# Special parameters and shell variables

## Special Parameters

|parameter|character|expansion description|
|---------|---------|---------------------|
|`*`|asterisk|The positional parameters starting from the first. When used inside doublequotes (see [quoting](../syntax/quoting)), like `"$*"`, it expands to all positional parameters *as one word*, delimited by the first character of the `IFS` variable (a space in this example): `"$1 $2 $3 $4"`.<br>If `IFS` is unset, the delimiter used will be always a space, if `IFS` is NULL, the delimiter will be nothing, which effectively concatenates all the positional parameters without any delimiter.<br>When used unquoted, it will just expand to the strings, one by one, not preserving the word boundaries (i.e. word splitting will split the text again, if it contains `IFS` characters.<br>See also the [scripting article about handling positional parameters](../scripting/posparams).|
|`@`|at-sign|The positional parameters starting from the first. When used inside doublequotes (see [quoting](../syntax/quoting)), like `"$@"`, it expands all positional parameters *as separate words*: `"$1" "$2" "$3" "$4"`<br>Without doublequotes, the behaviour is like the one of `*` without doublequotes.<br>See also the [scripting article about handling positional parameters](../scripting/posparams).|
|`#`|hash mark|Number of positional parameters (decimal)<br>See also the [scripting article about handling positional parameters](../scripting/posparams).|
|`?`|question mark|Status of the most recently executed foreground-pipeline (exit/return code)|
|`-`|dash|Current option flags set by the shell itself, on invocation, or using the [set builtin command](../commands/builtin/set). It\'s just a set of characters, like `himB` for `h`, `i`, `m` and `B`.|
|`$`|dollar-sign|The process ID (PID) of the shell. In an [explicit subshell](../syntax/ccmd/grouping_subshell) it expands to the PID of the current \"main shell\", not the subshell. This is different from `$BASHPID`!|
|`!`|exclamation mark|The process ID (PID) of the most recently executed background pipeline (like started with `command &`)|
|`0`|zero|The name of the shell or the shell script (filename). Set by the shell itself.<br>If Bash is started with a filename to execute (script), it\'s set to this filename. If started with the `-c <CMDLINE>` option (commandline given as argument), then `$0` will be the first argument after the given `<CMDLINE>`. Otherwise, it is set to the string given on invocation for `argv[0]`.<br>Unlike popular belief, `$0` is *not a positional parameter*.|
|`_`|underscore|A kind of catch-all parameter. Directly after shell invocation, it\'s set to the filename used to invoke Bash, or the absolute or relative path to the script, just like `$0` would show it. Subsequently, expands to the last argument to the previous command. Placed into the environment when executing commands, and set to the full pathname of these commands. When checking mail, this parameter holds the name of the mail file currently being checked.|

## Shell Variables

### BASH

|  Variable:      |`BASH`            |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

Expands to the full file name used to invoke the current instance of
Bash.

### BASHOPTS

|  Variable:      |`BASHOPTS`        |Since:       |4.1-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |yes|
|  Set by Bash:   |yes               |Default:     |n/a|

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](../commands/builtin/shopt.md). The options appearing
in `BASHOPTS` are those reported as on by `shopt`. If this variable is
in the environment when Bash starts up, each shell option in the list
will be enabled before reading any startup files.

Example content:

    cmdhist:expand_aliases:extquote:force_fignore:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath

This variable is read-only.

### BASHPID

|  Variable:      |`BASHPID`          |Since:       |4.0-alpha|
|  -------------- |------------------ |------------ |-----------|
|  Type:          |integer variable   |Read-only:   |yes|
|  Set by Bash:   |yes                |Default:     |n/a|

Always expands to the process ID of the current Bash process. This
differs from the special parameter `$` under certain circumstances, such
as subshells that do not require Bash to be re-initialized.

### BASH_ALIASES

|  Variable:      |`BASH_ALIASES`      |Since:       |unknown|
|  -------------- |------------------- |------------ |---------|
|  Type:          |associative array   |Read-only:   |no|
|  Set by Bash:   |yes                 |Default:     |n/a|

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; unsetting array elements cause
aliases to be removed from the alias list.

The associative key is the name of the alias as used with the `alias`
builtin command.

