Simple Commands¶

A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.

Pipelines¶

A pipeline is a sequence of one or more commands separated by one of the control operators | or |&. The format for a pipeline is:

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 the pipeline operator, 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. If a pipeline is executed synchronously, the shell waits for all commands in the pipeline to terminate before returning a value.

If the time reserved word precedes a pipeline, the shell reports the elapsed as well as user and system time consumed by its execution 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 value of the TIMEFORMAT variable is a format string that specifies how the timing information should be displayed; see the description of TIMEFORMAT below under Shell Variables.

When the shell is in posix mode, time may appear by itself as the only word in a simple command. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable specifies 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), and job control is not active, the last element of a pipeline may be run by the shell process.

Lists¶

A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or <newline>.

Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.

A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.

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

command2 is executed if, and only if, command1 returns an exit status of zero (success).

An OR list has the form

command2 is executed if, and only if, command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands¶

A compound command is one of the following. In most cases a list in a command's description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.

(list)

list is executed in a subshell (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 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 arithmetic 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 ]]

Evaluate the conditional expression expression and return a status of zero (true) or non-zero (false). 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. 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. If any part of the pattern is quoted, the quoted portion is matched as a string: every character in the quoted portion matches itself, instead of having any special pattern matching meaning.

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, as above. 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 match succeeds if the pattern 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:

for name [ [ in [ word ... ] ] ; ] do list ; done

First, expand The list of words following in, generating a list of items. Then, the variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set (see PARAMETERS below). The return status is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.

for (( expr1 ; expr2 ; expr3 )) ; do list ; done

First, evaluate the arithmetic expression expr1 according to the rules described below under ARITHMETIC EVALUATION. Then, repeatedly evaluate the arithmetic expression expr2 until it evaluates to zero. Each time expr2 evaluates to a non-zero value, execute list and evaluate the arithmetic expression expr3. 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 non-zero if any of the expressions is invalid.

Use the break and continue builtins (see SHELL BUILTIN COMMANDS below) to control loop execution.

select name [ in word ] ; do list ; done

First, expand the list of words following in, generating a list of items, and print the set of expanded words the standard error, each preceded by a number. If the in word is omitted, print the positional parameters (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 select sets the value of name to that word. If the line is empty, select displays the words and prompt again. If EOF is read, select completes and returns 1. Any other value sets name 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.

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

A case command first expands word, and tries to match it against each pattern in turn, proceeding from first to last, using the matching rules described under Pattern Matching below. A pattern list is a set of one or more patterns separated by , and the ) operator terminates the pattern list. 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. A clause is a pattern list and an associated list.

When a match is found, case executes the corresponding list. If the ;; operator terminates the case clause, the case command completes after the first match. Using ;& in place of ;; causes execution to continue with the list associated with the next pattern list. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list if the match succeeds. 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 the last list executed.

if list; then list; [ elif list; then list; ] ... [ else list; ] fi

The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.

while list-1; do list-2; done

until list-1; do list-2; done

The while command continuously executes the list list-2 as long as the last command in the list list-1 returns an exit status of zero. The until command is identical to the while command, except that the test is negated: list-2 is executed as long as the last command in list-1 returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last command executed in list-2, or zero if none was executed.

Coprocesses¶

A coprocess is a shell command preceded by the coproc reserved word. A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator, with a two-way pipe established between the executing shell and the coprocess.

The syntax for a coprocess is:

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

This form is preferred 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 may be used to wait for the coprocess to terminate.

Since the coprocess is created as an asynchronous command, the coproc command always returns success. The return status of a coprocess is the exit status of command.

Shell Function Definitions¶

A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters. Shell functions are declared as follows:

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. 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.)

Translating Strings¶

A double-quoted string preceded by a dollar sign ($"string") causes 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, and the string is treated as double-quoted as described above. 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 SHELL BUILTIN COMMANDS.

Positional Parameters¶

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed (see FUNCTIONS below).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPANSION below). Without braces, a digit following $ can only refer to one of the first nine positional parameters ($1-$9) or the special parameter $0 (see the next section).

Special Parameters¶

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed. Special parameters are denoted by one of the following characters.

*

($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where word expansions are performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS variable. That is, "$*" 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.

@

($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, such as the value portion of an assignment statement, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the expansion of the beginning part of the original word, and the expansion of the last parameter is joined with the expansion of the last part of the original word. When there are no positional parameters, "$@" 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 command.

-

($-) 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 parent 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

($0) Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke bash, as given by argument zero.

Shell Variables¶

The shell sets following variables:

_

($_, an underscore) This has a number of meanings depending on context. At shell startup, _ is set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, it expands to the last argument to the previous simple command executed in the foreground, after expansion. It is also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, $_ expands to 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, the shell enables each option in the list before reading any startup files. This variable is read-only.

BASHPID

Expands to the process ID of the current bash process. This differs from $$ under certain circumstances, such as subshells that do not require bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset.

BASH_ALIASES

An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. Elements added to this array appear in the alias list; however, unsetting array elements currently does not remove aliases from the alias list. If BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset.

BASH_ARGC

An array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed 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. Assignments to BASH_ARGC have no effect, and it may not be unset.

BASH_ARGV

An array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the shell pushes the supplied parameters 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. Assignments to BASH_ARGV have no effect, and it may not be unset.

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). Assigning a value to BASH_ARGV0 sets $0 to the same value. If BASH_ARGV0 is unset, it loses its special properties, even if it is subsequently reset.

BASH_CMDS

An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin. Adding elements to this array makes them appear in the hash table; however, unsetting array elements currently does not remove command names from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMMAND

Expands to 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.

BASH_EXECUTION_STRING

The command argument to the -c invocation option.

BASH_LINENO

An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number. Assignments to BASH_LINENO have no effect, and it may not be unset.

BASH_LOADABLES_PATH

A colon-separated list of directories in which the enable command. looks for dynamically loadable builtins.

BASH_MONOSECONDS

Each time this variable is referenced, it expands to the value returned by the system's monotonic clock, if one is available. If there is no monotonic clock, this is equivalent to EPOCHSECONDS. If BASH_MONOSECONDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_REMATCH

An array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression.

BASH_SOURCE

An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}. Assignments to BASH_SOURCE have no effect, and it may not be unset.

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_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.

BASH_TRAPSIG

Set to the signal number corresponding to the trap action being executed during its execution. See the description of trap under SHELL BUILTIN COMMANDS below for information about signal numbers and trap execution.

BASH_VERSINFO

A readonly array variable whose members hold version information for this instance of bash. The values assigned to the array members are as follows:

BASH_VERSION

Expands to a string describing the version of this instance of bash (e.g., 5.2.37(3)-release).

COMP_CWORD

An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion below).

COMP_KEY

The key (or final key of a key sequence) used to invoke the current completion function. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below).

COMP_LINE

The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below).

COMP_POINT

The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below).

COMP_TYPE

Set to an integer value corresponding to the type of attempted completion 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).

COMP_WORDBREAKS

The set of characters that the readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.

COMP_WORDS

An array variable (see Arrays below) consisting of the individual words in the current command line. The line is split into words as readline would split it, using COMP_WORDBREAKS as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion below).

COPROC

An array variable (see Arrays below) created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses above).

DIRSTACK

An array variable (see Arrays below) containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assigning to this variable does not change the current directory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.

EPOCHREALTIME

Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating-point value with micro-second granularity. Assignments to EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.

EPOCHSECONDS

Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently reset.

