lix/src/nix/flake.md

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Description

nix flake provides subcommands for creating, modifying and querying Nix flakes. Flakes are the unit for packaging Nix code in a reproducible and discoverable way. They can have dependencies on other flakes, making it possible to have multi-repository Nix projects.

A flake is a filesystem tree (typically fetched from a Git repository or a tarball) that contains a file named flake.nix in the root directory. flake.nix specifies some metadata about the flake such as dependencies (called inputs), as well as its outputs (the Nix values such as packages or NixOS modules provided by the flake).

Flake references

Flake references (flakerefs) are a way to specify the location of a flake. These have two different forms:

• An attribute set representation, e.g.

  {
    type = "github";
    owner = "NixOS";
    repo = "nixpkgs";
  }
  

  The only required attribute is type. The supported types are listed below.

• A URL-like syntax, e.g.

  github:NixOS/nixpkgs
  

  These are used on the command line as a more convenient alternative to the attribute set representation. For instance, in the command

  # nix build github:NixOS/nixpkgs#hello
  

  github:NixOS/nixpkgs is a flake reference (while hello is an output attribute). They are also allowed in the inputs attribute of a flake, e.g.

  inputs.nixpkgs.url = github:NixOS/nixpkgs;
  

  is equivalent to

  inputs.nixpkgs = {
    type = "github";
    owner = "NixOS";
    repo = "nixpkgs";
  };
  

Examples

Here are some examples of flake references in their URL-like representation:

• .: The flake in the current directory.
• /home/alice/src/patchelf: A flake in some other directory.
• nixpkgs: The nixpkgs entry in the flake registry.
• nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293: The nixpkgs entry in the flake registry, with its Git revision overridden to a specific value.
• github:NixOS/nixpkgs: The master branch of the NixOS/nixpkgs repository on GitHub.
• github:NixOS/nixpkgs/nixos-20.09: The nixos-20.09 branch of the nixpkgs repository.
• github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293: A specific revision of the nixpkgs repository.
• github:edolstra/nix-warez?dir=blender: A flake in a subdirectory of a GitHub repository.
• git+https://github.com/NixOS/patchelf: A Git repository.
• git+https://github.com/NixOS/patchelf?ref=master: A specific branch of a Git repository.
• git+https://github.com/NixOS/patchelf?ref=master&rev=f34751b88bd07d7f44f5cd3200fb4122bf916c7e: A specific branch and revision of a Git repository.
• https://github.com/NixOS/patchelf/archive/master.tar.gz: A tarball flake.

Flake reference attributes

The following generic flake reference attributes are supported:

• dir: The subdirectory of the flake in which flake.nix is located. This parameter enables having multiple flakes in a repository or tarball. The default is the root directory of the flake.

• narHash: The hash of the NAR serialisation (in SRI format) of the contents of the flake. This is useful for flake types such as tarballs that lack a unique content identifier such as a Git commit hash.

In addition, the following attributes are common to several flake reference types:

• rev: A Git or Mercurial commit hash.

• ref: A Git or Mercurial branch or tag name.

Finally, some attribute are typically not specified by the user, but can occur in locked flake references and are available to Nix code:

• revCount: The number of ancestors of the commit rev.

• lastModified: The timestamp (in seconds since the Unix epoch) of the last modification of this version of the flake. For Git/Mercurial flakes, this is the commit time of commit rev, while for tarball flakes, it's the most recent timestamp of any file inside the tarball.

Types

Currently the type attribute can be one of the following:

• path: arbitrary local directories, or local Git trees. The required attribute path specifies the path of the flake. The URL form is

  [path:]<path>(\?<params)?
  

  where path is an absolute path.

  path must be a directory in the file system containing a file named flake.nix.

  path generally must be an absolute path. However, on the command line, it can be a relative path (e.g. . or ./foo) which is interpreted as relative to the current directory. In this case, it must start with . to avoid ambiguity with registry lookups (e.g. nixpkgs is a registry lookup; ./nixpkgs is a relative path).

• git: Git repositories. The location of the repository is specified by the attribute url.

  They have the URL form

  git(+http|+https|+ssh|+git|+file|):(//<server>)?<path>(\?<params>)?
  

  The ref attribute defaults to resolving the HEAD reference.

  The rev attribute must denote a commit that exists in the branch or tag specified by the ref attribute, since Nix doesn't do a full clone of the remote repository by default (and the Git protocol doesn't allow fetching a rev without a known ref). The default is the commit currently pointed to by ref.

