Ruby comes by default with MacOS and is easy to install with apt-get but it is recomended using a version manager to manage your ruby installs.

There are two popular ones rvm and rbenv, it seems most people have moved to rbenv and its CLI is a bit easier so that is the one we are going to address.

First start by installing rbenv

#macOS
brew install rbenv
#Ubuntu 14.04+
sudo apt-get install autoconf bison build-essential libssl-dev libyaml-dev\
libreadline6-dev zlib1g-dev libncurses5-dev libffi-dev libgdbm3 libgdbm-dev
git clone https://github.com/rbenv/rbenv.git ~/.rbenv

Then load the necessary modifications to your PATH into your terminal environment file

echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc (~/.profile / ~/.zshrc)
echo 'eval "$(rbenv init -)"' >> ~/.bashrc (~/.profile / ~/.zshrc)
source ~/.bashrc (~/.profile / ~/.zshrc)

Then clone the ruby build plugin into your rbenv install

git clone https://github.com/rbenv/ruby-build.git ~/.rbenv/plugins/ruby-build

Now you can use rbenv to install your desired version on ruby

rbenv install 2.3.1

To set a version as the default version for your entire computer run

rbenv global 2.3.1

Once you have ruby installed it is recommended you install bundler which is the package manager for ruby

gem install bundler

You should now be good to go to start working on your ruby projects

To learn more see: https://github.com/rbenv/rbenv