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NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [command_string | file]
COPYRIGHT
Bash is Copyright (C) 1989-2018 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 Shell and
Utilities portion of the IEEE POSIX specification (IEEE Standard
1003.1). Bash can be configured to be POSIX-conformant by default.
OPTIONS
All of the single-character shell options documented in the
description of the set builtin command, including -o, can be used as
options when the shell is invoked. In addition, bash interprets the
following options when it is invoked:
-c If the -c option is present, then commands are read from the
first non-option argument command_string. If there are
arguments after the command_string, the first argument is
assigned to $0 and any remaining arguments are assigned to
the positional parameters. The assignment to $0 sets the
name of the shell, which is used in warning and error
messages.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell
(see INVOCATION below).
-r If the -r option is present, the shell becomes restricted
(see RESTRICTED SHELL below).
-s If the -s option is present, or if no arguments remain after
option processing, then commands are read from the standard
input. This option allows the positional parameters to be
set when invoking an interactive shell or when reading input
through a pipe.
-D A list of all double-quoted strings preceded by $ is printed
on the standard output. These are the strings that are
subject to language translation when the current locale is
not C or POSIX. This implies the -n option; no commands
will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the
shopt builtin (see SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of that option;
+O unsets it. If shopt_option is not supplied, the names
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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
filenames and arguments. An argument of - is equivalent to
--.
Bash also interprets a number of multi-character options. These
options must appear on the command line before the single-character
options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell
starts. Turns on extended debugging mode (see the description of
the extdebug option to the shopt builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po
(portable object) file format.
--dump-strings
Equivalent to -D.
--help
Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the 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 standard to match the standard (posix mode). See
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SEE ALSO below for a reference to a document that details how
posix mode affects bash's behavior.
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the
standard output and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor
the -s option has been supplied, the first argument is assumed to be
the name of a file containing shell 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
executes commands from this file, then exits. Bash's exit status is
the exit status of the last command executed in the script. If no
commands are executed, the exit status is 0. An attempt is first made
to open the file in the current directory, and, if no file is found,
then the shell searches the directories in PATH for the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or
one started with the --login option.
An interactive shell is one started without non-option arguments
(unless -s is specified) and without the -c option whose standard
input and error are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. PS1 is set and $-
includes i if bash is interactive, allowing a shell script or a
startup file to test this state.
The following paragraphs describe how bash executes its startup files.
If any of the files exist but cannot be read, bash reports an error.
Tildes are expanded in filenames as described below under Tilde
Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-
interactive shell with the --login option, it first reads and executes
commands from the file /etc/profile, if that file exists. After
reading that file, it looks for ~/.bash_profile, ~/.bash_login, and
~/.profile, in that order, and reads and executes commands from the
first one that exists and is readable. The --noprofile option may be
used when the shell is started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login
shell executes the exit builtin command, bash reads and executes
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commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, bash
reads and executes commands from ~/.bashrc, if that file exists. This
may be inhibited by using the --norc option. The --rcfile file option
will force bash to read and execute commands from file instead of
~/.bashrc.
When bash is started non-interactively, to run a shell script, for
example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the
filename.
If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well. When invoked as an
interactive login shell, or a non-interactive shell with the --login
option, it first attempts to read and execute commands from
/etc/profile and ~/.profile, in that order. The --noprofile option
may be used to inhibit this behavior. When invoked as an interactive
shell with the name sh, bash looks for the variable ENV, expands its
value if it is defined, and uses the expanded value as the name of a
file to read and 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 with its standard
input connected to a network connection, as when executed by the
remote shell daemon, usually rshd, or the secure shell daemon sshd.
If bash determines it is being run in this fashion, it reads and
executes commands from ~/.bashrc, 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 neither rshd nor sshd generally invoke
the shell with those options or allow them to be specified.
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If the shell is started with the effective user (group) id not equal
to the real user (group) id, and the -p option is not supplied, no
startup files are read, shell functions are not inherited from the
environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE
variables, if they appear in the environment, are ignored, and the
effective user id is set to the real user id. If the -p option is
supplied at invocation, the startup behavior is the same, but the
effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this
document.
blank
A space or tab.
word A sequence of characters considered as a single unit by the
shell. Also known as a token.
name A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character or an
underscore. Also referred to as an identifier.
metacharacter
A character that, when unquoted, separates words. One of the
following:
| & ; ( ) < > space tab newline
control operator
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell.
The following words are recognized as reserved when unquoted and
either the first word of a simple command (see SHELL GRAMMAR below) or
the third word of a case or for command:
! case coproc do done elif else esac fi for function if in select
then until while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments
followed by blank-separated words and redirections, and terminated by
a control operator. The first word specifies the command to be
executed, and is passed as argument zero. The remaining words are
passed as arguments to the invoked command.
The return value of a simple command is its exit status, or 128+n if
the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by one of
the control operators | or |&. The format for a pipeline is:
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[time [-p]] [ ! ] command [ [|||&] command2 ... ]
The standard output of command is connected via a pipe to the standard
input of command2. This connection is performed before any
redirections specified by the command (see REDIRECTION below). If |&
is used, command's standard error, in addition to its standard output,
is connected to command2's standard input through the pipe; it is
shorthand for 2>&1 |. This implicit redirection of the standard error
to the standard output is performed after any redirections specified
by the command.
The return status of a pipeline is the exit status of the last
command, unless the pipefail option is enabled. If pipefail is
enabled, the pipeline's return status is the value of the last
(rightmost) command to exit with a non-zero status, or zero if all
commands exit successfully. If the reserved word ! precedes a
pipeline, the exit status of that pipeline is the logical negation of
the exit status as described above. The shell waits for all commands
in the pipeline to terminate before returning a value.
If the time reserved word precedes a pipeline, the elapsed as well as
user and system time consumed by its execution are reported when the
pipeline terminates. The -p option changes the output format to that
specified by POSIX. When the shell is in posix mode, it does not
recognize time as a reserved word if the next token begins with a `-'.
The TIMEFORMAT variable may be set to a format string that specifies
how the timing information should be displayed; see the description of
TIMEFORMAT under Shell Variables below.
When the shell is in posix mode, time may be followed by a newline.
In this case, the shell displays the total user and system time
consumed by the shell and its children. The TIMEFORMAT variable may
be used to specify the format of the time information.
Each command in a pipeline is executed as a separate process (i.e., in
a subshell). See COMMAND EXECUTION ENVIRONMENT for a description of a
subshell environment. If the lastpipe option is enabled using the
shopt builtin (see the description of shopt below), the last element
of a pipeline may be run by the shell process.
Lists
A list is a sequence of one or more pipelines separated by one of the
operators ;, &, &&, or ||, and optionally terminated by one of ;, &,
or <newline>.
Of these list operators, && and || have equal precedence, followed by
; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list instead of a
semicolon to delimit commands.
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If a command is terminated by the control operator &, the shell
executes the command in the background in a subshell. The shell does
not wait for the command to finish, and the return status is 0. These
are referred to as asynchronous commands. Commands separated by a ;
are executed sequentially; the shell waits for each command to
terminate in turn. The return status is the exit status of the last
command executed.
AND and OR lists are sequences of one or more pipelines separated by
the && and || control operators, respectively. AND and OR lists are
executed with left associativity. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status
of zero (success).
An OR list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero exit
status. The return status of AND and OR lists is the exit status of
the last command executed in the list.
Compound Commands
A compound command is one of the following. In most cases a list in a
command's description may be separated from the rest of the command by
one or more newlines, and may be followed by a newline in place of a
semicolon.
(list)
list is executed in a subshell environment (see COMMAND EXECUTION
ENVIRONMENT below). Variable assignments and builtin commands
that affect the shell's environment do not remain in effect after
the command completes. The return status is the exit status of
list.
{ list; }
list is simply executed in the current shell environment. list
must be terminated with a newline or semicolon. This is known as
a group command. The return status is the exit status of list.
Note that unlike the metacharacters ( and ), { and } are reserved
words and must occur where a reserved word is permitted to be
recognized. Since they do not cause a word break, they must be
separated from list by whitespace or another shell metacharacter.
((expression))
The expression is evaluated according to the rules described
below under ARITHMETIC EVALUATION. If the value of the
expression is non-zero, the return status is 0; otherwise the
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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.
Conditional operators such as -f must be unquoted to be
recognized as primaries.
When used with [[, the < and > operators sort lexicographically
using the current locale.
When the == and != operators are used, the string to the right of
the operator is considered a pattern and matched according to the
rules described below under Pattern Matching, as if the extglob
shell option were enabled. The = operator is equivalent to ==.
If the nocasematch shell option is enabled, the match is
performed without regard to the case of alphabetic characters.
The return value is 0 if the string matches (==) or does not
match (!=) the pattern, and 1 otherwise. Any part of the pattern
may be quoted to force the quoted portion to be matched as a
string.
An additional binary operator, =~, is available, with the same
precedence as == and !=. When it is used, the string to the
right of the operator is considered a POSIX extended regular
expression and matched accordingly (as in regex(3)). The return
value is 0 if the string matches the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the
conditional expression's return value is 2. If the nocasematch
shell option is enabled, the match is performed without regard to
the case of alphabetic characters. Any part of the pattern may
be quoted to force the quoted portion to be matched as a string.
Bracket expressions in regular expressions must be treated
carefully, since normal quoting characters lose their meanings
between brackets. If the pattern is stored in a shell variable,
quoting the variable expansion forces the entire pattern to be
matched as a string. Substrings matched by parenthesized
subexpressions within the regular expression are saved in the
array variable BASH_REMATCH. The element of BASH_REMATCH with
index 0 is the portion of the string matching the entire regular
expression. The element of BASH_REMATCH with index n is the
portion of the string matching the nth parenthesized
subexpression.
Expressions may be combined using the following operators, listed
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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 positional parameter
that is set (see PARAMETERS below). The return status is the
exit status of the last command that executes. If the expansion
of the items following in results in an empty list, no commands
are executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to
the rules described below under ARITHMETIC EVALUATION. The
arithmetic expression expr2 is then evaluated repeatedly until it
evaluates to zero. Each time expr2 evaluates to a non-zero
value, list is executed and the arithmetic expression expr3 is
evaluated. If any expression is omitted, it behaves as if it
evaluates to 1. The return value is the exit status of the last
command in list that is executed, or false if any of the
expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of
items. The set of expanded words is printed on the standard
error, each preceded by a number. If the in word is omitted, the
positional parameters are printed (see PARAMETERS below). The
PS3 prompt is then displayed and a line read from the standard
input. If the line consists of a number corresponding to one of
the displayed words, then the value of name is set to that word.
If the line is empty, the words and prompt are displayed again.
If EOF is read, the command completes. Any other value read
causes name to be set to null. The line read is saved in the
variable REPLY. The list is executed after each selection until
a break command is executed. The exit status of select is the
exit status of the last command executed in list, or zero if no
commands were executed.
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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 matching rules described under
Pattern Matching below. The word is expanded using tilde
expansion, parameter and variable expansion, arithmetic
expansion, command substitution, process substitution and quote
removal. Each pattern examined is expanded using tilde
expansion, parameter and variable expansion, arithmetic
expansion, command substitution, and process substitution. If
the nocasematch shell option is enabled, the match is performed
without regard to the case of alphabetic characters. When a
match is found, the corresponding list is executed. If the ;;
operator is used, no subsequent matches are attempted after the
first pattern match. Using ;& in place of ;; causes execution to
continue with the list associated with the next set of patterns.
Using ;;& in place of ;; causes the shell to test the next
pattern list in the statement, if any, and execute any associated
list on a successful match. The exit status is zero if no
pattern matches. Otherwise, it is the exit status of the last
command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then
list is executed. Otherwise, each elif list is executed in turn,
and if its exit status is zero, the corresponding then list is
executed and the command completes. Otherwise, the else list is
executed, if present. The exit status is the exit status of the
last command executed, or zero if no condition tested true.
while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long
as the last command in the list list-1 returns an exit status of
zero. The until command is identical to the while command,
except that the test is negated: list-2 is executed as long as
the last command in list-1 returns a non-zero exit status. The
exit status of the while and until commands is the exit status of
the last command executed in list-2, or zero if none was
executed.
Coprocesses
A coprocess is a shell command preceded by the coproc reserved word.
A coprocess is executed asynchronously in a subshell, as if the
command had been terminated with the & control operator, with a two-
way pipe established between the executing shell and the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
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This creates a coprocess named NAME. If NAME is not supplied, the
default name is COPROC. NAME must not be supplied if command is a
simple command (see above); otherwise, it is interpreted as the first
word of the simple command. When the coprocess is executed, the shell
creates an array variable (see Arrays below) named NAME in the context
of the executing shell. The standard output of command is connected
via a pipe to a file descriptor in the executing shell, and that file
descriptor is assigned to NAME[0]. The standard input of command is
connected via a pipe to a file descriptor in the executing shell, and
that file descriptor is assigned to NAME[1]. This pipe is established
before any redirections specified by the command (see REDIRECTION
below). The file descriptors can be utilized as arguments to shell
commands and redirections using standard word expansions. Other than
those created to execute command and process substitutions, the file
descriptors are not available in subshells. The process ID of the
shell spawned to execute the coprocess is available as the value of
the variable NAME_PID. The wait builtin command may be used to wait
for the coprocess to terminate.
Since the coprocess is created as an asynchronous command, the coproc
command always returns success. The return status of a coprocess is
the exit status of command.
Shell Function Definitions
A shell function is an object that is called like a simple command and
executes a compound command with a new set of positional parameters.
Shell functions are declared as follows:
name () compound-command [redirection]
function name [()] compound-command [redirection]
This defines a function named name. The reserved word function
is optional. If the function reserved word is supplied, the
parentheses are optional. The body of the function is the
compound command compound-command (see Compound Commands above).
That command is usually a list of commands between { and }, but
may be any command listed under Compound Commands above, with one
exception: If the function reserved word is used, but the
parentheses are not supplied, the braces are required.
compound-command is executed whenever name is specified as the
name of a simple command. When in posix mode, name may not be
the name of one of the POSIX special builtins. Any redirections
(see REDIRECTION below) specified when a function is defined are
performed when the function is executed. The exit status of a
function definition is zero unless a syntax error occurs or a
readonly function with the same name already exists. When
executed, the exit status of a function is the exit status of the
last command executed in the body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is enabled (see SHELL
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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
interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or
words to the shell. Quoting can be used to disable special treatment
for special characters, to prevent reserved words from being
recognized as such, and to prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS has special
meaning to the shell and must be quoted if it is to represent itself.
When the command history expansion facilities are being used (see
HISTORY EXPANSION below), the history expansion character, usually !,
must be quoted to prevent history expansion.
There are three quoting mechanisms: the escape character, single
quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the
literal value of the next character that follows, with the exception
of <newline>. If a \<newline> pair appears, and the backslash is not
itself quoted, the \<newline> is treated as a line continuation (that
is, it is removed from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the literal value of
each character within the quotes. A single quote may not occur
between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of
all characters within the quotes, with the exception of $, `, \, and,
when history expansion is enabled, !. When the shell is in posix
mode, the ! has no special meaning within double quotes, even when
history expansion is enabled. The characters $ and ` retain their
special meaning within double quotes. The backslash retains its
special meaning only when followed by one of the following characters:
$, `, ", \, or <newline>. A double quote may be quoted within double
quotes by preceding it with a backslash. If enabled, history
expansion will be performed unless an ! appearing in double quotes is
escaped using a backslash. The backslash preceding the ! is not
removed.
The special parameters * and @ have special meaning when in double
quotes (see PARAMETERS below).
Words of the form $string are treated specially. The word expands to
string, with backslash-escaped characters replaced as specified by the
ANSI C standard. Backslash escape sequences, if present, are decoded
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as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\ single quote
\ double quote
\? question mark
\nnn the eight-bit character whose value is the octal value nnn
(one to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not
been present.
A double-quoted string preceded by a dollar sign ($string) will cause
the string to be translated according to the current locale. If the
current locale is C or POSIX, the dollar sign is ignored. If the
string is translated and replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a
number, or one of the special characters listed below under Special
Parameters. A variable is a parameter denoted by a name. A variable
has a value and zero or more attributes. Attributes are assigned
using the declare builtin command (see declare below in SHELL BUILTIN
COMMANDS).
A parameter is set if it has been assigned a value. The null string
is a valid value. Once a variable is set, it may be unset only by
using the unset builtin command (see SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
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command substitution, arithmetic expansion, and quote removal (see
EXPANSION below). If the variable has its integer attribute set, then
value is evaluated as an arithmetic expression even if the $((...))
expansion is not used (see Arithmetic Expansion below). Word
splitting is not performed, with the exception of "$@" 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
(declaration commands). When in posix mode, these builtins may appear
in a command after one or more instances of the command builtin and
retain these assignment statement properties.
In the context where an assignment statement is assigning a value to a
shell variable or array index, the += operator can be used to append
to or add to the variable's previous value. This includes arguments
to builtin commands such as declare that accept assignment statements
(declaration commands). When += is applied to a variable for which
the integer attribute has been set, value is evaluated as an
arithmetic expression and added to the variable's current value, which
is also evaluated. When += is applied to an array variable using
compound assignment (see Arrays below), the variable's value is not
unset (as it is when using =), and new values are appended to the
array beginning at one greater than the array's maximum index (for
indexed arrays) or added as additional key-value pairs in an
associative array. When applied to a string-valued variable, value is
expanded and appended to the variable's value.
A variable can be assigned the nameref attribute using the -n option
to the declare or local builtin commands (see the descriptions of
declare and local below) to create a nameref, or a reference to
another variable. This allows variables to be manipulated indirectly.
Whenever the nameref variable is referenced, assigned to, unset, or
has its attributes modified (other than using or changing the nameref
attribute itself), the operation is actually performed on the variable
specified by the nameref variable's value. A nameref is commonly used
within shell functions to refer to a variable whose name is passed as
an argument to the function. For instance, if a variable name is
passed to a shell function as its first argument, running
declare -n ref=$1
inside the function creates a nameref variable ref whose value is the
variable name passed as the first argument. References and
assignments to ref, and changes to its attributes, are treated as
references, assignments, and attribute modifications to the variable
whose name was passed as $1. If the control variable in a for loop
has the nameref attribute, the list of words can be a list of shell
variables, and a name reference will be established for each word in
the list, in turn, when the loop is executed. Array variables cannot
be given the nameref attribute. However, nameref variables can
reference array variables and subscripted array variables. Namerefs
can be unset using the -n option to the unset builtin. Otherwise, if
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unset is executed with the name of a nameref variable as an argument,
the variable referenced by the nameref variable will be unset.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits,
other than the single digit 0. Positional parameters are assigned
from the shell's arguments when it is invoked, and may be reassigned
using the set builtin command. Positional parameters may not be
assigned to with assignment statements. The positional parameters are
temporarily replaced when a shell function is executed (see FUNCTIONS
below).
When a positional parameter consisting of more than a single digit is
expanded, it must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When
the expansion is not within double quotes, each positional
parameter expands to a separate word. In contexts where it is
performed, those words are subject to further word splitting and
pathname expansion. When the expansion occurs within double
quotes, it expands to a single word with the value of each
parameter separated by the first character of the IFS special
variable. That is, "$*" is equivalent to "$1c$2c...", where c is
the first character of the value of the IFS variable. If IFS is
unset, the parameters are separated by spaces. If IFS is null,
the parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one. In
contexts where word splitting is performed, this expands each
positional parameter to a separate word; if not within double
quotes, these words are subject to word splitting. In contexts
where word splitting is not performed, this expands to a single
word with each positional parameter separated by a space. When
the expansion occurs within double quotes, each parameter expands
to a separate word. That is, "$@" is equivalent to "$1" "$2" ...
If the double-quoted expansion occurs within a word, the
expansion of the first parameter is joined with the beginning
part of the original word, and the expansion of the last
parameter is joined with the last part of the original word.
When there are no positional parameters, "$@" and $@ expand to
nothing (i.e., they are removed).
# Expands to the number of positional parameters in decimal.
? Expands to the exit status of the most recently executed
foreground pipeline.
- Expands to the current option flags as specified upon invocation,
by the set builtin command, or those set by the shell itself
(such as the -i option).
$ Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the subshell.
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! Expands to the process ID of the job most recently placed into
the background, whether executed as an asynchronous command or
using the bg builtin (see JOB CONTROL below).
0 Expands to the name of the shell or shell script. This is set at
shell initialization. If bash is invoked with a file of
commands, $0 is set to the name of that file. If bash is started
with the -c option, then $0 is set to the first argument after
the string to be executed, if one is present. Otherwise, it is
set to the filename used to invoke bash, as given by argument
zero.
_ At shell startup, set to the absolute pathname used to invoke the
shell or shell script being executed as passed in the environment
or argument list. Subsequently, expands to the last argument to
the previous simple command executed in the foreground, after
expansion. Also set to the full pathname used to invoke 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 filename used to invoke this instance of
bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -s option to the shopt
builtin command (see SHELL BUILTIN COMMANDS below). The options
appearing in BASHOPTS are those reported as on by shopt. If this
variable is in the environment when bash starts up, each shell
option in the list will be enabled before reading any startup
files. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This
differs from $$ under certain circumstances, such as subshells
that do not require bash to be re-initialized. Assignments to
BASHPID have no effect. If BASHPID is unset, it loses its
special properties, even if it is subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to the
internal list of aliases as maintained by the alias builtin.
Elements added to this array appear in the alias list; however,
unsetting array elements currently does not cause aliases to be
removed from the alias list. If BASH_ALIASES is unset, it loses
its special properties, even if it is subsequently reset.
BASH_ARGC
An array variable whose values are the number of parameters in
each frame of the current bash execution call stack. The number
of parameters to the current subroutine (shell function or script
executed with . or source) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed
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onto BASH_ARGC. The shell sets BASH_ARGC only when in extended
debugging mode (see the description of the extdebug option to the
shopt builtin below). Setting extdebug after the shell has
started to execute a script, or referencing this variable when
extdebug is not set, may result in inconsistent values.
