nix flake

Warning: This program is experimental and its interface is subject to change.

# Name

`nix flake` - manage Nix flakes

# Synopsis

`nix flake` [option...] subcommand

where subcommand is one of the following:

- `nix flake archive` - copy a flake and all its inputs to a store
- `nix flake check` - check whether the flake evaluates and run its tests
- `nix flake clone` - clone flake repository
- `nix flake info` - show flake metadata
- `nix flake init` - create a flake in the current directory from a template
- `nix flake lock` - create missing lock file entries
- `nix flake metadata` - show flake metadata
- `nix flake new` - create a flake in the specified directory from a template
- `nix flake prefetch` - download the source tree denoted by a flake reference into the Nix store
- `nix flake show` - show the outputs provided by a flake
- `nix flake update` - update flake lock file

# 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 `master`.

  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`.

  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 `http://`, `https://` or `file://` URLs and the extension must be `.zip`, `.tar`, `.tar.gz`, `.tar.xz`, `.tar.bz2` or `.tar.zst`.

- `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`

- `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 }: {

        defaultPackage.x86_64-linux =
          # 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).

## 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.