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INIT(8)		      Linux System Administrator's Manual	       INIT(8)

       init, telinit - process control initialization

       /sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
       /sbin/telinit [ -t SECONDS ] [ 0123456sSQqabcUu ]
       /sbin/telinit [ -e VAR[=VAL] ]

       Init  is	 the  parent  of all processes.	 Its primary role is to create
       processes from a script stored in  the  file  /etc/inittab  (see	 init‐
       tab(5)).	  This file usually has entries which cause init to spawn get‐
       tys on each line that users can log in.	It  also  controls  autonomous
       processes required by any particular system.

       A  runlevel is a software configuration of the system which allows only
       a selected group of processes to exist.	The processes spawned by  init
       for each of these runlevels are defined in the /etc/inittab file.  Init
       can be in one of eight runlevels: 0–6 and S  or	s.   The  runlevel  is
       changed	by having a privileged user run telinit, which sends appropri‐
       ate signals to init, telling it which runlevel to change to.

       Runlevels 0, 1, and 6 are reserved. Runlevel 0 is used to halt the sys‐
       tem, runlevel 6 is used to reboot the system, and runlevel 1 is used to
       get the system down into single user mode. Runlevel  S  is  not	really
       meant  to  be used directly, but more for the scripts that are executed
       when entering runlevel 1. For more information on this,	see  the  man‐
       pages for shutdown(8) and inittab(5).

       Runlevels  7-9  are  also  valid, though not really documented. This is
       because "traditional" Unix variants don't use  them.   In  case	you're
       curious,	 runlevels  S and s are in fact the same.  Internally they are
       aliases for the same runlevel.

       After init is invoked as the last step of the kernel boot sequence,  it
       looks for the file /etc/inittab to see if there is an entry of the type
       initdefault (see inittab(5)). The initdefault entry determines the ini‐
       tial  runlevel  of  the	system.	  If  there  is	 no  such entry (or no
       /etc/inittab at all), a runlevel must be entered at the system console.

       Runlevel S or s bring the system to single user mode and do not require
       an /etc/inittab file.  In single user mode, /sbin/sulogin is invoked on

       When entering single user mode, init initializes the consoles stty set‐
       tings  to sane values. Clocal mode is set. Hardware speed and handshak‐
       ing are not changed.

       When entering a multi-user mode for the first time, init	 performs  the
       boot  and  bootwait  entries to allow file systems to be mounted before
       users can log in.  Then all entries  matching  the  runlevel  are  pro‐

       When  starting  a  new  process,	 init  first  checks  whether the file
       /etc/initscript exists. If it does, it uses this script	to  start  the

       Each  time  a child terminates, init records the fact and the reason it
       died in /var/run/utmp and  /var/log/wtmp,  provided  that  these	 files

       After it has spawned all of the processes specified, init waits for one
       of its descendant processes to die, a powerfail signal, or until it  is
       signaled	 by  telinit to change the system's runlevel.  When one of the
       above three conditions occurs, it re-examines  the  /etc/inittab	 file.
       New entries can be added to this file at any time.  However, init still
       waits for one of the above three conditions to occur.  To  provide  for
       an  instantaneous response, the telinit Q or q command can wake up init
       to re-examine the /etc/inittab file.

       If init is not in single user mode  and	receives  a  powerfail	signal
       (SIGPWR),  it reads the file /etc/powerstatus. It then starts a command
       based on the contents of this file:

       F(AIL) Power is failing, UPS is providing the power. Execute the power‐
	      wait and powerfail entries.

       O(K)   The power has been restored, execute the powerokwait entries.

       L(OW)  The  power is failing and the UPS has a low battery. Execute the
	      powerfailnow entries.

       If /etc/powerstatus doesn't exist or contains anything  else  then  the
       letters F, O or L, init will behave as if it has read the letter F.

       Usage of SIGPWR and /etc/powerstatus is discouraged. Someone wanting to
       interact with init should use the /dev/initctl control  channel	-  see
       the  source  code  of the sysvinit package for more documentation about

       When init is requested to change the runlevel,  it  sends  the  warning
       signal SIGTERM to all processes that are undefined in the new runlevel.
       It then waits 5 seconds before forcibly terminating these processes via
       the  SIGKILL  signal.   Note that init assumes that all these processes
       (and their descendants) remain in the same  process  group  which  init
       originally  created for them.  If any process changes its process group
       affiliation it will not receive these signals.  Such processes need  to
       be terminated separately.

