Is there a linux command to block until a process exits?

Question

It's just what the question asks. Also, all I have is the PID, and the shell I am running the command from is not necessarily the shell that initially invoked the process. Any ideas?

Answer 1

while ps -p $PID >/dev/null 2>&1; do sleep 1; done

or

while ps -p $PID >/dev/null 2>&1; do :; done

Answer 2

It's not a very neat way to do it, but you may continuously issue kill(2) system calls to the specified pid, putting zero as a signal. The first time kill doesn't succeed by setting errno to ESRCH, you may conclude that the process has exited.