Linux and Unix init, telinit command
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Process control initialization.
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).
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 and inittab.
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). 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 pack-
age 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.
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.
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.
FILES
/etc/inittab
/etc/initscript
/dev/console
/var/run/utmp
/var/log/wtmp
/dev/initctl
/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]
/sbin/telinit [
-t sec ] [ 0123456sSQqabcUu ]
| -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 shows the current runlevel. |
| 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.
Caution: 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.
Tip: In order to use this command you must have root access.
telinit 5
If Linux boot into a runlevel of 3 (command-line), running the above command would switch into runlevel 5 (GUI) mode.