bash - GNU Bourne-Again SHell
bash [options] [command_string | file]
Bash is Copyright (C) 1989-2022 by the Free Software Foundation, Inc.
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.
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:
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.
If the -i option is present, the shell is
interactive.
Make bash act as if it had been invoked as a login shell (see
INVOCATION below).
If the -r option is present, the shell becomes
restricted (see
RESTRICTED SHELL below).
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.
Print shell input lines as they are read.
Print commands and their arguments as they are executed.
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.
shopt_option]shopt_option is one of the shell options accepted by the
shopt builtin (see
SHELL BUILTIN COMMANDS below). If
shopt_option is present, -O sets the value of
that option; +O unsets it. If shopt_option is
not supplied, the names and values of the shell options accepted by
shopt are printed on the standard output. If the
invocation option is +O, the output is displayed in a
format that may be reused as input.
A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as 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.
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).
Equivalent to -D, but the output is in the GNU
gettext po (portable object) file format.
Equivalent to -D.
Display a usage message on standard output and exit successfully.
filefileExecute commands from file instead of the system wide
initialization file /etc/bash.bashrc and the standard personal
initialization file ~/.bashrc if the shell is interactive
(see
INVOCATION below).
Equivalent to -l.
Do not use the GNU readline library to read command lines when the shell is interactive.
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).
Do not read and execute the system wide initialization file
/etc/bash.bashrc and the personal initialization file
~/.bashrc if the shell is interactive. This option is on by
default if the shell is invoked as sh.
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.
The shell becomes restricted (see
RESTRICTED SHELL below).
Equivalent to -v.
Show version information for this instance of bash on the standard output and exit successfully.
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.
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 $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in filenames as described below under Tilde Expansion in the
EXPANSION section.
When bash is invoked as an interactive login shell,
or as a non-interactive shell with the --login option,
it first reads and executes commands from the file
/etc/profile, if that file exists. After reading that file, it
looks for ~/.bash_profile, ~/.bash_login, and
~/.profile, in that order, and reads and executes commands from
the first one that exists and is readable. The
--noprofile option may be used when the shell is
started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login
shell executes the exit builtin command,
bash reads and executes commands from the file
~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started,
bash reads and executes commands from
/etc/bash.bashrc and ~/.bashrc, if these files exist.
This may be inhibited by using the --norc option. The
--rcfile file option will force
bash to read and execute commands from file
instead of /etc/bash.bashrc and ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks for the variable
BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:
if [ -n "$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 historical remote shell daemon, usually rshd, or the
secure shell daemon sshd. If bash determines
it is being run non-interactively in this fashion, it reads and executes
commands from /etc/bash.bashrc and ~/.bashrc, if these
files exist and are readable. It will not do this if invoked as
sh. The --norc option may be used to
inhibit this behavior, and the --rcfile option may be
used to force another file to be read, but neither rshd nor
sshd generally invoke the shell with those options or allow
them to be specified.
If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the
SHELLOPTS,
BASHOPTS,
CDPATH, and
GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.
The following definitions are used throughout the rest of this document.
A space or tab.
A sequence of characters considered as a single unit by the shell. Also known as a token.
A word consisting only of alphanumeric characters and
underscores, and beginning with an alphabetic character or an
underscore. Also referred to as an identifier.
A character that, when unquoted, separates words. One of the
following:
| & ; ( ) < > space tab newline
A token that performs a control function. It is one of the
following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>
Reserved words are words that have a special meaning to the
shell. The following words are recognized as reserved when unquoted and
either the first word of a command (see
SHELL GRAMMAR below), the third word of a case or select command (only in is valid), or the third word of a for command (only in and do are valid):
! case coproc do done elif else esac fi for function if in select then until while { } time [[ ]]
This section describes the syntax of the various forms of shell 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.
A pipeline is a sequence of one or more commands separated
by one of the control operators | or
|&. The format for a pipeline is:
[time [-p]] [ ! ]
command1[ [|⎪|&]command2... ]
The standard output of command1 is connected via a pipe to
the standard input of command2. This connection is performed
before any redirections specified by the command1(see
REDIRECTION below). If |& is
used, command1's standard error, in addition to its standard
output, is connected to command2's standard input through the
pipe; it is shorthand for 2>&1 |. This implicit
redirection of the standard error to the standard output is performed
after any redirections specified by command1.
The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as described above. 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 multi-command pipeline, where pipes are created, is
executed in a subshell, which is a separate process. See
COMMAND EXECUTION ENVIRONMENT for a description of subshells and a subshell environment. If the lastpipe option is enabled using the shopt builtin (see the description of shopt below), the last element of a pipeline may be run by the shell process when job control is not active.
A list is a sequence of one or more pipelines separated by
one of the operators ;, &,
&&, or ||, and optionally
terminated by one of ;, &, or
<newline>.
Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.
A sequence of one or more newlines may appear in a list
instead of a semicolon to delimit commands.
If a command is terminated by the control operator
&, the shell executes the command in the
background in a subshell. The shell does not wait for the
command to finish, and the return status is 0. These are referred to as
asynchronous commands. Commands separated by a
; are executed sequentially; the shell waits for each
command to terminate in turn. The return status is the exit status of
the last command executed.
AND and OR lists are sequences of one or more pipelines separated by the && and || control operators, respectively. AND and OR lists are executed with left associativity. An AND list has the form
command1&&command2
command2 is executed if, and only if, command1
returns an exit status of zero (success).
An OR list has the form
command1||command2
command2 is executed if, and only if, command1
returns a non-zero exit status. The return status of AND and OR lists is
the exit status of the last command executed in the list.
A compound command is one of the following. In most cases a
list in a command's description may be separated from the rest
of the command by one or more newlines, and may be followed by a newline
in place of a semicolon.
list)list is executed in a subshell (see
COMMAND EXECUTION ENVIRONMENT below for a
description of a subshell environment). Variable assignments and builtin
commands that affect the shell's environment do not remain in effect
after the command completes. The return status is the exit status of
list.
list; }list is 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 return
status is 1. The expression undergoes the same expansions as if
it were within double quotes, but double quote characters in
expression are not treated specially and are removed.
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. The words between the [[ and ]] do not undergo word splitting and pathname expansion. The shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words (the expansions that would occur if the words were enclosed in double quotes). 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.
See the description of the test builtin command (section
SHELL BUILTIN COMMANDS below) for the handling of parameters (i.e.
missing parameters).
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 (using the
POSIX regcomp and regexec interfaces usually described
in regex(3)). The return value is 0 if the string matches the
pattern, and 1 otherwise. If the regular expression is syntactically
incorrect, the conditional expression's return value is 2. If the
nocasematch shell option is enabled, the match is
performed without regard to the case of alphabetic characters. If any
part of the pattern is quoted, the quoted portion is matched literally.
This means every character in the quoted portion matches itself, instead
of having any special pattern matching meaning. If the pattern is stored
in a shell variable, quoting the variable expansion forces the entire
pattern to be matched literally. Treat bracket expressions in regular
expressions carefully, since normal quoting and pattern characters lose
their meanings between brackets.
The pattern will match if it matches any part of the string. Anchor the pattern using the ^ and $ regular expression operators to force it to match the entire string. The array variable
BASH_REMATCH records which parts of the string matched the pattern. The element of
BASH_REMATCH with index 0 contains the portion of the string matching the entire regular expression. Substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining
BASH_REMATCH indices. The element of
BASH_REMATCH with index n is the portion of
the string matching the nth parenthesized subexpression. Bash
sets
BASH_REMATCH in the global scope; declaring it as a local variable will lead to unexpected results.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
- (
expression)Returns the value of
expression. This may be used to override the normal precedence of operators.- !
expressionTrue if
expressionis false.expression1&&expression2True if both
expression1andexpression2are true.expression1||expression2True if either
expression1orexpression2is true.The && and || operators do not evaluate
expression2if the value ofexpression1is sufficient to determine the return value of the entire conditional expression.
name [ [ in [
word ... ] ] ; ] do list ;
doneThe 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.
expr1 ; expr2 ;
expr3 )) ; do list ;
doneFirst, 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.
name [ in
word ] ; do list ;
doneThe list of words following in is expanded,
generating a list of items, and 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). select then displays the
PS3 prompt and reads a line from the standard input.
If the line consists of a number corresponding to one of the displayed
words, then 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
select command completes and returns 1. 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.
word in [ [(]
pattern [ | pattern ] ... )
list ;; ] ... esacA 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, process substitution, and
quote removal. 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,
continuing the case statement execution as if the pattern list had not
matched. The exit status is zero if no pattern matches. Otherwise, it is
the exit status of the last command executed in list.
list; then
list; [ elif list;
then list; ] ... [ else
list; ] fiThe 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.
list-1; do
list-2; donelist-1; do
list-2; doneThe 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.
A coprocess is a shell command preceded by the
coproc reserved word. A coprocess is executed
asynchronously in a subshell, as if the command had been terminated with
the & control operator, with a two-way pipe
established between the executing shell and the coprocess.
The syntax for a coprocess is:
coproc [
NAME]command[redirections]
This creates a coprocess named NAME. command may be
either a simple command or a compound command (see above). NAME
is a shell variable name. If NAME is not supplied, the default
name is COPROC.
The recommended form to use for a coprocess is
coproc
NAME{command[redirections]; }
This form is recommended because simple commands result in the coprocess always being named COPROC, and it is simpler to use and more complete than the other compound commands.
If command is a compound command, NAME is optional.
The word following coproc determines whether that word
is interpreted as a variable name: it is interpreted as NAME if
it is not a reserved word that introduces a compound command. If
command is a simple command, NAME is not allowed; this
is to avoid confusion between NAME and the first word of the
simple command.
When the coprocess is executed, the shell creates an array variable
(see Arrays below) named NAME in the context
of the executing shell. The standard output of command is
connected via a pipe to a file descriptor in the executing shell, and
that file descriptor is assigned to NAME[0]. The standard input
of command is connected via a pipe to a file descriptor in the
executing shell, and that file descriptor is assigned to
NAME[1]. This pipe is established before any redirections
specified by the command (see
REDIRECTION below). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells.
The process ID of the shell spawned to execute the coprocess is
available as the value of the variable NAME_PID. The
wait builtin 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.