### BASH_ARGC

|  Variable:      |`BASH_ARGC`                       |Since:       |3.0|
|  -------------- |--------------------------------- |------------ |-----|
|  Type:          |integer indexed array             |Read-only:   |no|
|  Set by Bash:   |only in extended debugging mode   |Default:     |n/a|

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` builtin
command) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed onto
`BASH_ARGC`.

### BASH_ARGV

|  Variable:      |`BASH_ARGV`                       |Since:       |3.0|
|  -------------- |--------------------------------- |------------ |-----|
|  Type:          |integer indexed array             |Read-only:   |no|
|  Set by Bash:   |only in extended debugging mode   |Default:     |n/a|

An array variable containing all of the parameters in the current Bash
execution call stack.

The final parameter of the last subroutine call is at the top of the
stack; the first parameter of the initial call is at the bottom. When a
subroutine is executed, the parameters supplied are pushed onto
`BASH_ARGV`.

### BASH_ARGV0

|  Variable:      |`BASH_ARGV0`   |Since:       |5.0-alpha|
|  -------------- |-------------- |------------ |--------------|
|  Type:          |string         |Read-only:   |no|
|  Set by Bash:   |yes            |Default:     |same as `$0`|

Expands to the name of the shell or shell script - as the special
parameter `$0` does. Assignments to `BASH_ARGV0` causes the value to be
assigned to `$0`.

If this parameter is unset, it loses its special properties, even if
subsequently reset.

### BASH_CMDS

|  Variable:      |`BASH_CMDS`         |Since:       |unknown|
|  -------------- |------------------- |------------ |---------|
|  Type:          |associative array   |Read-only:   |no|
|  Set by Bash:   |yes                 |Default:     |n/a|

An associative array variable whose members correspond to the internal
hash table of commands as maintained by the `hash` builtin
command. Elements added to this array appear in
the hash table; unsetting array elements cause commands to be removed
from the hash table.

The associative key is the name of the command as used with the `hash`
builtin command.

### BASH_COMMAND

|  Variable:      |`BASH_COMMAND`    |Since:       |3.0|
|  -------------- |----------------- |------------ |-----|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

The command currently being executed or about to be executed, unless the
shell is executing a command as the result of a trap, in which case it
is the command executing at the time of the trap.

### BASH_COMPAT

|  Variable:      |`BASH_COMPAT`     |Since:       |4.3-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The value is used to set the shell's compatibility level. The value may
be a decimal number (e.g., `4.2`) or an integer (e.g., `42`)
corresponding to the desired compatibility level. If `BASH_COMPAT` is
unset or set to the empty string, the compatibility level is set to the
default for the current version. If `BASH_COMPAT` is set to a value that
is not one of the valid compatibility levels, the shell prints an error
message and sets the compatibility level to the default for the current
version. The valid compatibility levels correspond to the compatibility
options accepted by the shopt builtin. The current version is also a
valid value.

### BASH_EXECUTION_STRING

|  Variable:      |`BASH_EXECUTION_STRING`   |Since:       |3.0|
|  -------------- |------------------------- |------------ |-----|
|  Type:          |normal variable           |Read-only:   |no|
|  Set by Bash:   |yes                       |Default:     |n/a|

The command argument to the `-c` invocation option.

### BASH_LINENO

|  Variable:      |`BASH_LINENO`           |Since:       |3.0|
|  -------------- |----------------------- |------------ |-----|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable whose members are the line numbers in source files
corresponding to each member of `FUNCNAME`.

`${BASH_LINENO[$i]}` is the line number in the source file where
`${FUNCNAME[$ifP]}` was called. The corresponding source file name is
`${BASH_SOURCE[$i]}`. Use `LINENO` to obtain the current line number.

### BASH_REMATCH

|  Variable:      |`BASH_REMATCH`          |Since:       |3.0|
|  -------------- |----------------------- |------------ |-----|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

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.

Before Bash version 5.1-alpha this variable was readonly.

### BASH_SOURCE

|  Variable:      |`BASH_SOURCE`           |Since:       |3.0|
|  -------------- |----------------------- |------------ |-----|
|  Type:          |integer |indexed |array   Read-only:   no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable whose members are the source filenames corresponding
to the elements in the `FUNCNAME` array variable.

### BASH_SUBSHELL

|  Variable:      |`BASH_SUBSHELL`   |Since:       |3.0|
|  -------------- |----------------- |------------ |-----|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

Incremented by one each time a subshell or subshell environment is
spawned. The initial value is 0.

