supply stop valve

Being newbie to plumbing, I want to avoid need to copper cut pipe and reinstalling another compressor or whatever. To end usage of refrigerator's ice maker water supply line, simply want to loosen supply line's socket and adding value in middle without cutting anything. Attaching picture. Is...

Was attempting to set up my bidet at my new apartment in a very old building. Cranked the stop for the toilet until it should be good and closed; she's still flushing. Tightened the central screw-still flushing. The supply is an exposed pipe running along the baseboard, and it gets tapped for...