lars@lars-xenial2:~$ mysql -P "" -utestuser -ppass databasename
mysql: [Warning] mysql: Empty value for 'port' specified. Will throw an error in future versions
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1044 (42000): Access denied for user 'testuser'@'%' to database 'databasename'