set -A ARGS $@ defines ARGS as an array and initializes it with the script arguments.

$@ is the list of arguments itself.

FILE=${ARGS[$(expr $# - 1)]} assigns the last argument to the FILE variable. This is the way arrays are managed on ksh

$# is the number of arguments, so

expr $# -1 is the position of the last one within the array

Is this homework or something? Have a look to rule #6 on:
Simple rules of the UNIX.COM forums:


My intention was to give you some hints so try first to do it for yourself, then ask the forum.

Regards.