From Linux Man Pages
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 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 sys-
tem, 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, /sbin/sulogin is invoked 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 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 pro-
cess 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 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 /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 com-
mand 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/init.d/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 pro-
cesses 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.
RELATED
init.d(7), getty(1), login(1), sh(1), runlevel(8), shutdown(8), kill(1), inittab(5), initscript(5), utmp(5)
CATEGORY