Install/Upgrade a global Drush

# Browse to https://github.com/drush-ops/drush/releases and download the drush.phar attached to the latest 8.x release.

# 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.
drush init

• MAMP users, and anyone wishing to launch a non-default PHP, needs to edit ~/.bashrc so that the right PHP is in your $PATH.
• We have documented alternative ways to install, including Windows.
• If you need to pass custom php.ini values, run php -d foo=bar drush.phar --php-options=foo=bar
• Your shell now has useful bash aliases and tab completion for command names, site aliases, options, and arguments.
• A drushrc.php has been copied to ~/.drush above. Customize it to save typing and standardize options for commands.
• Upgrade using this same procedure.

Install a site-local Drush

In addition to the global Drush, it is recommended that Drupal 8 sites be built using Composer, with Drush listed as a dependency.

1. When you run drush, the global Drush is called first and then hands execution to the site-local Drush. This gives you the convenience of running drush without specifying the full path to the executable, without sacrificing the safety provided by a site-local Drush.
2. Optional: Copy the examples/drush.wrapper file to your project root and modify to taste. This is a handy launcher script; add --local here to turn off all global configuration locations, and maintain consistency over configuration/aliases/commandfiles for your team.
3. Note that if you have multiple Drupal sites on your system, it is possible to use a different version of Drush with each one.

Drupal Compatibility

Please see: https://www.drush.org/latest/install/#drupal-compatibility

EOL date for Drush 8 tbd, but estimated to be in concert with Drupal 7 EOL.