INIT(8) manual page
Table of Contents
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: 0en6 and S (a.k.a. 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 S, 0, 1, and
6 are reserved. Runlevel S is used to initialize the system on boot. When
starting runlevel S (on boot) or runlevel 1 (switching from a multi-user
runlevel) the system is entering ‘‘single-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.
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 initialize
the system 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 /run/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 /run, /run/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, /run/initctl.
- 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.
/etc/inittab
/etc/initscript
/dev/console
/var/run/utmp
/var/log/wtmp
/run/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.
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.
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)
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