### BASH_VERSINFO

|  Variable:      |`BASH_VERSINFO`         |Since:       |2.0|
|  -------------- |----------------------- |------------ |-----|
|  Type:          |integer indexed array   |Read-only:   |yes|
|  Set by Bash:   |yes                     |Default:     |n/a|

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

  -------------------- ----------------------------------------
  BASH_VERSINFO\[0\]   The major version number (the release)
  BASH_VERSINFO\[1\]   The minor version number (the version)
  BASH_VERSINFO\[2\]   The patch level
  BASH_VERSINFO\[3\]   The build version
  BASH_VERSINFO\[4\]   The release status (e.g., beta1)
  BASH_VERSINFO\[5\]   The value of `MACHTYPE`
  -------------------- ----------------------------------------

### BASH_VERSION

|  Variable:      |`BASH_VERSION`    |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

Expands to a string describing the version of this instance of Bash.

Since Bash 2.0 it includes the shell's "release status" (alpha\[N\],
beta\[N\], release).

### CHILD_MAX

|  Variable:      |`CHILD_MAX`       |Since:       |4.3-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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.

### COMP_CWORD

|  Variable:      |`COMP_CWORD`                                  |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |normal variable                               |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

An index into `COMP_WORDS` of the word containing the current cursor
position.

### COMP_KEY

|  Variable:      |`COMP_KEY`                                    |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |normal variable                               |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

The key (or final key of a key sequence) used to invoke the current
completion function.

### COMP_LINE

|  Variable:      |`COMP_LINE`                                   |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |normal variable                               |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

The current command line.

### COMP_POINT

|  Variable:      |`COMP_POINT`                                  |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |normal variable                               |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

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}`.

### COMP_TYPE

|  Variable:      |`COMP_TYPET`                                  |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |normal variable                               |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

Set to an integer value corresponding to the type of completion
attempted that caused a completion function to be called:

  ------- ---------------------------------------------------
  `TAB`   normal completion
  `?`     listing completions after successive tabs
  `!`     listing alternatives on partial word completion
  `@`     to list completions if the word is not unmodified
  `%`     for menu completion
  ------- ---------------------------------------------------

!!! warning "FIXME"
    where are the integer values?

### COMP_WORDBREAKS

|  Variable:      |`COMP_WORDBREAKS`   |Since:       |unknown|
|  -------------- |------------------- |------------ |---------|
|  Type:          |normal variable     |Read-only:   |no|
|  Set by Bash:   |yes                 |Default:     |n/a|

Reports the set of characters that the readline library treats as word
separators when performing word completion.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### COMP_WORDS

|  Variable:      |`COMP_WORDS`                                  |Since:       |unknown|
|  -------------- |--------------------------------------------- |------------ |---------|
|  Type:          |integer indexed array                         |Read-only:   |no|
|  Set by Bash:   |only for programmable completion facilities   |Default:     |n/a|

An array variable 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.

### COPROC

|  Variable:      |`COPROC`                |Since:       |unknown|
|  -------------- |----------------------- |------------ |---------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable created to hold the file descriptors for output from
and input to an unnamed coprocess.

### DIRSTACK

|  Variable:      |`DIRSTACK`              |Since:       |unknown|
|  -------------- |----------------------- |------------ |---------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable containing the current contents of the directory
stack.

Directories appear in the stack in the order they are displayed by the
dirs builtin. Assigning to members of this array variable may be used to
modify directories already in the stack, but the pushd and popd builtins
must be used to add and remove directories.

Assignment to this variable will not change the current directory.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### EPOCHREALTIME

|  Variable:      |`EPOCHREALTIME`    |Since:       |5.0-alpha|
|  -------------- |------------------ |------------ |-----------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

Expands to the number of seconds since Unix expoch as a floating point
value with micro-second granularity.

Assignments to this parameter are ignored. If this parameter is unset,
it loses its special properties, even if it is subsequently reset.

### EPOCHSECONDS

|  Variable:      |`EPOCHSECONDS`     |Since:       |5.0-alpha|
|  -------------- |------------------ |------------ |-----------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

Expands to the number of seconds since Unix expoch.

Assignments to this parameter are ignored. If this parameter is unset,
it loses its special properties, even if it is subsequently reset.

