/bin/telinit [ 0123456SsQqabc ]
At any given time, the system is in one of eight possible run-levels. A run-level is a software configuration of the system under which only a selected group of processes exist. The processes spawned by init for each of these run-levels are defined in /etc/inittab. init can be in one of the run-levels: 0 through 6 or S. (Run-levels S and s are identical.)
root can use either init or telinit to change the operating system's run-level by signaling the original init process (PID 1). (Note that telinit is a link to init.)
If a 0 through 6 is entered, init enters the corresponding run-level. Note that the run-levels 0, 1, 5, and 6 are reserved states for shutting the system down; run-levels 2, 3, and 4 are available as normal operating states.
If the file /etc/default/boot (see boot(F)) contains the string MAPKEY=YES, init invokes mapkey(M) to map the console keyboard. If the call to mapkey succeeds, the console is set to 8-bits no parity. If the call fails, and the string SERIAL8=YES appears in /etc/default/boot, a serial console device is assumed and set to 8-bits no parity. For additional information on keywords, see bootstring(HW) and boot(HW).
init and telinit accept one of the following arguments:
Note that init 0 should not be used if you have a USB keyboard attached to the console. During an init 0, the USB stack gets shut down and the system then waits for a keyboard keypress interrupt before rebooting. Since USB is gone, there's no way for a keypress on the USB keyboard to get serviced, and the system does not respond. If this occurs, you must either press the hardware reset button or power cycle the system.
Note that if the shell is terminated (via an end-of-file), init will only reinitialize to single-user mode if the /etc/inittab file does not exist.
If init enters a run-level other than single-user mode for the first time since the system last boooted, it runs entries in /etc/inittab which specify this run-level (or none) in their rstate field and which have boot and bootwait set in their action field. In this way, any special initialization of the operating system, such as mounting filesystems, can take place before users are allowed onto the system. init then scans /etc/inittab and executes all other entries that are to be processed for the new run-level.
In a multiuser environment, /etc/inittab is set up so that init will create a getty process for each terminal that the administrator sets up to respawn.
To spawn each process in /etc/inittab, init reads each entry and for each entry that should be respawned, it forks a child process. init spawns each process by forking a shell to run the job in. To set up the environment for this shell, init uses the /etc/initscript file which contains the definitions of some global variables, for example, TZ, HZ, and PATH. (For more information about /etc/initscript, see initscript(ADM).)
After init has spawned all of the processes specified by /etc/inittab, it waits for:
When init comes up at boot time and whenever the system changes from single-user mode to another run-level, init sets the ioctl(S) states of the virtual console to those modes saved in the file /etc/ioctl.syscon. This file is written by init whenever single-user mode is entered.
When a run-level change request is made, init sends the warning signal (SIGTERM) to all processes that are undefined in the target run-level. init waits 5 seconds before forcibly terminating these processes via the kill signal (SIGKILL).
The shell running on each terminal will terminate when the user types an end-of-file or hangs up. When init receives a signal telling it that a process it spawned has died, it records the fact and the reason it died in /etc/utmp and /etc/wtmp if it exists (see who(C)). A history of the processes spawned is kept in /etc/wtmp.
If init receives a power failure signal (SIGPWR),
it scans /etc/inittab for special entries of the type
powerfail or powerwait.
These entries are invoked (if the run-level permits this)
before any further processing takes place.
In this way init can perform various cleanup
and recording functions during the powerdown of the operating system.
When attempting to boot the system, failure of init to prompt for a new run-level may be because the virtual system console is linked to a device other than the physical system console.
The S or s state must not be used indiscriminately in the /etc/inittab file. A good rule to follow when modifying this file is to avoid adding this state to any line other than the initdefault.
The assignment of the console device may seem confusing at first. Whenever the system is rebooted, the first boot up messages will be displayed on the ``normal'' system console (tty01), then the prompt for going multiuser will be displayed on the the tty from which init S was last invoked, which could be any tty on the system. The system console device (/dev/syscon) remains linked to the tty from which the last init S is invoked. Rebooting the system does not reset this to tty01.
If the /etc/initscript file is not present, init will print a warning on the console and spawn the job without setting up the global environment.
The change to /etc/gettydefs, as described in the
``Limitations'' section of the
manual page, will permit terminals to pass 8 bits
to the system as long as the system is in
multiuser state (run-level greater than 1).
When the system changes to single-user state, the getty
is killed and the terminal attributes are lost.
To permit a terminal to pass 8 bits to the system in single-user state,
after you are in single-user state, type:
stty -istrip cs8
The /etc/TIMEZONE file should exist.
/etc/initscript tries to execute
this file to set the correct TZ variable for the system.
AT&T SVID Issue 2.
telinit is not part of any currently supported standard.