Linux Bash编程man帮助手册
Linux Bash编程man帮助手册
BASH(1) General Commands Manual BASH(1)
NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [command_string | file]
COPYRIGHT
Bash is Copyright (C) 1989-2020 by the Free Software Foundation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that executes commands read
from the standard input or from a file. Bash also incorporates useful features from
the Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the Shell and Utilities portion
of the IEEE POSIX specification (IEEE Standard 1003.1). Bash can be configured to be
POSIX-conformant by default.
OPTIONS
All of the single-character shell options documented in the description of the set
builtin command, including -o, can be used as options when the shell is invoked. In
addition, bash interprets the following options when it is invoked:
-c If the -c option is present, then commands are read from the first non-op‐
tion argument command_string. If there are arguments after the com‐
mand_string, the first argument is assigned to $0 and any remaining argu‐
ments are assigned to the positional parameters. The assignment to $0 sets
the name of the shell, which is used in warning and error messages.
-i If the -i option is present, the shell is interactive.
-l Make bash act as if it had been invoked as a login shell (see INVOCATION
below).
-r If the -r option is present, the shell becomes restricted (see RESTRICTED
SHELL below).
-s If the -s option is present, or if no arguments remain after option pro‐
cessing, then commands are read from the standard input. This option al‐
lows the positional parameters to be set when invoking an interactive shell
or when reading input through a pipe.
-D A list of all double-quoted strings preceded by $ is printed on the stan‐
dard output. These are the strings that are subject to language transla‐
tion when the current locale is not C or POSIX. This implies the -n op‐
tion; no commands will be executed.
[-+]O [shopt_option]
shopt_option is one of the shell options accepted by the shopt builtin (see
SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets the
value of that option; +O unsets it. If shopt_option is not supplied, the
names and values of the shell options accepted by shopt are printed on the
standard output. If the invocation option is +O, the output is displayed
in a format that may be reused as input.
-- A -- signals the end of options and disables further option processing.
Any arguments after the -- are treated as filenames and arguments. An ar‐
gument of - is equivalent to --.
Bash also interprets a number of multi-character options. These options must appear
on the command line before the single-character options to be recognized.
--debugger
Arrange for the debugger profile to be executed before the shell starts.
Turns on extended debugging mode (see the description of the extdebug option
to the shopt builtin below).
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext po (portable object)
file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit successfully.
--init-file file
--rcfile file
Execute commands from file instead of the standard personal initialization
file ~/.bashrc if the shell is interactive (see INVOCATION below).
--login
Equivalent to -l.
--noediting
Do not use the GNU readline library to read command lines when the shell is
interactive.
--noprofile
Do not read either the system-wide startup file /etc/profile or any of the
personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile.
By default, bash reads these files when it is invoked as a login shell (see
INVOCATION below).
--norc Do not read and execute the personal initialization file ~/.bashrc if the
shell is interactive. This option is on by default if the shell is invoked as
sh.
--posix
Change the behavior of bash where the default operation differs from the POSIX
standard to match the standard (posix mode). See SEE ALSO below for a refer‐
ence to a document that details how posix mode affects bash's behavior.
--restricted
The shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires
Produce the list of files that are required for the shell script to run. This
implies '-n' and is subject to the same limitations as compile time error
checking checking; Command substitutions, Conditional expressions and eval
builtin are not parsed so some dependencies may be missed.
--verbose
Equivalent to -v.
--version
Show version information for this instance of bash on the standard output and
exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither the -c nor the -s option has
been supplied, the first argument is assumed to be the name of a file containing
shell commands. If bash is invoked in this fashion, $0 is set to the name of the
file, and the positional parameters are set to the remaining arguments. Bash reads
and executes commands from this file, then exits. Bash's exit status is the exit
status of the last command executed in the script. If no commands are executed, the
exit status is 0. An attempt is first made to open the file in the current direc‐
tory, and, if no file is found, then the shell searches the directories in PATH for
the script.
INVOCATION
A login shell is one whose first character of argument zero is a -, or one started
with the --login option.
An interactive shell is one started without non-option arguments (unless -s is speci‐
fied) and without the -c option whose standard input and error are both connected to
terminals (as determined by isatty(3)), or one started with the -i option. PS1 is
set and $- includes i if bash is interactive, allowing a shell script or a startup
file to test this state.
The following paragraphs describe how bash executes its startup files. If any of the
files exist but cannot be read, bash reports an error. Tildes are expanded in file‐
names as described below under Tilde Expansion in the EXPANSION section.
When bash is invoked as an interactive login shell, or as a non-interactive shell
with the --login option, it first reads and executes commands from the file /etc/pro‐
file, if that file exists. After reading that file, it looks for ~/.bash_profile,
~/.bash_login, and ~/.profile, in that order, and reads and executes commands from
the first one that exists and is readable. The --noprofile option may be used when
the shell is started to inhibit this behavior.
When an interactive login shell exits, or a non-interactive login shell executes the
exit builtin command, bash reads and executes commands from the files ~/.bash_logout
and /etc/bash.bash_logout, if the files exists.
When an interactive shell that is not a login shell is started, bash reads and exe‐
cutes commands from ~/.bashrc, if that file exists. This may be inhibited by using
the --norc option. The --rcfile file option will force bash to read and execute com‐
mands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell script, for example, it looks
for the variable BASH_ENV in the environment, expands its value if it appears there,
and uses the expanded value as the name of a file to read and execute. Bash behaves
as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the filename.
If bash is invoked with the name sh, it tries to mimic the startup behavior of his‐
torical 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 fol‐
lows the POSIX standard for startup files. In this mode, interactive shells expand
the ENV variable and commands are read and executed from the file whose name is the
expanded value. No other startup files are read.
