nixpkgs/nixos/doc/manual/configuration/customizing-packages.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-customising-packages">
 <title>Customising Packages</title>

 <para>
  Some packages in Nixpkgs have options to enable or disable optional
  functionality or change other aspects of the package. For instance, the
  Firefox wrapper package (which provides Firefox with a set of plugins such as
  the Adobe Flash player) has an option to enable the Google Talk plugin. It
  can be set in <filename>configuration.nix</filename> as follows: <filename>
  nixpkgs.config.firefox.enableGoogleTalkPlugin = true; </filename>
 </para>

 <warning>
  <para>
   Unfortunately, Nixpkgs currently lacks a way to query available
   configuration options.
  </para>
 </warning>

 <para>
  Apart from high-level options, it’s possible to tweak a package in almost
  arbitrary ways, such as changing or disabling dependencies of a package. For
  instance, the Emacs package in Nixpkgs by default has a dependency on GTK 2.
  If you want to build it against GTK 3, you can specify that as follows:
<programlisting>
<xref linkend="opt-environment.systemPackages"/> = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
</programlisting>
  The function <varname>override</varname> performs the call to the Nix
  function that produces Emacs, with the original arguments amended by the set
  of arguments specified by you. So here the function argument
  <varname>gtk</varname> gets the value <literal>pkgs.gtk3</literal>, causing
  Emacs to depend on GTK 3. (The parentheses are necessary because in Nix,
  function application binds more weakly than list construction, so without
  them, <xref linkend="opt-environment.systemPackages"/> would be a list with
  two elements.)
 </para>

 <para>
  Even greater customisation is possible using the function
  <varname>overrideAttrs</varname>. While the <varname>override</varname>
  mechanism above overrides the arguments of a package function,
  <varname>overrideAttrs</varname> allows changing the
  <emphasis>attributes</emphasis> passed to <literal>mkDerivation</literal>.
  This permits changing any aspect of the package, such as the source code. For
  instance, if you want to override the source code of Emacs, you can say:
<programlisting>
<xref linkend="opt-environment.systemPackages"/> = [
  (pkgs.emacs.overrideAttrs (oldAttrs: {
    name = "emacs-25.0-pre";
    src = /path/to/my/emacs/tree;
  }))
];
</programlisting>
  Here, <varname>overrideAttrs</varname> takes the Nix derivation specified by
  <varname>pkgs.emacs</varname> and produces a new derivation in which the
  original’s <literal>name</literal> and <literal>src</literal> attribute
  have been replaced by the given values by re-calling
  <literal>stdenv.mkDerivation</literal>. The original attributes are
  accessible via the function argument, which is conventionally named
  <varname>oldAttrs</varname>.
 </para>

 <para>
  The overrides shown above are not global. They do not affect the original
  package; other packages in Nixpkgs continue to depend on the original rather
  than the customised package. This means that if another package in your
  system depends on the original package, you end up with two instances of the
  package. If you want to have everything depend on your customised instance,
  you can apply a <emphasis>global</emphasis> override as follows:
<screen>
nixpkgs.config.packageOverrides = pkgs:
  { emacs = pkgs.emacs.override { gtk = pkgs.gtk3; };
  };
</screen>
  The effect of this definition is essentially equivalent to modifying the
  <literal>emacs</literal> attribute in the Nixpkgs source tree. Any package in
  Nixpkgs that depends on <literal>emacs</literal> will be passed your
  customised instance. (However, the value <literal>pkgs.emacs</literal> in
  <varname>nixpkgs.config.packageOverrides</varname> refers to the original
  rather than overridden instance, to prevent an infinite recursion.)
 </para>
</section>