Nix Flakes

Updated at
May 8, 2024 9:17 PM
Published at
May 9, 2020
NOTE: All of this is completely unstable so please don’t adopt this just yet

Nix Flakes is an experimental branch of the Nix project that adds:

  • A central flake.nix entry-point to Nix projects.
  • Builtin dependency management
  • Is tied to Git
  • Per-commit evaluation caching
  • A new nix CLI.

Here are some notes that I took for myself on the subject.

Other sources



Add the following options to the NixOS configuration (on nixos-unstable):

{ pkgs, ... }:{
  # Enable the nix 2.0 CLI and flakes support feature-flags
  nix.extraOptions = ''
    experimental-features = nix-command flakes

Then run nixos-rebuild switch and that’s it.

Other systems

Install Nix 2.5.0 or later. Then edit either ~/.config/nix/nix.conf or /etc/nix/nix.conf and add:

experimental-features = nix-command flakes

This is needed to expose the Nix 2.0 CLI and flakes support that are hidden behind feature-flags.

Finally, if the Nix installation is in multi-user mode, don’t forget to restart the nix-daemon ( you can check that by running ps aux | grep nix-daemon to see if it’s running).

Basic project usage

NOTE: flake makes a strong assumption that the folder is a git repository. It doesn’t work outside of them.

In your repo, run nix flake init to generate the flake.nix file. Then run git add flake.nix to add it to the git staging area, otherwise nix will not recognize that the file exists.

TODO: add more usage examples here.

See also

Flake schema

The flake.nix file is a Nix file but that has special restrictions (more on that later).

It has 3 top-level attributes:

  • description which is self…describing
  • nixConfig allows to set per-project Nix configuration.
  • input is an attribute set of all the dependencies of the flake. The schema is described below.
  • output is a function of one argument that takes an attribute set of all the realized inputs, and outputs another attribute set which schema is described below.

nixConfig schema

Eg (from the commit message):

  nixConfig.bash-prompt-suffix = "ngi# ";
  nixConfig.substituters = [ "" ];

Input schema

This is not a complete schema but should be enough to get you started:

{ = {
    # Source of the input. It supports `github:` `gitlab:` and a number of    # other schemes
    url = "github:foo/bar/branch";
    # Turn off if the target is not a flake.
    flake = false;
    # Used to override inputs of the target if it is a flake.
    inputs = {
      # For example, here we declare to use the same version as the parent
      # nixpkgs. It's probably also possible to override the URL attribute.      
      nixpkgs.follows = "nixpkgs";

The bar input is then passes to the

Output schema

Here is what I found out while reading src/nix/ in CmdFlakeCheck.


  • <system> is something like "x86_64-linux".
  • <machine> is something like "mymachine".
  • <attr> is an attribute name like "hello".
  • <job> is a hydra job name like "release".
  • <flake> is a flake name like "nixpkgS".
  • <store-path> is a /nix/store.. path
{ self, ... }@inputs:
  # Executed by `nix flake check`
  checks."<system>"."<attr>" = derivation;
  # Executed by `nix build .#<name>`
  packages."<system>"."<attr>" = derivation;
  # Executed by `nix build .`
  defaultPackage."<system>" = derivation;
  # Executed by `nix run .#<name>
  apps."<system>"."<attr>" = {
    type = "app";
    program = "<store-path>";
  defaultApp."<system>" = { type = "app"; program = "..."; };

  # TODO: Not sure how it's being used
  legacyPackages = TODO;
  # TODO: Not sure how it's being used
  overlay = final: prev: { };
  # TODO: Same idea as overlay but several.
  overlays."<attr>" = final: prev: { };
  # TODO: Not sure how it's being used
  nixosModule = TODO;
  # TODO: Same idea as nixosModule but several
  nixosModules."<attr>" = TODO;
  # TODO: Not sure how it's being used
  nixosConfigurations."<machine>" = TODO;
  # TODO: Similar idea as for nixosModules but for hydra jobs.
  hydraJobs."<job>" = TODO;
  # Used by `nix flake init -t <flake>`
  defaultTemplate = {
    path = "<store-path>";
    description = "template description goes here?";
  # Used by `nix flake init -t <flake>#<attr>`
  templates."<attr>" = { path = "<store-path>"; description = ""; );

See also:

Building NixOS configurations with Flakes

There is a special, undocumented way to build NixOS configurations with flakes.

