rbenv README § Installation
https://github.com/rbenv/rbenv/blob/master/README.md#installation
> **Compatibility note**: rbenv is _incompatible_ with RVM. Please make
>   sure to fully uninstall RVM and remove any references to it from
>   your shell initialization files before installing rbenv.
> .
> For a more automated install, you can use
> [rbenv-installer](https://github.com/rbenv/rbenv-installer#rbenv-installer).
> If you prefer a manual approach, follow the steps below.
>
> This will get you going with the latest version of rbenv without needing
> a systemwide install.
>
> 1. Clone rbenv into `~/.rbenv`.
>
>     ~~~ sh
>     $ git clone https://github.com/rbenv/rbenv.git ~/.rbenv
>     ~~~
>
>     Optionally, try to compile dynamic bash extension to speed up rbenv. Don't
>     worry if it fails; rbenv will still work normally:
>
>     ~~~
>     $ cd ~/.rbenv && src/configure && make -C src
>     ~~~
>
> 2. Add `~/.rbenv/bin` to your `$PATH` for access to the `rbenv`
>    command-line utility.
>
>    * For **bash**:
>      ~~~ bash
>      $ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bash_profile
>      ~~~
>
>    * For **Ubuntu Desktop**:
>      ~~~ bash
>      $ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc
>      ~~~
>
>    * For **Zsh**:
>      ~~~ zsh
>      $ echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.zshrc
>      ~~~
>
>    * For **Fish shell**:
>      ~~~ fish
>      $ set -Ux fish_user_paths $HOME/.rbenv/bin $fish_user_paths
>      ~~~
>
> 3. Set up rbenv in your shell.
>
>    ~~~ sh
>    $ ~/.rbenv/bin/rbenv init
>    ~~~
>
>    Follow the printed instructions to [set up rbenv shell integration](#how-rbenv-hooks-into-your-shell).
>
> 4. Restart your shell so that PATH changes take effect. (Opening a new
>    terminal tab will usually do it.)
>
> 5. Verify that rbenv is properly set up using this
>    [rbenv-doctor](https://github.com/rbenv/rbenv-installer/blob/master/bin/rbenv-doctor) script:
>
>     ~~~ sh
>     $ curl -fsSL https://github.com/rbenv/rbenv-installer/raw/master/bin/rbenv-doctor | bash
>     Checking for `rbenv' in PATH: /usr/local/bin/rbenv
>     Checking for rbenv shims in PATH: OK
>     Checking `rbenv install' support: /usr/local/bin/rbenv-install (ruby-build 20170523)
>     Counting installed Ruby versions: none
>       There aren't any Ruby versions installed under `~/.rbenv/versions'.
>       You can install Ruby versions like so: rbenv install 2.2.4
>     Checking RubyGems settings: OK
>     Auditing installed plugins: OK
>     ~~~
>
> 6. _(Optional)_ Install [ruby-build][], which provides the
>    `rbenv install` command that simplifies the process of
>    [installing new Ruby versions](#installing-ruby-versions).

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