### EUID

|  Variable:      |`EUID`             |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |yes|
|  Set by Bash:   |yes                |Default:     |n/a|

Expands to the effective user ID of the current user, initialized at
shell startup.

!!! Danger "!"
    Do not rely on this variable when security is a concern.

### FUNCNAME

|  Variable:      |`FUNCNAME`                    |Since:       |2.04|
|  -------------- |----------------------------- |------------ |------|
|  Type:          |integer indexed array         |Read-only:   |no|
|  Set by Bash:   |only inside shell functions   |Default:     |n/a|

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 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
command](../commands/builtin/caller.md) displays the current call stack using
this information.

This variable exists only when a shell function is executing.

Assignments to this parameter have no effect and return an error status.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### GROUPS

|  Variable:      |`GROUPS`                |Since:       |2.01|
|  -------------- |----------------------- |------------ |------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable containing the list of groups of which the current
user is a member.

Assignments to this parameter have no effect and return an error status.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### HISTCMD

|  Variable:      |`HISTCMD`          |Since:       |1.14.0|
|  -------------- |------------------ |------------ |--------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

Expands to the history number (index in the history list) of the current
command.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### HOSTNAME

|  Variable:      |`HOSTNAME`        |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

Automatically set to the name of the current host.

### HOSTTYPE

|  Variable:      |`HOSTTYPE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |system-dependent|

Automatically set to a string that uniquely describes the type of
machine on which Bash is executing.

Example content:

    x86_64

### LINENO

|  Variable:      |`LINENO`           |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

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

### MACHTYPE

|  Variable:      |`MACHTYPE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |system-dependent|

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

Example content:

    x86_64-unknown-linux-gnu

### MAPFILE

|  Variable:      |`MAPFILE`               |Since:       |unknown|
|  -------------- |----------------------- |------------ |---------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable created to hold the text read by the [mapfile builtin
command](../commands/builtin/mapfile.md) when no variable name is supplied.

### OLDPWD

|  Variable:      |`OLDPWD`          |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

The previous working directory as set by the cd command.

### OPTARG

|  Variable:      |`OPTARG`          |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

The value of the last option argument processed by the `getopts` builtin
command.

### OPTIND

|  Variable:      |`OPTIND`           |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

The index of the next argument to be processed by the `getopts` builtin
command.

### OSTYPE

|  Variable:      |`OSTYPE`          |Since:       |unknown|
|  -------------- |----------------- |------------ |------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |system-dependent|

Automatically set to a string that describes the operating system on
which Bash is executing.

Example content:

    linux-gnu

### PIPESTATUS

|  Variable:      |`PIPESTATUS`            |Since:       |2.0|
|  -------------- |----------------------- |------------ |-----|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |yes                     |Default:     |n/a|

An array variable containing a list of exit status values from the
processes in the most-recently-executed foreground pipeline (which may
contain only a single command).

### PPID

|  Variable:      |`PPID`             |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |yes|
|  Set by Bash:   |yes                |Default:     |n/a|

The process ID of the shell's parent process.

### PWD

|  Variable:      |`PWD`             |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

The current working directory as set by the [cd builtin
command](../commands/builtin/cd.md).

### RANDOM

|  Variable:      |`RANDOM`           |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

Each time this parameter is referenced, a random integer between 0 and
32767 is generated. The sequence of random numbers may be initialized by
assigning a value to `RANDOM`.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### READLINE_LINE

|  Variable:      |`READLINE_LINE`   |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

The contents of the readline line buffer, for use with `bind -x`.

### READLINE_POINT

|  Variable:      |`READLINE_POINT`   |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |normal variable    |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

The position of the insertion point in the readline line buffer, for use
with `bind -x`.

### REPLY

|  Variable:      |`REPLY`                                                      |Since:       |unknown|
|  -------------- |------------------------------------------------------------ |------------ |---------|
|  Type:          |normal variable                                              |Read-only:   |no|
|  Set by Bash:   |only by the [read builtin command](../commands/builtin/read.md)   |Default:     |n/a|

Set to the line of input read by the [read builtin
command](../commands/builtin/read.md) when no arguments are supplied that
name target variables.

### SECONDS

|  Variable:      |`SECONDS`          |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |no|
|  Set by Bash:   |yes                |Default:     |n/a|

Each time this parameter is referenced, the number of seconds since
shell invocation is returned. If a value is assigned to SECONDS, the
value returned upon subsequent references is the number of seconds since
the assignment plus the value assigned.

