Wait for the termination of arbitrary processes. See also:
wait. More information: https://manned.org/waitpid.1.
waitpid {{pid1 pid2 ...}}
n seconds:waitpid {{[-t|--timeout]}} {{n}} {{pid1 pid2 ...}}
waitpid {{[-e|--exited]}} {{pid1 pid2 ...}}
n of the specified processes have exited:waitpid {{[-c|--count]}} {{n}} {{pid1 pid2 ...}}
waitpid {{[-h|--help]}}