minor changes, mostly inspired by shellcheck suggestions

ISTR that I didn't want to clutter things up with this (mostly trivial) MR shortly before the release, but having just noticed the branch in my local repo. I'll create an MR now to avoid it being lost.

I've tried to make sure that the many commits are independent, so that you should be able to cherry-pick the ones you like, while ignoring things you don't.

BTW I'm in no way emotionally invested in these changes, I just started out tweaking things that were being highlighted by shellcheck, and ended up having a bit of an OCD moment getting rid of all the flaws I could find, and putting them into a sensible set of commits.