  When git+file is used without specifying ref or rev, files are fetched directly from the local path as long as they have been added to the Git repository. If there are uncommitted changes, the reference is treated as dirty and a warning is printed.

  For example, the following are valid Git flake references:

  • git+https://example.org/my/repo
  • git+https://example.org/my/repo?dir=flake1
  • git+ssh://git@github.com/NixOS/nix?ref=v1.2.3
  • git://github.com/edolstra/dwarffs?ref=unstable&rev=e486d8d40e626a20e06d792db8cc5ac5aba9a5b4
  • git+file:///home/my-user/some-repo/some-repo

• mercurial: Mercurial repositories. The URL form is similar to the git type, except that the URL schema must be one of hg+http, hg+https, hg+ssh or hg+file.

• tarball: Tarballs. The location of the tarball is specified by the attribute url.

  In URL form, the schema must be tarball+http://, tarball+https:// or tarball+file://. If the extension corresponds to a known archive format (.zip, .tar, .tgz, .tar.gz, .tar.xz, .tar.bz2 or .tar.zst), then the tarball+ can be dropped.

• file: Plain files or directory tarballs, either over http(s) or from the local disk.

  In URL form, the schema must be file+http://, file+https:// or file+file://. If the extension doesn’t correspond to a known archive format (as defined by the tarball fetcher), then the file+ prefix can be dropped.

• github: A more efficient way to fetch repositories from GitHub. The following attributes are required:

  • owner: The owner of the repository.

  • repo: The name of the repository.

  These are downloaded as tarball archives, rather than through Git. This is often much faster and uses less disk space since it doesn't require fetching the entire history of the repository. On the other hand, it doesn't allow incremental fetching (but full downloads are often faster than incremental fetches!).

  The URL syntax for github flakes is:

  github:<owner>/<repo>(/<rev-or-ref>)?(\?<params>)?
  

  <rev-or-ref> specifies the name of a branch or tag (ref), or a commit hash (rev). Note that unlike Git, GitHub allows fetching by commit hash without specifying a branch or tag.

  Some examples:

  • github:edolstra/dwarffs
  • github:edolstra/dwarffs/unstable
  • github:edolstra/dwarffs/d3f2baba8f425779026c6ec04021b2e927f61e31

• sourcehut: Similar to github, is a more efficient way to fetch SourceHut repositories. The following attributes are required:

  • owner: The owner of the repository (including leading ~).

  • repo: The name of the repository.

  Like github, these are downloaded as tarball archives.

  The URL syntax for sourcehut flakes is:

  sourcehut:<owner>/<repo>(/<rev-or-ref>)?(\?<params>)?

  <rev-or-ref> works the same as github. Either a branch or tag name (ref), or a commit hash (rev) can be specified.

  Since SourceHut allows for self-hosting, you can specify host as a parameter, to point to any instances other than git.sr.ht.

  Currently, ref name resolution only works for Git repositories. You can refer to Mercurial repositories by simply changing host to hg.sr.ht (or any other Mercurial instance). With the caveat that you must explicitly specify a commit hash (rev).

  Some examples:

  • sourcehut:~misterio/nix-colors
  • sourcehut:~misterio/nix-colors/main
  • sourcehut:~misterio/nix-colors?host=git.example.org
  • sourcehut:~misterio/nix-colors/182b4b8709b8ffe4e9774a4c5d6877bf6bb9a21c
  • sourcehut:~misterio/nix-colors/21c1a380a6915d890d408e9f22203436a35bb2de?host=hg.sr.ht

• indirect: Indirections through the flake registry. These have the form

  [flake:]<flake-id>(/<rev-or-ref>(/rev)?)?
  

  These perform a lookup of <flake-id> in the flake registry. For example, nixpkgs and nixpkgs/release-20.09 are indirect flake references. The specified rev and/or ref are merged with the entry in the registry; see nix registry for details.

Flake format

As an example, here is a simple flake.nix that depends on the Nixpkgs flake and provides a single package (i.e. an installable derivation):

{
  description = "A flake for building Hello World";

  inputs.nixpkgs.url = github:NixOS/nixpkgs/nixos-20.03;

  outputs = { self, nixpkgs }: {

    packages.x86_64-linux.default =
      # Notice the reference to nixpkgs here.
      with import nixpkgs { system = "x86_64-linux"; };
      stdenv.mkDerivation {
        name = "hello";
        src = self;
        buildPhase = "gcc -o hello ./hello.c";
        installPhase = "mkdir -p $out/bin; install -t $out/bin hello";
      };

  };
}

The following attributes are supported in flake.nix:

• description: A short, one-line description of the flake.

