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INIT(8)                     System Manager's Manual                    INIT(8)

       init - process control initialization


       Init is invoked inside UNIX as the last step in the boot procedure.  It
       normally then runs  the  automatic  reboot  sequence  as  described  in
       reboot(8),  and  if this succeeds, begins multi-user operation.  If the
       reboot fails, it commences single user operation by giving  the  super-
       user  a  shell  on the console.  It is possible to pass parameters from
       the boot program to init so that single  user  operation  is  commenced
       immediately.   When such single user operation is terminated by killing
       the single-user shell (i.e. by hitting ^D), init runs  /etc/rc  without
       the  reboot  parameter.  This command file performs housekeeping opera-
       tions such as removing temporary  files,  mounting  file  systems,  and
       starting daemons.

       In  multi-user  operation,  init's role is to create a process for each
       terminal port on which a user may log in.  To begin such operations, it
       reads  the  file  /etc/ttys and forks several times to create a process
       for each terminal specified in the file.  Each of these processes opens
       the  appropriate terminal for reading and writing.  These channels thus
       receive file descriptors 0, 1 and 2, the standard input and output  and
       the  diagnostic  output.   Opening  the terminal will usually involve a
       delay, since the open is not completed until someone is dialed  up  and
       carrier  established on the channel.  If a terminal exists but an error
       occurs when trying to open the terminal init  complains  by  writing  a
       message to the system console; the message is repeated every 10 minutes
       for each such terminal until the terminal is shut off in /etc/ttys  and
       init  notified  (by  a  hangup,  as  described  below), or the terminal
       becomes accessible (init checks again every  minute).   After  an  open
       succeeds, /etc/getty is called with argument as specified by the second
       character of the ttys file line.   Getty  reads  the  user's  name  and
       invokes login to log in the user and execute the Shell.

       Ultimately  the  Shell  will terminate because of an end-of-file either
       typed explicitly or generated as a result of hanging up.  The main path
       of init, which has been waiting for such an event, wakes up and removes
       the appropriate entry from the file utmp, which records current  users,
       and  makes  an  entry  in  /usr/adm/wtmp,  which maintains a history of
       logins and logouts.  The wtmp entry is made only if a  user  logged  in
       successfully  on  the  line.  Then the appropriate terminal is reopened
       and getty is reinvoked.

       Init catches the hangup signal (signal SIGHUP)  and  interprets  it  to
       mean  that  the file /etc/ttys should be read again.  The Shell process
       on each line which used to be active in ttys but is no longer there  is
       terminated;  a  new  process  is  created  for  each  added line; lines
       unchanged in the file are undisturbed.  Thus it is possible to drop  or
       add  phone lines without rebooting the system by changing the ttys file
       and sending a hangup signal to the init process: use `kill -HUP 1.'

       Init will terminate multi-user operations and resume  single-user  mode
       if sent a terminate (TERM) signal, i.e. ``kill -TERM 1''.  If there are
       processes outstanding which are deadlocked (due to hardware or software
       failure), init will not wait for them all to die (which might take for-
       ever), but will time out after 30 seconds and print a warning message.

       Init will cease creating new getty's and allow the system to slowly die
       away,  if  it  is sent a terminal stop (TSTP) signal, i.e. ``kill -TSTP
       1''.  A later hangup will resume full multi-user operations, or a  ter-
       minate  will  initiate  a  single  user  shell.   This  hook is used by
       reboot(8) and halt(8).

       Init's role is so critical that if it  dies,  the  system  will  reboot
       itself  automatically.   If, at bootstrap time, the init process cannot
       be located, the system will loop in user mode at location 0x13.

       init: tty: cannot open.  A terminal which is turned on in the  rc  file
       cannot  be  opened,  likely  because the requisite lines are either not
       configured into the system or the associated device  was  not  attached
       during boot-time system configuration.

       WARNING:  Something  is  hung (wont die); ps axl advised.  A process is
       hung and could not be killed when the system was shutting  down.   This
       is usually caused by a process which is stuck in a device driver due to
       a persistent device error condition.

       /dev/console, /dev/tty*, /etc/utmp, /usr/adm/wtmp, /etc/ttys, /etc/rc

       login(1),  kill(1),  sh(1),  ttys(5),   crash(8V),   getty(8),   rc(8),
       reboot(8), halt(8), shutdown(8)

4th Berkeley Distribution        1 April 1981                          INIT(8)