A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters. Shell functions are declared as follows:
fname () compound-command
[redirection]fname [()]
compound-command [redirection]This defines a function named fname. The reserved word
function is optional. If the function
reserved word is supplied, the parentheses are optional. The
body of the function is the compound command
compound-command (see Compound Commands
above). That command is usually a list of commands between {
and }, but may be any command listed under Compound
Commands above. If the function reserved word
is used, but the parentheses are not supplied, the braces are
recommended. compound-command is executed whenever
fname is specified as the name of a simple command. When in
posix mode, fname must be a valid shell name
and may not be the name of one of the POSIX special builtins.
In default mode, a function name can be any unquoted shell word that
does not contain $. Any redirections (see
REDIRECTION below) specified when a function is defined are performed when the function is executed. The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. (See
FUNCTIONS below.)
In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see
SHELL BUILTIN COMMANDS below), a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells.
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).
Character sequences of the form $'string'
are treated as a special variant of single quotes. The sequence expands
to string, with backslash-escaped characters in string
replaced as specified by the ANSI C standard. Backslash escape
sequences, if present, are decoded as follows:
- \a
alert (bell)
- \b
backspace
- \e
- \E
an escape character
- \f
form feed
- \n
new line
- \r
carriage return
- \t
horizontal tab
- \v
vertical tab
- \\
backslash
- \'
single quote
- \"
double quote
- \?
question mark
- \
nnnthe eight-bit character whose value is the octal value
nnn(one to three octal digits)- \x
HHthe eight-bit character whose value is the hexadecimal value
HH(one or two hex digits)- \u
HHHHthe Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHH(one to four hex digits)- \U
HHHHHHHHthe Unicode (ISO/IEC 10646) character whose value is the hexadecimal value
HHHHHHHH(one to eight hex digits)- \c
xa control-
xcharacter
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign
($"string") will cause the string to be
translated according to the current locale. The gettext
infrastructure performs the lookup and translation, using the
LC_MESSAGES, TEXTDOMAINDIR, and
TEXTDOMAIN shell variables. If the current locale is
C or POSIX, if there are no
translations available, or if the string is not translated, the dollar
sign is ignored. This is a form of double quoting, so the string remains
double-quoted by default, whether or not it is translated and replaced.
If the noexpand_translation option is enabled using the
shopt builtin, translated strings are single-quoted
instead of double-quoted. See the description of shopt
below under
SHELLBUILTINCOMMANDS.
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, command substitution, arithmetic expansion, and
quote removal (see
EXPANSION below). If the variable has its
integer attribute set, then value is evaluated
as an arithmetic expression even if the $((...)) expansion is not used
(see Arithmetic Expansion below). Word splitting and
pathname expansion are not performed. Assignment statements may also
appear as arguments to the alias,
declare, typeset,
export, readonly, and
local builtin commands (declaration commands).
When in posix mode, these builtins may appear in a command
after one or more instances of the command builtin and
retain these assignment statement properties.
In the context where an assignment statement is assigning a value to
a shell variable or array index, the += operator 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 $1. If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference will be established for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref variables can reference array variables and subscripted array variables. Namerefs can be unset using the -n option to the unset builtin. Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable will be unset.
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell's arguments when it is invoked, and may be
reassigned using the set builtin command. Positional
parameters may not be assigned to with assignment statements. The
positional parameters are temporarily replaced when a shell function is
executed (see
FUNCTIONS below).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see
EXPANSION below).
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where it is performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the
IFS special variable. That is, "$*"
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.
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).
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.
The following variables are set by the shell:
At shell startup, set to the 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.
Expands to the full filename used to invoke this instance of bash.
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.
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.
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.
An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto
BASH_ARGC. The shell sets
BASH_ARGC only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values.
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.
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.
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.
The command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset.
The command argument to the -c invocation option.
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.
A colon-separated list of directories in which the shell looks for dynamically loadable builtins specified by the enable command.
An array variable whose members are assigned by the
=~ binary operator to the [[
conditional command. The element with index 0 is the portion of the
string matching the entire regular expression. The element with index
n is the portion of the string matching the nth
parenthesized subexpression.
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]}.
Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.
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.
Expands to a string describing the version of this instance of bash.
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).
The key (or final key of a key sequence) used to invoke the current completion function.
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).
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).
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
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).
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.
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).
An array variable (see Arrays below) created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses above).
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.
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.
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.
Expands to the effective user ID of the current user, initialized at shell startup. This variable is readonly.
An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is "main". This variable exists only when a shell function is executing. Assignments to
FUNCNAME have no effect. If
FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.
This variable can be used with BASH_LINENO and
BASH_SOURCE. Each element of FUNCNAME
has corresponding elements in BASH_LINENO and
BASH_SOURCE to describe the call stack. For instance,
${FUNCNAME[$i]} was called
from the file
${BASH_SOURCE[$i+1]} at line
number ${BASH_LINENO[$i]}.
The caller builtin displays the current call stack
using this information.
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.
The history number, or index in the history list, of the current command. Assignments to
HISTCMD are ignored. If
HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
Automatically set to the name of the current host.
Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.
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.
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.
An array variable (see Arrays below) created to hold the text read by the mapfile builtin when no variable name is supplied.
The previous working directory as set by the cd command.
The value of the last option argument processed by the getopts builtin command (see
SHELL BUILTIN COMMANDS below).
The index of the next argument to be processed by the getopts builtin command (see
SHELL BUILTIN COMMANDS below).
Automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent.
An array variable (see Arrays below) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command).
The process ID of the shell's parent. This variable is readonly.
The current working directory as set by the cd command.
Each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to
RANDOM initializes (seeds) the sequence of random numbers. If
RANDOM is unset, it loses its special properties, even if it is subsequently reset.
Any numeric argument given to a readline command that was defined using "bind -x" (see
SHELL BUILTIN COMMANDS below) when it was invoked.
The contents of the readline line buffer, for use with "bind -x" (see
SHELL BUILTIN COMMANDS below).
The position of the mark (saved insertion point) in the readline line buffer, for use with "bind -x" (see
SHELL BUILTIN COMMANDS below). The characters
between the insertion point and the mark are often called the
region.
The position of the insertion point in the readline line buffer, for use with "bind -x" (see
SHELL BUILTIN COMMANDS below).
Set to the line of input read by the read builtin command when no arguments are supplied.
Each time this parameter is referenced, it expands to the number of seconds since shell invocation. If a value is assigned to
SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. The number of seconds at shell invocation and the current time are always determined by querying the system clock. If
SECONDS is unset, it loses its special properties, even if it is subsequently reset.
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.
Incremented by one each time an instance of bash is started.
This variable expands to a 32-bit pseudo-random number each time it
is referenced. The random number generator is not linear on systems that
support /dev/urandom or arc4random, so each
returned number has no relationship to the numbers preceding it. The
random number generator cannot be seeded, so assignments to this
variable have no effect. If
SRANDOM is unset, it loses its special properties, even if it is subsequently reset.
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.
The value is used to set the shell's compatibility level. See
SHELL COMPATIBILITY MODE below for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If BASH_COMPAT is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. The valid values correspond to the compatibility levels described below under
SHELL COMPATIBILITY MODE. For example, 4.2 and 42 are valid values that correspond to the compat42 shopt option and set the compatibility level to 42. The current version is also a valid value.
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.
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.
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".
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.
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.
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.
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.
Expanded and executed similarly to
BASH_ENV (see INVOCATION above)
when an interactive shell is invoked in posix mode.
A colon-separated list of shell patterns (see Pattern Matching) defining the list of filenames to be ignored by command search using PATH. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH lookup. This does not affect the behavior of the [, test, and [[ commands. Full pathnames in the command hash table are not subject to EXECIGNORE. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the extglob shell option.
The default editor for the fc builtin command.
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:~" (Quoting is needed when assigning a value to this variable, which contains tildes).
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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>''.
Controls the action of an interactive shell on receipt of an
EOF character as the sole input. If set, the value is the number of consecutive
EOF characters which must be typed as the first characters on an input line before bash exits. If the variable exists but does not have a numeric value, or has no value, the default value is 10. If it does not exist,
EOF signifies the end of input to the shell.
The filename for the readline startup file,
overriding the default of ~/.inputrc (see
READLINE below).
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.
Used to determine the locale category for any category not specifically selected with a variable starting with LC_.
This variable overrides the value of
LANG and any other LC_ variable specifying a locale category.
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.
This variable determines the interpretation of characters and the behavior of character classes within pathname expansion and pattern matching.
This variable determines the locale used to translate double-quoted strings preceded by a $.
This variable determines the locale category used for number formatting.
This variable determines the locale category used for data and time formatting.
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.
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.
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.
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).
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.
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''.
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.
If this variable is set, and is an array, the value of each set element is executed as a command prior to issuing each primary prompt. If this is set but not an array variable, its value is used as a command to execute instead.
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.
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.
The value of this parameter is expanded (see
PROMPTING below) and used as the primary prompt string. The default value is ``\s-\v\$ ''.
The value of this parameter is expanded as with
PS1 and used as the secondary prompt string. The default is ``> ''.
The value of this parameter is used as the prompt for the select command (see
SHELL GRAMMAR above).
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 ``+ ''.
This variable expands to the full pathname to the shell. If it is not set when the shell starts, bash assigns to it the full pathname of the current user's login shell.
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]RThe elapsed time in seconds.
- %[
p][l]UThe number of CPU seconds spent in user mode.
- %[
p][l]SThe number of CPU seconds spent in system mode.
- %P
The CPU percentage, computed as (%U + %S) / %R.
The optional
pis a digit specifying theprecision, 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 ofpgreater than 3 are changed to 3. Ifpis not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form
MMmSS.FFs. The value ofpdetermines 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.
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.
If set, bash uses its value as the name of a directory in which bash creates temporary files for the shell's use.
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.
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.
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 array.
Arrays are assigned to using compound assignments of the form
name=(value1 ...
valuen), where each value may be of
the form [subscript]=string. Indexed array assignments
do not require anything but string. Each value in the
list is expanded using all the shell expansions described below
under
EXPANSION. When assigning to indexed arrays, if the optional brackets and subscript are supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the words in a compound
assignment may be either assignment statements, for which the subscript
is required, or a list of words that is interpreted as a sequence of
alternating keys and values: name=( key1
value1 key2 value2 ...). These are treated
identically to name=(
[key1]=value1 [key2]=value2
...). The first word in the list determines how the
remaining words are interpreted; all assignments in a list must be of
the same type. When using key/value pairs, the keys may not be missing
or empty; a final missing value is treated like the empty string.
This syntax is also accepted by the declare builtin.
Individual array elements may be assigned to using the
name[subscript]=value syntax introduced
above. When assigning to an indexed array, if name is
subscripted by a negative number, that number is interpreted as relative
to one greater than the maximum index of name, so negative
indices count back from the end of the array, and an index of -1
references the last element.