Bash attempts to determine when it is being run with its standard input connected to
a network connection, as when executed by the remote shell daemon, usually rshd, or
the secure shell daemon sshd. If bash determines it is being run in this fashion, it
reads and executes commands from ~/.bashrc, if that file exists and is readable. It
will not do this if invoked as sh. The --norc option may be used to inhibit this be‐
havior, 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 func‐
tions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and
GLOBIGNORE variables, if they appear in the environment, are ignored, and the effec‐
tive user id is set to the real user id. If the -p option is supplied at invocation,
the startup behavior is the same, but the effective user id is not reset.
DEFINITIONS
The following definitions are used throughout the rest of this document.
blank A space or tab.
word A sequence of characters considered as a single unit by the shell. Also known
as a token.
name A word consisting only of alphanumeric characters and underscores, and begin‐
ning with an alphabetic character or an underscore. Also referred to as an
identifier.
metacharacter
A character that, when unquoted, separates words. One of the following:
| & ; ( ) < > space tab newline
control operator
A token that performs a control function. It is one of the following symbols:
|| & && ; ;; ;& ;;& ( ) | |& <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to the shell. The following
words are recognized as reserved when unquoted and either the first word of a command
(see SHELL GRAMMAR below), the third word of a case or select command (only in is
valid), or the third word of a for command (only in and do are valid):
! case coproc do done elif else esac fi for function if in select then until while
{ } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable assignments followed by blank-
separated words and redirections, and terminated by a control operator. The first
word specifies the command to be executed, and is passed as argument zero. The re‐
maining words are passed as arguments to the invoked command.
The return value of a simple command is its exit status, or 128+n if the command is
terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated by one of the control op‐
erators | or |&. The format for a pipeline is:
[time [-p]] [ ! ] command [ [|⎪|&] command2 ... ]
The standard output of command is connected via a pipe to the standard input of com‐
mand2. This connection is performed before any redirections specified by the command
(see REDIRECTION below). If |& is used, command's standard error, in addition to its
standard output, is connected to command2's standard input through the pipe; it is
shorthand for 2>&1 |. This implicit redirection of the standard error to the stan‐
dard output is performed after any redirections specified by the command.
The return status of a pipeline is the exit status of the last command, unless the
pipefail option is enabled. If pipefail is enabled, the pipeline's return status is
the value of the last (rightmost) command to exit with a non-zero status, or zero if
all commands exit successfully. If the reserved word ! precedes a pipeline, the
exit status of that pipeline is the logical negation of the exit status as described
above. The shell waits for all commands in the pipeline to terminate before return‐
ing 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 op‐
tion 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 chil‐
dren. The TIMEFORMAT variable may be used to specify the format of the time informa‐
tion.
Each command in a pipeline is executed as a separate process (i.e., in a subshell).
See COMMAND EXECUTION ENVIRONMENT for a description of a subshell environment. If
the lastpipe option is enabled using the shopt builtin (see the description of shopt
below), the last element of a pipeline may be run by the shell process.
Lists
A list is a sequence of one or more pipelines separated by one of the operators ;, &,
&&, or ||, and optionally terminated by one of ;, &, or <newline>.
Of these list operators, && and || have equal precedence, followed by ; and &, which
have equal precedence.
A sequence of one or more newlines may appear in a list instead of a semicolon to de‐
limit 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. Com‐
mands 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 exe‐
cuted.
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 associativ‐
ity. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero (suc‐
cess).
An OR list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero exit status. The
return status of AND and OR lists is the exit status of the last command executed in
the list.
Compound Commands
A compound command is one of the following. In most cases a list in a command's de‐
scription may be separated from the rest of the command by one or more newlines, and
may be followed by a newline in place of a semicolon.
(list) list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT
below). Variable assignments and builtin commands that affect the shell's en‐
vironment 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 termi‐
nated with a newline or semicolon. This is known as a group command. The re‐
turn 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 per‐
mitted 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. This is exactly equivalent to
let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional ex‐
pression expression. Expressions are composed of the primaries described be‐
low under CONDITIONAL EXPRESSIONS. Word splitting and pathname expansion are
not performed on the words between the [[ and ]]; tilde expansion, parameter
and variable expansion, arithmetic expansion, command substitution, process
substitution, and quote removal are performed. Conditional operators such as
-f must be unquoted to be recognized as primaries.
When used with [[, the < and > operators sort lexicographically using the cur‐
rent locale.
When the == and != operators are used, the string to the right of the operator
is considered a pattern and matched according to the rules described below un‐
der Pattern Matching, as if the extglob shell option were enabled. The = op‐
erator 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 pat‐
tern, 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 con‐
sidered a POSIX extended regular expression and matched accordingly (using the
POSIX regcomp and regexec interfaces usually described in regex(3)). The re‐
turn value is 0 if the string matches the pattern, and 1 otherwise. If the
regular expression is syntactically incorrect, the conditional expression's
return value is 2. If the nocasematch shell option is enabled, the match is
performed without regard to the case of alphabetic characters. Any part of
the pattern may be quoted to force the quoted portion to be matched as a
string. Bracket expressions in regular expressions must be treated carefully,
since normal quoting characters lose their meanings between brackets. If the
pattern is stored in a shell variable, quoting the variable expansion forces
the entire pattern to be matched as a string.
The pattern will match if it matches any part of the string. Anchor the pat‐
tern 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 subexpres‐
sion.
