During summer season there is a weekly direct bus from Basel to Pula, the bus travels on Fridays from Basel, you can see details in the table below. At other periods of the year it is possible to from Basel to Pula via Rijeka, you can see schedule for those bus here. In either case the bus travel time is about 17-18 hours. If you plan also to travel back to Basel, you might as well buy a discounted return ticket