If this parameter is unset, it loses its special properties, even if it
is subsequently reset.

### SHELLOPTS

|  Variable:      |`SHELLOPTS`       |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |yes|
|  Set by Bash:   |yes               |Default:     |n/a|

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](../commands/builtin/set.md). The options appearing in `SHELLOPTS`
are those reported as on by `set -o`.

If this variable is in the environment when Bash starts up, each shell
option in the list will be enabled before reading any startup files.

### SHLVL

|  Variable:      |`SHLVL`           |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

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

### UID

|  Variable:      |`UID`              |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |integer variable   |Read-only:   |yes|
|  Set by Bash:   |yes                |Default:     |n/a|

Expands to the user ID of the current user, initialized at shell
startup.

:!: Do not rely on this variable when security is a concern.

### BASH_ENV

|  Variable:      |`BASH_ENV`        |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

If this parameter is set when Bash is executing a shell script, its
value is interpreted as a filename containing commands to initialize the
shell, as in `~/.bashrc`. The value of `BASH_ENV` is subjected to

-   [parameter expansion](../syntax/pe.md)
-   [command substitution](../syntax/expansion/cmdsubst.md)
-   [arithmetic expansion](../syntax/expansion/arith.md)

before being interpreted as a file name.

`PATH` is not used to search for the resultant file name.

### BASH_XTRACEFD

|  Variable:      |`BASH_XTRACEFD`   |Since:       |4.1-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

If set to an integer corresponding to a valid file descriptor, Bash will
write the trace output generated when `set -x` is enabled to that file
descriptor.

The file descriptor is closed when `BASH_XTRACEFD` is unset or assigned
a new value.

Unsetting `BASH_XTRACEFD` or assigning it the empty string causes the
trace output to be sent to the standard error. Note that setting
`BASH_XTRACEFD` to 2 (the standard error file descriptor) and then
unsetting it will result in the standard error being closed.

### CDPATH

|  Variable:      |`CDPATH`          |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The search path for the [cd builtin command](../commands/builtin/cd.md).

This is a colon-separated list of directories in which the shell looks
for destination directories specified by the `cd` command.

Example content:

    .:~:/usr

### COLUMNS

|  Variable:      |`COLUMNS`         |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |on `|SIGWINCH`     Default:     |n/a|

Used by the select compound command to determine the terminal width when
printing selection lists. Automatically set upon receipt of a
`SIGWINCH`.

### COMPREPLY

|  Variable:      |`COMPREPLY`             |Since:       |unknown|
|  -------------- |----------------------- |------------ |---------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |no                      |Default:     |n/a|

An array variable from which Bash reads the possible completions
generated by a shell function invoked by the programmable completion
facility.

### EMACS

|  Variable:      |`EMACS`           |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

|  Variable:      |`ENV`             |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

Similar to `BASH_ENV`: Used when the shell is invoked in POSIX(r) mode.

### FCEDIT

|  Variable:      |`FCEDIT`          |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The default editor for the `fc` builtin command.

### FIGNORE

|  Variable:      |`FIGNORE`         |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

A colon-separated list of suffixes to ignore when performing filename
completion. A filename whose suffix matches one of the entries in
`FIGNORE` is excluded from the list of matched filenames.

Example content:

    .o:~

### FUNCNEST

|  Variable:      |`FUNCNEST`        |Since:       |4.2-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

Negative values, 0 or non-numeric assignments have the effect as if
`FUNCNEST` was unset or empty: No nest control

### GLOBIGNORE

|  Variable:      |`GLOBIGNORE`      |Since:       |2.0|
|  -------------- |----------------- |------------ |-----|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

A colon-separated list of patterns defining the set of filenames to be
ignored by pathname expansion. If a filename matched by a pathname
expansion pattern also matches one of the patterns in `GLOBIGNORE`, it
is removed from the list of matches.

### HISTCONTROL

|  Variable:      |`HISTCONTROL`     |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

A colon-separated list of values controlling how commands are saved on
the history list:

| | |
|  ---------------|-------------------------------------------------------------------------|
| `ignorespace`   | lines which begin with a space character are not saved in the history list|
|  `ignoredups`   | don't save lines matching the previous history entry|
|  `ignoreboth`   | short for `ignorespace:ignoredups`|
|  `erasedups`    | remove all previous lines matching the current line from the history list before the current line is saved|

Any value not in the above list is ignored.

