So there is this idea with NixOS to install only the very base system in the global environment and augment these using Development Environments. And as I’m creating this blog using Github Pages aka Jekyll, writing in Markdown, and would like to be able to preview any changes locally, I of course need Jekyll running locally. Jekyll is even on nixpkgs, … but there are Jekyll plugins which aren’t bundled with this package and essential for correct rendering of e.g. @-mentions and source code blocks.

… so the obvious step was to create such a NixOS Development Environment, which has Ruby 2.2, Jekyll and all the required plugins installed. Turns out there even is a github-pages Gem, so we just need to “package” that. Packaging Ruby gems is pretty straight forward actually, …

so first let’s create a minimal Gemfile first:

source ''
gem 'github-pages'

The Github page has this , group: :jekyll_plugins thing in it, … I had to remove it, otherwise nix-shell complains that it cannot find the Jekyll gem file, once you try to run it (later).

Then we need to create Gemfile.lock by running bundler (from within a nix-shell that has bundler):

$ nix-shell -p bundler
$ bundler package --no-install --path vendor
$ rm -rf .bundler vendor
$ exit  # leave nix-shell

… and derive a Nix expression from Gemfile.lock like so (be sure to not accidentally run this command from within the other nix-shell, which would fail with strange SSL errors otherwise):

$ $(nix-build '<nixpkgs>' -A bundix)/bin/bundix
$ rm result   # nix-build created this (linking to bundix build)

… and last but not least we need a default.nix file which actually triggers the environment creation and also automatically starts jekyll serve after build:

with import <nixpkgs> { };

let jekyll_env = bundlerEnv rec {
    name = "jekyll_env";
    ruby = ruby_2_2;
    gemfile = ./Gemfile;
    lockfile = ./Gemfile.lock;
    gemset = ./gemset.nix;
  stdenv.mkDerivation rec {
    name = "jekyll_env";
    buildInputs = [ jekyll_env ];

    shellHook = ''
      exec ${jekyll_env}/bin/jekyll serve --watch

Take note of the exec in shellHook which actually replaces the shell which nix-shell is about to start by Jekyll itself, so once you stop it by pressing C-c the environment is immediately closed as well.

So we’re now ready to just start it all:

[stesie@faulobst:~/Projekte/]$ nix-shell 
Configuration file: /home/stesie/Projekte/
            Source: /home/stesie/Projekte/
       Destination: /home/stesie/Projekte/
 Incremental build: enabled
                    done in 0.147 seconds.
 Auto-regeneration: enabled for '/home/stesie/Projekte/'
Configuration file: /home/stesie/Projekte/
    Server address:
  Server running... press ctrl-c to stop.