nixfiles/modules/dynamic-motd.nix

{ lib, pkgs, config, ... }:
let
  inherit (lib) optionalAttrs filterAttrs genAttrs mkIf mkDefault;
  inherit (lib.my) mkOpt mkBoolOpt;

  cfg = config.my.dynamic-motd;

  scriptBin = pkgs.writeShellScript "dynamic-motd-script" cfg.script;
in
{
  options.my.dynamic-motd = with lib.types; {
    enable = mkBoolOpt true;
    services = mkOpt (listOf str) [ "login" "ssh" ];
    script = mkOpt (nullOr lines) null;
  };

  config = mkIf (cfg.enable && cfg.script != null) {
    security.pam.services = genAttrs cfg.services (s: {
      text = mkDefault
        ''
          session optional ${pkgs.pam}/lib/security/pam_exec.so stdout quiet ${scriptBin}
        '';
    });
  };
}