The += operator will append to an array variable when assigning using the compound assignment syntax; see
PARAMETERS above.
Any element of an array may be referenced using
${name[subscript]}. The braces are required to avoid
conflicts with pathname expansion. If subscript is
@ or *, the word expands to all
members of name. These subscripts differ only when the word
appears within double quotes. If the word is double-quoted,
${name[*]} expands to a single word with the value of each
array member separated by the first 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 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, removes the entire
array. unset name[subscript], where
subscript is * or @, behaves
differently depending on whether name is an indexed or
associative array. If name is an associative array, this unsets
the element with subscript * or @. If
name is an indexed array, unset removes all of the elements but
does not remove the array itself.
When using a variable name with a subscript as an argument to a command, such as with unset, without using the word expansion syntax described above, 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 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).
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 letters,
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 letters 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 (integer or letter). When the
increment is supplied, it is used as the difference between each term.
The default increment is 1 or -1 as appropriate.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. A { or , may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion, and inhibits brace expansion until the closing }.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Brace expansion introduces a slight incompatibility with historical
versions of sh. sh does not treat
opening or closing braces specially when they appear as part of a word,
and preserves them in the output. Bash removes braces
from words as a consequence of brace expansion. For example, a word
entered to sh as file{1,2} appears identically
in the output. 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).
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.
The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.
parameter}The value of parameter is substituted. The braces are
required when parameter is a positional parameter with more
than one digit, or when parameter is followed by a character
which is not to be interpreted as part of its name. The
parameter is a shell parameter as described above
PARAMETERS) or an array reference
(Arrays).
If the first character of parameter is an exclamation point
(!), 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.
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
@ or *, 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 @ or *,
the result is length positional parameters beginning at
offset. A negative offset is taken relative to one
greater than the greatest positional parameter, so an offset of -1
evaluates to the last positional parameter. It is an expansion error if
length evaluates to a number less than zero.
If parameter is an indexed array name subscripted by @ or *,
the result is the length members of the array beginning with
${parameter[offset]}. A negative offset is
taken relative to one greater than the maximum index of the specified
array. It is an expansion error if length evaluates to a number
less than zero.
Substring expansion applied to an associative array produces undefined results.
Substring indexing is zero-based unless the positional parameters are
used, in which case the indexing starts at 1 by default. If
offset is 0, and the positional parameters are used,
$0 is prefixed to the list.
prefix*}prefix@}Names matching prefix. Expands to the names of
variables whose names begin with prefix, separated by the first
character of the
IFS special variable. When @ is used and
the expansion appears within double quotes, each variable name expands
to a separate word.
name[@]}name[*]}List of array keys. If name is an array
variable, expands to the list of array indices (keys) assigned in
name. If name is not an array, expands to 0 if
name is set and null otherwise. When @ is used and the
expansion appears within double quotes, each key expands to a separate
word.
parameter}Parameter length. 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.
parameter/pattern/string}parameter//pattern/string}parameter/#pattern/string}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. string
undergoes tilde expansion, parameter and variable expansion, arithmetic
expansion, command and process substitution, and quote removal. The
match is performed using the rules described under Pattern
Matching below. In the first form above, only the first match
is replaced. If there are two slashes separating parameter and
pattern (the second form above), all matches of
pattern are replaced with string. If pattern
is preceded by # (the third form above), it must match
at the beginning of the expanded value of parameter. If
pattern is preceded by % (the fourth form
above), it must match at the end of the expanded value of
parameter. If the expansion of string is null, matches
of pattern are deleted. If string is null, matches of
pattern are deleted and the / following
pattern may be omitted.
If the patsub_replacement shell option is enabled
using shopt, any unquoted instances of
& in string are replaced with the matching
portion of pattern.
Quoting any part of string inhibits replacement in the
expansion of the quoted portion, including replacement strings stored in
shell variables. Backslash will escape & in
string; the backslash is removed in order to permit a literal
& in the replacement string. Backslash can also be
used to escape a backslash; \\ results in a literal
backslash in the replacement. Users should take care if string
is double-quoted to avoid unwanted interactions between the backslash
and double-quoting, since backslash has special meaning within double
quotes. Pattern substitution performs the check for unquoted
& after expanding string; shell
programmers should quote any occurrences of & they
want to be taken literally in the replacement and ensure any instances
of & they want to be replaced are unquoted.
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:
- U
The expansion is a string that is the value of
parameterwith lowercase alphabetic characters converted to uppercase.- u
The expansion is a string that is the value of
parameterwith the first character converted to uppercase, if it is alphabetic.- L
The expansion is a string that is the value of
parameterwith uppercase alphabetic characters converted to lowercase.- Q
The expansion is a string that is the value of
parameterquoted in a format that can be reused as input.- E
The expansion is a string that is the value of
parameterwith backslash escape sequences expanded as with the $'...' quoting mechanism.- P
The expansion is a string that is the result of expanding the value of
parameteras 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
parameterwith its attributes and value.- K
Produces a possibly-quoted version of the value of
parameter, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays above).- a
The expansion is a string consisting of flag values representing
parameter's attributes.- k
Like the K transformation, but expands the keys and values of indexed and associative arrays to separate words after word splitting.
If
parameteris @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. Ifparameteris 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 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 allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$((
expression))
The old format
$[expression] is deprecated
and will be removed in upcoming versions of bash.
The expression undergoes the same expansions as if it were
within double quotes, but double quote characters in expression
are not treated specially and are removed. All tokens in the expression
undergo parameter and variable expansion, command substitution, and
quote removal. The result is treated as the arithmetic expression to be
evaluated. 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 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.
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.
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.
After word splitting, unless the -f option has been
set, bash scans each word for the characters
*, ?, and [. If one
of these characters appears, and is not quoted, then the word is
regarded as a pattern, and replaced with 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. Note that when using range expressions like [a-z] (see below), letters of the other case may be included, depending on the setting of LC_COLLATE. When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. In order to match the filenames ``.'' and ``..'', the pattern must begin with ``.'' (for example, ``.?''), even if dotglob is set. If the globskipdots shell option is enabled, the filenames ``.'' and ``..'' are never matched, even if the pattern begins with a ``.''. When not matching pathnames, the ``.'' character is not treated specially. When matching a pathname, the slash character must always be matched explicitly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below under
Pattern Matching. See the description of shopt below under
SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob, globskipdots, failglob, and dotglob shell options.
The
GLOBIGNORE shell variable may be used to restrict
the set of file names matching a pattern. If
GLOBIGNORE is set, each matching file name that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches. If the nocaseglob option is set, the matching against the patterns in
GLOBIGNORE is performed without regard to case. The filenames ``.'' and ``..'' are always ignored when
GLOBIGNORE is set and not null. However, setting
GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option, so all other filenames beginning with a ``.'' 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
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, and the characters included in the range, are determined by the current locale and the values of theLC_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:], whereclassis 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 characterc.
Within [ and ], the syntax [.
symbol.] matches the collating symbolsymbol.
If the extglob shell option is enabled using the
shopt builtin, the shell recognizes several extended
pattern matching operators. In the following description, a
pattern-list is a list of one or more patterns separated by a
|. Composite patterns may be formed using one or more
of the following sub-patterns:
- ?(
pattern-list)Matches zero or one occurrence of the given patterns
- *(
pattern-list)Matches zero or more occurrences of the given patterns
- +(
pattern-list)Matches one or more occurrences of the given patterns
- @(
pattern-list)Matches one of the given patterns
- !(
pattern-list)Matches anything except one of the given patterns
Theextglob option changes the behavior of the parser, since the parentheses are normally treated as operators with syntactic meaning. To ensure that extended matching patterns are parsed correctly, make sure that extglob is enabled before parsing constructs containing the patterns, including shell functions and command substitutions.
When matching filenames, the dotglob shell option determines the set of filenames that are tested: when dotglob is enabled, the set of filenames includes all files beginning with ``.'', but ``.'' and ``..'' must be matched by a pattern or sub-pattern that begins with a dot; when it is disabled, the set does not include any filenames beginning with ``.'' unless the pattern or sub-pattern begins with a ``.''. As above, ``.'' only has a special meaning when matching filenames.
Complicated extended pattern matching against long strings is slow, especially when the patterns contain alternations and the strings contain multiple matches. Using separate matches against shorter strings, or using arrays of strings instead of a single long string, may be faster.
After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are removed.
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's lifetime manually. The
varredir_close shell option manages this behavior.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, quote removal, pathname expansion, and word splitting. If it expands to more than one word, bash reports an error.
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/
fdIf
fdis a valid integer, file descriptorfdis 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/portIf
hostis a valid hostname or Internet address, andportis an integer port number or service name, bash attempts to open the corresponding TCP socket.- /dev/udp/
host/portIf
hostis a valid hostname or Internet address, andportis an integer port number or service name, bash attempts to open the corresponding UDP socket.
A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
Note that the exec builtin command can make redirections take effect in the current shell.
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
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.
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
This construct allows both the standard output (file descriptor 1)
and the standard error output (file descriptor 2) to be redirected to
the file whose name is the expansion of word.
There are two formats for redirecting standard output and standard error:
&>
word
and
>&
word
Of the two forms, the first is preferred. This is semantically equivalent to
>
word2>&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.
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
>>
word2>&1
(see Duplicating File Descriptors below).
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.
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).
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.
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.
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 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, use a shell function (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 commands.
For almost every purpose, aliases are superseded by shell 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 (local variables).
Ordinarily, variables and their values are shared between the function
and its caller. If a variable is declared local, the
variable's visible scope is restricted to that function and its children
(including the functions it calls).
In the following description, the current scope is a
currently- executing function. Previous scopes consist of that
function's caller and so on, back to the "global" scope, where the shell
is not executing any shell function. Consequently, a local variable at
the current scope is a variable declared using the
local or declare builtins in the
function that is currently executing.
Local variables "shadow" variables with the same name declared at previous scopes. For instance, a local variable declared in a function hides 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.
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 (appearing as unset) until it is reset in that scope or until the function returns. Once the function returns, any instance of the variable at a previous scope 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 (see below how the localvar_unset shell option changes this behavior).
The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort.
If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. 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 child shell processes (those created when executing a separate shell invocation) 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.
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 --idvariable pre-increment and pre-decrement
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:exprconditional operator
assignment
expr1 , expr2comma
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.
