Install Drush is easy. You simply download the latest binary file(*.phar) from https://github.com/drush-ops/drush/releases. Set it executable and then run it.
# Download the latest version. wget https://github.com/drush-ops/drush/releases/download/8.4.1/drush.phar # Make it executable. chmod +x drush.phar # Move it to path where you can run it anywhere. mv drush.phar /usr/local/bin/drush # Test it out. It should show the version number. drush --version