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91 lines
4.1 KiB
XML
91 lines
4.1 KiB
XML
<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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,
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the Firefox wrapper package (which provides Firefox with a set of
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plugins such as the Adobe Flash player) has an option to enable the
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Google Talk plugin. It can be set in
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<literal>configuration.nix</literal> as follows:
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<literal>nixpkgs.config.firefox.enableGoogleTalkPlugin = true;</literal>
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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
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almost arbitrary ways, such as changing or disabling dependencies of
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a package. For instance, the Emacs package in Nixpkgs by default has
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a dependency on GTK 2. If you want to build it against GTK 3, you
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can specify that as follows:
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</para>
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<programlisting language="bash">
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environment.systemPackages = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
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</programlisting>
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<para>
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The function <literal>override</literal> performs the call to the
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Nix function that produces Emacs, with the original arguments
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amended by the set of arguments specified by you. So here the
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function argument <literal>gtk</literal> gets the value
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<literal>pkgs.gtk3</literal>, causing Emacs to depend on GTK 3. (The
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parentheses are necessary because in Nix, function application binds
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more weakly than list construction, so without them,
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<xref linkend="opt-environment.systemPackages" /> would be a list
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with 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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<literal>overrideAttrs</literal>. While the
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<literal>override</literal> mechanism above overrides the arguments
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of a package function, <literal>overrideAttrs</literal> allows
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changing the <emphasis>attributes</emphasis> passed to
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<literal>mkDerivation</literal>. This permits changing any aspect of
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the package, such as the source code. For instance, if you want to
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override the source code of Emacs, you can say:
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</para>
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<programlisting language="bash">
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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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<para>
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Here, <literal>overrideAttrs</literal> takes the Nix derivation
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specified by <literal>pkgs.emacs</literal> 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>. The
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original attributes are accessible via the function argument, which
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is conventionally named <literal>oldAttrs</literal>.
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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
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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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</para>
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<programlisting language="bash">
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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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</programlisting>
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<para>
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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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<literal>nixpkgs.config.packageOverrides</literal> refers to the
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original rather than overridden instance, to prevent an infinite
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recursion.)
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</para>
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</section>
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