Change File Extensions
Ubuntu 14.04
find . -name '*.JPEG' | awk -F "." '{print $2}' | xargs -I'{}' mv ./{}.JPEG ./{}.jpg
Ubuntu 18.04
find . -name '*.JPG' | awk -F "." '{print $2}' | awk -F "/" '{print $2}' | xargs -I'{}' mv ./{}.JPG ./{}.gif
Rename Files
rename 's/Content-to-be-Changed/Modified-Content/' filename
^ Start from the beginning
$ Start from the end
Alternatively, use
rename Content-to-be-Changed Modified-Content filename