Integer constants follow the C language definition, without suffixes
or character constants. Constants with a leading 0 are interpreted as
octal numbers. A leading 0x or 0X denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where the optional
base is a decimal number between 2 and 64 representing the
arithmetic base, and n is a number in that base. If
base# is omitted, then base 10 is used. When specifying
n, if a non-digit is required, the digits greater than 9 are
represented by the lowercase letters, the uppercase letters, @, and _,
in that order. If base is less than or equal to 36, lowercase
and uppercase letters may be used interchangeably to represent numbers
between 10 and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.
Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. The test and [ commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.
Expressions are formed from the 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.
fileTrue if file exists.
fileTrue if file exists and is a block special file.
fileTrue if file exists and is a character special file.
fileTrue if file exists and is a directory.
fileTrue if file exists.
fileTrue if file exists and is a regular file.
fileTrue if file exists and is set-group-id.
fileTrue if file exists and is a symbolic link.
fileTrue if file exists and its ``sticky'' bit is set.
fileTrue if file exists and is a named pipe (FIFO).
fileTrue if file exists and is readable.
fileTrue if file exists and has a size greater than zero.
fdTrue if file descriptor fd is open and refers to a
terminal.
fileTrue if file exists and its set-user-id bit is set.
fileTrue if file exists and is writable.
fileTrue if file exists and is executable.
fileTrue if file exists and is owned by the effective group
id.
fileTrue if file exists and is a symbolic link.
fileTrue if file exists and has been modified since it was last
read.
fileTrue if file exists and is owned by the effective user
id.
fileTrue if file exists and is a socket.
file1 -ef file2True if file1 and file2 refer to the same device
and inode numbers.
file1 -nt file2True if file1 is newer (according to modification date) than
file2, or if file1 exists and file2 does
not.
file1 -ot file2True if file1 is older than file2, or if
file2 exists and file1 does not.
optnameTrue if the shell option optname is enabled. See the list of
options under the description of the -o option to the
set builtin below.
varnameTrue if the shell variable varname is set (has been assigned
a value).
varnameTrue if the shell variable varname is set and is a name
reference.
stringTrue if the length of string is zero.
stringstringTrue if the length of string is non-zero.
string1 == string2string1 = string2True 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 != string2True if the strings are not equal.
string1 < string2True if string1 sorts before string2
lexicographically.
string1 > string2True if string1 sorts after string2
lexicographically.
arg1 OP arg2OP 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).
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right, in the following order.
The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.
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.
Redirections are performed as described above under
REDIRECTION.
The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.
If no command name results, the variable assignments affect the current shell environment. In the case of such a command (one that consists only of assignment statements and redirections), assignment statements are performed before redirections. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.
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, and the shell creates a new instance of itself 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.
The shell has an execution environment, which consists of
the following:
open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin
the current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation
the file creation mode mask as set by umask or inherited from the shell's parent
current traps set by trap
shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the environment
shell functions defined during execution or inherited from the shell's parent in the environment
options enabled at invocation (either by default or with command-line arguments) or by set
options enabled by shopt
shell aliases defined with alias
various process IDs, including those of background jobs, the value of $$, and the value of
PPID
When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
the shell's open files, plus any modifications and additions specified by redirections to the command
the current working directory
the file creation mode mask
shell variables and functions marked for export, along with variables exported for the command, passed in the environment
traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the shell's execution environment.
A subshell is a copy of the shell process.
Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline 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.
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.
The exit status of an executed command is the value returned by the
waitpid system call or equivalent function. Exit statuses fall
between 0 and 255, though, as explained below, the shell may use values
above 125 specially. Exit statuses from shell builtins and compound
commands are also limited to this range. Under certain circumstances,
the shell will use special values to indicate specific failure
modes.
For the shell's purposes, a command which exits with a zero exit
status has succeeded. An exit status of zero indicates success. A
non-zero exit status indicates failure. When a command terminates on a
fatal signal N, bash uses the value of
128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if
successful, and non-zero (false) if an error occurs while they
execute. All builtins return an exit status of 2 to indicate incorrect
usage, generally invalid options or missing arguments.
The exit status of the last command is available in the special parameter $?.
Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with a non-zero value. See also the exit builtin command below.
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.
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.
When job control is not enabled, and bash is waiting for a foreground command to complete, the shell receives keyboard-generated signals such as
SIGINT (usually generated by ^C) that users commonly intend to send to that command. This happens because the shell and the command are in the same process group as the terminal, and ^C sends
SIGINT to all processes in that process group.
When bash is running without job control enabled and receives
SIGINT while waiting for a foreground command, it waits until that foreground command terminates and then decides what to do about the
SIGINT:
If the command terminates due to the
SIGINT, bash concludes that the user meant to end the entire script, and acts on the
SIGINT (e.g., by running a
SIGINT trap or exiting itself);
If the command does not terminate due to
SIGINT, the program handled the
SIGINT itself and did not treat it as a fatal signal. In that case, bash does not treat
SIGINT as a fatal signal, either, instead assuming that the
SIGINT was used as part of the program's normal operation (e.g., emacs uses it to abort editing commands) or deliberately discarded. However, bash will run any trap set on
SIGINT, as it does with any other trapped signal it receives while it is waiting for the foreground command to complete, for compatibility.
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 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 job whose
command name begins with ce. 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 causes wait to wait until the job or process terminates before returning.
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 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
formatis passed tostrftime(3) and the result is inserted into the prompt string; an emptyformatresults 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 $0 (the portion following the final slash)
- \t
the current time in 24-hour HH:MM:SS format
- \T
the current time in 12-hour HH:MM:SS format
- \@
the current time in 12-hour am/pm format
- \A
the current time in 24-hour HH:MM format
- \u
the username of the current user
- \v
the version of bash (e.g., 2.00)
- \V
the release of bash, version + patch level (e.g., 2.00.0)
- \w
the value of the PWD shell variable ($PWD), with
$HOME abbreviated with a tilde (uses the value of the
PROMPT_DIRTRIM variable)
- \W
the basename of $PWD, with
$HOME abbreviated with a tilde
- \!
the history number of this command
- \#
the command number of this command
- \$
if the effective UID is 0, a #, otherwise a $
- \
nnnthe character corresponding to the octal number
nnn- \\
a backslash
- \[
begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
- \]
end a sequence of non-printing characters
The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see
HISTORY below), while the command number is the position in the sequence of commands executed during the current shell session. After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see the description of the shopt command under
SHELL BUILTIN COMMANDS below). This can have unwanted side effects if escaped portions of the string appear within command substitution or contain characters special to word expansion.
This is the library that handles reading input when using an interactive shell, unless the --noediting option is given at shell invocation. Line editing is also used when using the -e option to the read builtin. By default, the line editing commands are similar to those of Emacs. A vi-style line editing interface is also available. Line editing can be enabled at any time using the -o emacs or -o vi options to the set builtin (see
SHELL BUILTIN COMMANDS below). To turn off line editing after the shell is running, use the +o emacs or +o vi options to the set builtin.
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 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. If that file does not exist or cannot be
read, the ultimate default is /etc/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
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).
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 '
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
- \
nnnthe eight-bit character whose value is the octal value
nnn(one to three digits)- \x
HHthe 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 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-namevalue
or using the bind builtin command (see
SHELL BUILTIN COMMANDS below).
Except where noted, readline variables can take the values On or Off (without regard to case). Unrecognized variable names are ignored. When 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:
A string variable that controls the text color and background when
displaying the text in the active region (see the description of
enable-active-region below). This string must not take
up any physical character positions on the display, so it should consist
only of terminal escape sequences. It is output to the terminal before
displaying the text in the active region. This variable is reset to the
default value whenever the terminal type changes. The default value is
the string that puts the terminal in standout mode, as obtained from the
terminal's terminfo description. A sample value might be
"\e[01;33m".
A string variable that "undoes" the effects of
active-region-start-color and restores "normal"
terminal display appearance after displaying text in the active region.
This string must not take up any physical character positions on the
display, so it should consist only of terminal escape sequences. It is
output to the terminal after displaying the text in the active region.
This variable is reset to the default value whenever the terminal type
changes. The default value is the string that restores the terminal from
standout mode, as obtained from the terminal's terminfo description. A
sample value might be "\e[0m".
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.
If set to On, readline attempts to bind the control characters treated specially by the kernel's terminal driver to their readline equivalents.
If set to On, readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted.
If set to On, when listing completions, readline displays the common prefix of the set of possible completions using a different color. The color definitions are taken from the value of the LS_COLORS environment variable. If there is a color definition in $LS_COLORS for the custom suffix "readline-colored-completion-prefix", readline uses this color for the common prefix instead of its default.
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.
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.
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.
If set to On, readline performs filename matching and completion in a case-insensitive fashion.
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.
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.
This determines when the user is queried about viewing the number of possible completions generated by the possible-completions command. It may be set to any integer value greater than or equal to zero. If the number of possible completions is greater than or equal to the value of this variable, readline will ask whether or not the user wishes to view them; otherwise they are simply listed on the terminal. A zero value means readline should never ask; negative values are treated as zero.
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. This
variable is dependent on the LC_CTYPE locale category,
and may change if the locale is changed.
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.
When set to On, on operating systems that indicate they support it, readline echoes a character corresponding to a signal generated from the keyboard.
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.
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.
The point is the current cursor position, and mark
refers to a saved cursor position. The text between the point and mark
is referred to as the region. When this variable is set to
On, readline allows certain commands to designate the region as
active. When the region is active, readline highlights the text
in the region using the value of the
active-region-start-color, which defaults to the string
that enables the terminal's standout mode. The active region shows the
text inserted by bracketed-paste and any matching text found by
incremental and non-incremental history searches.
When set to On, readline configures the terminal to insert each paste into the editing buffer as a single string of characters, instead of treating each character as if it had been read from the keyboard. This prevents readline from executing any editing commands bound to key sequences appearing in the pasted text.
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.
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.
If set to On, tilde expansion is performed when readline attempts word completion.
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.
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.
When set to On, makes readline use a single line for display, scrolling the input horizontally on a single screen line when it becomes longer than the screen width rather than wrapping to a new line. This setting is automatically enabled for terminals of height 1.
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. This variable is dependent on the
LC_CTYPE locale category, and may change if the locale
is changed.
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.
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.
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.
If set to On, completed directory names have a slash appended.
If set to On, history lines that have been modified are displayed with a preceding asterisk (*).
If set to On, completed names which are symbolic links to directories have a slash appended (subject to the value of mark-directories).
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.
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.
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. This variable
is dependent on the LC_CTYPE locale category, and may
change if the locale is changed.
If set to On, readline uses an internal
more-like pager to display a screenful of possible completions
at a time.
