Let’s say I have a bunch of files with the extension “.abc” and I want to change all of them to end with “.doc”. I can rename each one individually in the Finder, but that could take a long time if there are a lot of files. Especially considering the warning the Finder gives you about changing the file extension.
A much faster way to do it is to launch the Terminal application, “cd” to the directory where your files exist, and use this command:
for f in *.abc; do mv ./”$f” “${f%abc}doc”; done
All of your “*.abc” files will be changed to “*.doc”.