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

NAME

       init, telinit - process control initialization

SYNOPSIS

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

DESCRIPTION

   Init
       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 gettys on each line that users can log in.  It also  controls	autonomous
       processes required by any particular system.

RUNLEVELS

       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  run-
       level is changed by having a privileged user run telinit, which sends appropriate signals to init, telling it which runlevel to change to.

       Runlevels  0, 1, and 6 are reserved. Runlevel 0 is used to halt the system, 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  exe-
       cuted when entering runlevel 1. For more information on this, see the manpages 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.

BOOTING

       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 initial 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 /dev/console.

       When  entering  single  user mode, init initializes the consoles stty settings to sane values. Clocal mode is set. Hardware speed and hand-
       shaking 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 processed.

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

       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
       exist.

CHANGING RUNLEVELS

       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 powerwait 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 this.

       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 pro-
       cesses (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.

TELINIT

       /sbin/telinit  is linked to /sbin/init.	It takes a one-character argument and signals init to perform the appropriate action.  The follow-
       ing 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 vari-
       able 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 follows
       that instead of calling telinit one can also just use init instead as a shortcut.

ENVIRONMENT

       Init sets the following environment variables for all its children:

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

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

       RUNLEVEL
	      The current system runlevel.

       PREVLEVEL
	      The previous runlevel (useful after a runlevel switch).

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

BOOTFLAGS

       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" environment variable to "yes". Note that you cannot use this for	any  secu-
	    rity 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 command 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.

INTERFACE

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

SIGNALS

       Init reacts to several signals:

       SIGHUP
	    Has the same effect as telinit q.

       SIGUSR1
	    On receipt of this signals, init closes and re-opens its control fifo, /dev/initctl. Useful for bootscripts when /dev is remounted.

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

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

CONFORMING TO

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

FILES

       /etc/inittab
       /etc/initscript
       /dev/console
       /var/run/utmp
       /var/log/wtmp
       /dev/initctl

WARNINGS

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

DIAGNOSTICS

       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
       command	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.

AUTHOR

       Miquel van Smoorenburg (miquels@cistron.nl), initial manual page by Michael Haardt (u31b3hs@pool.informatik.rwth-aachen.de).

SEE ALSO

       init.d(7), getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), inittab(5), initscript(5), utmp(5)

								    29 Jul 2004 							   INIT(8)