Inspect the composition.nix file

If you open the composition.nix file, you will find the following:

{ pkgs, ... }: {
  roles = {
    foo = { pkgs, ... }:
      {
        # add needed package
        # environment.systemPackages = with pkgs; [ socat ];
      };
  };
  testScript = ''
    foo.succeed("true")
  '';
}

The composition is a function that takes a set as input ({ pkgs, ... }) and returns a set containing:

• a testScript string

• a roles set of NixOS configurations

What interest us for the moment is the roles set. In the example above, we define a single role named foo with an empty configuration. We can add packages to the environment by uncommenting the environment.systemPackages line:

{ pkgs, ... }: {
  roles = {
    foo = { pkgs, ... }:
      {
        # add needed package
        environment.systemPackages = with pkgs; [ socat ];
      };
  };
  testScript = ''
    foo.succeed("true")
  '';
}