4. Meta-attributes

Nix packages can declare meta-attributes that contain information about a package such as a description, its homepage, its license, and so on. For instance, the GNU Hello package has a meta declaration like this:

meta = {
  description = "A program that produces a familiar, friendly greeting";
  longDescription = ''
    GNU Hello is a program that prints "Hello, world!" when you run it.
    It is fully customizable.
  homepage = http://www.gnu.org/software/hello/manual/;
  license = "GPLv3+";

Meta-attributes are not passed to the builder of the package. Thus, a change to a meta-attribute doesn’t trigger a recompilation of the package. The value of a meta-attribute must a string.

The meta-attributes of a package can be queried from the command-line using nix-env:

$ nix-env -qa hello --meta --xml
<?xml version='1.0' encoding='utf-8'?>
  <item attrPath="hello" name="hello-2.3" system="i686-linux">
    <meta name="description" value="A program that produces a familiar, friendly greeting" />
    <meta name="homepage" value="http://www.gnu.org/software/hello/manual/" />
    <meta name="license" value="GPLv3+" />
    <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;" />

nix-env knows about the description field specifically:

$ nix-env -qa hello --description
hello-2.3  A program that produces a familiar, friendly greeting

4.1. Standard meta-attributes

The following meta-attributes have a standard interpretation:


A short (one-line) description of the package. This is shown by nix-env -q --description and also on the Nixpkgs release pages.

Don’t include a period at the end. Don’t include newline characters. Capitalise the first character. For brevity, don’t repeat the name of package — just describe what it does.

Wrong: "libpng is a library that allows you to decode PNG images."

Right: "A library for decoding PNG images"

An arbitrarily long description of the package.
The package’s homepage. Example: http://www.gnu.org/software/hello/manual/
The license for the package. See below for the allowed values.

A list of names and e-mail addresses of the maintainers of this Nix expression, e.g. ``[“Alice

<alice@example.org>” “Bob <bob@example.com>”]``. If you are the

maintainer of multiple packages, you may want to add yourself to `pkgs/lib/maintainers.nix <https://github.com/NixOS/nixpkgs/blob/master/pkgs/lib/maintainers.nix>`__ and write something like ``[stdenv.lib.maintainers.alice

The priority of the package, used by nix-env to resolve file name conflicts between packages. See the Nix manual page for nix-env for details. Example: "10" (a low-priority package).

The list of Nix platform types on which the package is supported. If this attribute is set, the package will refuse to build, and won’t show up in ``nix-env

-qa`` output, on any platform not listed here. An example is:
meta.platforms = [ "x86_64-linux" "i686-linux" "x86_64-darwin" ];

The set lib.platforms defines various common lists of platforms types, so it’s more typical to write:

meta.platforms = stdenv.lib.platforms.linux ++ stdenv.lib.platforms.darwin;

The list of Nix platform types for which the Hydra instance at hydra.nixos.org should build the package. (Hydra is the Nix-based continuous build system.) It defaults to the value of meta.platforms. Thus, the only reason to set meta.hydraPlatforms is if you want hydra.nixos.org to build the package on a subset of meta.platforms, or not at all, e.g.

meta.platforms = stdenv.lib.platforms.linux;
meta.hydraPlatforms = [];
If set to true, the package is marked as “broken”, meaning that it won’t show up in nix-env -qa, and cannot be built or installed. Sush packages should be removed from Nixpkgs eventually unless they are fixed.

4.2. Licenses


This is just a first attempt at standardising the license attribute.

The meta.license attribute must be one of the following:

GNU General Public License; version not specified.
GNU General Public License, version 2.
GNU General Public License, version 2 or higher.
GNU General Public License, version 3.
GNU General Public License, version 3 or higher.
Catch-all for licenses that are essentially similar to the original BSD license with the advertising clause removed, i.e. permissive non-copyleft free software licenses. This includes the X11 (“MIT”) License.
The Perl 5 license (Artistic License, version 1 and GPL, version 1 or later).
Catch-all for free software licenses not listed above.
Catch-all for free, copyleft software licenses not listed above.
Catch-all for free, non-copyleft software licenses not listed above.

Unfree package that can be redistributed in binary form. That is, it’s legal to redistribute the output of the derivation. This means that the package can be included in the Nixpkgs channel.

Sometimes proprietary software can only be redistributed unmodified. Make sure the builder doesn’t actually modify the original binaries; otherwise we’re breaking the license. For instance, the NVIDIA X11 drivers can be redistributed unmodified, but our builder applies patchelf to make them work. Thus, its license is unfree and it cannot be included in the Nixpkgs channel.

Unfree package that cannot be redistributed. You can build it yourself, but you cannot redistribute the output of the derivation. Thus it cannot be included in the Nixpkgs channel.
This package supplies unfree, redistributable firmware. This is a separate value from unfree-redistributable because not everybody cares whether firmware is free.