How do RVM and rbenv actually work?

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

Answer 1

So to summarise the excellent answers above, the main practical difference between RVM and rbenv is when the version of Ruby is selected.

rbenv:

rbenv adds a shim to the start of your path, a command with the same name as Ruby. When you type ruby at a command line the shim is run instead (because it is also called "ruby" and comes first in the path). The shim looks for an environment variable or .rbenv_version file to tell it which version of Ruby to delegate to.

RVM:

RVM allows you to set a version of Ruby directly by calling rvm use. In addition, it also overrides the cd system command. When you cd into a folder that contains a .rvmrc file, the code inside the .rvmrc file is executed. This can be used to set a Ruby version, or anything else you fancy.

Other differences:

There are of course other differences. RVM has gemsets out of the box, while rbenv requires just a little more hacking (but not much). Both are functional solutions to the problem.