While in Barcelona I am staying in the Vila 2 residence of the Universitat Autonoma de Barcelona (UAB) campus. I have a very nice furnished room with a kitchen.
I have found four good places to shop for food nearby. The two nearest are the Supermercat de la Vila (left below) in the main student residence area. This has most of what you need and has good hours. There is a laundromat in this area as well. The other direction, along the back road through the underpass under the train tracks in Bellaterra village is the Condis supermarket (right). This has a larger selection and good fresh fruit, vegetables and meat.
If you reached Condis then you know where the Bellaterra train station is. The S2 train (and the S55 in rush hours) stop here. Both take you Placa Catalunya in the centre of tourist Barcelona (maybe it's considered the centre for locals as well, I don't know). If you go 3 stops towards Barcelona you get to Sant Cugat (this is a one zone trip from Bellaterra). There is a nice bar (Bar Catalunya) in the square right at the train station. From this square starts a pedestrian shopping area that is quite nice. In this area (you'll want to google a map to get specific directions) you'll find Mercadona (picture left below), a higher quality food place with some organic and specialty foods. When you take the train back to Bellaterra make sure to get on the S2 (Sabadell) or S55 (UAB) trains. The S1 and S5 also stop here but won't get you home.