## Flake outputs

the `outputs` in `flake.nix` are what a flake produces as part of its build. Each flake can have many different outputs simultaneously, including but not limited to:

- Nix packages: named `apps.<system>.<name>`, `packages.<system>.<name>`, or `legacyPackages.<system>.<name>`
  - we can build a package by command `nix build .#<name>`
- Nix Helper Functions: named `lib`., which means a library for other flakes.
- Nix development environments: named `devShells`
  - `devShells` can be used by command `nix develop`, will be introduced later.
- NixOS configuration: named `nixosConfiguration`
  - `nixosConfiguration` will be used by command `nixos-rebuild switch --flake .#<name>`
- Nix templates: named `templates`
  - templates can be used by command `nix flake init --template <reference>`
- Other user defined outputs, may be parsed by other nix-related tools.

An example copy from NixOS Wiki:

```nix
{ 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 = ""; };
}
```