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Manage your rubies with direnv and ruby-install

direnv is just a shell extension that manages your environment variables depending on the folder you live in. In this article we will explore how it can be used in combination with ruby-install to manage and select the version of ruby that you want to use in a project.

The setup

First install direnv. This is the quick version on OSX + Bash:

brew install direnv
echo 'eval $(direnv hook bash)' >> .bashrc
exec $0

Then use ruby-install to install a couple of ruby versions. We're also creating a couple of aliases for convenience.

brew install ruby-install
ruby-install ruby 1.9
ruby-install ruby 2.0
cd ~/.rubies
ln -s 1.9.3-p448 1.9.3
ln -s 1.9.3-p448 1.9
ln -s 2.0.0-p247 2.0.0
ln -s 2.0.0-p247 2.0

The end goal is that each project will have an ".envrc" file that contains a descriptive syntax like use ruby 1.9.3 to selects the right ruby version for the project.

For that regard we are going to use a couple of commands available in the direnv stdlib and expand it a bit in the ~/.direnvrc file.

Add this to the ~/.direnvrc file (you have to create it if it doesn't exist):

# Usage: use ruby <version>
#
# Loads the specified ruby version into the environent
#
use_ruby() {
  local ruby_dir=$HOME/.rubies/$1
  load_prefix $ruby_dir
  layout ruby
}

That's it. Now in any project you can run direnv edit . and add use ruby 1.9.3 or use ruby 2.0 in the file like you want and direnv will select the right ruby version when you enter the project's folder.

A bit of explanation

The last part probably needs a bit more explanation. We make use of a couple of commands that are part of the stdlib which is availabe in the execution context of an envrc.

use is a command dispatch that's just there to build the use something something dsl so that use ruby <version> will translate into use_ruby <version>.

load_prefix will add a couple of things into the environment, notably add <prefix>/bin into the PATH. This is what makes the specified ruby available.

And finally layout ruby who like use translates into the layout_ruby function call. It's used to decribe common project layouts. In the stdlib, the ruby layout will configure rubygems (with the GEM_HOME environment variable) to install all the gems into the .direnv/ruby/RUBY_VERSION folder under the project root. This is a bit similar to rvm's gemsets except that they live inside your project's folder. It also configures bundler to install wrapper shims into the .direnv/bin folder which allows you to invoke the commands directly instead of prefixing your ruby programs with bundle exec all the time.

Conclusion

As you see this approach is not restricted to ruby. You could have various versions of python installed under ~/.pythons and a use_python defined in your ~/.direnvrc. Or perl, php, ... This is the good thing about direnv, it's not restricted to a single language.

Actually, wouldn't it be great to have all your project's dependencies available when you enter the project folder ? Not only your ruby version but also the exact redis or mysql or ... version that you want to use, without having to start a VM. I think that's definitely possible using something like the Nix package manager, something that still needs to be explored in a future post.