Reserved variable names used for Bash Scripts.
These are:
$0-$9: Reference to each command-line Argument.$#: The number of arguments passed to the Script.$@: The list of command-line Arguments.$$: Process ID of the currently running process.$?: Exit status of the script. Is equal to 0 for success and 1 for failure.
The special variables use the Internal Field Separator to identify when an argument ends and the next begins.