Init is the parent of all processes


unisys18:~$ pstree


init-+-atd
     |-bash
     |-cron
     |-events/0-+-aio/0
     |          |-ata/0
     |          |-kblockd/0
     |          |-khelper
     |          `-2*[pdflush]
     |-exim4
     |-inetd
     |-khubd
     |-5*[kjournald]
     |-klogd
     |-kseriod
     |-ksoftirqd/0
     |-kswapd0
     |-lpd
     |-portmap
     |-rpc.statd
     |-slapd
     |-sshd---sshd---sshd---bash---pstree
     `-syslogd


unisys18:~$ pstree -p -l

shows the process tree with their PID in the long (non-truncated) format. /usr/bin/pstree comes with package psmisc.

top