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INIT(8) Linux System Administrator’s Manual INIT(8)
NAME
init, telinit - process control initialization
SYNOPSIS
/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
/sbin/telinit [ -t sec ] [ 0123456sSQqabcUu ]
DESCRIPTION
Init
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.
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 run-
levels are defined in the /etc/inittab file. Init can be in one of eight run-
levels: 0â€â€“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 "tradi-
tional" 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.
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, 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 handshaking 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 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 pow-
erfail 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 sig-
nals 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
Has the same effect as telinit q.
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
/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 termi-
nal 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 AT cistron.nl), initial manual page by Michael Haardt
(u31b3hs AT pool.de).
SEE ALSO
getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), inittab(5),
initscript(5), utmp(5)
18 April 2003 INIT(8)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 01:38 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)