# Modularity

The NixOS configuration mechanism is modular. If your `configuration.nix` becomes too big, you can split it into multiple files. Likewise, if you have multiple NixOS configurations (e.g. for different computers) with some commonality, you can move the common configuration into a shared file.

Modules have exactly the same syntax as `configuration.nix`. In fact, `configuration.nix` is itself a module. You can use other modules by including them from `configuration.nix`, e.g.:

    { config, pkgs, ... }:

    { imports = [ ./vpn.nix ./kde.nix ];
      services.httpd.enable = true;
      environment.systemPackages = [ pkgs.emacs ];
      ...
    }

Here, we include two modules from the same directory, `vpn.nix` and `kde.nix`. The latter might look like this:

    { config, pkgs, ... }:

    { services.xserver.enable = true;
      services.xserver.displayManager.sddm.enable = true;
      services.xserver.desktopManager.plasma5.enable = true;
      environment.systemPackages = [ pkgs.vim ];
    }

Note that both `configuration.nix` and `kde.nix` define the option . When multiple modules define an option, NixOS will try to merge the definitions. In the case of , that’s easy: the lists of packages can simply be concatenated. The value in `configuration.nix` is merged last, so for list-type options, it will appear at the end of the merged list. If you want it to appear first, you can use `mkBefore`:

    boot.kernelModules = mkBefore [ "kvm-intel" ];

This causes the `kvm-intel` kernel module to be loaded before any other kernel modules.

For other types of options, a merge may not be possible. For instance, if two modules define , `nixos-rebuild` will give an error:

    The unique option `services.httpd.adminAddr' is defined multiple times, in `/etc/nixos/httpd.nix' and `/etc/nixos/configuration.nix'.

When that happens, it’s possible to force one definition take precedence over the others:

    services.httpd.adminAddr = pkgs.lib.mkForce "bob@example.org";

When using multiple modules, you may need to access configuration values defined in other modules. This is what the `config` function argument is for: it contains the complete, merged system configuration. That is, `config` is the result of combining the configurations returned by every module If you’re wondering how it’s possible that the (indirect) result of a function is passed as an input to that same function: that’s because Nix is a lazy language — it only computes values when they are needed. This works as long as no individual configuration value depends on itself. . For example, here is a module that adds some packages to only if is set to `true` somewhere else:

    { config, pkgs, ... }:

    { environment.systemPackages =
        if config.services.xserver.enable then
          [ pkgs.firefox
            pkgs.thunderbird
          ]
        else
          [ ];
    }

With multiple modules, it may not be obvious what the final value of a configuration option is. The command `nixos-option` allows you to find out:

    $ nixos-option services.xserver.enable
    true

    $ nixos-option boot.kernelModules
    [ "tun" "ipv6" "loop" ... ]

Interactive exploration of the configuration is possible using `nix repl`, a read-eval-print loop for Nix expressions. A typical use:

    $ nix repl '<nixpkgs/nixos>'

    nix-repl> config.networking.hostName
    "mandark"

    nix-repl> map (x: x.hostName) config.services.httpd.virtualHosts
    [ "example.org" "example.gov" ]

While abstracting your configuration, you may find it useful to generate modules using code, instead of writing files. The example below would have the same effect as importing a file which sets those options.

    { config, pkgs, ... }:

    let netConfig = hostName: {
      networking.hostName = hostName;
      networking.useDHCP = false;
    };

    in

    { imports = [ (netConfig "nixos.localdomain") ]; }