When variables are used they are referred to with the $ symbol in front of them. There are several useful variables available in the shell program. Here are a few:

 

$$ = The PID number of the process executing the shell.

$? = Exit status variable.

$0 = The name of the command you used to call a program.

$1 = The first argument on the command line.

$2 = The second argument on the command line.

$n = The nth argument on the command line.

$* = All the arguments on the command line.

$# The number of command line arguments.

The “shift” command can be used to shift command line arguments to the left, ie $1 becomes the value of $2, $3 shifts into $2, etc. The command, “shift 2” will shift 2 places meaning the new value of $1 will be the old value of $3 and so forth.