How to create a /etc file, only when it's not available

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1

I know how to create a file, but the file can only be created. Never should the file be overwritten on a nixos-rebuild boot/switch.

environment.etc."foo.env" = {
	text = ''
		bar = 1
		# bar = 2
	'';
	mode = "0770";
};
2

I’ve solved it with a systemd service.

Please share if you find a better solution.

    systemd.services.create-foo-conf = {
      script = ''
        cat > /etc/foo.conf << EOF
        bar = 1
        # bar = 2
        EOF
        chown 1000:100 /etc/foo.conf
        chmod 770 /etc/foo.conf
      '';
      wantedBy = [ "multi-user.target" ];
      after = [ "network-online.target" ];
      wants = [ "network-online.target" ];
      unitConfig.ConditionPathExists = "!/etc/foo.conf";
      serviceConfig.Type = "oneshot";
    };
3

I’m curious why you don’t want the file to be over-written? Feels like there is part of the story you didn’t share.

I do agree, that your systemd service approach is a solution to the problem “create /etc/file, but only if it is not present”

4

This really seems like a strange thing to want, yes. However, assuming it really is what you want, I can’t really think of a better solution than the systemd service.

5

I have 3 kiosks in a production environment, which run the same nixos config (except static ip, disk id)
I want a 4th pc to function as a spare. When needed someone can swap them.

I can map a keybinding to change it in the file
or I could access remotely and change the value in the file.(currently the system doesn’t have internet access)

I gave each kiosk a numpad keyboard

6

Sounds like a job for specializations. I’d map the keybind to switch specializations instead, then you can declare the file differently in the specialization.

7

good idea

I’ll lookup how to change default specialization and reboot.

Can this work on a system without the config?
I build the config on my laptop and push it nixos-rebuild ... target

8

you can tell systemd-tmpfiles to link a file if and only if the target doesn’t exist - this might be the simplest and cleanest solution

9

specialisation work, but it will only allow to switch once?

ls /run/current-system/specialisation/
foo1  foo2

sudo /run/current-system/specialisation/foo2/bin/switch-to-configuration switch

When I try to run it a second time, the folder is empty:

ls /run/current-system/specialisation/


    environment.etc."foo.conf" = lib.mkDefault {
      mode = "0777";
      text = ''
        bar = 1
      '';
    };
    specialisation = {
      foo1.configuration = {
        system.nixos.tags = [ "foo1" ];
        environment.etc."foo.conf" = lib.mkForce {
          mode = "0777";
          text = ''
            bar = 1
          '';
        };
      };
      foo2.configuration = {
        system.nixos.tags = [ "foo2" ];
        environment.etc."foo.conf" = lib.mkForce {
          mode = "0777";
          text = ''
            bar = 2
          '';
        };
      };
    };
10 
sudo /nix/var/nix/profiles/system/specialisation/foo2/bin/switch-to-configuration switch

This works

11

Assuming you don’t update the system without rebooting it, getting it from /run/booted-system would work. Or you could grab it from /nix/var/nix/profiles/system, I suppose.

I understand why it can’t just link them all together circularly… even though that’s a bit annoying here.

Incidentally, you should probably use switch-to-configuration test, so it doesn’t bother recreating boot menus and such.

12

After a reboot the specialisation isn’t active anymore.
tried with switch & boot

Is this a bug or normal behavior?

13

I believe that’s normal. You boot into the non-specialized config unless you choose otherwise in the bootloader. There’s no state tracking across reboots for which specialization you’re in.

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