let

  fromEnv = name: default:
    let env = builtins.getEnv name; in
    if env == "" then default else env;
    
  configuration = import (fromEnv "NIXOS_CONFIG" /etc/nixos/configuration.nix);
  
  nixpkgs = fromEnv "NIXPKGS" /etc/nixos/nixpkgs;

  system = import system/system.nix { inherit configuration nixpkgs; };

in

{ inherit (system)
    activateConfiguration
    bootStage2
    etc
    grubMenuBuilder
    kernel
    modulesTree
    system
    systemPath
    config
    ;

  nix = system.config.environment.nix;
  
  nixFallback = (import nixpkgs {}).nixUnstable;

  manifests = system.config.installer.manifests; # exported here because nixos-rebuild uses it

  tests = system.config.tests;
}