<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-custom-packages">
  <title>Adding Custom Packages</title>
  <para>
    It’s possible that a package you need is not available in NixOS. In
    that case, you can do two things. Either you can package it with
    Nix, or you can try to use prebuilt packages from upstream. Due to
    the peculiarities of NixOS, it is important to note that building
    software from source is often easier than using pre-built
    executables.
  </para>
  <section xml:id="sec-custom-packages-nix">
    <title>Building with Nix</title>
    <para>
      This can be done either in-tree or out-of-tree. For an in-tree
      build, you can clone the Nixpkgs repository, add the package to
      your clone, and (optionally) submit a patch or pull request to
      have it accepted into the main Nixpkgs repository. This is
      described in detail in the
      <link xlink:href="https://nixos.org/nixpkgs/manual">Nixpkgs
      manual</link>. In short, you clone Nixpkgs:
    </para>
    <programlisting>
$ git clone https://github.com/NixOS/nixpkgs
$ cd nixpkgs
</programlisting>
    <para>
      Then you write and test the package as described in the Nixpkgs
      manual. Finally, you add it to
      <xref linkend="opt-environment.systemPackages" />, e.g.
    </para>
    <programlisting language="bash">
environment.systemPackages = [ pkgs.my-package ];
</programlisting>
    <para>
      and you run <literal>nixos-rebuild</literal>, specifying your own
      Nixpkgs tree:
    </para>
    <programlisting>
# nixos-rebuild switch -I nixpkgs=/path/to/my/nixpkgs
</programlisting>
    <para>
      The second possibility is to add the package outside of the
      Nixpkgs tree. For instance, here is how you specify a build of the
      <link xlink:href="https://www.gnu.org/software/hello/">GNU
      Hello</link> package directly in
      <literal>configuration.nix</literal>:
    </para>
    <programlisting language="bash">
environment.systemPackages =
  let
    my-hello = with pkgs; stdenv.mkDerivation rec {
      name = &quot;hello-2.8&quot;;
      src = fetchurl {
        url = &quot;mirror://gnu/hello/${name}.tar.gz&quot;;
        sha256 = &quot;0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6&quot;;
      };
    };
  in
  [ my-hello ];
</programlisting>
    <para>
      Of course, you can also move the definition of
      <literal>my-hello</literal> into a separate Nix expression, e.g.
    </para>
    <programlisting language="bash">
environment.systemPackages = [ (import ./my-hello.nix) ];
</programlisting>
    <para>
      where <literal>my-hello.nix</literal> contains:
    </para>
    <programlisting language="bash">
with import &lt;nixpkgs&gt; {}; # bring all of Nixpkgs into scope

stdenv.mkDerivation rec {
  name = &quot;hello-2.8&quot;;
  src = fetchurl {
    url = &quot;mirror://gnu/hello/${name}.tar.gz&quot;;
    sha256 = &quot;0wqd8sjmxfskrflaxywc7gqw7sfawrfvdxd9skxawzfgyy0pzdz6&quot;;
  };
}
</programlisting>
    <para>
      This allows testing the package easily:
    </para>
    <programlisting>
$ nix-build my-hello.nix
$ ./result/bin/hello
Hello, world!
</programlisting>
  </section>
  <section xml:id="sec-custom-packages-prebuilt">
    <title>Using pre-built executables</title>
    <para>
      Most pre-built executables will not work on NixOS. There are two
      notable exceptions: flatpaks and AppImages. For flatpaks see the
      <link linkend="module-services-flatpak">dedicated section</link>.
      AppImages will not run <quote>as-is</quote> on NixOS. First you
      need to install <literal>appimage-run</literal>: add to
      <literal>/etc/nixos/configuration.nix</literal>
    </para>
    <programlisting language="bash">
environment.systemPackages = [ pkgs.appimage-run ];
</programlisting>
    <para>
      Then instead of running the AppImage <quote>as-is</quote>, run
      <literal>appimage-run foo.appimage</literal>.
    </para>
    <para>
      To make other pre-built executables work on NixOS, you need to
      package them with Nix and special helpers like
      <literal>autoPatchelfHook</literal> or
      <literal>buildFHSUserEnv</literal>. See the
      <link xlink:href="https://nixos.org/nixpkgs/manual">Nixpkgs
      manual</link> for details. This is complex and often doing a
      source build is easier.
    </para>
  </section>
</section>
