{pkgs, ...}: {
  # Set your time zone.
  time.timeZone = "Asia/Shanghai";

  # Select internationalisation properties.
  i18n.defaultLocale = "en_US.UTF-8";

  i18n.extraLocaleSettings = {
    LC_ADDRESS = "zh_CN.UTF-8";
    LC_IDENTIFICATION = "zh_CN.UTF-8";
    LC_MEASUREMENT = "zh_CN.UTF-8";
    LC_MONETARY = "zh_CN.UTF-8";
    LC_NAME = "zh_CN.UTF-8";
    LC_NUMERIC = "zh_CN.UTF-8";
    LC_PAPER = "zh_CN.UTF-8";
    LC_TELEPHONE = "zh_CN.UTF-8";
    LC_TIME = "zh_CN.UTF-8";
  };

  # ssh-agent is used to pull my private secrets repo from github when depoloying my nixos config.
  programs.ssh.startAgent = true;

  # List packages installed in system profile. To search, run:
  # $ nix search wget
  environment.systemPackages = with pkgs; [
    neovim # Do not forget to add an editor to edit configuration.nix! The Nano editor is also installed by default.
    git
    gnumake
    wget
    curl
    nix-output-monitor
  ];

  users.groups = {
    "ryan" = {};
  };

  # Don't allow mutation of users outside the config.
  users = {
    mutableUsers = false;
    # Define a user account. Don't forget to set a password with ‘passwd’.
    users = {
      ryan = {
        # generated by `mkpasswd -m scrypt`
        # we have to use initialHashedPassword here, if your'are using tmpfs for /
        initialHashedPassword = "$7$CU..../....Sdl/JRH..9eIvZ6mE/52r.$xeR6lyvTcVVKt28Owcoc/vPOOECcYSiq1xjw/QCz2t0";
        isNormalUser = true;
        description = "ryan";
        extraGroups = ["ryan" "networkmanager" "wheel"];
      };
      root.initialHashedPassword = "$7$CU..../....X6uvZYnFD.i1CqqFFNl4./$4vgqzIPyw5XBr0aCDFbY/UIRRJr7h5SMGoQ/ZvX3FP2";
    };
  };

  # make ryan a trusted user so he can set custom nix substituters url(cache mirror) to speed up nixos-rebuild.
  nix.settings.trusted-users = ["ryan"];
}