• inputs: An attrset specifying the dependencies of the flake (described below).

• outputs: A function that, given an attribute set containing the outputs of each of the input flakes keyed by their identifier, yields the Nix values provided by this flake. Thus, in the example above, inputs.nixpkgs contains the result of the call to the outputs function of the nixpkgs flake.

  In addition to the outputs of each input, each input in inputs also contains some metadata about the inputs. These are:

  • outPath: The path in the Nix store of the flake's source tree.

  • rev: The commit hash of the flake's repository, if applicable.

  • revCount: The number of ancestors of the revision rev. This is not available for github repositories, since they're fetched as tarballs rather than as Git repositories.

  • lastModifiedDate: The commit time of the revision rev, in the format %Y%m%d%H%M%S (e.g. 20181231100934). Unlike revCount, this is available for both Git and GitHub repositories, so it's useful for generating (hopefully) monotonically increasing version strings.

  • lastModified: The commit time of the revision rev as an integer denoting the number of seconds since 1970.

  • narHash: The SHA-256 (in SRI format) of the NAR serialization of the flake's source tree.

  The value returned by the outputs function must be an attribute set. The attributes can have arbitrary values; however, various nix subcommands require specific attributes to have a specific value (e.g. packages.x86_64-linux must be an attribute set of derivations built for the x86_64-linux platform).

• nixConfig: a set of nix.conf options to be set when evaluating any part of a flake. In the interests of security, only a small set of whitelisted options (currently bash-prompt, bash-prompt-prefix, bash-prompt-suffix, and flake-registry) are allowed to be set without confirmation so long as accept-flake-config is not set in the global configuration.

Flake inputs

The attribute inputs specifies the dependencies of a flake, as an attrset mapping input names to flake references. For example, the following specifies a dependency on the nixpkgs and import-cargo repositories:

# A GitHub repository.
inputs.import-cargo = {
  type = "github";
  owner = "edolstra";
  repo = "import-cargo";
};

# An indirection through the flake registry.
inputs.nixpkgs = {
  type = "indirect";
  id = "nixpkgs";
};

Alternatively, you can use the URL-like syntax:

inputs.import-cargo.url = github:edolstra/import-cargo;
inputs.nixpkgs.url = "nixpkgs";

Each input is fetched, evaluated and passed to the outputs function as a set of attributes with the same name as the corresponding input. The special input named self refers to the outputs and source tree of this flake. Thus, a typical outputs function looks like this:

outputs = { self, nixpkgs, import-cargo }: {
  ... outputs ...
};

It is also possible to omit an input entirely and only list it as expected function argument to outputs. Thus,

outputs = { self, nixpkgs }: ...;

without an inputs.nixpkgs attribute is equivalent to

inputs.nixpkgs = {
  type = "indirect";
  id = "nixpkgs";
};

Repositories that don't contain a flake.nix can also be used as inputs, by setting the input's flake attribute to false:

inputs.grcov = {
  type = "github";
  owner = "mozilla";
  repo = "grcov";
  flake = false;
};

outputs = { self, nixpkgs, grcov }: {
  packages.x86_64-linux.grcov = stdenv.mkDerivation {
    src = grcov;
    ...
  };
};

Transitive inputs can be overridden from a flake.nix file. For example, the following overrides the nixpkgs input of the nixops input:

inputs.nixops.inputs.nixpkgs = {
  type = "github";
  owner = "my-org";
  repo = "nixpkgs";
};

It is also possible to "inherit" an input from another input. This is useful to minimize flake dependencies. For example, the following sets the nixpkgs input of the top-level flake to be equal to the nixpkgs input of the dwarffs input of the top-level flake:

inputs.nixpkgs.follows = "dwarffs/nixpkgs";

The value of the follows attribute is a /-separated sequence of input names denoting the path of inputs to be followed from the root flake.

Overrides and follows can be combined, e.g.

inputs.nixops.inputs.nixpkgs.follows = "dwarffs/nixpkgs";

sets the nixpkgs input of nixops to be the same as the nixpkgs input of dwarffs. It is worth noting, however, that it is generally not useful to eliminate transitive nixpkgs flake inputs in this way. Most flakes provide their functionality through Nixpkgs overlays or NixOS modules, which are composed into the top-level flake's nixpkgs input; so their own nixpkgs input is usually irrelevant.

