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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.

Fabio Laganà Linux, Shell Linux, Variables