BASH_ARGV
An array variable containing all of the parameters in the current
bash execution call stack. The final parameter of the last
subroutine call is at the top of the stack; the first parameter
of the initial call is at the bottom. When a subroutine is
executed, the parameters supplied are pushed onto BASH_ARGV. The
shell sets BASH_ARGV only when in extended debugging mode (see
the description of the extdebug option to the shopt builtin
below). Setting extdebug after the shell has started to execute
a script, or referencing this variable when extdebug is not set,
may result in inconsistent values.
BASH_ARGV0
When referenced, this variable expands to the name of the shell
or shell script (identical to $0; see the description of special
parameter 0 above). Assignment to BASH_ARGV0 causes the value
assigned to also be assigned to $0. If BASH_ARGV0 is unset, it
loses its special properties, even if it is subsequently reset.
BASH_CMDS
An associative array variable whose members correspond to the
internal hash table of commands as maintained by the hash
builtin. Elements added to this array appear in the hash table;
however, unsetting array elements currently does not cause
command names to be removed from the hash table. If BASH_CMDS is
unset, it loses its special properties, even if it is
subsequently reset.
BASH_COMMAND
The command currently being executed or about to be executed,
unless the shell is executing a command as the result of a trap,
in which case it is the command executing at the time of the
trap.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source
files where each corresponding member of FUNCNAME was invoked.
${BASH_LINENO[$i]} is the line number in the source file
(${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or
${BASH_LINENO[$i-1]} if referenced within another shell
function). Use LINENO to obtain the current line number.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the shell looks
for dynamically loadable builtins specified by the enable
command.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary
operator to the [[ conditional command. The element with index 0
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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 where
the corresponding shell function names in the FUNCNAME array
variable are defined. The shell function ${FUNCNAME[$i]} is
defined in the file ${BASH_SOURCE[$i]} and called from
${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment
when the shell begins executing in that environment. The initial
value is 0.
BASH_VERSINFO
A readonly array variable whose members hold version information
for this instance of bash. The values assigned to the array
members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of
bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current
cursor position. This variable is available only in shell
functions invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the
current completion function.
COMP_LINE
The current command line. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the
beginning of the current command. If the current cursor position
is at the end of the current command, the value of this variable
is equal to ${#COMP_LINE}. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called: TAB,
for normal completion, ?, for listing completions after
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successive tabs, !, for listing alternatives on partial word
completion, @, to list completions if the word is not unmodified,
or %, for menu completion. This variable is available only in
shell functions and external commands invoked by the programmable
completion facilities (see Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word
separators when performing word completion. If COMP_WORDBREAKS
is unset, it loses its special properties, even if it is
subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual
words in the current command line. The line is split into words
as readline would split it, using COMP_WORDBREAKS as described
above. This variable is available only in shell functions
invoked by the programmable completion facilities (see
Programmable Completion below).
COPROC
An array variable (see Arrays below) created to hold the file
descriptors for output from and input to an unnamed coprocess
(see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current
contents of the directory stack. Directories appear in the stack
in the order they are displayed by the dirs builtin. Assigning
to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd builtins
must be used to add and remove directories. Assignment to this
variable will not change the current directory. If DIRSTACK is
unset, it loses its special properties, even if it is
subsequently reset.
EPOCHREALTIME
Each time this parameter is referenced, it expands to the number
of seconds since the Unix Epoch (see time(3)) as a floating point
value with micro-second granularity. Assignments to
EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses
its special properties, even if it is subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the number
of seconds since the Unix Epoch (see time(3)). Assignments to
EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its
special properties, even if it is subsequently reset.
EUID Expands to the effective user ID of the current user, initialized
at shell startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions
currently in the execution call stack. The element with index 0
is the name of any currently-executing shell function. The
bottom-most element (the one with the highest index) is "main".
This variable exists only when a shell function is executing.
Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it
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loses its special properties, even if it is subsequently reset.
This variable can be used with BASH_LINENO and BASH_SOURCE. Each
element of FUNCNAME has corresponding elements in BASH_LINENO and
BASH_SOURCE to describe the call stack. For instance,
${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at
line number ${BASH_LINENO[$i]}. The caller builtin displays the
current call stack using this information.
GROUPS
An array variable containing the list of groups of which the
current user is a member. Assignments to GROUPS have no effect.
If GROUPS is unset, it loses its special properties, even if it
is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current
command. If HISTCMD is unset, it loses its special properties,
even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of
machine on which bash is executing. The default is system-
dependent.
LINENO
Each time this parameter is referenced, the shell substitutes a
decimal number representing the current sequential line number
(starting with 1) within a script or function. When not in a
script or function, the value substituted is not guaranteed to be
meaningful. If LINENO is unset, it loses its special properties,
even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system
type on which bash is executing, in the standard GNU cpu-
company-system format. The default is system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text
read by the mapfile builtin when no variable name is supplied.
OLDPWD
The previous working directory as set by the cd command.
OPTARG
The value of the last option argument processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND
The index of the next argument to be processed by the getopts
builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE
Automatically set to a string that describes the operating system
on which bash is executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit
status values from the processes in the most-recently-executed
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foreground pipeline (which may contain only a single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM
Each time this parameter is referenced, a random integer between
0 and 32767 is generated. The sequence of random numbers may be
initialized by assigning a value to RANDOM. If RANDOM is unset,
it loses its special properties, even if it is subsequently
reset.
READLINE_LINE
The contents of the readline line buffer, for use with "bind -x"
(see SHELL BUILTIN COMMANDS below).
READLINE_POINT
The position of the insertion point in the readline line buffer,
for use with "bind -x" (see SHELL BUILTIN COMMANDS below).
REPLY
Set to the line of input read by the read builtin command when no
arguments are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds
since shell invocation is returned. If a value is assigned to
SECONDS, the value returned upon subsequent references is the
number of seconds since the assignment plus the value assigned.
If SECONDS is unset, it loses its special properties, even if it
is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in
the list is a valid argument for the -o option to the set builtin
command (see SHELL BUILTIN COMMANDS below). The options
appearing in SHELLOPTS are those reported as on by set -o. If
this variable is in the environment when bash starts up, each
shell option in the list will be enabled before reading any
startup files. This variable is read-only.
SHLVL
Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialized at shell
startup. This variable is readonly.
The following variables are used by the shell. In some cases, bash
assigns a default value to a variable; these cases are noted below.
BASH_COMPAT
The value is used to set the shell's compatibility level. See
the description of the shopt builtin below under SHELL BUILTIN
COMMANDS for a description of the various compatibility levels
and their effects. The value may be a decimal number (e.g., 4.2)
or an integer (e.g., 42) corresponding to the desired
compatibility level. If BASH_COMPAT is unset or set to the empty
string, the compatibility level is set to the default for the
current version. If BASH_COMPAT is set to a value that is not
one of the valid compatibility levels, the shell prints an error
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message and sets the compatibility level to the default for the
current version. The valid compatibility levels correspond to
the compatibility options accepted by the shopt builtin described
below (for example, compat42 means that 4.2 and 42 are valid
values). The current version is also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell script,
its value is interpreted as a filename containing commands to
initialize the shell, as in ~/.bashrc. The value of BASH_ENV is
subjected to parameter expansion, command substitution, and
arithmetic expansion before being interpreted as a filename.
PATH is not used to search for the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor,
bash will write the trace output generated when set -x is enabled
to that file descriptor. The file descriptor is closed when
BASH_XTRACEFD is unset or assigned a new value. Unsetting
BASH_XTRACEFD or assigning it the empty string causes the trace
output to be sent to the standard error. Note that setting
BASH_XTRACEFD to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.
CDPATH
The search path for the cd command. This is a colon-separated
list of directories in which the shell looks for destination
directories specified by the cd command. A sample value is
".:~:/usr".
CHILD_MAX
Set the number of exited child status values for the shell to
remember. Bash will not allow this value to be decreased below a
POSIX-mandated minimum, and there is a maximum value (currently
8192) that this may not exceed. The minimum value is system-
dependent.
COLUMNS
Used by the select compound command to determine the terminal
width when printing selection lists. Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions
generated by a shell function invoked by the programmable
completion facility (see Programmable Completion below). Each
array element contains one possible completion.
EMACS
If bash finds this variable in the environment when the shell
starts with value "t", it assumes that the shell is running in an
Emacs shell buffer and disables line editing.
ENV Similar to BASH_ENV; used when the shell is invoked in posix
mode.
EXECIGNORE
A colon-separated list of shell patterns (see Pattern Matching)
defining the list of filenames to be ignored by command search
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using PATH. Files whose full pathnames match one of these
patterns are not considered executable files for the purposes of
completion and command execution via PATH lookup. This does not
affect the behavior of the [, test, and [[ commands. Full
pathnames in the command hash table are not subject to
EXECIGNORE. Use this variable to ignore shared library files
that have the executable bit set, but are not executable files.
The pattern matching honors the setting of the extglob shell
option.
FCEDIT
The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing
filename completion (see READLINE below). A filename whose
suffix matches one of the entries in FIGNORE is excluded from the
list of matched filenames. A sample value is ".o:~".
FUNCNEST
If set to a numeric value greater than 0, defines a maximum
function nesting level. Function invocations that exceed this
nesting level will cause the current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of file names
to be ignored by pathname expansion. If a file name matched by a
pathname expansion pattern also matches one of the patterns in
GLOBIGNORE, it is removed from the list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are
saved on the history list. If the list of values includes
ignorespace, lines which begin with a space character are not
saved in the history list. A value of ignoredups causes lines
matching the previous history entry to not be saved. A value of
ignoreboth is shorthand for ignorespace and ignoredups. A value
of erasedups causes all previous lines matching the current line
to be removed from the history list before that line is saved.
Any value not in the above list is ignored. If HISTCONTROL is
unset, or does not include a valid value, all lines read by the
shell parser are saved on the history list, subject to the value
of HISTIGNORE. The second and subsequent lines of a multi-line
compound command are not tested, and are added to the history
regardless of the value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see
HISTORY below). The default value is ~/.bash_history. If unset,
the command history is not saved when a shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When
this variable is assigned a value, the history file is truncated,
if necessary, to contain no more than that number of lines by
removing the oldest entries. The history file is also truncated
to this size after writing it when a shell exits. If the value
is 0, the history file is truncated to zero size. Non-numeric
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values and numeric values less than zero inhibit truncation. The
shell sets the default value to the value of HISTSIZE after
reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which command
lines should be saved on the history list. Each pattern is
anchored at the beginning of the line and must match the complete
line (no implicit `*' is appended). Each pattern is tested
against the line after the checks specified by HISTCONTROL are
applied. In addition to the normal shell pattern matching
characters, `&' matches the previous history line. `&' may be
escaped using a backslash; the backslash is removed before
attempting a match. The second and subsequent lines of a multi-
line compound command are not tested, and are added to the
history regardless of the value of HISTIGNORE. The pattern
matching honors the setting of the extglob shell option.
HISTSIZE
The number of commands to remember in the command history (see
HISTORY below). If the value is 0, commands are not saved in the
history list. Numeric values less than zero result in every
command being saved on the history list (there is no limit). The
shell sets the default value to 500 after reading any startup
files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a
format string for strftime(3) to print the time stamp associated
with each history entry displayed by the history builtin. If
this variable is set, time stamps are written to the history file
so they may be preserved across shell sessions. This uses the
history comment character to distinguish timestamps from other
history lines.
HOME The home directory of the current user; the default argument for
the cd builtin command. The value of this variable is also used
when performing tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that
should be read when the shell needs to complete a hostname. The
list of possible hostname completions may be changed while the
shell is running; the next time hostname completion is attempted
after the value is changed, bash adds the contents of the new
file to the existing list. If HOSTFILE is set, but has no value,
or does not name a readable file, bash attempts to read
/etc/hosts to obtain the list of possible hostname completions.
When HOSTFILE is unset, the hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting
after expansion and to split lines into words with the read
builtin command. The default value is ``<space><tab><newline>''.
IGNOREEOF
Controls the action of an interactive shell on receipt of an EOF
character as the sole input. If set, the value is the number of
consecutive EOF characters which must be typed as the first
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characters on an input line before bash exits. If the variable
exists but does not have a numeric value, or has no value, the
default value is 10. If it does not exist, EOF signifies the end
of input to the shell.
INPUTRC
The filename for the readline startup file, overriding the
default of ~/.inputrc (see READLINE below).
INSIDE_EMACS
If this variable appears in the environment when the shell
starts, bash assumes that it is running inside an Emacs shell
buffer and may disable line editing, depending on the value of
TERM.
LANG Used to determine the locale category for any category not
specifically selected with a variable starting with LC_.
LC_ALL
This variable overrides the value of LANG and any other LC_
variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used when sorting
the results of pathname expansion, and determines the behavior of
range expressions, equivalence classes, and collating sequences
within pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of characters and the
behavior of character classes within pathname expansion and
pattern matching.
LC_MESSAGES
This variable determines the locale used to translate double-
quoted strings preceded by a $.
LC_NUMERIC
This variable determines the locale category used for number
formatting.
LC_TIME
This variable determines the locale category used for data and
time formatting.
LINES
Used by the select compound command to determine the column
length for printing selection lists. Automatically set if the
checkwinsize option is enabled or in an interactive shell upon
receipt of a SIGWINCH.
MAIL If this parameter is set to a file or directory name and the
MAILPATH variable is not set, bash informs the user of the
arrival of mail in the specified file or Maildir-format
directory.
MAILCHECK
Specifies how often (in seconds) bash checks for mail. The
default is 60 seconds. When it is time to check for mail, the
shell does so before displaying the primary prompt. If this
variable is unset, or set to a value that is not a number greater
than or equal to zero, the shell disables mail checking.
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MAILPATH
A colon-separated list of filenames to be checked for mail. The
message to be printed when mail arrives in a particular file may
be specified by separating the filename from the message with a
`?'. When used in the text of the message, $_ expands to the
name of the current mailfile. Example:
MAILPATH=/var/mail/bfox?"You have mail":~/shell-mail?"$_ has
mail!"
Bash can be configured to supply a default value for this
variable (there is no value by default), but the location of the
user mail files that it uses is system dependent (e.g.,
/var/mail/$USER).
OPTERR
If set to the value 1, bash displays error messages generated by
the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTERR is initialized to 1 each time the shell is invoked or a
shell script is executed.
PATH The search path for commands. It is a colon-separated list of
directories in which the shell looks for commands (see COMMAND
EXECUTION below). A zero-length (null) directory name in the
value of PATH indicates the current directory. A null directory
name may appear as two adjacent colons, or as an initial or
trailing colon. The default path is system-dependent, and is set
by the administrator who installs bash. A common value is
``/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin''.
POSIXLY_CORRECT
If this variable is in the environment when bash starts, the
shell enters posix mode before reading the startup files, as if
the --posix invocation option had been supplied. If it is set
while the shell is running, bash enables posix mode, as if the
command set -o posix had been executed. When the shell enters
posix mode, it sets this variable if it was not already set.
PROMPT_COMMAND
If set, the value is executed as a command prior to issuing each
primary prompt.
PROMPT_DIRTRIM
If set to a number greater than zero, the value is used as the
number of trailing directory components to retain when expanding
the \w and \W prompt string escapes (see PROMPTING below).
Characters removed are replaced with an ellipsis.
PS0 The value of this parameter is expanded (see PROMPTING below) and
displayed by interactive shells after reading a command and
before the command is executed.
PS1 The value of this parameter is expanded (see PROMPTING below) and
used as the primary prompt string. The default value is
``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1 and used as
the secondary prompt string. The default is ``> ''.
PS3 The value of this parameter is used as the prompt for the select
command (see SHELL GRAMMAR above).
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PS4 The value of this parameter is expanded as with PS1 and the value
is printed before each command bash displays during an execution
trace. The first character of the expanded value of PS4 is
replicated multiple times, as necessary, to indicate multiple
levels of indirection. The default is ``+ ''.
SHELL
The full pathname to the shell is kept in this environment
variable. If it is not set when the shell starts, bash assigns
to it the full pathname of the current user's login shell.
TIMEFORMAT
The value of this parameter is used as a format string specifying
how the timing information for pipelines prefixed with the time
reserved word should be displayed. The % character introduces an
escape sequence that is expanded to a time value or other
information. The escape sequences and their meanings are as
follows; the braces denote optional portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user mode.
%[p][l]S The number of CPU seconds spent in system mode.
%P The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of
fractional digits after a decimal point. A value of 0 causes no
decimal point or fraction to be output. At most three places
after the decimal point may be specified; values of p greater
than 3 are changed to 3. If p is not specified, the value 3 is
used.
The optional l specifies a longer format, including minutes, of
the form MMmSS.FFs. The value of p determines whether or not the
fraction is included.
If this variable is not set, bash acts as if it had the value
$\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS. If the value is null, no
timing information is displayed. A trailing newline is added
when the format string is displayed.
TMOUT
If set to a value greater than zero, TMOUT is treated as the
default timeout for the read builtin. The select command
terminates if input does not arrive after TMOUT seconds when
input is coming from a terminal. In an interactive shell, the
value is interpreted as the number of seconds to wait for a line
of input after issuing the primary prompt. Bash terminates after
waiting for that number of seconds if a complete line of input
does not arrive.
TMPDIR
If set, bash uses its value as the name of a directory in which
bash creates temporary files for the shell's use.
auto_resume
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This variable controls how the shell interacts with the user and
job control. If this variable is set, single word simple
commands without redirections are treated as candidates for
resumption of an existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning with the string
typed, the job most recently accessed is selected. The name of a
stopped job, in this context, is the command line used to start
it. If set to the value exact, the string supplied must match
the name of a stopped job exactly; if set to substring, the
string supplied needs to match a substring of the name of a
stopped job. The substring value provides functionality
analogous to the %? job identifier (see JOB CONTROL below). If
set to any other value, the supplied string must be a prefix of a
stopped job's name; this provides functionality analogous to the
%string job identifier.
histchars
The two or three characters which control history expansion and
tokenization (see HISTORY EXPANSION below). The first character
is the history expansion character, the character which signals
the start of a history expansion, normally `!'. The second
character is the quick substitution character, which is used as
shorthand for re-running the previous command entered,
substituting one string for another in the command. The default
is `^'. The optional third character is the character which
indicates that the remainder of the line is a comment when found
as the first character of a word, normally `#'. The history
comment character causes history substitution to be skipped for
the remaining words on the line. It does not necessarily cause
the shell parser to treat the rest of the line as a comment.
Arrays
Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array; the declare builtin will
explicitly declare an array. There is no maximum limit on the size of
an array, nor any requirement that members be indexed or assigned
contiguously. Indexed arrays are referenced using integers (including
arithmetic expressions) and are zero-based; associative arrays are
referenced using arbitrary strings. Unless otherwise noted, indexed
array indices must be non-negative integers.
An indexed array is created automatically if any variable is assigned
to using the syntax name[subscript]=value. The subscript is treated
as an arithmetic expression that must evaluate to a number. To
explicitly declare an indexed array, use declare -a name (see SHELL
BUILTIN COMMANDS below). declare -a name[subscript] is also accepted;
the subscript is ignored.
Associative arrays are created using declare -A name.
Attributes may be specified for an array variable using the declare
and readonly builtins. Each attribute applies to all members of an
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array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen), where each value is of the form
[subscript]=string. Indexed array assignments do not require anything
but string. When assigning to indexed arrays, if the optional
brackets and subscript are supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the subscript is required.
This syntax is also accepted by the declare builtin. Individual array
elements may be assigned to using the name[subscript]=value syntax
introduced above. When assigning to an indexed array, if name is
subscripted by a negative number, that number is interpreted as
relative to one greater than the maximum index of name, so negative
indices count back from the end of the array, and an index of -1
references the last element.
Any element of an array may be referenced using ${name[subscript]}.
The braces are required to avoid conflicts with pathname expansion.
If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double
quotes. If the word is double-quoted, ${name[*]} expands to a single
word with the value of each array member separated by the first
character of the IFS special variable, and ${name[@]} expands each
element of name to a separate word. When there are no array members,
${name[@]} expands to nothing. If the double-quoted expansion occurs
within a word, the expansion of the first parameter is joined with the
beginning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word. This is
analogous to the expansion of the special parameters * and @ (see
Special Parameters above). ${#name[subscript]} expands to the length
of ${name[subscript]}. If subscript is * or @, the expansion is the
number of elements in the array. If the subscript used to reference
an element of an indexed array evaluates to a number less than zero,
it is interpreted as relative to one greater than the maximum index of
the array, so negative indices count back from the end of the array,
and an index of -1 references the last element.
Referencing an array variable without a subscript is equivalent to
referencing the array with a subscript of 0. Any reference to a
variable using a valid subscript is legal, and bash will create an
array if necessary.
An array variable is considered set if a subscript has been assigned a
value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as the
values. ${!name[@]} and ${!name[*]} expand to the indices assigned in
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array variable name. The treatment when in double quotes is similar
to the expansion of the special parameters @ and * within double
quotes.
The unset builtin is used to destroy arrays. unset name[subscript]
destroys the array element at index subscript, for both indexed and
associative arrays. Negative subscripts to indexed arrays are
interpreted as described above. Unsetting the last element of an
array variable does not unset the variable. unset name, where name is
an array, or unset name[subscript], where subscript is * or @, removes
the entire array.
When using a variable name with a subscript as an argument to a
command, such as with unset, without using the word expansion syntax
described above, the argument is subject to pathname expansion. If
pathname expansion is not desired, the argument should be quoted.
