init, telinit - process control initialization


/sbin/init [ -t sec ] [ 0123456SsQq ] /sbin/telinit [ -t sec ] [ 0123456sSQqabc ]



Init is the father 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 run level 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 run levels are defined in the /etc/inittab file. Init can be in one of eight run levels, 0\(en6 and S or s. The run level is changed by having a privileged user run /sbin/telinit, which sends appropriate signals to init, telling it which run level to change to.

After init is invoked as the last step of the kernel booting, it looks for the file /etc/inittab to see if there is an entry of the type initdefault (see inittab(5)). initdefault determines the initial run level of the system. If there is no such entry or no /etc/inittab at all, a run level has to be entered at the system console.

Run level S or s bring the system to single user mode and do not require an /etc/initttab file. In single user mode, /bin/sh is invoked on /dev/console.

When entering single user mode, init reads the console's ioctl(2) states from /etc/ If this file does not exist, init initializes the line at 9600 baud and with CLOCAL settings. When init leaves single user mode, it stores the console's ioctl settings in this file so it can re-use them for the next single-user session.

When entering a multi user mode 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 run level are processed.

When starting a new process, init first looks if 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 if 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 /sbin/telinit to change the system's run level. 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 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, special powerfail entries are invoked.

When init is requested to change the run level, it sends the warning signal SIGTERM to all processes that are undefined in the new run level. It then waits 5 seconds before forcibly terminating these processes via the kill signal SIGKILL. 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 /sbin/telinit:

0,1,2,3,4,5 or 6
tell /sbin/init to switch to the specified run level.

tell /sbin/init to process only those /etc/inittab file entries having run level a,b or c.

Q or q
tells /sbin/init to re-examine the /etc/inittab file.

S or s
tells /sbin/init to switch to single user mode.

/sbin/telinit can also tell init how much time it should wait between sending processes the TERM and the KILL signal; default this is 5 seconds, but it can be changed by the -t sec option.

/sbin/telinit can be invoked only by users with appropriate privileges.


Run levels 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 ment to use directly, but more for the scripts that are executed when entering runlevel 1.For more information on this, see the manpages for shutdown(1) 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 wonder, runlevels S and s are in fact the same. Internally they are aliases for the same runlevel - this is just a leftover from the systems the author used to use when writing sysvinit.




Init sets the following environment variables for all its children:

Defaults to /bin:/usr/bin:

As the name says. Useful to determine if a script runs directly from init.

The current system runlevel.

The previous runlevel (useful after a runlevel switch).


It is possible to pass a number of flags to init from the boot monitor (eg. LILO). Init accepts the following flags:

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.

Runlevel to boot into.

-b, emergency
Boot directly into a single user shell without running any other startup scripts.


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 systems 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.


If /sbin/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 (, initial manual page by Michael Haardt (


getty(1), login(1), sh(1), who(1), shutdown(1), kill(2), inittab(5), initscript(5), utmp(5)