EUID

Expands to the effective user ID of the current user, initialized at shell startup. This variable is readonly.

FUNCNAME

An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is “main”. This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it 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. Assignments to HISTCMD have no effect. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.

HOSTNAME

Automatically set to the name of the current host.

HOSTTYPE

Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.

LINENO

Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function. When not in a script or function, the value substituted is not guaranteed to be meaningful. If LINENO is unset, it loses its special properties, even if it is subsequently reset.

MACHTYPE

Automatically set to a string that fully describes the system type on which bash is executing, in the standard GNU cpu-company-system format. The default is system-dependent.

MAPFILE

An array variable (see Arrays below) created to hold the text read by the mapfile builtin when no variable name is supplied.

OLDPWD

The previous working directory as set by the cd command.

OPTARG

The value of the last option argument processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).

OPTIND

The index of the next argument to be processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).

OSTYPE

Automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent.

PIPESTATUS

An array variable (see Arrays below) containing a list of exit status values from the commands in the most-recently-executed foreground pipeline, which may consist of only a simple command (see SHELL GRAMMAR above). Bash sets PIPESTATUS after executing multi-element pipelines, timed and negated pipelines, simple commands, subshells created with the ( operator, the [[ and (( compound commands, and after error conditions that result in the shell aborting command execution.

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. Seeding the random number generator with the same constant value produces the same sequence of values. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.

READLINE_ARGUMENT

Any numeric argument given to a readline command that was defined using “bind -x” (see SHELL BUILTIN COMMANDS below) when it was invoked.

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 between 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, 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 at one-second resolution. 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, the shell enables each option in the list before reading any startup files. This variable is read-only.

SHLVL

Incremented by one each time an instance of bash is started.

SRANDOM

Each time it is referenced, this variable expands to a 32-bit pseudo-random number. The random number generator is not linear on systems that support /dev/urandom or arc4random(3), 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.

UID

Expands to the user ID of the current user, initialized at shell startup. This variable is readonly.

The shell uses the following variables. 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 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. A subset of 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.

BASH_ENV

If this parameter is set when bash is executing a shell script, its expanded value is interpreted as a filename containing commands to initialize the shell before it reads and executes commands from the script. 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 writes the trace output generated when “set -x” is enabled to that file descriptor, instead of the standard error. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.

CDPATH

The search path for the cd command. This is a colon-separated list of directories where the shell looks for directories specified as arguments to the cd command. A sample value is “.:~:/usr”.

CHILD_MAX

Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent.

COLUMNS

Used by the select compound command to determine the terminal width when printing selection lists. Automatically set if the checkwinsize option is enabled or in an interactive shell upon receipt of a SIGWINCH.

COMPREPLY

An array variable from which bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion below). Each array element contains one possible completion.

EMACS

If bash finds this variable in the environment when the shell starts with value “t”, it assumes that the shell is running in an Emacs shell buffer and disables line editing.

ENV

Expanded and executed similarly to BASH_ENV (see INVOCATION above) when an interactive shell is invoked in posix mode.

EXECIGNORE

A colon-separated list of shell patterns (see Pattern Matching) defining the set 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.

FCEDIT

The default editor for the fc builtin command.

FIGNORE

A colon-separated list of suffixes to ignore when performing filename completion (see READLINE below). A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value is “.o:~” (Quoting is needed when assigning a value to this variable, which contains tildes).

FUNCNEST

If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level cause the current command to abort.

GLOBIGNORE

A colon-separated list of patterns defining the set of file names to be ignored by pathname expansion. If a file name matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches. The pattern matching honors the setting of the extglob shell option.

GLOBSORT

Controls how the results of pathname expansion are sorted. The value of this variable specifies the sort criteria and sort order for the results of pathname expansion. If this variable is unset or set to the null string, pathname expansion uses the historical behavior of sorting by name, in ascending lexicographic order as determined by the LC_COLLATE shell variable.

If set, a valid value begins with an optional +, which is ignored, or -, which reverses the sort order from ascending to descending, followed by a sort specifier. The valid sort specifiers are name, numeric, size, mtime, atime, ctime, and blocks, which sort the files on name, names in numeric rather than lexicographic order, file size, modification time, access time, inode change time, and number of blocks, respectively. If any of the non-name keys compare as equal (e.g., if two files are the same size), sorting uses the name as a secondary sort key.

For example, a value of -mtime sorts the results in descending order by modification time (newest first).

The numeric specifier treats names consisting solely of digits as numbers and sorts them using their numeric value (so “2” sorts before “10”, for example). When using numeric, names containing non-digits sort after all the all-digit names and are sorted by name using the traditional behavior.

A sort specifier of nosort disables sorting completely; bash returns the results in the order they are read from the file system, ignoring any leading -.

If the sort specifier is missing, it defaults to name, so a value of + is equivalent to the null string, and a value of - sorts by name in descending order. Any invalid value restores the historical sorting behavior.

HISTCONTROL

A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ignorespace, lines which begin with a space character are not saved in the history list. A value of ignoredups causes lines matching the previous history entry not to 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, bash saves all lines read by the shell parser on the history list, subject to the value of HISTIGNORE. If the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of HISTCONTROL. If the first line was not saved, the second and subsequent lines of the command are not saved either.

HISTFILE

The name of the file in which command history is saved (see HISTORY below). Bash assigns a default value of ~/.bash_history. If HISTFILE is unset or null, the shell does not save the command history when it 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 or by the history builtin. 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.

HISTIGNORE

A colon-separated list of patterns used to decide which command lines should be saved on the history list. If a command line matches one of the patterns in the value of HISTIGNORE, it is not saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (bash does not implicitly append a “*”). 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. A backslash escapes the “&”; the backslash is removed before attempting a match. If the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of HISTIGNORE. If the first line was not saved, the second and subsequent lines of the command are not saved either. The pattern matching honors the setting of the extglob shell option.

HISTSIZE

The number of commands to remember in the command history (see HISTORY below). If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files.

HISTTIMEFORMAT

If this variable is set and not null, its value is used as a format string for strftime(3) to print the time stamp associated with each history entry displayed by the history builtin. If this variable is set, the shell writes time stamps to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.

HOME

The home directory of the current user; the default argument for the cd builtin command. The value of this variable is also used when performing tilde expansion.

HOSTFILE

Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, or does not name a readable file, bash attempts to read /etc/hosts to obtain the list of possible hostname completions. When HOSTFILE is unset, bash clears the the hostname list.

IFS

The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. Word splitting is described below under EXPANSION. The default value is “<space><tab><newline>”.

IGNOREEOF

Controls the action of an interactive shell on receipt of an EOF character as the sole input. If set, the value is the number of consecutive EOF characters which must be typed as the first characters on an input line before bash exits. If the variable is set but does not have a numeric value, or the value is null, the default value is 10. If it is unset, EOF signifies the end of input to the shell.

INPUTRC

The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE below).

INSIDE_EMACS

If this variable appears in the environment when the shell starts, bash assumes that it is running inside an Emacs shell buffer and may disable line editing, depending on the value of TERM.

LANG

Used to determine the locale category for any category not specifically selected with a variable starting with LC_.

LC_ALL

This variable overrides the value of LANG and any other LC_ variable specifying a locale category.

LC_COLLATE

This variable determines the collation order used when sorting the results of pathname expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within pathname expansion and pattern matching.

LC_CTYPE

This variable determines the interpretation of characters and the behavior of character classes within pathname expansion and pattern matching.

