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