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 comm,pid,ppid,pgid,sid | grep $$
sh 40280 32530 40280 32530
ps 40287 40280 40287 32530
grep 40288 40280 40287 32530