The Classic Backup Trick

cp config.yml{,.bak} expands to cp config.yml config.yml.bak. A classic trick for making a quick backup before editing a config file.

The key is the empty string before the comma. {,.bak} is a list expansion of "empty string" and ".bak", generating config.yml and config.yml.bak.

mv file.txt{,.old} for renaming, diff config.yml{,.bak} for diffing. The applications are endless. Brace expansion makes everyday operations just a little smarter.