If `HISTCONTROL` is unset, or does not include a valid value, all lines
read by the shell parser are saved on the history list, subject to the
value of `HISTIGNORE`. The second and subsequent lines of a multi-line
compound command are not tested, and are added to the history regardless
of the value of `HISTCONTROL`.

### HISTFILE

|  Variable:      |`HISTFILE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |---------------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     \'\' \~/.bash_history\'\'

The |name of the file in which command history is saved.|

If unset, the command history is not saved when an interactive shell
exits.

### HISTFILESIZE

|  Variable:      |`HISTFILESIZE`    |Since:       |unknown|
|  -------------- |----------------- |------------ |------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     `HISTSIZE`

The |maximum number of lines contained in the history file.|

When this variable is assigned a value, the history file is truncated,
if necessary, by removing the oldest entries, to contain no more than
the given number of lines. If the given number of lines is 0 (zero), the
file is truncated to zero size. Non-numeric values and numeric values
less than zero inhibit truncation.

The history file is also truncated to this size after writing it when an
interactive shell exits.

### HISTIGNORE

|  Variable:      |`HISTIGNORE`      |Since:       |2.0|
|  -------------- |----------------- |------------ |-----|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

A colon-separated list of patterns used to decide which command lines
should be saved on the history list. Each pattern is anchored at the
beginning of the line and must match the complete line (no implicit
'`*`' is appended).

Each pattern is tested against the line after the checks specified by
`HISTCONTROL` are applied.

In addition to the normal shell pattern matching characters, "&"
matches the previous history line. "&" may be escaped using a
backslash; the backslash is removed before attempting a match.

The second and subsequent lines of a multi-line compound command are not
tested, and are added to the history regardless of the value of
`HISTIGNORE`.

### HISTSIZE

|  Variable:      |`HISTSIZE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |-----------------------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     |set at compile time (default 500)|

The number of commands to remember in the command history.

If the number is set to 0 (zero), then the history list is disabled. If
the number is set to any negative number, then the history list is
unlimited.

### HISTTIMEFORMAT

|  Variable:      |`HISTTIMEFORMAT`   |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |normal variable    |Read-only:   |no|
|  Set by Bash:   |no                 |Default:     |n/a|

If this variable is set and not null, its value is used as a format
string for `strftime(3)` to print the time stamp associated with each
history entry displayed by the history builtin.

If this variable is set, time stamps are written to the history file so
they may be preserved across shell sessions. This uses the history
comment character to distinguish timestamps from other history lines.

### HOME

|  Variable:      |`HOME`            |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The home directory of the current user.

The default argument for the [cd builtin command](../commands/builtin/cd.md).

The value of this variable is also used when performing [tilde
expansion](../syntax/expansion/tilde.md).

### HOSTFILE

|  Variable:      |`HOSTFILE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

Contains the name of a file in the same format as `/etc/hosts` that
should be read when the shell needs to complete a hostname.

The list of possible hostname completions may be changed while the shell
is running. the next time hostname completion is attempted after the
value is changed, Bash adds the contents of the new file to the existing
list.

If `HOSTFILE` is set, but has no value, or does not name a readable
file, Bash attempts to read `/etc/hosts` to obtain the list of possible
hostname completions.

When `HOSTFILE` is unset, the hostname list is cleared.

### IFS

|  Variable:      |`IFS`             |Since:       |unknown|
|  -------------- |----------------- |------------ |-------------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |`<space><tab><newline>`|

The Internal Field Separator |that is used for word splitting after|
expansion and to split lines into words with the read builtin command.

### IGNOREEOF

|  Variable:      |`IGNOREEOF`       |Since:       |unknown|
|  -------------- |----------------- |------------ |-------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |10 (when invalid)|

Controls the action of an interactive shell on receipt of an `EOF`
character (e.g. by Ctrl-D) as the sole input.

If set, the value is the number of consecutive EOF characters which must
be typed as the first characters on an input line before Bash exits.

If the variable exists but does not have a numeric value, or has no
value, the default value is 10.

If it does not exist, `EOF` signifies the end of input to the shell.