LC_MESSAGES

This variable determines the locale used to translate double-quoted strings preceded by a $.

LC_NUMERIC

This variable determines the locale category used for number formatting.

LC_TIME

This variable determines the locale category used for data and time formatting.

LINES

Used by the select compound command to determine the column length for printing selection lists. Automatically set if the checkwinsize option is enabled or in an interactive shell upon receipt of a SIGWINCH.

MAIL

If the value is set to a file or directory name and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file or Maildir-format directory.

MAILCHECK

Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.

MAILPATH

A colon-separated list of filenames to be checked for mail. The message to be printed when mail arrives in a particular file may be specified by separating the filename from the message with a “?”. When used in the text of the message, $_ expands to the name of the current mailfile. For example:

MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'

OPTERR

If set to the value 1, bash displays error messages generated by the getopts builtin command (see SHELL BUILTIN COMMANDS below). OPTERR is initialized to 1 each time the shell is invoked or a shell script is executed.

PATH

The search path for commands. It is a colon-separated list of directories in which the shell looks for commands (see COMMAND EXECUTION below). A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. The default path is system-dependent, and is set by the administrator who installs bash. A common value is “/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin”.

POSIXLY_CORRECT

If this variable is in the environment when bash starts, the shell enters posix mode before reading the startup files, as if the --posix invocation option had been supplied. If it is set while the shell is running, bash enables posix mode, as if the command “set -o posix” had been executed. When the shell enters posix mode, it sets this variable if it was not already set.

PROMPT_COMMAND

If 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.

PROMPT_DIRTRIM

If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see PROMPTING below). Characters removed are replaced with an ellipsis.

PS0

The value of this parameter is expanded (see PROMPTING below) and displayed by interactive shells after reading a command and before the command is executed.

PS1

The value of this parameter is expanded (see PROMPTING below) and used as the primary prompt string. The default value is “\s-\v\$ ”.

PS2

The value of this parameter is expanded as with PS1 and used as the secondary prompt string. The default is “> ”.

PS3

The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR above).

PS4

The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace. The first character of the expanded value of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is “+ ”.

SHELL

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.

TIMEFORMAT

The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The % character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the brackets denote optional portions.

The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. time prints at most six digits after the decimal point; values of p greater than 6 are changed to 6. If p is not specified, time prints three digits after the decimal point.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

If this variable is not set, bash acts as if it had the value $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'. If the value is null, bash does not display any timing information. A trailing newline is added when the format string is displayed.

TMOUT

If set to a value greater than zero, the read builtin uses the value as its default timeout. The select command terminates if input does not arrive after TMOUT seconds when input is coming from a terminal. In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if a complete line of input does not arrive.

TMPDIR

If set, bash uses its value as the name of a directory in which bash creates temporary files for the shell's use.

auto_resume

This variable controls how the shell interacts with the user and job control. If this variable is set, simple commands consisting of only a single word, 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 or containing the word, this selects the most recently accessed job. The name of a stopped job, in this context, is the command line used to start it, as displayed by jobs. If set to the value exact, the word must match the name of a stopped job exactly; if set to substring, the word 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 (e.g., prefix), the word must be a prefix of a stopped job's name; this provides functionality analogous to the %string job identifier.

histchars

The two or three characters which control history expansion, quick substitution, and tokenization (see HISTORY EXPANSION below). The first character is the history expansion character, the character which begins a history expansion, normally “!”. The second character is the quick substitution character, normally “^”. When it appears as the first character on the line, history substitution repeats the previous command, replacing one string with another. 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 disables history substitution for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.

Arrays¶

Bash provides one-dimensional indexed and associative array variables. Any variable may be used as an indexed array; the declare builtin explicitly declares 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 arithmetic expressions that must expand to an integer 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 greater than or equal to zero. 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 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 the shell expansions described below under EXPANSION, but values that are valid variable assignments including the brackets and subscript do not undergo brace expansion and word splitting, as with individual variable assignments.

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 appends to an array variable when assigning using the compound assignment syntax; see PARAMETERS above.

An array element is 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, unless noted in the description of a builtin or word expansion. 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 expansion of the original word, and the expansion of the last parameter is joined with the last part of the expansion 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 valid; bash creates 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] unsets 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] behaves differently depending on whether name is an indexed or associative array when subscript is * or @. 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, (e.g., unset a[4]), the argument is subject to pathname expansion. Quote the argument if pathname expansion is not desired (e.g., unset 'a[4]').

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. Other builtins accept array name arguments as well (e.g., mapfile); see the descriptions of individual builtins below for details. The shell provides a number of builtin array variables.

Brace Expansion¶

Brace expansion is a mechanism to generate arbitrary strings sharing a common prefix and suffix, either of which can be empty. This mechanism is similar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded are formed from 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; brace expansion preserves left to right order. 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. If either x or y begins with 0, each generated term will contain the same number of digits, zero-padding where necessary. 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:

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. Bash outputs that word as file1 file2 after brace expansion. Start bash with the +B option or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below) for strict sh compatibility.

Tilde Expansion¶

If a word begins with an unquoted tilde character (“~”), all of the characters preceding the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the shell parameter HOME. If HOME is unset, the tilde expands to the home directory of the user executing the shell 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 shell substitutes the value of the shell variable OLDPWD, if it is set. 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 characters following the tilde in the tilde-prefix as an argument. If the characters following the tilde in the tilde-prefix consist of a number without a leading “+” or “-”, tilde expansion assumes “+”.

The results of tilde expansion are treated as if they were quoted, so the replacement is not subject to word splitting and pathname expansion.

If the login name is invalid, or the tilde expansion fails, the tilde-prefix is unchanged.

Bash checks each variable assignment for unquoted tilde-prefixes immediately following a : or the first =, and performs tilde expansion in these cases. Consequently, one may use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

Bash also performs tilde expansion on words satisfying the conditions of variable assignments (as described above under PARAMETERS) when they appear as arguments to simple commands. Bash does not do this, except for the declaration commands listed above, when in posix mode.

Parameter Expansion¶

The “$” character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.

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.

The basic form of parameter expansion is

${parameter}

which substitutes the value of parameter. 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 new parameter 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, for compatibility. 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 tests 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, and the expansion is the final value of parameter. Positional parameters and special parameters may not be assigned in this way.

${parameter:?word}

Display Error if Null or Unset. If parameter is null or unset, the shell writes the expansion of word (or a message to that effect if word is not present) to the standard error and, if it is not interactive, exits with a non-zero status. An interactive shell does not exit, but does not execute the command associated with the expansion. 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. The value of parameter is not used.

${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 (or 0 if there are no positional parameters). 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. Substitutes the length in characters of the expanded value of parameter. 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 and matched against the expanded value of parameter using the rules described under Pattern Matching below. The longest match of pattern in the expanded value is replaced with string. string undergoes tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, and quote removal.

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 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 escapes & 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.

Like the pattern removal operators, double quotes surrounding the replacement string quote the expanded characters, while double quotes enclosing the entire parameter substitution do not, since the expansion is performed in a context that doesn't take any enclosing double quotes into account.

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:

Command Substitution¶

Command substitution allows the output of a command to replace the command itself. There are two standard forms:

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).

With the old-style backquote form of substitution, 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.

There is an alternate form of command substitution:

which executes command in the current execution environment and captures its output, again with trailing newlines removed.

