-p] [name[=value] ...]
typeset [-afFirtx] [-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, additional options are ignored. 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 vari-
ables attributes:
-a Each name is an array variable (see Arrays above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evalu-
ation (see ARITHMETIC EVALUATION ) is performed when the
variable is assigned a value.
-r Make names readonly. These names cannot then be
assigned values by subsequent assignment statements or
unset.
-t Give each name the trace attribute. Traced functions
inherit the DEBUG trap from the calling shell. The
trace attribute has no special meaning for variables.
-x Mark names for export to subsequent commands via the
environment.
Using ‘+’ instead of ‘-’ turns off the attribute instead, with
the exception that +a may not be used to destroy an array vari-
able. When used in a function, makes each name local, as with
the local command. If a variable name is followed by =value,
the value of the variable is set to value. The return value is
0 unless an invalid option is encountered, an attempt is made
to define a function using ‘‘-f foo=bar’’, an attempt is made
to assign a value to a readonly variable, an attempt is made to
assign a value to an array variable without using the compound
assignment syntax (see Arrays above), one of the names is not a
valid shell variable name, an attempt is made to turn off read-
only status for a readonly variable, an attempt is made to turn
off array status for an array variable, or an attempt is made
to display a non-existent function with -f.
dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remembered
directories. The default display is on a single line with
directory names separated by spaces. Directories are added to
the list with the pushd command; the popd command removes
entries from the list.
+n Displays the nth entry counting from the left of the
list shown by dirs when invoked without options, start-
ing with zero.
-n Displays the nth entry counting from the right of the
list shown by dirs when invoked without options, start-
ing 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, pre-
fixing each entry with its index in the stack.
The return value is 0 unless an invalid option is supplied or n
indexes beyond the end of the directory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of
active jobs. If the -h option is given, each jobspec is not
removed from the table, but is marked so that SIGHUP is not
sent to the job if the shell receives a SIGHUP. If no jobspec
is present, and neither the -a nor the -r option is 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 trail-
ing newline is suppressed. If the -e option is given, inter-
pretation 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 dynami-
cally determine whether or not echo expands these escape char-
acters 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 trailing newline
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn the eight-bit character whose value is the octal value
nnn (zero to three octal digits)
\nnn the eight-bit character whose value is the octal value
nnn (one to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
enable [-adnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell
builtin to be executed without specifying a full pathname, even
though the shell normally searches for builtins before disk
commands. If -n is used, each name is disabled; otherwise,
names are enabled. For example, to use the test binary found
via the PATH instead of the shell builtin version, run ‘‘enable
-n test’’. The -f option means to load the new builtin command
name from shared object filename, on systems that support
dynamic loading. The -d option will delete a builtin previ-
ously loaded with -f. If no name arguments are given, or if
the -p option is supplied, a list of shell builtins is printed.
With no other option arguments, the list consists of all
enabled shell builtins. If -n is supplied, only disabled
builtins are printed. If -a is supplied, the list printed
includes all builtins, with an indication of whether or not
each is enabled. If -s is supplied, the output is restricted
to the POSIX special builtins. The return value is 0 unless a
name is not a shell builtin or there is an error loading a new
builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a single com-
mand. This command is then read and executed by the shell, and
its exit status is returned as the value of eval. If there are
no args, or only null arguments, eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process
is created. The arguments become the arguments to command. If
the -l option is supplied, the shell places a dash at the
beginning of the zeroth arg 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 com-
mand cannot be executed for some reason, a non-interactive
shell exits, unless the shell option execfail 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 redirections take effect in the current shell,
and the return status is 0. If there is a redirection error,
the return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is omitted,
the exit status is that of the last command executed. A trap
on EXIT is executed before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the envi-
ronment of subsequently executed commands. If the -f option is
given, the names refer to functions. If no names are given, or
if the -p option is supplied, a list of all names that are
exported in this shell is printed. The -n option causes the
export property to be removed from each name. If a variable
name is followed by =word, the value of the variable is set to
word. export returns an exit status of 0 unless an invalid
option is encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that is not a
function.