Lock files

Inputs specified in flake.nix are typically "unlocked" in the sense that they don't specify an exact revision. To ensure reproducibility, Nix will automatically generate and use a lock file called flake.lock in the flake's directory. The lock file contains a graph structure isomorphic to the graph of dependencies of the root flake. Each node in the graph (except the root node) maps the (usually) unlocked input specifications in flake.nix to locked input specifications. Each node also contains some metadata, such as the dependencies (outgoing edges) of the node.

For example, if flake.nix has the inputs in the example above, then the resulting lock file might be:

{
  "version": 7,
  "root": "n1",
  "nodes": {
    "n1": {
      "inputs": {
        "nixpkgs": "n2",
        "import-cargo": "n3",
        "grcov": "n4"
      }
    },
    "n2": {
      "inputs": {},
      "locked": {
        "owner": "edolstra",
        "repo": "nixpkgs",
        "rev": "7f8d4b088e2df7fdb6b513bc2d6941f1d422a013",
        "type": "github",
        "lastModified": 1580555482,
        "narHash": "sha256-OnpEWzNxF/AU4KlqBXM2s5PWvfI5/BS6xQrPvkF5tO8="
      },
      "original": {
        "id": "nixpkgs",
        "type": "indirect"
      }
    },
    "n3": {
      "inputs": {},
      "locked": {
        "owner": "edolstra",
        "repo": "import-cargo",
        "rev": "8abf7b3a8cbe1c8a885391f826357a74d382a422",
        "type": "github",
        "lastModified": 1567183309,
        "narHash": "sha256-wIXWOpX9rRjK5NDsL6WzuuBJl2R0kUCnlpZUrASykSc="
      },
      "original": {
        "owner": "edolstra",
        "repo": "import-cargo",
        "type": "github"
      }
    },
    "n4": {
      "inputs": {},
      "locked": {
        "owner": "mozilla",
        "repo": "grcov",
        "rev": "989a84bb29e95e392589c4e73c29189fd69a1d4e",
        "type": "github",
        "lastModified": 1580729070,
        "narHash": "sha256-235uMxYlHxJ5y92EXZWAYEsEb6mm+b069GAd+BOIOxI="
      },
      "original": {
        "owner": "mozilla",
        "repo": "grcov",
        "type": "github"
      },
      "flake": false
    }
  }
}

This graph has 4 nodes: the root flake, and its 3 dependencies. The nodes have arbitrary labels (e.g. n1). The label of the root node of the graph is specified by the root attribute. Nodes contain the following fields:

• inputs: The dependencies of this node, as a mapping from input names (e.g. nixpkgs) to node labels (e.g. n2).

• original: The original input specification from flake.lock, as a set of builtins.fetchTree arguments.

• locked: The locked input specification, as a set of builtins.fetchTree arguments. Thus, in the example above, when we build this flake, the input nixpkgs is mapped to revision 7f8d4b088e2df7fdb6b513bc2d6941f1d422a013 of the edolstra/nixpkgs repository on GitHub.

  It also includes the attribute narHash, specifying the expected contents of the tree in the Nix store (as computed by nix hash-path), and may include input-type-specific attributes such as the lastModified or revCount. The main reason for these attributes is to allow flake inputs to be substituted from a binary cache: narHash allows the store path to be computed, while the other attributes are necessary because they provide information not stored in the store path.

• flake: A Boolean denoting whether this is a flake or non-flake dependency. Corresponds to the flake attribute in the inputs attribute in flake.nix.

The original and locked attributes are omitted for the root node. This is because we cannot record the commit hash or content hash of the root flake, since modifying flake.lock will invalidate these.

The graph representation of lock files allows circular dependencies between flakes. For example, here are two flakes that reference each other:

{
  inputs.b = ... location of flake B ...;
  # Tell the 'b' flake not to fetch 'a' again, to ensure its 'a' is
  # *this* 'a'.
  inputs.b.inputs.a.follows = "";
  outputs = { self, b }: {
    foo = 123 + b.bar;
    xyzzy = 1000;
  };
}

and

{
  inputs.a = ... location of flake A ...;
  inputs.a.inputs.b.follows = "";
  outputs = { self, a }: {
    bar = 456 + a.xyzzy;
  };
}

Lock files transitively lock direct as well as indirect dependencies. That is, if a lock file exists and is up to date, Nix will not look at the lock files of dependencies. However, lock file generation itself does use the lock files of dependencies by default.

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