I came across an interesting thread on the NixOS subreddit today that helped me fix a problem that I didn’t even know I had with my NixOS system configuration.

Every now and then, I’ll try to quickly do nix-shell -p somepackage, and it will fail, because somepackage couldn’t be found.

I go and check https://search.nixos.org and it’s there. Huh. Weird.

So I just go and add it to my environment.systemPackages to install it, and remove it if it turns out I don’t really need it.

This alternative step works because my system configuration flake is pointing to a different version of nixpkgs than the nix-shell command.

So what’s the solution? Let’s make nix-shell use the same version of nixpkgs as the system configuration flake!

{
  inputs,
  ...
}: {
  nix = {
    registry = {
      nixpkgs = {
        flake = inputs.nixpkgs;
      };
    };

    nixPath = [
      "nixpkgs=${inputs.nixpkgs.outPath}"
      "nixos-config=/etc/nixos/configuration.nix"
      "/nix/var/nix/profiles/per-user/root/channels"
    ];
  };
}

Of course, if you also have a nixpkgs-unstable input in your flake and would rather that nix-shell -p always pulled in packages from there, you can reference nixpkgs-unstable here as well without forcing your whole system configuration to track the changes on the nixpkgs master branch.

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