If you’re not already using RVM, I’d highly recommend it as it makes it easier to manage different versions of ruby. You can have different projects running on different versions of ruby and switch rubies with ease. One way to make this easier is to make use of the .rvmrc file. It will automatically switch to the ruby version that your project is running on when you cd into it.

Suppose we have a rails project called donut running on ruby 1.9.2.

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cd donut

Once in the rails root of the project, create a file called .rvmrc and add this:

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rvm use ruby-1.9.2-p0@donut --create

To list known rubies you can use this:

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rvm list known

Now when you cd into the directory, it will automatically use ruby 1.9.2.

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cd ..
cd donut

info: Using ruby 1.9.2 p0 with gemset donut

Notice that it is using the gemset donut. Gemsets are basically compartmentalized ruby setups. Gems installed in this gemset will be separated from other projects (assuming they use different gemsets). You can read more about it here