### INPUTRC

|  Variable:      |`INPUTRC`         |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The filename for the readline startup file, overriding the default of
`~/.inputrc`.

### LANG

|  Variable:      |`LANG`            |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

### LC_ALL

|  Variable:      |`LC_ALL`          |Since:       |unknown|
  -------------- ----------------- ------------ ---------
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

### LC_COLLATE

|  Variable:      |`LC_COLLATE`      |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

|  Variable:      |`LC_CTYPE`        |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

### LC_MESSAGES

|  Variable:      |`LC_MESSAGES`     |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

### LC_NUMERIC

|  Variable:      |`LC_NUMERIC`      |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

This variable determines the locale category used for number formatting.

### LINES

|  Variable:      |`LINES`           |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |on `|SIGWINCH`     Default:     |n/a|

Used by the select compound command to determine the column length for
printing selection lists. Automatically set upon receipt of a
`SIGWINCH`.

### MAIL

|  Variable:      |`MAIL`            |Since:       |unknown|
|  -------------- |----------------- |------------ |------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |system-dependent|

If this parameter is set to a file or directory name and the `MAILPATH`
variable is not set, Bash informs the user of the arrival of mail in the
specified file or Maildir-format direc‐ tory.

### MAILCHECK

|  Variable:      |`MAILCHECK`       |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |60|

Specifies |how often (in seconds) Bash checks for mail.|

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

|  Variable:      |`MAILPATH`        |Since:       |unknown|
|  -------------- |----------------- |------------ |------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |system-dependent|

A colon-separated list of file names to be checked for mail.

The message to be printed when mail arrives in a particular file may be
specified by separating the file name from the message with a '?'
(question mark).

When used in the text of the message, `$_` expands to the name of the
current mailfile.

Example content:

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

### OPTERR

|  Variable:      |`OPTERR`          |Since:       |unknown|
|  -------------- |----------------- |------------ |--------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |1 (set on startup)|

If set to the value 1, Bash displays error messages generated by the
`getopts` builtin command.

`OPTERR` is initialized to 1 each time the shell is invoked or a shell
script is executed.

### PATH

|  Variable:      |`PATH`            |Since:       |unknown|
|  -------------- |----------------- |------------ |----------------------------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |system-dependent (set on compile time)|

The search path for commands. This is a colon-separated list of
directories in which the shell looks for commands.

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.

There can be a static path compiled in for use in a restricted shell.

### POSIXLY_CORRECT

|  Variable:      |`POSIXLY_CORRECT`   |Since:       |unknown|
|  -------------- |------------------- |------------ |---------|
|  Type:          |normal variable     |Read-only:   |no|
|  Set by Bash:   |no                  |Default:     |n/a|

If this variable is in the environment when Bash starts, the shell
enters posix mode before reading the startup files, as if the `--posix`
invocation option had been supplied.

If it is set while the shell is running, Bash enables posix mode, as if
the command `set -o posix` had been executed.

### PROMPT_COMMAND

|  Variable:      |`PROMPT_COMMAND`   |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |normal variable    |Read-only:   |no|
|  Set by Bash:   |no                 |Default:     |n/a|

If set, the value is executed as a command prior to issuing each primary
prompt.

### PROMPT_COMMANDS

|  Variable:      |`PROMPT_COMMANDS`       |Since:       |5.1-alpha|
|  -------------- |----------------------- |------------ |-----------|
|  Type:          |integer indexed array   |Read-only:   |no|
|  Set by Bash:   |no                      |Default:     |n/a|

If set, each element is executed as a command prior to issuing each
primary prompt (like `PROMPT_COMMAND`, just as array).

### PROMPT_DIRTRIM

|  Variable:      |`PROMPT_DIRTRIM`   |Since:       |unknown|
|  -------------- |------------------ |------------ |---------|
|  Type:          |normal variable    |Read-only:   |no|
|  Set by Bash:   |no                 |Default:     |n/a|

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.

Characters removed are replaced with an ellipsis.

### PS0

|  Variable:      |`PS0`             |Since:       |4.4.0|
|  -------------- |----------------- |------------ |--------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     |\"\'\'\'\'\"|

Expanded |and displayed by interactive shells after reading a complete|
command but before executing it.

