pmiddend.nix

{ pkgs ? import <nixpkgs> {} }:
let
  test = pkgs.writeTextFile {
    name = "weird";
    text = ''
      #!${pkgs.stdenv.shell}
      echo RUNNING
      echo contents > testfile
    '';
  };
in
  pkgs.stdenv.mkDerivation {
    name = "test";
    phases = "installPhase";
    buildInputs = [ test ];
    installPhase = "cp testfile $out";
  }