init, telinit - process control initialization
/sbin/init [ -a ] [ -s ] [ -b ] [ -z
xxx ] [ 0123456Ss ]
/sbin/telinit [ -t SECONDS ] [ 0123456sSQqabcUu ]
/sbin/telinit [ -e VAR[=VAL] ]
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.
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 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.
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.
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 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.
/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 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 follows that instead of calling
telinit one can also just use init instead as a shortcut.
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.
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.
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.
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.
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.
/etc/inittab
/etc/initscript
/dev/console
/var/run/utmp
/var/log/wtmp
/dev/initctl
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.
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.
Miquel van Smoorenburg (miquels@cistron.nl), initial manual page by Michael
Haardt (u31b3hs@pool.informatik.rwth-aachen.de).
getty(1), login(1), sh(1), runlevel(8),
shutdown(8), kill(1), inittab(5), initscript(5),
utmp(5)