github上的rbenv文档--

Basic GitHub Checkout

This will get you going with the latest version of rbenv and make it easy to fork and contribute any changes back upstream.

Check out rbenv into ~/.rbenv.

$ git clone https://github.com/sstephenson/rbenv.git ~/.rbenv
Add ~/.rbenv/bin to your $PATH for access to the rbenv command-line utility.

$ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
Ubuntu Desktop note: Modify your ~/.bashrc instead of ~/.bash_profile.

Zsh note: Modify your ~/.zshrc file instead of ~/.bash_profile.

Add rbenv init to your shell to enable shims and autocompletion.

$ echo 'eval "$(rbenv init -)"' >> ~/.bash_profile
Same as in previous step, use ~/.bashrc on Ubuntu, or ~/.zshrc for Zsh.

Restart your shell so that PATH changes take effect. (Opening a new terminal tab will usually do it.) Now check if rbenv was set up:

$ type rbenv
#=> "rbenv is a function"
(Optional) Install ruby-build, which provides the rbenv install command that simplifies the process of installing new Ruby versions.

Upgrading

If you've installed rbenv manually using git, you can upgrade your installation to the cutting-edge version at any time.

$ cd ~/.rbenv
$ git pull
To use a specific release of rbenv, check out the corresponding tag:

$ cd ~/.rbenv
$ git fetch
$ git checkout v0.3.0

$rbenv install --list     如果发现没有自己想要装的版本
$cd ~/.rbenv
$git pull
$cd plugins
$cd ruby-build
$git pull
$rbenv install --list
$rbenv install 2.1.2
$ruby -v
$rbenv global 2.1.2
$rbenv versions
还有一种方式是
$export RBENV_VERSION=2.1.2
但是这种方式只在当前终端窗口有效
$gem install rails
这时候会发现速度超级慢,
$gem list
$gem source -l
来到网页看一下吧 ruby.taobao.org,接着按照网页提示操作
$gem sources --remove http://rubygems.org/
....
$gem install rails