Federico Cargnelutti

Ruby Version Manager (RVM) is a tool that allows you to install multiple versions of Ruby and have multiple versions of the same interpreter. Very handy for those who have to maintain different applications using different versions of Ruby.

To start, download RVM and install the latest stable version of Ruby:

$ echo insecure >> ~/.curlrc
$ curl -L https://get.rvm.io | bash -s stable --ruby
$ source ~/.bash_profile

Install an old version of Ruby:

$ rvm install 1.8.6
$ rvm use 1.8.6 --default
$ ruby -v
ruby 1.8.6

Create a Gem set and install an old version of Rails:

$ rvm gemset create rails123
$ gem install rails -v 1.2.3
$ rails -v
Rails 1.2.3

Switch back to your system:

$ rvm system
$ rails -v
Rails 2.3.5

Switch back to your RVM environment:

$ rvm 1.8.6@rails123

And, if you want to remove Rails 1.2.3, just delete the Gem set:

$ rvm gemset delete rails123

Alternatively to RVM, you also might look into rbenv.