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of commands from first
to last is selected from the history list. First and last may
be specified as a string (to locate the last command beginning
with that string) or as a number (an index into the history
list, where a negative number is used as an offset from the
current command number). If last is not specified it is set to
the current command for 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 listing. The
-r option reverses the order of the commands. If the -l option
is given, the commands are listed on standard output. Other-
wise, the editor given by ename is invoked on a file containing
those commands. If ename is not given, the value of the FCEDIT
variable is used, and the value of EDITOR if FCEDIT is not set.
If neither variable is set, vi is used. When editing is com-
plete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance
of pat is replaced by rep. A useful alias to use with this is
‘‘r="fc -s"’’, so that typing ‘‘r cc’’ runs the last command
beginning with ‘‘cc’’ and typing ‘‘r’’ re-executes the last
command.
If the first form is used, the return value is 0 unless an
invalid option is encountered or first or last specify history
lines out of range. If the -e option is supplied, the return
value is the value of the last command executed or failure if
an error occurs with the temporary file of commands. If the
second form is used, the return status is that of the command
re-executed, unless cmd does not specify a valid history line,
in which case fc returns failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the current job.
If jobspec is not present, the shell’s notion of the current
job is used. The return value is that of the command placed
into the foreground, or failure if run when job control is dis-
abled or, when run with job control enabled, if jobspec does
not specify a valid job or jobspec specifies a job that was
started without job control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional parame-
ters. optstring contains the option characters to be recog-
nized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it
by white space. The colon and question mark characters may not
be used as option characters. Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist, and the index of the next argument
to be processed into the variable OPTIND. OPTIND is initial-
ized to 1 each time the shell or a shell script is invoked.
When an option requires an argument, getopts places that argu-
ment into the variable OPTARG. The shell does not reset OPTIND
automatically; it must be manually reset between multiple calls
to getopts within the same shell invocation if a new set of
parameters is to be used.
When the end of options is encountered, getopts exits with a
return value greater than zero. OPTIND is set to the index of
the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.
getopts can report errors in two ways. If the first character
of optstring is a colon, silent error reporting is used. In
normal operation diagnostic messages are printed when invalid
options or missing option arguments are encountered. If the
variable OPTERR is set to 0, no error messages will be dis-
played, even if the first character of optstring is not a
colon.
If an invalid option is seen, getopts places ? into name and,
if not silent, prints an error message and unsets OPTARG. If
getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent,
a question mark (?) is placed in name, OPTARG is unset, and a
diagnostic message is printed. If getopts is silent, then a
colon (:) is placed in name and OPTARG is set to the option
character found.
getopts returns true if an option, specified or unspecified, is
found. It returns false if the end of options is encountered
or an error occurs.
hash [-lr] [-p filename] [-dt] [name]
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 file-
name 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 multi-
ple name arguments are supplied with -t, the name is printed
before the hashed full pathname. The -l option causes output
to be displayed in a format that may be reused as input. If no
arguments are given, or if only -l is supplied, information
about remembered commands is printed. The return status is
true unless a name is not found or an invalid option is sup-
plied.
help [-s] [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. The -s option restricts the information
displayed to a short usage synopsis. The return status is 0
unless no command matches pattern.
history [n]
history -c
history -d offset
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line
numbers. Lines listed with a * have been modified. An argu-
ment 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 sup-
plied, 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, overwrit-
ing 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 is set, the time stamp information asso-
ciated with each history entry is written to the history file.
The return value is 0 unless an invalid option is encountered,
an error occurs while reading or writing the history file, an
invalid offset is supplied as an argument to -d, or the history
expansion supplied as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the
following meanings:
-l List process IDs in addition to the normal information.
-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 information about
that job. The return status is 0 unless an invalid option is
encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found
in command or args with the corresponding process group ID, and
executes command passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the processes
named by pid or jobspec. sigspec is either a case-insensitive
signal name such as SIGKILL (with or without the SIG prefix) or
a signal number; signum is a signal number. If sigspec is not
present, then SIGTERM is assumed. An argument of -l lists the
signal names. If any arguments are supplied when -l is given,
the names of the signals corresponding to the arguments are
listed, and the return status is 0. The exit_status argument
to -l is a number specifying either a signal number or the exit
status of a process terminated by a signal. kill returns true
if at least one signal was successfully sent, or false if an
error occurs or an invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see
ARITHMETIC EVALUATION). If the last arg evaluates to 0, let
returns 1; 0 is returned otherwise.
local [option] [name[=value] ...]
