Special Parameters

$<digit>

${<number>}

Positional parameter are assigned from the shell's arguments when it is invoked, and may be reassigned using the set builtin command. The shift command shifts the parameters FIXME.

$0

Special positional parameter: holds the Proccess ID of the current process

$*

Expands to the positional parameters, starting from one. It is equivalent to "$1 $2 ..."
Almost every time $@ should be used in place of $*

$@

Expands to the positional parameters, starting from one. It is equivalent to "$1" "$2" ...

$#

Expands to the number of positional parameters

$?

Expands to the exit status of the most recently executed foreground pipeline

$$

Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the invoking shell, not the subshell

$!

Expands to the process ID of the most recently executed background (asynchronous) command