The declare, local, and readonly builtins each accept a -a option to
specify an indexed array and a -A option to specify an associative
array. If both options are supplied, -A takes precedence. The read
builtin accepts a -a option to assign a list of words read from the
standard input to an array. The set and declare builtins display
array values in a way that allows them to be reused as assignments.
EXPANSION
Expansion is performed on the command line after it has been split
into words. There are seven kinds of expansion performed: brace
expansion, tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, word splitting, and pathname
expansion.
The order of expansions is: brace expansion; tilde expansion,
parameter and variable expansion, arithmetic expansion, and command
substitution (done in a left-to-right fashion); word splitting; and
pathname expansion.
On systems that can support it, there is an additional expansion
available: process substitution. This is performed at the same time
as tilde, parameter, variable, and arithmetic expansion and command
substitution.
After these expansions are performed, quote characters present in the
original word are removed unless they have been quoted themselves
(quote removal).
Only brace expansion, word splitting, and pathname expansion can
increase the number of words of the expansion; other expansions expand
a single word to a single word. The only exceptions to this are the
expansions of "$@" and "${name[@]}", and, in most cases, $* and
${name[*]} as explained above (see PARAMETERS).
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Brace Expansion
Brace expansion is a mechanism by which arbitrary strings may be
generated. This mechanism is similar to pathname expansion, but the
filenames generated need not exist. 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[..incr]}, where x and y are
either integers or single characters, and incr, an optional increment,
is an integer. When integers are supplied, the expression expands to
each number between x and y, inclusive. Supplied integers may be
prefixed with 0 to force each term to have the same width. When
either x or y begins with a zero, the shell attempts to force all
generated terms to contain the same number of digits, zero-padding
where necessary. When characters are supplied, the expression expands
to each character lexicographically between x and y, inclusive, using
the default C locale. Note that both x and y must be of the same
type. When the increment is supplied, it is used as the difference
between each term. The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result.
It is strictly textual. Bash does not apply any syntactic
interpretation to the context of the expansion or the text between the
braces.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.
A { or , may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with
parameter expansion, the string ${ is not considered eligible for
brace expansion, and inhibits brace expansion until the closing }.
This construct is typically used as shorthand when the common prefix
of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical
versions of sh. sh does not treat opening or closing braces specially
when they appear as part of a word, and preserves them in the output.
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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
directory of the user executing the shell is substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
If the tilde-prefix is a `~+', the value of the shell variable PWD
replaces the tilde-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
immediately following a : or the first =. In these cases, tilde
expansion is also performed. Consequently, one may use filenames with
tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell
assigns the expanded value.
Bash also performs tilde expansion on words satisfying the conditions
of variable assignments (as described above under PARAMETERS) when
they appear as arguments to simple commands. Bash does not do this,
except for the declaration commands listed above, when in posix mode.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name or symbol
to be expanded may be enclosed in braces, which are optional but serve
to protect the variable to be expanded from characters immediately
following it which could be interpreted as part of the name.
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When braces are used, the matching ending brace is the first `}' not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
${parameter}
The value of parameter is substituted. The braces are required
when parameter is a positional parameter with more than one
digit, or when parameter is followed by a character which is not
to be interpreted as part of its name. The parameter is a shell
parameter as described above PARAMETERS) or an array reference
(Arrays).
If the first character of parameter is an exclamation point (!), and
parameter is not a nameref, it introduces a level of indirection.
Bash uses the value formed by expanding the rest of parameter as the
new parameter; this is then expanded and that value is used in the
rest of the expansion, rather than the expansion of the original
parameter. This is known as indirect expansion. The value is subject
to tilde expansion, parameter expansion, command substitution, and
arithmetic expansion. If parameter is a nameref, this expands to the
name of the parameter referenced by parameter instead of performing
the complete indirect expansion. The exceptions to this are the
expansions of ${!prefix*} and ${!name[@]} described below. The
exclamation point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, word is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, using the forms documented
below (e.g., :-), bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is
unset.
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion
of word is substituted. Otherwise, the value of parameter is
substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter. The value of
parameter is then substituted. Positional parameters and special
parameters may not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset,
the expansion of word (or a message to that effect if word is not
present) is written to the standard error and the shell, if it is
not interactive, exits. Otherwise, the value of parameter is
substituted.
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${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is
substituted, otherwise the expansion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length characters of the
value of parameter starting at the character specified by offset.
If parameter is @, an indexed array subscripted by @ or *, or an
associative array name, the results differ as described below.
If length is omitted, expands to the substring of the value of
parameter starting at the character specified by offset and
extending to the end of the value. length and offset are
arithmetic expressions (see ARITHMETIC EVALUATION below).
If offset evaluates to a number less than zero, the value is used
as an offset in characters from the end of the value of
parameter. If length evaluates to a number less than zero, it is
interpreted as an offset in characters from the end of the value
of parameter rather than a number of characters, and the
expansion is the characters between offset and that result. Note
that a negative offset must be separated from the colon by at
least one space to avoid being confused with the :- expansion.
If parameter is @, the result is length positional parameters
beginning at offset. A negative offset is taken relative to one
greater than the greatest positional parameter, so an offset of
-1 evaluates to the last positional parameter. It is an
expansion error if length evaluates to a number less than zero.
If parameter is an indexed array name subscripted by @ or *, the
result is the length members of the array beginning with
${parameter[offset]}. A negative offset is taken relative to one
greater than the maximum index of the specified array. It is an
expansion error if length evaluates to a number less than zero.
Substring expansion applied to an associative array produces
undefined results.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1 by default. If
offset is 0, and the positional parameters are used, $0 is
prefixed to the list.
${!prefix*}
${!prefix@}
Names matching prefix. Expands to the names of variables whose
names begin with prefix, separated by the first character of the
IFS special variable. When @ is used and the expansion appears
within double quotes, each variable name expands to a separate
word.
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${!name[@]}
${!name[*]}
List of array keys. If name is an array variable, expands to the
list of array indices (keys) assigned in name. If name is not an
array, expands to 0 if name is set and null otherwise. When @ is
used and the expansion appears within double quotes, each key
expands to a separate word.
${#parameter}
Parameter length. The length in characters of the value of
parameter is substituted. If parameter is * or @, the value
substituted is the number of positional parameters. If parameter
is an array name subscripted by * or @, the value substituted is
the number of elements in the array. If parameter is an indexed
array name subscripted by a negative number, that number is
interpreted as relative to one greater than the maximum index of
parameter, so negative indices count back from the end of the
array, and an index of -1 references the last element.
${parameter#word}
${parameter##word}
Remove matching prefix pattern. The word is expanded to produce
a pattern just as in pathname expansion, and matched against the
expanded value of parameter using the rules described under
Pattern Matching below. If the pattern matches the beginning of
the value of parameter, then the result of the expansion is the
expanded value of parameter with the shortest matching pattern
(the ``#'' case) or the longest matching pattern (the ``##''
case) deleted. If parameter is @ or *, the pattern removal
operation is applied to each positional parameter in turn, and
the expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the pattern removal operation
is applied to each member of the array in turn, and the expansion
is the resultant list.
${parameter%word}
${parameter%%word}
Remove matching suffix pattern. The word is expanded to produce
a pattern just as in pathname expansion, and matched against the
expanded value of parameter using the rules described under
Pattern Matching below. If the pattern matches a trailing
portion of the expanded value of parameter, then the result of
the expansion is the expanded value of parameter with the
shortest matching pattern (the ``%'' case) or the longest
matching pattern (the ``%%'' case) deleted. If parameter is @ or
*, the pattern removal operation is applied to each positional
parameter in turn, and the expansion is the resultant list. If
parameter is an array variable subscripted with @ or *, the
pattern removal operation is applied to each member of the array
in turn, and the expansion is the resultant list.
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${parameter/pattern/string}
Pattern substitution. The pattern is expanded to produce a
pattern just as in pathname expansion, Parameter is expanded and
the longest match of pattern against its value is replaced with
string. The match is performed using the rules described under
Pattern Matching below. If pattern begins with /, all matches of
pattern are replaced with string. Normally only the first match
is replaced. If pattern begins with #, it must match at the
beginning of the expanded value of parameter. If pattern begins
with %, it must match at the end of the expanded value of
parameter. If string is null, matches of pattern are deleted and
the / following pattern may be omitted. If the nocasematch shell
option is enabled, the match is performed without regard to the
case of alphabetic characters. If parameter is @ or *, the
substitution operation is applied to each positional parameter in
turn, and the expansion is the resultant list. If parameter is
an array variable subscripted with @ or *, the substitution
operation is applied to each member of the array in turn, and the
expansion is the resultant list.
${parameter^pattern}
${parameter^^pattern}
${parameter,pattern}
${parameter,,pattern}
Case modification. This expansion modifies the case of
alphabetic characters in parameter. The pattern is expanded to
produce a pattern just as in pathname expansion. Each character
in the expanded value of parameter is tested against pattern,
and, if it matches the pattern, its case is converted. The
pattern should not attempt to match more than one character. The
^ operator converts lowercase letters matching pattern to
uppercase; the , operator converts matching uppercase letters to
lowercase. The ^^ and ,, expansions convert each matched
character in the expanded value; the ^ and , expansions match and
convert only the first character in the expanded value. If
pattern is omitted, it is treated like a ?, which matches every
character. If parameter is @ or *, the case modification
operation is applied to each positional parameter in turn, and
the expansion is the resultant list. If parameter is an array
variable subscripted with @ or *, the case modification operation
is applied to each member of the array in turn, and the expansion
is the resultant list.
${parameter@operator}
Parameter transformation. The expansion is either a
transformation of the value of parameter or information about
parameter itself, depending on the value of operator. Each
operator is a single letter:
Q The expansion is a string that is the value of parameter
quoted in a format that can be reused as input.
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E The expansion is a string that is the value of parameter
with backslash escape sequences expanded as with the $'...'
quoting mechanism.
P The expansion is a string that is the result of expanding
the value of parameter as if it were a prompt string (see
PROMPTING below).
A The expansion is a string in the form of an assignment
statement or declare command that, if evaluated, will
recreate parameter with its attributes and value.
a The expansion is a string consisting of flag values
representing parameter's attributes.
If parameter is @ or *, the operation is applied to each
positional parameter in turn, and the expansion is the resultant
list. If parameter is an array variable subscripted with @ or *,
the operation is applied to each member of the array in turn, and
the expansion is the resultant list.
The result of the expansion is subject to word splitting and
pathname expansion as described below.
Command Substitution
Command substitution allows the output of a command to replace the
command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command in a subshell
environment and replacing the command substitution with the standard
output of the command, with any trailing newlines deleted. Embedded
newlines are not deleted, but they may be removed during word
splitting. The command substitution $(cat file) can be replaced by
the equivalent but faster $(< file).
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
backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
pathname expansion are not performed on the results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic
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expansion is:
$((expression))
The expression is treated as if it were within double quotes, but a
double quote inside the parentheses is not treated specially. All
tokens in the expression undergo parameter and variable expansion,
command substitution, and quote removal. The result is treated as the
arithmetic expression to be evaluated. Arithmetic expansions may be
nested.
The evaluation is performed according to the rules listed below under
ARITHMETIC EVALUATION. If expression is invalid, bash prints a
message indicating failure and no substitution occurs.
Process Substitution
Process substitution allows a process's input or output to be referred
to using a filename. It takes the form of <(list) or >(list). The
process list is run asynchronously, and its input or output appears as
a filename. This filename is passed as an argument to the current
command as the result of the expansion. If the >(list) form is used,
writing to the file 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. Process substitution is supported on systems that
support named pipes (FIFOs) or the /dev/fd method of naming open
files.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
Word Splitting
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within
double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and splits the
results of the other expansions into words using these characters as
field terminators. If IFS is unset, or its value is exactly
<space><tab><newline>, the default, then sequences of <space>, <tab>,
and <newline> at the beginning and end of the results of the previous
expansions are ignored, and any sequence of IFS characters not at the
beginning or end serves to delimit words. If IFS has a value other
than the default, then sequences of the whitespace characters space,
tab, and newline are ignored at the beginning and end of the word, as
long as the whitespace character is in the value of IFS (an IFS
whitespace character). Any character in IFS that is not IFS
whitespace, along with any adjacent IFS whitespace characters,
delimits a field. A sequence of IFS whitespace characters is also
treated as a delimiter. If the value of IFS is null, no word
splitting occurs.
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Explicit null arguments ("" or ) are retained and passed to commands
as empty strings. Unquoted implicit null arguments, resulting from
the expansion of parameters that have no values, are removed. If a
parameter with no value is expanded within double quotes, a null
argument results and is retained and passed to a command as an empty
string. When a quoted null argument appears as part of a word whose
expansion is non-null, the null argument is removed. That is, the
word -d becomes -d after word splitting and null argument removal.
Note that if no expansion occurs, no splitting is performed.
Pathname Expansion
After word splitting, unless the -f option has been set, bash scans
each word for the characters *, ?, and [. If one of these characters
appears, then the word is regarded as a pattern, and replaced with an
alphabetically sorted list of filenames matching the pattern (see
Pattern Matching below). If no matching filenames are found, and the
shell option nullglob is not enabled, the word is left unchanged. If
the nullglob option is set, and no matches are found, the word is
removed. If the failglob shell option is set, and no matches are
found, an error message is printed and the command is not executed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters. 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. The filenames ``.'' and
``..'' must always be matched explicitly, even if dotglob is set. In
other cases, the ``.'' character is not treated specially. When
matching a pathname, the slash character must always be matched
explicitly by a slash in the pattern, but in other matching contexts
it can be matched by a special pattern character as described below
under Pattern Matching. See the description of shopt below under
SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob,
failglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the set of file
names matching a pattern. If GLOBIGNORE is set, each matching file
name that also matches one of the patterns in GLOBIGNORE is removed
from the list of matches. If the nocaseglob option is set, the
matching against the patterns in GLOBIGNORE is performed without
regard to case. The filenames ``.'' and ``..'' are always ignored
when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a
non-null value has the effect of enabling the dotglob shell option, so
all other filenames beginning with a ``.'' will match. To get the old
behavior of ignoring filenames beginning with a ``.'', make ``.*'' one
of the patterns in GLOBIGNORE. The dotglob option is disabled when
GLOBIGNORE is unset. The pattern matching honors the setting of the
extglob shell option.
Pattern Matching
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Any character that appears in a pattern, other than the special
pattern characters described below, matches itself. The NUL character
may not occur in a pattern. A backslash escapes the following
character; the escaping backslash is discarded when matching. The
special pattern characters must be quoted if they are to be matched
literally.
The special pattern characters have the following meanings:
* Matches any string, including the null string. When the
globstar shell option is enabled, and * is used in a
pathname expansion context, two adjacent *s used as a single
pattern will match all files and zero or more directories
and subdirectories. If followed by a /, two adjacent *s
will match only directories and subdirectories.
? Matches any single character.
[...]
Matches any one of the enclosed characters. A pair of
characters separated by a hyphen denotes a range expression;
any character that falls between those two characters,
inclusive, using the current locale's collating sequence and
character set, is matched. If the first character following
the [ is a ! or a ^ then any character not enclosed is
matched. The sorting order of characters in range
expressions is determined by the current locale and the
values of the LC_COLLATE or LC_ALL shell variables, if set.
To obtain the traditional interpretation of range
expressions, where [a-d] is equivalent to [abcd], set value
of the LC_ALL shell variable to C, or enable the
globasciiranges shell option. A - may be matched by
including it as the first or last character in the set. A ]
may be matched by including it as the first character in the
set.
Within [ and ], character classes can be specified using the
syntax [:class:], where class is one of the following
classes defined in the POSIX standard:
alnum alpha ascii blank cntrl digit graph lower print punct
space upper word xdigit
A character class matches any character belonging to that
class. The word character class matches letters, digits,
and the character _.
Within [ and ], an equivalence class can be specified using
the syntax [=c=], which matches all characters with the same
collation weight (as defined by the current locale) as the
character c.
Within [ and ], the syntax [.symbol.] matches the collating
symbol symbol.
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If the extglob shell option is enabled using the shopt builtin,
several extended pattern matching operators are recognized. In the
following description, a pattern-list is a list of one or more
patterns separated by a |. Composite patterns may be formed using one
or more of the following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given patterns
*(pattern-list)
Matches zero or more occurrences of the given patterns
+(pattern-list)
Matches one or more occurrences of the given patterns
@(pattern-list)
Matches one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
Complicated extended pattern matching against long strings is slow,
especially when the patterns contain alternations and the strings
contain multiple matches. Using separate matches against shorter
strings, or using arrays of strings instead of a single long string,
may be faster.
Quote Removal
After the preceding expansions, all unquoted occurrences of the
characters \, and " that did not result from one of the above
expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell. Redirection allows
commands' file handles to be duplicated, opened, closed, made to refer
to different files, and can change the files the command reads from
and writes to. Redirection may also be used to modify file handles in
the current shell execution environment. The following redirection
operators may precede or appear anywhere within a simple command or
may follow a command. Redirections are processed in the order they
appear, from left to right.
Each redirection that may be preceded by a file descriptor number may
instead be preceded by a word of the form {varname}. In this case,
for each redirection operator except >&- and <&-, the shell will
allocate a file descriptor greater than or equal to 10 and assign it
to varname. If >&- or <&- is preceded by {varname}, the value of
varname defines the file descriptor to close. If {varname} is
supplied, the redirection persists beyond the scope of the command,
allowing the shell programmer to manage the file descriptor himself.
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is <, the
redirection refers to the standard input (file descriptor 0). If the
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first character of the redirection operator is >, the redirection
refers to the standard output (file descriptor 1).
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter and variable expansion, command
substitution, arithmetic expansion, quote removal, pathname expansion,
and word splitting. If it expands to more than one word, bash reports
an error.
Note that the order of redirections is significant. For example, the
command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist,
while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard
error was duplicated from the standard output before the standard
output was redirected to dirlist.
Bash handles several filenames specially when they are used in
redirections, as described in the following table. If the operating
system on which bash is running provides these special files, bash
will use them; otherwise it will emulate them internally with the
behavior described below.
/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.
/dev/stdin
File descriptor 0 is duplicated.
/dev/stdout
File descriptor 1 is duplicated.
/dev/stderr
File descriptor 2 is duplicated.
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is
an integer port number or service name, bash attempts to
open the corresponding TCP socket.
/dev/udp/host/port
If host is a valid hostname or Internet address, and port is
an integer port number or service name, bash attempts to
open the corresponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with
care, as they may conflict with file descriptors the shell uses
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internally.
Redirecting Input
Redirection of input causes the file whose name results from the
expansion of word to be opened for reading on file descriptor n, or
the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
Redirecting Output
Redirection of output causes the file whose name results from the
expansion of word to be opened for writing on file descriptor n, or
the standard output (file descriptor 1) if n is not specified. If the
file does not exist it is 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
This construct allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be redirected to the
file whose name is the expansion of word.
There are two formats for redirecting standard output and standard
error:
&>word
and
>&word
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Of the two forms, the first is preferred. This is semantically
equivalent to
>word 2>&1
When using the second form, word may not expand to a number or -. If
it does, other redirection operators apply (see Duplicating File
Descriptors below) for compatibility reasons.
Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and
the standard error output (file descriptor 2) to be appended to the
file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
Here Documents
This type of redirection instructs the shell to read input from the
current source until a line containing only delimiter (with no
trailing blanks) is seen. All of the lines read up to that point are
then used as the standard input (or file descriptor n if n is
specified) for a command.
The format of here-documents is:
[n]<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution, arithmetic
expansion, or pathname expansion is performed on word. If any part of
word is quoted, the delimiter is the result of quote removal on word,
and the lines in the here-document are not expanded. If word is
unquoted, all lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion, 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.
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Here Strings
A variant of here documents, the format is:
[n]<<<word
The word undergoes tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal.
Pathname expansion and word splitting are not performed. The result
is supplied as a single string, with a newline appended, to the
command on its standard input (or file descriptor n if n is
specified).
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one
or more digits, the file descriptor denoted by n is made to be a copy
of that file descriptor. If the digits in word do not specify a file
descriptor open for input, a redirection error occurs. If word
evaluates to -, file descriptor n is closed. If n is not specified,
the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not
specified, the standard output (file descriptor 1) is used. If the
digits in word do not specify a file descriptor open for output, a
redirection error occurs. If word evaluates to -, file descriptor n
is closed. As a special case, if n is omitted, and word does not
expand to one or more digits or -, the standard output and standard
error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard
input (file 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.
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Opening File Descriptors for Reading and Writing
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for
both reading and writing on file descriptor n, or on file descriptor 0
if n is not specified. If the file does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when it is used as
the first word of a simple command. The shell maintains a list of
aliases that may be set and unset with the alias and unalias builtin
commands (see SHELL BUILTIN COMMANDS below). The first word of each
simple command, if unquoted, is checked to see if it has an alias. If
so, that word is replaced by the text of the alias. The characters /,
$, `, and = and any of the shell metacharacters or quoting characters
listed above may not appear in an alias name. The replacement text
may contain any valid shell input, including shell metacharacters.
The first word of the replacement text is tested for aliases, but a
word that is identical to an alias being expanded is not expanded a
second time. This means that one may alias ls to ls -F, for instance,
and bash does not try to recursively expand the replacement text. If
the last character of the alias value is a blank, then the next
command word following the alias is also checked for alias expansion.
Aliases are created and listed with the alias command, and removed
with the unalias command.
There is no mechanism for using arguments in the replacement text. If
arguments are needed, a shell function should be used (see FUNCTIONS
below).
Aliases are not expanded when the shell is not interactive, unless the
expand_aliases shell option is set using shopt (see the description of
shopt under SHELL BUILTIN COMMANDS below).
The rules concerning the definition and use of aliases are somewhat
confusing. Bash always reads at least one complete line of input, and
all lines that make up a compound command, before executing any of the
commands on that line or the compound command. Aliases are expanded
when a command is read, not when it is executed. Therefore, an alias
definition appearing on the same line as another command does not take
effect until the next line of input is read. The commands following
the alias definition on that line are not affected by the new alias.
