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 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 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 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
/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 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 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/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.
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.
