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Flake

Introduction to Flake

Overview of Flakes (and why you want it)

Flakes is a few things:

  • flake.nix: a Nix file, with a specific structure to describe inputs and outputs for a Nix project
  • See NixOS Wiki - Flakes - Input Schema for flake input examples
  • See NixOS Wiki - Flakes - Output Schema for flake output examples
  • flake.lock: a manifest that "locks" inputs and records the exact versions in use
  • CLI support for flake-related features
  • pure (by default) evaluations

This ultimately enables:

  • properly hermetic builds
  • fully reproducable and portable Nix projects
  • faster Nix operations due to evaluation caching enabled by pure evaluations)

This removes the need for:

  • using niv or other tooling to lock dependencies
  • manually documenting or scripting to ensure NIX_PATH is set consistently for your team
  • the need for the "the impure eval tree of sorrow" that comes with all of today's Nix impurities

Important Related Reading

  • NixOS Wiki - Flakes
  • a somewhat haphazard collection of factoids/snippets related to flakes
  • particularly look at: Flake Schema, and it's two sections: Input Schema, Output Schema
  • Tweag - NixOS flakes
  • this article describes how to enable flake support in nix and nix-daemon
  • reading this article is a pre-requisite
  • this README.md assumes you've enabled flakes system-wide
  • omit using boot.isContainer = true; on configuration.nix (as the article suggests) if you want to use nixos-rebuild rather than nixos-container

Nix CLI - Flakes Usage

Nix is in flakes mode when:

  • --flake is used with the nixos-rebuild command
  • or, when nix build is used with an argument like '.#something' (the hash symbol separates the flake source from the attribute to build)

When in this mode:

  • Nix flake commands will implicitly take a directory path, it expects a flake.nix inside
  • when you see: nix build '.#something', the . means current directory, and #something means to build the something output attribute

Useful Commands and Examples

nixos-rebuild

  • nixos-rebuild build --flake '.#'
  • looks for flake.nix in . (current dir)
  • since it's nixos-rebuild, it automatically tries to build:
  • #nixosConfigurations.{hostname}.config.system.build.toplevel
  • nixos-rebuild build --flake '/code/nixos-config#mysystem'
  • looks for flake.nix in /code/nixos-config
  • since it's nixos-rebuild, it automatically tries to build:
  • #nixosConfigurations.mysystem.config.system.build.toplevel
  • (note that this time we specifically asked, and got to build the mysystem config)

nix build

  • nix build 'github:colemickens/nixpkgs-wayland#obs-studio'
  • looks for flake.nix in (a checkout of github.com/colemickens/nixpkgs-wayland)
  • builds and run the first attribute found:
  • #obs-studio
  • #packages.{currentSystem}.obs-studio
  • TODO: finish fleshing out this list

nix flake

  • nix flake update --recreate-lock-file
  • updates all inputs and recreating flake.lock
  • nix flake update --update-input nixpkgs
  • updates a single input to latest and recording it in flake.lock