Difference Between cron:run and setup:cron:run in Magento 2

The cron.sh is pointing to the cron.php file, so you should be pointing your cronjob task to the .sh file.

Basically inside the PHP file resides all the logic for the cron to retrieve jobs from Magento and the sh file invokes the PHP file.

The system doesn't know what language the file is, so it's having trouble executing it.

First you need to find where your PHP executable is. To do this use the which command:

# which php
/usr/bin/php

In my case the executable is located at /usr/bin/php so my crontab would look like

15 * * * * /usr/bin/php /var/www/public_html/cron.php

However, it's recommended that you use the cron.sh that's provided. It has some additional checks so that you don't have multiple process running. You should also have the cron run every minute or job might be missed. Because of Magento's unique cron system, this doesn't necessarily mean it's doing a lot of work, it's just checking the schedule to see if anything should be done at the moment. Try changing your crontab to

* * * * * /bin/bash /var/www/public_html/cron.sh