Mixing stable and unstable packages on flake-based NixOS system

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1

Hello everyone. My system is based on flakes (nix-channel is disabled). How can I use some packages from nixos-unstable while keeping the rest of my system on the stable branch? I’d like to achieve something like this:

environment.systemPackages = with pkgs; [
    firefox
    thunderbird
    unstable.osu-lazer-bin
];

In other words, I wanna be able to access osu-lazer-bin from the unstable branch via pkgs.unstable.osu-lazer-bin.
I’ve already seen multiple approaches to achieve this, but none of them worked for me, maybe it’s because my system uses flakes, idk.

translated with llama 3.1 405b :3

2

You should be able to add the unstable nixpkgs as part of your inputs, then import it and add that package directly. Something like this:

 inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/release-24.05";
    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixpkgs-unstable";
 };

 outputs = {nixpkgs, nixpkgs-unstable, ... }:
 let
   system = "x86_64-linux";
   pkgs = import nixpkgs { inherit system; };
   pkgsUnstable = import nixpkgs-unstable { inherit system; };
 in
 {
    # now along with `pkgs` you also have `pkgsUnstable`
    # and can do: pkgsUnstable.osu-lazer-bin
 }
3

Something like this should work:

  overlay-unstable = final: prev: {
    unstable = import nixpkgs-unstable {
      inherit system;
      config.allowUnfree = true;
    };
  };

  pkgs = import nixpkgs {
    inherit system;
    config = {
      allowUnfree = true;
    };
    overlays = [
      overlay-unstable
    ];
  };

Then you can do pkgs.firefox or pkgs.unstable.firefox.

2 Likes
4

Never set pkgs directly in the NixOS module system, as the other answers are indicating.

Here’s a couple of concepts you need to know:

  1. If you want to access some inputs, or some other value across your config without creating an explicit option for it, use specialArgs or _module.args. (For things that you import, you must pass them in specialArgs specifically to avoid infrec.)
  2. Changing pkgs requires an overlay; however, for this application, you may not necessarily need one if you are willing to get away from the pkgs.unstable syntax specifically.

Which means the following:

  1. Add an entry to your inputs, and pass them to your config via specialArgs, in your flake.nix:

    inputs = {
  nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
  nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
};

outputs = { nixpkgs, ... }@inputs:
  nixosConfigurations = {
    foo = nixpkgs.lib.nixosSystem {
      #...
      specialArgs = { inherit inputs; };
    };
  };
};

  2. Since you need an unfree package specifically, allow the unfree package somewhere in your NixOS config:

    {
  nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
    "osu-lazer-bin"
  ];
}

    or if you want, you can just blanket allow all unfree packages, that’s up to you:

    {
  nixpkgs.config.allowUnfree = true;
}

  3. Then, use that config when importing the unstable nixpkgs, and then use the appropriate module arg in your package list.
    a. EITHER create a new module arg:

    { config, inputs, pkgs, pkgsUnstable, ... }:
{
  # this allows you to access `pkgsUnstable` anywhere in your config
  _module.args.pkgsUnstable = import inputs.nixpkgs-unstable {
    inherit (pkgs.stdenv.hostPlatform) system;
    inherit (config.nixpkgs) config;
  };

  environment.systemPackages = [
    pkgsUnstable.osu-lazer-bin
  ];
}

    b. OR, add an overlay to modify the existing nixpkgs instance (pkgs):

    { inputs, pkgs, ... }:
{
  nixpkgs.overlays = [
    (final: _: {
      # this allows you to access `pkgs.unstable` anywhere in your config
      unstable = import inputs.nixpkgs-unstable {
        inherit (final.stdenv.hostPlatform) system;
        inherit (final) config;
      };
    })
  ];

  environment.systemPackages = [
    pkgs.unstable.osu-lazer-bin
  ];
}

I’m personally against 3b, because an overlay for this is completely unnecessary and will likely slow down eval.

PS this has got to be one of the most-asked questions, it’s silly that we are still having to rewrite the answers from scratch in 2024…

4 Likes
5

Is that what is happening here?

You could add an article to nix.dev or the wiki.

6

Perfect! Used option 3a, this should definitely be written somewhere in the docs, at least not official.

7

Hello, I am a bit new to flakes and the nix syntax. The tutorial i followed led me to a different outputs syntax so i am not sure how i can follow your step 1. Can you help? I am figuring this stuff out as i go so there are some holes in my understanding.

{
  description = "My system configuration";
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.05";
    unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs, ... }:
    let
      lib = nixpkgs.lib;
    in {
      nixosConfigurations = {
        tartarus = lib.nixosSystem {
          system = "x86_64-linux";
          modules = [ ./configuration.nix ];
          # revision = self.shortRev or self.dirtyShortRev or self.lastModified or "unknown";
        };
      };
    };
}