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Core Options

These options are provided by default. They reflect what Nix expects, plus a small number of helpful options, notably perSystem.

Options

debug

Whether to add the attributes debug, allSystems and currentSystem to the flake output. When true, this allows inspection of options via nix repl.

$ nix repl
nix-repl> :lf .
nix-repl> currentSystem._module.args.pkgs.hello
«derivation /nix/store/7vf0d0j7majv1ch1xymdylyql80cn5fp-hello-2.12.1.drv»

Each of debug, allSystems.<system> and currentSystem is an attribute set consisting of the config attributes, plus the extra attributes _module, config, options, extendModules. So note that these are not part of the config parameter, but are merged in for debugging convenience.

  • debug: The top-level options
  • allSystems: The perSystem submodule applied to the configured systems.
  • currentSystem: Shortcut into allSystems. Only available in impure mode. Works for arbitrary system values.

See Expore and debug option values for more examples.

Type: boolean

Default: false

Declared by:

flake

Raw flake output attributes. Any attribute can be set here, but some attributes are represented by options, to provide appropriate configuration merging.

Type: lazy attribute set of raw value

Declared by:

flake.packages

See perSystem.packages for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake.apps

See perSystem.apps for description and examples.

Type: lazy attribute set of lazy attribute set of (submodule)

Default: { }

Declared by:

flake.apps.<name>.<name>.meta

Metadata information about the app. Standardized in Nix at https://github.com/NixOS/nix/pull/11297.

Note: nix flake check is only aware of the description attribute in meta.

Type: lazy attribute set of raw value

Default: { }

Declared by:

flake.apps.<name>.<name>.program

A path to an executable or a derivation with meta.mainProgram.

Type: string or package convertible to it

Declared by:

flake.apps.<name>.<name>.type

A type tag for apps consumers.

Type: value “app” (singular enum)

Default: "app"

Declared by:

flake.checks

See perSystem.checks for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake.devShells

See perSystem.devShells for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake.flakeModule

Alias of flakeModules.default.

Type: submodule

Declared by:

flake.flakeModules

flake-parts modules for use by other flakes.

If the flake defines only one module, it should be flakeModules.default.

You can not read this option in defining the flake’s own imports. Instead, you can put the module in question into its own file or let binding and reference it both in imports and export it with this option.

See Dogfood a Reusable Module for details and an example.

Type: lazy attribute set of module

Default: { }

Declared by:

flake.formatter

An attribute set of per system a package used by nix fmt.

Type: lazy attribute set of package

Default: { }

Declared by:

flake.legacyPackages

See perSystem.legacyPackages for description and examples.

Type: lazy attribute set of lazy attribute set of raw value

Default: { }

Declared by:

flake.modules

Groups of modules published by the flake.

The outer attributes declare the class of the modules within it. The special attribute generic does not declare a class, allowing its modules to be used in any module class.

Type: lazy attribute set of lazy attribute set of module

Example:

{
  # NixOS configurations are modules with class "nixos"
  nixos = {
    # You can define a module right here:
    noBoot = { config, ... }: {
      boot.loader.enable = false;
    };
    # Or you may refer to it by file
    autoDeploy = ./nixos/auto-deploy.nix;
    # Or maybe you need both
    projectIcarus = { config, pkgs, ... }: {
      imports = [ ./nixos/project-icarus.nix ];
      services.project-icarus.package =
        withSystem pkgs.stdenv.hostPlatform.system ({ config, ... }:
          config.packages.default
        );
    };
  };
  # Flake-parts modules
  # If you're not just publishing a module, but also using it locally,
  # create a let binding to declare it before calling `mkFlake` so you can
  # use it in both places.
  flake = {
    foo = someModule;
  };
  # Modules that can be loaded anywhere
  generic = {
    my-pkgs = { _module.args.my-pkgs = …; };
  };
}

Declared by:

flake.nixosConfigurations

Instantiated NixOS configurations. Used by nixos-rebuild.

nixosConfigurations is for specific machines. If you want to expose reusable configurations, add them to nixosModules in the form of modules (no lib.nixosSystem), so that you can reference them in this or another flake’s nixosConfigurations.

Type: lazy attribute set of raw value

Default: { }

Example:

{
  my-machine = inputs.nixpkgs.lib.nixosSystem {
    # system is not needed with freshly generated hardware-configuration.nix
    # system = "x86_64-linux";  # or set nixpkgs.hostPlatform in a module.
    modules = [
      ./my-machine/nixos-configuration.nix
      config.nixosModules.my-module
    ];
  };
}

Declared by:

flake.nixosModules

NixOS modules.

You may use this for reusable pieces of configuration, service modules, etc.

Type: lazy attribute set of module

Default: { }

Declared by:

flake.overlays

An attribute set of overlays.

Note that the overlays themselves are not mergeable. While overlays can be composed, the order of composition is significant, but the module system does not guarantee sufficiently deterministic definition ordering, across versions and when changing imports.

Type: lazy attribute set of function that evaluates to a(n) function that evaluates to a(n) lazy attribute set of unspecified value

Default: { }

Example:

{
  default = final: prev: {};
}

Declared by:

partitionedAttrs

A set of flake output attributes that are taken from a partition instead of the default top level flake-parts evaluation.

The attribute name refers to the flake output attribute name, and the value is the name of the partition to use.

The flake attributes are overridden with lib.mkForce priority.

