This nice little Perl script is the most logical way that I’ve seen to rename a bunch of files. Well for UNIX/Perl geeks at least. You use nice little Perlish regular expression replacements to do the renaming. E.g.:
ren-regexp 's/JPG/jpg/' *
http://www.michael-forman.com/perl/ren-regexp.html (broken)
http://replay.waybackmachine.org/20090223051758/http://michael-forman.com/perl/ren-regexp.html
https://github.com/msabramo/ren-regexp
As pointed out by Jud, here’s another tool, which offers some additional features:
The python version “remv’ is my personal favorite. It has a few more features (like recursive renaming) that I find to be very helpful. https://moin.conectiva.com.br/ReMv