HOWTO change the extension of lots of files while keeping the original filename

With rename you get all the power of perl regex substitution in a simple interface. e.g. to backup all the .yaml files in a directory you could use:

rename --no-act 's/(.*)\.yaml$/$1.yaml.bak/' *.yaml

Remove the --no-act to actually make the changes.

The next logical progression is to want to do this more than once, so adding a timestamp to the backup is desirable, but I couldn't think of a way to make rename do this (you can't put backticks in the regex for instance). So here's a workaround

find . -name *.yaml -exec mv {} {}.bak.`date +%Y-%m-%dT%H:%M:%S%z` \;