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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
       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, 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

       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 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

       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:

	    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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