What is the difference between the two commands nice and renice to manage process priority?
nice launches a new command with a modified nice level (lower priority than it would have otherwise had, or higher priority if you have permission). You specify which command to launch by providing it as an argument to nice itself. nice actually execs that command, so nice itself doesn't terminate until the command does.
renice changes the priority of an existing running process, then terminates immediately. You specify which process by giving its PID (as well as the new desired nice level) as an argument to renice.
Note also that nice takes a relative nice level (i.e. less or more than before) whereas renice requires an absolute level.
man niceandman renicewould have given you your answer. – terdon♦ Feb 28 '16 at 18:44