The character c following the open brace must be a space, tab, newline, or |, and the close brace must be in a position where a reserved word may appear (i.e., preceded by a command terminator such as semicolon). Bash allows the close brace to be joined to the remaining characters in the word without being followed by a shell metacharacter as a reserved word would usually require.

Any side effects of command take effect immediately in the current execution environment and persist in the current environment after the command completes (e.g., the exit builtin exits the shell).

This type of command substitution superficially resembles executing an unnamed shell function: local variables are created as when a shell function is executing, and the return builtin forces command to complete; however, the rest of the execution environment, including the positional parameters, is shared with the caller.

If the first character following the open brace is a |, the construct expands to the value of the REPLY shell variable after command executes, without removing any trailing newlines, and the standard output of command remains the same as in the calling shell. Bash creates REPLY as an initially-unset local variable when command executes, and restores REPLY to the value it had before the command substitution after command completes, as with any local variable.

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, bash does not perform word splitting and pathname expansion on the results.

Arithmetic Expansion¶

Arithmetic expansion evaluates an arithmetic expression and substitutes the result. The format for arithmetic expansion is:

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 to standard error indicating failure, does not perform the substitution, and does not execute the command associated with the expansion.

Process Substitution¶

Process substitution allows a process's input or output to be referred to using a filename. It takes the form of <(list) or >(list). The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion.

If the >(list) form is used, writing to the file provides input for list. If the <(list) form is used, reading the file obtains the output of list. No space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

Word Splitting¶

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. Words that were not expanded are not split.

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.

An IFS whitespace character is whitespace as defined above (see Definitions) that appears in the value of IFS. Space, tab, and newline are always considered IFS whitespace, even if they don't appear in the locale's space category.

If IFS is unset, field splitting acts as if its value were <space><tab><newline>, and treats these characters as IFS whitespace. If the value of IFS is null, no word splitting occurs, but implicit null arguments (see below) are still removed.

Word splitting begins by removing sequences of IFS whitespace characters from the beginning and end of the results of the previous expansions, then splits the remaining words.

If the value of IFS consists solely of IFS whitespace, any sequence of IFS whitespace characters delimits a field, so a field consists of characters that are not unquoted IFS whitespace, and null fields result only from quoting.

If IFS contains a non-whitespace character, then any character in the value of IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. This means that adjacent non-IFS-whitespace delimiters produce a null field. A sequence of IFS whitespace characters also delimits a field.

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. Expanding a parameter with no value within double quotes produces a null field, which 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, word splitting removes the null argument portion, leaving the non-null expansion. That is, the word “-d''” becomes “-d” after word splitting and null argument removal.

Pathname Expansion¶

After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with a sorted list of filenames matching the pattern (see Pattern Matching below) subject to the value of the GLOBSORT shell variable.

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, bash prints an error message and does not execute the command. 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 .. never match, 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 “.” 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 GLOBIGNORE pattern matching honors the setting of the extglob shell option.

The GLOBSORT shell variable controls how the results of pathname expansion are sorted, as described above.

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:

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:

The extglob 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 “.”. If the globskipdots shell option is enabled, the filenames . and .. never appear in the set. 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.

Quote Removal¶

After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are removed.

Redirecting Input¶

Redirecting input opens the file whose name results from the expansion of word 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:

Redirecting Output¶

Redirecting output opens the file whose name results from the expansion of word 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:

If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the redirection fails 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, bash attempts the redirection even if the file named by word exists.

Appending Redirected Output¶

Redirecting output in this fashion opens the file whose name results from the expansion of word 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:

Redirecting Standard Output and Standard Error¶

This construct redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

There are two formats for redirecting standard output and standard error:

Of the two forms, the first is preferred. This is semantically equivalent to

When using the second form, word may not expand to a number or -. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.

Appending Standard Output and Standard Error¶

This construct appends both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

The format for appending standard output and standard error is:

This is semantically equivalent to

(see Duplicating File Descriptors below).

Here Documents¶

This type of redirection instructs the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). All of the lines read up to that point then become 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

The shell does not perform 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, the delimiter is word itself, and the here-document text is treated similarly to a double-quoted string: all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \<newline> is treated literally, and \ must be used to quote the characters \, $, and `; however, double quote characters have no special meaning.

If the redirection operator is <<-, then the shell strips all leading tab characters from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

If the delimiter is not quoted, the \<newline> sequence is treated as a line continuation: the two lines are joined and the backslash-newline is removed. This happens while reading the here-document, before the check for the ending delimiter, so joined lines can form the end delimiter.

Here Strings¶

A variant of here documents, the format is:

[n]<<<word

The word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Pathname expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified).

Duplicating File Descriptors¶

The redirection operator

is used to duplicate input file descriptors. If word expands to one or more digits, file descriptor n is made to be a copy of that file descriptor. It is a redirection error if the digits in word do not specify a file descriptor open for input. If word evaluates to -, file descriptor n is closed. If n is not specified, this uses the standard input (file descriptor 0).

The operator

is used similarly to duplicate output file descriptors. If n is not specified, this uses the standard output (file descriptor 1). It is a redirection error if the digits in word do not specify a file descriptor open for output. 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 -, this redirects the standard output and standard error as described previously.

Moving File Descriptors¶

The redirection operator

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

moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.

Opening File Descriptors for Reading and Writing¶

The redirection operator

opens the file whose name is the expansion of word 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.

Readline Notation¶

This section uses Emacs-style editing concepts and uses its notation for 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. The Meta key is often labeled “Alt” or “Option”.

On keyboards without a Meta key, M-x means ESC x, i.e., press and release the Escape key, then press and release the x key, in sequence. This makes ESC the meta prefix. The combination M-C-x means ESC Control-x: press and release the Escape key, then press and hold the Control key while pressing the x key, then release both.

On some keyboards, the Meta key modifier produces characters with the eighth bit (0200) set. You can use the enable-meta-key variable to control whether or not it does this, if the keyboard allows it. On many others, the terminal or terminal emulator converts the metafied key to a key sequence beginning with ESC as described in the preceding paragraph.

If your Meta key produces a key sequence with the ESC meta prefix, you can make M-key key bindings you specify (see Readline Key Bindings below) do the same thing by setting the force-meta-prefix variable.

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) makes that command act in a backward direction. Commands whose behavior with arguments deviates from this are noted below.

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. Readline has the concept of an active region: when the region is active, readline redisplay highlights the region using the value of the active-region-start-color variable. The enable-active-region variable turns this on and off. Several commands set the region to active; those 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 accumulate the deleted text into one unit, which can be yanked all at once. Commands which do not kill text separate the chunks of text on the kill ring.

Readline Initialization¶

Readline is customized by putting commands in an initialization file (the inputrc file). The name of this file is taken from the value of the INPUTRC shell variable. If that variable is unset, the default is ~/.inputrc. If that file does not exist or cannot be read, readline looks for /etc/inputrc. When a program that uses the readline library starts up, readline reads the initialization file and sets the key bindings and variables found there, before reading any user input.

There are only a few basic constructs allowed in the inputrc 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 in this section may be changed using key binding commands in the inputrc file. Programs that use the readline library, including bash, may add their own commands and bindings.

For example, placing

into the inputrc would make M-C-u execute the readline command universal-argument.

Key bindings may contain the following symbolic character names: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive backspace), SPACE, SPC, 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 difference between a macro and a command is that a macro is enclosed in single or double quotes.

