Using Ruby

Several versions of Ruby interpreters are available on Nix, as well as over 250 gems and many applications written in Ruby. The attribute ruby refers to the default Ruby interpreter, which is currently MRI 2.6. It's also possible to refer to specific versions, e.g. ruby_2_y, jruby, or mruby.

In the Nixpkgs tree, Ruby packages can be found throughout, depending on what they do, and are called from the main package set. Ruby gems, however are separate sets, and there's one default set for each interpreter (currently MRI only).

There are two main approaches for using Ruby with gems. One is to use a specifically locked Gemfile for an application that has very strict dependencies. The other is to depend on the common gems, which we'll explain further down, and rely on them being updated regularly.

The interpreters have common attributes, namely gems, and withPackages. So you can refer to ruby.gems.nokogiri, or ruby_2_7.gems.nokogiri to get the Nokogiri gem already compiled and ready to use.

Since not all gems have executables like nokogiri, it's usually more convenient to use the withPackages function like this: ruby.withPackages (p: with p; [ nokogiri ]). This will also make sure that the Ruby in your environment will be able to find the gem and it can be used in your Ruby code (for example via ruby or irb executables) via require "nokogiri" as usual.

Temporary Ruby environment with nix-shell

Rather than having a single Ruby environment shared by all Ruby development projects on a system, Nix allows you to create separate environments per project. nix-shell gives you the possibility to temporarily load another environment akin to a combined chruby or rvm and bundle exec.

There are two methods for loading a shell with Ruby packages. The first and recommended method is to create an environment with ruby.withPackages and load that.

$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])"

The other method, which is not recommended, is to create an environment and list all the packages directly.

$ nix-shell -p ruby.gems.nokogiri ruby.gems.pry

Again, it's possible to launch the interpreter from the shell. The Ruby interpreter has the attribute gems which contains all Ruby gems for that specific interpreter.

Load Ruby environment from .nix expression

As explained in the Nix manual, nix-shell can also load an expression from a .nix file. Say we want to have Ruby 2.6, nokogori, and pry. Consider a shell.nix file with:

with import <nixpkgs> {};
ruby.withPackages (ps: with ps; [ nokogiri pry ])

What's happening here?

  1. We begin with importing the Nix Packages collections. import <nixpkgs> imports the <nixpkgs> function, {} calls it and the with statement brings all attributes of nixpkgs in the local scope. These attributes form the main package set.
  2. Then we create a Ruby environment with the withPackages function.
  3. The withPackages function expects us to provide a function as an argument that takes the set of all ruby gems and returns a list of packages to include in the environment. Here, we select the packages nokogiri and pry from the package set.

Execute command with --run

A convenient flag for nix-shell is --run. It executes a command in the nix-shell. We can e.g. directly open a pry REPL:

$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "pry"

Or immediately require nokogiri in pry:

$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "pry -rnokogiri"

Or run a script using this environment:

$ nix-shell -p "ruby.withPackages (ps: with ps; [ nokogiri pry ])" --run "ruby example.rb"

Using nix-shell as shebang

In fact, for the last case, there is a more convenient method. You can add a shebang to your script specifying which dependencies nix-shell needs. With the following shebang, you can just execute ./example.rb, and it will run with all dependencies.

#! /usr/bin/env nix-shell
#! nix-shell -i ruby -p "ruby.withPackages (ps: with ps; [ nokogiri rest-client ])"

require 'nokogiri'
require 'rest-client'

body = RestClient.get('').body
puts Nokogiri::HTML(body).at('h1').text

Developing with Ruby

Using an existing Gemfile

In most cases, you'll already have a Gemfile.lock listing all your dependencies. This can be used to generate a gemset.nix which is used to fetch the gems and combine them into a single environment. The reason why you need to have a separate file for this, is that Nix requires you to have a checksum for each input to your build. Since the Gemfile.lock that bundler generates doesn't provide us with checksums, we have to first download each gem, calculate its SHA256, and store it in this separate file.

So the steps from having just a Gemfile to a gemset.nix are:

$ bundle lock
$ bundix

If you already have a Gemfile.lock, you can simply run bundix and it will work the same.

To update the gems in your Gemfile.lock, you may use the bundix -l flag, which will create a new Gemfile.lock in case the Gemfile has a more recent time of modification.

Once the gemset.nix is generated, it can be used in a bundlerEnv derivation. Here is an example you could use for your shell.nix:

# ...
  gems = bundlerEnv {
    name = "gems-for-some-project";
    gemdir = ./.;
in mkShell { packages = [ gems gems.wrappedRuby ]; }

With this file in your directory, you can run nix-shell to build and use the gems. The important parts here are bundlerEnv and wrappedRuby.

The bundlerEnv is a wrapper over all the gems in your gemset. This means that all the /lib and /bin directories will be available, and the executables of all gems (even of indirect dependencies) will end up in your $PATH. The wrappedRuby provides you with all executables that come with Ruby itself, but wrapped so they can easily find the gems in your gemset.