This behavior is also an issue when functions are executed. Aliases
are expanded when a function definition is read, not when the function
is executed, because a function definition is itself a 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
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commands.
For almost every purpose, aliases are superseded by shell functions.
FUNCTIONS
A shell function, defined as described above under SHELL GRAMMAR,
stores a series of commands for later execution. When the name of a
shell function is used as a simple command name, the list of commands
associated with that function name is executed. Functions are
executed in the context of the current shell; no new process is
created to interpret them (contrast this with the execution of a shell
script). When a function is executed, the arguments to the function
become the positional parameters during its execution. The special
parameter # is updated to reflect the change. Special parameter 0 is
unchanged. The first element of the FUNCNAME variable is set to the
name of the function while the function is executing.
All other aspects of the shell execution environment are identical
between a function and its caller with these exceptions: the DEBUG and
RETURN traps (see the description of the trap builtin under SHELL
BUILTIN COMMANDS below) are not inherited unless the function has been
given the trace attribute (see the description of the declare builtin
below) or the -o functrace shell option has been enabled with the set
builtin (in which case all functions inherit the DEBUG and RETURN
traps), and the ERR trap is not inherited unless the -o errtrace shell
option has been enabled.
Variables local to the function may be declared with the local builtin
command. Ordinarily, variables and their values are shared between
the function and its caller. If a variable is declared local, the
variable's visible scope is restricted to that function and its
children (including the functions it calls). Local variables "shadow"
variables with the same name declared at previous scopes. For
instance, a local variable declared in a function hides a global
variable of the same name: references and assignments refer to the
local variable, leaving the global variable unmodified. When the
function returns, the global variable is once again visible.
The shell uses dynamic scoping to control a variable's visibility
within functions. With dynamic scoping, visible variables and their
values are a result of the sequence of function calls that caused
execution to reach the current function. The value of a variable that
a function sees depends on its value within its caller, if any,
whether that caller is the "global" scope or another shell function.
This is also the value that a local variable declaration "shadows",
and the value that is restored when the function returns.
For example, if a variable var is declared as local in function func1,
and func1 calls another function func2, references to var made from
within func2 will resolve to the local variable var from func1,
shadowing any global variable named var.
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The unset builtin also acts using the same dynamic scope: if a
variable is local to the current scope, unset will unset it; otherwise
the unset will refer to the variable found in any calling scope as
described above. If a variable at the current local scope is unset,
it will remain so until it is reset in that scope or until the
function returns. Once the function returns, any instance of the
variable at a previous scope will become visible. If the unset acts
on a variable at a previous scope, any instance of a variable with
that name that had been shadowed will become visible.
The FUNCNEST variable, if set to a numeric value greater than 0,
defines a maximum function nesting level. Function invocations that
exceed the limit cause the entire command to abort.
If the builtin command return is executed in a function, the function
completes and execution resumes with the next command after the
function call. 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).
Functions may be exported so that subshells automatically have them
defined with the -f option to the export builtin. A function
definition may be deleted using the -f option to the unset builtin.
Functions may be recursive. The FUNCNEST variable may be used to
limit the depth of the function call stack and restrict the number of
function invocations. By default, no limit is imposed on the number
of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated, under certain
circumstances (see the let and declare builtin commands, the ((
compound command, 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
- + unary minus and plus
++id --id
variable pre-increment and pre-decrement
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! ~ logical and bitwise negation
** exponentiation
* / %
multiplication, division, remainder
+ - addition, subtraction
<< >>
left and right bitwise shifts
<= >= < >
comparison
== !=
equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional operator
= *= /= %= += -=
assignment
expr1 , expr2
comma
Shell variables are allowed as operands; parameter expansion is
performed before the expression is evaluated. Within an expression,
shell variables may also be referenced by name without using the
parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter
expansion syntax. The value of a variable is evaluated as an
arithmetic expression when it is referenced, or when a variable which
has been given the integer attribute using declare -i is assigned a
value. A null value evaluates to 0. A shell variable need not have
its integer attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers. A
leading 0x or 0X denotes hexadecimal. Otherwise, numbers take the
form [base#]n, where the optional base is a decimal number between 2
and 64 representing the arithmetic base, and n is a number in that
base. If base# is omitted, then base 10 is used. When specifying n,
the digits greater than 9 are represented by the lowercase letters,
the uppercase letters, @, and _, in that order. If base is less than
or equal to 36, lowercase and uppercase letters may be used
interchangeably to 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
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and arithmetic comparisons. The test abd [ commands determine their
behavior based on the number of arguments; see the descriptions of
those commands for any other command-specific actions.
Expressions are formed from the following unary or binary primaries.
Bash handles several filenames specially when they are used in
expressions. If the operating system on which bash is running
provides these special files, bash will use them; otherwise it will
emulate them internally with this behavior: If any file argument to
one of the primaries is of the form /dev/fd/n, then file descriptor n
is checked. If the file argument to one of the primaries is one of
/dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2,
respectively, is checked.
Unless otherwise specified, primaries that operate on files follow
symbolic links and operate on the target of the link, rather than the
link itself.
When used with [[, the < and > operators sort lexicographically using
the current locale. The test command sorts using ASCII ordering.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special file.
-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than zero.
-t fd
True if file descriptor fd is open and refers to a terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
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-x file
True if file exists and is executable.
-G file
True if file exists and is owned by the effective group id.
-L file
True if file exists and is a symbolic link.
-N file
True if file exists and has been modified since it was last read.
-O file
True if file exists and is owned by the effective user id.
-S file
True if file exists and is a socket.
file1 -ef file2
True if file1 and file2 refer to the same device and inode
numbers.
file1 -nt file2
True if file1 is newer (according to modification date) than
file2, or if file1 exists and file2 does not.
file1 -ot file2
True if file1 is older than file2, or if file2 exists and file1
does not.
-o optname
True if the shell option optname is enabled. See the list of
options under the description of the -o option to the set builtin
below.
-v varname
True if the shell variable varname is set (has been assigned a
value).
-R varname
True if the shell variable varname is set and is a name
reference.
-z string
True if the length of string is zero.
string
-n string
True if the length of string is non-zero.
string1 == string2
string1 = string2
True if the strings are equal. = should be used with the test
command for POSIX conformance. When used with the [[ command,
this performs pattern matching as described above (Compound
Commands).
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographically.
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string1 > string2
True if string1 sorts after string2 lexicographically.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic
binary operators return true if arg1 is equal to, not equal to,
less than, less than or equal to, greater than, or greater than
or equal to arg2, respectively. Arg1 and arg2 may be positive or
negative integers. When used with the [[ command, Arg1 and Arg2
are evaluated as arithmetic expressions (see ARITHMETIC
EVALUATION above).
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
1. The words that the parser has marked as variable assignments
(those preceding the command name) and redirections are saved for
later processing.
2. The words that are not variable assignments or redirections are
expanded. If any words remain after expansion, the first word is
taken to be the name of the command and the remaining words are
the arguments.
3. Redirections are performed as described above under REDIRECTION.
4. The text after the = in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, and quote removal before being assigned to the
variable.
If no command name results, the variable assignments affect the
current shell environment. Otherwise, the variables are added to the
environment of the executed command and do not affect the current
shell environment. If any of the assignments attempts to assign a
value to a readonly variable, an error occurs, and the command exits
with a non-zero status.
If no command name results, redirections are performed, but do not
affect the current shell environment. A redirection error causes the
command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as
described below. Otherwise, the command exits. If one of the
expansions contained a command substitution, the exit status of the
command is the exit status of the last command substitution performed.
If there were no command substitutions, the command exits with a
status of zero.
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COMMAND EXECUTION
After a command has been split into words, if it results in a simple
command and an optional list of arguments, the following actions are
taken.
If the command name contains no slashes, the shell attempts to locate
it. If there exists a shell function by that name, that function is
invoked as described above in FUNCTIONS. If the name does not match a
function, the shell searches for it in the list of shell builtins. If
a match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and contains no
slashes, bash searches each element of the PATH for a directory
containing an executable file by that name. Bash uses a hash table to
remember the full pathnames of executable files (see hash under SHELL
BUILTIN COMMANDS below). A full search of the directories in PATH is
performed only if the command is not found in the hash table. If the
search is unsuccessful, the shell searches for a defined shell
function named command_not_found_handle. If that function exists, it
is invoked in a separate execution environment with the original
command and the original command's arguments as its arguments, and the
function's exit status becomes the exit status of that subshell. If
that function is not defined, the shell prints an error message and
returns an exit status of 127.
If the search is successful, or if the command name contains one or
more slashes, the shell executes the named program in a separate
execution environment. Argument 0 is set to the name given, and the
remaining arguments to the command are set to the arguments given, if
any.
If this execution fails because the file is not in executable format,
and the file is not a directory, it is assumed to be a shell script, a
file containing shell commands. 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
consist of a single optional argument following the interpreter name
on the first line of the program, followed by the name of the program,
followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of the
following:
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+ open files inherited by the shell at invocation, as modified by
redirections supplied to the exec builtin
+ the current working directory as set by cd, pushd, or popd, or
inherited by the shell at invocation
+ the file creation mode mask as set by umask or inherited from the
shell's parent
+ current traps set by trap
+ shell parameters that are set by variable assignment or with set
or inherited from the shell's parent in the environment
+ shell functions defined during execution or inherited from the
shell's parent in the environment
+ options enabled at invocation (either by default or with
command-line arguments) or by set
+ options enabled by shopt
+ shell aliases defined with alias
+ various process IDs, including those of background jobs, the
value of $$, and the value of PPID
When a simple command other than a builtin or shell function is to be
executed, it is invoked in a separate execution environment that
consists of the following. Unless otherwise noted, the values are
inherited from the shell.
+ the shell's open files, plus any modifications and additions
specified by redirections to the command
+ the current working directory
+ the file creation mode mask
+ shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
+ traps caught by the shell are reset to the values inherited from
the shell's parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the
shell's execution environment.
Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that is a
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duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a
pipeline are also executed in a subshell environment. Changes made to
the subshell environment cannot affect the shell's execution
environment.
Subshells spawned to execute command substitutions inherit the value
of the -e option from the parent shell. When not in posix mode, bash
clears the -e option in such subshells.
If a command is followed by a & and job control is not active, the
default standard input for the command is the empty file /dev/null.
Otherwise, the invoked command inherits the file descriptors of the
calling shell as modified by redirections.
ENVIRONMENT
When a program is invoked it is given an array of strings called the
environment. This is a list of name-value pairs, of the form
name=value.
The shell provides several ways to manipulate the environment. On
invocation, the shell scans its own environment and creates a
parameter for each name found, automatically marking it for export to
child processes. Executed commands inherit the environment. The
export and declare -x commands allow parameters and functions to be
added to and deleted from the environment. If the value of a
parameter in the environment is modified, the new value becomes part
of the environment, replacing the old. The environment inherited by
any executed command consists of the shell's initial environment,
whose values may be modified in the shell, less any pairs removed by
the unset command, plus any additions via the export and declare -x
commands.
The environment for any simple command or function may be augmented
temporarily by prefixing it with parameter 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 filename of the command and passed to that command in its
environment.
EXIT STATUS
The exit status of an executed command is the value returned by the
waitpid system call or equivalent function. Exit statuses fall
between 0 and 255, though, as explained below, the shell may use
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values above 125 specially. Exit statuses from shell builtins and
compound commands are also limited to this range. Under certain
circumstances, the shell will use special values to indicate specific
failure modes.
For the shell's purposes, a command which exits with a zero exit
status has succeeded. An exit status of zero indicates success. A
non-zero exit status indicates failure. When a command terminates on
a fatal signal N, bash uses the value of 128+N as the exit status.
If a command is not found, the child process created to execute it
returns a status of 127. If a command is found but is not executable,
the return status is 126.
If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if successful, and
non-zero (false) if an error occurs while they execute. All builtins
return an exit status of 2 to indicate incorrect usage, generally
invalid options or missing arguments.
Bash itself returns the exit status of the last command executed,
unless a syntax error occurs, in which case it exits with a non-zero
value. See also the exit builtin command below.
SIGNALS
When bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that kill 0 does not kill an interactive shell), and
SIGINT is caught and handled (so that the wait builtin is
interruptible). In all cases, bash ignores SIGQUIT. If job control
is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Non-builtin commands run by bash have signal handlers set to the
values inherited by the shell from its parent. When job control is
not in effect, asynchronous commands ignore SIGINT and SIGQUIT in
addition to these inherited handlers. Commands run as a result of
command substitution ignore the keyboard-generated job control signals
SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before exiting,
an interactive shell resends the SIGHUP to all jobs, running or
stopped. Stopped jobs are sent SIGCONT to ensure that they receive
the SIGHUP. To prevent the shell from sending the signal to a
particular job, it should be removed from the jobs table with the
disown builtin (see SHELL BUILTIN COMMANDS below) or marked to not
receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt, bash sends a
SIGHUP to all jobs when an interactive login shell exits.
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If bash is waiting for a command to complete and receives a signal for
which a trap has been set, the trap will not be executed until the
command completes. When bash is waiting for an asynchronous command
via the wait builtin, the reception of a signal for which a trap has
been set will cause the wait builtin to return immediately with an
exit status greater than 128, immediately after which the trap is
executed.
JOB CONTROL
Job control refers to the ability to selectively stop (suspend) the
execution of processes and continue (resume) their execution at a
later point. A user typically employs this facility via an
interactive interface supplied jointly by the operating system
kernel's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a table of
currently executing jobs, which may be listed with the jobs command.
When bash starts a job asynchronously (in the background), it prints a
line that looks like:
[1] 25647
indicating that this job is job number 1 and that the process ID of
the last process in the pipeline associated with this job is 25647.
All of the processes in a single pipeline are members of the same job.
Bash uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job control,
the operating system maintains the notion of a current terminal
process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group ID)
receive keyboard-generated signals such as SIGINT. These processes
are said to be in the foreground. Background processes are those
whose process group ID differs from the terminal's; such processes are
immune to keyboard-generated signals. Only foreground processes are
allowed to read from or, if the user so specifies with stty tostop,
write to the terminal. Background processes which attempt to read
from (write to when stty tostop is in effect) the terminal are sent a
SIGTTIN (SIGTTOU) signal by the kernel's terminal driver, which,
unless caught, suspends the process.
If the operating system on which bash is running supports job control,
bash contains facilities to use it. Typing the suspend character
(typically ^Z, Control-Z) while a process is running causes that
process to be stopped and returns control to bash. Typing the delayed
suspend character (typically ^Y, Control-Y) causes the process to be
stopped when it attempts to read input from the terminal, and control
to be returned to bash. The user may then manipulate the state of
this job, using the bg command to continue it in the background, the
fg command to continue it in the foreground, or the kill command to
kill it. A ^Z takes effect immediately, and has the additional side
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effect of causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The
character % introduces a job specification (jobspec). Job number n
may be referred to as %n. A job may also be referred to using a
prefix of the name used to start it, or using a substring that appears
in its command line. For example, %ce refers to a stopped ce job. If
a prefix matches more than one job, bash reports an error. Using
%?ce, on the other hand, refers to any job containing the string ce in
its command line. If the substring matches more than one job, bash
reports an error. The symbols %% and %+ refer to the shell's notion
of the current job, which is the last job stopped while it was in the
foreground or started in the background. The previous job may be
referenced using %-. If there is only a single job, %+ and %- can
both be used to refer to that job. In output pertaining to jobs
(e.g., the output of the jobs command), the current job is always
flagged with a +, and the previous job with a -. A single % (with no
accompanying job specification) also refers to the current job.
Simply naming a job can be used to bring it into the foreground: %1 is
a synonym for ``fg %1'', bringing job 1 from the background into the
foreground. Similarly, ``%1 &'' resumes job 1 in the background,
equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state. Normally,
bash waits until it is about to print a prompt before reporting
changes in a job's status so as to not interrupt any other output. If
the -b option to the set builtin command is enabled, bash reports such
changes immediately. Any trap on SIGCHLD is executed for each child
that exits.
If an attempt to exit bash is made while jobs are stopped (or, if the
checkjobs shell option has been enabled using the shopt builtin,
running), the shell prints a warning message, and, if the checkjobs
option is enabled, lists the jobs and their statuses. The jobs
command may then be used to inspect their status. If a second attempt
to exit is made without an intervening command, the shell does not
print another warning, and any stopped jobs are terminated.
When the shell is waiting for a job or process using the wait builtin,
and job control is enabled, wait will return when the job changes
state. The -f option will force wait to wait until the job or process
terminates before returning.
PROMPTING
When executing interactively, bash displays the primary prompt PS1
when it is ready to read a command, and the secondary prompt PS2 when
it needs more input to complete a command. Bash displays PS0 after it
reads a command but before executing it. Bash displays PS4 as
described above before tracing each command when the -x option is
enabled. Bash allows these prompt strings to be customized by
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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 (uses the value of the PROMPT_DIRTRIM variable)
\W the basename of the current working directory, with $HOME
abbreviated with a tilde
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a $
\nnn the character corresponding to the octal number nnn
\\ a backslash
\[ begin a sequence of non-printing characters, which could be
used to embed a terminal control sequence into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually different: the
history number of a command is its position in the history list, which
may include commands restored from the history file (see HISTORY
below), while the command number is the position in the sequence of
commands executed during the current shell session. After the string
is decoded, it is expanded via parameter expansion, command
substitution, arithmetic expansion, and quote removal, subject to the
value of the promptvars shell option (see the description of the shopt
command under SHELL BUILTIN COMMANDS below).
READLINE
This is the library that handles reading input when using an
interactive shell, unless the --noediting option is given at shell
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invocation. Line editing is also used when using the -e option to the
read builtin. By default, the line editing commands are similar to
those of Emacs. A vi-style line editing interface is also available.
Line editing can be enabled at any time using the -o emacs or -o vi
options to the set builtin (see SHELL BUILTIN COMMANDS below). To
turn off line editing after the shell is running, use the +o emacs or
+o vi options to the set builtin.
Readline Notation
In this section, the Emacs-style notation is used to denote
keystrokes. Control keys are denoted by C-key, e.g., C-n means
Control-N. Similarly, meta keys are denoted by M-key, so M-x means
Meta-X. (On keyboards without a meta key, M-x means ESC x, i.e.,
press the Escape key then the x key. This makes ESC the meta prefix.
The combination M-C-x means ESC-Control-x, or press the Escape key
then hold the Control key while pressing the x key.)
Readline commands may be given numeric arguments, which normally act
as a repeat count. Sometimes, however, it is the sign of the argument
that is significant. Passing a negative argument to a command that
acts in the forward direction (e.g., kill-line) causes that command to
act in a backward direction. Commands whose behavior with arguments
deviates from this are noted below.
When a command is described as killing text, the text deleted is saved
for possible future retrieval (yanking). The killed text is saved in
a kill ring. Consecutive kills cause the text to be accumulated into
one unit, which can be yanked all at once. Commands which do not kill
text separate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initialization file
(the inputrc file). The name of this file is taken from the value of
the INPUTRC variable. If that variable is unset, the default is
~/.inputrc. When a program which uses the readline library starts up,
the initialization file is read, and the key bindings and variables
are set. There are only a few basic constructs allowed in the
readline initialization file. Blank lines are ignored. Lines
beginning with a # are comments. Lines beginning with a $ indicate
conditional constructs. Other lines denote key bindings and variable
settings.
The default key-bindings may be changed with an inputrc file. Other
programs that use this library may add their own commands and
bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
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into the inputrc would make M-C-u execute the readline command
universal-argument.
The following symbolic character names are recognized: RUBOUT, DEL,
ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.
In addition to command names, readline allows keys to be bound to a
string that is inserted when the key is pressed (a macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc file is simple.
All that is required is the name of the command or the text of a macro
and a key sequence to which it should be bound. The name may be
specified in one of two ways: as a symbolic key name, possibly with
Meta- or Control- prefixes, or as a key sequence.
When using the form keyname:function-name or macro, keyname is the
name of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function universal-argument,
M-DEL is bound to the function backward-kill-word, and C-o is bound to
run the macro expressed on the right hand side (that is, to insert the
text ``> output'' into the line).
In the second form, "keyseq":function-name or macro, keyseq differs
from keyname above in that strings denoting an entire key sequence may
be specified by placing the sequence within double quotes. Some GNU
Emacs style key escapes can be used, as in the following example, but
the symbolic character names are not recognized.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function
universal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text
``Function Key 1''.
The full set of GNU Emacs style escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\ literal
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In addition to the GNU Emacs style escape sequences, a second set of
backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the eight-bit character whose value is the octal value nnn
(one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
When entering the text of a macro, single or double quotes must be
used to indicate a macro definition. Unquoted text is assumed to be a
function name. In the macro body, the backslash escapes described
above are expanded. Backslash will quote any other character in the
macro text, including " and .
Bash allows the current readline key bindings to be displayed or
modified with the bind builtin command. The editing mode may be
switched during interactive use by using the -o option to the set
builtin command (see SHELL BUILTIN COMMANDS below).
Readline Variables
Readline has variables that can be used to further customize its
behavior. A variable may be set in the inputrc file with a statement
of the form
set variable-name value
Except where noted, readline variables can take the values On or Off
(without regard to case). Unrecognized variable names are ignored.
When a variable value is read, empty or null values, "on" (case-
insensitive), and "1" are equivalent to On. All other values are
equivalent to Off. The variables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring the terminal
bell. If set to none, readline never rings the bell. If set to
visible, readline uses a visible bell if one is available. If
set to audible, readline attempts to ring the terminal's bell.
bind-tty-special-chars (On)
If set to On, readline attempts to bind the control characters
treated specially by the kernel's terminal driver to their
readline equivalents.
blink-matching-paren (Off)
If set to On, readline attempts to briefly move the cursor to an
opening parenthesis when a closing parenthesis is inserted.
