Install Drush 8 using Composer

Installing Drush for developing Drupal-site is a must-have for each developer.

Drush is a command line scripting interface for Drupal. By using a lot of useful commands, you can interact with code like modules, themes, profiles.
You can run update.php, execute sql-queries, clear the cache and a lot more!

Since we can get started now with developing Drupal 8 sites it's necessary to have Drush 8 installed. Drush 8 is compatible with Drupal 6, 7 and 8.
Drush 8 needs PHP 5.4.5 or higher to be installed.

Before we can install Drush 8, we have to install Composer globally. Composer is a tool for dependency management in PHP. It allows you to declare the libraries your project depends on and it will manage (install/update) them for you.

Run this commands in your Terminal window:

curl -sS https://getcomposer.org/installer | php
mv composer.phar /usr/local/bin/composer

Now we need to add Composer's directory path to our bash_profile:

$ export PATH="$HOME/.composer/vendor/bin:$PATH"

Reload your terminal window or re-source the configuration you just added with this command:

$ source ~/.bash_profile

Installing Composer was that easy! Installing Drush will be even more easy!
We need to install Drush froml the latest HEAD with only one command:

$ composer global require drush/drush:dev-master

Now Drush is installed, we can check if everything is installed correctly by using this command:

$ drush status

This will output some configuration info and the Drush version.

Good luck!