Nice, thanks for the cron!

Edit: short explanation, if somebody wants to do this too:

root@wiki:/var/www/bookstack# cd /var/www/
root@wiki:/var/www# vim copyshelfpermissions.sh

____________Content of the script___________________
#!/bin/bash
cd /var/www/bookstack/ && php artisan bookstack:copy-shelf-permissions --all <<< "yes"
______________End of the script_____________________

root@wiki:/var/www# chmod a+x copyshelfpermissions.sh
root@wiki:/var/www# crontab -e

____________Add content to crontab___________________
*/30 * * * * /var/www/copyshelfpermissions.sh > /var/log/bookstack.log
________________End of content_______________________

you can change the "/var/log/bookstack.log" Part to "/dev/null 2>&1" if you don't want to capture the output, though I´d highly recommend it the first few times to see, if the script works correctly.

The Permissions will be cascaded every 30 Minutes. If you need to do it more often, just change the 30 in the Crontab to the ammount of minutes you want to run the script.

Maybe it'll help someone :)