Readline Key Bindings¶

The syntax for controlling key bindings in the inputrc file is simple. All that is required is the name of the command or the text of a macro and a key sequence to which it should be bound. The key sequence may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- prefixes, or as a key sequence composed of one or more characters enclosed in double quotes. The key sequence and name are separated by a colon. There can be no whitespace between the name and the colon.

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 none of the symbolic character names are 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 available when specifying key sequences is

In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:

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. The backslash escapes described above are expanded in the macro body. Backslash quotes any other character in the macro text, including " and '.

Bash will display or modify the current readline key bindings with the bind builtin command. The -o emacs or -o vi options to the set builtin (see SHELL BUILTIN COMMANDS below) change the editing mode during interactive use.

Readline Variables¶

Readline has variables that can be used to further customize its behavior. A variable may be set in the inputrc file with a statement of the form

Except where noted, readline variables can take the values On or Off (without regard to case). Unrecognized variable names are ignored. When readline reads a variable value, empty or null values, “on” (case-insensitive), and “1” are equivalent to On. All other values are equivalent to Off.

The bind -V command lists the current readline variable names and values (see SHELL BUILTIN COMMANDS below).

The variables and their default values are:

active-region-start-color

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”.

active-region-end-color

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”.

bell-style (audible)

Controls what happens when readline wants to ring the terminal bell. If set to none, readline never rings the bell. If set to visible, readline uses a visible bell if one is available. If set to audible, readline attempts to ring the terminal's bell.

bind-tty-special-chars (On)

If set to On, readline attempts to bind the control characters that are treated specially by the kernel's terminal driver to their readline equivalents. These override the default readline bindings described here. Type “stty -a” at a bash prompt to see your current terminal settings, including the special control characters (usually cchars).

blink-matching-paren (Off)

If set to On, readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted.

colored-completion-prefix (Off)

If set to On, when listing completions, readline displays the common prefix of the set of possible completions using a different color. The color definitions are taken from the value of the LS_COLORS environment variable. 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.

colored-stats (Off)

If set to On, readline displays possible completions using different colors to indicate their file type. The color definitions are taken from the value of the LS_COLORS environment variable.

comment-begin (“#”)

The string that the readline insert-comment command inserts. This command is bound to M-# in emacs mode and to # in vi command mode.

completion-display-width (-1)

The number of screen columns used to display possible matches when performing completion. The value is ignored if it is less than 0 or greater than the terminal screen width. A value of 0 causes matches to be displayed one per line. The default value is -1.

completion-ignore-case (Off)

If set to On, readline performs filename matching and completion in a case-insensitive fashion.

completion-map-case (Off)

If set to On, and completion-ignore-case is enabled, readline treats hyphens (-) and underscores (_) as equivalent when performing case-insensitive filename matching and completion.

completion-prefix-display-length (0)

The maximum 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, readline replaces common prefixes longer than this value with an ellipsis when displaying possible completions. If a completion begins with a period, and eadline is completing filenames, it uses three underscores instead of an ellipsis.

completion-query-items (100)

This determines when the user is queried about viewing the number of possible completions generated by the possible-completions command. It may be set to any integer value greater than or equal to zero. If the number of possible completions is greater than or equal to the value of this variable, readline asks whether or not the user wishes to view them; otherwise readline simply lists them on the terminal. A zero value means readline should never ask; negative values are treated as zero.

convert-meta (On)

If set to On, readline converts characters it reads that have the eighth bit set to an ASCII key sequence by clearing the eighth bit and prefixing it with an escape character (converting the character to have the meta prefix). The default is On, but readline sets it to Off if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and may change if the locale changes. This variable also affects key bindings; see the description of force-meta-prefix below.

disable-completion (Off)

If set to On, readline inhibits word completion. Completion characters are inserted into the line as if they had been mapped to self-insert.

echo-control-characters (On)

When set to On, on operating systems that indicate they support it, readline echoes a character corresponding to a signal generated from the keyboard.

editing-mode (emacs)

Controls whether readline uses a set of key bindings similar to Emacs or vi. editing-mode can be set to either emacs or vi.

emacs-mode-string (@)

If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when emacs editing mode is active. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.

enable-active-region (On)

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 variable, 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.

enable-bracketed-paste (On)

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 is called bracketed-paste mode; it prevents readline from executing any editing commands bound to key sequences appearing in the pasted text.

enable-keypad (Off)

When set to On, readline tries to enable the application keypad when it is called. Some systems need this to enable the arrow keys.

enable-meta-key (On)

When set to On, readline tries to enable any meta modifier key the terminal claims to support. On many terminals, the Meta key is used to send eight-bit characters; this variable checks for the terminal capability that indicates the terminal can enable and disable a mode that sets the eighth bit of a character (0200) if the Meta key is held down when the character is typed (a meta character).

expand-tilde (Off)

If set to On, readline performs tilde expansion when it attempts word completion.

force-meta-prefix (Off)

If set to On, readline modifies its behavior when binding key sequences containing \M- or Meta- (see Key Bindings above) by converting a key sequence of the form \M-C or Meta-C to the two-character sequence ESC C (adding the meta prefix). If force-meta-prefix is set to Off (the default), readline uses the value of the convert-meta variable to determine whether to perform this conversion: if convert-meta is On, readline performs the conversion described above; if it is Off, readline converts C to a meta character by setting the eighth bit (0200).

history-preserve-point (Off)

If set to On, the history code attempts to place point at the same location on each history line retrieved with previous-history or next-history.

history-size (unset)

Set the maximum number of history entries saved in the history list. If set to zero, any existing history entries are deleted and no new entries are saved. If set to a value less than zero, the number of history entries is not limited. By default, bash sets the the maximum number of history entries to the value of the HISTSIZE shell variable. Setting history-size to a non-numeric value will set the maximum number of history entries to 500.

horizontal-scroll-mode (Off)

Setting this variable 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.

input-meta (Off)

If set to On, readline enables eight-bit input (that is, it does not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default is Off, but readline sets it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and its value may change if the locale changes. The name meta-flag is a synonym for input-meta.

isearch-terminators (“C-[C-j”)

The string of characters that should terminate an incremental search without subsequently executing the character as a command. If this variable has not been given a value, the characters ESC and C-j terminate an incremental search.

keymap (emacs)

Set the current readline keymap. The set of valid keymap names is emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command, and vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-standard. The default value is emacs; the value of editing-mode also affects the default keymap.

keyseq-timeout (500)

Specifies the duration readline will wait for a character when reading an ambiguous key sequence (one that can form a complete key sequence using the input read so far, or can take additional input to complete a longer key sequence). If readline does not receive any input within the timeout, it uses 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 waits until another key is pressed to decide which key sequence to complete.

mark-directories (On)

If set to On, completed directory names have a slash appended.

mark-modified-lines (Off)

If set to On, readline displays history lines that have been modified with a preceding asterisk (*).

mark-symlinked-directories (Off)

If set to On, completed names which are symbolic links to directories have a slash appended, subject to the value of mark-directories.

match-hidden-files (On)

This variable, when set to On, forces readline to match files whose names begin with a “.” (hidden files) when performing filename completion. If set to Off, the user must include the leading “.” in the filename to be completed.

menu-complete-display-prefix (Off)

If set to On, menu completion displays the common prefix of the list of possible completions (which may be empty) before cycling through the list.

output-meta (Off)

If set to On, readline displays characters with the eighth bit set directly rather than as a meta-prefixed escape sequence. The default is Off, but readline sets it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and its value may change if the locale changes.

page-completions (On)

If set to On, readline uses an internal pager resembling more(1) to display a screenful of possible completions at a time.