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colored-completion-prefix (Off)
If set to On, when listing completions, readline displays the
common prefix of the set of possible completions using a
different color. The color definitions are taken from the value
of the LS_COLORS environment variable.
colored-stats (Off)
If set to On, readline displays possible completions using
different colors to indicate their file type. The color
definitions are taken from the value of the LS_COLORS environment
variable.
comment-begin (``#'')
The string that is inserted when the readline insert-comment
command is executed. This command is bound to M-# in emacs mode
and to # in vi command mode.
completion-display-width (-1)
The number of screen columns used to display possible matches
when performing completion. The value is ignored if it is less
than 0 or greater than the terminal screen width. A value of 0
will cause matches to be displayed one per line. The default
value is -1.
completion-ignore-case (Off)
If set to On, readline performs filename matching and completion
in a case-insensitive fashion.
completion-map-case (Off)
If set to On, and completion-ignore-case is enabled, readline
treats hyphens (-) and underscores (_) as equivalent when
performing case-insensitive filename matching and completion.
completion-prefix-display-length (0)
The length in characters of the common prefix of a list of
possible completions that is displayed without modification.
When set to a value greater than zero, common prefixes longer
than this value are replaced with an ellipsis when displaying
possible completions.
completion-query-items (100)
This determines when the user is queried about viewing the number
of possible completions generated by the possible-completions
command. It may be set to any integer value greater than or
equal to zero. If the number of possible completions is greater
than or equal to the value of this variable, the user is asked
whether or not he wishes to view them; otherwise they are simply
listed on the terminal.
convert-meta (On)
If set to On, readline will convert characters with the eighth
bit set to an ASCII key sequence by stripping the eighth bit and
prefixing an escape character (in effect, using escape as the
meta prefix). The default is On, but readline will set it to Off
if the locale contains eight-bit characters.
disable-completion (Off)
If set to On, readline will inhibit word completion. Completion
characters will be inserted into the line as if they had been
mapped to self-insert.
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echo-control-characters (On)
When set to On, on operating systems that indicate they support
it, readline echoes a character corresponding to a signal
generated from the keyboard.
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.
emacs-mode-string (@)
If the show-mode-in-prompt variable is enabled, this string is
displayed immediately before the last line of the primary prompt
when emacs editing mode is active. The value is expanded like a
key binding, so the standard set of meta- and control prefixes
and backslash escape sequences is available. Use the \1 and \2
escapes to begin and end sequences of non-printing characters,
which can be used to embed a terminal control sequence into the
mode string.
enable-bracketed-paste (Off)
When set to On, readline will configure the terminal in a way
that will enable it to insert each paste into the editing buffer
as a single string of characters, instead of treating each
character as if it had been read from the keyboard. This can
prevent pasted characters from being interpreted as editing
commands.
enable-keypad (Off)
When set to On, readline will try to enable the application
keypad when it is called. Some systems need this to enable the
arrow keys.
enable-meta-key (On)
When set to On, readline will try to enable any meta modifier key
the terminal claims to support when it is called. On many
terminals, the meta key is used to send eight-bit characters.
expand-tilde (Off)
If set to On, tilde expansion is performed when readline attempts
word completion.
history-preserve-point (Off)
If set to On, the history code attempts to place point at the
same location on each history line retrieved with previous-
history or next-history.
history-size (unset)
Set the maximum number of history entries saved in the history
list. If set to zero, any existing history entries are deleted
and no new entries are saved. If set to a value less than zero,
the number of history entries is not limited. By default, the
number of history entries is set to the value of the HISTSIZE
shell variable. If an attempt is made to set history-size to a
non-numeric value, the maximum number of history entries will be
set to 500.
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
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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 eighth 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. The default is Off,
but readline will set it to On if the locale contains eight-bit
characters.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an incremental
search without subsequently executing the character as a command.
If this variable has not been given a value, the characters ESC
and C-J will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid keymap names
is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command,
and vi-insert. vi is equivalent to vi-command; emacs is
equivalent to emacs-standard. The default value is emacs; the
value of editing-mode also affects the default keymap.
keyseq-timeout (500)
Specifies the duration readline will wait for a character when
reading an ambiguous key sequence (one that can form a complete
key sequence using the input read so far, or can take additional
input to complete a longer key sequence). If no input is
received within the timeout, readline will use the shorter but
complete key sequence. The value is specified in milliseconds,
so a value of 1000 means that readline will wait one second for
additional input. If this variable is set to a value less than
or equal to zero, or to a non-numeric value, readline will wait
until another key is pressed to decide which key sequence to
complete.
mark-directories (On)
If set to On, completed directory names have a slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified are displayed
with a preceding asterisk (*).
mark-symlinked-directories (Off)
If set to On, completed names which are symbolic links to
directories have a slash appended (subject to the value of
mark-directories).
match-hidden-files (On)
This variable, when set to On, causes readline to match files
whose names begin with a `.' (hidden files) when performing
filename completion. If set to Off, the leading `.' must be
supplied by the user in the filename to be completed.
menu-complete-display-prefix (Off)
If set to On, menu completion displays the common prefix of the
list of possible completions (which may be empty) before cycling
through the list.
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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.
The default is Off, but readline will set it to On if the locale
contains eight-bit characters.
page-completions (On)
If set to On, readline uses an internal more-like pager to
display a screenful of possible completions at a time.
print-completions-horizontally (Off)
If set to On, readline will display completions with matches
sorted horizontally in alphabetical order, rather than down the
screen.
revert-all-at-newline (Off)
If set to On, readline will undo all changes to history lines
before returning when accept-line is executed. By default,
history lines may be modified and retain individual undo lists
across calls to readline.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion functions. If
set to On, words which have more than one possible completion
cause the matches to be listed immediately instead of ringing the
bell.
show-all-if-unmodified (Off)
This alters the default behavior of the completion functions in a
fashion similar to show-all-if-ambiguous. If set to On, words
which have more than one possible completion without any possible
partial completion (the possible completions don't share a common
prefix) cause the matches to be listed immediately instead of
ringing the bell.
show-mode-in-prompt (Off)
If set to On, add a string to the beginning of the prompt
indicating the editing mode: emacs, vi command, or vi insertion.
The mode strings are user-settable (e.g., emacs-mode-string).
skip-completed-text (Off)
If set to On, this alters the default completion behavior when
inserting a single match into the line. It's only active when
performing completion in the middle of a word. If enabled,
readline does not insert characters from the completion that
match characters after point in the word being completed, so
portions of the word following the cursor are not duplicated.
vi-cmd-mode-string ((cmd))
If the show-mode-in-prompt variable is enabled, this string is
displayed immediately before the last line of the primary prompt
when vi editing mode is active and in command mode. The value is
expanded like a key binding, so the standard set of meta- and
control prefixes and backslash escape sequences is available.
Use the \1 and \2 escapes to begin and end sequences of non-
printing characters, which can be used to embed a terminal
control sequence into the mode string.
vi-ins-mode-string ((ins))
If the show-mode-in-prompt variable is enabled, this string is
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displayed immediately before the last line of the primary prompt
when vi editing mode is active and in insertion mode. The value
is expanded like a key binding, so the standard set of meta- and
control prefixes and backslash escape sequences is available.
Use the \1 and \2 escapes to begin and end sequences of non-
printing characters, which can be used to embed a terminal
control sequence into the mode string.
visible-stats (Off)
If set to On, a character denoting a file's type as reported by
stat(2) is appended to the filename when listing possible
completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the conditional
compilation features of the C preprocessor which allows key bindings
and variable settings to be performed as the result of tests. There
are four parser directives used.
$if The $if construct allows bindings to be made based on the editing
mode, the terminal being used, or the application using readline.
The text of the test, after any comparison operator,
extends to the end of the line; unless otherwise noted, no
characters are required to isolate it.
mode The mode= form of the $if directive is used to test whether
readline is in emacs or vi mode. This may be used in
conjunction with the set keymap command, for instance, to
set bindings in the emacs-standard and emacs-ctlx keymaps
only if readline is starting out in emacs mode.
term The term= form may be used to include terminal-specific key
bindings, perhaps to bind the key sequences output by the
terminal's function keys. The word on the right side of the
= is tested against both the full name of the terminal and
the portion of the terminal name before the first -. This
allows sun to match both sun and sun-cmd, for instance.
version
The version test may be used to perform comparisons against
specific readline versions. The version expands to the
current readline version. The set of comparison operators
includes =, (and ==), !=, <=, >=, <, and >. The version
number supplied on the right side of the operator consists
of a major version number, an optional decimal point, and an
optional minor version (e.g., 7.1). If the minor version is
omitted, it is assumed to be 0. The operator may be
separated from the string version and from the version
number argument by whitespace.
application
The application construct is used to include application-
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specific settings. Each program using the readline library
sets the application name, and an initialization file can
test for a particular value. This could be used to bind key
sequences to functions useful for a specific program. For
instance, the following command adds a key sequence that
quotes the current or previous word in bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
variable
The variable construct provides simple equality tests for
readline variables and values. The permitted comparison
operators are =, ==, and !=. The variable name must be
separated from the comparison operator by whitespace; the
operator may be separated from the value on the right hand
side by whitespace. Both string and boolean variables may
be tested. Boolean variables must be tested against the
values on and off.
$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 following
directive would read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the command history
(see HISTORY below) for lines containing a specified string. There
are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the
search string. As each character of the search string is typed,
readline displays the next entry from the history matching the string
typed so far. An incremental search requires only as many characters
as needed to find the desired history entry. The characters present
in the value of the isearch-terminators variable are used to terminate
an incremental search. If that 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
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original line. When the search is terminated, the history entry
containing the search string becomes the current line.
To find other matching entries in the history list, type Control-S or
Control-R as appropriate. This will search backward or forward in the
history for the next entry matching the search string typed so far.
Any other key sequence bound to a readline command will terminate the
search and execute that command. For instance, a newline will
terminate the search and accept the line, thereby executing the
command from the history list.
Readline remembers the last incremental search string. If two
Control-Rs are typed 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).
shell-forward-word
Move forward to the end of the next word. Words are delimited by
non-quoted shell metacharacters.
shell-backward-word
Move back to the start of the current or previous word. Words
are delimited by non-quoted shell metacharacters.
previous-screen-line
Attempt to move point to the same physical screen column on the
previous physical screen line. This will not have the desired
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effect if the current Readline line does not take up more than
one physical line or if point is not greater than the length of
the prompt plus the screen width.
next-screen-line
Attempt to move point to the same physical screen column on the
next physical screen line. This will not have the desired effect
if the current Readline line does not take up more than one
physical line or if the length of the current Readline line is
not greater than the length of the prompt plus the screen width.
clear-screen (C-l)
Clear the screen leaving the current line at the top of the
screen. With an argument, refresh the current line without
clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is. If this line
is non-empty, add it to the history list according to the state
of the HISTCONTROL variable. If the line is a modified history
line, then restore the history line to its original state.
previous-history (C-p)
Fetch the previous command from the history list, moving back in
the list.
next-history (C-n)
Fetch the next command from the history list, moving forward in
the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the line currently
being entered.
reverse-search-history (C-r)
Search backward starting at the current line and moving `up'
through the history as necessary. This is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and moving `down'
through the history as necessary. This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the current line
using a non-incremental search for a string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-incremental search
for a string supplied by the user.
history-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. This is a
non-incremental search.
history-search-backward
Search backward through the history for the string of characters
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between the start of the current line and the point. This is a
non-incremental search.
history-substring-search-backward
Search backward through the history for the string of characters
between the start of the current line and the current cursor
position (the point). The search string may match anywhere in a
history line. This is a non-incremental search.
history-substring-search-forward
Search forward through the history for the string of characters
between the start of the current line and the point. The search
string may match anywhere in a history line. This is a non-
incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command (usually the
second word on the previous line) at point. With an argument n,
insert the nth word from the previous command (the words in the
previous command begin with word 0). A negative argument inserts
the nth word from the end of the previous command. Once the
argument n is computed, the argument is extracted as if the "!n"
history expansion had been specified.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command (the last word
of the previous history entry). With a numeric argument, behave
exactly like yank-nth-arg. Successive calls to yank-last-arg
move back through the history list, inserting the last word (or
the word specified by the argument to the first call) of each
line in turn. Any numeric argument supplied to these successive
calls determines the direction to move through the history. A
negative argument switches the direction through the history
(back or forward). The history expansion facilities are used to
extract the last word, as if the "!$" history expansion had been
specified.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs alias and
history expansion as well as all of the shell word expansions.
See HISTORY EXPANSION below for a description of history
expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See HISTORY
EXPANSION below for a description of history expansion.
magic-space
Perform history expansion on the current line and insert a space.
See HISTORY EXPANSION below for a description of history
expansion.
alias-expand-line
Perform alias expansion on the current line. See ALIASES above
for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
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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. A
numeric argument, if supplied, specifies the history entry to use
instead of the current line.
edit-and-execute-command (C-x C-e)
Invoke an editor on the current command line, and execute the
result as shell commands. Bash attempts to invoke $VISUAL,
$EDITOR, and emacs as the editor, in that order.
Commands for Changing Text
end-of-file (usually C-d)
The character indicating end-of-file as set, for example, by
``stty''. If this character is read when there are no characters
on the line, and point is at the beginning of the line, Readline
interprets it as the end of input and returns EOF.
delete-char (C-d)
Delete the character at point. If this function is bound to the
same character as the tty EOF character, as C-d commonly is, see
above for the effects.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given a numeric
argument, save the deleted text on the kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the cursor is at
the end of the line, in which case the character behind the
cursor is deleted.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim. This is how
to insert 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
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argument, capitalize the previous word, but do not move point.
overwrite-mode
Toggle overwrite mode. With an explicit positive numeric
argument, switches to overwrite mode. With an explicit non-
positive numeric argument, switches to insert mode. This command
affects only emacs mode; vi mode does overwrite differently.
Each call to readline() starts in insert mode. In overwrite
mode, characters bound to self-insert replace the text at point
rather than pushing the text to the right. Characters bound to
backward-delete-char replace the character before point with a
space. By default, this command is unbound.
Killing and Yanking
kill-line (C-k)
Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the line. The
killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter where point
is.
kill-word (M-d)
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind point. Word boundaries are the same as
those used by backward-word.
shell-kill-word
Kill from point to the end of the current word, or if between
words, to the end of the next word. Word boundaries are the same
as those used by shell-forward-word.
shell-backward-kill-word
Kill the word behind point. Word boundaries are the same as
those used by shell-backward-word.
unix-word-rubout (C-w)
Kill the word behind point, using white space as a word boundary.
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.
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copy-backward-word
Copy the word before point to the kill buffer. The word
boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer. The word
boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only works following
yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulating, or start a
new argument. M-- starts a negative argument.
universal-argument
This is another way to specify an argument. If this command is
followed by one or more digits, optionally with a leading minus
sign, those digits define the argument. If the command is
followed by digits, executing universal-argument again ends the
numeric argument, but is otherwise ignored. As a special case,
if this command is immediately followed by a character that is
neither a digit nor minus sign, the argument count for the next
command is multiplied by four. The argument count is initially
one, so executing this function the first time makes the argument
count four, a second time makes the argument count sixteen, and
so on.
Completing
complete (TAB)
Attempt to perform completion on the text before point. Bash
attempts completion treating the text as a variable (if the text
begins with $), username (if the text begins with ~), hostname
(if the text begins with @), or command (including aliases and
functions) in turn. If none of these produces a match, filename
completion is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that would have
been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be completed with a
single match from the list of possible completions. Repeated
execution of menu-complete steps through the list of possible
completions, inserting each match in turn. At the end of the
list of completions, the bell is rung (subject to the setting of
bell-style) and the original text is restored. An argument of n
moves n positions forward in the list of matches; a negative
argument may be used to move backward through the list. This
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command is intended to be bound to TAB, but is unbound by
default.
menu-complete-backward
Identical to menu-complete, but moves backward through the list
of possible completions, as if menu-complete had been given a
negative argument. This command is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the beginning or
end of the line (like delete-char). If at the end of the line,
behaves identically to possible-completions. This command is
unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point, treating
it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating it as a
username.
possible-username-completions (C-x ~)
List the possible completions of the text before point, treating
it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating it as a
shell variable.
possible-variable-completions (C-x $)
List the possible completions of the text before point, treating
it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treating it as a
hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before point, treating
it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treating it as a
command name. Command completion attempts to match the text
against aliases, reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before point, treating
it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, comparing the text
against lines from the history list for possible completion
matches.
dabbrev-expand
Attempt menu completion on the text before point, comparing the
text against lines from the history list for possible completion
matches.
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complete-into-braces (M-{)
Perform filename completion and insert the list of possible
completions enclosed within braces so the list is available to
the shell (see Brace Expansion above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current keyboard
macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current keyboard macro
and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making the
characters in the macro appear as if typed at the keyboard.
print-last-kbd-macro ()
Print the last keyboard macro defined in a format suitable for
the inputrc file.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and incorporate any
bindings or variable assignments found there.
abort (C-g)
Abort the current editing command and ring the terminal's bell
(subject to the setting of bell-style).
do-lowercase-version (M-A, M-B, M-x, ...)
If the metafied character x is uppercase, run the command that is
bound to the corresponding metafied lowercase character. The
behavior is undefined if x is already lowercase.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each line.
revert-line (M-r)
Undo all changes made to this line. This is like executing the
undo command enough times to return the line to its initial
state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the point. If a numeric argument is supplied,
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.
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character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence
of that character. A negative count searches for subsequent
occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as
those defined for keys like Home and End. Such sequences begin
with a Control Sequence Indicator (CSI), usually ESC-[. If this
sequence is bound to "\[", keys producing such sequences will
have no effect unless explicitly bound to a readline command,
instead of inserting stray characters into the editing buffer.
This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline
comment-begin variable is inserted at the beginning of the
current line. If a numeric argument is supplied, this command
acts as a toggle: if the characters at the beginning of the line
do not match the value of comment-begin, the value is inserted,
otherwise the characters in comment-begin are deleted from the
beginning of the line. In either case, the line is accepted as
if a newline had been typed. The default value of comment-begin
causes this command to make the current line a shell comment. If
a numeric argument causes the comment character to be removed,
the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname
expansion, with an asterisk implicitly appended. This pattern is
used to generate a list of matching filenames for possible
completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname
expansion, and the list of matching filenames is inserted,
replacing the word. If a numeric argument is supplied, an
asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by
glob-expand-word is displayed, and the line is redrawn. If a
numeric argument is supplied, an asterisk is appended before
pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline
output stream. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an inputrc
file.
dump-variables
Print all of the settable readline variables and their values to
the readline output stream. If a numeric argument is supplied,
the output is formatted in such a way that it can be made part of
an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the
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strings they output. If a numeric argument is supplied, the
output is formatted in such a way that it can be made part of an
inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
Programmable Completion
When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined using
the complete builtin (see SHELL BUILTIN COMMANDS below), the
programmable completion facilities are invoked.
First, the command name is identified. If the command word is the
empty string (completion attempted at the beginning of an empty line),
any compspec defined with the -E option to complete is used. If a
compspec has been defined for that command, the compspec is used to
generate the list of possible completions for the word. If the
command word is a full pathname, a compspec for the full pathname is
searched for first. If no compspec is found for the full pathname, an
attempt is made to find a compspec for the portion following the final
slash. If those searches do not result in a compspec, any compspec
defined with the -D option to complete is used as the default. If
there is no default compspec, bash attempts alias expansion on the
command word as a final resort, and attempts to find a compspec for
the command word from any successful expansion.
Once a compspec has been found, it is used to generate the list of
matching words. If a compspec is not found, the default bash
completion as described above under Completing is performed.
First, the actions specified by the compspec are used. Only matches
which are prefixed by the word being completed are returned. When the
-f or -d option is used for filename or directory name completion, the
shell variable FIGNORE is used to filter the matches.
Any completions specified by a pathname expansion pattern to the -G
option are generated next. The words generated by the pattern need
not match the word being completed. The GLOBIGNORE shell variable is
not used to filter the matches, but the FIGNORE variable is used.
Next, the string specified as the argument to the -W option is
considered. The string is first split using the characters in the IFS
special variable as delimiters. Shell quoting is honored. Each word
is then expanded using brace expansion, tilde expansion, parameter and
variable expansion, command substitution, and arithmetic expansion, as
described above under EXPANSION. The results are split using the
rules described above under Word Splitting. The results of the
expansion are prefix-matched against the word being completed, and the
matching words become the possible completions.
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After these matches have been generated, any shell function or command
specified with the -F and -C options is invoked. When the command or
function is invoked, the COMP_LINE, COMP_POINT, COMP_KEY, and
COMP_TYPE variables are assigned values as described above under Shell
Variables. If a shell function is being invoked, the COMP_WORDS and
COMP_CWORD variables are also set. When the function or command is
invoked, the first argument ($1) is the name of the command whose
arguments are being completed, the second argument ($2) is the word
being completed, and the third argument ($3) is the word preceding the
word being completed on the current command line. No filtering of the
generated completions against the word being completed is performed;
the function or command has complete freedom in generating the
matches.
Any function specified with -F is invoked first. The function may use
any of the shell facilities, including the compgen builtin described
below, to generate the matches. It must put the possible completions
in the COMPREPLY array variable, one per array element.
Next, any command specified with the -C option is invoked in an
environment equivalent to command substitution. It should print a
list of completions, one per line, to the standard output. Backslash
may be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter
specified with the -X option is applied to the list. The filter is a
pattern as used for pathname expansion; a & in the pattern is replaced
with the text of the word being completed. A literal & may be escaped
with a backslash; the backslash is removed before attempting a match.
Any 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. If the nocasematch shell option
is enabled, the match is performed without regard to the case of
alphabetic characters.
