How do I find information on bash special parameters ($* $@ $# $? $- $$ $! $0 $_)?
bash, shell, special-characters
Solution
Documentation on Bash special parameters:
$* $@ $# $? $- $$ $! $0 $_
can be found in the Bash Reference Manual, specifically in section 3.4.2, "Special Parameters". If you have the bash documentation installed on your system, you can type
% info bash
and then search for "Special Parameters".
As rici points out in a comment, within the `info` command you can also find the special parameters via the index: type `i` and then type the single character (excluding the `$`), then Enter. This doesn't work for `?`, and searching for `!` finds a different section first (typing `,` to find the next entry works). (This still works reasonably well after I apply my patch.)
It's unfortunate, IMHO, that this section refers to these parameters without the leading `$` character. (I've just submitted a patch that changes this.)
A brief summary (but read the manual for details):
- `$*`: Expands to the positional parameters starting with `$1`.
- `$@`: Also expands to the positional parameters, but behaves differently when enclosed in double quotes.
- `$#`: Expands to the number of positional parameters in decimal.
- `$?`: Expands to the exit status of the most recent command. (Similar to `$status` in csh and tcsh.)
- `$-`: Expands to the current option flags.
- `$!`: Expands to the process ID of the most recent background command.
- `$0`: Expands to the name of the shell or script. (Note that `$0`, unlike `$1` et al, is not a positional parameter.)
- `$_`: Initially set to the absolute pathname use to invoke the shell or shell script, later set to the last argument of the previous command. (There's more; see the manual.)
UPDATE :
As of bash version 4.3, released 2014-02-26, the bash documentation is annotated to show the full names of these variables. In release 4.2:
`#'
Expands to the number of positional parameters in decimal.
In release 4.3:
`#'
($#) Expands to the number of positional parameters in decimal.
Problem
(I've seen a number of questions here about Bash special parameters. It can be difficult to search for things like `$*`, both in the Bash manual and via Google. This question is intended to be a general reference for these questions.) The Bash shell defines a number of "special parameters" (which is itself a bit confusing, since most of us think of them as "variables", not "parameters"). References to them consist of a dollar sign followed by some punctuation character. Google searches for strings consisting of punctuation characters are notoriously difficult, and there are no occurrences of, for example, `$?` in the Bash Reference Manual. How can I find information on particular Bash special parameters?