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
       inittab(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.

       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
       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 executed
       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.

       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

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

       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
              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

       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
       argument 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
       follows 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"
            environment 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
            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.

       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.

       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{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.


       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.

       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.

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

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

                                  29 Jul 2004                          INIT(8)