Test module

modules/default.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ./hello.nix
+  ];
+}

modules/hello.nix

@@ -0,0 +1,23 @@
+{ lib, pkgs, config, ...}:
+
+with lib;
+let
+  cfg = config.services.hello;
+
+in {
+  options.services.hello = {
+    enable = mkEnableOption "Hello service.";
+    greeter = mkOption {
+      type = types.str;
+      default = "world";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [
+      (pkgs.writeShellScriptBin "hello" ''
+        echo "'Hello, ${escapeShellArg cfg.greeter}!'"
+      '');
+    ];
+  };
+}