Expressions may be combined using the following operators, listed in decreas‐
ing order of precedence:
( expression )
Returns the value of expression. This may be used to override the nor‐
mal precedence of operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are true.
expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expres‐
sion1 is sufficient to determine the return value of the entire conditional
expression.
for name [ [ in [ word ... ] ] ; ] do list ; done
The list of words following in is expanded, generating a list of items. The
variable name is set to each element of this list in turn, and list is exe‐
cuted each time. If the in word is omitted, the for command executes list
once for each positional parameter that is set (see PARAMETERS below). The
return status is the exit status of the last command that executes. If the
expansion of the items following in results in an empty list, no commands are
executed, and the return status is 0.
for (( expr1 ; expr2 ; expr3 )) ; do list ; done
First, the arithmetic expression expr1 is evaluated according to the rules de‐
scribed below under ARITHMETIC EVALUATION. The arithmetic expression expr2 is
then evaluated repeatedly until it evaluates to zero. Each time expr2 evalu‐
ates to a non-zero value, list is executed and the arithmetic expression expr3
is evaluated. If any expression is omitted, it behaves as if it evaluates to
1. The return value is the exit status of the last command in list that is
executed, or false if any of the expressions is invalid.
select name [ in word ] ; do list ; done
The list of words following in is expanded, generating a list of items. The
set of expanded words is printed on the standard error, each preceded by a
number. If the in word is omitted, the positional parameters are printed (see
PARAMETERS below). The PS3 prompt is then displayed and a line read from the
standard input. If the line consists of a number corresponding to one of the
displayed words, then the value of name is set to that word. If the line is
empty, the words and prompt are displayed again. If EOF is read, the command
completes. Any other value read causes name to be set to null. The line read
is saved in the variable REPLY. The list is executed after each selection un‐
til a break command is executed. The exit status of select is the exit status
of the last command executed in list, or zero if no commands were executed.
case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
A case command first expands word, and tries to match it against each pattern
in turn, 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 re‐
moval. Each pattern examined is expanded using tilde expansion, parameter and
variable expansion, arithmetic expansion, command substitution, and process
substitution. If the nocasematch shell option is enabled, the match is per‐
formed 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. Other‐
wise, it is the exit status of the last command executed in list.
if list; then list; [ elif list; then list; ] ... [ else list; ] fi
The if list is executed. If its exit status is zero, the then list is exe‐
cuted. Otherwise, each elif list is executed in turn, and if its exit status
is zero, the corresponding then list is executed and the command completes.
Otherwise, the else list is executed, if present. The exit status is the exit
status of the last command executed, or zero if no condition tested true.
while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long as the last
command in the list list-1 returns an exit status of zero. The until command
is identical to the while command, except that the test is negated: list-2 is
executed as long as the last command in list-1 returns a non-zero exit status.
The exit status of the while and until commands is the exit status of the last
command executed in list-2, or zero if none was executed.
Coprocesses
A coprocess is a shell command preceded by the coproc reserved word. A coprocess is
executed asynchronously in a subshell, as if the command had been terminated with the
& control operator, with a two-way pipe established between the executing shell and
the coprocess.
The format for a coprocess is:
coproc [NAME] command [redirections]
This creates a coprocess named NAME. If NAME is not supplied, the default name is
COPROC. NAME must not be supplied if command is a simple command (see above); other‐
wise, it is interpreted as the first word of the simple command. When the coprocess
is executed, the shell creates an array variable (see Arrays below) named NAME in the
context of the executing shell. The standard output of command is connected via a
pipe to a file descriptor in the executing shell, and that file descriptor is as‐
signed 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 REDI‐
RECTION 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 sub‐
shells. The process ID of the shell spawned to execute the coprocess is available as
the value of the variable NAME_PID. The wait builtin command may be used to wait for
the coprocess to terminate.
Since the coprocess is created as an asynchronous command, the coproc command always
returns success. The return status of a coprocess is the exit status of command.
Shell Function Definitions
A shell function is an object that is called like a simple command and executes a
compound command with a new set of positional parameters. Shell functions are de‐
clared as follows:
fname () compound-command [redirection]
function fname [()] compound-command [redirection]
This defines a function named fname. The reserved word function is optional.
If the function reserved word is supplied, the parentheses are optional. The
body of the function is the compound command compound-command (see Compound
Commands above). That command is usually a list of commands between { and },
but may be any command listed under Compound Commands above, with one excep‐
tion: If the function reserved word is used, but the parentheses are not sup‐
plied, the braces are required. compound-command is executed whenever fname
is specified as the name of a simple command. When in posix mode, fname must
be a valid shell name and may not be the name of one of the POSIX special
builtins. In default mode, a function name can be any unquoted shell word
that does not contain $. Any redirections (see REDIRECTION below) specified
when a function is defined are performed when the function is executed. The
exit status of a function definition is zero unless a syntax error occurs or a
readonly function with the same name already exists. When executed, the exit
status of a function is the exit status of the last command executed in the
body. (See FUNCTIONS below.)
COMMENTS
In a non-interactive shell, or an interactive shell in which the interactive_comments
option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word be‐
ginning with # causes that word and all remaining characters on that line to be ig‐
nored. An interactive shell without the interactive_comments option enabled does not
allow comments. The interactive_comments option is on by default in interactive
shells.
QUOTING
Quoting is used to remove the special meaning of certain characters or words to the
shell. Quoting can be used to disable special treatment for special characters, to
prevent reserved words from being recognized as such, and to prevent parameter expan‐
sion.
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 . If a \
pair appears, and the backslash is not itself quoted, the \ is treated as a
line continuation (that is, it is removed from the input stream and effectively ig‐
nored).
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 en‐
abled, !. When the shell is in posix mode, the ! has no special meaning within dou‐
ble quotes, even when history expansion is enabled. The characters $ and ` retain
their special meaning within double quotes. The backslash retains its special mean‐
ing only when followed by one of the following characters: $, `, ", \, or .
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 PARAM‐
ETERS below).
