I needed to quickly rename a bunch of file extensions in a directory. This one liner made quick work of it:
for old in *.JPG; do cp $old `basename $old .JPG`.jpg; done
basename, when given a file name and a extension spits out something like this:
user@server$ basename derp.txt .txt derp user@server$
Which we then use to append the proper extension (.jpg) in the example and done.