- regular expressions, including the hairy stuff like back references or lookarounds. When you need it, you're glad you know it exists, event if you need a few

- regular expressions, including the hairy stuff like back references or lookarounds. When you need it, you're glad you know it exists, event if you need a few minutes to look for the syntax. - shell scripting + Linux commands. Not having friction when writing a shell script is amazing, it's extremely productive. - awk. I rarely use it (I use cut, sed, or perl one liners in priority), but to make some non trivial changes it's great to be able to keep a state between parsed lines. - git. I've never had to delete/re-clone a repository since I've read the Git Book. - SQL querying. Same thing, I have no friction when I need to write group by, CTEs, or other intermediate stuff. If you do frontend stuff: - CSS. It's a quirky language so you need to learn how it works properly, not hacking stuff here and there. I've never had any issues doing anything in CSS since I learned it for real.