| INIT(8) | Linux System Administrator's Manual | INIT(8) |
init, telinit - process control initialization
/sbin/init [ -a ] [ -s ] [ -b ] [
-z xxx ] [ 0123456Ss ]
/sbin/init [ --version ]
/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 (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 (reload) 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:
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 initctl manual page 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 3 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:
telinit can tell init how long it should wait between sending processes the SIGTERM and SIGKILL signals. The default is 3 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:
It is possible to pass a number of flags to init from the boot monitor (eg. LILO or GRUB). Init accepts the following flags:
Init listens on a fifo in /dev, /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:
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), initctl(5), inittab(5), initscript(5), utmp(5)
| 29 Jul 2004 |