Built-in Shell Variables (and Arrays)

The $ sign is actually not part of the variable name, although some built-in variables are always referenced this way. We do the same.

$0

The first element passed to the shell is the command name.

$n

The n^th argument passed on the command line. If n ≥ 10, then the syntax must be ${n}.

$*

All the arguments on the command line. The values are separated by the first character in the shell variable IFS: (${1} … ${n}). See also: the IFS entry in Other Shell Variables.

$@

All the arguments on the command line. The values are individually quoted: ("${1}" … "${n}").

$#

The number of command-line arguments.

$?

The exit value of the last executed command.

$_

The last argument of the previous command.

$!

The process ID of the most recent background process.

BASH

The full path used to invoke this instance of Bash.

BASH_REMATCH

This is actually an indexed array, not a variable.

If a [[ "${string}" =~ ${regex} ]] statement is true (i.e. when the ${regex} matches the ${string}), then BASH_REMATCH[0] contains the entire matched substring and the BASH_REMATCH[n] contain the matched sections from the left to the right, from number 1 to n.

See also: Regular Expressions.

BASH_VERSION

A string describing the Bash version.

Examples of use:

• If more than one version of Bash are installed on the computer, then the running one can be retrieved by:

  bash -c 'echo $BASH_VERSION'
  

• To prompt only the numeric “major.minor.patch” part of the version string:

  echo ${BASH_VERSION%%[^0-9.]*}
  

• To verify if the running Bash is at least version 4:

  if (( $(bash -c 'echo ${BASH_VERSION%%.*}') >= 4 )); then
    echo "yes"
  else
    echo "no"
  fi

• To verify if Bash is at least of version 3.2:

  if printf '%s\n%s\n' "$BASH_VERSION" "3.2" | sort -rVC; then
    echo "yes"
  else
    echo "no"
  fi

HOSTNAME

The name of the current host.

MAPFILE

The default array for the built-in mapfile and readarray commands.

OPTARG

The value of arguments to last option processed by the built-in getopts command.

OPTIND

The numerical index of OPTARG in a built-in getopts command.

PWD

The current working directory (“print working directory”). It can be changed by the built-in cd command.

2022-02-12