I am new to Ubuntu and systemd (was a long time away from Linux) and I was trying out
systemctl list-unit-files - oddly it failed with the message "Not a directory". The command
systemctl list-units seems to work fine though. I googled around, and looked here, but I couldn't find a mention of this error condition. My installation (16.04.1 LTE) is only a day old, so I hardly think it could be corruption yet.
What is this error, and how should I have gone about finding what was wrong?