Words of the form $' string' are treated specially. The word expands to string, with
backslash-escaped characters replaced as specified by the ANSI C standard. Backslash
escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\? question mark
\nnn the eight-bit character whose value is the octal value nnn (one to
three octal digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or
two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal
value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal
value HHHHHHHH (one to eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($"string") will cause the string to
be translated according to the current locale. The gettext infrastructure performs
the message catalog lookup and translation, using the LC_MESSAGES and TEXTDOMAIN
shell variables. If the current locale is C or POSIX, or if there are no transla‐
tions available, the dollar sign is ignored. If the string is translated and re‐
placed, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a name, a number, or one of
the special characters listed below under Special Parameters. A variable is a param‐
eter denoted by a name. A variable has a value and zero or more attributes. At‐
tributes 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 com‐
mand (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
$((...)) expansion is not used (see Arithmetic Expansion below). Word splitting is
not performed, with the exception of "$@" as explained below under Special Parame‐
ters. Pathname expansion is not performed. Assignment statements may also appear as
arguments to the alias, declare, typeset, export, readonly, and local builtin com‐
mands (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 assign‐
ment 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 ac‐
cept assignment statements (declaration commands). When += is applied to a variable
for which the integer attribute has been set, value is evaluated as an arithmetic ex‐
pression 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 ar‐
rays) 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 manipu‐
lated 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 $1. If the control variable in a for loop has
the nameref attribute, the list of words can be a list of shell variables, and a name
reference will be established for each word in the list, in turn, when the loop is
executed. Array variables cannot be given the nameref attribute. However, nameref
variables can reference array variables and subscripted array variables. Namerefs
can be unset using the -n option to the unset builtin. Otherwise, if unset is exe‐
cuted with the name of a nameref variable as an argument, the variable referenced by
the nameref variable will be unset.
Positional Parameters
A positional parameter is a parameter denoted by one or more digits, other than the
single digit 0. Positional parameters are assigned from the shell's arguments when
it is invoked, and may be reassigned using the set builtin command. Positional pa‐
rameters may not be assigned to with assignment statements. The positional parame‐
ters are temporarily replaced when a shell function is executed (see FUNCTIONS be‐
low).
When a positional parameter consisting of more than a single digit is expanded, it
must be enclosed in braces (see EXPANSION below).
Special Parameters
The shell treats several parameters specially. These parameters may only be refer‐
enced; assignment to them is not allowed.
* Expands to the positional parameters, starting from one. When the expansion
is not within double quotes, each positional parameter expands to a separate
word. In contexts where it is performed, those words are subject to further
word splitting and pathname expansion. When the expansion occurs within dou‐
ble quotes, it expands to a single word with the value of each parameter sepa‐
rated by the first character of the IFS special variable. That is, "$*" is
equivalent to "$1c$2c...", where c is the first character of the value of the
IFS variable. If IFS is unset, the parameters are separated by spaces. If
IFS is null, the parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from one. In contexts where
word splitting is performed, this expands each positional parameter to a sepa‐
rate word; if not within double quotes, these words are subject to word split‐
ting. In contexts where word splitting is not performed, this expands to a
single word with each positional parameter separated by a space. When the ex‐
pansion occurs within double quotes, each parameter expands to a separate
word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted ex‐
pansion occurs within a word, the expansion of the first parameter is joined
with the beginning part of the original word, and the expansion of the last
parameter is joined with the last part of the original word. When there are
no positional parameters, "$@" and $@ expand to nothing (i.e., they are re‐
moved).
# 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).
0 Expands to the name of the shell or shell script. This is set at shell ini‐
tialization. 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. Other‐
wise, it is set to the filename used to invoke bash, as given by argument
zero.
Shell Variables
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.
BASH Expands to the full filename used to invoke this instance of bash.
BASHOPTS
A colon-separated list of enabled shell options. Each word in the list is a
valid argument for the -s option to the shopt builtin command (see SHELL
BUILTIN COMMANDS below). The options appearing in BASHOPTS are those reported
as on by shopt. If this variable is in the environment when bash starts up,
each shell option in the list will be enabled before reading any startup
files. This variable is read-only.
BASHPID
Expands to the process ID of the current bash process. This differs from $$
under certain circumstances, such as subshells that do not require bash to be
re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset,
it loses its special properties, even if it is subsequently reset.
BASH_ALIASES
An associative array variable whose members correspond to the internal list of
aliases as maintained by the alias builtin. Elements added to this array ap‐
pear in the alias list; however, unsetting array elements currently does not
cause aliases to be removed from the alias list. If BASH_ALIASES is unset, it
loses its special properties, even if it is subsequently reset.
BASH_ARGC
An array variable whose values are the number of parameters in each frame of
the current bash execution call stack. The number of parameters to the cur‐
rent 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 ex‐
tended debugging mode (see the description of the extdebug option to the shopt
builtin below). Setting extdebug after the shell has started to execute a
script, or referencing this variable when extdebug is not set, may result in
inconsistent values.
BASH_ARGV
An array variable containing all of the parameters in the current bash execu‐
tion call stack. The final parameter of the last subroutine call is at the
top of the stack; the first parameter of the initial call is at the bottom.
When a subroutine is executed, the parameters supplied are pushed onto
BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see
the description of the extdebug option to the shopt builtin below). Setting
extdebug after the shell has started to execute a script, or referencing this
variable when extdebug is not set, may result in inconsistent values.
BASH_ARGV0
When referenced, this variable expands to the name of the shell or shell
script (identical to $0; see the description of special parameter 0 above).
Assignment to BASH_ARGV0 causes the value assigned to also be assigned to $0.
If BASH_ARGV0 is unset, it loses its special properties, even if it is subse‐
quently reset.
