The Concrete5 documentation says:
Clearing the concrete5 cache is as easy as executing this command:
concrete/bin/concrete5 c5:clear-cache
But after running this command as root the website answers with an error 500. Why?
If you enable error display you will see:
Cache path is not writable
The c5:clear-cache command doesn't give write permissions on the cache path to the web server.
You can solve this by resetting the correct ownership with:
chown -R www-data:www-data /var/www/concrete5/application/files/
chown -R www-data:www-data /var/www/concrete5/application/config/
chown -R www-data:www-data /var/www/concrete5/packages/
chown -R www-data:www-data /var/www/concrete5/updates/