Finally, any prefix and suffix specified with the -P and -S options
are added to each member of the completion list, and the result is
returned to the readline completion code as the list of possible
completions.
If the previously-applied actions do not generate any matches, and the
-o dirnames option was supplied to complete when the compspec was
defined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when the compspec
was defined, directory name completion is attempted and any matches
are added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned
to the completion code as the full set of possible completions. The
default bash completions are not attempted, and the readline default
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of filename completion is disabled. If the -o bashdefault option was
supplied to complete when the compspec was defined, the bash default
completions are attempted if the compspec generates no matches. If
the -o default option was supplied to complete when the compspec was
defined, readline's default completion will be performed if the
compspec (and, if attempted, the default bash completions) generate no
matches.
When a compspec indicates that directory name completion is desired,
the programmable completion functions force readline to append a slash
to completed names which are symbolic links to directories, subject to
the value of the mark-directories readline variable, regardless of the
setting of the mark-symlinked-directories readline variable.
There is some support for dynamically modifying completions. This is
most useful when used in combination with a default completion
specified with complete -D. It's possible for shell functions
executed as completion handlers to indicate that completion should be
retried by returning an exit status of 124. If a shell function
returns 124, and changes the compspec associated with the command on
which completion is being attempted (supplied as the first argument
when the function is executed), programmable completion restarts from
the beginning, with an attempt to find a new compspec for that
command. This allows a set of completions to be built dynamically as
completion is attempted, rather than being loaded all at once.
For instance, assuming that there is a library of compspecs, each kept
in a file corresponding to the name of the command, the following
default completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default
HISTORY
When the -o history option to the set builtin is enabled, the shell
provides access to the command history, the list of commands
previously typed. The value of the HISTSIZE variable is used as the
number of commands to save in a history list. The text of the last
HISTSIZE 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
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the number of lines specified by the value of HISTFILESIZE. If
HISTFILESIZE is unset, or set to null, a non-numeric value, or a
numeric value less than zero, the history file is not truncated. When
the history file is read, lines beginning with the history comment
character followed immediately by a digit are interpreted as
timestamps for the following history line. These timestamps are
optionally displayed depending on the value of the HISTTIMEFORMAT
variable. When a shell with history enabled exits, the last $HISTSIZE
lines are copied from the history list to $HISTFILE. If the
histappend shell option is enabled (see the description of shopt under
SHELL BUILTIN COMMANDS below), the lines are appended to the history
file, otherwise the history file is overwritten. If HISTFILE is
unset, or if the history file is unwritable, the history is not saved.
If the HISTTIMEFORMAT variable is set, time stamps are written to the
history file, marked with the history comment character, so they may
be preserved across shell sessions. This uses the history comment
character to distinguish timestamps from other history lines. After
saving the history, the history file is truncated to contain no more
than HISTFILESIZE lines. If HISTFILESIZE is unset, or set to null, a
non-numeric value, or a numeric value less than zero, the history file
is not truncated.
The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used
to list or edit and re-execute a portion of the history list. The
history builtin may be used to display or modify the history list and
manipulate the history file. When using command-line editing, search
commands are available in each editing mode that provide access to the
history list.
The shell allows control over which commands are saved on the history
list. The HISTCONTROL and HISTIGNORE variables may be set to cause
the shell to save only a subset of the commands entered. The cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness. The
lithist shell option causes the shell to save the command with
embedded newlines instead of semicolons. See the description of the
shopt builtin below under SHELL BUILTIN COMMANDS for information on
setting and unsetting shell options.
HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the
history expansion in csh. This section describes what syntax features
are available. This feature is enabled by default for interactive
shells, and can be disabled using the +H option to the set builtin
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
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previous commands quickly.
History expansion is performed immediately after a complete line is
read, before the shell breaks it into words, and is performed on each
line individually without taking quoting on previous lines into
account. It takes place in two parts. The first is to determine
which line from the history list to use during substitution. The
second is to select portions of that line for inclusion into the
current one. The line selected from the history is the event, and the
portions of that line that are acted upon are words. Various
modifiers are available to manipulate the selected words. The line is
broken into words in the same fashion as when reading input, so that
several metacharacter-separated words surrounded by quotes are
considered one word. History expansions are introduced by the
appearance of the history expansion character, which is ! by default.
Only backslash (\) and single quotes can quote the history expansion
character, but the history expansion character is also treated as
quoted if it immediately precedes the closing double quote in a
double-quoted string.
Several characters inhibit history expansion if found immediately
following the history expansion character, even if it is unquoted:
space, tab, newline, carriage return, and =. If the extglob shell
option is enabled, ( will also inhibit expansion.
Several shell options settable with the shopt builtin may be used to
tailor the behavior of history expansion. If the histverify shell
option is enabled (see the description of the shopt builtin below),
and readline is being used, history substitutions are not immediately
passed to the shell parser. Instead, the expanded line is reloaded
into the readline editing buffer for further modification. If
readline is being used, and the histreedit shell option is enabled, a
failed history substitution will be reloaded into the readline editing
buffer for correction. The -p option to the history builtin command
may be used to see what a history expansion will do before using it.
The -s option to the history builtin may be used to add commands to
the end of the history list without actually executing them, so that
they are available for subsequent recall.
The shell allows control of the various characters used by the history
expansion mechanism (see the description of histchars above under
Shell Variables). The shell uses the history comment character to
mark history timestamps when writing the history file.
Event Designators
An event designator is a reference to a command line entry in the
history list. Unless the reference is absolute, events are relative
to the current position in the history list.
! Start a history substitution, except when followed by a blank,
newline, carriage return, = or ( (when the extglob shell option
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is enabled using the shopt builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command preceding the current position
in the history list starting with string.
!?string[?]
Refer to the most recent command preceding the current position
in the history list containing string. The trailing ? may be
omitted if string is followed immediately by a newline.
^string1^string2^
Quick substitution. Repeat the previous command, replacing
string1 with string2. Equivalent to ``!!:s/string1/string2/''
(see Modifiers below).
!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the event. A :
separates the event specification from the word designator. It may be
omitted if the word designator begins with a ^, $, *, -, or %. Words
are numbered from the beginning of the line, with the first word being
denoted by 0 (zero). Words are inserted into the current line
separated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last word. This is usually the last argument, but will
expand to the zeroth word if there is only one word in the line.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for `1-$'.
It is not an error to use * if there is just one word in the
event; the empty string is returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specification, the
previous command is used as the event.
Modifiers
After the optional word designator, there may appear a sequence of one
or more of the following modifiers, each preceded by a `:'.
h Remove a trailing filename component, leaving only the head.
t Remove all leading filename components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
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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 single backslash
will quote the &. If old is null, it is set to the last old
substituted, or, if no previous history substitutions took place,
the last string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is
used in conjunction with `:s' (e.g., `:gs/old/new/') or `:&'. If
used with `:s', any delimiter can be used in place of /, and the
final delimiter is optional if it is the last character of the
event line. An a may be used as a synonym for g.
G Apply the following `s' modifier once to each word in the event
line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this
section as accepting options preceded by - accepts -- to signify the
end of the options. The :, true, false, and test/[ builtins do not
accept options and do not treat -- specially. The exit, logout,
return, break, continue, let, and shift builtins accept and process
arguments beginning with - without requiring --. Other builtins that
accept arguments but are not specified as accepting options interpret
arguments beginning with - as invalid options and require -- to
prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments
and performing any specified redirections. The return status is
zero.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell
environment and return the exit status of the last command
executed from filename. If filename does not contain a slash,
filenames in PATH are used to find the directory containing
filename. The file searched for in PATH need not be executable.
When bash is not in posix mode, the current directory is searched
if no file is found in PATH. If the sourcepath option to the
shopt builtin command is turned off, the PATH is not searched.
If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the positional
parameters are unchanged. If the -T option is enabled, source
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inherits any trap on DEBUG; if it is not, any DEBUG trap string
is saved and restored around the call to source, and source
unsets the DEBUG trap while it executes. If -T is not set, and
the sourced file changes the DEBUG trap, the new value is
retained when source completes. The return status is the status
of the last command exited within the script (0 if no commands
are executed), and false if filename is not found or cannot be
read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of
aliases in the form alias name=value on standard output. When
arguments are supplied, an alias is defined for each name whose
value is given. A trailing space in value causes the next word
to be checked for alias substitution when the alias is expanded.
For each name in the argument list for which no value is
supplied, the name and value of the alias is printed. Alias
returns true unless a name is given for which no alias has been
defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had
been started with &. If jobspec is not present, the shell's
notion of the current job is used. bg jobspec returns 0 unless
run when job control is disabled or, when run with job control
enabled, any specified jobspec was not found or was started
without job control.
bind [-m keymap] [-lpsvPSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] keyseq:readline-command
Display current readline key and function bindings, bind a key
sequence to a readline function or macro, or set a readline
variable. Each non-option argument is a command as it would
appear in .inputrc, but each binding or command must be passed as
a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent
bindings. Acceptable keymap names are emacs,
emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move,
vi-command, and vi-insert. vi is equivalent to vi-command
(vi-move is also a synonym); emacs is equivalent to
emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way
that they can be re-read.
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-P List current readline function names and bindings.
-s Display readline key sequences bound to macros and the
strings they output in such a way that they can be re-read.
-S Display readline key sequences bound to macros and the
strings they output.
-v Display readline variable names and values in such a way
that they can be re-read.
-V List current readline variable names and values.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
-x keyseq:shell-command
Cause shell-command to be executed whenever keyseq is
entered. When shell-command is executed, the shell sets the
READLINE_LINE variable to the contents of the readline line
buffer and the READLINE_POINT variable to the current
location of the insertion point. If the executed command
changes the value of READLINE_LINE or READLINE_POINT, those
new values will be reflected in the editing state.
-X List all key sequences bound to shell commands and the
associated commands in a format that can be reused as input.
The return value is 0 unless an unrecognized option is given or
an error occurred.
break [n]
Exit from within a for, while, until, or select loop. If n is
specified, break n levels. n must be _ 1. If n is greater than
the number of enclosing loops, all enclosing loops are exited.
The return value is 0 unless n is not greater than or equal to 1.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it arguments, and
return its exit status. This is useful when defining a function
whose name is the same as a shell builtin, retaining the
functionality of the builtin within the function. The cd builtin
is commonly redefined this way. The return status is false if
shell-builtin is not a shell builtin command.
caller [expr]
Returns the context of any active subroutine call (a shell
function or a script executed with the . or source builtins).
Without expr, caller displays the line number and source filename
of the current subroutine call. If a non-negative integer is
supplied as expr, caller displays the line number, subroutine
name, and source file corresponding to that position in the
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current execution call stack. This extra information may be
used, for example, to print a stack trace. The current frame is
frame 0. The return value is 0 unless the shell is not executing
a subroutine call or expr does not correspond to a valid position
in the call stack.
cd [-L|[-P [-e]] [-@]] [dir]
Change the current directory to dir. if dir is not supplied, the
value of the HOME shell variable is the default. Any additional
arguments following dir are ignored. The variable CDPATH defines
the search path for the directory containing dir: each directory
name in CDPATH is searched for dir. Alternative directory names
in CDPATH are separated by a colon (:). A null directory name in
CDPATH is the same as the current directory, i.e., ``.''. If dir
begins with a slash (/), then CDPATH is not used. The -P option
causes cd to use the physical directory structure by resolving
symbolic links while traversing dir and before processing
instances of .. in dir (see also the -P option to the set builtin
command); the -L option forces symbolic links to be followed by
resolving the link after processing instances of .. in dir. If
.. appears in dir, it is processed by removing the immediately
previous pathname component from dir, back to a slash or the
beginning of dir. If the -e option is supplied with -P, and the
current working directory cannot be successfully determined after
a successful directory change, cd will return an unsuccessful
status. On systems that support it, the -@ option presents the
extended attributes associated with a file as a directory. An
argument of - is converted to $OLDPWD before the directory change
is attempted. If a non-empty directory name from CDPATH is used,
or if - is the first argument, and the directory change is
successful, the absolute pathname of the new working directory is
written to the standard output. The return value is true if the
directory was successfully changed; false otherwise.
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell function
lookup. Only builtin commands or commands found in the PATH are
executed. If the -p option is given, the search for command is
performed using a default value for PATH that is guaranteed to
find all of the standard utilities. If either the -V or -v
option is supplied, a description of command is printed. The -v
option causes a single word indicating the command or filename
used to invoke command to be displayed; the -V option produces a
more verbose description. If the -V or -v option is supplied,
the exit status is 0 if command was found, and 1 if not. If
neither option is supplied and an error occurred or command
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
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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
programmable completion code had generated them directly from a
completion specification with the same flags. If word is
specified, only those completions matching word will be
displayed.
The return value is true unless an invalid option is supplied, or
no matches were generated.
[-W wordlist] [-F function] [-C command]
complete [-abcdefgjksuv] [-o comp-
option] [-DEI] [-A action] [-G globpat]
[-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...]
Specify how arguments to each name should be completed. If the
-p option is supplied, or if no options 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 -D option indicates that other
supplied options and actions should apply to the ``default''
command completion; that is, completion attempted on a command
for which no completion has previously been defined. The -E
option indicates that other supplied options and actions should
apply to ``empty'' command completion; that is, completion
attempted on a blank line. The -I option indicates that other
supplied options and actions should apply to completion on the
inital non-assignment word on the line, or after a command
delimiter such as ; or |, which is usually command name
completion. If multiple options are supplied, the -D option
takes precedence over -E, and both take precedence over -I. If
any of -D, -E, or -I are supplied, any other name arguments are
ignored; these completions only apply to the case specified by
the option.
The process of applying these completion specifications when word
completion is attempted is described above under Programmable
Completion.
Other options, if specified, have the following meanings. The
arguments to the -G, -W, and -X options (and, if necessary, the
-P and -S options) should be quoted to protect them from
expansion before the complete builtin is invoked.
-o comp-option
The comp-option controls several aspects of the
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compspec's behavior beyond the simple generation of
completions. comp-option may be one of:
bashdefault
Perform the rest of the default bash completions
if the compspec generates no matches.
default Use readline's default filename completion if the
compspec generates no matches.
dirnames
Perform directory name completion if the compspec
generates no matches.
filenames
Tell readline that the compspec generates
filenames, so it can perform any
filename-specific processing (like adding a slash
to directory names, quoting special characters,
or suppressing trailing spaces). Intended to be
used with shell functions.
noquote Tell readline not to quote the completed words if
they are filenames (quoting filenames is the
default).
nosort Tell readline not to sort the list of possible
completions alphabetically.
nospace Tell readline not to append a space (the default)
to words completed at the end of the line.
plusdirs
After any matches defined by the compspec are
generated, directory name completion is attempted
and any matches are added to the results of the
other actions.
-A action
The action may be one of the following to generate a list
of possible completions:
alias Alias names. May also be specified as -a.
arrayvar
Array variable names.
binding Readline key binding names.
builtin Names of shell builtin commands. May also be
specified as -b.
command Command names. May also be specified as -c.
directory
Directory names. May also be specified as -d.
disabled
Names of disabled shell builtins.
enabled Names of enabled shell builtins.
export Names of exported shell variables. May also be
specified as -e.
file File names. May also be specified as -f.
function
Names of shell functions.
group Group names. May also be specified as -g.
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helptopic
Help topics as accepted by the help builtin.
hostname
Hostnames, as taken from the file specified by
the HOSTFILE shell variable.
job Job names, if job control is active. May also be
specified as -j.
keyword Shell reserved words. May also be specified as
-k.
running Names of running jobs, if job control is active.
service Service names. May also be specified as -s.
setopt Valid arguments for the -o option to the set
builtin.
shopt Shell option names as accepted by the shopt
builtin.
signal Signal names.
stopped Names of stopped jobs, if job control is active.
user User names. May also be specified as -u.
variable
Names of all shell variables. May also be
specified as -v.
-C command
command is executed in a subshell environment, and its
output is used as the possible completions.
-F function
The shell function function is executed in the current
shell environment. When the function is executed, the
first argument ($1) is the name of the command whose
arguments are being completed, the second argument ($2)
is the word being completed, and the third argument ($3)
is the word preceding the word being completed on the
current command line. When it finishes, the possible
completions are retrieved from the value of the COMPREPLY
array variable.
-G globpat
The pathname expansion pattern globpat is expanded to
generate the possible completions.
-P prefix
prefix is added at the beginning of each possible
completion after all other options have been applied.
-S suffix
suffix is appended to each possible completion after all
other options have been applied.
-W wordlist
The wordlist is split using the characters in the IFS
special variable as delimiters, and each resultant word
is expanded. Shell quoting is honored within wordlist,
in order to provide a mechanism for the words to contain
shell metacharacters or characters in the value of IFS.
The possible completions are the members of the resultant
list which match the word being completed.
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-X filterpat
filterpat is a pattern as used for pathname expansion.
It is applied to the list of possible completions
generated by the preceding options and arguments, and
each completion matching filterpat is removed from the
list. A leading ! in filterpat negates the pattern; in
this case, any completion not matching filterpat is
removed.
The return value is true unless an invalid option is supplied, an
option other than -p or -r is supplied without a name argument,
an attempt is made to remove a completion specification for a
name for which no specification exists, or an error occurs adding
a completion specification.
compopt [-o option] [-DEI] [+o option] [name]
Modify completion options for each name according to the options,
or for the currently-executing completion if no names are
supplied. If no options are given, display the completion
options for each name or the current completion. The possible
values of option are those valid for the complete builtin
described above. The -D option indicates that other supplied
options should apply to the ``default'' command completion; that
is, completion attempted on a command for which no completion has
previously been defined. The -E option indicates that other
supplied options should apply to ``empty'' command completion;
that is, completion attempted on a blank line. The -I option
indicates that other supplied options should apply to completion
on the inital non-assignment word on the line, or after a command
delimiter such as ; or |, which is usually command name
completion.
The return value is true unless an invalid option is supplied, an
attempt is made to modify the options for a name for which no
completion specification exists, or an output error occurs.
continue [n]
Resume the next iteration of the enclosing for, while, until, or
select loop. If n is specified, resume at the nth enclosing
loop. n must be _ 1. If n is greater than the number of
enclosing loops, the last enclosing loop (the ``top-level'' loop)
is resumed. The return value is 0 unless n is not greater than
or equal to 1.
declare [-aAfFgilnrtux] [-p] [name[=value] ...]
typeset [-aAfFgilnrtux] [-p] [name[=value] ...]
Declare variables and/or give them attributes. If no names are
given then display the values of variables. The -p option will
display the attributes and values of each name. When -p is used
with name arguments, additional options, other than -f and -F,
are ignored. When -p is supplied without name arguments, it will
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display the attributes and values of all variables having the
attributes specified by the additional options. If no other
options are supplied with -p, declare will display the attributes
and values of all shell variables. The -f option will restrict
the display to shell functions. The -F option inhibits the
display of function definitions; only the function name and
attributes are printed. If the extdebug shell option is enabled
using shopt, the source file name and line number where each name
is defined are displayed as well. The -F option implies -f. The
-g option forces variables to be created or modified at the
global scope, even when declare is executed in a shell function.
It is ignored in all other cases. The following options can be
used to restrict output to variables with the specified attribute
or to give variables attributes:
-a Each name is an indexed array variable (see Arrays above).
-A Each name is an associative array variable (see Arrays
above).
-f Use function names only.
-i The variable is treated as an integer; arithmetic evaluation
(see ARITHMETIC EVALUATION above) is performed when the
variable is assigned a value.
-l When the variable is assigned a value, all upper-case
characters are converted to lower-case. The upper-case
attribute is disabled.
-n Give each name the nameref attribute, making it a name
reference to another variable. That other variable is
defined by the value of name. All references, assignments,
and attribute modifications to name, except those using or
changing the -n attribute itself, are performed on the
variable referenced by name's value. The nameref attribute
cannot be applied to array variables.
-r Make names readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
-t Give each name the trace attribute. Traced functions
inherit the DEBUG and RETURN traps from the calling shell.
The trace attribute has no special meaning for variables.
-u When the variable is assigned a value, all lower-case
characters are converted to upper-case. The lower-case
attribute is disabled.
-x Mark names for export to subsequent commands via the
environment.
Using `+' instead of `-' turns off the attribute instead, with
the exceptions that +a and +A may not be used to destroy array
variables and +r will not remove the readonly attribute. When
used in a function, declare and typeset make each name local, as
with the local command, unless the -g option is supplied. If a
variable name is followed by =value, the value of the variable is
set to value. When using -a or -A and the compound assignment
syntax to create array variables, additional attributes do not
take effect until subsequent assignments. The return value is 0
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unless an invalid option is encountered, an attempt is made to
define a function using ``-f foo=bar'', an attempt is made to
assign a value to a readonly variable, an attempt is made to
assign a value to an array variable without using the compound
assignment syntax (see Arrays above), one of the names is not a
valid shell variable name, an attempt is made to turn off
readonly status for a readonly variable, an attempt is made to
turn off array status for an array variable, or an attempt is
made to display a non-existent function with -f.
dirs [-clpv] [+n] [-n]
Without options, 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. The current directory is always the first
directory in the stack.
-c Clears the directory stack by deleting all of the entries.
-l Produces a listing using full pathnames; the default listing
format uses a tilde to denote the home directory.
-p Print the directory stack with one entry per line.
-v Print the directory stack with one entry per line, prefixing
each entry with its index in the stack.
+n Displays the nth entry counting from the left of the list
shown by dirs when invoked without options, starting with
zero.
-n Displays the nth entry counting from the right of the list
shown by dirs when invoked without options, starting with
zero.
