Hacker News Reader

Comment by evgpbfhnr | Hacker News Reader

Note if the author reads this (perfect is the enemy of good, feel free to ignore): You can make udev run systemd services directly, replacing RUN=... by e.g. TAG+="systemd", ENV{SYSTEMD_WANTS}+="foo.service"

That might require you also remove the 'enabled = true' for the service, and make sure gphoto2 dies when the camera is unplugged?

Anyway, it was a nice read :)

View on HN

Using your suggestion I ended up with the following for a Canon 7D:

  services.udev.extraRules = ''
    ACTION=="add", \
    ATTR{idVendor}=="04a9", \
    ATTR{idProduct}=="319a", \
    ENV{SYSTEMD_WANTS}+="external_webcam.service", \
    SUBSYSTEM=="usb", \
    TAG+="systemd"
  '';

  systemd.services.external_webcam = {
    enable = true;
    script = ''
      ${pkgs.gphoto2}/bin/gphoto2 --stdout --capture-movie |
        ${pkgs.ffmpeg}/bin/ffmpeg -i - -vcodec rawvideo -pix_fmt yuv420p -f v4l2  /dev/video0
    '';
    wantedBy = ["multi-user.target"];
  };

Worked first try! [1]

[1] https://gitlab.com/engmark/root/-/commit/ae59aad118e6effcf47...

Wow thanks for the suggestion! I knew there must be some way to do it with fewer steps but didn't quite figure out what it was so I really appreciate this comment. When I get a chance I'll test what you said out!

Make sure it doesn't end up stopping the service on system activation, if you do that.

Huh, I was solving similar problem some time ago. I needed to run some pulseaudio commands (to split stereo input into 2 mono ones) after USB device was reconnected and my solution was RUN script adding a systemd timer (the commands had to be ran after pulseaudio registered new device) that just ran it few seconds after device was connected

Is BindsTo= the right way to have systemd shut the service down if the device disappears?

It's been a while but yes BindsTo is a good idea if you can express the device in a reliable way. Unfortunately for USB devices the path will look like something that depends on the USB port it's been plugged in (I only have a sound card on my hands, but something like these two alternatives on two different ports):

    sys-devices-pci0000:00-0000:00:14.0-usb1-1\x2d4-1\x2d4:1.0-sound-card1-controlC1.device
    sys-devices-pci0000:00-0000:00:14.0-usb1-1\x2d1-1\x2d1:1.0-sound-card1-controlC1.device

notice the usb-`1-1:4` became `1-1:1`

That makes BindsTo unpractical unless you're sure you'll always plug in the camera on the same port everytime, so it might not be best.

This made me look up how I'd do this if the needs arise, so I haven't tried, but `StopWhenUnneeded=true` in the service definition looks like it would stop it properly when the device "wanting" it is gone? cf. https://unix.stackexchange.com/questions/528803/systemd-does...

I learned something new :)

Reply on news.ycombinator.com