init(1M) init(1M)
NAME
init - process control initialization
SYNOPSIS
/usr/sbin/init [0123456SsQqabc]
DESCRIPTION
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
/usr/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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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 /usr/sbin/su is
invoked and a message is generated on the physical console saying
where the virtual console has been relocated. Use init 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.
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init(1M) init(1M)
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 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 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.
FILES
/etc/inittab
/var/adm/utmp
/var/adm/wtmp
/etc/ioctl.syscon
/dev/console
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init(1M) init(1M)
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.
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 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 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.
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