Bash Special Parameters Reference

---
title: Bash Special Parameters Reference
author: "GNU Free Documentation: 3.4.2 Special Parameters"
date: February 9, 2020
source: https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html#Special-Parameters
notoc: true
---

> The shell treats several parameters specially. These parameters may only be
> referenced; assignment to them is not allowed.

- [`*` \(asterisk\)](#-asterisk)
- [`@` \(at sign\)](#-at-sign)
- [`#` \(hash\)](#-hash)
- [`?` \(question mark\)](#-question-mark)
- [`-` \(hyphen\)](#--hyphen)
- [`$` \(dollar symbol\)](#%24-dollar-symbol)
- [`!` \(exclamation mark\)](#-exclamation-mark)
- [`0` \(zero\)](#0-zero)
- [`_` \(underscore\)](#_-underscore)

<a id="-asterisk"></a>

## `*` (asterisk)

**(\$\*)** Expands to the positional parameters, starting from one. When the
expansion is not within double quotes, each positional parameter expands to a
separate word.

In contexts where it is performed, those words are subject to further word
splitting and pathname expansion. When the expansion occurs within double
quotes, it expands to a single word with the value of each parameter separated
by the first character of the `IFS` special variable.

That is, `"$*"` is equivalent to `"$1c$2c..."`, where `c`{.variable} 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.

<a id="-at-sign"></a>

## `@` (at sign)

**(\$@)** Expands to the positional parameters, starting from one. In contexts
where word splitting is performed, this expands each positional parameter to a
separate word; if not within double quotes, these words are subject to word
splitting.

In contexts where word splitting is not performed, this expands to a single word
with each positional parameter separated by a space.

When the expansion occurs within double quotes, 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 beginning part of the
original word, and the expansion of the last parameter is joined with the last
part of the original word.

When there are no positional parameters, `"$@"` and `$@` expand to nothing
(i.e., they are removed).

<a id="-hash"></a>

## `#` (hash)

**(\$\#)** Expands to the number of positional parameters in decimal.

<a id="-question-mark"></a>

## `?` (question mark)

**(\$?)** Expands to the exit status of the most recently executed foreground
pipeline.

<a id="--hyphen"></a>

## `-` (hyphen)

**(\$-)** Expands to the current option flags as specified upon invocation, by
the `set` built-in command, or those set by the shell itself (such as the
`-i`{.sample} option).

<a id="%24-dollar-symbol"></a>

## `$` (dollar symbol)

**(\$\$)** Expands to the process of the shell. In a `()` sub-shell, it expands
to the process of the invoking shell, not the sub-shell.

<a id="-exclamation-mark"></a>

## `!` (exclamation mark)

**(\$!)** Expands to the process of the job most recently placed into the
background, whether executed as an asynchronous command or using the `bg`
built-in.

<a id="0-zero"></a>

## `0` (zero)

**(\$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 (see
[Shell Scripts](https://www.gnu.org/software/bash/manual/html_node/Shell-Scripts.html#Shell-Scripts)),
`$0` is set to the name of that file.

If Bash is started with the `-c`{.sample} option (see
[Invoking Bash](https://www.gnu.org/software/bash/manual/html_node/Invoking-Bash.html#Invoking-Bash)),
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.

<a id="_-underscore"></a>

## `_` (underscore)

**(\$\_)** At shell startup, set to the absolute pathname used to invoke the
shell or shell script being executed as passed in the environment or argument
list.

Subsequently, expands to the last argument to the previous simple command
executed in the foreground, after expansion.

Also set to the full pathname used to invoke each command executed and placed in
the environment exported to that command.

When checking mail, this parameter holds the name of the mail file.

---

> Source:
> [GNU Free Documentation: 3.4.2 Special Parameters](https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html#Special-Parameters)