nixos-config/components/nixos/upgrade-diff.nix

# MIT Jörg Thalheim - https://github.com/Mic92/dotfiles/blob/c6cad4e57016945c4816c8ec6f0a94daaa0c3203/nixos/modules/upgrade-diff.nix
{ config, lib, pkgs, ... }:
{

  options.components.nixos.update-diff.enable = lib.mkOption {
    type = lib.types.bool;
    default = config.components.nixos.enable;
  };

  config = lib.mkIf (config.components.nixos.update-diff.enable) {
    system.activationScripts.diff = {
      supportsDryActivation = true;
      text = ''
        if [[ -e /run/current-system ]]; then
          echo "--- diff to current-system"
          ${pkgs.nvd}/bin/nvd --nix-bin-dir=${config.nix.package}/bin diff /run/current-system "$systemConfig"
          echo "---"
        fi
      '';
    };
  };
}