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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 get-
       tys 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  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.

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 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, /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-
       cessed.

       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 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
       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 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.

TELINIT
       /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.

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" 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.

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 doc-
       umented 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/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
       /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 pro-
       cesses 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  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.

AUTHOR
       Miquel  van  Smoorenburg  (miquelsATcistron.nl),  initial manual page by
       Michael Haardt (u31b3hsATpool.de).

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



                                  29 Jul 2004                          INIT(8)