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.