BASH_CMDS
An associative array variable whose members correspond to the internal hash
table of commands as maintained by the hash builtin. Elements added to this
array appear in the hash table; however, unsetting array elements currently
does not cause command names to be removed from the hash table. If BASH_CMDS
is unset, it loses its special properties, even if it is subsequently reset.
BASH_COMMAND
The command currently being executed or about to be executed, unless the shell
is executing a command as the result of a trap, in which case it is the com‐
mand executing at the time of the trap. If BASH_COMMAND is unset, it loses
its special properties, even if it is subsequently reset.
BASH_EXECUTION_STRING
The command argument to the -c invocation option.
BASH_LINENO
An array variable whose members are the line numbers in source files where
each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the
line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]}
was called (or ${BASH_LINENO[$i-1]} if referenced within another shell func‐
tion). Use LINENO to obtain the current line number.
BASH_LOADABLES_PATH
A colon-separated list of directories in which the shell looks for dynamically
loadable builtins specified by the enable command.
BASH_REMATCH
An array variable whose members are assigned by the =~ binary operator to the
[[ conditional command. The element with index 0 is the portion of the string
matching the entire regular expression. The element with index n is the por‐
tion of the string matching the nth parenthesized subexpression.
BASH_SOURCE
An array variable whose members are the source filenames where the correspond‐
ing shell function names in the FUNCNAME array variable are defined. The
shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and
called from ${BASH_SOURCE[$i+1]}.
BASH_SUBSHELL
Incremented by one within each subshell or subshell environment when the shell
begins executing in that environment. The initial value is 0. If BASH_SUB‐
SHELL is unset, it loses its special properties, even if it is subsequently
reset.
BASH_VERSINFO
A readonly array variable whose members hold version information for this in‐
stance of bash. The values assigned to the array members are as follows:
BASH_VERSINFO[0] The major version number (the release).
BASH_VERSINFO[1] The minor version number (the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g., beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
BASH_VERSION
Expands to a string describing the version of this instance of bash.
COMP_CWORD
An index into ${COMP_WORDS} of the word containing the current cursor posi‐
tion. This variable is available only in shell functions invoked by the pro‐
grammable completion facilities (see Programmable Completion below).
COMP_KEY
The key (or final key of a key sequence) used to invoke the current completion
function.
COMP_LINE
The current command line. This variable is available only in shell functions
and external commands invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_POINT
The index of the current cursor position relative to the beginning of the cur‐
rent command. If the current cursor position is at the end of the current
command, the value of this variable is equal to ${#COMP_LINE}. This variable
is available only in shell functions and external commands invoked by the pro‐
grammable completion facilities (see Programmable Completion below).
COMP_TYPE
Set to an integer value corresponding to the type of completion attempted that
caused a completion function to be called: TAB, for normal completion, ?, for
listing completions after successive tabs, !, for listing alternatives on par‐
tial word completion, @, to list completions if the word is not unmodified, or
%, for menu completion. This variable is available only in shell functions
and external commands invoked by the programmable completion facilities (see
Programmable Completion below).
COMP_WORDBREAKS
The set of characters that the readline library treats as word separators when
performing word completion. If COMP_WORDBREAKS is unset, it loses its special
properties, even if it is subsequently reset.
COMP_WORDS
An array variable (see Arrays below) consisting of the individual words in the
current command line. The line is split into words as readline would split
it, using COMP_WORDBREAKS as described above. This variable is available only
in shell functions invoked by the programmable completion facilities (see Pro‐
grammable Completion below).
COPROC An array variable (see Arrays below) created to hold the file descriptors for
output from and input to an unnamed coprocess (see Coprocesses above).
DIRSTACK
An array variable (see Arrays below) containing the current contents of the
directory stack. Directories appear in the stack in the order they are dis‐
played 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 vari‐
able will not change the current directory. If DIRSTACK is unset, it loses
its special properties, even if it is subsequently reset.
EPOCHREALTIME
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see time(3)) as a floating point value with micro-second
granularity. Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME is
unset, it loses its special properties, even if it is subsequently reset.
EPOCHSECONDS
Each time this parameter is referenced, it expands to the number of seconds
since the Unix Epoch (see time(3)). Assignments to EPOCHSECONDS are ignored.
If EPOCHSECONDS is unset, it loses its special properties, even if it is sub‐
sequently reset.
EUID Expands to the effective user ID of the current user, initialized at shell
startup. This variable is readonly.
FUNCNAME
An array variable containing the names of all shell functions currently in the
execution call stack. The element with index 0 is the name of any currently-
executing shell function. The bottom-most element (the one with the highest
index) is "main". This variable exists only when a shell function is execut‐
ing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses
its special properties, even if it is subsequently reset.
This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of
FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe
the call stack. For instance, ${FUNCNAME[$i]} was called from the file
${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin
displays the current call stack using this information.
GROUPS An array variable containing the list of groups of which the current user is a
member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses
its special properties, even if it is subsequently reset.
HISTCMD
The history number, or index in the history list, of the current command. As‐
signments to HISTCMD are ignored. If HISTCMD is unset, it loses its special
properties, even if it is subsequently reset.
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely describes the type of machine on
which bash is executing. The default is system-dependent.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number
representing the current sequential line number (starting with 1) within a
script or function. When not in a script or function, the value substituted
is not guaranteed to be meaningful. If LINENO is unset, it loses its special
properties, even if it is subsequently reset.
MACHTYPE
Automatically set to a string that fully describes the system type on which
bash is executing, in the standard GNU cpu-company-system format. The default
is system-dependent.
MAPFILE
An array variable (see Arrays below) created to hold the text read by the map‐
file builtin when no variable name is supplied.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts builtin command
(see SHELL BUILTIN COMMANDS below).
