init(1M) UNIX System V(Essential Utilities) init(1M)
NAME
init, telinit - process control initialization
SYNOPSIS
/sbin/init [0123456SsQqabc]
/sbin/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)].
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 is defined in /etc/inittab. init can be in one
of eight run levels, 0-6 and S or s (run levels S and s are identical).
The run level changes when a privileged user runs /sbin/init. This
user-spawned init sends appropriate signals to the original init spawned
by the operating system when the system was booted, telling it which run
level to change to.
The following are the arguments to init.
0 shut the machine down so it is safe to remove the power.
Have the machine remove power if it can.
1 put the system in system administrator mode. All file
systems are mounted. Only a small set of essential kernel
processes are left running. This mode is for administrative
tasks such as installing optional utility packages. All
files are accessible and no users are logged in on the
system.
2 put the system in multi-user mode. All multi-user
environment terminal processes and daemons are spawned.
This state is commonly referred to as the multi-user state.
3 start the remote file sharing processes and daemons. Mount
and advertise remote resources. Run level 3 extends multi-
user mode and is known as the remote-file-sharing state.
4 is available to be defined as an alternative multi-user
environment configuration. It is not necessary for system
operation and is usually not used.
5 Stop the UNIX system and go to the firmware monitor.
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init(1M) UNIX System V(Essential Utilities) init(1M)
6 Stop the UNIX system and reboot to the state defined by the
initdefault entry in /etc/inittab.
a,b,c process only those /etc/inittab 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 re-examine /etc/inittab.
S,s enter single-user mode. When this occurs, the terminal
which executed this command becomes the system console.
This is the only run level that doesn't require the
existence of a properly formatted /etc/inittab file. If
this file does not exist, then by default the only legal run
level that init can enter is the single-user mode. When the
system comes up to S or s, file systems for users' files are
not mounted and only essential kernel processes are running.
When the system comes down to S or s, all mounted file
systems remain mounted, and all processes started by init
that should only be running in multi-user mode are killed.
In addition, any process that has a utmp entry will be
killed. This last condition insures that all port monitors
started by the SAC are killed and all services started by
these port monitors, including ttymon login services, are
killed. Other processes not started directly by init will
remain running. For example, cron remains running.
When a UNIX system is booted, init is invoked and the following occurs.
First, init looks in /etc/inittab for the initdefault entry [see
inittab(4)]. If there is one, init will usually use the run level
specified in that entry as the initial run level to enter. If there is
no initdefault entry in /etc/inittab, init requests that the user enter a
run level from the virtual system console. If an S or s is entered, init
goes to the single-user state. In the single-user state the virtual
console terminal is assigned to the user's terminal and is opened for
reading and writing. The command /sbin/su is invoked and a message is
generated on the physical console saying where the virtual console has
been relocated. 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
the /etc/inittab file does not exist.
If a 0 through 6 is entered, init enters the corresponding run level.
Run levels 0, 5, and 6 are reserved states for shutting the system down.
Run levels 2, 3, and 4 are available as multi-user operating states.
If this is the first time since power up that init has entered a run
level other than single-user state, init first scans /etc/inittab for
boot and bootwait entries [see inittab(4)]. These entries are performed
before any other processing of /etc/inittab takes place, providing that
the run level entered matches that of the entry. In this way any special
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init(1M) UNIX System V(Essential Utilities) init(1M)
initialization of the operating system, such as mounting file systems,
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
that run level.
To spawn each process in /etc/inittab, 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 /etc/inittab, init waits for
one of its descendant processes to die, a powerfail signal, or a signal
from another init or telinit process to change the system's run level.
When one of these conditions occurs, init re-examines /etc/inittab. New
entries can be added to /etc/inittab at any time; however, init still
waits for one of the above three conditions to occur before re-examining
/etc/inittab. To get around this, init Q or init q command wakes init to
re-examine /etc/inittab immediately.
When init comes up at boot time and whenever the system changes from the
single-user state to another run state, init sets the ioctl(2) states of
the virtual console to those modes saved in the file /etc/ioctl.syscon.
This file is written by init whenever the single-user state 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 five seconds before forcibly terminating these processes via
the kill signal (SIGKILL).
When init receives a signal telling it that a process it spawned has
died, it records the fact and the reason it died in /var/adm/utmp and
/var/adm/wtmp if it exists [see who(1)]. A history of the processes
spawned is kept in /var/adm/wtmp.
If init receives a powerfail signal (SIGPWR) it scans /etc/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.
telinit
telinit, which is linked to /sbin/init, is used to direct the actions of
init. It takes a one-character argument and signals init to take the
appropriate action.
FILES
/etc/inittab
/var/adm/utmp
/var/adm/wtmp
/etc/ioctl.syscon
/dev/console
SEE ALSO
ttymon(1M), shutdown(1M), inittab(4), utmp(4), utmpx(4), termio(7).
login(1), sh(1), stty(1), who(1) in the User's Reference Manual.
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init(1M) UNIX System V(Essential Utilities) init(1M)
kill(2) in the Programmer's Reference Manual.
DIAGNOSTICS
If init finds that it is respawning an entry from /etc/inittab more than
ten times in two 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 five
minutes has elapsed or it receives a signal from a user-spawned init or
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 /etc/inittab.
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.
NOTES
init and telinit can be run only by a privileged user.
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.
If a default state is not specified in the initdefault entry in
/etc/inittab, state 6 is entered. Consequently, the system will loop,
that is, it will go to firmware and reboot continuously.
If the utmp file cannot be created when booting the system, the system
will boot to state ``s'' regardless of the state specified in the
initdefault entry in /etc/inittab. This can happen if the /var
filesystem is not accessible.
In the event of a file table overflow condition, init uses the file
descriptor associated with /etc/inittab that was retained from the last
time it accessed the file. This prevents init from going into single
user mode when it cannot obtain a file descriptor to open /etc/inittab.
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