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87 lines
4.0 KiB
XML
87 lines
4.0 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-customising-packages">
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<title>Customising Packages</title>
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<para>
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Some packages in Nixpkgs have options to enable or disable optional
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functionality or change other aspects of the package. For instance, the
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Firefox wrapper package (which provides Firefox with a set of plugins such as
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the Adobe Flash player) has an option to enable the Google Talk plugin. It
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can be set in <filename>configuration.nix</filename> as follows: <filename>
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nixpkgs.config.firefox.enableGoogleTalkPlugin = true; </filename>
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</para>
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<warning>
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<para>
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Unfortunately, Nixpkgs currently lacks a way to query available
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configuration options.
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</para>
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</warning>
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<para>
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Apart from high-level options, it’s possible to tweak a package in almost
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arbitrary ways, such as changing or disabling dependencies of a package. For
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instance, the Emacs package in Nixpkgs by default has a dependency on GTK 2.
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If you want to build it against GTK 3, you can specify that as follows:
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<programlisting>
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<xref linkend="opt-environment.systemPackages"/> = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
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</programlisting>
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The function <varname>override</varname> performs the call to the Nix
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function that produces Emacs, with the original arguments amended by the set
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of arguments specified by you. So here the function argument
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<varname>gtk</varname> gets the value <literal>pkgs.gtk3</literal>, causing
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Emacs to depend on GTK 3. (The parentheses are necessary because in Nix,
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function application binds more weakly than list construction, so without
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them, <xref linkend="opt-environment.systemPackages"/> would be a list with
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two elements.)
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</para>
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<para>
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Even greater customisation is possible using the function
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<varname>overrideAttrs</varname>. While the <varname>override</varname>
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mechanism above overrides the arguments of a package function,
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<varname>overrideAttrs</varname> allows changing the
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<emphasis>attributes</emphasis> passed to <literal>mkDerivation</literal>.
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This permits changing any aspect of the package, such as the source code. For
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instance, if you want to override the source code of Emacs, you can say:
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<programlisting>
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<xref linkend="opt-environment.systemPackages"/> = [
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(pkgs.emacs.overrideAttrs (oldAttrs: {
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name = "emacs-25.0-pre";
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src = /path/to/my/emacs/tree;
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}))
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];
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</programlisting>
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Here, <varname>overrideAttrs</varname> takes the Nix derivation specified by
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<varname>pkgs.emacs</varname> and produces a new derivation in which the
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original’s <literal>name</literal> and <literal>src</literal> attribute
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have been replaced by the given values by re-calling
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<literal>stdenv.mkDerivation</literal>. The original attributes are
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accessible via the function argument, which is conventionally named
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<varname>oldAttrs</varname>.
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</para>
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<para>
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The overrides shown above are not global. They do not affect the original
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package; other packages in Nixpkgs continue to depend on the original rather
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than the customised package. This means that if another package in your
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system depends on the original package, you end up with two instances of the
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package. If you want to have everything depend on your customised instance,
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you can apply a <emphasis>global</emphasis> override as follows:
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<screen>
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nixpkgs.config.packageOverrides = pkgs:
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{ emacs = pkgs.emacs.override { gtk = pkgs.gtk3; };
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};
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</screen>
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The effect of this definition is essentially equivalent to modifying the
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<literal>emacs</literal> attribute in the Nixpkgs source tree. Any package in
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Nixpkgs that depends on <literal>emacs</literal> will be passed your
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customised instance. (However, the value <literal>pkgs.emacs</literal> in
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<varname>nixpkgs.config.packageOverrides</varname> refers to the original
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rather than overridden instance, to prevent an infinite recursion.)
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</para>
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</section>
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