Sets the Process Group ID
Process groups are useful for culling all child processes. In particular, you can kill all processes in a process group with a single syscall (kill(-getpgrp()))
Generally you'll want to create a new process group, but it's also useful for attaching to existing ones.
Use 0 to create a new proces group id.`
$ ./setpgid 0 sh
$ ps xao pid,ppid,pgid,sid | grep $$
9647 18575 9647 18575
9648 9647 9648 18575
9649 9647 9648 18575