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RedHat 9 (Linux i386) - man page for init (redhat section 8)

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 inittab(5)).  This file  usually  has  entries
       which cause init to spawn gettys on each line that users can log in.  It also controls au-
       tonomous 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 sig-
       nals 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, a root shell is opened on /dev/console.

       When  entering  single  user  mode,  init  reads  the  console's  ioctl(2)   states   from
       /etc/ioctl.save.  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 systems 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 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 instanta-
       neous response, the telinit Q or q command can wake up init to re-examine the /etc/inittab

       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

       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

       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.

       /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

       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 processes  the  SIGTERM
       and  SIGKILL  signals.	The default is 5 seconds, but this can be changed with the -t sec

       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   /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 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 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 communi-
       cate 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 ar-

       Init reacts to several signals:

	    Init looks for /etc/initrunlvl and /var/log/initrunlvl.  If one of these files  exist
	    and  contain an ASCII runlevel, init switches to the new runlevel.	This is for back-
	    wards 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 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  acti-
	    vates 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


       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  (miquels@cistron.nl),	initial  manual  page  by  Michael Haardt

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

					  23 August 2001				  INIT(8)

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