### PS1

|  Variable:      |`PS1`             |Since:       |unknown|
|  -------------- |----------------- |------------ |--------------------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     |\"\'\'\\s-\\v\\\$ \'\'\"|

The |value of this parameter is expanded and used as the primary prompt|
string. See [Controlling the
Prompt](https://www.gnu.org/software/bash/manual/bash.html#Controlling-the-Prompt).

### PS2

|  Variable:      |`PS2`             |Since:       |unknown|
|  -------------- |----------------- |------------ |-----------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     |\"\'\'> \'\'\"|

The |value of this parameter is expanded as with PS1 and used as the|
secondary prompt string.

### PS3

|  Variable:      |`PS3`             |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The value of this parameter is used as the prompt for the select
command.

### PS4

|  Variable:      |`PS4`             |Since:       |unknown|
|  -------------- |----------------- |------------ |----------------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |if unset          |Default:     \"\'\'+ \'\'\"

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

### SHELL

|  Variable:      |`SHELL`           |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The full pathname to the shell is kept in this environment variable. If
it is not set when the shell starts, Bash assigns the full pathname of
the current user's login shell.

### SRANDOM

|  Variable:      |`SRANDOM`         |Since:       |5.1-alpha|
|  -------------- |----------------- |------------ |-----------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |yes               |Default:     |n/a|

A variable that delivers a 32bit random number. The random number
generation uses platform specific generators in the background and a
builtin fallback generator.

### TIMEFORMAT

|  Variable:      |`TIMEFORMAT`      |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The value of this parameter is used as a format string specifying how
the timing information for pipelines prefixed with the time reserved
word should be displayed.

The % character introduces an escape sequence that is expanded to a time
value or other information. The escape sequences and their meanings are
as follows, the braces denote optional portions:

|            |                                              |
|:-----------|:---------------------------------------------|
| `%%`       | a literal `%` (percent sign)                 |
| `%[p][l]R` | elapsed time in seconds                      |
| `%[p][l]U` | number of CPU seconds spent in user mode     |
| `%[p][l]S` | number of CPU seconds spent in system mode   |
| `%P`       | CPU percentage, computed as `(%U + %S) / %R` |

The optional modifiers (p and l) are:

|  |  |
|--|--|
| `p` | A digit specifying the precision. A value of 0 causes no decimal point or fraction to be output. At most three digits after the decimal point are shown. If not specified, the value 3 is used. |
| `l` | A longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.                                                                   |

If this variable is not set, Bash acts as if it had the value

    $'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'

If the value is null, no timing information is displayed.

A trailing newline is added when the format string is displayed.

### TMOUT

|  Variable:      |`TMOUT`           |Since:       |2.05b|
|  -------------- |----------------- |------------ |-------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

If set to a value greater than zero, `TMOUT` is treated as the default
timeout for the [read builtin command](../commands/builtin/read.md).

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 input after issuing the primary prompt. Bash
terminates after waiting for that number of seconds if input does not
arrive.

### TMPDIR

|  Variable:      |`TMPDIR`          |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

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

|  Variable:      |`auto_resume`     |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

This variable controls how the shell interacts with the user and job
control. If this variable is set, single word simple commands without
redirections are treated as candidates for resumption of an existing
stopped job. There is no ambiguity allowed; if there is more than one
job beginning with the string typed, the job most recently accessed is
selected. The name of a stopped job, in this context, is the command
line used to start it. If set to the value exact, the string supplied
must match the name of a stopped job exactly; if set to substring, the
string supplied needs to match a substring of the name of a stopped job.
The substring value provides functionality analogous to the %? job
identifier.

If set to any other value, the supplied string must be a prefix of a
stopped job's name; this provides functionality analogous to the
`%string` job identifier.

### histchars

|  Variable:      |`histchars`       |Since:       |unknown|
|  -------------- |----------------- |------------ |---------|
|  Type:          |normal variable   |Read-only:   |no|
|  Set by Bash:   |no                |Default:     |n/a|

The two or three characters which control history expansion and
tokenization.

The first character is the history expansion character, the character
which signals the start of a history expansion, normally '!'
(exlamation mark).

The second character is the quick substitution character, which is used
as shorthand for re-running the previous command entered, substi tuting
one string for another in the command. The default is '^' (carret).

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 '#' (hash mark). The history comment character causes
history substitution to be skipped for the remaining words on the line.
It does not necessarily cause the shell parser to treat the rest of the
line as a comment.