Using defaults and "${VAR:?}"

[stuartpb]

If you don't assert the existence of certain important variables, your script could inadvertently wipe the user's home directory:

https://github.com/ValveSoftware/steam-for-linux/issues/3671

https://news.ycombinator.com/item?id=8896186

[stuartpb]

There's also set -u

[progrium]

I generally don't like set -u, but I do like "${foo:?}" when needed.

[stuartpb]

I'm not into set -u either, but it's worth noting for people who want to write their own style guides. (It's also worth noting as a "critical section" wrapper that can be paired with set +u.)