How do I change a lot of file names all at once, and using the linux command line?  Look no further.  Let’s say you have a bunch of images in your current directory (let’s create them like so for testing purposes):

for x in {1..100}; do touch “image$x.gif”; done

Now that you have all 100 gif images files, just run the following command to change them to have the .jpg file extension:

for x in *.gif; do mv “$x” “${x%.gif}.jpg”; done

And presto, there you go!  Your .gif files are now .jpg files.