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 sec ] [ 0123456sSQqabcUu ]

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, a root shell is  opened
       on /dev/console.

       When  entering  single user mode, init reads the console's ioctl(2) states from /etc/ioctl.save. 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 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
	      Ss12345, otherwise request would be silently ignored.

       telinit can also 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 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 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   /usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin

       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
	    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.

       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/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
       work.

FILES

       /etc/inittab
       /etc/initscript
       /dev/console
       /etc/ioctl.save
       /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

       getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), inittab(5), initscript(5), utmp(5)

								  23 August 2001							   INIT(8)