Communicate with Systemd process through socket

Help
1

I’m new to both systemd and NixOs and chances are that the problem below lies more on my misunderstanding about systemd process and sockets.

I have the following script /tmp/myservice.sh:

#! /usr/bin/env bash

while read received_cmd
do
	echo "Received command ${received_cmd}"
done

And the following configuration in my configuration.nix:

  # Custom service
  systemd.user.sockets.myservice = {
    description = "Socket to communicate with myservice";
    listenStreams = [ "/tmp/myservice.socket" ];
  };
  systemd.user.services.myservice = {
    description = "A simple service example";
    serviceConfig = {
      Type = "simple";
      ExecStart = "${pkgs.bash}/bin/bash /tmp/myservice.sh";
       StandardInput = "socket";
       StandardOutput = "socket";
       StandardError = "journal";
    };
  };

The idea is to understand how to communicate with a background service, here using an unix file socket. The script works well launch from the shell and reading stdin and I thought that by setting StandardInput = "socket" it would read from the socket the same way.

Nevertheless, when I run nc -U /tmp/myservice.socket the command returns right away and I have the following output:

$ journalctl --user -u myservice
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21941]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21942]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21943]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21944]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: Started A simple service example.
Oct 29 08:40:16 shiny bash[21945]: /tmp/myservice.sh: line 3: read: read error: 0: Invalid argument
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Succeeded.
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Start request repeated too quickly.
Oct 29 08:40:16 shiny systemd[1689]: myservice.service: Failed with result 'start-limit-hit'.
Oct 29 08:40:16 shiny systemd[1689]: Failed to start A simple service example.

Did I misunderstand how sockets work? Why read fails to read from the socket? Should I use another mechanism to communicate with my background service (as I said, it’s my first background service so I may do unconventional things here)?

2

Looks like a syntax error in your service script. You seem to have copied the text from a pager (aka less) and the details are cut off.

Could you provide the full line?

E.g.
journalctl --since '2020-10-29 08:40:16' | grep 'Invalid argument' | head -n1 | xclip -i -sel clip

3

Thanks for pointing it out @boxofrox! Sorry for the wrong copy/paste. I edited the original post.

4

Can you please share how generated units look like?