264 lines
9.2 KiB
XML
264 lines
9.2 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 = stdenv.lib.licenses.gpl3Plus;
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maintainers = [ stdenv.lib.maintainers.eelco ];
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platforms = stdenv.lib.platforms.all;
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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 --json
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{
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"hello": {
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"meta": {
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"description": "A program that produces a familiar, friendly greeting",
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"homepage": "http://www.gnu.org/software/hello/manual/",
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"license": {
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"fullName": "GNU General Public License version 3 or later",
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"shortName": "GPLv3+",
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"url": "http://www.fsf.org/licensing/licenses/gpl.html"
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},
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"longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
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"maintainers": [
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"Ludovic Court\u00e8s <ludo@gnu.org>"
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],
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"platforms": [
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"i686-linux",
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"x86_64-linux",
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"armv5tel-linux",
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"armv7l-linux",
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"mips64el-linux",
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"x86_64-darwin",
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"i686-cygwin",
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"i686-freebsd",
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"x86_64-freebsd",
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"i686-openbsd",
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"x86_64-openbsd"
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],
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"position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/ex-2/default.nix:14"
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},
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"name": "hello-2.9",
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"system": "x86_64-linux"
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}
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}
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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. One from the
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attribute set defined in <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix">
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<filename>nixpkgs/lib/licenses.nix</filename></link>. Example:
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<literal>stdenv.lib.licenses.gpl3</literal>. For details, see
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<xref linkend='sec-meta-license'/>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>maintainers</varname></term>
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<listitem><para>A list of names and e-mail addresses of the
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maintainers of this Nix expression. If
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you would like to be a maintainer of a package, you may want to add
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yourself to <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/maintainers.nix"><filename>nixpkgs/lib/maintainers.nix</filename></link>
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and write something like <literal>[ stdenv.lib.maintainers.alice
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stdenv.lib.maintainers.bob ]</literal>.</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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<varlistentry>
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<term><varname>platforms</varname></term>
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<listitem><para>The list of Nix platform types on which the
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package is supported. Hydra builds packages according to the
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platform specified. If no platform is specified, the package does
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not have prebuilt binaries. An example is:
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<programlisting>
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meta.platforms = stdenv.lib.platforms.linux;
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</programlisting>
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Attribute Set <varname>stdenv.lib.platforms</varname> in
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<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/platforms.nix">
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<filename>nixpkgs/lib/platforms.nix</filename></link> defines various common
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lists of platforms types.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>hydraPlatforms</varname></term>
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<listitem><para>The list of Nix platform types for which the Hydra
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instance at <literal>hydra.nixos.org</literal> will build the
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package. (Hydra is the Nix-based continuous build system.) It
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defaults to the value of <varname>meta.platforms</varname>. Thus,
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the only reason to set <varname>meta.hydraPlatforms</varname> is
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if you want <literal>hydra.nixos.org</literal> to build the
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package on a subset of <varname>meta.platforms</varname>, or not
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at all, e.g.
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<programlisting>
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meta.platforms = stdenv.lib.platforms.linux;
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meta.hydraPlatforms = [];
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</programlisting>
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>broken</varname></term>
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<listitem><para>If set to <literal>true</literal>, the package is
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marked as “broken”, meaning that it won’t show up in
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<literal>nix-env -qa</literal>, and cannot be built or installed.
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Such packages should be removed from Nixpkgs eventually unless
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they are fixed.</para></listitem>
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</varlistentry>
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</variablelist>
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</section>
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<section xml:id="sec-meta-license"><title>Licenses</title>
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<para>The <varname>meta.license</varname> attribute should preferrably contain
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a value from <varname>stdenv.lib.licenses</varname> defined in
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<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix">
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<filename>nixpkgs/lib/licenses.nix</filename></link>,
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or in-place license description of the same format if the license is
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unlikely to be useful in another expression.
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A few generic options are available, although it's typically better
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to indicate the specific license:
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<variablelist>
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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>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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