$ ./698259.pl
  PID  PPID  PGRP   SID CMD
 2963  2960  2963  2963 bash
 4074  2963  4074  2963  \_ /usr/bin/perl ./698259.pl
 4075  4074  4075  2963      \_ /usr/bin/perl ./698259.pl
 4076  4075  4075  2963      |   \_ bash -c cat; echo
 4077  4076  4075  2963      |       \_ cat
 4078  4074  4074  2963      \_ ps f -s 2963 -o pid,ppid,pgrp,sid,cmd
killed children
  PID  PPID  PGRP   SID CMD
 2963  2960  2963  2963 bash
 4074  2963  4074  2963  \_ /usr/bin/perl ./698259.pl
 4079  4074  4074  2963      \_ ps f -s 2963 -o pid,ppid,pgrp,sid,cmd