Would it be safe to use a truck if it had not been used for a long time?

Would it be safe to use a truck if it had not been used for a long time?
It depends on what "a long time" is, and how/where it was stored. Before driving a vehicle that hasn't been moved in a long time (3 months or more) you need to make sure that the vehicle operates properly. This means running, shifting, turning, braking, all the lights, horn, and dashboard gauges need to be operating correctly. If you aren't sure about something, then either have the vehicle towed to where it needs to go, or ask a mechanic to look at it for you before you try to drive it.