See the partitions options to understand the purpose.

Type: attribute set of string

Default: { }

Example:

{
  checks = "dev";
  devShells = "dev";
  herculesCI = "dev";
}

Declared by:

partitions

By partitioning the flake, you can avoid fetching inputs that are not needed for the evaluation of a particular attribute.

Each partition is a distinct module system evaluation. This allows attributes of the final flake to be defined by multiple sets of modules, so that for example the packages attribute can be evaluated without loading development related inputs.

While the module system does a good job at preserving laziness, the fact that a development related import can define packages means that in order to evaluate packages, you need to evaluate at least to the point where you can conclude that the development related import does not actually define a packages attribute. While the actual evaluation is cheap, it can only happen after fetching the input, which is not as cheap.

Type: attribute set of (submodule)

Default: { }

Example:

{
  dev = {
    extraInputsFlake = ./dev;
    module = ./dev/flake-module.nix;
  };
}

Declared by:

partitions.<name>.extraInputs

Extra inputs to add to the inputs module argument in the partition.

This can be used as a workaround for the fact that transitive inputs are locked in the “end user” flake. That’s not desirable for inputs they don’t need, such as development inputs.

Type: lazy attribute set of raw value

Default: if extraInputsFlake is set, then builtins.getFlake extraInputsFlake, else { }

Declared by:

partitions.<name>.extraInputsFlake

Location of a flake whose inputs to add to the inputs module argument in the partition. Note that flake follows are resolved without any awareness of inputs that are not in the flake. As a consequence, a follows entry in the flake inputs can not refer to inputs that are not in that specific flake.

Implementation note: if the type of extraInputsFlake is a path, it is loaded with an expression-based reimplementation of builtins.getFlake, as getFlake is incapable of loading paths in pure mode as of writing.

Type: raw value

Example: ./dev

Declared by:

partitions.<name>.module

A re-evaluation of the flake-parts top level modules.

You may define config definitions, imports, etc here, and it can be read like any other submodule.

Type: submodule

Default: { }

Example:

{
  imports = [
    ./dev/flake-module.nix
  ];
}

Declared by:

perInput

A function that pre-processes flake inputs.

It is called for users of perSystem such that inputs'.${name} = config.perInput system inputs.${name}.

This is used for inputs' and self'.

The attributes returned by the perInput function definitions are merged into a single namespace (per input), so each module should return an attribute set with usually only one or two predictable attribute names. Otherwise, the inputs' namespace gets polluted.

Type: function that evaluates to a(n) function that evaluates to a(n) lazy attribute set of unspecified value

Declared by:

perSystem

A function from system to flake-like attributes omitting the <system> attribute.

Modules defined here have access to the suboptions and some convenient module arguments.

Type: module

Declared by:

perSystem.packages

An attribute set of packages to be built by nix build.

nix build .#<name> will build packages.<name>.

Type: lazy attribute set of package

Default: { }

Declared by:

perSystem.apps

Programs runnable with nix run <name>.

Type: lazy attribute set of (submodule)

Default: { }

Example:

{
  default.program = "${config.packages.hello}/bin/hello";
}

Declared by:

perSystem.apps.<name>.meta

Metadata information about the app. Standardized in Nix at https://github.com/NixOS/nix/pull/11297.

Note: nix flake check is only aware of the description attribute in meta.

Type: lazy attribute set of raw value

Default: { }

Declared by:

perSystem.apps.<name>.program

A path to an executable or a derivation with meta.mainProgram.

Type: string or package convertible to it

Declared by:

perSystem.apps.<name>.type

A type tag for apps consumers.

Type: value “app” (singular enum)

Default: "app"

Declared by:

perSystem.checks

Derivations to be built by nix flake check.

Type: lazy attribute set of package

Default: { }

Declared by:

perSystem.debug

Values to return in e.g. allSystems.<system> when debug = true.

Type: lazy attribute set of raw value

Declared by:

perSystem.devShells

An attribute set of packages to be used as shells. nix develop .#<name> will run devShells.<name>.

Type: lazy attribute set of package

Default: { }

Example:

{
  default = pkgs.mkShell {
    nativeBuildInputs = with pkgs; [ wget bat cargo ];
  };
}

Declared by:

perSystem.formatter

A package used by nix fmt.

Type: null or package

Default: null

Declared by:

perSystem.legacyPackages

An attribute set of unmergeable values. This is also used by nix build .#<attrpath>.

Type: lazy attribute set of raw value

Default: { }

Declared by:

systems

All the system types to enumerate in the flake output subattributes.

In other words, all valid values for system in e.g. packages.<system>.foo.

Type: list of string

Declared by:

transposition

A helper that defines transposed attributes in the flake outputs.

When you define transposition.foo = { };, definitions are added to the effect of (pseudo-code):

flake.foo.${system} = (perSystem system).foo;
perInput = system: inputFlake: inputFlake.foo.${system};

Transposition is the operation that swaps the indices of a data structure. Here it refers specifically to the transposition between

perSystem: .${system}.${attribute}
outputs:   .${attribute}.${system}

It also defines the reverse operation in perInput.

Type: lazy attribute set of (submodule)

Declared by:

transposition.<name>.adHoc

Whether to provide a stub option declaration for perSystem.<name>.

The stub option declaration does not support merging and lacks documentation, so you are recommended to declare the perSystem.<name> option yourself and avoid adHoc.

Type: boolean

Default: false

Declared by: