I would like to install a grab bar in my 1950's era house in the shower. The shower has a hot and cold water knob, a knob between them to select shower or bath faucet and the shower head is in a vertical line up from the center. To the left of the tub is a toilet with an old fashioned flusher (no tank, comercial like flusher). I would like to put the grab bar in the shower on the wall of the knobs/spout or over the toilet, but I have plaster walls and don't know where the pipes run. Help!

Chances are that all the water supply lines are below the hot/cold handles and below the toilet inlet valve, except for the one vertical one going up to the shower. Vent pipes could be anywhere but most likely there's just one right behind the toilet. You find a stud with a stud finder or drill teeny holes, or use molly bolts. If you have tile, it can be a little tricky. There are grab bars with huge suction cups that have a clamp to create the suction. I don't know if they work or not.