The return value is 0 unless an invalid option is supplied or n
indexes beyond the end of the directory stack.
disown [-ar] [-h] [jobspec ... | pid ... ]
Without options, remove each jobspec from the table of active
jobs. If jobspec is not present, and neither the -a nor the -r
option is supplied, the current job is used. If the -h option is
given, each jobspec is not removed from the table, but is marked
so that SIGHUP is not sent to the job if the shell receives a
SIGHUP. If no jobspec is supplied, the -a option means to remove
or mark all jobs; the -r option without a jobspec argument
restricts operation to running jobs. The return value is 0
unless a jobspec does not specify a valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a newline. The
return status is 0 unless a write error occurs. If -n is
specified, the trailing newline is suppressed. If the -e option
is given, interpretation of the following backslash-escaped
characters is enabled. The -E option disables the interpretation
of these escape characters, even on systems where they are
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interpreted by default. The xpg_echo shell option may be used to
dynamically determine whether or not echo expands these escape
characters by default. echo does not interpret -- to mean the
end of options. echo interprets the following escape sequences:
\a alert (bell)
\b backspace
\c suppress further output
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\0nnn
the eight-bit character whose value is the octal value nnn
(zero to three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
\uHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
enable [-a] [-dnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though
the shell normally searches for builtins before disk commands.
If -n is used, each name is disabled; otherwise, names are
enabled. For example, to use the test binary found via the PATH
instead of the shell builtin version, run ``enable -n test''.
The -f option means to load the new builtin command name from
shared object filename, on systems that support dynamic loading.
The -d option will delete a builtin previously loaded with -f.
If no name arguments are given, or if the -p option is supplied,
a list of shell builtins is printed. With no other option
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
command. This command is then read and executed by the shell,
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and its exit status is returned as the value of eval. If there
are no args, or only null arguments, eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No new process
is created. The arguments become the arguments to command. If
the -l option is supplied, the shell places a dash at the
beginning of the zeroth argument passed to command. This is what
login(1) does. The -c option causes command to be executed with
an empty environment. If -a is supplied, the shell passes name
as the zeroth argument to the executed command. If command
cannot be executed for some reason, a non-interactive shell
exits, unless the execfail shell option is enabled. In that
case, it returns failure. An interactive shell returns failure
if the file cannot be executed. A subshell exits unconditionally
if exec fails. If command is not specified, any redirections
take effect in the current shell, and the return status is 0. If
there is a redirection error, the return status is 1.
exit [n]
Cause the shell to exit with a status of n. If n is omitted, the
exit status is that of the last command executed. A trap on EXIT
is executed before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export to the
environment of subsequently executed commands. If the -f option
is given, the names refer to functions. If no names are given,
or if the -p option is supplied, a list of names of all exported
variables is printed. The -n option causes the export property
to be removed from each name. If a variable name is followed by
=word, the value of the variable is set to word. export returns
an exit status of 0 unless an invalid option is encountered, one
of the names is not a valid shell variable name, or -f is
supplied with a name that is not a function.
fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]
The first form selects a range of commands from first to last
from the history list and displays or edits and re-executes them.
First and last may be specified as a string (to locate the last
command beginning with that string) or as a number (an index into
the history list, where a negative number is used as an offset
from the current command number). If last is not specified, it
is set to the current command for listing (so that ``fc -l -10''
prints the last 10 commands) and to first 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
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-r option reverses the order of the commands. If the -l option
is given, the commands are listed on standard output. Otherwise,
the editor given by ename is invoked on a file containing those
commands. If ename is not given, the value of the FCEDIT
variable is used, and the value of EDITOR if FCEDIT is not set.
If neither variable is set, vi is used. When editing is
complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance of
pat is replaced by rep. Command is interpreted the same as first
above. A useful alias to use with this is ``r="fc -s"'', so that
typing ``r cc'' runs the last command beginning with ``cc'' and
typing ``r'' re-executes the last command.
If the first form is used, the return value is 0 unless an
invalid option is encountered or first or last specify history
lines out of range. If the -e option is supplied, the return
value is the value of the last command executed or failure if an
error occurs with the temporary file of commands. If the second
form is used, the return status is that of the command re-
executed, unless cmd does not specify a valid history line, in
which case fc returns failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the current job.
If jobspec is not present, the shell's notion of the current job
is used. The return value is that of the command placed into the
foreground, or failure if run when job control is disabled or,
when run with job control enabled, if jobspec does not specify a
valid job or jobspec specifies a job that was started without job
control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional
parameters. optstring contains the option characters to be
recognized; if a character is followed by a colon, the option is
expected to have an argument, which should be separated from it
by white space. The colon and question mark characters may not
be used as option characters. Each time it is invoked, getopts
places the next option in the shell variable name, initializing
name if it does not exist, and the index of the next argument to
be processed into the variable OPTIND. OPTIND is initialized to
1 each time the shell or a shell script is invoked. When an
option requires an argument, getopts places that argument into
the variable OPTARG. The shell does not reset OPTIND
automatically; it must be manually reset between multiple calls
to getopts within the same shell invocation 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
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the first non-option argument, and name is set to ?.
getopts normally parses the positional parameters, but if more
arguments are given in args, getopts parses those instead.
getopts can report errors in two ways. If the first character of
optstring is a colon, silent error reporting is used. In normal
operation, diagnostic messages are printed when invalid options
or missing option arguments are encountered. If the variable
OPTERR is set to 0, no error messages will be displayed, even if
the first character of optstring is not a colon.
If an invalid option is seen, getopts places ? into name and, if
not silent, prints an error message and unsets OPTARG. If
getopts is silent, the option character found is placed in OPTARG
and no diagnostic message is printed.
If a required argument is not found, and getopts is not silent, a
question mark (?) is placed in name, OPTARG is unset, and a
diagnostic message is printed. If getopts is silent, then a
colon (:) is placed in name and OPTARG is set to the option
character found.
getopts returns true if an option, specified or unspecified, is
found. It returns false if the end of options is encountered or
an error occurs.
hash [-lr] [-p filename] [-dt] [name]
Each time hash is invoked, the full pathname of the command name
is determined by searching the directories in $PATH and
remembered. Any previously-remembered pathname is discarded. If
the -p option is supplied, no path search is performed, and
filename is used as the full filename of the command. The -r
option causes the shell to forget all remembered locations. The
-d option causes the shell to forget the remembered location of
each name. If the -t option is supplied, the full pathname to
which each name corresponds is printed. If multiple name
arguments are supplied with -t, the name is printed before the
hashed full pathname. The -l option causes output to be
displayed in a format that may be reused as input. If no
arguments are given, or if only -l is supplied, information about
remembered commands is printed. The return status is true unless
a name is not found or an invalid option is supplied.
help [-dms] [pattern]
Display helpful information about builtin commands. If pattern
is specified, help gives detailed help on all commands matching
pattern; otherwise help for all the builtins and shell control
structures is printed.
-d Display a short description of each pattern
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-m Display the description of each pattern in a manpage-like
format
-s Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches pattern.
history [n]
history -c
history -d offset
history -d start-end
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list with line
numbers. Lines listed with a * have been modified. An argument
of n lists only the last n lines. If the shell variable
HISTTIMEFORMAT is set and not null, it is used as a format string
for strftime(3) to display the time stamp associated with each
displayed history entry. No intervening blank is printed between
the formatted time stamp and the history line. If filename is
supplied, it is used as the name of the history file; if not, the
value of HISTFILE is used. Options, if supplied, have the
following meanings:
-c Clear the history list by deleting all the entries.
-d offset
Delete the history entry at position offset. If offset is
negative, it is interpreted as relative to one greater than
the last history position, so negative indices count back
from the end of the history, and an index of -1 refers to
the current history -d command.
-d start-end
Delete the history entries between positions start and end,
inclusive. Positive and negative values for start and end
are interpreted as described above.
-a Append the ``new'' history lines to the history file. These
are history lines entered since the beginning of the current
bash session, but not already appended to the history file.
-n Read the history lines not already read from the history
file into the current history list. These are lines
appended to the history file since the beginning of the
current bash session.
-r Read the contents of the history file and append them to the
current history list.
-w Write the current history list to the history file,
overwriting the history file's contents.
-p Perform history substitution on the following args and
display the result on the standard output. Does not store
the results in the history list. Each arg must be quoted to
disable normal history expansion.
-s Store the args in the history list as a single entry. The
last command in the history list is removed before the args
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are added.
If the HISTTIMEFORMAT variable is set, the time stamp information
associated with each history entry is written to the history
file, marked with the history comment character. When the
history file is read, lines beginning with the history comment
character followed immediately by a digit are interpreted as
timestamps for the following history entry. The return value is
0 unless an invalid option is encountered, an error occurs while
reading or writing the history file, an invalid offset is
supplied as an argument to -d, or the history expansion supplied
as an argument to -p fails.
jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options have the
following meanings:
-l List process IDs in addition to the normal information.
-n Display information only about jobs that have changed status
since the user was last notified of their status.
-p List only the process ID of the job's process group leader.
-r Display only running jobs.
-s Display only stopped jobs.
If jobspec is given, output is restricted to information about
that job. The return status is 0 unless an invalid option is
encountered or an invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any jobspec found in
command or args with the corresponding process group ID, and
executes command passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...
kill -l|-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. The -L option is equivalent to -l. kill
returns true if at least one signal was successfully sent, or
false if an error occurs or an invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated (see
ARITHMETIC EVALUATION above). If the last arg evaluates to 0,
let returns 1; 0 is returned otherwise.
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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. If name is -, the set of shell options is made
local to the function in which local is invoked: shell options
changed using the set builtin inside the function are restored to
their original values when the function returns. 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.
callback] [-c quantum] [array]
mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]
readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
Read lines from the standard input into the indexed array
variable array, or from file descriptor fd if the -u option is
supplied. The variable MAPFILE is the default array. Options,
if supplied, have the following meanings:
-d The first character of delim is used to terminate each input
line, rather than newline. If delim is the empty string,
mapfile will terminate a line when it reads a NUL character.
-n Copy at most count lines. If count is 0, all lines are
copied.
-O Begin assigning to array at index origin. The default index
is 0.
-s Discard the first count lines read.
-t Remove a trailing delim (default newline) from each line
read.
-u Read lines from file descriptor fd instead of the standard
input.
-C Evaluate callback each time quantum lines are read. The -c
option specifies quantum.
-c Specify the number of lines read between each call to
callback.
If -C is specified without -c, the default quantum is 5000. When
callback is evaluated, it is supplied the index of the next array
element to be assigned and the line to be assigned to that
element as additional arguments. callback is evaluated after the
line is read but before the array element is assigned.
If not supplied with an explicit origin, mapfile will clear array
before assigning to it.
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mapfile returns successfully unless an invalid option or option
argument is supplied, array is invalid or unassignable, or if
array is not an indexed array.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no arguments,
removes the top directory from the stack, and performs a cd to
the new top directory. Arguments, if supplied, have the
following meanings:
-n Suppresses the normal change of directory when removing
directories from the stack, so that only the stack is
manipulated.
+n Removes the nth entry counting from the left of the list
shown by dirs, starting with zero. For example: ``popd +0''
removes the first directory, ``popd +1'' the second.
-n Removes the nth entry counting from the right of the list
shown by dirs, starting with zero. For example: ``popd -0''
removes the last directory, ``popd -1'' the next to last.
If the popd command is successful, a dirs is performed as well,
and the return status is 0. popd returns false if an invalid
option is encountered, the directory stack is empty, a non-
existent directory stack entry is specified, or the directory
change fails.
printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format. The -v option causes the output to be
assigned to the variable var rather than being printed to the
standard output.
The format is a character string which contains three types of
objects: plain characters, which are simply copied to standard
output, character escape sequences, which are converted and
copied to the standard output, and format specifications, each of
which causes printing of the next successive argument. In
addition to the standard printf(1) format specifications, printf
interprets the following extensions:
%b causes printf to expand backslash escape sequences in the
corresponding argument in the same way as echo -e.
%q causes printf to output the corresponding argument in a
format that can be reused as shell input.
%(datefmt)T
causes printf to output the date-time string resulting from
using datefmt as a format string for strftime(3). The
corresponding argument is an integer representing the number
of seconds since the epoch. Two special argument values may
be used: -1 represents the current time, and -2 represents
the time the shell was invoked. If no argument is
specified, conversion behaves as if -1 had been given. This
is an exception to the usual printf behavior.
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Arguments to non-string format specifiers are treated as C
constants, except that a leading plus or minus sign is allowed,
and if the leading character is a single or double quote, the
value is the ASCII value of the following character.
The format is reused as necessary to consume all of the
arguments. If the format requires more arguments than are
supplied, the extra format specifications behave as if a zero
value or null string, as appropriate, had been supplied. The
return value is zero on success, non-zero on failure.
pushd [-n] [+n] [-n]
pushd [-n] [dir]
Adds a directory to the top of the directory stack, or rotates
the stack, making the new top of the stack the current working
directory. With no arguments, pushd exchanges the top two
directories and returns 0, unless the directory stack is empty.
Arguments, if supplied, have the following meanings:
-n Suppresses the normal change of directory when rotating or
adding directories to the stack, so that only the stack is
manipulated.
+n Rotates the stack so that the nth directory (counting from
the left of the list shown by dirs, starting with zero) is
at the top.
-n Rotates the stack so that the nth directory (counting from
the right of the list shown by dirs, starting with zero) is
at the top.
dir Adds dir to the directory stack at the top, making it the
new current working directory as if it had been supplied as
the argument to the cd builtin.
If the pushd command is successful, a dirs is performed as well.
If the first form is used, pushd returns 0 unless the cd to dir
fails. With the second form, pushd returns 0 unless the
directory stack is empty, a non-existent directory stack element
is specified, or the directory change to the specified new
current directory fails.
pwd [-LP]
Print the absolute pathname of the current working directory.
The pathname printed contains no symbolic links if the -P option
is supplied or the -o physical option to the set builtin command
is enabled. If the -L option is used, the pathname printed may
contain symbolic links. The return status is 0 unless an error
occurs while reading the name of the current directory or an
invalid option is supplied.
prompt] [-t timeout] [-u fd] [name ...]
read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p
One line is read from the standard input, or from the file
descriptor fd supplied as an argument to the -u option, split
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into words as described above under Word Splitting, and the first
word is assigned to the first name, the second word to the second
name, and so on. If there are more words than names, the
remaining words and their intervening delimiters are assigned to
the last name. If there are fewer words read from the input
stream than names, the remaining names are assigned empty values.
The characters in IFS are used to split the line into words using
the same rules the shell uses for expansion (described above
under Word Splitting). The backslash character (\) may be used
to remove any special meaning for the next character read and for
line continuation. Options, if supplied, have the following
meanings:
-a aname
The words are assigned to sequential indices of the array
variable aname, starting at 0. aname is unset before any
new values are assigned. Other name arguments are ignored.
-d delim
The first character of delim is used to terminate the input
line, rather than newline. If delim is the empty string,
read will terminate a line when it reads a NUL character.
-e If the standard input is coming from a terminal, readline
(see READLINE above) is used to obtain the line. Readline
uses the current (or default, if line editing was not
previously active) editing settings, but uses Readline's
default filename completion.
-i text
If readline is being used to read the line, text is placed
into the editing buffer before editing begins.
-n nchars
read returns after reading nchars characters rather than
waiting for a complete line of input, but honors a delimiter
if fewer than nchars characters are read before the
delimiter.
-N nchars
read returns after reading exactly nchars characters rather
than waiting for a complete line of input, unless EOF is
encountered or read times out. Delimiter characters
encountered in the input are not treated specially and do
not cause read to return until nchars characters are read.
The result is not split on the characters in IFS; the intent
is that the variable is assigned exactly the characters read
(with the exception of backslash; see the -r option below).
-p prompt
Display prompt on standard error, without a trailing
newline, before attempting to read any input. The prompt is
displayed only if input is coming from a terminal.
-r Backslash does not act as an escape character. The
backslash is considered to be part of the line. In
particular, a backslash-newline pair may not then be used as
a line continuation.
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-s Silent mode. If input is coming from a terminal, characters
are not echoed.
-t timeout
Cause read to time out and return failure if a complete line
of input (or a specified number of characters) is not read
within timeout seconds. timeout may be a decimal number
with a fractional portion following the decimal point. This
option is only effective if read is reading input from a
terminal, pipe, or other special file; it has no effect when
reading from regular files. If read times out, read saves
any partial input read into the specified variable name. If
timeout is 0, read returns immediately, without trying to
read any data. The exit status is 0 if input is available
on the specified file descriptor, non-zero otherwise. The
exit status is greater than 128 if the timeout is exceeded.
-u fd
Read input from file descriptor fd.
If no names are supplied, the line read is assigned to the
variable REPLY. The exit status is zero, unless end-of-file is
encountered, read times out (in which case the status is greater
than 128), a variable assignment error (such as assigning to a
readonly variable) occurs, or an invalid file descriptor is
supplied as the argument to -u.
readonly [-aAf] [-p] [name[=word] ...]
The given names are marked readonly; the values of these names
may not be changed by subsequent assignment. If the -f option is
supplied, the functions corresponding to the names are so marked.
The -a option restricts the variables to indexed arrays; the -A
option restricts the variables to associative arrays. If both
options are supplied, -A takes precedence. If no name arguments
are given, or if the -p option is supplied, a list of all
readonly names is printed. The other options may be used to
restrict the output to a subset of the set of readonly names.
The -p option causes output to be displayed in a format that may
be reused as input. If a variable name is followed by =word, the
value of the variable is set to word. The return status is 0
unless an invalid option is encountered, one of the names is not
a valid shell variable name, or -f is supplied with a name that
is not a function.
return [n]
Causes a function to stop executing and return the value
specified by n to its caller. If n is omitted, the return status
is that of the last command executed in the function body. If
return is executed by a trap handler, the last command used to
determine the status is the last command executed before the trap
handler. If return is executed during a DEBUG trap, the last
command used to determine the status is the last command executed
by the trap handler before return was invoked. If return is used
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outside a function, but during execution of a script by the .
(source) command, it causes the shell to stop executing that
script and return either n or the exit status of the last command
executed within the script as the exit status of the script. If
n is supplied, the return value is its least significant 8 bits.
The return status is non-zero if return is supplied a non-numeric
argument, or is used outside a function and not during execution
of a script by . or source. Any command associated with the
RETURN trap is executed before execution resumes after the
function or script.
set [--abefhkmnptuvxBCEHPT] [-o option-name] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [arg ...]
Without options, the name and value of each shell variable are
displayed in a format that can be reused as input for setting or
resetting the currently-set variables. Read-only variables
cannot be reset. In posix mode, only shell variables are listed.
The output is sorted according to the current locale. When
options are specified, they set or unset shell attributes. Any
arguments remaining after option processing are treated as values
for the positional parameters and are assigned, in order, to $1,
$2, ... $n. Options, if specified, have the following meanings:
-a Each variable or function that is created or modified is
given the export attribute and marked for export to the
environment of subsequent commands.
-b Report the status of terminated background jobs
immediately, rather than before the next primary prompt.
This is effective only when job control is enabled.
-e Exit immediately if a pipeline (which may consist of a
single simple command), a list, or a compound command
(see SHELL GRAMMAR above), exits with a non-zero status.
The shell does not exit if the command that fails is part
of the command list immediately following a while or
until keyword, part of the test following the if or elif
reserved words, part of any command executed in a && or
|| list except the command following the final && or ||,
any command in a pipeline but the last, or if the
command's return value is being inverted with !. If a
compound command other than a subshell returns a non-zero
status because a command failed while -e was being
ignored, the shell does not exit. A trap on ERR, if set,
is executed before the shell exits. This option applies
to the shell environment and each subshell environment
separately (see COMMAND EXECUTION ENVIRONMENT above), and
may cause subshells to exit before executing all the
commands in the subshell.
If a compound command or shell function executes in a
context where -e is being ignored, none of the commands
executed within the compound command or function body
will be affected by the -e setting, even if -e is set and
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a command returns a failure status. If a compound
command or shell function sets -e while executing in a
context where -e is ignored, that setting will not have
any effect until the compound command or the command
containing the function call completes.
-f Disable pathname expansion.
-h Remember the location of commands as they are looked up
for execution. This is enabled by default.
-k All arguments in the form of assignment statements are
placed in the environment for a command, not just those
that precede the command name.
-m Monitor mode. Job control is enabled. This option is on
by default for interactive shells on systems that support
it (see JOB CONTROL above). All processes run in a
separate process group. When a background job completes,
the shell prints a line containing its exit status.
-n Read commands but do not execute them. This may be used
to check a shell script for syntax errors. This is
ignored by interactive shells.
-o option-name
The option-name can be one of the following:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line editing
interface. This is enabled by default when the
shell is interactive, unless the shell is started
with the --noediting option. This also affects
the editing interface used for read -e.
errexit Same as -e.
errtrace
Same as -E.
functrace
Same as -T.
hashall Same as -h.
histexpand
Same as -H.
history Enable command history, as described above under
HISTORY. This option is on by default in
interactive shells.
ignoreeof
The effect is as if the shell command
``IGNOREEOF=10'' had been executed (see Shell
Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
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noglob Same as -f.
nolog Currently ignored.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
pipefail
If set, the return value of a pipeline is the
value of the last (rightmost) command to exit
with a non-zero status, or zero if all commands
in the pipeline exit successfully. This option
is disabled by default.
posix Change the behavior of bash where the default
operation differs from the POSIX standard to
match the standard (posix mode). See SEE ALSO
below for a reference to a document that details
how posix mode affects bash's behavior.
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
This also affects the editing interface used for
read -e.
xtrace Same as -x.
If -o is supplied with no option-name, the values of the
current options are printed. If +o is supplied with no
option-name, a series of set commands to recreate the
current option settings is displayed on the standard
output.
