use rbenv to manage your Ruby versions within Emacs

(add-to-list 'load-path (expand-file-name "/path/to/rbenv.el/"))
(require 'rbenv)
(global-rbenv-mode)

• global-rbenv-mode activate / deactivate rbenv.el (The current Ruby version is shown in the modeline)
• rbenv-use-global will activate your global ruby
• rbenv-use allows you to choose what ruby version you want to use
• rbenv-use-corresponding searches for .ruby-version and activates the corresponding ruby

rbenv installation directory By default rbenv.el assumes that you installed rbenv into ~/.rbenv. If you use a different installation location you can customize rbenv.el to search in the right place:

(setq rbenv-installation-dir "/usr/local/rbenv")

IMPORTANT:: Currently you need to set this variable before you load rbenv.el

the modeline rbenv.el will show you the active ruby in the modeline. If you don't like this feature you can disable it:

(setq rbenv-show-active-ruby-in-modeline nil)

The default modeline representation is the ruby version (colored red) in square brackets. You can change the format by customizing the variable:

;; this will remove the colors
(setq rbenv-modeline-function 'rbenv--modeline-plain)

You can also define your own function to format the ruby version as you like.

If you want to read more about rbenv.el check out the following links:

• Use the right Ruby with emacs and rbenv by Yves Senn