For each argument, a local variable named name is created, and
assigned value. The option can be any of the options accepted
by declare. When local is used within a function, it causes
the variable name to have a visible scope restricted to that
function and its children. With no operands, local writes a
list of local variables to the standard output. It is an error
to use local when not within a 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.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments,
removes the top directory from the stack, and performs a cd to
the new top directory. Arguments, if supplied, have the fol-
lowing meanings:
+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 sec-
ond.
-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.
-n Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is
manipulated.
If the popd command is successful, a dirs is performed as well,
and the return status is 0. popd returns false if an invalid
option is encountered, the directory stack is empty, a non-
existent directory stack entry is specified, or the directory
change fails.
printf format [arguments]
Write the formatted arguments to the standard output under the
control of the format. The format is a character string which
contains three types of objects: plain characters, which are
simply copied to standard output, character escape sequences,
which are converted and copied to the standard output, and for-
mat specifications, each of which causes printing of the next
successive 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 out-
put, 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 format is reused as necessary to consume all of the argu-
ments. If the format requires more arguments than are sup-
plied, 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] [dir]
pushd [-n] [+n] [-n]
Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory. With no arguments, exchanges the top two directo-
ries and returns 0, unless the directory stack is empty. Argu-
ments, if supplied, have the following meanings:
+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.
-n Suppresses the normal change of directory when adding
directories to the stack, so that only the stack is
manipulated.
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 ele-
ment is specified, or the directory change to the specified new
current directory fails.
pwd [-LP]
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P
option is supplied or the -o physical option to the set builtin
command is enabled. If the -L option is used, the pathname
printed may contain symbolic links. The return status is 0
unless an error occurs while reading the name of the current
directory or an invalid option is supplied.
read [-ers] [-u fd] [-t timeout] [-a aname] [-p prompt] [-n nchars]
[-d delim] [name ...]
One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u option, and the
first word is assigned to the first name, the second word to
the second name, and so on, with leftover words and their
intervening separators assigned to the last name. If there are
fewer words read from the input stream than names, the remain-
ing 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 charac-
ter read and for line continuation. Options, if supplied, have
the following meanings:
-a aname
The words are assigned to sequential indices of the
array variable aname, starting at 0. aname is unset
before any new values are assigned. Other name argu-
ments 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, read-
line (see READLINE above) is used to obtain the line.
-n nchars
read returns after reading nchars characters rather than
waiting for a complete line of input.
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input. The
prompt is displayed only if input is coming from a ter-
minal.
-r Backslash does not act as an escape character. The
backslash is considered to be part of the line. In par-
ticular, a backslash-newline pair may not be used as a
line continuation.
-s Silent mode. If input is coming from a terminal, char-
acters 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. This
option has no effect if read is not reading input from
the terminal or a pipe.
-u fd Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the
variable REPLY. The return code is zero, unless end-of-file is
encountered, read times out, or an invalid file descriptor is
supplied as the argument to -u.
readonly [-apf] [name[=word] ...]
The given names are marked readonly; the values of these names
may not be changed by subsequent assignment. If the -f option
is supplied, the functions corresponding to the names are so
marked. The -a option restricts the variables to 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 vari-
able 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 specified 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 outside 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 [--abefhkmnptuvxBCHP] [-o option] [arg ...]
Without options, the name and value of each shell variable are
displayed in a format that can be reused as input. The output
is sorted according to the current locale. When options are
specified, they set or unset shell attributes. Any arguments
remaining after the options are processed are treated as values
for the positional parameters and are assigned, in order, to
$1, $2, ... $n. Options, if specified, have the following
meanings:
-a Automatically mark variables and functions which are
modified or created for export to the environment of
subsequent commands.
-b Report the status of terminated background jobs immedi-
ately, rather than before the next primary prompt.
This is effective only when job control is enabled.
