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 information 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.  This user-spawned init sends appropriate signals to the original 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.  First init looks in /etc/inittab for the initdefault entry (see inittab(4)). If there is one, init uses the run-level specified in that entry as the initial run-level to enter.  If this entry is not in /etc/inittab, init requests that the user enter a run-level from the virtual system console, /dev/console.  If an S or an s is entered, init goes into the SINGLE USER state.  This is the only run-level that doesn’t require the existence of a properly formatted /etc/inittab file.  If it doesn’t exist, then by default the only legal run-level that init can enter is the SINGLE USER state.  In the SINGLE USER state the virtual console terminal /dev/console is opened for reading and writing and the command /bin/su is invoked immediately.  To exit from the SINGLE USER state, use either init or telinit, to signal init to change the run-level of the system.  Note that if the shell is terminated (via an end-of-file), init will only re-initialize to the SINGLE USER state. 

If init fails to prompt for a new run-level when attempting to boot the system, it may be the result of the device /dev/console being linked to a device other than the physical system console (/dev/contty).  If this occurs, init can be forced to relink /dev/console if you type a delete on the physical system console. 

When init prompts for the new run-level, the operator may enter only one of the digits 0 through 6 or the letters S or s.  If S or s is entered, init operates as previously described in the SINGLE USER state with the additional result that /dev/console is linked to the user’s terminal line, thus making it the virtual system console.  A message is generated on the physical console, /dev/contty, 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/console, to those modes saved in the file /etc/ioctl.syscon.  This file is written by init whenever the SINGLE USER state is entered. 

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. Note that, on the 3B2 Computer, the run-levels 0, 1, 5, and 6 are reserved states for shutting the system down; the run-levels 2, 3, and 4 are available as normal operating states. 

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 defined to contain all of the terminal processes and daemons that are spawned in the multi-user environment.  Hence, it is commonly referred to as the MULTI-USER state.  Run-level 3 is defined to start up remote file sharing processes and daemons as well as mount and advertise remote resources.  So, run-level 3 extends multi-user mode and is known as the Remote File Sharing state.  Run-level 4 is available to be defined as an alternative multi-user environment configuration, however, it is not necessary for system operation and is usually unused. 

In a MULTI-USER environment, the inittab file is set up so that init will create a process for each terminal on the system that the administrator sets up to respawn. 

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 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. 

To spawn each process in the inittab file, init reads each entry and for each entry that 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 these 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 get around this, init Q or init q command wakes init to re-examine the inittab file immediately. 

If init receives a powerfail signal (SIGPWR) it scans inittab for special entries of the type powerfail and powerwait. 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 during the powerdown of the operating system.  Note that in the SINGLE-USER state only powerfail and powerwait entries are executed. 

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 5 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.  These are pseudo-states, which may be defined to run certain commands, but which do not cause the current run-level to change. 

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/console, is changed to the terminal from which the command was executed. 

FILES

/etc/inittab
/etc/utmp
/etc/wtmp
/etc/ioctl.syscon
/dev/console
/dev/contty

SEE ALSO

getty(1M), inittab(4), kill(2), login(1), sh(1), termio(7), utmp(4), who(1). 

DIAGNOSTICS

If init finds that it is 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 in the entry, and generate an error message on the system console.  It will then refuse to respawn this entry until either 5 minutes has elapsed or it receives a signal from a user-spawned 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. 

WARNINGS

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

BUGS

Attempting to relink /dev/console with /dev/contty by typing a delete on the system console does not work. 

September 02, 1992