The radical changes that Drupal 8 brought forward have been reflected in Drush 7 as well. Drush 7 now uses composer to download its dependencies. Before installing Drupal 8 you must make sure that Drush 7 has been installed. This is because Drupal 8 can be downloaded and installed using Drush like in the previous versions of Drush and also, Drupal 8 only supports Drush 7. Unlike earlier versions, Drush 7 cannot be installed using "sudo apt-get install drush" nor can it be updated by running "drush self-update" because that will result in updating to Drush 6 and no more. Hence the purpose of this article.
Drush is a powerful tool to manage routine tasks in a Drupal site. Downloading, installing or enabling modules, running cron and much more can be accomplished using Drush. This is why its important to have Drush 7 before a Drupal 8 installation. Drupal 8 requires minimum version of PHP 5.4. Apache requirements haven't been mentioned, but I recommend apache2.4. After this we can begin installing Drush 7.
Installing Composer