OctoPi not using Additional Serial Ports (Solved, but a puzzle)

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1

I use Klipper firmware on the printer, which I understand works through a kind of virtual serial port.
Installation instructions say to put '~/printer_data/comms/klippy.serial' in the "Additional serial ports" box on the Octoprint settings.
It's been working fine, up until just now: after a break of a week or few I went to print something and found it was failing to connect.
Looking at the terminal I could see that it was looking on /dev/ttyACM0 and /dev/ttyS0 but it wasn't looking anywhere else.
Part way through writing this post, it occurred to me to try the full path, and that worked, but I thought I would finish the post in case anyone else has similar problems.

Also because I am puzzled as to why it happened and I'm hoping someone can explain, or suggest how I can diagnose the problem.

2

Have you tried?

/home/pi/printer_data/comms/klippy.serial

in Additional serial ports?

3

Yes, sorry: was I not clear?
When I put the full path in, it works fine, but when I referenced it from ~/ it doesn't work.

4

Sorry, not on first sight.

BTW: I never used the relative path.

5

I just followed the instructions. That isn't a relative path: ~/ is Home... or at least, I thought it was.

6

There are some process contexts where ~/ is not defined. Perhaps OctoPrint is one of those.