First, change flake.nix to output a configuration. This uses the nixosConfigurations key. The nixpkgs flake includes a helper for that:

  outputs = { nixpkgs, ... }: {
    nixosConfigurations.mymachine = nixpkgs.lib.nixosSystem {
      modules = [
        # Point this to your original configuration.
      # Select the target system here.
      system = "x86_64-linux";

Then to switch configurations, use nixos-rebuild --flake .#mymachine switch, from the same repository where the flake.nix file is located.

To switch a remote configuration, use:

nixos-rebuild --flake .#mymachine \  --target-host mymachine-hostname --build-host localhost \  switch
NOTE: Remote building seems to be broken at the moment, which is why the build host is set to “localhost”.

Super fast nix-shell

One of the nix feature of the Flake edition is that Nix evaluations are cached.

Let’s say that your project has a shell.nix file that looks like this:

{ pkgs ? import <nixpkgs> { } }:
with pkgs;
mkShell {
  buildInputs = [

  shellHook = ''
    # ...

Running nix-shell can be a bit slow and take 1-3 seconds.

Now create a flake.nix file in the same repository:

  description = "my project description";

  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, flake-utils }:
        let pkgs = nixpkgs.legacyPackages.${system}; in
          devShell = import ./shell.nix { inherit pkgs; };

Run git add flake.nix so that Nix recognizes it.

And finally, run nix develop. This is what replaces the old nix-shell invocation.

Exit and run again, this command should now be super fast.

NOTE: TODO: there is an alternative version where the defaultPackage is a pkgs.buildEnv that contains all the dependencies. And then nix shell is used to open the environment.

Direnv integration

direnv 2.29.0 and later ship with the use flake builtin function. Just add that to your .envrc and you’re good to go!

The nice thing about this approach is that evaluation is cached.

Optimize the reloads

Nix Flakes has a Nix evaluation caching mechanism. Is it possible to expose that somehow to automatically trigger direnv reloads?

With the previous solution, direnv would only reload iff the flake.nix or flake.lock files have changed. This is not completely precise as the flake.nix file might import other files in the repository.

Using with GitHub Actions


Pushing Flake inputs to Cachix

Flake inputs can also be cached in the Nix binary cache!

nix flake archive --json \  | jq -r '.path,(.inputs|to_entries[].value.path)' \  | cachix push $cache_name

How to build specific attributes in a flake repository?

When in the repository top-level, run nix build .#<attr>. It will look in the legacyPackages and packages output attributes for the corresponding derivation.

Eg, in nixpkgs:

nix build .#hello

Building all the derivations of a flake

Traditionally we would run nix-build ci.nix or something equivalent but flakes only support pointing nix build to a single derivation.

I am not 100% confident on this answer: it looks like exposing the derivation in the checks output attribute, and then running nix flake check does the trick.

Some file is not found

Flakes only takes files into account if they are either in the git tree. You don’t necessarily have to commit the files, adding them in the git staging area with git add is enough.

Pure evaluation

Because the evaluation in Flakes is “pure”, a few things are disabled.

Pure evaluation can also be enabled by using --option pure-eval true on standard nix CLIs. Eg:

$ nix-instantiate --option pure-eval true --eval --expr '(builtins.currentTime)'
error: --- EvalError -------------------------------------------------------------- nix-instantiate
at: (1:2) from string

     1| (builtins.currentTime)
      |  ^

attribute 'currentTime' missing

To find these out I searched for evalSettings.pureEval in the “src/libexpr” folder of the Nix repo.

All these builtins are not defined in pure evaluation:

  • builtins.currentTime -> int
  • builtins.currentSystem -> str

Some more special behaviours:

  • builtins.getEnv str -> str returns empty strings.
  • builtins.storePath throws '__storePath' is not allowed in pure evaluation mode
  • builtins.filterSource and builtins.path has some condition in it, I don’t know exactly which.
  • builtins.fetchTree also has some conditions.
  • <foo> throws cannot look up '<foo>' in pure evaluation mode (use'--impure' to override)

Running unfree packages

Because Flakes are pure by default, something like nix run nixpkgs#steam will complain that it’s unfree, even if NIXPKGS_ALLOW_UNFREE=1 is set in the environment.

The workaround is to disable the pure evaluation with the --impure flag like so:

NIXPKGS_ALLOW_UNFREE=1 nix run --impure nixpkgs#steam