How to install Ruby 2.1.4 on Ubuntu 14.04

Best is to install it using rvm(ruby version manager).
Run following commands in a terminal:

sudo apt-get update
sudo apt-get install build-essential make curl
\curl -L https://get.rvm.io | bash -s stable
source ~/.bash_profile
rvm install ruby-2.1.4

Then check ruby versions installed and in use:

rvm list
rvm use --default ruby-2.1.4

Also you can directly add ruby bin path to PATH variable. Ruby is installed in

$HOME/.rvm/rubies export PATH=$PATH:$HOME/.rvm/rubies/ruby-2.1.4/bin

First of all, install the prerequisite libraries:

sudo apt-get update
sudo apt-get install git-core curl zlib1g-dev build-essential libssl-dev libreadline-dev libyaml-dev libsqlite3-dev sqlite3 libxml2-dev libxslt1-dev libcurl4-openssl-dev python-software-properties libffi-dev

Then install rbenv, which is used to install Ruby:

cd
git clone https://github.com/rbenv/rbenv.git ~/.rbenv
echo 'export PATH="$HOME/.rbenv/bin:$PATH"' >> ~/.bashrc
echo 'eval "$(rbenv init -)"' >> ~/.bashrc
exec $SHELL

git clone https://github.com/rbenv/ruby-build.git ~/.rbenv/plugins/ruby-build
echo 'export PATH="$HOME/.rbenv/plugins/ruby-build/bin:$PATH"' >> ~/.bashrc
exec $SHELL

rbenv install 2.3.1
rbenv global 2.3.1
ruby -v

Then (optional) tell Rubygems to not install local documentation:

echo "gem: --no-ri --no-rdoc" > ~/.gemrc

Credits: https://gorails.com/setup/ubuntu/14.10

Warning!!! There are issues with Gnome-Shell. See comment below.

There is a PPA with up-to-date versions of Ruby 2.x for Ubuntu 12.04+:

$ sudo apt-add-repository ppa:brightbox/ruby-ng
$ sudo apt-get update
$ sudo apt-get install ruby2.4

$ ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux-gnu]