File: init.8

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'\" -*- coding: UTF-8 -*-
.\" Copyright (C) 1998-2004 Miquel van Smoorenburg.
.\"
.\" This program is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
.\" the Free Software Foundation; either version 2 of the License, or
.\" (at your option) any later version.
.\"
.\" This program is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public License
.\" along with this program; if not, write to the Free Software
.\" Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
.\"
.\"{{{}}}
.\"{{{  Title
.TH INIT 8 "29 Jul 2004" "" "Linux System Administrator's Manual"
.\"}}}
.\"{{{  Name
.SH NAME
init, telinit \- process control initialization
.\"}}}
.\"{{{  Synopsis
.SH SYNOPSIS
.B /sbin/init
.RB [ " -a " ]
.RB [ " -s " ]
.RB [ " -b " ]
[ \fB\-z\fP \fIxxx\fP ]
.RB [ " 0123456Ss " ]
.br
.B /sbin/telinit
[ \fB\-t\fP \fISECONDS\fP ]
.RB [ " 0123456sSQqabcUu " ]
.br
.B /sbin/telinit
[ \fB\-e\fP \fIVAR\fP[\fB=\fP\fIVAL\fP] ]
.\"}}}
.\"{{{  Description
.SH DESCRIPTION
.\"{{{  init
.SS Init
.B Init
is the parent of all processes.  Its primary role is to create processes
from a script stored in the file \fB/etc/inittab\fP (see
\fIinittab\fP(5)).  This file usually has entries which cause \fBinit\fP
to spawn \fBgetty\fPs on each line that users can log in.  It also
controls autonomous processes required by any particular system.
.PP
.\"{{{ Runlevels
.SH RUNLEVELS
A \fIrunlevel\fP is a software configuration of the system which allows
only a selected group of processes to exist.  The processes spawned by
\fBinit\fP for each of these runlevels are defined in the
\fB/etc/inittab\fP file.  \fBInit\fP can be in one of eight runlevels:
\fB0\(en6\fP and \fBS\fP (a.k.a. \fBs\fP).  The runlevel is
changed by having a privileged user run \fBtelinit\fP, which sends
appropriate signals to \fBinit\fP, telling it which runlevel to change
to.
.PP
Runlevels \fBS\fP, \fB0\fP, \fB1\fP, and \fB6\fP are reserved.
Runlevel S is used to initialize the system on boot.
When starting runlevel S (on boot)
or runlevel 1 (switching from a multi-user runlevel)
the system is entering ``single-user mode'', after which the
current runlevel is S.
Runlevel 0 is used to halt the system;
runlevel 6 is used to reboot the system.
.PP
After booting through S the system automatically enters one of
the multi-user runlevels 2 through 5, unless there was some
problem that needs to be fixed by the administrator in
single-user mode.
Normally after entering single-user mode
the administrator performs maintenance and then reboots the system.
.PP
For more information,
see the manpages for \fBshutdown\fP(8) and \fBinittab\fP(5).
.PP
Runlevels 7-9 are also valid, though not really documented. This is
because "traditional" Unix variants don't use them.
.PP
Runlevels \fIS\fP and \fIs\fP are the same.
Internally they are aliases for the same runlevel.
.\"}}}
.PP
.SH BOOTING
After \fBinit\fP is invoked as the last step of the kernel boot sequence,
it looks for the file \fB/etc/inittab\fP to see if there is an entry of the
type \fBinitdefault\fP (see \fIinittab\fP(5)). The \fBinitdefault\fP entry
determines the initial runlevel of the system.  If there is no such
entry (or no \fB/etc/inittab\fP at all), a runlevel must be
entered at the system console.
.PP
Runlevel \fBS\fP or \fBs\fP initialize the system
and do not require an \fB/etc/inittab\fP file.
.PP
In single user mode, \fB/sbin/sulogin\fP is invoked on \fB/dev/console\fP.
.PP
When entering single user mode, \fBinit\fP initializes the consoles
\fBstty\fP settings to sane values. Clocal mode is set. Hardware
speed and handshaking are not changed.
.PP
When entering a multi-user mode for the first time, \fBinit\fP performs the
\fBboot\fP and \fBbootwait\fP entries to allow file systems to be
mounted before users can log in.  Then all entries matching the runlevel
are processed.
.PP
When starting a new process, \fBinit\fP first checks whether the file
\fI/etc/initscript\fP exists. If it does, it uses this script to
start the process.
