nixos-flake-example

This shows how to build the same config, with and without flakes.

It also shows that flake.nix is basically just some syntax.

flake fundamentals

Nix is in flakes mode when:

1. nixos-rebuild <cmd> --flake '.#' is used
2. nix build '.#something' the hash-tag syntax is used

This automatically loads from a flake.nix in the specified dir. See these examples:

These three examples, for me, are all the same source, but accessed in different ways:

• nix build '.#nixosConfigurations.mysystem'

  (loads flake.nix from . (current dir))

• nix build '/home/cole/code/nixos-flake-example#nixosConfigurations.mysystem'

  (loads flake.nix from /home/cole/code/nixos-flake-example)

• nix build 'github.com:colemickens/nixos-flake-example#nixosConfigurations.mysystem'

  (nix will clone my github repo, then load flake.nix from flake.nix in the root of that repo checkout)

more tips

1. nixos-rebuild build --flake '.#' will automatically try to find and build the attribute: .#nixosConfigurations.your_hostname (assuming your machines hostname is your_hostname)

overview

Note that these produce the same output:

1. with flakes:

   nix build '.#nixosConfigurations.mysystem.config.system.build.toplevel'
   readlink -f result
   /nix/store/0imi716z1qd04pfh4zdw6mb0gnxmakjs-nixos-system-nixos-21.03.20201020.007126e
   
   nixos-rebuild build --flake '.#mysystem'
   readlink -f result
   /nix/store/0imi716z1qd04pfh4zdw6mb0gnxmakjs-nixos-system-nixos-21.03.20201020.007126e
   

   Note, nixos-rebuild is basically just some magic to build the right derivation and then set it as a system profile, and activate it.

2. without flakes:

   export NIX_PATH=nixos-config=$(pwd)/configuration.nix:nixpkgs=https://github.com/nixos/nixpkgs/archive/nixos-unstable.tar.gz
   
   
   /nix/store/zidq625i13hvbbs8alkklj8k6a191xix-nixos-system-nixos-21.03pre-git
   

   Note, same path same inner system, just much slower due to no eval cache. (they should be identical, but the flake version suffix is slightly different)

They build the same thing, the flake.nix just moves the redirection from the NixOS channel system into the flake instead.

Note, if you come back and run this later, you may need to tell nix to update the nixpkgs that it has pinned in flake.lock by running nix flake update --update-input nixpkgs. The non-flake example is going to re-download the nixos-unstable build when the cache expires. This could cause any hash differences if they're on different revs. (again, another reason to have control of it via flakes, and can lock it directly in the source.)