prefer-visible-bell

See bell-style.

print-completions-horizontally (Off)

If set to On, readline displays completions with matches sorted horizontally in alphabetical order, rather than down the screen.

revert-all-at-newline (Off)

If set to On, readline will undo all changes to history lines before returning when executing accept-line. By default, history lines may be modified and retain individual undo lists across calls to readline.

search-ignore-case (Off)

If set to On, readline performs incremental and non-incremental history list searches in a case-insensitive fashion.

show-all-if-ambiguous (Off)

This alters the default behavior of the completion functions. If set to On, words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell.

show-all-if-unmodified (Off)

This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. If set to On, words which have more than one possible completion without any possible partial completion (the possible completions don't share a common prefix) cause the matches to be listed immediately instead of ringing the bell.

show-mode-in-prompt (Off)

If set to On, add a string to the beginning of the prompt indicating the editing mode: emacs, vi command, or vi insertion. The mode strings are user-settable (e.g., emacs-mode-string).

skip-completed-text (Off)

If set to On, this alters the default completion behavior when inserting a single match into the line. It's only active when performing completion in the middle of a word. If enabled, readline does not insert characters from the completion that match characters after point in the word being completed, so portions of the word following the cursor are not duplicated.

vi-cmd-mode-string ((cmd))

If the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in command mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.

vi-ins-mode-string ((ins))

If the show-mode-in-prompt variable is enabled, this string is 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. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string.

visible-stats (Off)

If set to On, a character denoting a file's type as reported by stat(2) is appended to the filename when listing possible completions.

Readline Conditional Constructs¶

Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are four parser directives available.

$if

The $if construct allows bindings to be made based on the editing mode, the terminal being used, or the application using readline. The text of the test, after any comparison operator, extends to the end of the line; unless otherwise noted, no characters are required to isolate it.

$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif

$else

Commands in this branch of the $if directive are executed if the test fails.

$endif

This command, as seen in the previous example, terminates an $if command.

$include

This directive takes a single filename as an argument and reads commands and key bindings from that file. For example, the following directive would read /etc/inputrc:

$include  /etc/inputrc

Searching¶

Readline provides commands for searching through the command history (see HISTORY below) for lines containing a specified string. There are two search modes: incremental and non-incremental.

Incremental searches begin before the user has finished typing the search string. As each character of the search string is typed, readline displays the next entry from the history matching the string typed so far. An incremental search requires only as many characters as needed to find the desired history entry. When using emacs editing mode, type C-r to search backward in the history for a particular string. Typing C-s searches forward through the history. 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, ESC and C-j terminate an incremental search. C-g aborts an incremental search and restores 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 C-r or C-s as appropriate. This searches 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 terminates the search and executes that command. For instance, a newline terminates the search and accepts the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.

Readline remembers the last incremental search string. If two C-rs are typed without any intervening characters defining a new search string, readline uses any remembered search string.

Non-incremental searches read the entire search string before starting to search for matching history entries. The search string may be typed by the user or be part of the contents of the current line.

Readline Command Names¶

The following is a list of the names of the commands and the default key sequences to which they are bound. Command names without an accompanying key sequence are unbound by default.

In the following descriptions, point refers to the current cursor position, and mark refers to a cursor position saved by the set-mark command. The text between the point and mark is referred to as the region. Readline has the concept of an active region: when the region is active, readline redisplay highlights the region using the value of the active-region-start-color variable. The enable-active-region readline variable turns this on and off. Several commands set the region to active; those are noted below.

Commands for Moving¶

beginning-of-line (C-a)

Move to the start of the current line. This may also be bound to the Home key on some keyboards.

end-of-line (C-e)

Move to the end of the line. This may also be bound to the End key on some keyboards.

forward-char (C-f)

Move forward a character. This may also be bound to the right arrow key on some keyboards.

backward-char (C-b)

Move back a character. This may also be bound to the left arrow key on some keyboards.

forward-word (M-f)

Move forward to the end of the next word. Words are composed of alphanumeric characters (letters and digits).

backward-word (M-b)

Move back to the start of the current or previous word. Words are composed of alphanumeric characters (letters and digits).

shell-forward-word

Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.

shell-backward-word

Move back to the start of the current or previous word. Words are delimited by non-quoted shell metacharacters.

previous-screen-line

Attempt to move point to the same physical screen column on the previous physical screen line. This will not have the desired effect if the current readline line does not take up more than one physical line or if point is not greater than the length of the prompt plus the screen width.

next-screen-line

Attempt to move point to the same physical screen column on the next physical screen line. This will not have the desired effect if the current readline line does not take up more than one physical line or if the length of the current readline line is not greater than the length of the prompt plus the screen width.

clear-display (M-C-l)

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-screen (C-l)

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.

redraw-current-line

Refresh the current line.

Commands for Manipulating the History¶

accept-line (Newline, Return)

Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list according to the state of the HISTCONTROL and HISTIGNORE variables. If the line is a modified history line, restore the history line to its original state.

previous-history (C-p)

Fetch the previous command from the history list, moving back in the list. This may also be bound to the up arrow key on some keyboards.

next-history (C-n)

Fetch the next command from the history list, moving forward in the list. This may also be bound to the down arrow key on some keyboards.

beginning-of-history (M-<)

Move to the first line in the history.

end-of-history (M->)

Move to the end of the input history, i.e., the line currently being entered.

operate-and-get-next (C-o)

Accept the current line for execution as if a newline had been entered, 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.

fetch-history

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.

reverse-search-history (C-r)

Search backward starting at the current line and moving “up” through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.

forward-search-history (C-s)

Search forward starting at the current line and moving “down” through the history as necessary. This is an incremental search. This command sets the region to the matched text and activates the region.

non-incremental-reverse-search-history (M-p)

Search backward through the history starting at the current line using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.

non-incremental-forward-search-history (M-n)

Search forward through the history using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.

history-search-backward

Search backward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. This may be bound to the Page Up key on some keyboards.

history-search-forward

Search forward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. This may be bound to the Page Down key on some keyboards.

history-substring-search-backward

Search backward 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.

history-substring-search-forward

Search forward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search.

yank-nth-arg (M-C-y)

Insert the first argument to the previous command (usually the second word on the previous line) at point. With an argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A negative argument inserts the nth word from the end of the previous command. Once the argument n is computed, this uses the history expansion facilities to extract the nth word, as if the “!n” history expansion had been specified.

yank-last-arg (M-., M-_)

Insert the last argument to the previous command (the last word of the previous history entry). With a numeric argument, behave exactly like yank-nth-arg. Successive calls to yank-last-arg move back through the history list, inserting the last word (or the word specified by the argument to the first call) of each line in turn. Any numeric argument supplied to these successive calls determines the direction to move through the history. A negative argument switches the direction through the history (back or forward). This uses the history expansion facilities to extract the last word, as if the “!$” history expansion had been specified.

shell-expand-line (M-C-e)

Expand the line by performing shell word expansions. This performs alias and history expansion, $'string' and $"string" quoting, tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, word splitting, and quote removal. An explicit argument suppresses command and process substitution. See HISTORY EXPANSION below for a description of history expansion.

history-expand-line (M-^)

Perform history expansion on the current line. See HISTORY EXPANSION below for a description of history expansion.

magic-space

Perform history expansion on the current line and insert a space. See HISTORY EXPANSION below for a description of history expansion.

alias-expand-line

Perform alias expansion on the current line. See ALIASES above for a description of alias expansion.

history-and-alias-expand-line

Perform history and alias expansion on the current line.

insert-last-argument (M-., M-_)

A synonym for yank-last-arg.

edit-and-execute-command (C-x C-e)

Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke $VISUAL, $EDITOR, and emacs as the editor, in that order.

