Base flake structure

The base structure of the flake is fairly simple:

{
  description = "Build the blog";
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/22.11";
  };
  outputs = inputs: with inputs; let
    system = "x86_64-linux";
    pkgs = import nixpkgs { inherit system; };
    # Utility to run a script easily in the flakes app
    simple_script = name: add_deps: text: let
      exec = pkgs.writeShellApplication {
        inherit name text;
        runtimeInputs = with pkgs; [
            gnumake     # Required by some dependencies in order to build
            bundler
        ] ++ add_deps;
      };
    in {
      type = "app";
      program = "${exec}/bin/${name}";
    };
  in {
    apps.${system} = {
      default = simple_script "serve_blog" [] ''
        # nix run -> Serve the website locally
      '';
      generate = simple_script "generate_blog_env" [] ''
          # nix run .#generate
          #   Will be used to re-generate the Ruby environment after modifying
          #     a dependency, the version number, etc ...
      '';
    };
  };
}

The workflow

At initialisation, and after that every time you modify a dependency, bump a version, or modify something in the Ruby environment, call nix run .#generate

It will use bundler and bundix to generate the Gemfile, Gemfile.lock,gemset.nix and even .bundle/config.
This way, we only need to configure variables inside the flake, the rest will magically work.

Once this is done, you can serve the website locally by calling nix run

It will use the Ruby environment we created using the generate script, and calljekyll serve --trace

Generating the Ruby environment

First for the environment generation, we will generate the Gemfile file using good ol' nix variables:

github-pages-version = 227;
inc_gems = {
  minima = "2.5";
  webrick = "1.7";          # Required for the jekyll serve to work
};
inc_plugins = {
  jekyll-feed = "0.12";
};
# Generation of the Gemfile
generate_gemfile = let
  gems = builtins.concatStringsSep "\n" (pkgs.lib.attrsets.mapAttrsToList (name: version:
    "gem \"${name}\", \"~> ${version}\""
  ) inc_gems);
  plugins = builtins.concatStringsSep "\n" (pkgs.lib.attrsets.mapAttrsToList (name: version:
    "  gem \"${name}\", \"~> ${version}\""
  ) inc_plugins);
in
  ''
    # This file is autogenerated by the flake.nix file, do not edit
    source "https://rubygems.org"
    # Gems dependencies to be installed
    ${gems}
    gem "github-pages", "~> ${builtins.toString github-pages-version}", group: :jekyll_plugins
    # Github Pages plugins
    group :jekyll_plugins do
    ${plugins}
    end
  '';

This Gemfile will be used by bundler to create a Gemfile.lock to pin the dependencies, which in turn will be used by bundix to generate a gemset.nix, the nixified ruby gems.

All these files are used by the nix bundlerEnv function like this:

# The build environment
env = pkgs.bundlerEnv {
  name = "blog";
  ruby = pkgs.ruby_3_1;
  gemdir = ./.;
};

Now, let's tie the two pieces together in the generate_blog_env script:

generate = simple_script "generate_blog_env" [ pkgs.bundix ] ''
    set -e
    rm -f gemset.nix Gemfile Gemfile.lock .bundle/config
    cat << EOF > Gemfile
    ${generate_gemfile}
    EOF
    bundler update
    bundler lock
    bundler package
    bundix --magic -d -l
    rm -rf vendor
'';

So far so good.

Bundle configuration and platform issues

Nowadays, bundler doesn't like command line flags to be used, and it prefers to read from either the .bundle/config file, or directly from environment variables.

In the generate_blog_env script, I adapted the flags passed to generate the configuration before generating the gemset.nix file like so:

export BUNDLE_PATH=vendor
export BUNDLE_CACHE_ALL=true
export BUNDLE_NO_INSTALL=true
export BUNDLE_FORCE_RUBY_PLATFORM=true

Notice the BUNDLE_FORCE_RUBY_PLATFORM, it is needed because some dependencies (at least nokogiri) are platform-specific and will raise an issue if it detects the machine platform. This configuration will ensure that it won't raise any compatibility issue.

Using the environment

Yay ! The environment builds without any trouble !

Now let's serve the website locally using jekyll. In the serve_blog script, we simply have to write:

''
    echo "Bundler env: ${env}"      # For debugging / exploration of the files
    ${env}/bin/bundler exec ${env}/bin/jekyll serve --trace
''

And now we can serve the whole thing locally without any troubles !

It finally builds

The full flake source can be seen here

All this setup is the result of hours or research and debugging, and I unfortunately wasn't able to find all the links and pages that I used, so I cannot credit them the way I'd like to. However a big Thank you is in order for all these people

This setup builds perfectly for my (weird) setup, but it's far from clean, optimized, and could maybe not work for you.
However feel free to reach me if you have some trouble, or if you wish to make this setup better, for now it only lies in my blog repo, but why not having a repository specially made for building a Github pages + jerkyll setup using Nix.