OPTIND The index of the next argument to be processed by the getopts builtin command
(see SHELL BUILTIN COMMANDS below).
OSTYPE Automatically set to a string that describes the operating system on which
bash is executing. The default is system-dependent.
PIPESTATUS
An array variable (see Arrays below) containing a list of exit status values
from the processes in the most-recently-executed foreground pipeline (which
may contain only a single command).
PPID The process ID of the shell's parent. This variable is readonly.
PWD The current working directory as set by the cd command.
RANDOM Each time this parameter is referenced, 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.
READLINE_LINE
The contents of the readline line buffer, for use with "bind -x" (see SHELL
BUILTIN COMMANDS below).
READLINE_MARK
The position of the mark (saved insertion point) in the readline line buffer,
for use with "bind -x" (see SHELL BUILTIN COMMANDS below). The characters be‐
tween the insertion point and the mark are often called the region.
READLINE_POINT
The position of the insertion point in the readline line buffer, for use with
"bind -x" (see SHELL BUILTIN COMMANDS below).
REPLY Set to the line of input read by the read builtin command when no arguments
are supplied.
SECONDS
Each time this parameter is referenced, the number of seconds since shell in‐
vocation is returned. If a value is assigned to SECONDS, the value returned
upon subsequent references is the number of seconds since the assignment plus
the value assigned. The number of seconds at shell invocation and the current
time is always determined by querying the system clock. If SECONDS is unset,
it loses its special properties, even if it is subsequently reset.
SHELLOPTS
A colon-separated list of enabled shell options. Each word in the list is a
valid argument for the -o option to the set builtin command (see SHELL BUILTIN
COMMANDS below). The options appearing in SHELLOPTS are those reported as on
by set -o. If this variable is in the environment when bash starts up, each
shell option in the list will be enabled before reading any startup files.
This variable is read-only.
SHLVL Incremented by one each time an instance of bash is started.
SRANDOM
This variable expands to a 32-bit pseudo-random number each time it is refer‐
enced. 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 as‐
signments to this variable have no effect. If SRANDOM is unset, it loses its
special properties, even if it is subsequently reset.
UID Expands to the user ID of the current user, initialized at shell startup.
This variable is readonly.
The following variables are used by the shell. In some cases, bash assigns a default
value to a variable; these cases are noted below.
BASH_COMPAT
The value is used to set the shell's compatibility level. See SHELL COMPATI‐
BILITY 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 de‐
fault 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
BSHELLCOMPATIBILITYMODE. For example, 4.2 and 42 are valid values that corre‐
spond to the compat42 shopt option and set the compatibility level to 42. The
current version is also a valid value.
BASH_ENV
If this parameter is set when bash is executing a shell script, its value is
interpreted as a filename containing commands to initialize the shell, as in
~/.bashrc. The value of BASH_ENV is subjected to parameter expansion, command
substitution, and arithmetic expansion before being interpreted as a filename.
PATH is not used to search for the resultant filename.
BASH_XTRACEFD
If set to an integer corresponding to a valid file descriptor, bash will write
the trace output generated when set -x is enabled to that file descriptor.
The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new
value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the
trace output to be sent to the standard error. Note that setting
BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it
will result in the standard error being closed.
CDPATH The search path for the cd command. This is a colon-separated list of direc‐
tories in which the shell looks for destination directories specified by the
cd command. A sample value is ".:~:/usr".
CHILD_MAX
Set the number of exited child status values for the shell to remember. Bash
will not allow this value to be decreased below a POSIX-mandated minimum, and
there is a maximum value (currently 8192) that this may not exceed. The mini‐
mum value is system-dependent.
COLUMNS
Used by the select compound command to determine the terminal width when
printing selection lists. Automatically set if the checkwinsize option is en‐
abled or in an interactive shell upon receipt of a SIGWINCH.
COMPREPLY
An array variable from which bash reads the possible completions generated by
a shell function invoked by the programmable completion facility (see Program‐
mable Completion below). Each array element contains one possible completion.
EMACS If bash finds this variable in the environment when the shell starts with
value "t", it assumes that the shell is running in an Emacs shell buffer and
disables line editing.
ENV Expanded and executed similarly to BASH_ENV (see INVOCATION above) when an in‐
teractive shell is invoked in posix mode.
EXECIGNORE
A colon-separated list of shell patterns (see Pattern Matching) defining the
list of filenames to be ignored by command search 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 exe‐
cutable files. The pattern matching honors the setting of the extglob shell
option.
FCEDIT The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when performing filename comple‐
tion (see READLINE below). A filename whose suffix matches one of the entries
in FIGNORE is excluded from the list of matched filenames. A sample value is
".o:~".
FUNCNEST
If set to a numeric value greater than 0, defines a maximum function nesting
level. Function invocations that exceed this nesting level will cause the
current command to abort.
GLOBIGNORE
A colon-separated list of patterns defining the set of file names to be ig‐
nored by pathname expansion. If a file name matched by a pathname expansion
pattern also matches one of the patterns in GLOBIGNORE, it is removed from the
list of matches.
HISTCONTROL
A colon-separated list of values controlling how commands are saved on the
history list. If the list of values includes ignorespace, lines which begin
with a space character are not saved in the history list. A value of ig‐
noredups 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 com‐
pound command are not tested, and are added to the history regardless of the
value of HISTCONTROL.
HISTFILE
The name of the file in which command history is saved (see HISTORY below).
The default value is ~/.bash_history. If unset, the command history is not
saved when a shell exits.
