When you tell rbenv to install a Ruby it does not know about, you will get an error message.

$ rbenv install 2.1.2
ruby-build: definition not found: 2.1.2

You can list all available versions with `rbenv install --list'.

If the version you're looking for is not present, first try upgrading
ruby-build. If it's still missing, open a request on the ruby-build
issue tracker: https://github.com/sstephenson/ruby-build/issues

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(Fun fact: Recent versions of ruby-build will give you a more helpful error message which just tells you the following.)

On Linux, you probably installed ruby-build as an rbenv plugin. Updating is easy:

git -C ~/.rbenv/plugins/ruby-build pull

Done. Rbenv will know about the new versions immediately.