-e Exit immediately if a simple command (see SHELL GRAMMAR
above) exits with a non-zero status. The shell does
not exit if the command that fails is part of the com-
mand list immediately following a while or until key-
word, part of the test in an if statement, part of a &&
or ││ list, or if the command’s return value is being
inverted via !. A trap on ERR, if set, is executed
before the shell exits.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up
for execution. This is enabled by default.
-k All arguments in the form of assignment statements are
placed in the environment for a command, not just those
that precede the command name.
-m Monitor mode. Job control is enabled. This option is
on by default for interactive shells on systems that
support it (see JOB CONTROL above). Background pro-
cesses run in a separate process group and a line con-
taining their exit status is printed upon their comple-
tion.
-n Read commands but do not execute them. This may be
used to check a shell script for syntax errors. This
is ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing inter-
face. This is enabled by default when the
shell is interactive, unless the shell is
started with the --noediting option.
errtrace
Same as -E.
functrace
Same as -T.
errexit Same as -e.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above
under HISTORY. This option is on by default in
interactive shells.
ignoreeof
The effect is as if the shell command
‘‘IGNOREEOF=10’’ had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f. nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all commands
in the pipeline exit successfully. This option
is disabled by default.
posix Change the behavior of bash where the default
operation differs from the POSIX 1003.2 stan-
dard to match the standard (‘posix mode).
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
xtrace Same as -x.
If -o is supplied with no option-name, the values of
the current options are printed. If +o is supplied
with no option-name, a series of set commands to recre-
ate the current option settings is displayed on the
standard output.
-p Turn on privileged mode. In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS
variable, if it appears in the environment, is 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 as an error when performing
parameter expansion. If expansion is attempted on an
unset variable, the shell prints an error message, and,
if not interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, for command, case
command, select command, or arithmetic for command,
display the expanded value of PS4, followed by the com-
mand and its expanded arguments or associated word
list.
-B The shell performs brace expansion (see Brace Expansion
above). This is on by default.
-C If set, bash does not overwrite an existing file with
the >, >&, and <> redirection operators. This may be
overridden when creating output files by using the
redirection operator >| instead of >.
-E If set, any trap on ERR is inherited by shell func-
tions, 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 logi-
cal chain of directories when performing commands which
change the current directory.
-T If set, any trap on DEBUG is inherited by shell func-
tions, command substitutions, and commands executed in
a subshell environment. The DEBUG trap is normally not
inherited in such cases.
-- If no arguments follow this option, then the positional
parameters are unset. Otherwise, the positional param-
eters are set to the args, even if some of them begin
with a -.
- Signal the end of options, cause all remaining args to
be assigned to the positional parameters. The -x and
-v options are turned off. If there are no args, the
positional parameters remain unchanged.
The options are off by default unless otherwise noted. Using +
rather than - causes these options to be turned off. The
options can also be specified as 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 following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status
indicates whether the optname is set or unset. If mul-
tiple 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 dis-
play is limited to those options which are set or unset,
respectively. Unless otherwise noted, the shopt options are
disabled (unset) by default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting
options, the return status is zero unless an optname is not a
valid shell option.
The list of shopt options is:
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 transposed 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 interac-
tive 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 per-
formed.
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.
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 inter-
active shells.
extdebug
If set, behavior intended for use by debuggers is
enabled:
1. The -F option to the declare builtin displays
the source file name and line number correspond-
ing to each function name supplied as an argu-
ment.
2. If the command run by the DEBUG trap returns a
non-zero value, the next command is skipped and
not executed.
3. If the command run by the DEBUG trap returns a
value of 2, and the shell is executing in a sub-
routine (a shell function or a shell script exe-
cuted by the . or source builtins), a call to
return is simulated.
extglob If set, the extended pattern matching features
described above under Pathname Expansion are enabled.
extquote
If set, $’string’ and $"string" quoting is performed
within ${parameter} expansions enclosed in double
quotes. This option is enabled by default.
failglob
If set, patterns which fail to match filenames during
pathname expansion result in an expansion error.
force_fignore
If set, the suffixes specified by the FIGNORE shell
variable cause words to be ignored when performing word
completion even if the ignored words are the only pos-
sible completions. See SHELL VARIABLES above for a
description of FIGNORE. This option is enabled by
default.
gnu_errfmt
If set, shell error messages are written in the stan-
dard GNU error message format.
histappend
If set, the history list is appended to the file named
by the value of the HISTFILE variable when the shell
exits, rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of his-
tory substitution are not immediately passed to the
shell parser. Instead, the resulting line is loaded
into the readline editing buffer, allowing further mod-
ification.
hostcomplete
If set, and readline is being used, bash will attempt
to perform hostname completion when a word containing a
@ is being completed (see Completing under READLINE
above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an
interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to cause that
word and all remaining characters on that line to be
ignored in an interactive shell (see COMMENTS above).
