init

init (PID 1)

init is...

• the first user level process started after the kernel has started (i.e. kernel running, device drivers & data structures initialized)
• usually located at /sbin/init

when init starts up, it...

• checks filesystems
• cleans up /tmp
• looks for /etc/inittab file, sets the runlevel as stipulated by the inittab file ( WebHome ), and starts services
• starts getty ( WebHome ) for each terminal

during normal system operation, init ...

• adopts all orphan processes (orphans become child of init)
• restarts getty for each terminal when a user logs out

you can see this visually by examining the output of the command pstree which produces a tree of processes, providing parent/child relationships at a glance:

$pstree -p 
init(1)─┬─acpid(1984)
        ├─auditd(20764)─┬─audispd(20766)───{audispd}(20767)
        │               └─{auditd}(20765)
        ├─crond(2176)
        ├─dbus-daemon(1949)
        ├─denyhosts.py(11836)
        ├─httpd(624)─┬─httpd(2973)
        │            ├─httpd(7785)
        │            ├─httpd(9278)
        │            ├─httpd(14645)
        │            ├─httpd(14787)
        │            ├─httpd(26891)
        │            ├─httpd(28084)
        │            ├─httpd(28684)
        │            ├─httpd(29070)
        │            └─httpd(29071)
        ├─irqbalance(1807)
        ├─master(974)─┬─pickup(28545)
        │             └─qmgr(977)
        ├─mingetty(2267)
        ├─mingetty(2268)
        ├─mingetty(2269)
        ├─mingetty(2270)
        ├─mingetty(2271)
        ├─mingetty(2272)
        ├─ntpd(10416)
        ├─python(892)─┬─python(895)
        │             ├─python(896)
        │             ├─python(897)
        │             ├─python(898)
        │             ├─python(899)
        │             ├─python(900)
        │             ├─python(901)
        │             └─python(902)
        ├─rklogd(11154)
        ├─rsyslogd(11150)───{rsyslogd}(11874)
        ├─sshd(8412)───sshd(25778)───sshd(25780)───bash(25781)───pstree(32007)
        └─udevd(616)

init command

change current runlevel to 5:

init 5

-- RobertSajan - 03 Oct 2008