Thank you, Norman. This is the output I got from the Mint terminal:
DISTRIB_DESCRIPTION="Linux Mint 19.3 Tricia"
PRETTY_NAME="Linux Mint 19.3"
cat: /etc/upstream-release: Is a directory
This is fixed by doing '/etc/os-release' instead of using the wildcard version that was one of the samples I'd seen. Now I just have to make sure that that doesn't break the other distros :-)
Incidentally, the prime reason I was doing this was that, on non-Ubuntu releases, it seems I have to widen my window a tad or it truncates my toolbar. While Mint was ignoring the nested code in the error test, since it counts as Ubuntu it didn't have to do anything special anyway. But then I decided to get a string with the distro name, to add to the app's logging feature (which previously only reported "Linux".) That's when I caught this :-)
Thank you too, Tim. Your reply also came in while I was typing :-)