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While the word 'simply' is usually added to encourage readers, it often has the opposite effect and may even appear condescending, especially when the reader runs into trouble trying to apply the suggestions from the documentation. It is almost always an improvement to simply drop the word from the sentence. (there are more possible improvements like this, we can apply those in separate PRs)
134 lines
4 KiB
Markdown
134 lines
4 KiB
Markdown
# Modularity {#sec-modularity}
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The NixOS configuration mechanism is modular. If your
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`configuration.nix` becomes too big, you can split it into multiple
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files. Likewise, if you have multiple NixOS configurations (e.g. for
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different computers) with some commonality, you can move the common
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configuration into a shared file.
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Modules have exactly the same syntax as `configuration.nix`. In fact,
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`configuration.nix` is itself a module. You can use other modules by
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including them from `configuration.nix`, e.g.:
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```nix
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{ config, pkgs, ... }:
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{ imports = [ ./vpn.nix ./kde.nix ];
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services.httpd.enable = true;
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environment.systemPackages = [ pkgs.emacs ];
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...
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}
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```
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Here, we include two modules from the same directory, `vpn.nix` and
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`kde.nix`. The latter might look like this:
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```nix
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{ config, pkgs, ... }:
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{ services.xserver.enable = true;
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services.xserver.displayManager.sddm.enable = true;
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services.xserver.desktopManager.plasma5.enable = true;
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environment.systemPackages = [ pkgs.vim ];
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}
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```
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Note that both `configuration.nix` and `kde.nix` define the option
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[](#opt-environment.systemPackages). When multiple modules define an
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option, NixOS will try to *merge* the definitions. In the case of
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[](#opt-environment.systemPackages) the lists of packages will be
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concatenated. The value in `configuration.nix` is
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merged last, so for list-type options, it will appear at the end of the
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merged list. If you want it to appear first, you can use `mkBefore`:
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```nix
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boot.kernelModules = mkBefore [ "kvm-intel" ];
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```
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This causes the `kvm-intel` kernel module to be loaded before any other
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kernel modules.
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For other types of options, a merge may not be possible. For instance,
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if two modules define [](#opt-services.httpd.adminAddr),
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`nixos-rebuild` will give an error:
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```plain
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The unique option `services.httpd.adminAddr' is defined multiple times, in `/etc/nixos/httpd.nix' and `/etc/nixos/configuration.nix'.
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```
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When that happens, it's possible to force one definition take precedence
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over the others:
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```nix
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services.httpd.adminAddr = pkgs.lib.mkForce "bob@example.org";
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```
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When using multiple modules, you may need to access configuration values
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defined in other modules. This is what the `config` function argument is
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for: it contains the complete, merged system configuration. That is,
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`config` is the result of combining the configurations returned by every
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module. (If you're wondering how it's possible that the (indirect) *result*
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of a function is passed as an *input* to that same function: that's
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because Nix is a "lazy" language --- it only computes values when
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they are needed. This works as long as no individual configuration
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value depends on itself.)
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For example, here is a module that adds some packages to
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[](#opt-environment.systemPackages) only if
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[](#opt-services.xserver.enable) is set to `true` somewhere else:
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```nix
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{ config, pkgs, ... }:
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{ environment.systemPackages =
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if config.services.xserver.enable then
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[ pkgs.firefox
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pkgs.thunderbird
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]
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else
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[ ];
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}
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```
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With multiple modules, it may not be obvious what the final value of a
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configuration option is. The command `nixos-option` allows you to find
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out:
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```ShellSession
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$ nixos-option services.xserver.enable
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true
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$ nixos-option boot.kernelModules
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[ "tun" "ipv6" "loop" ... ]
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```
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Interactive exploration of the configuration is possible using `nix
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repl`, a read-eval-print loop for Nix expressions. A typical use:
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```ShellSession
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$ nix repl '<nixpkgs/nixos>'
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nix-repl> config.networking.hostName
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"mandark"
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nix-repl> map (x: x.hostName) config.services.httpd.virtualHosts
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[ "example.org" "example.gov" ]
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```
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While abstracting your configuration, you may find it useful to generate
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modules using code, instead of writing files. The example below would
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have the same effect as importing a file which sets those options.
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```nix
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{ config, pkgs, ... }:
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let netConfig = hostName: {
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networking.hostName = hostName;
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networking.useDHCP = false;
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};
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in
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{ imports = [ (netConfig "nixos.localdomain") ]; }
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```
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