What is Nix with an example

I think the best way to describe nix is be drawing parallels to files like package.json or requirements.txt. These files are used to declare a “environment” where our code will run. This means packages, versions, dev and prod dependencies, etc.

For example take a look at the flake.nix for this project:

# flake.nix
{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };
  outputs = # ...
        devShells.default = pkgs.mkShell {
          packages = [
            nodejs
            pkgs.git
          ];
        };

        packages.default = pkgs.buildNpmPackage {
          src = ./.;
          nodejs = nodejs;
          npmDepsHash = "sha256-OjRs5EVIORxkKg077FTwqV2M6sdvrSnth7dclBc5xik=";
          npmBuildScript = "build";
          installPhase = ''
            runHook preInstall
            mkdir -p $out
            cp -r dist/* $out/
            runHook postInstall
          '';
        };
  # ...
}

This seems like a lot to take in but you should think of it as a function. There are inputs and there are outputs. In the inputs we define the dependencies of our project, which in our case are nixpkgs and flake-utils.

For the outputs we expose two main things: devShells and packages.default. First devShells is a development environment where I develop the site. It has nodejs and git installed. This way when I type nix develop in the terminal, I will have a shell with nodejs and git available to me.

As for the packages.default, this is the actual build path for this site. We use a provided function called buildNpmPackage which will build our site, and then we install it in the installPhase, which copies the built files to the output path.

How to leverage Nix

Now with this defined, I have the guarantee from Nix that no matter what machine I run this on, I will have 1) the same environment to develop (no more it works on my machine) and 2) the same build output. And I leverage this with github workflows:

# .github/workflows/pages.yml
# ...
      - name: Install Nix
        uses: DeterminateSystems/nix-installer-action@v17

      - name: Set up Nix cache
        uses: DeterminateSystems/magic-nix-cache-action@v13

      - name: Build site
        run: nix build

This workflow is interesting because it does not have any language specifics for building the site, thus it could be the same workflow even if I was using Python, Rust, etc. Not only do build artefacts will be cached but if it builds today it will build in the future. So I can be sure of maximum stability and if I am thinking of hosting this site for a long time (which I am) this initial effort will pay off in the long run.