Rename file extensions from a shell script

.zshrc

function rext () {
	if [ $# -lt 3 ]; then
		echo 1>&2 "$0 needs 3 arguments as DIR SOURCE_EXT DEST_EXT"
	else
		for file in "$1"/*.$2; do
		    mv "$file" "${file%.$2}.$3"
		done	
	fi
}