Question

I am interested in how RVM and rbenv actually work.

Obviously they swap between different versions of Ruby and gemsets, but how is this achieved? I had assumed they were simply updating symlinks, but having delved into the code (and I must admit my knowledge of Bash is superficial) they appear to be doing more than this.

Answer 1

Short explanation: rbenv works by hooking into your environment’s PATH. The concept is simple, but the devil is in the details; full scoop below.

First, rbenv creates shims for all the commands (ruby, irb, rake, gem and so on) across all your installed versions of Ruby, that is for all files in ~/.rbenv/versions/*/bin. This process is called rehashing. Every time you install a new version of Ruby or install a gem that provides a command, run rbenv rehash to make sure any new commands are shimmed.

These shims live in a single directory (~/.rbenv/shims by default). To use rbenv, you need only add the shims directory to the front of your PATH:

export PATH="$HOME/.rbenv/shims:$PATH"

Then any time you run ruby from the command line, or run a script whose shebang reads #!/usr/bin/env ruby, your operating system will find ~/.rbenv/shims/ruby first and run it instead of any other ruby executable you may have installed.

Each shim is a tiny Bash script that in turn runs rbenv exec. So with rbenv in your path, irb is equivalent to rbenv exec irb, and ruby -e "puts 42" is equivalent to rbenv exec ruby -e "puts 42".

The rbenv exec command figures out what version of Ruby you want to use, then runs the corresponding command for that version. Here’s how:

1. If the RBENV_VERSION environment variable is set, its value determines the version of Ruby to use.
2. If the current working directory has an .rbenv-version file, its contents are used to set the RBENV_VERSION environment variable.
3. If there is no .rbenv-version file in the current directory, rbenv searches each parent directory for an .rbenv-version file until it hits the root of your filesystem. If one is found, its contents are used to set the RBENV_VERSION environment variable.
4. If RBENV_VERSION is still not set, rbenv tries to set it using the contents of the ~/.rbenv/version file.
5. If no version is specified anywhere, rbenv assumes you want to use the "system" Ruby—i.e. whatever version would be run if rbenv weren’t in your path.

(You can set a project-specific Ruby version with the rbenv local command, which creates a .rbenv-version file in the current directory. Similarly, the rbenv global command modifies the ~/.rbenv/version file.)

Armed with an RBENV_VERSION environment variable, rbenv adds ~/.rbenv/versions/$RBENV_VERSION/bin to the front of your PATH, then execs the command and arguments passed to rbenv exec. Voila!

For a thorough look at exactly what happens under the hood, try setting RBENV_DEBUG=1 and running a Ruby command. Every Bash command that rbenv runs will be written to your terminal.

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Now, rbenv is just concerned with switching versions, but a thriving ecosystem of plugins will help you do everything from installing Ruby to setting up your environment, managing "gemsets" and even automating bundle exec.

I am not quite sure what IRC support has to do with switching Ruby versions, and rbenv is designed to be simple and understandable enough not to require support. But should you ever need help, the issue tracker and Twitter are just a couple of clicks away.

Disclosure: I am the author of rbenv, ruby-build, and rbenv-vars.