209 lines
6.5 KiB
XML
209 lines
6.5 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="chap-meta">
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<title>Meta-attributes</title>
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<para>Nix packages can declare <emphasis>meta-attributes</emphasis>
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that contain information about a package such as a description, its
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homepage, its license, and so on. For instance, the GNU Hello package
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has a <varname>meta</varname> declaration like this:
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<programlisting>
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meta = {
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description = "A program that produces a familiar, friendly greeting";
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longDescription = ''
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GNU Hello is a program that prints "Hello, world!" when you run it.
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It is fully customizable.
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'';
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homepage = http://www.gnu.org/software/hello/manual/;
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license = "GPLv3+";
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};
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</programlisting>
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</para>
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<para>Meta-attributes are not passed to the builder of the package.
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Thus, a change to a meta-attribute doesn’t trigger a recompilation of
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the package. The value of a meta-attribute must a string.</para>
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<para>The meta-attributes of a package can be queried from the
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command-line using <command>nix-env</command>:
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<screen>
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$ nix-env -qa hello --meta --xml
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<?xml version='1.0' encoding='utf-8'?>
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<items>
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<item attrPath="hello" name="hello-2.3" system="i686-linux">
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<meta name="description" value="A program that produces a familiar, friendly greeting" />
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<meta name="homepage" value="http://www.gnu.org/software/hello/manual/" />
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<meta name="license" value="GPLv3+" />
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<meta name="longDescription" value="GNU Hello is a program that prints &quot;Hello, world!&quot; when you run it.&#xA;It is fully customizable.&#xA;" />
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</item>
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</items>
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</screen>
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<command>nix-env</command> knows about the
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<varname>description</varname> field specifically:
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<screen>
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$ nix-env -qa hello --description
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hello-2.3 A program that produces a familiar, friendly greeting
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</screen>
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</para>
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<section><title>Standard meta-attributes</title>
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<para>The following meta-attributes have a standard
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interpretation:</para>
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<variablelist>
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<varlistentry>
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<term><varname>description</varname></term>
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<listitem><para>A short (one-line) description of the package.
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This is shown by <command>nix-env -q --description</command> and
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also on the Nixpkgs release pages.</para>
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<para>Don’t include a period at the end. Don’t include newline
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characters. Capitalise the first character. For brevity, don’t
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repeat the name of package — just describe what it does.</para>
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<para>Wrong: <literal>"libpng is a library that allows you to decode PNG images."</literal></para>
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<para>Right: <literal>"A library for decoding PNG images"</literal></para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>longDescription</varname></term>
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<listitem><para>An arbitrarily long description of the
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package.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>homepage</varname></term>
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<listitem><para>The package’s homepage. Example:
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<literal>http://www.gnu.org/software/hello/manual/</literal></para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>license</varname></term>
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<listitem><para>The license for the package. See below for the
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allowed values.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>priority</varname></term>
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<listitem><para>The <emphasis>priority</emphasis> of the package,
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used by <command>nix-env</command> to resolve file name conflicts
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between packages. See the Nix manual page for
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<command>nix-env</command> for details. Example:
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<literal>"10"</literal> (a low-priority
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package).</para></listitem>
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</varlistentry>
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</variablelist>
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</section>
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<section><title>Licenses</title>
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<note><para>This is just a first attempt at standardising the license
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attribute.</para></note>
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<para>The <varname>meta.license</varname> attribute must be one of the
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following:
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<variablelist>
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<varlistentry>
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<term><varname>GPL</varname></term>
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<listitem><para>GNU General Public License; version not
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specified.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>GPLv2</varname></term>
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<listitem><para>GNU General Public License, version
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2.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>GPLv2+</varname></term>
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<listitem><para>GNU General Public License, version
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2 or higher.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>GPLv3</varname></term>
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<listitem><para>GNU General Public License, version
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3.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>GPLv3+</varname></term>
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<listitem><para>GNU General Public License, version
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3 or higher.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>free</varname></term>
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<listitem><para>Catch-all for free software licenses not listed
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above.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>free-copyleft</varname></term>
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<listitem><para>Catch-all for free, copyleft software licenses not
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listed above.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>unfree-redistributable</varname></term>
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<listitem><para>Unfree package that can be redistributed in binary
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form. That is, it’s legal to redistribute the
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<emphasis>output</emphasis> of the derivation. This means that
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the package can be included in the Nixpkgs
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channel.</para>
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<para>Sometimes proprietary software can only be redistributed
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unmodified. Make sure the builder doesn’t actually modify the
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original binaries; otherwise we’re breaking the license. For
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instance, the NVIDIA X11 drivers can be redistributed unmodified,
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but our builder applies <command>patchelf</command> to make them
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work. Thus, its license is <varname>unfree</varname> and it
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cannot be included in the Nixpkgs channel.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>unfree</varname></term>
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<listitem><para>Unfree package that cannot be redistributed. You
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can build it yourself, but you cannot redistribute the output of
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the derivation. Thus it cannot be included in the Nixpkgs
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channel.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>unfree-redistributable-firmware</varname></term>
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<listitem><para>This package supplies unfree, redistributable
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firmware. This is a separate value from
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<varname>unfree-redistributable</varname> because not everybody
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cares whether firmware is free.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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</section>
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</chapter>
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