146 lines
5.4 KiB
XML
146 lines
5.4 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-modularity">
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<title>Modularity</title>
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<para>
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The NixOS configuration mechanism is modular. If your
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<filename>configuration.nix</filename> becomes too big, you can split it into
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multiple 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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</para>
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<para>
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Modules have exactly the same syntax as
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<filename>configuration.nix</filename>. In fact,
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<filename>configuration.nix</filename> is itself a module. You can use other
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modules by including them from <filename>configuration.nix</filename>, e.g.:
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<programlisting>
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{ config, pkgs, ... }:
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{ imports = [ ./vpn.nix ./kde.nix ];
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<xref linkend="opt-services.httpd.enable"/> = true;
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<xref linkend="opt-environment.systemPackages"/> = [ pkgs.emacs ];
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<replaceable>...</replaceable>
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}
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</programlisting>
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Here, we include two modules from the same directory,
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<filename>vpn.nix</filename> and <filename>kde.nix</filename>. The latter
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might look like this:
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<programlisting>
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{ config, pkgs, ... }:
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{ <xref linkend="opt-services.xserver.enable"/> = true;
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<xref linkend="opt-services.xserver.displayManager.sddm.enable"/> = true;
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<xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/> = true;
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}
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</programlisting>
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Note that both <filename>configuration.nix</filename> and
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<filename>kde.nix</filename> define the option
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<xref linkend="opt-environment.systemPackages"/>. When multiple modules
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define an option, NixOS will try to <emphasis>merge</emphasis> the
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definitions. In the case of <xref linkend="opt-environment.systemPackages"/>,
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that’s easy: the lists of packages can simply be concatenated. The value in
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<filename>configuration.nix</filename> is merged last, so for list-type
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options, it will appear at the end of the merged list. If you want it to
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appear first, you can use <varname>mkBefore</varname>:
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<programlisting>
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<xref linkend="opt-boot.kernelModules"/> = mkBefore [ "kvm-intel" ];
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</programlisting>
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This causes the <literal>kvm-intel</literal> kernel module to be loaded
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before any other kernel modules.
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</para>
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<para>
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For other types of options, a merge may not be possible. For instance, if two
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modules define <xref linkend="opt-services.httpd.adminAddr"/>,
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<command>nixos-rebuild</command> will give an error:
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<screen>
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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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</screen>
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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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<programlisting>
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<xref linkend="opt-services.httpd.adminAddr"/> = pkgs.lib.mkForce "bob@example.org";
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</programlisting>
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</para>
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<para>
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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 <varname>config</varname> function
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argument is for: it contains the complete, merged system configuration. That
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is, <varname>config</varname> is the result of combining the configurations
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returned by every module
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<footnote xml:id="footnote-nix-is-lazy">
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<para>
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If you’re wondering how it’s possible that the (indirect)
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<emphasis>result</emphasis> of a function is passed as an
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<emphasis>input</emphasis> to that same function: that’s because Nix is a
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“lazy” language — it only computes values when they are needed. This
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works as long as no individual configuration value depends on itself.
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</para>
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</footnote>
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. For example, here is a module that adds some packages to
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<xref linkend="opt-environment.systemPackages"/> only if
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<xref linkend="opt-services.xserver.enable"/> is set to
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<literal>true</literal> somewhere else:
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<programlisting>
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{ config, pkgs, ... }:
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{ <xref linkend="opt-environment.systemPackages"/> =
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if config.<xref linkend="opt-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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</programlisting>
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</para>
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<para>
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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 <option>nixos-option</option> allows you
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to find out:
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<screen>
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<prompt>$ </prompt>nixos-option <xref linkend="opt-services.xserver.enable"/>
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true
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<prompt>$ </prompt>nixos-option <xref linkend="opt-boot.kernelModules"/>
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[ "tun" "ipv6" "loop" <replaceable>...</replaceable> ]
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</screen>
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Interactive exploration of the configuration is possible using <command>nix
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repl</command>, a read-eval-print loop for Nix expressions. A typical use:
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<screen>
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<prompt>$ </prompt>nix repl '<nixpkgs/nixos>'
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<prompt>nix-repl> </prompt>config.<xref linkend="opt-networking.hostName"/>
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"mandark"
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<prompt>nix-repl> </prompt>map (x: x.hostName) config.<xref linkend="opt-services.httpd.virtualHosts"/>
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[ "example.org" "example.gov" ]
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</screen>
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</para>
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<para>
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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 have
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the same effect as importing a file which sets those options.
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<programlisting>
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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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</programlisting>
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</para>
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</section>
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