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 spawngettys 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 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.
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 initdefaultentry determines the initial runlevel of the system. If there is no such entry (or no /etc/inittabat 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 reads the console’s ioctl(2) states from /etc/ioctl.save. 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 for the first time, init performs the boot and bootwaitentries 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.
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 qcommand 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:
- Power is failing, UPS is providing the power. Execute the powerwait and powerfailentries.
- The power has been restored, execute the powerokwait entries.
- 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 withinit 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.
/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.
- 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.