This option is enabled by default.
lithist If set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded new-
lines rather than using semicolon separators where pos-
sible.
login_shell
The shell sets this option if it is started as a login
shell (see INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has
been accessed since the last time it was checked, the
message ‘‘The mail in mailfile has been read’’ is dis-
played.
no_empty_cmd_completion
If set, and readline is being used, bash will not
attempt to search the PATH for possible completions
when completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Path-
name Expansion above).
nullglob
If set, bash allows patterns which match no files (see
Pathname Expansion above) to expand to a null string,
rather than themselves.
progcomp
If set, the programmable completion facilities (see
Programmable Completion above) are enabled. This
option is enabled by default.
promptvars
If set, prompt strings undergo parameter expansion,
command substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING
above. This option is enabled by default.
restricted_shell
The shell sets this option if it is started in
restricted mode (see RESTRICTED SHELL below). The
value may not be changed. This is not reset when the
startup files are executed, allowing the startup files
to discover whether or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when
the shift count exceeds the number of positional param-
eters.
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. The -f option says not to complain if this is a login
shell; just suspend anyway. The return status is 0 unless the
shell is a login shell and -f is not supplied, or if job con-
trol 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 CONDITIONAL EXPRESSIONS.
Expressions may be combined using the following operators,
listed in decreasing order of precedence.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used to override
the normal precedence of operators.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a set of
rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is
not null.
2 arguments
If the first argument is !, the expression is true if
and only if the second argument is null. If the first
argument is one of the unary conditional operators
listed above under CONDITIONAL EXPRESSIONS, the expres-
sion 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 conditional
operators listed above under CONDITIONAL EXPRESSIONS,
the result of the expression is the result of the binary
test using the first and third arguments as operands.
If the first argument is !, the value is the negation of
the two-argument test using the second and third argu-
ments. 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. The -a and -o operators are considered binary
operators in this case.
4 arguments
If the first argument is !, the result is the negation
of the three-argument expression composed of the remain-
ing 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 cor-
responding numbers. Each sigspec is either a signal name
defined in <signal.h>, or a signal number. Signal names are
case insensitive and the SIG prefix is optional. If a sigspec
is EXIT (0) the command arg is executed on exit from the shell.
If a sigspec is DEBUG, the command arg is executed before every
simple command, for command, case command, select command,
every arithmetic for command, and before the first command exe-
cutes in a shell function (see SHELL GRAMMAR above). Refer to
the description of the extglob option to the shopt builtin for
details of its effect on the DEBUG trap. If a sigspec is ERR,
the command arg is executed whenever a simple command has a
non-zero exit status, subject to the following conditions. The
ERR trap is not executed if the failed command is part of the
command list immediately following a while or until keyword,
part of the test in an if statement, part of a && or ││ list,
or if the command’s return value is being inverted via !.
These are the same conditions obeyed by the errexit option. If
a sigspec is RETURN, the command arg is executed each time a
shell function or a script executed with the . or source
builtins finishes executing. Signals ignored upon entry to the
shell cannot be trapped or reset. Trapped signals are reset to
their original values in a child process when it is created.