HISTFILESIZE
The maximum number of lines contained in the history file. When this variable
is assigned a value, the history file is truncated, if necessary, to contain
no more than that number of lines by removing the oldest entries. The history
file is also truncated to this size after writing it when a shell exits. If
the value is 0, the history file is truncated to zero size. Non-numeric val‐
ues and numeric values less than zero inhibit truncation. The shell sets the
default value to the value of HISTSIZE after reading any startup files.
HISTIGNORE
A colon-separated list of patterns used to decide which command lines should
be saved on the history list. Each pattern is anchored at the beginning of
the line and must match the complete line (no implicit `*' is appended). Each
pattern is tested against the line after the checks specified by HISTCONTROL
are applied. In addition to the normal shell pattern matching characters, `&'
matches the previous history line. `&' may be escaped using a backslash; the
backslash is removed before attempting a match. The second and subsequent
lines of a multi-line compound command are not tested, and are added to the
history regardless of the value of HISTIGNORE. The pattern matching honors
the setting of the extglob shell option.
HISTSIZE
The number of commands to remember in the command history (see HISTORY below).
If the value is 0, commands are not saved in the history list. Numeric values
less than zero result in every command being saved on the history list (there
is no limit). The shell sets the default value to 500 after reading any
startup files.
HISTTIMEFORMAT
If this variable is set and not null, its value is used as a format string for
strftime(3) to print the time stamp associated with each history entry dis‐
played by the history builtin. If this variable is set, time stamps are writ‐
ten to the history file so they may be preserved across shell sessions. This
uses the history comment character to distinguish timestamps from other his‐
tory lines.
HOME The home directory of the current user; the default argument for the cd
builtin command. The value of this variable is also used when performing
tilde expansion.
HOSTFILE
Contains the name of a file in the same format as /etc/hosts that should be
read when the shell needs to complete a hostname. The list of possible host‐
name completions may be changed while the shell is running; the next time
hostname completion is attempted after the value is changed, bash adds the
contents of the new file to the existing list. If HOSTFILE is set, but has no
value, or does not name a readable file, bash attempts to read /etc/hosts to
obtain the list of possible hostname completions. When HOSTFILE is unset, the
hostname list is cleared.
IFS The Internal Field Separator that is used for word splitting after expansion
and to split lines into words with the read builtin command. The default
value is ``' , the default, then sequences of
, , and 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 se‐
quences of the whitespace characters space, tab, and newline are ignored at the be‐
ginning 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 ' )
Set the mark to the point. If a numeric argument is supplied, the mark is set
to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor position is set to the saved
position, and the old cursor position is saved as the mark.
character-search (C-])
A character is read and point is moved to the next occurrence of that charac‐
ter. A negative count searches for previous occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the previous occurrence of that
character. A negative count searches for subsequent occurrences.
skip-csi-sequence
Read enough characters to consume a multi-key sequence such as those defined
for keys like Home and End. Such sequences begin with a Control Sequence In‐
dicator (CSI), usually ESC-[. If this sequence is bound to "\[", keys produc‐
ing such sequences will have no effect unless explicitly bound to a readline
command, instead of inserting stray characters into the editing buffer. This
is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)
Without a numeric argument, the value of the readline comment-begin variable
is inserted at the beginning of the current line. If a numeric argument is
supplied, this command acts as a toggle: if the characters at the beginning of
the line do not match the value of comment-begin, the value is inserted, oth‐
erwise the characters in comment-begin are deleted from the beginning of the
line. In either case, the line is accepted as if a newline had been typed.
The default value of comment-begin causes this command to make the current
line a shell comment. If a numeric argument causes the comment character to
be removed, the line will be executed by the shell.
glob-complete-word (M-g)
The word before point is treated as a pattern for pathname expansion, with an
asterisk implicitly appended. This pattern is used to generate a list of
matching filenames for possible completions.
glob-expand-word (C-x *)
The word before point is treated as a pattern for pathname expansion, and the
list of matching filenames is inserted, replacing the word. If a numeric ar‐
gument is supplied, an asterisk is appended before pathname expansion.
glob-list-expansions (C-x g)
The list of expansions that would have been generated by glob-expand-word is
displayed, and the line is redrawn. If a numeric argument is supplied, an as‐
terisk is appended before pathname expansion.
dump-functions
Print all of the functions and their key bindings to the readline output
stream. If a numeric argument is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and their values to the readline
output stream. If a numeric argument is supplied, the output is formatted in
such a way that it can be made part of an inputrc file.
dump-macros
Print all of the readline key sequences bound to macros and the strings they
output. If a numeric argument is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
display-shell-version (C-x C-v)
Display version information about the current instance of bash.
Programmable Completion
When word completion is attempted for an argument to a 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 (com‐
pletion 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 de‐
fault. 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 Com‐
pleting is performed.
First, the actions specified by the compspec are used. Only matches which are pre‐
fixed 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 gener‐
ated next. The words generated by the pattern need not match the word being com‐
pleted. 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 ex‐
pansion, 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 de‐
scribed 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 argu‐
ment ($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 per‐
formed; 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 equiva‐
lent 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 expan‐
sion; 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 comple‐
tion is attempted.
If the -o plusdirs option was supplied to complete when the compspec was defined, di‐
rectory 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 comple‐
tion 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 vari‐
able.
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 comple‐
tion should be retried by returning an exit status of 124. If a shell function re‐
turns 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 cor‐
responding to the name of the command, the following default completion function
would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default
HISTORY
When the -o history option to the set builtin is enabled, the shell provides access
to the command history, the list of commands previously typed. The value of the
HISTSIZE variable is used as the number of commands to save in a history list. The
text of the last HISTSIZE commands (default 500) is saved. The shell stores each
command in the history list prior to parameter and variable expansion (see EXPANSION
above) but after history expansion is performed, subject to the values of the shell
variables HISTIGNORE and HISTCONTROL.