If set to On, readline will display completions with matches sorted horizontally in alphabetical order, rather than down the screen.
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.
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.
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.
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).
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.
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.
If the show-mode-in-prompt variable is enabled, this string
is displayed immediately before the last line of the primary prompt when
vi editing mode is active and in insertion mode. The value is expanded
like a key binding, so the standard set of meta- and control prefixes
and backslash escape sequences is available. 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.
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 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.
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.
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.
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.
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.
The application construct is used to include
application-specific settings. Each program using the readline library
sets the application name, and an initialization file can test
for a particular value. This could be used to bind key sequences to
functions useful for a specific program. For instance, the following
command adds a key sequence that quotes the current or previous word in
bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endifThe 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.
This command, as seen in the previous example, terminates an $if command.
Commands in this branch of the $if directive are executed if the test fails.
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/inputrcReadline 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 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.
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.
Move to the start of the current line.
Move to the end of the line.
Move forward a character.
Move back a character.
Move forward to the end of the next word. Words are composed of alphanumeric characters (letters and digits).
Move back to the start of the current or previous word. Words are composed of alphanumeric characters (letters and digits).
Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.
Move back to the start of the current or previous word. Words are delimited by non-quoted shell metacharacters.
Attempt to move point to the same physical screen column on the previous physical screen line. This will not have the desired effect if the current readline line does not take up more than one physical line or if point is not greater than the length of the prompt plus the screen width.
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 the screen and, if possible, the terminal's scrollback buffer, then redraw the current line, leaving the current line at the top of the screen.
Clear the screen, then redraw the current line, leaving the current line at the top of the screen. With an argument, refresh the current line without clearing the screen.
Refresh the current line.
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.
Fetch the previous command from the history list, moving back in the list.
Fetch the next command from the history list, moving forward in the list.
Move to the first line in the history.
Move to the end of the input history, i.e., the line currently being entered.
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.
With a numeric argument, fetch that entry from the history list and make it the current line. Without an argument, move back to the first entry in the history list.
Search backward starting at the current line and moving `up' through the history as necessary. This is an incremental search.
Search forward starting at the current line and moving `down' through the history as necessary. This is an incremental search.
Search backward through the history starting at the current line using a non-incremental search for a string supplied by the user.
Search forward through the history using a non-incremental search for a string supplied by the user.
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.
Search backward through the history for the string of characters between the start of the current line and the point. This is a non-incremental search.
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.
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.
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.
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.
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.
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description of history expansion.
Perform history expansion on the current line and insert a space. See
HISTORY EXPANSION below for a description of history expansion.
Perform alias expansion on the current line. See
ALIASES above for a description of alias expansion.
Perform history and alias expansion on the current line.
A synonym for yank-last-arg.
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.
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 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.
Delete the character behind the cursor. When given a numeric argument, save the deleted text on the kill ring.
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.
Add the next character typed to the line verbatim. This is how to insert characters like C-q, for example.
Insert a tab character.
Insert the character typed.
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.
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.
Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move point.
Lowercase the current (or following) word. With a negative argument, lowercase the previous word, but do not move point.
Capitalize the current (or following) word. With a negative argument, capitalize the previous word, but do not move point.
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.
Kill the text from point to the end of the line.
Kill backward to the beginning of the line.
Kill backward from point to the beginning of the line. The killed text is saved on the kill-ring.
Kill all characters on the current line, no matter where point is.
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.
Kill the word behind point. Word boundaries are the same as those used by backward-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.
Kill the word behind point. Word boundaries are the same as those used by shell-backward-word.
Kill the word behind point, using white space as a word boundary. The killed text is saved on the kill-ring.
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 all spaces and tabs around point.
Kill the text in the current region.
Copy the text in the region to the kill buffer.
Copy the word before point to the kill buffer. The word boundaries are the same as backward-word.
Copy the word following point to the kill buffer. The word boundaries are the same as forward-word.
Yank the top of the kill ring into the buffer at point.
Rotate the kill ring, and yank the new top. Only works following yank or yank-pop.
Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative 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.
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.
List the possible completions of the text before point.
Insert all completions of the text before point that would have been generated by possible-completions.
Similar to complete, but replaces the word to be
completed with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the
list of possible completions, inserting each match in turn. At the end
of the list of completions, the bell is rung (subject to the setting of
bell-style) and the original text is restored. An
argument of n moves n positions forward in the list of
matches; a negative argument may be used to move backward through the
list. This command is intended to be bound to TAB, but
is unbound by default.
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.
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.
Attempt filename completion on the text before point.
List the possible completions of the text before point, treating it as a filename.
Attempt completion on the text before point, treating it as a username.
List the possible completions of the text before point, treating it as a username.
Attempt completion on the text before point, treating it as a shell variable.
List the possible completions of the text before point, treating it as a shell variable.
Attempt completion on the text before point, treating it as a hostname.
List the possible completions of the text before point, treating it as a hostname.
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.
List the possible completions of the text before point, treating it as a command name.
Attempt completion on the text before point, comparing the text against lines from the history list for possible completion matches.
Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.
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).
Begin saving the characters typed into the current keyboard macro.
Stop saving the characters typed into the current keyboard macro and store the definition.
Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.
Print the last keyboard macro defined in a format suitable for the
inputrc file.
Read in the contents of the inputrc file, and incorporate
any bindings or variable assignments found there.
Abort the current editing command and ring the terminal's bell (subject to the setting of bell-style).
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.
Metafy the next character typed.
ESC f is equivalent to Meta-f.
Incremental undo, separately remembered for each line.
Undo all changes made to this line. This is like executing the undo command enough times to return the line to its initial state.
Perform tilde expansion on the current word.
Set the mark to the point. If a numeric argument is supplied, the mark is set to that position.
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.
A character is read and point is moved to the next occurrence of that character. A negative argument searches for previous occurrences.
A character is read and point is moved to the previous occurrence of that character. A negative argument searches for subsequent occurrences.
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-[.
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.
Perform spelling correction on the current word, treating it as a directory or filename, in the same way as the cdspell shell option. Word boundaries are the same as those used by shell-forward-word.
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.
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.
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.
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.
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.
Print all of the readline key sequences bound to macros and the
strings they output. If a numeric argument is supplied, the output is
formatted in such a way that it can be made part of an inputrc
file.
Display version information about the current instance of bash.
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.
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 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
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 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.
The shell supports a history expansion feature that is similar to the history expansion in csh. This section describes what syntax features are available. This feature is enabled by default for interactive shells, and can be disabled using the +H option to the set builtin command (see
SHELL BUILTIN COMMANDS below). Non-interactive shells do not perform history expansion by default.
History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.
History expansion is performed immediately after a complete line is
read, before the shell breaks it into words, 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.
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 is enabled using the shopt builtin).
nRefer to command line n.
nRefer to the current command minus n.
Refer to the previous command. This is a synonym for `!-1'.
stringRefer to the most recent command preceding the current position in
the history list starting with string.
string[?]Refer to the most recent command preceding the current position in
the history list containing string. The trailing
? may be omitted if string is followed
immediately by a newline. If string is missing, the string from
the most recent search is used; it is an error if there is no previous
search string.
string1^string2^Quick substitution. Repeat the previous command, replacing
string1 with string2. Equivalent to
``!!:s^string1^string2^'' (see
Modifiers below).
The entire command line typed so far.
Word designators 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 ^, $, *, -, or %. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces.
The zeroth word. For the shell, this is the command word.
nThe nth word.
The first argument. That is, word 1.
The last word. This is usually the last argument, but will expand to the zeroth word if there is only one word in the line.
The first word matched by the most recent `?string?' search,
if the search string begins with a character that is part of a word.
x-yA range of words; `-y' abbreviates `0-y'.
All of the words but the zeroth. This is a synonym for
`1-$'. It is not an error to use * if there is
just one word in the event; the empty string is returned in that
case.
Abbreviates x-$.
Abbreviates x-$ like x*, but omits the last
word. If x is missing, it defaults to 0.
If a word designator is supplied without an event specification, the previous command is used as the event.
After the optional word designator, there may appear a sequence of one or more of the following modifiers, each preceded by a `:'. These modify, or edit, the word or words selected from the history event.
Remove a trailing filename component, leaving only the head.
Remove all leading filename components, leaving the tail.
Remove a trailing suffix of the form .xxx, leaving the
basename.
Remove all but the trailing suffix.
Print the new command but do not execute it.
Quote the substituted words, escaping further substitutions.
Quote the substituted words as with q, but break into words at blanks and newlines. The q and x modifiers are mutually exclusive; the last one supplied is used.
old/new/Substitute new for the first occurrence of old in
the event line. Any character may be used as the delimiter in place of
/. The final delimiter is optional if it is the last character of the
event line. 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. If
new is null, each matching old is deleted.
Repeat the previous substitution.
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.
Apply the following `s' or `&' modifier once to each word in the event line.
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]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, but filename does not need to be executable.
The file searched for in
PATH need not be executable. When
bash is not in posix mode, it searches the
current directory 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, . inherits any
trap on DEBUG; if it is not, any DEBUG
trap string is saved and restored around the call to .,
and . unsets the DEBUG trap while it
executes. If -T is not set, and the sourced file
changes the DEBUG trap, the new value is retained when
. 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.
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.
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.
keymap]
[-lpsvPSVX]keymap]
[-q function] [-u
function] [-r keyseq]keymap]
-f filenamekeymap]
-x keyseq:shell-commandkeymap]
keyseq:function-namekeymap]
keyseq:readline-commandreadline-command-lineDisplay 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 a readline initialization
file such as .inputrc, but each binding or command must be
passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.
Options, if supplied, have the following meanings:
- -m
keymapUse
keymapas the keymap to be affected by the subsequent bindings. Acceptablekeymapnames are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, andvi-insert.viis equivalent tovi-command(vi-moveis also a synonym);emacsis equivalent toemacs-standard.- -l
List the names of all readline functions.
- -p
Display readline function names and bindings in such a way that they can be re-read.
- -P
List current readline function names and bindings.
- -s
Display readline key sequences bound to macros and the strings they output in such a way that they can be re-read.
- -S
Display readline key sequences bound to macros and the strings they output.
- -v
Display readline variable names and values in such a way that they can be re-read.
- -V
List current readline variable names and values.
