To rename a bunch of files from "*.foo" to "*.bar" (like windows' REN *.foo *.bar) in unix.

Looking at the solution, it is easy to adapt it to more powerful character substitutions on a 1 to 1 basis.

If you want to test what will be executed before doing so, just drop the last pipe.

ls -d *.foo | sed -e 's/.*/mv & &/' -e 's/foo$/bar/' | sh