nixpkgs/doc/stdenv/meta.chapter.md

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 = with lib; {
  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 = "https://www.gnu.org/software/hello/manual/";
  license = licenses.gpl3Plus;
  maintainers = [ maintainers.eelco ];
  platforms = platforms.all;
};

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 be a string.

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

$ nix-env -qa hello --json
{
    "hello": {
        "meta": {
            "description": "A program that produces a familiar, friendly greeting",
            "homepage": "https://www.gnu.org/software/hello/manual/",
            "license": {
                "fullName": "GNU General Public License version 3 or later",
                "shortName": "GPLv3+",
                "url": "http://www.fsf.org/licensing/licenses/gpl.html"
            },
            "longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
            "maintainers": [
                "Ludovic Court\u00e8s <ludo@gnu.org>"
            ],
            "platforms": [
                "i686-linux",
                "x86_64-linux",
                "armv5tel-linux",
                "armv7l-linux",
                "mips32-linux",
                "x86_64-darwin",
                "i686-cygwin",
                "i686-freebsd",
                "x86_64-freebsd",
                "i686-openbsd",
                "x86_64-openbsd"
            ],
            "position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/default.nix:14"
        },
        "name": "hello-2.9",
        "system": "x86_64-linux"
    }
}

nix-env knows about the description field specifically:

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

Standard meta-attributes

It is expected that each meta-attribute is one of the following:

description

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"

longDescription

An arbitrarily long description of the package.

branch

Release branch. Used to specify that a package is not going to receive updates that are not in this branch; for example, Linux kernel 3.0 is supposed to be updated to 3.0.X, not 3.1.

homepage

The package’s homepage. Example: https://www.gnu.org/software/hello/manual/

downloadPage

The page where a link to the current version can be found. Example: https://ftp.gnu.org/gnu/hello/

changelog

A link or a list of links to the location of Changelog for a package. A link may use expansion to refer to the correct changelog version. Example: "https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${version}"

license

The license, or licenses, for the package. One from the attribute set defined in nixpkgs/lib/licenses.nix. At this moment using both a list of licenses and a single license is valid. If the license field is in the form of a list representation, then it means that parts of the package are licensed differently. Each license should preferably be referenced by their attribute. The non-list attribute value can also be a space delimited string representation of the contained attribute shortNames or spdxIds. The following are all valid examples:

• Single license referenced by attribute (preferred) lib.licenses.gpl3Only.
• Single license referenced by its attribute shortName (frowned upon) "gpl3Only".
• Single license referenced by its attribute spdxId (frowned upon) "GPL-3.0-only".
• Multiple licenses referenced by attribute (preferred) with lib.licenses; [ asl20 free ofl ].
• Multiple licenses referenced as a space delimited string of attribute shortNames (frowned upon) "asl20 free ofl".

For details, see Licenses.

maintainers

A list of the maintainers of this Nix expression. Maintainers are defined in nixpkgs/maintainers/maintainer-list.nix. There is no restriction to becoming a maintainer, just add yourself to that list in a separate commit titled “maintainers: add alice”, and reference maintainers with maintainers = with lib.maintainers; [ alice bob ].

priority

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).

platforms

The list of Nix platform types on which the package is supported. Hydra builds packages according to the platform specified. If no platform is specified, the package does not have prebuilt binaries. An example is:

meta.platforms = lib.platforms.linux;

Attribute Set lib.platforms defines various common lists of platforms types.

tests

::: {.warning} This attribute is special in that it is not actually under the meta attribute set but rather under the passthru attribute set. This is due to how meta attributes work, and the fact that they are supposed to contain only metadata, not derivations. :::

An attribute set with as values tests. A test is a derivation, which builds successfully when the test passes, and fails to build otherwise. A derivation that is a test needs to have meta.timeout defined.

The NixOS tests are available as nixosTests in parameters of derivations. For instance, the OpenSMTPD derivation includes lines similar to:

{ /* ... */, nixosTests }:
{
  # ...
  passthru.tests = {
    basic-functionality-and-dovecot-integration = nixosTests.opensmtpd;
  };
}

timeout

A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, it can fail due to breaking the timeout. However, all computers do not have the same computing power, hence some builders may decide to apply a multiplicative factor to this value. When filling this value in, try to keep it approximately consistent with other values already present in nixpkgs.

hydraPlatforms

The list of Nix platform types for which the Hydra instance at hydra.nixos.org will 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 = lib.platforms.linux;
meta.hydraPlatforms = [];

broken

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. Such packages should be removed from Nixpkgs eventually unless they are fixed.

updateWalker

If set to true, the package is tested to be updated correctly by the update-walker.sh script without additional settings. Such packages have meta.version set and their homepage (or the page specified by meta.downloadPage) contains a direct link to the package tarball.

Licenses

The meta.license attribute should preferably contain a value from lib.licenses defined in nixpkgs/lib/licenses.nix, or in-place license description of the same format if the license is unlikely to be useful in another expression.

Although it’s typically better to indicate the specific license, a few generic options are available:

lib.licenses.free, "free"

Catch-all for free software licenses not listed above.

lib.licenses.unfreeRedistributable, "unfree-redistributable"

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.

lib.licenses.unfree, "unfree"

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.

lib.licenses.unfreeRedistributableFirmware, "unfree-redistributable-firmware"

This package supplies unfree, redistributable firmware. This is a separate value from unfree-redistributable because not everybody cares whether firmware is free.