Commands for Changing Text¶

end-of-file (usually C-d)

The character indicating end-of-file as set, for example, by stty(1). If this character is read when there are no characters on the line, and point is at the beginning of the line, readline interprets it as the end of input and returns EOF.

delete-char (C-d)

Delete the character at point. If this function is bound to the same character as the tty EOF character, as C-d commonly is, see above for the effects. This may also be bound to the Delete key on some keyboards.

backward-delete-char (Rubout)

Delete the character behind the cursor. When given a numeric argument, save the deleted text on the kill ring.

forward-backward-delete-char

Delete the character under the cursor, unless the cursor is at the end of the line, in which case the character behind the cursor is deleted.

quoted-insert (C-q, C-v)

Add the next character typed to the line verbatim. This is how to insert characters like C-q, for example.

tab-insert (C-v TAB)

Insert a tab character.

self-insert (a, b, A, 1, !, ...)

Insert the character typed.

bracketed-paste-begin

This function is intended to be bound to the “bracketed paste” escape sequence sent by some terminals, and such a binding is assigned by default. It allows readline to insert the pasted text as a single unit without treating each character as if it had been read from the keyboard. The pasted characters are inserted as if each one was bound to self-insert instead of executing any editing commands.

Bracketed paste sets the region to the inserted text and activates the region.

transpose-chars (C-t)

Drag the character before point forward over the character at point, moving point forward as well. If point is at the end of the line, then this transposes the two characters before point. Negative arguments have no effect.

transpose-words (M-t)

Drag the word before point past the word after point, moving point over that word as well. If point is at the end of the line, this transposes the last two words on the line.

shell-transpose-words (M-C-t)

Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line. Word boundaries are the same as shell-forward-word and shell-backward-word.

upcase-word (M-u)

Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move point.

downcase-word (M-l)

Lowercase the current (or following) word. With a negative argument, lowercase the previous word, but do not move point.

capitalize-word (M-c)

Capitalize the current (or following) word. With a negative argument, capitalize the previous word, but do not move point.

overwrite-mode

Toggle overwrite mode. With an explicit positive numeric 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, but may be bound to the Insert key on some keyboards.

Killing and Yanking¶

kill-line (C-k)

Kill the text from point to the end of the current line. With a negative numeric argument, kill backward from the cursor to the beginning of the line.

backward-kill-line (C-x Rubout)

Kill backward to the beginning of the current line. With a negative numeric argument, kill forward from the cursor to the end of the line.

unix-line-discard (C-u)

Kill backward from point to the beginning of the line, saving the killed text on the kill-ring.

kill-whole-line

Kill all characters on the current line, no matter where point is.

kill-word (M-d)

Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as those used by forward-word.

backward-kill-word (M-Rubout)

Kill the word behind point. Word boundaries are the same as those used by backward-word.

shell-kill-word (M-C-d)

Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as those used by shell-forward-word.

shell-backward-kill-word

Kill the word behind point. Word boundaries are the same as those used by shell-backward-word.

unix-word-rubout (C-w)

Kill the word behind point, using white space as a word boundary, saving the killed text on the kill-ring.

unix-filename-rubout

Kill the word behind point, using white space and the slash character as the word boundaries, saving the killed text on the kill-ring.

delete-horizontal-space (M-\)

Delete all spaces and tabs around point.

kill-region

Kill the text in the current region.

copy-region-as-kill

Copy the text in the region to the kill buffer, so it can be yanked immediately.

copy-backward-word

Copy the word before point to the kill buffer. The word boundaries are the same as backward-word.

copy-forward-word

Copy the word following point to the kill buffer. The word boundaries are the same as forward-word.

yank (C-y)

Yank the top of the kill ring into the buffer at point.

yank-pop (M-y)

Rotate the kill ring, and yank the new top. Only works following yank or yank-pop.

Numeric Arguments¶

digit-argument (M-0, M-1, ..., M--)

Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument.

universal-argument

This is another way to specify an argument. If this command is followed by one or more digits, optionally with a leading minus sign, those digits define the argument. If the command is followed by digits, executing universal-argument again ends the numeric argument, but is otherwise ignored. As a special case, if this command is immediately followed by a character that is neither a digit nor minus sign, the argument count for the next command is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four, a second time makes the argument count sixteen, and so on.

Completing¶

complete (TAB)

Attempt to perform completion on the text before point. Bash attempts completion by first checking for any programmable completions for the command word (see Programmable Completion below), otherwise 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, functions, and builtins) in turn. If none of these produces a match, it falls back to filename completion.

possible-completions (M-?)

List the possible completions of the text before point. When displaying completions, readline sets the number of columns used for display to the value of completion-display-width, the value of the shell variable COLUMNS, or the screen width, in that order.

insert-completions (M-*)

Insert all completions of the text before point that would have been generated by possible-completions, separated by a space.

menu-complete

Similar to complete, but replaces the word to be completed with a single match from the list of possible completions. Repeatedly executing menu-complete steps through the list of possible completions, inserting each match in turn. At the end of the list of completions, menu-complete rings the bell (subject to the setting of bell-style) and restores the original text. An argument of n moves n positions forward in the list of matches; a negative argument moves backward through the list. This command is intended to be bound to TAB, but is unbound by default.

menu-complete-backward

Identical to menu-complete, but moves backward through the list of possible completions, as if menu-complete had been given a negative argument. This command is unbound by default.

export-completions

Perform completion on the word before point as described above and write the list of possible completions to readline's output stream using the following format, writing information on separate lines:

If there are no matches, the first line will be “0”, and this command does not print any output after the S:E. If there is only a single match, this prints a single line containing it. If there is more than one match, this prints the common prefix of the matches, which may be empty, on the first line after the S:E, then the matches on subsequent lines. In this case, N will include the first line with the common prefix.

The user or application should be able to accommodate the possibility of a blank line. The intent is that the user or application reads N lines after the line containing S:E to obtain the match list. This command is unbound by default.

delete-char-or-list

Deletes the character under the cursor if not at the beginning or end of the line (like delete-char). At the end of the line, it behaves identically to possible-completions. This command is unbound by default.

complete-filename (M-/)

Attempt filename completion on the text before point.

possible-filename-completions (C-x /)

List the possible completions of the text before point, treating it as a filename.

complete-username (M-~)

Attempt completion on the text before point, treating it as a username.

possible-username-completions (C-x ~)

List the possible completions of the text before point, treating it as a username.

complete-variable (M-$)

Attempt completion on the text before point, treating it as a shell variable.

possible-variable-completions (C-x $)

List the possible completions of the text before point, treating it as a shell variable.

complete-hostname (M-@)

Attempt completion on the text before point, treating it as a hostname.

possible-hostname-completions (C-x @)

List the possible completions of the text before point, treating it as a hostname.

complete-command (M-!)

Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that order.

possible-command-completions (C-x !)

List the possible completions of the text before point, treating it as a command name.

dynamic-complete-history (M-TAB)

Attempt completion on the text before point, comparing the text against history list entries for possible completion matches.

dabbrev-expand

Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.

complete-into-braces (M-{)

Perform filename completion and insert the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion above).

