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 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 sig-
nals 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 "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.
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 val-
ues. 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 instanta-
neous 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 communi-
cate 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 acti-
vates 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 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 (), initial manual page by Michael Haardt
().
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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