LBB controller described runway incursion event when traffic crossed a runway without clearance; the reporter indicating staffing shortages and combined position operations limit controller's awareness capabilities.

Narrative

I instructed Air Carrier X to taxi from the terminal ramp to Runway 08 via Taxiway F; M; and to hold short of Taxiway J (for an aircraft that I knew would be exiting the runway). The pilot of Air Carrier X read back the instructions at the same instance that Air Carrier Y was on final reporting birds. While I was typing in the Flight Data Input/Output (FDIO) system; attempting to amend a flight plan; I looked up and observed Aircraft X on Taxiway F; on the West side of Runway 17R; facing West. The aircraft had obviously just crossed Runway 17R @ Taxiway F. I advised the aircraft that he had gone the wrong way; instructed the aircraft turn around (holding short of the runway); then proceeded with traffic as normal. The pilot made no indication that he knew he had even made a mistake. There was another landing aircraft on about 6-7 mile final. Recommendation; more awareness and less complacency on the part of pilots. Just because it's a low activity time doesn't mean that the same hazards of collision do not exist. The pilots were obviously either unconcerned; or didn't care enough about their location/directions to even ask. The Air Carrier Y pilots are always in a hurry to get the next phase of their flight started; even though they have not finished the current leg; and still have flocks of geese on final to contend with etc. Amending just 1 flight plan requires heads-down time; and amending routes. This takes away (since we work Local/Ground/Flight Data combined a majority of the time; no matter the traffic) from the controller's ability to spot those pesky 'little things' like flocks of geese on final; jets crossing the runway; etc. The combined positions in the tower continue to be a problem as a result of many years of unbelievably low Certified Professional Controller staffing. In addition; the reluctance and/or unwillingness of controllers to report this type of event; for fear of retribution that they know has happened before and will happen again; is also a major systemic contributor to the problem; since pilots are no longer much concerned about it; and they continue to fly; putting their aircraft and passengers in unsafe situations; unconcerned. I had another regional jet a few weeks ago; in broad daylight; cleared to cross Runway 08 just stop right in the middle of the runway; requiring me to send an aircraft around. With fully half the aircraft out in the middle of the runway; the pilots just slammed on the brakes; coming to a complete stop. I say it and say it and say it: Those combined positions are a VERY poor practice; requiring the Local Controller to take his eyes out of the air and away from the runways and aircraft; to perform required duties of two other positions at the same time. Events such as these can be EXTREMELY hazardous. Fear of retribution and apathy of reporting incidents; etc.; also will contribute to the problem getting rapidly worse.