One common issue that you might have is that you have Ruby 2.6, but also bundler in your gemset. That leads to a conflict for /bin/bundle and /bin/bundler. You can resolve this by wrapping either your Ruby or your gems in a lowPrio call. So in order to give the bundler from your gemset priority, it would be used like this:

# ...
mkShell { buildInputs = [ gems (lowPrio gems.wrappedRuby) ]; }

Gem-specific configurations and workarounds

In some cases, especially if the gem has native extensions, you might need to modify the way the gem is built.

This is done via a common configuration file that includes all of the workarounds for each gem.

This file lives at /pkgs/development/ruby-modules/gem-config/default.nix, since it already contains a lot of entries, it should be pretty easy to add the modifications you need for your needs.

In the meanwhile, or if the modification is for a private gem, you can also add the configuration to only your own environment.

Two places that allow this modification are the ruby derivation, or bundlerEnv.

Here's the ruby one:

{ pg_version ? "10", pkgs ? import <nixpkgs> { } }:
  myRuby = pkgs.ruby.override {
    defaultGemConfig = pkgs.defaultGemConfig // {
      pg = attrs: {
        buildFlags =
        [ "--with-pg-config=${pkgs."postgresql_${pg_version}"}/bin/pg_config" ];
in myRuby.withPackages (ps: with ps; [ pg ])

And an example with bundlerEnv:

{ pg_version ? "10", pkgs ? import <nixpkgs> { } }:
  gems = pkgs.bundlerEnv {
    name = "gems-for-some-project";
    gemdir = ./.;
    gemConfig = pkgs.defaultGemConfig // {
      pg = attrs: {
        buildFlags =
        [ "--with-pg-config=${pkgs."postgresql_${pg_version}"}/bin/pg_config" ];
in mkShell { buildInputs = [ gems gems.wrappedRuby ]; }

And finally via overlays:

{ pg_version ? "10" }:
  pkgs = import <nixpkgs> {
    overlays = [
      (self: super: {
        defaultGemConfig = super.defaultGemConfig // {
          pg = attrs: {
            buildFlags = [
in pkgs.ruby.withPackages (ps: with ps; [ pg ])

Then we can get whichever postgresql version we desire and the pg gem will always reference it correctly:

$ nix-shell --argstr pg_version 9_4 --run 'ruby -rpg -e "puts PG.library_version"'

$ nix-shell --run 'ruby -rpg -e "puts PG.library_version"'

Of course for this use-case one could also use overlays since the configuration for pg depends on the postgresql alias, but for demonstration purposes this has to suffice.

Platform-specific gems

Right now, bundix has some issues with pre-built, platform-specific gems: bundix PR #68. Until this is solved, you can tell bundler to not use platform-specific gems and instead build them from source each time:

  • globally (will be set in ~/.config/.bundle/config):
$ bundle config set force_ruby_platform true
  • locally (will be set in <project-root>/.bundle/config):
$ bundle config set --local force_ruby_platform true

Adding a gem to the default gemset

Now that you know how to get a working Ruby environment with Nix, it's time to go forward and start actually developing with Ruby. We will first have a look at how Ruby gems are packaged on Nix. Then, we will look at how you can use development mode with your code.

All gems in the standard set are automatically generated from a single Gemfile. The dependency resolution is done with bundler and makes it more likely that all gems are compatible to each other.

In order to add a new gem to nixpkgs, you can put it into the /pkgs/development/ruby-modules/with-packages/Gemfile and run ./maintainers/scripts/update-ruby-packages.

To test that it works, you can then try using the gem with:

NIX_PATH=nixpkgs=$PWD nix-shell -p "ruby.withPackages (ps: with ps; [ name-of-your-gem ])"

Packaging applications

A common task is to add a ruby executable to nixpkgs, popular examples would be chef, jekyll, or sass. A good way to do that is to use the bundlerApp function, that allows you to make a package that only exposes the listed executables, otherwise the package may cause conflicts through common paths like bin/rake or bin/bundler that aren't meant to be used.

The absolute easiest way to do that is to write a Gemfile along these lines:

source '' do
  gem 'mdl'

If you want to package a specific version, you can use the standard Gemfile syntax for that, e.g. gem 'mdl', '0.5.0', but if you want the latest stable version anyway, it's easier to update by simply running the bundle lock and bundix steps again.

Now you can also make a default.nix that looks like this:

{ bundlerApp }:

bundlerApp {
  pname = "mdl";
  gemdir = ./.;
  exes = [ "mdl" ];

All that's left to do is to generate the corresponding Gemfile.lock and gemset.nix as described above in the Using an existing Gemfile section.

Packaging executables that require wrapping

Sometimes your app will depend on other executables at runtime, and tries to find it through the PATH environment variable.

In this case, you can provide a postBuild hook to bundlerApp that wraps the gem in another script that prefixes the PATH.

Of course you could also make a custom gemConfig if you know exactly how to patch it, but it's usually much easier to maintain with a simple wrapper so the patch doesn't have to be adjusted for each version.

Here's another example:

{ lib, bundlerApp, makeWrapper, git, gnutar, gzip }:

bundlerApp {
  pname = "r10k";
  gemdir = ./.;
  exes = [ "r10k" ];

  nativeBuildInputs = [ makeWrapper ];

  postBuild = ''
    wrapProgram $out/bin/r10k --prefix PATH : ${lib.makeBinPath [ git gnutar gzip ]}