- -f
filenameRead key bindings from
filename.- -q
functionQuery about which keys invoke the named
function.- -u
functionUnbind all keys bound to the named
function.- -r
keyseqRemove any current binding for
keyseq.- -x
keyseq:shell-commandCause
shell-commandto be executed wheneverkeyseqis entered. Whenshell-commandis executed, the shell sets theREADLINE_LINE variable to the contents of the readline line buffer and the
READLINE_POINT and
READLINE_MARK variables to the current location of the insertion point and the saved insertion point (the mark), respectively. The shell assigns any numeric argument the user supplied to the
READLINE_ARGUMENT variable. If there was no argument, that variable is not set. If the executed command changes the value of any of
READLINE_LINE,
READLINE_POINT, or
READLINE_MARK, those new values will be reflected in the editing state.
- -X
List all key sequences bound to shell commands and the associated commands in a format that can be reused as input.
The return value is 0 unless an unrecognized option is given or an error occurred.
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.
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.
expr]Returns the context of any active subroutine call (a shell function
or a script executed with the . or
source builtins). Without expr,
caller displays the line number and source filename of
the current subroutine call. If a non-negative integer is supplied as
expr, caller displays the line number,
subroutine name, and source file corresponding to that position in the
current execution call stack. This extra information may be used, for
example, to print a stack trace. The current frame is frame 0. The
return value is 0 unless the shell is not executing a subroutine call or
expr does not correspond to a valid position in the call
stack.
dir]Change the current directory to dir. if dir is not
supplied, the value of the
HOME shell variable is the default. 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. If the directory change is successful, cd sets the value of the PWD environment variable to the new directory name, and sets the OLDPWD environment variable to the value of the current working directory before the change. The return value is true if the directory was successfully changed; false otherwise.
command
[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.
option] [word]Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete builtin with the exception of
-p and -r, and write the matches to
the standard output. When using the -F or
-C options, the various shell variables set by the
programmable completion facilities, while available, will not have
useful values.
The matches will be generated in the same way as if the 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.
comp-option] [-DEI]
[-A action] [-G
globpat] [-W wordlist]
[-F function] [-C
command] [-X filterpat]
[-P prefix] [-S
suffix] name [name ...]
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 initial non-assignment word on
the line, or after a command delimiter such as ; or
|, which is usually command name completion. If
multiple options are supplied, the -D option takes
precedence over -E, and both take precedence over
-I. If any of -D, -E,
or -I are supplied, any other name arguments
are ignored; these completions only apply to the case specified by the
option.
The process of applying these completion specifications when word completion is attempted is described
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-optionThe
comp-optioncontrols several aspects of the compspec's behavior beyond the simple generation of completions.comp-optionmay 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
actionThe
actionmay be one of the following to generate a list of possible completions:
- alias
Alias names. May also be specified as -a.
- arrayvar
Array variable names.
- binding
Readline key binding names.
- builtin
Names of shell builtin commands. May also be specified as -b.
- command
Command names. May also be specified as -c.
- directory
Directory names. May also be specified as -d.
- disabled
Names of disabled shell builtins.
- enabled
Names of enabled shell builtins.
- export
Names of exported shell variables. May also be specified as -e.
- file
File names. May also be specified as -f.
- function
Names of shell functions.
- group
Group names. May also be specified as -g.
- helptopic
Help topics as accepted by the help builtin.
- hostname
Hostnames, as taken from the file specified by the
HOSTFILE shell variable.
- job
Job names, if job control is active. May also be specified as -j.
- keyword
Shell reserved words. May also be specified as -k.
- running
Names of running jobs, if job control is active.
- service
Service names. May also be specified as -s.
- setopt
Valid arguments for the -o option to the set builtin.
- shopt
Shell option names as accepted by the shopt builtin.
- signal
Signal names.
- stopped
Names of stopped jobs, if job control is active.
- user
User names. May also be specified as -u.
- variable
Names of all shell variables. May also be specified as -v.
- -C
command
commandis executed in a subshell environment, and its output is used as the possible completions. Arguments are passed as with the -F option.- -F
functionThe shell function
functionis 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 theCOMPREPLY array variable.
- -G
globpatThe pathname expansion pattern
globpatis expanded to generate the possible completions.- -P
prefix
prefixis added at the beginning of each possible completion after all other options have been applied.- -S
suffix
suffixis appended to each possible completion after all other options have been applied.- -W
wordlistThe
wordlistis split using the characters in theIFS 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 ofIFS. The possible completions are the members of the resultant list which match the word being completed.
- -X
filterpat
filterpatis 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 matchingfilterpatis removed from the list. A leading ! infilterpatnegates the pattern; in this case, any completion not matchingfilterpatis removed.The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a
nameargument, an attempt is made to remove a completion specification for anamefor which no specification exists, or an error occurs adding a completion specification.
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
initial 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.
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.
name[=value] ...]name[=value] ...]Declare variables and/or give them attributes. If no names
are given then display the values of variables. The -p
option will display the attributes and values of each name.
When -p is used with name arguments,
additional options, other than -f and
-F, are ignored. When -p is supplied
without name arguments, it will display the attributes and
values of all variables having the attributes specified by the
additional options. If no other options are supplied with
-p, declare will display the
attributes and values of all shell variables. The -f
option will restrict the display to shell functions. The
-F option inhibits the display of function definitions;
only the function name and attributes are printed. If the
extdebug shell option is enabled using
shopt, the source file name and line number where 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 -I option causes local
variables to inherit the attributes (except the nameref
attribute) and value of any existing variable with the same
name at a surrounding scope. If there is no existing variable,
the local variable is initially unset. The following options can be used
to restrict output to variables with the specified attribute or to give
variables attributes:
- -a
Each
nameis an indexed array variable (see Arrays- -A
Each
nameis an associative array variable (see Arrays- -f
Use function names only.
- -i
The variable is treated as an integer; arithmetic evaluation (see
ARITHMETIC EVALUATION 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
namethenamerefattribute, making it a name reference to another variable. That other variable is defined by the value ofname. All references, assignments, and attribute modifications toname, except those using or changing the -n attribute itself, are performed on the variable referenced byname'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
namethetraceattribute. 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
namelocal, 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 tovalue. When using -a or -A and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments. The return value is 0 unless an invalid option is encountered, an attempt is made to define a function using ``-f foo=bar'', an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Arrays one of thenamesis 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.
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.
Clears the directory stack by deleting all of the entries.
Produces a listing using full pathnames; the default listing format uses a tilde to denote the home directory.
Print the directory stack with one entry per line.
Print the directory stack with one entry per line, prefixing each entry with its index in the stack.
nDisplays the nth entry counting from the left of the list
shown by dirs when invoked without options, starting
with zero.
nDisplays 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.
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.
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 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:
alert (bell)
backspace
suppress further output
an escape character
form feed
new line
carriage return
horizontal tab
vertical tab
backslash
nnnthe eight-bit character whose value is the octal value nnn
(zero to three octal digits)
HHthe eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
HHHHthe Unicode (ISO/IEC 10646) character whose value is the hexadecimal
value HHHH (one to four hex digits)
HHHHHHHHthe Unicode (ISO/IEC 10646) character whose value is the hexadecimal
value HHHHHHHH (one to eight hex digits)
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. Bash will use the value of the
BASH_LOADABLES_PATH variable as a colon-separated list
of directories in which to search for filename. The default is
system-dependent. 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. If no
options are supplied and a name is not a shell builtin,
enable will attempt to load name from a shared
object named name, as if the command were ``enable -f name
name . The return value is 0 unless a name is not a shell
builtin or there is an error loading a new builtin from a shared
object.
arg ...]The args are read and concatenated together into a single
command. This command is then read and executed by the shell, and its
exit status is returned as the value of eval. If there
are no args, or only null arguments, eval
returns 0.
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.
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.
name[=word]] ...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.
ename]
[-lnr] [first] [last]pat=rep]
[cmd]The first form selects a range of commands from first to
last from the history list and displays or edits and
re-executes them. First and last may be specified as a
string (to locate the last command beginning with that string) or as a
number (an index into the history list, where a negative number is used
as an offset from the current command number). When listing, a
first or last of 0 is equivalent to -1 and -0 is
equivalent to the current command (usually the fc
command); otherwise 0 is equivalent to -1 and -0 is invalid. If
last is not specified, it is set to the current command for
listing (so that ``fc -l -10'' prints the last 10 commands) and to
first otherwise. If first is not specified, it is set
to the previous command for editing and -16 for listing.
The -n option suppresses the command numbers when
listing. The -r option reverses the order of the
commands. If the -l option is given, the commands are
listed on standard output. 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.
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.
optstring name [arg
...]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 the first non-option
argument, and name is set to ?.
getopts normally parses the positional parameters,
but if more arguments are supplied as arg values,
getopts parses those instead.
getopts can report errors in two ways. If the first
character of optstring is a colon, 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.
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.
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.
Display a short description of each pattern
Display the description of each pattern in a manpage-like
format
Display only a short usage synopsis for each pattern
The return status is 0 unless no command matches
pattern.
n]offsetstart-endfilename]arg [arg
...]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
offsetDelete the history entry at position
offset. Ifoffsetis 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-endDelete the range of history entries between positions
startandend, inclusive. Positive and negative values forstartandendare 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
argsand display the result on the standard output. Does not store the results in the history list. Eachargmust be quoted to disable normal history expansion.- -s
Store the
argsin the history list as a single entry. The last command in the history list is removed before theargsare 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
offsetor range is supplied as an argument to -d, or the history expansion supplied as an argument to -p fails.
jobspec ...
]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
jobspecis given, output is restricted to information about that job. The return status is 0 unless an invalid option is encountered or an invalidjobspecis supplied.If the -x option is supplied, jobs replaces any
jobspecfound incommandorargswith the corresponding process group ID, and executescommandpassing itargs, returning its exit status.
sigspec | -n
signum | -sigspec]
[pid | jobspec]
...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.
arg [arg
...]Each arg is an arithmetic expression to be evaluated
(see
ARITHMETIC EVALUATION If the last arg
evaluates to 0, let returns 1; 0 is returned
otherwise.
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.
The restore is effected as if a series of set commands were
executed to restore the values that were in place before the
function. With no operands, local writes a list of
local variables to the standard output. 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.
