-- configuration.nix
imports = [ ./hardware-configuration.nix
            ./shell-scripts/hello.nix ];

-- /shell-scripts/hello.nix file
{ pkgs, ... }:
let
  helloWorld = pkgs.writeScriptBin "helloWorld" ''
    #!${pkgs.stdenv.shell}
    echo Hello World!!
  '';
in {
  environment.systemPackages = [ helloWorld ];
}