Systemd services should allow running commands as root

ConradMearns · July 2, 2020, 4:32am

Hi ya’ll, I’m trying to run a service that executes a script as root. Having no luck.

I’m trying to make a test script to blink the builtin LED on a Raspberry Pi 4. Only root has access to these kinds of devices.

This is my sd-image.nix, root-ssh.nix just has some mDNS and SSH keys inside.

{ config, pkgs, ... }:{

  imports = [
    <nixpkgs/nixos/modules/installer/cd-dvd/sd-image-raspberrypi4.nix>
    ./root-ssh.nix
  ];

  config.networking.hostName = "mo";

  config.users.users.sprout = {
    isNormalUser = true;
    extraGroups = [ "wheel" ];
  };
  
  config.networking.wireless.enable = false;

  config.systemd.services.led-test = {
    wantedBy = [ "multi-user.target" ]; 
    after = [ "network.target.service" ];
    description = "Test LED";
    serviceConfig = {
      Type = "simple";
      User = "root";
      Group = "root";
      PermissionsStartOnly = true;
      DynamicUser = true;
      ExecStart = pkgs.writeScriptBin "blink" ''
        #!${pkgs.stdenv.shell}
        echo none > /sys/class/leds/led0/trigger
        while true; do
          echo 1 > /sys/class/leds/led0/brightness
          sleep 0.5
          echo 0 > /sys/class/leds/led0/brightness
          sleep 0.5
        done
      '';

    };
  };
}

Pretty sleep deprived, so I’ve just slapped a bunch of things into my service - though it sounds like there isn’t actually a way to do this? (I event tried to sneak in the ExecStart ‘+’ → ExecStart=+/nix/store/z7455b2injbwy3l6kjqq5kw00fpl56fw-blink)

Any thoughts? I just want some blinkin lights