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 dont use them. In case youre curious, runlevels S and s are in fact the same. Internally they are aliases for the same runlevel.
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.
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.|
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.
|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 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 inits 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_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.|
|-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.|
|Boot directly into a single user shell without running any other startup scripts.|
|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.|
|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.|
|Has the same effect as telinit q.|
|On receipt of this signals, init closes and re-opens its control fifo, /dev/initctl. Useful for bootscripts when /dev is remounted.|
|Normally the kernel sends this signal to init when CTRL-ALT-DEL is pressed. It activates the ctrlaltdel action.|
The kernel sends this signal when the KeyboardSignal key is hit.
It activates the kbrequest action.
/etc/inittab /etc/initscript /dev/console /var/run/utmp /var/log/wtmp /dev/initctl