       /sbin/telinit  is linked to /sbin/init.	It takes a one-character argu‐
       ment and signals init to perform the appropriate action.	 The following
       arguments serve as directives to telinit:

       0,1,2,3,4,5 or 6
	      tell init to switch to the specified run level.

       a,b,c  tell init to process only those /etc/inittab file entries having
	      runlevel a,b or c.

       Q or q tell init to re-examine the /etc/inittab file.

       S or s tell init to switch to single user mode.

       U or u tell init to re-execute itself (preserving the  state).  No  re-
	      examining	 of /etc/inittab file happens. Run level should be one
	      of Ss0123456 otherwise request would be silently ignored.

       telinit can tell init how long it should wait between sending processes
       the  SIGTERM  and  SIGKILL signals.  The default is 5 seconds, but this
       can be changed with the -t option.

       telinit -e tells init  to  change  the  environment  for	 processes  it
       spawns.	 The  argument	of -e is either of the form VAR=VAL which sets
       variable VAR to value VAL, or of the  form  VAR	(without  an  equality
       sign) which unsets variable VAR.

       telinit can be invoked only by users with appropriate privileges.

       The  init  binary  checks  if  it  is init or telinit by looking at its
       process id; the real init's process id is always 1.  From this it  fol‐
       lows that instead of calling telinit one can also just use init instead
       as a shortcut.

       Init sets the following environment variables for all its children:

       PATH   /bin:/usr/bin:/sbin:/usr/sbin

	      As the name says. Useful to determine if a script runs  directly
	      from init.

	      The current system runlevel.

	      The previous runlevel (useful after a runlevel switch).

	      The  system  console.  This is really inherited from the kernel;
	      however if it is not set init will set  it  to  /dev/console  by

       It  is possible to pass a number of flags to init from the boot monitor
       (eg. LILO). Init accepts the following flags:

       -s, S, single
	    Single user mode boot. In this mode /etc/inittab is	 examined  and
	    the	 bootup rc scripts are usually run before the single user mode
	    shell is started.

       1-5  Runlevel to boot into.

       -b, emergency
	    Boot directly into a single user shell without running  any	 other
	    startup scripts.

       -a, auto
	    The	 LILO  boot loader adds the word "auto" to the command line if
	    it booted the kernel with the default command line	(without  user
	    intervention).  If this is found init sets the "AUTOBOOT" environ‐
	    ment variable to "yes". Note that you  cannot  use	this  for  any
	    security  measures - of course the user could specify "auto" or -a
	    on the command line manually.

       -z xxx
	    The argument to -z is ignored. You can use this to expand the com‐
	    mand  line	a  bit, so that it takes some more space on the stack.
	    Init can then manipulate the command line so that ps(1) shows  the
	    current runlevel.

       Init  listens  on  a fifo in /dev, /dev/initctl, for messages.  Telinit
       uses this to communicate with init. The interface is not very well doc‐
       umented	or  finished. Those interested should study the initreq.h file
       in the src/ subdirectory of the init source code tar archive.

       Init reacts to several signals:

	    Has the same effect as telinit q.

	    On receipt of this signals, init closes and re-opens  its  control
	    fifo, /dev/initctl. Useful for bootscripts when /dev is remounted.

	    Normally the kernel sends this signal to init when CTRL-ALT-DEL is
	    pressed. It activates the ctrlaltdel action.

	    The kernel sends this signal when the KeyboardSignal key  is  hit.
	    It activates the kbrequest action.

       Init  is	 compatible  with the System V init. It works closely together
       with the	 scripts  in  the  directories	/etc/init.d  and  /etc/rc{run‐
       level}.d.   If  your  system  uses  this	 convention, there should be a
       README file in the directory /etc/init.d explaining how	these  scripts


       Init  assumes that processes and descendants of processes remain in the
       same process group which was originally created for them.  If the  pro‐
       cesses change their group, init can't kill them and you may end up with
       two processes reading from one terminal line.

       If init finds that it is continuously respawning an entry more than  10
       times  in  2 minutes, it will assume that there is an error in the com‐
       mand string, generate an error  message	on  the	 system	 console,  and
       refuse  to  respawn this entry until either 5 minutes has elapsed or it
       receives a signal.  This prevents it from eating	 up  system  resources
       when  someone  makes  a typographical error in the /etc/inittab file or
       the program for the entry is removed.

       Miquel van Smoorenburg (,  initial  manual  page  by
       Michael Haardt (

       getty(1),  login(1),  sh(1),  runlevel(8),  shutdown(8), kill(1), init‐
       tab(5), initscript(5), utmp(5)

				  29 Jul 2004			       INIT(8)

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