These instructions are to share Drush amongst all sites in a shared server.
There are loads of resources on how to install Drush but it can all get a bit confusing, for example:
This documetation is for Drush 8.
# Browse to https://github.com/drush-ops/drush/releases and download the drush.phar attached to the latest 8.x release.
At this time of this writing it’s 8.2.1
# Test your install.
php drush.phar core-status
# Rename to `drush` instead of `php drush.phar`. Destination can be anywhere on $PATH.
chmod +x drush.phar
sudo mv drush.phar /usr/local/bin/drush
# Optional. Enrich the bash startup file with completion and aliases.