INIT linux command manual

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 sec ] [ 0123456sSQqabcUu ]

       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 or 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 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  con-

       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 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 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 pow-
              erwait 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  run-
       level.  It then waits 5 seconds before forcibly terminating these pro-
       cesses 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 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  hav-
              ing 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 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 shortcut.

       Init sets the following environment variables for all its children:

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

              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" envi-
            ronment 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

            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.

       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  (, initial manual page by
       Michael Haardt (

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

                                18 April 2003                         INIT(8)