INIT(8) Linux System Administrator's Manual INIT(8)
NAME
init, telinit - process control initialization
SYNOPSIS
/sbin/init [ 0123456Ss ]
/sbin/telinit [ -t sec ] [ 0123456sSQqabcUu ]
DESCRIPTION
Init
Init is the father 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 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 docu-
mented. 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 - this is just a leftover from the systems
the author used to use when writing sysvinit.
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 init-
tab(5)). The initdefault entry determines the initial run-
level 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/initttab file. In single user
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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 sys-
tems 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 power-
fail signal, or until it is signaled by telinit to change
the system's runlevel. When one of the above three condi-
tions 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 power-
fail signal, special powerfail entries are invoked.
When init is requested to change the runlevel, it sends
the warning signal SIGTERM to all processes that are unde-
fined in the new runlevel. It then waits 5 seconds before
forcibly terminating these processes via the SIGKILL sig-
nal. 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 appro-
priate action. The following arguments serve as direc-
tives to telinit:
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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 hap-
pens. 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:
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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 run-
ning any other startup scripts.
INTERFACE
Init listens on a fifo in /dev, /dev/initctl, for mes-
sages. 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/initrun-
lvl. 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
it's 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.
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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 cre-
ated 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 mes-
sage 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/init-
tab file or the program for the entry is removed.
AUTHOR
Miquel van Smoorenburg (miquels@cistron.nl), initial man-
ual page by Michael Haardt (u31b3hs@pool.informatik.rwth-
aachen.de).
SEE ALSO
getty(1), login(1), sh(1), who(1), shutdown(8), kill(1),
inittab(5), initscript(5), utmp(5)
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