INIT(8)                      System Administration                     INIT(8)

NAME

       init - start and kill processes

DESCRIPTION

       init  is the component of the UNIX operating system that is responsible
       for creating processes once the system has  finished  booting  and  for
       maintaining those processes, including killing them.

       At  any  time, the system is in one of 12 states or run levels. At each
       run level, a different group of processes is allowed  to  run.   Gener‐
       ally,  when  the  run  level changes, init kills the processes which it
       created under the old run level, and creates a new set for the new  run
       level.

       Run   levels   are   identified  with  single  characters  in  the  set
       [bs0123456789].  B and S are also accepted as equivalents to b  and  s.
       Some  of  these  run  levels have special meaning and are listed below.
       Except for b, all run levels can have their meaning  changed  via  runl
       commands in /etc/inittab.

       Except  for  levels  0,  5 and 6, the actual state of the machine after
       init switches to a level is completely defined by the  processes  which
       run,  which are configurable in /etc/inittab.  It is therefore possible
       to use all but those three run levels for purposes other than their in‐
       tended  one. Their meaning as described here are the conventional mean‐
       ing. It is encouraged that the conventions be followed.

       b (boot)
              This is the run level that is selected by default. Any  initial‐
              ization  processes  should be run under this level.  By default,
              init switches to run level s (single user) after  starting  pro‐
              cesses  at  this level. The bootlevel command in /etc/rchost may
              be used to modify this behavior.

       0      This is the system shutdown level. Switching to this level  will
              cause  the  system to shut down after any level 0 processes have
              been run.

       1      This run level is defined as the administration state.

       2, 3, 4
              These are multi user run levels.  Level 2 is standard.  Level  3
              is multi user with networking, and level 4 is an alternate multi
              user set up.

       5      Under UNIX, this is the firmware monitor run level.  Under  GNO,
              switching to this level causes GNO to quit. The disparity arises
              from the fact that there is no such thing as quitting UNIX.

       6      Switching to this level causes the system to reboot.

       s      This is the single user run level.

       7, 8   These are local run levels. Any local terminals are  active,  as
              well  as  any  local area networks (level 8), but no dialups are
              active and no modem based networking is available.

       9      This level is undefined.

SIGNALS

       The following signals can be sent to init to force certain  conditions.
       Note  that  these are intended as low level commands; init(1) should be
       used to issue these signals.

       SIGHUP Causes init to reread /etc/inittab,  kill  processes  that  have
              been removed from it, and start processes that were newly added.

              init  depends  on  the process names (see inittab(5)) to compare
              the old copy of the table with the new one.

       SIGTERM
              Causes init to switch to the kill default run level. This  is  0
              unless it is changed in /etc/inittab.  Normally, the effect is a
              system shutdown.

       SIGUSR1
              Causes init to try to receive a message  (see  procrecv(2))  and
              act on it. The message is typically sent by init(1) and contains
              status inquiries or run level change requests.

       SIGUSR2
              Causes init to exit its sigpause(2) and check  for  any  pending
              events (which is useful if a timing bug has caused init to go to
              sleep while it had pending events).

NOTES

       Under real UNIX, there are fewer run levels.

       When switching to some run levels, init  automatically  issues  a  runl
       command  to  carry  out the apropriate action for that run level.  This
       action can be overridden with another runl command, possibly n to  can‐
       cel any special action at all.

       If  init  becomes  idle  and has no children, it switch to the idle run
       level, which is 5 by default.

       If a LOG_EMERG message is received by syslogd(8), init will  switch  to
       the panic run level, 6 by default.

       When  a  child  of  init dies, any entry it has in the utmp(5) database
       will be removed before any other action is taken. Also, stray bogus en‐
       tries are removed whenever they are noticed.

       At  startup  time, init tries to read the file /etc/rchost and set cer‐
       tain system globals according to the information in that file.

FILES

       /etc/inittab
       /usr/sbin/initd
       /var/adm/utmp
       /var/adm/wtmp
       /etc/rchost

SEE ALSO

       init(1), inittab(5), rchost(5), utmp(5), getty(8), syslogd(8)

AUTHOR

       Phillip Vandry, <vandry@cam.org>



GNO                             27 October 1993                        INIT(8)