On startup, the history is initialized from the file named by the variable HISTFILE
(default ~/.bash_history). The file named by the value of HISTFILE is truncated, if
necessary, to contain no more than 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 time‐
stamps are optionally displayed depending on the value of the HISTTIMEFORMAT vari‐
able. 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 un‐
set, or if the history file is unwritable, the history is not saved. If the HIST‐
TIMEFORMAT 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 com‐
mand-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 COM‐
MANDS for information on setting and unsetting shell options.
HISTORY EXPANSION
The shell supports a history expansion feature that is similar to the history expan‐
sion in csh. This section describes what syntax features are available. This fea‐
ture 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-interac‐
tive shells do not perform history expansion by default.
History expansions introduce words from the history list into the input stream, mak‐
ing 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 se‐
lected 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 sev‐
eral metacharacter-separated words surrounded by quotes are considered one word.
History expansions are introduced by the appearance of the history expansion charac‐
ter, which is ! by default. Only backslash (\) and single quotes can quote the his‐
tory 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 his‐
tory expansion character, even if it is unquoted: space, tab, newline, carriage re‐
turn, 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 be‐
havior of history expansion. If the histverify shell option is enabled (see the de‐
scription of the shopt builtin below), and readline is being used, history substitu‐
tions 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 be‐
fore 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 avail‐
able 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 his‐
tory file.
Event Designators
An event designator is a reference to a command line entry in the history list. Un‐
less 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, car‐
riage return, = or ( (when the extglob shell option is enabled using the shopt
builtin).
!n Refer to command line n.
!-n Refer to the current command minus n.
!! Refer to the previous command. This is a synonym for `!-1'.
!string
Refer to the most recent command preceding the current position in the history
list starting with string.
!?string[?]
Refer to the most recent command preceding the current position in the history
list containing string. The trailing ? may be omitted if string is followed
immediately by a newline. If string is missing, the string from the most re‐
cent 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
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 designa‐
tor begins with a ^, $, *, -, or %. Words are numbered from the beginning of the
line, with the first word being denoted by 0 (zero). Words are inserted into the
current line separated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last word. This is usually the last argument, but will expand to the ze‐
roth 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-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym for `1-$'. It is not an
error to use * if there is just one word in the event; the empty string is re‐
turned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word. If x is missing, it de‐
faults to 0.
If a word designator is supplied without an event specification, the previous command
is used as the event.
Modifiers
After the optional word designator, there may appear a sequence of one or more of the
following modifiers, each preceded by a `:'. These modify, or edit, the word or
words selected from the history event.
h Remove a trailing filename component, leaving only the head.
t Remove all leading filename components, leaving the tail.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at blanks and new‐
lines. The q and x modifiers are mutually exclusive; the last one supplied is
used.
s/old/new/
Substitute new for the first occurrence of old in the event line. Any charac‐
ter may be used as the delimiter in place of /. The final delimiter is op‐
tional 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 re‐
placed 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 match‐
ing old is deleted.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is used in con‐
junction with `:s' (e.g., `:gs/old/new/') or `:&'. If used with `:s', any de‐
limiter can be used in place of /, and the final delimiter is optional if it
is the last character of the event line. An a may be used as a synonym for g.
G Apply the following `s' or `&' modifier once to each word in the event line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in this section as accepting
options preceded by - accepts -- to signify the end of the options. The :, true,
false, and test/[ builtins do not accept options and do not treat -- specially. The
exit, logout, return, break, continue, let, and shift builtins accept and process ar‐
guments beginning with - without requiring --. Other builtins that accept arguments
but are not specified as accepting options interpret arguments beginning with - as
invalid options and require -- to prevent this interpretation.
: [arguments]
No effect; the command does nothing beyond expanding arguments and performing
any specified redirections. The return status is zero.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current shell environment and
return the exit status of the last command executed from filename. If file‐
name does not contain a slash, filenames in PATH are used to find the direc‐
tory containing filename. The file searched for in PATH need not be exe‐
cutable. When bash is not in posix mode, the current directory is searched if
no file is found in PATH. If the sourcepath option to the shopt builtin com‐
mand is turned off, the PATH is not searched. If any arguments are supplied,
they become the positional parameters when filename is executed. Otherwise
the positional parameters are unchanged. If the -T option is enabled, source
inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and
restored around the call to source, and source unsets the DEBUG trap while it
executes. If -T is not set, and the sourced file changes the DEBUG trap, the
new value is retained when source completes. The return status is the status
of the last command exited within the script (0 if no commands are executed),
and false if filename is not found or cannot be read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option prints the list of aliases in
the form alias name=value on standard output. When arguments are supplied, an
alias is defined for each name whose value is given. A trailing space in
value causes the next word to be checked for alias substitution when the alias
is expanded. For each name in the argument list for which no value is sup‐
plied, the name and value of the alias is printed. Alias returns true unless
a name is given for which no alias has been defined.
bg [jobspec ...]
Resume each suspended job jobspec in the background, as if it had been started
with &. If jobspec is not present, the shell's notion of the current job is
used. bg jobspec returns 0 unless run when job control is disabled or, when
run with job control enabled, any specified jobspec was not found or was
started without job control.
bind [-m keymap] [-lpsvPSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] keyseq:readline-command
Display current readline key and function bindings, bind a key sequence to a
readline function or macro, or set a readline variable. Each non-option argu‐
ment is a command as it would appear in .inputrc, but each binding or command
must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'.
Options, if supplied, have the following meanings:
-m keymap
Use keymap as the keymap to be affected by the subsequent bindings.
Acceptable keymap names are emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent
to vi-command (vi-move is also a synonym); emacs is equivalent to
emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings in such a way that they
can be re-read.
-P List current readline function names and bindings.