Exit a login shell.
delim] [-n
count] [-O origin]
[-s count] [-t] [-u
fd] [-C callback]
[-c quantum]
[array]delim] [-n
count] [-O origin]
[-s count] [-t] [-u
fd] [-C callback]
[-c quantum]
[array]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
delimis used to terminate each input line, rather than newline. Ifdelimis the empty string, mapfile will terminate a line when it reads a NUL character.- -n
Copy at most
countlines. Ifcountis 0, all lines are copied.- -O
Begin assigning to
arrayat indexorigin. The default index is 0.- -s
Discard the first
countlines read.- -t
Remove a trailing
delim(default newline) from each line read.- -u
Read lines from file descriptor
fdinstead of the standard input.- -C
Evaluate
callbackeach timequantumlines are read. The -c option specifiesquantum.- -c
Specify the number of lines read between each call to
callback.If -C is specified without -c, the default quantum is 5000. When
callbackis 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.callbackis evaluated after the line is read but before the array element is assigned.If not supplied with an explicit origin, mapfile will clear
arraybefore assigning to it.mapfile returns successfully unless an invalid option or option argument is supplied,
arrayis invalid or unassignable, or ifarrayis not an indexed array.
n]
[-n]Removes entries from the directory stack. The elements are numbered from 0 starting at the first directory listed by dirs. With no arguments, popd removes the top directory from the stack, and changes to the new top directory. Arguments, if supplied, have the following meanings:
Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated.
nRemoves the nth entry counting from the left of the
list shown by dirs, starting with zero, from
the stack. For example: ``popd +0'' removes the first directory, ``popd
+1'' the second.
nRemoves the nth entry counting from the right of the
list shown by dirs, starting with zero. For
example: ``popd -0'' removes the last directory, ``popd -1'' the next to
last.
If the top element of the directory stack is modified, and the
-n option was not supplied, popd uses the cd
builtin to change to the directory at the top of the stack. If the
cd fails, popd returns a non-zero value.
Otherwise, popd returns false if an invalid option is encountered, the directory stack is empty, or a non-existent directory stack entry is specified.
If the popd command is successful, bash runs dirs to show the final contents of the directory stack, and the return status is 0.
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:
causes printf to expand backslash escape sequences in the
corresponding argument in the same way as echo
-e.
causes printf to output the corresponding
argument in a format that can be reused as shell
input.
like %q, but applies any supplied precision to the
argument before quoting it.
datefmt)Tcauses 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.
The %b, %q, and %T directives all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original.
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.
n]
[-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 elements of the directory stack. 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.
- +
nRotates the stack so that the
nth directory (counting from the left of the list shown by dirs, starting with zero) is at the top.- -
nRotates the stack so that the
nth directory (counting from the right of the list shown by dirs, starting with zero) is at the top.dirAdds
dirto the directory stack at the topAfter the stack has been modified, if the -n option was not supplied, pushd uses the cd builtin to change to the directory at the top of the stack. If the cd fails, pushd returns a non-zero value.
Otherwise, if no arguments are supplied, pushd returns 0 unless the directory stack is empty. When rotating the directory stack, pushd returns 0 unless the directory stack is empty or a non-existent directory stack element is specified.
If the pushd command is successful, bash runs dirs to show the final contents of the directory stack.
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.
aname]
[-d delim] [-i text]
[-n nchars] [-N
nchars] [-p prompt]
[-t timeout] [-u fd]
[name ...]One line is read from the standard input, or from the file descriptor
fd supplied as an argument to the -u option,
split into words as described 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 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
anameThe words are assigned to sequential indices of the array variable
aname, starting at 0.anameis unset before any new values are assigned. Othernamearguments are ignored.- -d
delimThe first character of
delimis used to terminate the input line, rather than newline. Ifdelimis 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 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
textIf readline is being used to read the line,
textis placed into the editing buffer before editing begins.- -n
ncharsread returns after reading
ncharscharacters rather than waiting for a complete line of input, but honors a delimiter if fewer thanncharscharacters are read before the delimiter.- -N
ncharsread returns after reading exactly
ncharscharacters 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 untilncharscharacters 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
promptDisplay
prompton 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.
- -s
Silent mode. If input is coming from a terminal, characters are not echoed.
- -t
timeoutCause read to time out and return failure if a complete line of input (or a specified number of characters) is not read within
timeoutseconds.timeoutmay 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 variablename. Iftimeoutis 0, read returns immediately, without trying to read any data. The exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise. The exit status is greater than 128 if the timeout is exceeded.- -u
fdRead input from file descriptor
fd.If no
namesare supplied, the line read, without the ending delimiter but otherwise unmodified, is assigned to the variableREPLY. 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.
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.
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 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.
option-name] [--] [-] [arg
...]option-name] [--] [-] [arg
...]Without options, display the name and value of each shell variable in
a format that can be reused as input for setting or resetting the
currently-set variables. Read-only variables cannot be reset. In
posix mode, only shell variables are listed.
The output is sorted according to the current locale. When options are
specified, they set or unset shell attributes. Any arguments remaining
after option processing are treated as values for the positional
parameters and are assigned, in order, to $1,
$2, ... $n.
Options, if specified, have the following meanings:
- -a
Each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands.
- -b
Report the status of terminated background jobs immediately, rather than before the next primary prompt. This is effective only when job control is enabled.
- -e
Exit immediately if a
pipeline(which may consist of a singlesimple command), alist, or acompound command(seeSHELL GRAMMAR 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 and may cause subshells to exit before executing all the commands in the subshell.
If a compound command or shell function executes in a context where -e is being ignored, none of the commands executed within the compound command or function body will be affected by the -e setting, even if -e is set and a command returns a failure status. If a compound command or shell function sets -e while executing in a context where -e is ignored, that setting will not have any effect until the compound command or the command containing the function call completes.
- -f
Disable pathname expansion.
- -h
Remember the location of commands as they are looked up for execution. This is enabled by default.
- -k
All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.
- -m
Monitor mode. Job control is enabled. This option is on by default for interactive shells on systems that support it (see
JOB CONTROL 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-nameThe
option-namecan 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 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
- keyword
Same as -k.
- monitor
Same as -m.
- noclobber
Same as -C.
- noexec
Same as -n.
- noglob
Same as -f.
- nolog
Currently ignored.
- notify
Same as -b.
- nounset
Same as -u.
- onecmd
Same as -t.
- physical
Same as -P.
- pipefail
If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.
- posix
Change the behavior of bash where the default operation differs from the POSIX standard to match the standard (
posix mode). SeeSEE ALSO 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 nooption-name, a series of set commands to recreate the current option settings is displayed on the standard output.- -p
Turn on
privilegedmode. 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.
- -r
Enable restricted shell mode. This option cannot be unset once it has been set.
- -t
Exit after reading and executing one command.
- -u
Treat unset variables and parameters other than the special parameters "@" and "*", or array variables subscripted with "@" or "*", as an error when performing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status.
- -v
Print shell input lines as they are read.
- -x
After expanding each
simple command, for command, case command, select command, or arithmetic for command, display the expanded value ofPS4, followed by the command and its expanded arguments or associated word list.
- -B
The shell performs brace expansion (see Brace Expansion 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 noargs, 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.
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.
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:
Enable (set) each optname.
Disable (unset) each optname.
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.
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:
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.
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.
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.
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.
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.
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 The shell always postpones exiting if any jobs are stopped.
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.
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 under
HISTORY.
These control aspects of the shell's compatibility mode (see
SHELL COMPATIBILITY MODE
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.
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.
If set, bash attempts spelling correction on directory names during word completion if the directory name initially supplied does not exist.
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.
If set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command. An interactive shell does not exit if exec fails.
If set, aliases are expanded as described under
ALIASES. This option is enabled by default for interactive shells.
If set at shell invocation, or in a shell startup file, arrange to execute the debugger profile before the shell starts, identical to the --debugger option. If set after invocation, behavior intended for use by debuggers is enabled:
The -F option to the declare builtin displays the source file name and line number corresponding to each function name supplied as an argument.
If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.
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.
BASH_ARGC and
BASH_ARGV are updated as described in their descriptions
Function tracing is enabled: command substitution, shell functions,
and subshells invoked with ( command )
inherit the DEBUG and RETURN traps.
Error tracing is enabled: command substitution, shell functions, and
subshells invoked with ( command )
inherit the ERR trap.
If set, the extended pattern matching features described under Pathname Expansion are enabled.
If set, $'string' and
$"string" quoting is performed within
${parameter} expansions
enclosed in double quotes. This option is enabled by default.
If set, patterns which fail to match filenames during pathname expansion result in an expansion error.
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 for a description of
FIGNORE. This option is enabled by default.
If set, range expressions used in pattern matching bracket expressions (see
Pattern Matching behave as if in the traditional C locale when performing comparisons. That is, the current locale's collating 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.
If set, pathname expansion will never match the filenames ``.'' and ``..'', even if the pattern begins with a ``.''. This option is enabled by default.
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.
If set, shell error messages are written in the standard GNU error message format.
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.
If set, and readline is being used, a user is given the opportunity to re-edit a failed history substitution.
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.
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 This is enabled by default.
If set, bash will send
SIGHUP to all jobs when an interactive login shell exits.
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.
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 This option is enabled by default.
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.
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.
If set, local variables inherit the value and attributes of a variable of the same name that exists at a previous scope before any new value is assigned. The nameref attribute is not inherited.
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.
The shell sets this option if it is started as a login shell (see
INVOCATION The value may not be changed.
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.
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.
If set, bash matches filenames in a case-insensitive fashion when performing pathname expansion (see Pathname Expansion
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.
If set, bash encloses the translated results of $"..." quoting in single quotes instead of double quotes. If the string is not translated, this has no effect.
If set, bash allows patterns which match no files (see Pathname Expansion to expand to a null string, rather than themselves.
If set, bash expands occurrences of & in the replacement string of pattern substitution to the text matched by the pattern, as described under Parameter Expansion This option is enabled by default.
If set, the programmable completion facilities (see Programmable Completion are enabled. This option is enabled by default.
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.
If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described in
PROMPTING This option is enabled by default.
The shell sets this option if it is started in restricted mode (see
RESTRICTED SHELL 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.
If set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters.
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.
If set, the shell automatically closes file descriptors assigned
using the {varname} redirection syntax
(see
REDIRECTION instead of leaving them open when the command completes.
If set, the echo builtin expands backslash-escape sequences by default.
Suspend the execution of this shell until it receives a
SIGCONT signal. A login shell, or a shell without job control enabled, cannot be suspended; the -f option can be used to override this and force the suspension. The return status is 0 unless the shell is a login shell or job control is not enabled and -f is not supplied.
exprexpr ]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 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.
- !
exprTrue if
expris false.- (
expr)Returns the value of
expr. This may be used to override the normal precedence of operators.expr1-aexpr2True if both
expr1andexpr2are true.expr1-oexpr2True if either
expr1orexpr2is true.test and [ evaluate conditional expressions using a set of rules based on the number of arguments.
- 0 arguments
The expression is false.
