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nixos-flake-example

warning

WARNING: You should understand that:

  • there is currently no path to flakes being stable

  • we can't even experiment with flakes alternatives without committing to pure-eval as a stepping stone

  • the people that CARE about the problems solved by flakes seem to care about solving this problem

  • the people using impurity everywhere don't seem very motivated to dive into this problem-space

  • as a result, the tooling is completely fractured and stagnant

  • besides some UX fixes, bugfixes, and other feature-work in Nix, this has more or less been the case for years

  • I'm tired of seeing (often self-proclaimed, repeated-from-others) FUD by non-flakes users

    Anyway, now you know. Proceed at your own caution.

overview

NOTE: nixflk is a better example repo for a full NixOS config layout, this repo is mostly to provide more context+examples around flakes, and to show that you can produce the same EXACT system with flakes as with nix-build, if you know what to do.

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)

Flakes is a few things:

  • flake.nix: a Nix file, with a specific structure to describe inputs and outputs for a Nix project
  • 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
  • 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

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:

These represent an example, minimal NixOS system configuration.

The easiest way to build it, without cloning this repo:

nix build 'github:colemickens/nixos-flake-example#nixosConfigurations.mysystem.config.system.build.toplevel'

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...