Flakes - how to programmatically get the config

Managed to solve my previous flake issues. What is left now are just tests. Before the rewrite I did
something like export NIXOS_CONFIG and then invoked nixos = import <nixpkgs/nixos> {} and did some assertions on nixos.config - like that there are all necessary users present etc. Now I wonder how to “port” this to flakes (since there is no /etc/nixos/configuration.nix anymore). What I’d like basically is inside nix code obtain modules config (for current computer). Thanks.

Again answering myself but oh well at least I learned something new. Seems I totally missed there is builtins.getFlake.

So now I do something like:

{ hostname ? "default", pkgs ? import <nixpkgs> { } }:
  flake = builtins.getFlake (toString /etc/nixos);
  lib = pkgs.lib;
  users = flake.nixosConfigurations.${hostname}.config.users.users;
  normalUsers = builtins.filter (x: x.isNormalUser) (lib.toList (lib.attrValues users));
  num = toString (builtins.length normalUsers);
pkgs.mkShell {
  shellHook = ''
    echo "${num}"
    exit 0

And to invoke it I do:

nix-shell test.nix --arg hostname \"$(hostname)\"

Of course this is just an approximation, I have more test cases and if they fail I return exit code != 0.

Open for suggestions how to do this in a more elegant fashion, possibly through nix command.

1 Like

You might consider implementing these as checks inside your flake itself! Then you can do nix flake check and get your “tests” run for free. I think you’d put the check in the “checks” attribute.

This one is testing/exercising a Rust program, but you should be able to do the same general strategy you’ve shown above in the flake directly. A Flake for your Crate