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  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 docu­
       mented. 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 init­
       tab(5)). The initdefault entry determines the initial run­
       level  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/initttab file.   In  single  user
       ioctl(2) states from /etc/ If  this  file  does
       not exist, init initializes 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 sys­
       tems 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.


       After  it has spawned all of the processes specified, init
       waits for one of its descendant processes to die, a power­
       fail  signal, or until it is signaled by telinit to change
       the system's runlevel.  When one of the above three condi­
       tions  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  power­
       fail  signal (SIGPWR), it reads the file /etc/powerstatus.
       It then starts a command based on  the  contents  of  this

       F(AIL) Power  is failing, UPS is providing the power. Exe­
              cute 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. Some­
       one   wanting   to  interact  with  init should  use  the
       /dev/initctl control channel - see the source code of  the
       the warning signal SIGTERM to all processes that are unde­
       fined in the new runlevel.  It then waits 5 seconds before
       forcibly terminating these processes via the SIGKILL sig­
       nal.  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 appro­
       priate action.  The following arguments  serve  as  direc­
       tives 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 hap­
              pens. 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

       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

       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

              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.


       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 run­
            ning any other startup scripts.

       -a, auto
            The LILO boot loader adds the word booted the  kernel
            with the default command line (without user interven­
            tion).  If this is found init sets  the  variable  to
            measures  -  ofcourse  the user could specify 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 run­


       Init listens on a fifo in  /dev,  /dev/initctl,  for  mes­
       sages.   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:

            Init looks for /etc/initrunlvl and  /var/log/initrun­
            lvl. 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.

            On  receipt of this signals, init closes and re-opens
            it's   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

            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 cre­
       ated 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
       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/init­
       tab file or the program for the entry is removed.


       Miquel  van Smoorenburg (, initial man­
       ual page by Michael Haardt  (u31b3hs@pool.informatik.rwth-


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