Drush command to check if Drupal is installed? Is there any way to use Drush to determine if Drupal is installed, and have it return true or false? The boolean output would be used elsewhere in a bash script's logic.

Bonus: it needs to work for both Drupal 7 and Drupal 8.
$ cd /path/to/drupal6or7or8
$ drush status bootstrap | grep -q Successful


Then, $? will be 0 if Drupal has been installed, or 1 if there is no site, or it does not have a database, or the site has not been installed.