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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 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  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 (a.k.a. s).   The  runlevel
       is  changed by having a privileged user run telinit, which sends appro-
       priate signals to init, telling it which runlevel to change to.

       Runlevels S, 0, 1, and 6 are reserved.  Runlevel S is used to  initial-
       ize the system on boot.  When starting runlevel S (on boot) or runlevel
       1 (switching from a multi-user runlevel) the system is entering  ``sin-
       gle-user  mode'', after which the current runlevel is S.  Runlevel 0 is
       used to halt the system; runlevel 6 is used to reboot the system.

       After booting through S the system  automatically  enters  one  of  the
       multi-user  runlevels  2  through 5, unless there was some problem that
       needs to be fixed by the administrator in single-user  mode.   Normally
       after  entering single-user mode the administrator performs maintenance
       and then reboots the system.

       For more information, 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.

       Runlevels S and s are 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 initialize the system and do not require an  /etc/init-
       tab file.

       In single user mode, /sbin/sulogin is invoked on /dev/console.

       When entering single user mode, init initializes the consoles stty set-
       tings to sane values. Clocal mode is set. Hardware speed and  handshak-
       ing 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 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 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  pro-
       cess  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

       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" 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:

            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{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.

       On a Debian system, entering runlevel 1  causes  all  processes  to  be
       killed  except  for kernel threads and the script that does the killing
       and other processes in its session.  As a consequence of this, it isn't
       safe  to  return from runlevel 1 to a multi-user runlevel: daemons that
       were started in runlevel S and are needed for normal operation  are  no
       longer running.  The system should be rebooted.

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

       getty(1), login(1), sh(1),  runlevel(8),  shutdown(8),  kill(1),  init-
       tab(5), initscript(5), utmp(5)

                                  29 Jul 2004                          INIT(8)