So this is short script used to rename extension of files i found on tldp.#!/bin/bashMysterious=65case $# in 0|1) echo Usage: `basename $0` old_file_extension new_file_extension exit $Mysterious ;;esacfor filename in *.$1do mv $filename ${filename%$1}$2doneexit 0I don't understand about the value of Mysterious variable. Why did the script's owner choose 65 and why we had to use case $# in 0 or 1? I changed value 65 into 100 and it still worked. 
What's the meaning of this variable in Bash script?
shell script
The person who wrote that script apparently wrongly thought that the exit status could represent a security issue.  If they had named it parmError or something like that it would have made more sense.Here's a better way to do this that also supports spaces and special chars:#!/bin/bashif test $# -ne 2then        echo Usage: $0 old_file_extension new_file_extension        exit 2fifor filename in *.$1do        mv $filename ${filename%$1}$2doneNote that there's no need to return 0 as it's the default.