-p Turn on privileged mode. In this mode, the $ENV and
$BASH_ENV files are not processed, shell functions are
not inherited from the environment, and the SHELLOPTS,
BASHOPTS, CDPATH, and GLOBIGNORE variables, if they
appear in the environment, are ignored. If the shell is
started with the effective user (group) id not equal to
the real user (group) id, and the -p option is not
supplied, these actions are taken and the effective user
id is set to the real user id. If the -p option is
supplied at startup, the effective user id is not reset.
Turning this option off causes the effective user and
group ids to be set to the real user and group ids.
-t Exit after reading and executing one command.
-u Treat unset variables and parameters other than the
special parameters "@" and "*" as an error when
performing parameter expansion. If expansion is
attempted on an unset variable or parameter, the shell
prints an error message, and, if not interactive, exits
with a non-zero status.
-v Print shell input lines as they are read.
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-x After expanding each simple command, for command, case
command, select command, or arithmetic for command,
display the expanded value of PS4, followed by the
command and its expanded arguments or associated word
list.
-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 functions,
command substitutions, and commands executed in a
subshell environment. The ERR trap is normally not
inherited in such cases.
-H Enable ! style history substitution. This option is on
by default when the shell is interactive.
-P If set, the shell does not resolve symbolic links when
executing commands such as cd that change the current
working directory. It uses the physical directory
structure instead. By default, bash follows the logical
chain of directories when performing commands which
change the current directory.
-T If set, any traps on DEBUG and RETURN are inherited by
shell functions, command substitutions, and commands
executed in a subshell environment. The DEBUG and RETURN
traps are normally not inherited in such cases.
-- If no arguments follow this option, then the positional
parameters are unset. Otherwise, the positional
parameters are set to the args, even if some of them
begin with a -.
- Signal the end of options, cause all remaining args to be
assigned to the positional parameters. The -x and -v
options are turned off. If there are no args, the
positional parameters remain unchanged.
The options are off by default unless otherwise noted. Using +
rather than - causes these options to be turned off. The options
can also be specified as 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.
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shopt [-pqsu] [-o] [optname ...]
Toggle the values of settings controlling optional shell
behavior. The settings can be either those listed below, or, if
the -o option is used, those available with the -o option to the
set builtin command. With no options, or with the -p option, a
list of all settable options is displayed, with an indication of
whether or not each is set; if optnames are supplied, the output
is restricted to those options. The -p option causes output to
be displayed in a form that may be reused as input. Other
options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the return status
indicates whether the optname is set or unset. If multiple
optname arguments are given with -q, the return status is
zero if all optnames are enabled; non-zero otherwise.
-o Restricts the values of optname to be those defined for the
-o option to the set builtin.
If either -s or -u is used with no optname arguments, shopt shows
only those options which are set or unset, respectively. Unless
otherwise noted, the shopt options are disabled (unset) by
default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise. When setting or unsetting
options, the return status is zero unless an optname is not a
valid shell option.
The list of shopt options is:
assoc_expand_once
If set, the shell suppresses multiple evaluation of
associative array subscripts during arithmetic expression
evaluation, while executing builtins that can perform
variable assignments, and while executing builtins that
perform array dereferencing.
autocd If set, a command name that is the name of a directory is
executed as if it were the argument to the cd command.
This option is only used by interactive shells.
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 filename is printed, and the
command proceeds. This option is only used by
interactive shells.
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checkhash
If set, bash checks that a command found in the hash
table exists before trying to execute it. If a hashed
command no longer exists, a normal path search is
performed.
checkjobs
If set, bash lists the status of any stopped and running
jobs before exiting an interactive shell. If any jobs
are running, this causes the exit to be deferred until a
second exit is attempted without an intervening command
(see JOB CONTROL above). The shell always postpones
exiting if any jobs are stopped.
checkwinsize
If set, bash checks the window size after each external
(non-builtin) command and, if necessary, updates the
values of LINES and COLUMNS. This option is enabled by
default.
cmdhist If set, bash attempts to save all lines of a multiple-
line command in the same history entry. This allows easy
re-editing of multi-line commands. This option is
enabled by default, but only has an effect if command
history is enabled, as described above under HISTORY.
compat31
If set, bash changes its behavior to that of version 3.1
with respect to quoted arguments to the [[ conditional
command's =~ operator and locale-specific string
comparison when using the [[ conditional command's < and
> operators. Bash versions prior to bash-4.1 use ASCII
collation and strcmp(3); bash-4.1 and later use the
current locale's collation sequence and strcoll(3).
compat32
If set, bash changes its behavior to that of version 3.2
with respect to locale-specific string comparison when
using the [[ conditional command's < and > operators (see
previous item) and the effect of interrupting a command
list. Bash versions 3.2 and earlier continue with the
next command in the list after one terminates due to an
interrupt.
compat40
If set, bash changes its behavior to that of version 4.0
with respect to locale-specific string comparison when
using the [[ conditional command's < and > operators (see
description of compat31) and the effect of interrupting a
command list. Bash versions 4.0 and later interrupt the
list as if the shell received the interrupt; previous
versions continue with the next command in the list.
compat41
If set, bash, when in posix mode, treats a single quote
in a double-quoted parameter expansion as a special
character. The single quotes must match (an even number)
and the characters between the single quotes are
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considered quoted. This is the behavior of posix mode
through version 4.1. The default bash behavior remains
as in previous versions.
compat42
If set, bash does not process the replacement string in
the pattern substitution word expansion using quote
removal.
compat43
If set, bash does not print a warning message if an
attempt is made to use a quoted compound array assignment
as an argument to declare, makes word expansion errors
non-fatal errors that cause the current command to fail
(the default behavior is to make them fatal errors that
cause the shell to exit), and does not reset the loop
state when a shell function is executed (this allows
break or continue in a shell function to affect loops in
the caller's context).
compat44
If set, bash saves the positional parameters to BASH_ARGV
and BASH_ARGC before they are used, regardless of whether
or not extended debugging mode is enabled.
complete_fullquote
If set, bash quotes all shell metacharacters in filenames
and directory names when performing completion. If not
set, bash removes metacharacters such as the dollar sign
from the set of characters that will be quoted in
completed filenames when these metacharacters appear in
shell variable references in words to be completed. This
means that dollar signs in variable names that expand to
directories will not be quoted; however, any dollar signs
appearing in filenames will not be quoted, either. This
is active only when bash is using backslashes to quote
completed filenames. This variable is set by default,
which is the default bash behavior in versions through
4.2.
direxpand
If set, bash replaces directory names with the results of
word expansion when performing filename completion. This
changes the contents of the readline editing buffer. If
not set, bash attempts to preserve what the user typed.
dirspell
If set, bash attempts spelling correction on directory
names during word completion if the directory name
initially supplied does not exist.
dotglob If set, bash includes filenames beginning with a `.' in
the results of pathname expansion. The filenames ``.''
and ``..'' must always be matched explicitly, even if
dotglob is set.
execfail
If set, a non-interactive shell will not exit if it
cannot execute the file specified as an argument to the
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exec builtin command. An interactive shell does not exit
if exec fails.
expand_aliases
If set, aliases are expanded as described above under
ALIASES. This option is enabled by default for
interactive shells.
extdebug
If set at shell invocation, arrange to execute the
debugger profile before the shell starts, identical to
the --debugger option. If set after invocation, behavior
intended for use by debuggers is enabled:
1. The -F option to the declare builtin displays the
source file name and line number corresponding to
each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a non-
zero value, the next command is skipped and not
executed.
3. If the command run by the DEBUG trap returns a value
of 2, and the shell is executing in a subroutine (a
shell function or a shell script executed by the .
or source builtins), the shell simulates a call to
return.
4. BASH_ARGC and BASH_ARGV are updated as described in
their descriptions above.
5. Function tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution,
shell functions, and subshells invoked with (
command ) inherit the ERR trap.
extglob If set, 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
possible completions. See SHELL VARIABLES above for a
description of FIGNORE. This option is enabled by
default.
globasciiranges
If set, range expressions used in pattern matching
bracket expressions (see Pattern Matching above) behave
as if in the traditional C locale when performing
comparisons. That is, the current locale's collating
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sequence is not taken into account, so b will not collate
between A and B, and upper-case and lower-case ASCII
characters will collate together.
globstar
If set, the pattern ** used in a pathname expansion
context will match all files and zero or more directories
and subdirectories. If the pattern is followed by a /,
only directories and subdirectories match.
gnu_errfmt
If set, shell error messages are written in the standard
GNU error message format.
histappend
If set, the history list is appended to the file named by
the value of the HISTFILE variable when the shell exits,
rather than overwriting the file.
histreedit
If set, and readline is being used, a user is given the
opportunity to re-edit a failed history substitution.
histverify
If set, and readline is being used, the results of
history substitution are not immediately passed to the
shell parser. Instead, the resulting line is loaded into
the readline editing buffer, allowing further
modification.
hostcomplete
If set, and readline is being used, bash will attempt to
perform hostname completion when a word containing a @ is
being completed (see Completing under READLINE above).
This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs when an
interactive login shell exits.
inherit_errexit
If set, command substitution inherits the value of the
errexit option, instead of unsetting it in the subshell
environment. This option is enabled when posix mode is
enabled.
interactive_comments
If set, allow a word beginning with # to cause that word
and all remaining characters on that line to be ignored
in an interactive shell (see COMMENTS above). This
option is enabled by default.
lastpipe
If set, and job control is not active, the shell runs the
last command of a pipeline not executed in the background
in the current shell environment.
lithist If set, and the cmdhist option is enabled, multi-line
commands are saved to the history with embedded newlines
rather than using semicolon separators where possible.
localvar_inherit
If set, local variables inherit the value and attributes
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of a variable of the same name that exists at a previous
scope before any new value is assigned. The nameref
attribute is not inherited.
localvar_unset
If set, calling unset on local variables in previous
function scopes marks them so subsequent lookups find
them unset until that function returns. This is identical
to the behavior of unsetting local variables at the
current function scope.
login_shell
The shell sets this option if it is started as a login
shell (see INVOCATION above). The value may not be
changed.
mailwarn
If set, and a file that bash is checking for mail has
been accessed since the last time it was checked, the
message ``The mail in mailfile has been read'' is
displayed.
no_empty_cmd_completion
If set, and readline is being used, bash will not attempt
to search the PATH for possible completions when
completion is attempted on an empty line.
nocaseglob
If set, bash matches filenames in a case-insensitive
fashion when performing pathname expansion (see Pathname
Expansion above).
nocasematch
If set, bash matches patterns in a case-insensitive
fashion when performing matching while executing case or
[[ conditional commands, when performing pattern
substitution word expansions, or when filtering possible
completions as part of programmable completion.
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.
progcomp_alias
If set, and programmable completion is enabled, bash
treats a command name that doesn't have any completions
as a possible alias and attempts alias expansion. If it
has an alias, bash attempts programmable completion using
the command word resulting from the expanded alias.
promptvars
If set, prompt strings undergo parameter expansion,
command substitution, arithmetic expansion, and quote
removal after being expanded as described in PROMPTING
above. This option is enabled by default.
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restricted_shell
The shell sets this option if it is started in restricted
mode (see RESTRICTED SHELL below). The value may not be
changed. This is not reset when the startup files are
executed, allowing the startup files to discover whether
or not a shell is restricted.
shift_verbose
If set, the shift builtin prints an error message when
the shift count exceeds the number of positional
parameters.
sourcepath
If set, the source (.) builtin uses the value of PATH to
find the directory containing the file supplied as an
argument. This option is enabled by default.
xpg_echo
If set, the echo builtin expands backslash-escape
sequences by default.
suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT
signal. A login shell cannot be suspended; the -f option can be
used to override this and force the suspension. The return
status is 0 unless the shell is a login shell and -f is not
supplied, or if job control is not enabled.
test expr
[ expr ]
Return a status of 0 (true) or 1 (false) depending on the
evaluation of the conditional expression expr. Each operator and
operand must be a separate argument. Expressions are composed of
the primaries described above under CONDITIONAL EXPRESSIONS.
test does not accept any options, nor does it accept and ignore
an argument of -- as signifying the end of options.
Expressions may be combined using the following operators, listed
in decreasing order of precedence. The evaluation depends on the
number of arguments; see below. Operator precedence is used when
there are five or more arguments.
! expr
True if expr is false.
( expr )
Returns the value of expr. This may be used to override the
normal 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.
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0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not
null.
2 arguments
If the first argument is !, the expression is true if and
only if the second argument is null. If the first argument
is one of the unary conditional operators listed above under
CONDITIONAL EXPRESSIONS, the expression is true if the unary
test is true. If the first argument is not a valid unary
conditional operator, the expression is false.
3 arguments
The following conditions are applied in the order listed.
If the second argument is one of the binary conditional
operators listed above under CONDITIONAL EXPRESSIONS, the
result of the expression is the result of the binary test
using the first and third arguments as operands. The -a and
-o operators are considered binary operators when there are
three arguments. If the first argument is !, the value is
the negation of the two-argument test using the second and
third arguments. If the first argument is exactly ( and the
third argument is exactly ), the result is the one-argument
test of the second argument. Otherwise, the expression is
false.
4 arguments
If the first argument is !, the result is the negation of
the three-argument expression composed of the remaining
arguments. Otherwise, the expression is parsed and
evaluated according to precedence using the rules listed
above.
5 or more arguments
The expression is parsed and evaluated according to
precedence using the rules listed above.
When used with test or [, the < and > operators sort
lexicographically using ASCII ordering.
times
Print the accumulated user and system times for the shell and for
processes run from the shell. The return status is 0.
trap [-lp] [[arg] sigspec ...]
The command arg is to be read and executed when the shell
receives signal(s) sigspec. If arg is absent (and there is a
single sigspec) or -, each specified signal is reset to its
original disposition (the value it had upon entrance to the
shell). If arg is the null string the signal specified by each
sigspec is ignored by the shell and by the commands it invokes.
If arg is not present and -p has been supplied, then the trap
commands associated with each sigspec are displayed. If no
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arguments are supplied or if only -p is given, trap prints the
list of commands associated with each signal. The -l option
causes the shell to print a list of signal names and their
corresponding numbers. Each sigspec is either a signal name
defined in <signal.h>, or a signal number. Signal names are case
insensitive and the SIG prefix is optional.
If a sigspec is EXIT (0) the command arg is executed on exit from
the shell. If a sigspec is DEBUG, the command arg is executed
before every simple command, for command, case command, select
command, every arithmetic for command, and before the first
command executes in a shell function (see SHELL GRAMMAR above).
Refer to the description of the extdebug option to the shopt
builtin for details of its effect on the DEBUG trap. If a
sigspec is RETURN, the command arg is executed each time a shell
function or a script executed with the . or source builtins
finishes executing.
If a sigspec is ERR, the command arg is executed whenever a
pipeline (which may consist of a single simple command), a list,
or a compound command returns a non-zero exit status, subject to
the following conditions. The ERR trap is not executed if the
failed command is part of the command list immediately following
a while or until keyword, part of the test in an if statement,
part of a command executed in a && or || list except the command
following the final && or ||, any command in a pipeline but the
last, or if the command's return value is being inverted using !.
These are the same conditions obeyed by the errexit (-e) option.
Signals ignored upon entry to the shell cannot be trapped or
reset. Trapped signals that are not being ignored are reset to
their original values in a subshell or subshell environment when
one is created. The return status is false if any sigspec is
invalid; otherwise trap returns true.
type [-aftpP] name [name ...]
With no options, indicate how each name would be interpreted if
used as a command name. If the -t option is used, type prints a
string which is one of alias, keyword, function, builtin, or file
if name is an alias, shell reserved word, function, builtin, or
disk file, respectively. If the name is not found, then nothing
is printed, and an exit status of false is returned. If the -p
option is used, type either returns the name of the disk file
that would be executed if name were specified as a command name,
or nothing if ``type -t name'' would not return file. The -P
option forces a PATH search for each name, even if ``type -t
name'' would not return file. If a command is hashed, -p and -P
print the hashed value, which is not necessarily the file that
appears first in PATH. If the -a option is used, type prints all
of the places that contain an executable named name. This
includes aliases and functions, if and only if the -p option is
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not also used. The table of hashed commands is not consulted
when using -a. The -f option suppresses shell function lookup,
as with the command builtin. type returns true if all of the
arguments are found, false if any are not found.
ulimit [-HSabcdefiklmnpqrstuvxPT [limit]]
Provides control over the resources available to the shell and to
processes started by it, on systems that allow such control. The
-H and -S options specify that the hard or soft limit is set for
the given resource. A hard limit cannot be increased by a non-
root user once it is set; a soft limit may be increased up to the
value of the hard limit. If neither -H nor -S is specified, both
the soft and hard limits are set. The value of limit can be a
number in the unit specified for the resource or one of the
special values hard, soft, or unlimited, which stand for the
current hard limit, the current soft limit, and no limit,
respectively. If limit is 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
-b The maximum socket buffer size
-c The maximum size of core files created
-d The maximum size of a process's data segment
-e The maximum scheduling priority ("nice")
-f The maximum size of files written by the shell and its
children
-i The maximum number of pending signals
-k The maximum number of kqueues that may be allocated
-l The maximum size that may be locked into memory
-m The maximum resident set size (many systems do not honor
this limit)
-n The maximum number of open file descriptors (most systems do
not allow this value to be set)
-p The pipe size in 512-byte blocks (this may not be set)
-q The maximum number of bytes in POSIX message queues
-r The maximum real-time scheduling priority
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to a single user
-v The maximum amount of virtual memory available to the shell
and, on some systems, to its children
-x The maximum number of file locks
-P The maximum number of pseudoterminals
-T The maximum number of threads
If limit is given, and the -a option is not used, limit is the
new value of the specified resource. If no option is given, then
-f is assumed. Values are in 1024-byte increments, except for
-t, which is in seconds; -p, which is in units of 512-byte
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blocks; -P, -T, -b, -k, -n, and -u, which are unscaled values;
and, when in posix mode, -c and -f, which are in 512-byte
increments. The return status is 0 unless an invalid option or
argument is supplied, or an error occurs while setting a new
limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If mode begins with
a digit, it is interpreted as an octal number; otherwise it is
interpreted as a symbolic mode mask similar to that accepted by
chmod(1). If mode is omitted, the current value of the mask is
printed. The -S option causes the mask to be printed in symbolic
form; the default output is an octal number. If the -p option is
supplied, and mode is omitted, the output is in a form that may
be reused as input. The return status is 0 if the mode was
successfully changed or if no mode argument was supplied, and
false otherwise.
unalias [-a] [name ...]
Remove each name from the list of defined aliases. If -a is
supplied, all alias definitions are removed. The return value is
true unless a supplied name is not a defined alias.
unset [-fv] [-n] [name ...]
For each name, remove the corresponding variable or function. If
the -v option is given, each name refers to a shell variable, and
that variable is removed. Read-only variables may not be unset.
If -f is specified, each name refers to a shell function, and the
function definition is removed. If the -n option is supplied,
and name is a variable with the nameref attribute, name will be
unset rather than the variable it references. -n has no effect
if the -f option is supplied. If no options are supplied, each
name refers to a variable; if there is no variable by that name,
any function with that name is unset. Each unset variable or
function is removed from the environment passed to subsequent
commands. If any of COMP_WORDBREAKS, RANDOM, SECONDS, LINENO,
HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they lose their
special properties, even if they are subsequently reset. The
exit status is true unless a name is readonly.
wait [-fn] [id ...]
Wait for each specified child process and return its termination
status. Each id 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 id is not given, all currently active child
processes are waited for, and the return status is zero. If the
-n option is supplied, wait waits for any job to terminate and
returns its exit status. If the -f option is supplied, and job
control is enabled, wait forces id to terminate before returning
its status, instead of returning when it changes status. If id
specifies a non-existent process or job, the return status is
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127. Otherwise, the return status is the exit status of the last
process or job waited for.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option is supplied
at invocation, the shell becomes restricted. A restricted shell is
used to set up an environment more controlled than the standard shell.
It behaves identically to bash with the exception that the following
are disallowed or not performed:
+ changing directories with cd
+ setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV
+ specifying command names containing /
+ specifying a filename containing a / as an argument to the .
builtin command
+ specifying a filename containing a slash as an argument to the -p
option to the hash builtin command
+ importing function definitions from the shell environment at
startup
+ parsing the value of SHELLOPTS from the shell environment at
startup
+ redirecting output using the >, >|, <>, >&, &>, and >>
redirection operators
+ using the exec builtin command to replace the shell with another
command
+ adding or deleting builtin commands with the -f and -d options to
the enable builtin command
+ using the enable builtin command to enable disabled shell
builtins
+ specifying the -p option to the command builtin command
+ turning off restricted mode with set +r or set +o restricted.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see
COMMAND EXECUTION above), rbash turns off any restrictions in the
shell spawned to execute the script.
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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
IEEE --
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities,
http://pubs.opengroup.org/onlinepubs/9699919799/
http://tiswww.case.edu/~chet/bash/POSIX -- a description of posix mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login shells
~/.bash_profile
The personal initialization file, executed for login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed when a login
shell exits
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet.ramey@case.edu
BUG REPORTS
If you find a bug in bash, you should report it. But first, you
should make sure that it really is a bug, and that it appears in the
latest version of bash. The latest version is always available from
ftp://ftp.gnu.org/pub/gnu/bash/.
Once you have determined that a bug actually exists, use the bashbug
command to submit a bug report. If you have a fix, you are encouraged
to mail that as well! Suggestions and `philosophical' bug reports may
be mailed to bug-bash@gnu.org or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
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The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into the template
it provides for filing a bug report.
Comments and bug reports concerning this manual page should be
directed to chet.ramey@case.edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and traditional
versions of sh, mostly because of the POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command sequences of the form `a ; b ; c' are
not handled gracefully when process suspension is attempted. When a
process is stopped, the shell immediately executes the next command in
the sequence. It suffices to place the sequence of commands between
parentheses to force it into a subshell, which may be stopped as a
unit.
Array variables may not (yet) be exported.
There may be only one active coprocess at a time.
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