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 = with 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.

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 in CommonMark Markdown.

`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” in the same pull request, and reference maintainers with maintainers = with lib.maintainers; [ alice bob ].

`mainProgram`

The name of the main binary for the package. This affects the binary nix run executes. Example: "rg"

`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.

`badPlatforms`

The list of Nix platform types on which the package is known not to be buildable. Hydra will never create prebuilt binaries for these platform types, even if they are in meta.platforms. In general it is preferable to set meta.platforms = lib.platforms.all and then exclude any platforms on which the package is known not to build. For example, a package which requires dynamic linking and cannot be linked statically could use this:

meta.platforms = lib.platforms.all;
meta.badPlatforms = [ lib.systems.inspect.patterns.isStatic ];

The lib.meta.availableOn function can be used to test whether or not a package is available (i.e. buildable) on a given platform. Some packages use this to automatically detect the maximum set of features with which they can be built. For example, systemd requires dynamic linking, and has a meta.badPlatforms setting similar to the one above. Packages which can be built with or without systemd support will use lib.meta.availableOn to detect whether or not systemd is available on the hostPlatform for which they are being built; if it is not available (e.g. due to a statically-linked host platform like pkgsStatic) this support will be disabled by default.

`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 tests as values. A test is a derivation that builds when the test passes and fails to build otherwise.

You can run these tests with:

$ cd path/to/nixpkgs
$ nix-build -A your-package.tests

Package tests

Tests that are part of the source package are often executed in the installCheckPhase.

Prefer passthru.tests for tests that are introduced in nixpkgs because:

passthru.tests tests the 'real' package, independently from the environment in which it was built
we can run passthru.tests independently
installCheckPhase adds overhead to each build

For more on how to write and run package tests, see Coding conventions.

NixOS tests

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;
  };
}

NixOS tests run in a VM, so they are slower than regular package tests. For more information see NixOS module tests.

Alternatively, you can specify other derivations as tests. You can make use of the optional parameter to inject the correct package without relying on non-local definitions, even in the presence of overrideAttrs. Here that's finalAttrs.finalPackage, but you could choose a different name if finalAttrs already exists in your scope.

(mypkg.overrideAttrs f).passthru.tests will be as expected, as long as the definition of tests does not rely on the original mypkg or overrides it in all places.

# my-package/default.nix
{ stdenv, callPackage }:
stdenv.mkDerivation (finalAttrs: {
  # ...
  passthru.tests.example = callPackage ./example.nix { my-package = finalAttrs.finalPackage; };
})

# my-package/example.nix
{ runCommand, lib, my-package, ... }:
runCommand "my-package-test" {
  nativeBuildInputs = [ my-package ];
  src = lib.sources.sourcesByRegex ./. [ ".*.in" ".*.expected" ];
} ''
  my-package --help
  my-package <example.in >example.actual
  diff -U3 --color=auto example.expected example.actual
  mkdir $out
''

`timeout`

A timeout (in seconds) for building the derivation. If the derivation takes longer than this time to build, Hydra will fail it 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.

meta attributes are not stored in the instantiated derivation. Therefore, this setting may be lost when the package is used as a dependency. To be effective, it must be presented directly to an evaluation process that handles the meta.timeout attribute.

`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 search.nixos.org, and cannot be built or installed unless the environment variable NIXPKGS_ALLOW_BROKEN is set. Such unconditionally-broken packages should be removed from Nixpkgs eventually unless they are fixed.

The value of this attribute can depend on a package's arguments, including stdenv. This means that broken can be used to express constraints, for example:

Does not cross compile

 meta.broken = !(stdenv.buildPlatform.canExecute stdenv.hostPlatform)

Broken if all of a certain set of its dependencies are broken

meta.broken = lib.all (map (p: p.meta.broken) [ glibc musl ])

This makes broken strictly more powerful than meta.badPlatforms. However meta.availableOn currently examines only meta.platforms and meta.badPlatforms, so meta.broken does not influence the default values for optional dependencies.

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.

Source provenance

The value of a package's meta.sourceProvenance attribute specifies the provenance of the package's derivation outputs.

If a package contains elements that are not built from the original source by a nixpkgs derivation, the meta.sourceProvenance attribute should be a list containing one or more value from lib.sourceTypes defined in nixpkgs/lib/source-types.nix.

Adding this information helps users who have needs related to build transparency and supply-chain security to gain some visibility into their installed software or set policy to allow or disallow installation based on source provenance.

The presence of a particular sourceType in a package's meta.sourceProvenance list indicates that the package contains some components falling into that category, though the absence of that sourceType does not guarantee the absence of that category of sourceType in the package's contents. A package with no meta.sourceProvenance set implies it has no known sourceTypes other than fromSource.

The meaning of the meta.sourceProvenance attribute does not depend on the value of the meta.license attribute.