nixos-and-flakes-book/docs/other-usage-of-flakes/outputs.md
2023-07-04 13:56:18 +08:00

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# Flake Outputs
In `flake.nix`, the `outputs` section defines the different outputs that a flake can produce during its build process. A flake can have multiple outputs simultaneously, which can include but are not limited to the following:
- Nix packages: These are named `apps.<system>.<name>`, `packages.<system>.<name>`, or `legacyPackages.<system>.<name>`. You can build a specific package using the command `nix build .#<name>`.
- Nix helper functions: These are named `lib.<name>` and serve as libraries for other flakes to use.
- Nix development environments: These are named `devShells` and provide isolated development environments. They can be accessed using the command `nix develop`.
- NixOS configurations: These are named `nixosConfiguration` and represent specific NixOS system configurations. You can activate a configuration using the command `nixos-rebuild switch --flake .#<name>`.
- Nix templates: These are named `templates` and can be used as a starting point for creating new projects. You can generate a project using the command `nix flake init --template <reference>`.
- Other user-defined outputs: These outputs can be defined by the user and may be used by other Nix-related tools.
Here's an example excerpt from the NixOS Wiki that demonstrates the structure of the `outputs` section:
```nix
{
inputs = {
# ......
};
outputs = { self, ... }@inputs: {
# Executed by `nix flake check`
checks."<system>"."<name>" = derivation;
# Executed by `nix build .#<name>`
packages."<system>"."<name>" = derivation;
# Executed by `nix build .`
packages."<system>".default = derivation;
# Executed by `nix run .#<name>`
apps."<system>"."<name>" = {
type = "app";
program = "<store-path>";
};
# Executed by `nix run . -- <args?>`
apps."<system>".default = { type = "app"; program = "..."; };
# Formatter (alejandra, nixfmt or nixpkgs-fmt)
formatter."<system>" = derivation;
# Used for nixpkgs packages, also accessible via `nix build .#<name>`
legacyPackages."<system>"."<name>" = derivation;
# Overlay, consumed by other flakes
overlays."<name>" = final: prev: { };
# Default overlay
overlays.default = {};
# Nixos module, consumed by other flakes
nixosModules."<name>" = { config }: { options = {}; config = {}; };
# Default module
nixosModules.default = {};
# Used with `nixos-rebuild --flake .#<hostname>`
# nixosConfigurations."<hostname>".config.system.build.toplevel must be a derivation
nixosConfigurations."<hostname>" = {};
# Used by `nix develop .#<name>`
devShells."<system>"."<name>" = derivation;
# Used by `nix develop`
devShells."<system>".default = derivation;
# Hydra build jobs
hydraJobs."<attr>"."<system>" = derivation;
# Used by `nix flake init -t <flake>#<name>`
templates."<name>" = {
path = "<store-path>";
description = "template description goes here?";
};
# Used by `nix flake init -t <flake>`
templates.default = { path = "<store-path>"; description = ""; };
};
}
```