Drush installation problem
Did you get drush installed on your system? Check it with the following command.

drush --version


If it is a lower version please update drush using composer, if composer already installed skip the step to install composer.

cd ~
curl -sS https://getcomposer.org/installer | php
export PATH="$HOME/.composer/vendor/bin:$PATH"
source ~/.bashrc
composer global require drush/drush:8.*
drush --version