In this guide we are installing Drush, the command line interface of Drupal. According the the developers:

Drush is a command line shell and scripting interface for Drupal, a veritable Swiss Army knife designed to make life easier for those of us who spend some of our working hours hacking away at the command prompt.

Prerequisites

• A Drupal setup

Install dependencies

Run the commands as the admin user:

$ sudo apt -y install curl unzip
$ curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer

Install Drush

Switch to the webadmin user:

$ sudo su - webadmin

Install Drush:

$ cd cms/
$ composer require drush/drush

Them we need to add drush in the $PATH:

$ mkdir ~/bin
$ cd ~/bin
$ ln -s ../cms/vendor/bin/drush

You can run drush to check the status of your Drupal install, with:

$ drush status

NOTE: This command must be run under the cms directory. References

• http://docs.drush.org/en/master/
• http://www.bravo-kernel.com/2014/08/how-to-install-composer-on-debian/