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 SECONDS ] [ 0123456sSQqabcUu ]
/sbin/telinit [ -e VAR[=VAL] ]
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 init‐
tab(5)). This file usually has entries which cause init to spawn get‐
tys 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 (a.k.a. s). The runlevel
is changed by having a privileged user run telinit, which sends appro‐
priate signals to init, telling it which runlevel to change to.
Runlevels S, 0, 1, and 6 are reserved. Runlevel S is used to initial‐
ize the system on boot. When starting runlevel S (on boot) or runlevel
1 (switching from a multi-user runlevel) the system is entering ``sin‐
gle-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.
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.
For more information, 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.
Runlevels S and s are 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 ini‐
tial 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 initialize the system and do not require an /etc/init‐
tab file.
In single user mode, /sbin/sulogin is invoked on /dev/console.
When entering single user mode, init initializes the consoles stty set‐
tings to sane values. Clocal mode is set. Hardware speed and handshak‐
ing 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 pro‐
cessed.
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 power‐
wait 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 argu‐
ment 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 Ss0123456 otherwise request would be silently ignored.
telinit can 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 option.
telinit -e tells init to change the environment for processes it
spawns. The argument of -e is either of the form VAR=VAL which sets
variable VAR to value VAL, or of the form VAR (without an equality
sign) which unsets variable VAR.
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 fol‐
lows 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 /bin:/usr/bin:/sbin:/usr/sbin
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" environ‐
ment 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 com‐
mand 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 doc‐
umented 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{run‐
level}.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 pro‐
cesses change their group, init can't kill them and you may end up with
two processes reading from one terminal line.
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
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 com‐
mand 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 ALSOgetty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), init‐
tab(5), initscript(5), utmp(5)
29 Jul 2004 INIT(8)
