nixos-flake-example

This readme starts out with an attempt to explain and justify flakes. It also contains some examples of nix cli flakes syntax and tips for adopting flakes in your project.

Finally, at the end of the readme is an example NixOS config with a supporting flake.nix, and instructions to build it with and without flakes support at the same time.

• Overview of Flakes (and why you want it)
• Important Related Reading
• Nix CLI - Flakes Usage
  • Useful Commands and Examples
    • nixos-rebuild
    • nix build
    • nix flake
  • Auto-coercion examples
• Tips for Porting to Flakes
• Example NixOS Config with optional Flake support

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

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

Auto-coercion examples

Nix CLI will try to be ... smart and auto-coerce some output attribute paths for you.

• nix build '/some/path#obs-studio':
  • builds and run the first attribute found:
    • /some/path#obs-studio
    • /some/path#packages.x86_64-linux.obs-studio
    • /some/path#legacyPackages.x86_64-linux.obs-studio
    • TODO: finish fleshing out this list
    • TODO: not sure about search order, presumably the bare one would be priority

Tips for Porting to Flakes

Remove Impurities - Since nix flakes does a 'pure' build by default,

• NIX_PATH is ignored
• <nixpkgs> imports do not work, and explicitly error
• local user nixpkgs config (~/.config/{nix,nixpkgs}) are ignore
• unpinned imports (aka, fetchTarball without a pinned rev+sha256) are forbidden

To fix these:

• specify all remote imports in flake.nix instead of using fetchTarball
  • the config in this repo shows an example of using the overlay from nixpkgs-wayland.
  • TODO: investigate getFlake vs passing inputs in specialArgs

Example NixOS Config with optional Flake support

Consider the nixos configuration in this repo:

• ./configuration.nix
• ./hardware-configuration.nix

These represent an example, minimal NixOS system configuration.

Let's prove that we can build this config, with and without flakes:

• Using nixos-rebuild:

  # with flakes
  unset NIX_PATH
  nixos-rebuild build --flake '.#mysystem'
  readlink -f ./result
  /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  
  # !! for this next step, match the git SHA1 to what the flake.lock uses
  #    otherwise you'll have a hash mismatch due to different nixpkgs
  
  # without flakes
  export NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
  nixos-rebuild build
  readlink -f ./result
  /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  

• Using nix build:

  # with flakes
  unset NIX_PATH
  nix build '.#nixosConfigurations.mysystem.config.system.build.toplevel
  readlink -f ./result
  /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  
  # without flakes
  export NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
  nix-build '<nixos/nixpkgs>' -A config.system.build.toplevel
  readlink -f ./result
  /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  

• The ./check.sh script automates this process:

  cole@slynux ~/code/nixos-flake-example master* 7s
  ❯ ./check.sh     
  
  :: Updating the 'nixpkgs' input in flake.nix
  + nix flake update --update-input nixpkgs
  + set +x
  
  :: Using 'nixos-rebuild' to build the 'mysystem' toplevel
  + nixos-rebuild build --flake '.#mysystem'
  warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
  building the system configuration...
  warning: Git tree '/home/cole/code/nixos-flake-example' is dirty
  + set +x
  
  :: Using rev=007126eef72271480cb7670e19e501a1ad2c1ff2 for <nixpkgs> (extracted from flake.nix)
  
  :: Setting NIX_PATH to the same values flakes is using
  + NIX_PATH=nixpkgs=https://github.com/nixos/nixpkgs/archive/007126eef72271480cb7670e19e501a1ad2c1ff2.tar.gz:nixos-config=/home/cole/code/nixos-flake-example/configuration.nix
  + nix-build '<nixpkgs/nixos>' -A config.system.build.toplevel
  /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  + set +x
  
  flake: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  clssc: /nix/store/gg1jhmzqndqa0rfnwfdbnzrn8f74ckr6-nixos-system-mysystem-21.03pre-git
  

Flake Feedback/Ponderings

• Is the hash tag syntax really worth it?

  • For example, is:
    • nix build 'github:colemickens/nixpkgs-wayland#obs-studio'
  • really better than:
    • nix build --flake 'github:colemickens/nixpkgs-wayland' 'obs-studio' ?

• Are the auto-coercion rules for attribute paths worth it? They definitely add some mental overhead...