Special Shell Script Variables

There are some variables which are set internally by the shell and which are available to the user:

$1 - $9   These variables are the positional parameters.

$0        The name of the command currently being executed.

$#        The number of positional arguments given to this invocation of the shell.

$?        The exit status of the last command executed is given as a decimal string.
       When a command completes successfully, it returns the exit status of 0
       (zero), otherwise it returns a non-zero exit status.

$$        The process number of this shell - useful for including in filenames, to
       make them unique.

$!        The process id of the last command run in the background.

$-        The current options supplied to this invocation of the shell.

$*        A string containing all the arguments to the shell, starting at $1.

$@@       Same as above, except when quoted.