The return status is false if any sigspec is invalid; otherwise
trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if
used as a command name. If the -t option is used, type prints
a string which is one of alias, keyword, function, builtin, or
file if name is an alias, shell reserved word, function,
builtin, or disk file, respectively. If the name is not found,
then nothing is printed, and an exit status of false is
returned. If the -p option is used, type either returns the
name of the disk file that would be executed if name were spec-
ified 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
executable named name. This includes aliases and functions, if
and only if the -p option is not also used. The table of
hashed commands is not consulted when using -a. The -f option
suppresses shell function lookup, as with the command builtin.
type returns true if any of the arguments are found, false if
none are found.
ulimit [-SHacdflmnpstuv [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
once it is set; a soft limit may be increased up to the value
of the hard limit. If neither -H nor -S is specified, both the
soft and hard limits are set. The value of limit can be a num-
ber in the unit specified for the resource or one of the spe-
cial values hard, soft, or unlimited, which stand for the cur-
rent 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 specified, the limit
name and unit are printed before the value. Other options are
interpreted as follows:
-a All current limits are reported
-c The maximum size of core files created
-d The maximum size of a process’s data segment
-f The maximum size of files created by the shell
-l The maximum size that may be locked into memory
-m The maximum resident set size (has no effect on Linux)
-n The maximum number of open file descriptors (most sys-
tems do not allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be set)
-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
If limit is given, it is the new value of the specified
resource (the -a option is display only). If no option is
given, then -f is assumed. Values are in 1024-byte increments,
except for -t, which is in seconds, -p, which is in units of
512-byte blocks, and -n and -u, which are unscaled values. The
return status is 0 unless an invalid option or argument is sup-
plied, or an error occurs while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins
with a digit, it is interpreted as an octal number; otherwise
it is interpreted as a symbolic mode mask similar to that
accepted by chmod(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 num-
ber. If the -p option is supplied, and mode is omitted, the
output is in a form that may be reused as input. The return
status is 0 if the mode was successfully changed or if no mode
argument was supplied, and false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a is
supplied, all alias definitions are removed. The return value
is true unless a supplied name is not a defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or function.
If no options are supplied, or the -v option is given, each
name refers to a shell variable. Read-only variables may not
be unset. If -f is specifed, each name refers to a shell func-
tion, and the function definition is removed. Each unset vari-
able or function is removed from the environment passed to sub-
sequent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD,
FUNCNAME, GROUPS, or DIRSTACK are unset, they lose their spe-
cial properties, even if they are subsequently reset. The exit
status is true unless a name is readonly.
wait [n]
Wait for the specified process and return its termination sta-
tus. n may be a process ID or a job specification; if a job
spec is given, all processes in that job’s pipeline are waited
for. If n is not given, all currently active child processes
are waited for, and the return status is zero. If n specifies
a non-existent process or job, the return status is 127. Oth-
erwise, the return status is the exit status of the last pro-
cess or job waited for.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied
at invocation, the shell becomes restricted. A restricted shell is
used to set up an environment more controlled than the standard shell.
It behaves identically to bash with the exception that the following
are disallowed or not performed:
· changing directories with cd
· setting or unsetting the values of SHELL, PATH, ENV, or
BASH_ENV
· specifying command names containing /
· specifying a file name containing a / as an argument to the .
builtin command
· Specifying a filename containing a slash as an argument to the
-p option to the hash builtin command
· importing function definitions from the shell environment at
startup
· parsing the value of SHELLOPTS from the shell environment at
startup
· redirecting output using the >, >|, <>, >&, &>, and >> redirec-
tion operators
· using the exec builtin command to replace the shell with
another command
· adding or deleting builtin commands with the -f and -d options
to the enable builtin command
· Using the enable builtin command to enable disabled shell
builtins
· specifying the -p option to the command builtin command
· turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see
COMMAND EXECUTION above), rbash turns off any restrictions in the
shell spawned to execute the script.
SEE ALSO
Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utili-
ties, IEEE
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.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@po.CWRU.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/.
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 traditional ver-
sions of sh, mostly because of the POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command sequences of the form ‘a ; b ; c’ are
not handled gracefully when process suspension is attempted. When a
process is stopped, the shell immediately executes the next command in
the sequence. It suffices to place the sequence of commands between
parentheses to force it into a subshell, which may be stopped as a
unit.
Commands inside of $(...) command substitution are not parsed until
substitution is attempted. This will delay error reporting until some
time after the command is entered. For example, unmatched parenthe-
ses, even inside shell comments, will result in error messages while
the construct is being read.
Array variables may not (yet) be exported.
GNU Bash-3.0 2004 June 26 BASH(1)
