Core Options

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

Options

debug

flake

flake.packages

flake.apps

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

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

flake.checks

flake.devShells

flake.formatter

flake.legacyPackages

flake.nixosConfigurations

flake.nixosModules

flake.overlays

perInput

perSystem

perSystem.packages

perSystem.apps

perSystem.apps.<name>.program

perSystem.apps.<name>.type

perSystem.checks

perSystem.debug

perSystem.devShells

perSystem.formatter

perSystem.legacyPackages

perSystem.overlayAttrs

systems

transposition

transposition.<name>.adHoc

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-parts/modules/debug.nix

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-parts/modules/packages.nix, flake-parts/modules/overlays.nix, flake-parts/modules/nixosModules.nix, flake-parts/modules/nixosConfigurations.nix, flake-parts/modules/legacyPackages.nix, flake-parts/modules/formatter.nix, flake-parts/modules/flake.nix, flake-parts/modules/devShells.nix, flake-parts/modules/checks.nix, flake-parts/modules/apps.nix

flake.packages

See perSystem.packages for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake-parts/modules/packages.nix

flake.apps

See perSystem.apps for description and examples.

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

Default: { }

Declared by:

flake-parts/modules/apps.nix

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-parts/modules/apps.nix

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

A type tag for apps consumers.

Type: value “app” (singular enum)

Default: "app"

Declared by:

flake-parts/modules/apps.nix

flake.checks

See perSystem.checks for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake-parts/modules/checks.nix

flake.devShells

See perSystem.devShells for description and examples.

Type: lazy attribute set of lazy attribute set of package

Default: { }

Declared by:

flake-parts/modules/devShells.nix

flake.formatter

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

Type: lazy attribute set of package

Default: { }

Declared by:

flake-parts/modules/formatter.nix

flake.legacyPackages

See perSystem.legacyPackages for description and examples.

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

Default: { }

Declared by:

flake-parts/modules/legacyPackages.nix

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-parts/modules/nixosConfigurations.nix

flake.nixosModules

NixOS modules.

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

Type: lazy attribute set of unspecified value

Default: { }

Declared by:

flake-parts/modules/nixosModules.nix

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:

flake-parts/modules/overlays.nix

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:

flake-parts/modules/perSystem.nix

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:

flake-parts/extras/easyOverlay.nix, flake-parts/modules/withSystem.nix, flake-parts/modules/perSystem.nix, flake-parts/modules/packages.nix, flake-parts/modules/legacyPackages.nix, flake-parts/modules/formatter.nix, flake-parts/modules/devShells.nix, flake-parts/modules/debug.nix, flake-parts/modules/checks.nix, flake-parts/modules/apps.nix

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:

flake-parts/modules/packages.nix

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:

flake-parts/modules/apps.nix

perSystem.apps.<name>.program

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

Type: string or package convertible to it

Declared by:

flake-parts/modules/apps.nix

perSystem.apps.<name>.type

A type tag for apps consumers.

Type: value “app” (singular enum)

Default: "app"

Declared by:

flake-parts/modules/apps.nix

perSystem.checks

Derivations to be built by nix flake check.

Type: lazy attribute set of package

Default: { }

Declared by:

flake-parts/modules/checks.nix

perSystem.debug

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

Type: lazy attribute set of raw value

Declared by:

flake-parts/modules/formatter.nix

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: { }

Declared by:

flake-parts/modules/devShells.nix

perSystem.formatter

A package used by nix fmt.

Type: null or package

Default: null

Declared by:

flake-parts/modules/formatter.nix

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:

flake-parts/modules/legacyPackages.nix

perSystem.overlayAttrs

Attributes to add to overlays.default.

The overlays.default overlay will re-evaluate perSystem with the “prev” (or “super”) overlay argument value as the pkgs module argument. The easyOverlay module also adds the final module argument, for the result of applying the overlay.

When not in an overlay, final defaults to pkgs plus the generated overlay. This requires Nixpkgs to be re-evaluated, which is more expensive than setting pkgs to a Nixpkgs that already includes the necessary overlays that are required for the flake itself.

See Overlays.

Type: lazy attribute set of raw value

Default: { }

Declared by:

flake-parts/extras/easyOverlay.nix

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:

flake-parts/modules/perSystem.nix

transposition

A helper that defines transposed attributes in the flake outputs.

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:

flake-parts/modules/transposition.nix

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:

flake-parts/modules/transposition.nix