Renaming multiple files

For [zsh][1] there is zmv module which must be loaded with autoload; make sure extendedglob is on

autoload -U zmv
setopt extendedglob

Lower case the filename in the current directory

zmv '(*)' '${(L)1}'

Lower case the extension for each file in the current directory:

zmv '(*.JPG)' '${1:r}.${(L)1:e}'

or using recursive traversal:

zmv '(**/)(*.JPG)' '$1${2:r}.${(L)2:e}'

On Debian/Ubuntu there is [rename][2] tool.

apt-get install rename

Lower case the entire filename in the current directory

rename 'y/A-Z/a-z/' *

If you get Argument list too long error, combine it with find

find . -exec rename 'y/A-Z/a-z/' {} \;

On case insensitive filesystems such as OS X's HFS+, you need to add the -f flag

rename -f 'y/A-Z/a-z/' *