- 1 argument
The expression is true if and only if the argument is not null.
- 2 arguments
If the first argument is !, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators listed 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 under
CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test using the first and third arguments as operands. The -a and -o operators are considered binary operators when there are three arguments. If the first argument is !, the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly ( and the third argument is exactly ), the result is the one-argument test of the second argument. Otherwise, the expression is false.
- 4 arguments
The following conditions are applied in the order listed. If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. the two-argument test using the second and third arguments. If the first argument is exactly ( and the fourth argument is exactly ), the result is the two-argument test of the second and third arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.
- 5 or more arguments
The expression is parsed and evaluated according to precedence using the rules listed above.
When used with test or [, the < and > operators sort lexicographically using ASCII ordering.
Print the accumulated user and system times for the shell and for processes run from the shell. The return status is 0.
arg]
sigspec ...]The command arg is to be read and executed when the shell
receives signal(s) sigspec. If arg is absent (and
there is a single sigspec) or
-, each specified signal is reset to its original
disposition (the value it had upon entrance to the shell). If
arg is the null string the signal specified by each
sigspec is ignored by the shell and by the commands it invokes.
If arg is not present and -p has been
supplied, then the trap commands associated with each sigspec
are displayed. If no arguments are supplied or if only
-p is given, trap prints the list of
commands associated with each signal. The -l option
causes the shell to print a list of signal names and their corresponding
numbers. Each sigspec is either a signal name defined in
<signal.h>, 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 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.
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 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.
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, if appropriate, are
printed before the value. Other options are interpreted as follows:
- -a
All current limits are reported; no limits are set
- -b
The maximum socket buffer size
- -c
The maximum size of core files created
- -d
The maximum size of a process's data segment
- -e
The maximum scheduling priority ("nice")
- -f
The maximum size of files written by the shell and its children
- -i
The maximum number of pending signals
- -k
The maximum number of kqueues that may be allocated
- -l
The maximum size that may be locked into memory
- -m
The maximum resident set size (many systems do not honor this limit)
- -n
The maximum number of open file descriptors (most systems do not allow this value to be set)
- -p
The pipe size in 512-byte blocks (this may not be set)
- -q
The maximum number of bytes in POSIX message queues
- -r
The maximum real-time scheduling priority
- -s
The maximum stack size
- -t
The maximum amount of cpu time in seconds
- -u
The maximum number of processes available to a single user
- -v
The maximum amount of virtual memory available to the shell and, on some systems, to its children
- -x
The maximum number of file locks
- -P
The maximum number of pseudoterminals
- -R
The maximum time a real-time process can run before blocking, in microseconds
- -T
The maximum number of threads
If
limitis given, and the -a option is not used,limitis 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; -R, which is in microseconds; -p, which is in units of 512-byte blocks; -P, -T, -b, -k, -n, and -u, which are unscaled values; and, when in posix mode, -c and -f, which are in 512-byte increments. The return status is 0 unless an invalid option or argument is supplied, or an error occurs while setting a new limit.
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.
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.
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, a function with that name, if any, is unset. Each unset variable
or function is removed from the environment passed to subsequent
commands. If any of
BASH_ALIASES,
BASH_ARGV0,
BASH_CMDS,
BASH_COMMAND,
BASH_SUBSHELL,
BASHPID,
COMP_WORDBREAKS,
DIRSTACK,
EPOCHREALTIME,
EPOCHSECONDS,
FUNCNAME,
GROUPS,
HISTCMD,
LINENO,
RANDOM,
SECONDS, or
SRANDOM are unset, they lose their special
properties, even if they are subsequently reset. The exit status is true
unless a name is readonly or may not be unset.
varname]
[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, wait waits for all running
background jobs and the last-executed process substitution, if
its process id is the same as $!, and the return status
is zero. If the -n option is supplied, wait
waits for a single job from the list of ids
or, if no ids are supplied, any job,
to complete and returns its exit status. If none of the supplied
arguments is a child of the shell, or if no arguments are supplied and
the shell has no unwaited-for children, the exit status is 127. If the
-p option is supplied, the process or job identifier of the
job for which the exit status is returned is assigned to the
variable varname named by the option argument.
The variable will be unset initially, before any assignment. This is
useful only when the -n option is supplied. Supplying
the -f option, when job control is enabled, forces
wait to wait for 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 127. If wait is interrupted by a signal,
the return status will be greater than 128, as described under
SIGNALS Otherwise, the return status is the exit status
of the last process or job waited for.
Bash-4.0 introduced the concept of a shell compatibility level, specified as a set of options to the shopt builtin ( compat31, compat32, compat40, compat41, and so on). There is only one current compatibility level -- each option is mutually exclusive. The compatibility level is intended to allow users to select behavior from previous versions that is incompatible with newer versions while they migrate scripts to use current features and behavior. It's intended to be a temporary solution.
This section does not mention behavior that is standard for a particular version (e.g., setting compat32 means that quoting the rhs of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2 and subsequent versions).
If a user enables, say, compat32, it may affect the behavior of other compatibility levels up to and including the current compatibility level. The idea is that each compatibility level controls behavior that changed in that version of bash, but that behavior may have been present in earlier versions. For instance, the change to use locale-based comparisons with the [[ command came in bash-4.1, and earlier versions used ASCII-based comparisons, so enabling compat32 will enable ASCII-based comparisons as well. That granularity may not be sufficient for all uses, and as a result users should employ compatibility levels carefully. Read the documentation for a particular feature to find out the current behavior.
Bash-4.3 introduced a new shell variable:
BASH_COMPAT. The value assigned to this variable (a
decimal version number like 4.2, or an integer corresponding to the
compatNN option, like 42) determines
the compatibility level.
Starting with bash-4.4, Bash has begun deprecating older compatibility levels. Eventually, the options will be removed in favor of
BASH_COMPAT.
Bash-5.0 is the final version for which there will be an individual shopt option for the previous version. Users should use
BASH_COMPAT on bash-5.0 and later versions.
The following table describes the behavior changes controlled by each
compatibility level setting. The compatNN
tag is used as shorthand for setting the compatibility
level to NN using one of the following
mechanisms. For versions prior to bash-5.0, the compatibility
level may be set using the corresponding
compatNN shopt option. For
bash-4.3 and later versions, the
BASH_COMPAT variable is preferred, and it is required for bash-5.1 and later versions.
quoting the rhs of the [[ command's regexp matching operator (=~) has no special effect
interrupting a command list such as "a ; b ; c" causes the execution of the next command in the list (in bash-4.0 and later versions, the shell acts as if it received the interrupt, so interrupting one command in a list aborts the execution of the entire list)
the < and > operators to the [[ command do
not consider the current locale when comparing strings; they
use ASCII ordering. Bash versions prior to bash-4.1 use ASCII collation
and strcmp(3); bash-4.1 and later use the current locale's
collation sequence and strcoll(3).
in posix mode, time may be followed by options
and still be recognized as a reserved word (this is POSIX
interpretation 267)
in posix mode, the parser requires that an even
number of single quotes occur in the word
portion of a double-quoted parameter expansion and
treats them specially, so that characters within the single quotes are
considered quoted (this is POSIX interpretation 221)
the replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2
in posix mode, single quotes are considered special when
expanding the word portion of a double-quoted parameter
expansion and can be used to quote a closing brace or other
special character (this is part of POSIX interpretation 221); in later
versions, single quotes are not special within double-quoted word
expansions
the shell does not print a warning message if an attempt is made to use a quoted compound assignment as an argument to declare (e.g., declare -a foo='(1 2)'). Later versions warn that this usage is deprecated
word expansion errors are considered non-fatal errors that cause the current command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit)
when executing a shell function, the loop state (while/until/etc.) is not reset, so break or continue in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this
the shell sets up the values used by
BASH_ARGV and
BASH_ARGC so they can expand to the shell's positional parameters even if extended debugging mode is not enabled
a subshell inherits loops from its parent context, so break or continue will cause the subshell to exit. Bash-5.0 and later reset the loop state to prevent the exit
variable assignments preceding builtins like export and readonly that set attributes continue to affect variables with the same name in the calling environment even if the shell is not in posix mode
Bash-5.1 changed the way
$RANDOM is generated to introduce slightly more randomness. If the shell compatibility level is set to 50 or lower, it reverts to the method from bash-5.0 and previous versions, so seeding the random number generator by assigning a value to
RANDOM will produce the same sequence as in bash-5.0
If the command hash table is empty, bash versions prior to bash-5.1 printed an informational message to that effect, even when producing output that can be reused as input. Bash-5.1 suppresses that message when the -l option is supplied.
The unset builtin treats attempts to unset array subscripts @ and * differently depending on whether the array is indexed or associative, and differently than in previous versions.
If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. It behaves identically to bash with the exception that the following are disallowed or not performed:
changing directories with cd
setting or unsetting the values of
SHELL,
PATH,
HISTFILE,
ENV, or
BASH_ENV
specifying command names containing /
specifying a filename containing a / as an argument to the . builtin command
specifying a filename containing a slash as an argument to the history builtin command
specifying a filename containing a slash as an argument to the -p option to the hash builtin command
importing function definitions from the shell environment at startup
parsing the 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 shopt -u restricted_shell.
These restrictions are enforced after any startup files are read.
rbash turns off any restrictions in the shell spawned to execute the script.
Bash Reference Manual, Brian Fox and Chet
RameyThe Gnu Readline Library, Brian Fox and Chet
RameyThe Gnu History Library, Brian Fox and Chet
Rameysh(1), ksh(1),
csh(1)emacs(1),
vi(1)readline(3) /bin/bash The bash executable
/etc/profile The systemwide initialization file, executed for login shells
/etc/bash.bashrc The systemwide per-interactive-shell startup file
/etc/bash.bash.logout The systemwide login shell cleanup file, executed when a login shell exits
~/.bash_profile The personal initialization file, executed for login shells
~/.bashrc The individual per-interactive-shell startup file
~/.bash_logout The individual login shell cleanup file, executed when a login shell exits
~/.bash_history The default value of HISTFILE, the file in which bash saves the command history
~/.inputrc Individual readline initialization file
Brian Fox, Free Software Foundation
bfox@gnu.org
Chet Ramey, Case Western Reserve University
chet.ramey@case.edu
If you find a bug in bash, you should report it. But
first, you should make sure that it really is a bug, and that it appears
in the latest version of bash. The latest version is
always available from ftp://ftp.gnu.org/pub/gnu/bash/
and
http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz.
Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report. If you have a fix, you
are encouraged to mail that as well! 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:
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.
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.