Drush is a command-line utility that allows for non-graphical access to the Drupal website. You can use it to automatically download and install themes and modules, clear the Drupal cache, upgrade the site and more. Tripal v3 supports Drush. For this tutorial we will use Drush and therefore we want the most recent version installed. Drush can be found on a GitHub page at https://github.com/drush-ops/drush.
An extensive list of platforms and their corresponding install instructions can be found here: https://www.drupal.org/node/1791676. Below are quick links for installation of Drush on Ubuntu and CentOS