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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 sec ]	[ 0123456sSQqabcUu ]

       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 be-
       cause "traditional" Unix	variants don't use them.  In case you're curi-
       ous,  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, a root shell is opened on

       When entering single user  mode,	 init  reads  the  console's  ioctl(2)
       states from /etc/ If this file does not exist, init initial-
       izes the	line at	9600 baud and with CLOCAL settings.  When init	leaves
       single  user  mode, it stores the console's ioctl settings in this file
       so it can re-use	them for the next single-user session.

       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  ex-

       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-ex-
	      amining of /etc/inittab file happens. Run	level should be	one of
	      Ss12345, otherwise request would be silently ignored.

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

       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   /usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin

	      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 de-

       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 se-
	    curity 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:

	    Init looks for /etc/initrunlvl and /var/log/initrunlvl.  If	one of
	    these files	exist and contain an ASCII runlevel, init switches  to
	    the	new runlevel.  This is for backwards compatibility only! .  In
	    the	normal case (the files don't exist) init behaves like  telinit
	    q was executed.

	    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)

				23 August 2001			       INIT(8)


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