.PP
Each time a child terminates, \fBinit\fP records the fact and the reason
it died in \fB/var/run/utmp\fP and \fB/var/log/wtmp\fP,
provided that these files exist.
.SH CHANGING RUNLEVELS
After it has spawned all of the processes specified, \fBinit\fP waits
for one of its descendant processes to die, a powerfail signal, or until
it is signaled by \fBtelinit\fP to change the system's runlevel.  
When one of the above three conditions occurs, it re-examines
the \fB/etc/inittab\fP file.  New entries can be added to this file at
any time.  However, \fBinit\fP still waits for one of the above three
conditions to occur.  To provide for an instantaneous response, the
\fBtelinit Q\fP or \fBq\fP command can wake up \fBinit\fP to re-examine the
\fB/etc/inittab\fP file.
.PP
If \fBinit\fP is not in single user mode and receives a powerfail
signal (SIGPWR), it reads the file \fB/etc/powerstatus\fP. It then starts
a command based on the contents of this file:
.IP F(AIL)
Power is failing, UPS is providing the power. Execute the \fBpowerwait\fP
and \fBpowerfail\fP entries.
.IP O(K)
The power has been restored, execute the \fBpowerokwait\fP entries.
.IP L(OW)
The power is failing and the UPS has a low battery. Execute the
\fBpowerfailnow\fP entries.
.PP
If /etc/powerstatus doesn't exist or contains anything else then the
letters \fBF\fP, \fBO\fP or \fBL\fP, init will behave as if it has read
the letter \fBF\fP.
.PP
Usage of \fBSIGPWR\fP and \fB/etc/powerstatus\fP is discouraged. Someone
wanting to interact with \fBinit\fP should use the \fB/run/initctl\fP
control channel - see the source code of the \fBsysvinit\fP package
for more documentation about this.
.PP
When \fBinit\fP is requested to change the runlevel, it sends the
warning signal \s-1\fBSIGTERM\fP\s0 to all processes that are undefined
in the new runlevel.  It then waits 5 seconds before forcibly
terminating these processes via the \s-1\fBSIGKILL\fP\s0 signal.
Note that \fBinit\fP assumes that all these processes (and their
descendants) remain in the same process group which \fBinit\fP
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
.SH TELINIT
\fB/sbin/telinit\fP is linked to \fB/sbin/init\fP.  It takes a
one-character argument and signals \fBinit\fP to perform the appropriate
action.  The following arguments serve as directives to
\fBtelinit\fP:
.IP "\fB0\fP,\fB1\fP,\fB2\fP,\fB3\fP,\fB4\fP,\fB5\fP or \fB6\fP"
tell \fBinit\fP to switch to the specified run level.
.IP \fBa\fP,\fBb\fP,\fBc\fP
tell \fBinit\fP to process only those \fB/etc/inittab\fP file
entries having runlevel \fBa\fP,\fBb\fP or \fBc\fP.
.IP "\fBQ\fP or \fBq\fP"
tell \fBinit\fP to re-examine the \fB/etc/inittab\fP file.
.IP "\fBS\fP or \fBs\fP"
tell \fBinit\fP to switch to single user mode.
.IP "\fBU\fP or \fBu\fP"
tell \fBinit\fP to re-execute itself (preserving the state). No re-examining of 
\fB/etc/inittab\fP file happens. Run level should be one of
\fBSs0123456\fP
otherwise request would be silently ignored.
.PP
\fBtelinit\fP can tell \fBinit\fP 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 \fB-t\fP option.
.PP
\fBtelinit -e\fP tells \fBinit\fP to change the environment
for processes it spawns.
The argument of \fB-e\fP is either of the form \fIVAR\fP=\fIVAL\fP
which sets variable \fIVAR\fP to value \fIVAL\fP,
or of the form \fIVAR\fP
(without an equality sign)
which unsets variable \fIVAR\fP.
.PP
\fBtelinit\fP can be invoked only by users with appropriate
privileges.
.PP
The \fBinit\fP binary checks if it is \fBinit\fP or \fBtelinit\fP by looking
at its \fIprocess id\fP; the real \fBinit\fP's process id is always \fB1\fP.
From this it follows that instead of calling \fBtelinit\fP one can also
just use \fBinit\fP instead as a shortcut.
