После установки ruby из rvm система говорит, что его нет

Перезапустить терминал

оО спасибо, помогло...

На потом :

Поставь себе в .bashrc

source то-место_куда ставил_rvm/scripts/rvm

rvm

Брось каку! Пользуй rbenv.

А чем rbenv лучше rvm? Спасибо.

https://github.com/sstephenson/rbenv/wiki/Why-rbenv?

rbenv does…

Provide support for specifying application-specific Ruby versions.
Let you change the global Ruby version on a per-user basis.
Allow you to override the Ruby version with an environment variable.

In contrast with RVM, rbenv does not…

Need to be loaded into your shell. Instead, rbenv's shim approach works by adding a directory to your $PATH.
Override shell commands like cd or require prompt hacks. That's dangerous and error-prone.
Have a configuration file. There's nothing to configure except which version of Ruby you want to use.
Install Ruby. You can build and install Ruby yourself, or use ruby-build to automate the process.
Manage gemsets. Bundler is a better way to manage application dependencies. If you have projects that are not yet using Bundler you can install the rbenv-gemset plugin.
Require changes to Ruby libraries for compatibility. The simplicity of rbenv means as long as it's in your $PATH, nothing else needs to know about it.

Не говорите глупостей. У каждого свои цели.

Я и не говорил.

Брось каку! Пользуй rbenv.

Собирай Ruby сам, устанавливай зависимости сам, удивляйся, почему Ruby собрался без поддержки [library name] тоже сам...

Не пользуйся rbenv-build, нет, не надо.