Special parameters

Special parameters are predefined parameters that have symbolic names and provide specific information about the shell environment. They are not user-defined variables and cannot be assigned values with the assignment syntax.

Below are the special parameters and their meanings:

• @: All positional parameters.

  • Expands to all positional parameters as separate fields. Useful for passing all arguments as is to a utility or function.
  • When expanded outside double quotes, the result is subject to field splitting and pathname expansion. To preserve each parameter as a separate field, use "$@". If there are no positional parameters, "$@" expands to zero fields.
  • In contexts where only one field is expected (such as in the content of a here document), @ expands to a single field with all positional parameters joined by the first character of the IFS variable (defaults to space if unset, or no separator if IFS is empty).

  $ set foo 'bar bar' baz # three positional parameters
  $ for value in "$@"; do echo "[$value]"; done
  [foo]
  [bar bar]
  [baz]
  $ for value in $@; do echo "[$value]"; done
  [foo]
  [bar]
  [bar]
  [baz]
  

• *: All positional parameters.

  • Similar to @, but in double quotes, * expands to a single field containing all positional parameters joined by the first character of IFS.

  $ set foo 'bar bar' baz # three positional parameters
  $ for value in "$*"; do echo "[$value]"; done
  [foo bar bar baz]
  $ for value in $*; do echo "[$value]"; done
  [foo]
  [bar]
  [bar]
  [baz]
  

• #: Number of positional parameters.

  $ set foo 'bar bar' baz
  $ echo "$#"
  3
  

• ?: Exit status of the last command.

• -: Current shell options.

  • Expands to the short names of all currently set shell options, concatenated together. Options without a short name are omitted. For example, if -i and -m are set, the value is im.

• $: Process ID of the current shell.

  • Set when the shell starts and remains constant, even in subshells.

• !: Process ID of the last asynchronous command.

  • Updated when an asynchronous command is started or resumed.

• 0: Name of the shell or script being executed.

  • Set at shell startup and remains constant.
  • If neither the -c nor -s shell option is active, the value of 0 is the first operand in the shell invocation (the script pathname).
  • If the -c option is used and a second operand is present, that operand is used as 0.
  • Otherwise, 0 is set to the first argument passed to the shell, usually the shell’s name.