NAME

init, telinit − process control initialization

SYNOPSIS

/etc/init [0123456SsQq]

/etc/telinit [0123456sSQqabc]

DESCRIPTION

Init

Init is a general process spawner. Its primary role is to create processes from a script stored in the file /etc/inittab (see inittab(4)). This file usually has init spawn getty’s on each line that a user may log in on.  It also controls autonomous processes required by any particular system.

Init considers the system to be in a run-level at any given time.  A run-level can be viewed as a software configuration of the system where each configuration allows only a selected group of processes to exist.  The processes spawned by init for each of these run-levels is defined in the inittab file.  Init can be in one of eight run-levels, 0−6 and S or s.  The run-level is changed by having a privileged user run /etc/init (which is linked to /etc/telinit). This user spawned init sends appropriate signals to the orginal init spawned by the operating system when the system was rebooted, telling it which run-level to change to.

Init is invoked inside the UNIX System as the last step in the boot procedure.  The first thing init does is to look for /etc/inittab and see if there is an entry of the type initdefault (see inittab(4)). If there is, init uses the run-level specified in that entry as the initial run-level to enter.  If this entry is not in inittab or inittab is not found, init requests that the user enter a run-level from the virtual system console, /dev/syscon.  If an S (s) is entered, init goes into the SINGLE USER level.  This is the only run-level that doesn’t require the existence of a properly formatted inittab file.  If /etc/inittab doesn’t exist, then by default the only legal run-level that init can enter is the SINGLE USER level.  In the SINGLE USER level the virtual console terminal /dev/syscon is opened for reading and writing and the command /bin/su is invoked immediately.  To exit from the SINGLE USER run-level one of two options can be elected. First, if the shell is terminated (via an end-of-file), init will reprompt for a new run-level.  Second, the init or telinit command can signal init and force it to change the run-level of the system. 

When attempting to boot the system, failure of init to prompt for a new run-level may be due to the fact that the device /dev/syscon is linked to a device other than the physical system teletype (/dev/systty).  If this occurs, init can be forced to relink /dev/syscon by typing a delete on the system teletype which is co-located with the processor. 

When init prompts for the new run-level, the operator may only enter one of the digits 0 through 6 or the letters S or s.  If S is entered init operates as previously described in SINGLE USER mode with the additional result that /dev/syscon is linked to the user’s terminal line, thus making it the virtual system console.  A message is generated on the physical console, /dev/systty, saying where the virtual terminal has been relocated. 

When init comes up initially and whenever it switches out of SINGLE USER state to normal run states, it sets the ioctl(2) states of the virtual console, /dev/syscon, to those modes saved in the file /etc/ioctl.syscon.  This file is written by init whenever SINGLE USER mode is entered.  If this file doesn’t exist when init wants to read it, a warning is printed and default settings are assumed. 

If a 0 through 6 is entered init enters the corresponding run-level. Any other input will be rejected and the user will be re-prompted. If this is the first time init has entered a run-level other than SINGLE USER, init first scans inittab for special entries of the type boot and bootwait. These entries are performed, providing the run-level entered matches that of the entry before any normal processing of inittab takes place.  In this way any special initialization of the operating system, such as mounting file systems, can take place before users are allowed onto the system.  The inittab file is scanned to find all entries that are to be processed for that run-level.

Run-level 2 is usually defined by the user to contain all of the terminal processes and daemons that are spawned in the multi-user environment. 

In a multi-user environment, the inittab file is usually set up so that init will create a process for each terminal on the system. 

For terminal processes, ultimately the shell will terminate because of an end-of-file either typed explicitly or generated as the result of hanging up.  When init receives a child death 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(1)). A history of the processes spawned is kept in /etc/wtmp if such a file exists. 

To spawn each process in the inittab file, init reads each entry and for each entry which should be respawned, it forks a child process.  After it has spawned all of the processes specified by the inittab file, init waits for one of its descendant processes to die, a powerfail signal, or until init is signaled by init or telinit to change the system’s run-level.  When one of the above three conditions occurs, init re-examines the inittab file.  New entries can be added to the inittab file at any time; however, init still waits for one of the above three conditions to occur.  To provide for an instantaneous response the init Q or init q command can wake init to re-examine the inittab file. 

If init receives a powerfail signal (SIGPWR) and is not in SINGLE USER mode, it scans inittab for special powerfail entries.  These entries are invoked (if the run-levels permit) before any further processing takes place.  In this way init can perform various cleanup and recording functions whenever the operating system experiences a power failure.  It is important to note that the powerfail entries should not use devices that must first be initialized (e.g. dzb lines) after a power failure has occurred. 

When init is requested to change run-levels (via telinit), init sends the warning signal (SIGTERM) to all processes that are undefined in the target run-level. Init waits 20 seconds before forcibly terminating these processes via the kill signal (SIGKILL). 

Telinit

Telinit, which is linked to /etc/init, is used to direct the actions of init. It takes a one character argument and signals init via the kill system call to perform the appropriate action.  The following arguments serve as directives to init.

0−6 tells init to place the system in one of the run-levels 0−6. 

a,b,c tells init to process only those /etc/inittab file entries having the a, b or c run-level set. 

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

s,S tells init to enter the single user environment.  When this level change is effected, the virtual system teletype, /dev/syscon, is changed to the terminal from which the command was executed. 

Telinit can only be run by someone who is super-user or a member of group sys. 

FILES

/etc/inittab
/etc/utmp
/etc/wtmp
/etc/ioctl.syscon
/dev/syscon
/dev/systty

SEE ALSO

getty(1M), login(1), sh(1), who(1), kill(2), inittab(4), utmp(4). 

DIAGNOSTICS

If init finds that it is continuously respawning an entry from /etc/inittab more than 10 times in 2 minutes, it will assume that there is an error in the command string, and 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 from a user init (telinit). This prevents init from eating up system resources when someone makes a typographical error in the inittab file or a program is removed that is referenced in the inittab. 

Version 2.1  —  January 02, 1985