The exit statement will exit the current shell script. It can be given a numeric argument which is the script's exit status. If omitted the exit status of the last run command is used. 0 (zero) signifies success, non-zero signifies failure. For example:
if [ $# -ne 2 ] # "$#" is number of parameters- here we test # whether it is not equal to two then echo "Usage $0 \<file1\> \<file2\>" #not two parameters # so print message exit 2 # and fail ($0 is # name of command). fi <rest of script>
This script is supposed to take two positional arguments. It will exit with status 2 (error) rather than 0 (success) if it is not called with two parameters.