Abount inittab & init

#man inittab
INITTAB(5)                                     Linux System Administrator's Manual                                     INITTAB(5)
NAME
       inittab - format of the inittab file used by the sysv-compatible init process
DESCRIPTION
       The  inittab  file  describes  which  processes  are started at bootup and during normal operation (e.g. /etc/init.d/boot,
       /etc/init.d/rc, gettys...).  Init(8) distinguishes multiple runlevels, each of which can have its  own  set  of  processes
       that  are  started.   Valid runlevels are 0-6 plus A, B, and C for ondemand entries.  An entry in the inittab file has the
       following format:
              id:runlevels:action:process
       Lines beginning with '#' are ignored.
       id     is a unique sequence of 1-4 characters which identifies an entry in inittab (for versions of sysvinit compiled with
              the old libc5 (< 5.2.18) or a.out libraries the limit is 2 characters).
              Note:  traditionally, for getty and other login processes, the value of the id field is kept the same as the suffix
              of the corresponding tty, e.g. 1 for tty1. Some ancient login accounting programs might expect this, though I can't
              think of any.
       runlevels
              lists the runlevels for which the specified action should be taken.
       action describes which action should be taken.
       process
              specifies  the process to be executed.  If the process field starts with a '+' character, init will not do utmp and
              wtmp accounting for that process.  This is needed for gettys that insist on doing their own utmp/wtmp housekeeping.
              This is also a historic bug.
       The  runlevels field may contain multiple characters for different runlevels.  For example, 123 specifies that the process
       should be started in runlevels 1, 2, and 3.  The runlevels for ondemand entries may contain an A, B, or C.  The  runlevels
       field of sysinit, boot, and bootwait entries are ignored.
       When  the  system runlevel is changed, any running processes that are not specified for the new runlevel are killed, first
       with SIGTERM, then with SIGKILL.
       Valid actions for the action field are:
       respawn
              The process will be restarted whenever it terminates (e.g. getty).
       wait   The process will be started once when the specified runlevel is entered and init will wait for its termination.
       once   The process will be executed once when the specified runlevel is entered.
       boot   The process will be executed during system boot.  The runlevels field is ignored.
       bootwait
              The process will be executed during system boot, while init waits for its termination  (e.g.  /etc/rc).   The  run-
              levels field is ignored.
       off    This does nothing.
       ondemand
              A  process  marked  with  an ondemand runlevel will be executed whenever the specified ondemand runlevel is called.
              However, no runlevel change will occur (ondemand runlevels are 'a', 'b', and 'c').
       initdefault
              An initdefault entry specifies the runlevel which should be entered after system boot.  If none exists,  init  will
              ask for a runlevel on the console. The process field is ignored.
       sysinit
              The  process  will  be  executed during system boot. It will be executed before any boot or  bootwait entries.  The
              runlevels field is ignored.
       powerwait
              The process will be executed when the power goes down. Init is usually informed about this by a process talking  to
              a UPS connected to the computer.  Init will wait for the process to finish before continuing.
       powerfail
              As for powerwait, except that init does not wait for the process's completion.
       powerokwait
              This process will be executed as soon as init is informormed that the power has been restored.
       powerfailnow
              This  process will be executed when init is told that the battery of the external UPS is almost empty and the power
              is failing (provided that the external UPS and the monitoring process are able to detect this condition).
       ctrlaltdel
              The process will be executed when init receives the SIGINT signal.  This means that someone on the  system  console
              has  pressed  the  CTRL-ALT-DEL key combination. Typically one wants to execute some sort of shutdown either to get
              into single-user level or to reboot the machine.
       kbrequest
              The process will be executed when init receives a signal from the keyboard handler that a special  key  combination
              was pressed on the console keyboard.
              The  documentation  for this function is not complete yet; more documentation can be found in the kbd-x.xx packages
              (most recent was kbd-0.94 at the time of this writing). Basically you want to map some keyboard combination to  the
              "KeyboardSignal" action. For example, to map Alt-Uparrow for this purpose use the following in your keymaps file:
              alt keycode 103 = KeyboardSignal
EXAMPLES
       This is an example of a inittab which resembles the old Linux inittab:
              # inittab for linux
              id:1:initdefault:
              rc::bootwait:/etc/rc
              1:1:respawn:/etc/getty 9600 tty1
              2:1:respawn:/etc/getty 9600 tty2
              3:1:respawn:/etc/getty 9600 tty3
              4:1:respawn:/etc/getty 9600 tty4
       This inittab file executes /etc/rc during boot and starts gettys on tty1-tty4.
       A more elaborate inittab with different runlevels (see the comments inside):
              # Level to run in
              id:2:initdefault:
              # Boot-time system configuration/initialization script.
              si::sysinit:/etc/init.d/rcS
              # What to do in single-user mode.
              ~:S:wait:/sbin/sulogin
              # /etc/init.d executes the S and K scripts upon change
              # of runlevel.
              #
              # Runlevel 0 is halt.
              # Runlevel 1 is single-user.
              # Runlevels 2-5 are multi-user.
              # Runlevel 6 is reboot.
              l0:0:wait:/etc/init.d/rc 0
              l1:1:wait:/etc/init.d/rc 1
              l2:2:wait:/etc/init.d/rc 2
              l3:3:wait:/etc/init.d/rc 3
              l4:4:wait:/etc/init.d/rc 4
              l5:5:wait:/etc/init.d/rc 5
              l6:6:wait:/etc/init.d/rc 6
              # What to do at the "3 finger salute".
              ca::ctrlaltdel:/sbin/shutdown -t1 -h now
              # Runlevel 2,3: getty on virtual consoles
              # Runlevel   3: getty on terminal (ttyS0) and modem (ttyS1)
              1:23:respawn:/sbin/getty tty1 VC linux
              2:23:respawn:/sbin/getty tty2 VC linux
              3:23:respawn:/sbin/getty tty3 VC linux
              4:23:respawn:/sbin/getty tty4 VC linux
              S0:3:respawn:/sbin/getty -L 9600 ttyS0 vt320
              S1:3:respawn:/sbin/mgetty -x0 -D ttyS1
FILES
       /etc/inittab
AUTHOR
       Init  was written by Miquel van Smoorenburg ().  This manual page was written by Sebastian Lederer (led-
       ) and modified by Michael Haardt ().
SEE ALSO
       init(8), telinit(8)
                                                           Dec 4, 2001                                                 INITTAB(5)