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!!!INIT


!!NAME


init, telinit - process control initialization
!!SYNOPSIS


__/sbin/init__ [[ __-a__ ] [[ __-s__ ] [[ __-b__ ] [[ __-z xxx__ ] [[ __0123456Ss__ ]

__/sbin/telinit__ [[ __-t sec__ ] [[__0123456sSQqabcUu__ ]


!!DESCRIPTION

__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
__getty__s 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 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 documented.
This is because
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 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, __/sbin/sulogin__ is invoked 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.

!!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
__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. 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 argument 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 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 ''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.
!!ENVIRONMENT


__Init__ sets the following environment variables for all
its children:


__PATH__


''/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin''


__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"
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.
!!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 documented 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__


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 backwards compatibility only!'' .
In the normal case (the files don't exist) init behaves like
__telinit q__ was executed.


__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{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.
!!FILES


/etc/inittab
/etc/initscript
/dev/console
/etc/ioctl.save
/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 processes 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 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.
!!AUTHOR


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

!!SEE ALSO


getty(1), login(1), sh(1),
runlevel(8), shutdown(8), kill(1),
inittab(5), initscript(5),
utmp(5)
----
26 pages link to init(8):
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Last edited on Friday, August 13, 2004 7:44:47 pm by JohnMcPherson
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