Keyboard Macros¶

start-kbd-macro (C-x ()

Begin saving the characters typed into the current keyboard macro.

end-kbd-macro (C-x ))

Stop saving the characters typed into the current keyboard macro and store the definition.

call-last-kbd-macro (C-x e)

Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.

print-last-kbd-macro ()

Print the last keyboard macro defined in a format suitable for the inputrc file.

Miscellaneous¶

re-read-init-file (C-x C-r)

Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there.

abort (C-g)

Abort the current editing command and ring the terminal's bell (subject to the setting of bell-style).

do-lowercase-version (M-A, M-B, M-x, ...)

If the metafied character x is uppercase, run the command that is bound to the corresponding metafied lowercase character. The behavior is undefined if x is already lowercase.

prefix-meta (ESC)

Metafy the next character typed. ESC f is equivalent to Meta-f.

undo (C-_, C-x C-u)

Incremental undo, separately remembered for each line.

revert-line (M-r)

Undo all changes made to this line. This is like executing the undo command enough times to return the line to its initial state.

tilde-expand (M-&)

Perform tilde expansion on the current word.

set-mark (C-@, M-<space>)

Set the mark to the point. If a numeric argument is supplied, set the mark to that position.

exchange-point-and-mark (C-x C-x)

Swap the point with the mark. Set the current cursor position to the saved position, then set the mark to the old cursor position.

character-search (C-])

Read a character and move point to the next occurrence of that character. A negative argument searches for previous occurrences.

character-search-backward (M-C-])

Read a character and move point to the previous occurrence of that character. A negative argument 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. CSI sequences begin with a Control Sequence Indicator (CSI), usually ESC [. If this sequence is bound to “\e[”, keys producing CSI sequences 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, insert the value of the readline comment-begin variable 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, insert the value; otherwise delete the characters in comment-begin 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.

spell-correct-word (C-x s)

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.

glob-complete-word (M-g)

Treat the word before point as a pattern for pathname expansion, with an asterisk implicitly appended, then use the pattern to generate a list of matching file names for possible completions.

glob-expand-word (C-x *)

Treat the word before point as a pattern for pathname expansion, and insert the list of matching file names, replacing the word. If a numeric argument is supplied, append a * before pathname expansion.

glob-list-expansions (C-x g)

Display the list of expansions that would have been generated by glob-expand-word and redisplay the line. If a numeric argument is supplied, append a * 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 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.

execute-named-command (M-x)

Read a bindable readline command name from the input and execute the function to which it's bound, as if the key sequence to which it was bound appeared in the input. If this function is supplied with a numeric argument, it passes that argument to the function it executes.

display-shell-version (C-x C-v)

Display version information about the current instance of bash.

Programmable Completion¶

When a user attempts word completion for a command or an argument to a command for which a completion specification (a compspec) has been defined using the complete builtin (see SHELL BUILTIN COMMANDS below), readline invokes the programmable completion facilities.

First, bash identifies the command name. 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 the empty string (completion attempted at the beginning of an empty line), bash uses any compspec defined with the -E option to complete. The -I option to complete indicates that the command word is the first non-assignment word on the line, or after a command delimiter such as ; or |. This usually indicates command name completion.

If the command word is a full pathname, bash searches for a compspec for the full pathname first. If there is no compspec for the full pathname, bash attempts to find a compspec for the portion following the final slash. If those searches do not result in a compspec, or if there is no compspec for the command word, bash uses any compspec defined with the -D option to complete as the default. If there is no default compspec, bash performs alias expansion on the command word as a final resort, and attempts to find a compspec for the command word resulting from any successful expansion.

If a compspec is not found, bash performs its default completion as described above under Completing. Otherwise, once a compspec has been found, bash uses it to generate the list of matching words.

First, bash performs the actions specified by the compspec. This only returns matches which are prefixes of the word being completed. When the -f or -d option is used for filename or directory name completion, bash uses the shell variable FIGNORE to filter the matches.

Next, programmable completion generates matches specified by a pathname expansion pattern supplied as an argument to the -G option. The words generated by the pattern need not match the word being completed. Bash uses the FIGNORE variable to filter the matches, but does not use the GLOBIGNORE shell variable.

Next, completion considers the string specified as the argument to the -W option. The string is first split using the characters in the IFS special variable as delimiters. This honors shell quoting within the string, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of IFS. 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 possible completions.

After these matches have been generated, bash executes any shell function or command specified with the -F and -C options. When the command or function is invoked, bash assigns values to the COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables as described above under Shell Variables. If a shell function is being invoked, bash also sets the COMP_WORDS and COMP_CWORD variables. 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. There is no filtering of the generated completions against the word being completed; the function or command has complete freedom in generating the matches and they do not need to match a prefix of the word.

Any function specified with -F is invoked first. The function may use any of the shell facilities, including the compgen and compopt builtins 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 will escape a newline, if necessary. These are added to the set of possible completions.

After generating all of the possible completions, bash applies any filter specified with the -X option to the completions in 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 is removed from the list. A leading ! negates the pattern; in this case bash removes any completion that does not match the pattern. If the nocasematch shell option is enabled, bash performs the match without regard to the case of alphabetic characters.

Finally, programmable completion adds any prefix and suffix specified with the -P and -S options, respectively, to each completion, and returns the result to readline 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, bash attempts directory name completion.

If the -o plusdirs option was supplied to complete when the compspec was defined, bash attempts directory name completion and adds any matches to the set of possible completions.

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 and the readline default of filename completion are disabled. If the -o bashdefault option was supplied to complete when the compspec was defined, and the compspec generates no matches, bash attempts its default completions. If the compspec and, if attempted, the default bash completions generate no matches, and the -o default option was supplied to complete when the compspec was defined, programmable completion performs readline's default completion.

The options supplied to complete and compopt can control how readline treats the completions. For instance, the -o fullquote option tells readline to quote the matches as if they were filenames. See the description of complete below for details.

When a compspec indicates that it wants directory name completion, 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 functions 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 can be used to build a set of completions dynamically as completion is attempted, rather than loading them 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

Event Designators¶

An event designator is a reference to an entry in the history list. The event designator consists of the portion of the word beginning with the history expansion character and ending with the word designator if present, or the end of the word. 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, (.

!n

Refer to history list entry n.

!-n

Refer to the current entry minus n.

!!

Refer to the previous entry. 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, this uses the string from the most recent search; 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. They are optional; if the word designator isn't supplied, the history expansion uses the entire event. A : separates the event specification from the word designator. It may be omitted if the word designator begins with a ^, $, *, -, or %. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces.

0 (zero)

The zeroth word. For the shell, this is the command word.

n

The nth word.

^

The first argument: 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. By default, searches begin at the end of each line and proceed to the beginning, so the first word matched is the one closest to the end of the line.

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; it expands to the empty string in that case.

x*

Abbreviates x-$.

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, equivalent to !!.

Modifiers¶

After the optional word designator, the expansion may include 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 pathname component, leaving only the head.

t

Remove all leading pathname 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 newlines. The q and x modifiers are mutually exclusive; expansion uses the last one supplied.

s/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. A single backslash quotes the delimiter in old and new. If & appears in new, it is replaced with old. A single backslash quotes 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.

g

Cause changes to be applied over the entire event line. This is used in conjunction with “:s” (e.g., “:gs/old/new/”) or “:&”. If used with “:s”, any delimiter can be used in place of /, and the final delimiter is optional if it is the last character of the event line. An a may be used as a synonym for g.

G

Apply the following “s” or “&” modifier once to each word in the event line.