.\"}}}
.\"}}}
.SH ENVIRONMENT
\fBInit\fP sets the following environment variables for all its children:
.IP \fBPATH\fP
\fI/bin:/usr/bin:/sbin:/usr/sbin\fP
.IP \fBINIT_VERSION\fP
As the name says. Useful to determine if a script runs directly from \fBinit\fP.
.IP \fBRUNLEVEL\fP
The current system runlevel.
.IP \fBPREVLEVEL\fP
The previous runlevel (useful after a runlevel switch).
.IP \fBCONSOLE\fP
The system console. This is really inherited from the kernel; however
if it is not set \fBinit\fP will set it to \fB/dev/console\fP by default.
.SH BOOTFLAGS
It is possible to pass a number of flags to \fBinit\fP from the
boot monitor (eg. LILO). \fBInit\fP accepts the following flags:
.TP 0.5i
.B -s, S, single
Single user mode boot. In this mode \fI/etc/inittab\fP is
examined and the bootup rc scripts are usually run before
the single user mode shell is started.
.PP
.TP 0.5i
.B 1-5
Runlevel to boot into.
.PP
.TP 0.5i
.B -b, emergency
Boot directly into a single user shell without running any
other startup scripts.
.PP
.TP 0.5i
.B -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 \fBinit\fP 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.
.PP
.TP 0.5i
.BI "-z " xxx
The argument to \fB-z\fP is ignored. You can use this to expand the command
line a bit, so that it takes some more space on the stack. \fBInit\fP
can then manipulate the command line so that \fBps\fP(1) shows
the current runlevel.
.PP
.SH INTERFACE
Init listens on a \fIfifo\fP in /run, \fI/run/initctl\fP, for messages.
\fBTelinit\fP uses this to communicate with init. The interface is not
very well documented or finished. Those interested should study the
\fIinitreq.h\fP file in the \fIsrc/\fP subdirectory of the \fBinit\fP
source code tar archive.
.SH SIGNALS
Init reacts to several signals:
.TP 0.5i
.B SIGHUP
Has the same effect as \fBtelinit q\fP.
.PP
.TP 0.5i
.B SIGUSR1
On receipt of this signals, init closes and re-opens its control fifo,
\fB/run/initctl\fP.
.TP 0.5i
.B SIGINT
Normally the kernel sends this signal to init when CTRL-ALT-DEL is
pressed. It activates the \fIctrlaltdel\fP action.
.TP 0.5i
.B SIGWINCH
The kernel sends this signal when the \fIKeyboardSignal\fP key is hit.
It activates the \fIkbrequest\fP action.
\"{{{  Conforming to
.SH CONFORMING TO
\fBInit\fP is compatible with the System V init. It works closely
together with the scripts in the directories
\fI/etc/init.d\fP and \fI/etc/rc{runlevel}.d\fP.
If your system uses this convention, there should be a \fIREADME\fP
file in the directory \fI/etc/init.d\fP explaining how these scripts work.
.\"}}}
.\"{{{  Files
.SH FILES
.nf
/etc/inittab
/etc/initscript
/dev/console
/var/run/utmp
/var/log/wtmp
/run/initctl
.fi
.\"}}}
.\"{{{  Warnings
.SH WARNINGS
\fBInit\fP 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, \fBinit\fP can't
kill them and you may end up with two processes reading from one
terminal line.
.PP
On a Debian system, entering runlevel 1 causes all processes
to be killed except for kernel threads and the script that does
the killing and other processes in its session.
As a consequence of this, it isn't safe to return from runlevel 1
to a multi-user runlevel: daemons that were started in runlevel S
and are needed for normal operation are no longer running.
The system should be rebooted.
.\"}}}
.\"{{{  Diagnostics
.SH DIAGNOSTICS
If \fBinit\fP 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 \fB/etc/inittab\fP file
or the program for the entry is removed.
.\"}}}
.\"{{{  Author
.SH AUTHOR
Miquel van Smoorenburg (miquels@cistron.nl), initial manual
page by Michael Haardt (u31b3hs@pool.informatik.rwth-aachen.de).
.\"}}}
.\"{{{  See also
.SH "SEE ALSO"
.BR getty (1),
.BR login (1),
.BR sh (1),
.BR runlevel (8),
.BR shutdown(8),
.BR kill (1),
.BR inittab (5),
.BR initscript (5),
.BR utmp (5)
.\"}}}