depot/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 = {
    description = "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 = lib.licenses.gpl3Plus;
    maintainers = with lib.maintainers; [ eelco ];
    platforms = lib.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

If the package is to be submitted to Nixpkgs, please check out the requirements for meta attributes in the contributing documentation.

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

description

A short (one-line) description of the package. This is displayed on search.nixos.org.

The general requirements of a description are:

• Be short, just one sentence.
• Be capitalized.
• Not start with definite ("The") or indefinite ("A"/"An") article.
• Not start with the package name.
  • More generally, it should not refer to the package name.
• Not end with a period (or any punctuation for that matter).
• Provide factual information.
  • Avoid subjective language.

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

Right: "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.

sourceProvenance

A list containing the type or types of source inputs from which the package is built, e.g. original source code, pre-built binaries, etc.

For details, see Source provenance.

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 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.platformPatterns.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.

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.

lib.sourceTypes.fromSource

Package elements which are produced by a nixpkgs derivation which builds them from source code.

lib.sourceTypes.binaryNativeCode

Native code to be executed on the target system's CPU, built by a third party. This includes packages which wrap a downloaded AppImage or Debian package.

lib.sourceTypes.binaryFirmware

Code to be executed on a peripheral device or embedded controller, built by a third party.

lib.sourceTypes.binaryBytecode

Code to run on a VM interpreter or JIT compiled into bytecode by a third party. This includes packages which download Java .jar files from another source.