gno/usr.man/man8/init.8

.\"
.\" $Id: init.8,v 1.1 1997/02/28 05:12:12 gdr Exp $
.\"
.TH INIT 8 "27 October 1993" GNO "System Administration"
.SH NAME
\fBinit\fR \- start and kill processes
.SH DESCRIPTION
.B init
is the component of the UNIX operating system that is responsible
for creating processes once the system has finished booting and for
maintaining those processes, including killing them.
.sp
At any time, the system is in one of 12 states or run levels. At
each run level, a different group of processes is allowed to run.
Generally, when the run level changes, init kills the processes which
it created under the old run level, and creates a new set for the new
run level.
.sp
Run levels are identified with single characters in the set
.RB [ bs0123456789 ].
.BR B 
and 
.BR S
are also accepted as equivalents to 
.BR b
and 
.BR s .
Some of these run levels have special meaning and are listed below. 
Except for 
.BR b ,
all run levels can have their meaning changed via 
.IR runl
commands in 
.BR /etc/inittab .
.sp
Except for levels 
.BR 0 , 
.BR 5 
and 
.BR 6 ,
the actual state of the machine after
.B init
switches to a level is completely defined by the processes which run,
which are configurable in
.BR /etc/inittab .
It is therefore possible to use all but those three run levels for
purposes other than their intended one. Their meaning as described
here are the conventional meaning. It is encouraged that the
conventions be followed.
.IP "\fBb\fR (boot)"
This is the run level that is selected by default. Any
initialization processes should be run under this level.
By default, init switches to run level 
.BR s
(single user) after
starting processes at this level. The
.B bootlevel
command in
.I /etc/rchost
may be used to modify this behavior.
.IP \fB0\fR
This is the system shutdown level. Switching to this level will cause
the system to shut down after any level 
.BR 0
processes have been run.
.IP \fB1\fR
This run level is defined as the administration state.
.IP "\fB2\fR, \fB3\fR, \fB4\fR"
These are multi user run levels.
Level 
.BR 2
is standard. Level 
.BR 3
is multi user with networking, and level 
.BR 4
is an alternate multi user set up.
.IP \fB5\fR
Under UNIX, this is the firmware monitor run level. Under GNO, switching
to this level causes GNO to quit. The disparity arises from the fact
that there is no such thing as quitting UNIX.
.IP \fB6\fR
Switching to this level causes the system to reboot.
.IP \fBs\fR
This is the single user run level.
.IP "\fB7\fR, \fB8\fR"
These are local run levels. Any local terminals are active, as well as
any local area networks (level
.BR 8 ),
but no dialups are active and no
modem based networking is available.
.IP \fB9\fR
This level is undefined.
.SH SIGNALS
The following signals can be sent to init to force certain conditions.
Note that these are intended as low level commands;
.BR init (1)
should be used to issue these signals.
.IP \fBSIGHUP\fR
Causes init to reread 
.BR /etc/inittab ,
kill processes that have been removed from it, and start processes that 
were newly added.
.sp 1
.BR init
depends on the process names (see
.BR inittab (5))
to compare the old copy of the table with the new one.
.IP \fBSIGTERM\fR
Causes 
.BR init
to switch to the kill default run level. This is 
.BR 0 
unless it is changed in 
.BR /etc/inittab .
Normally, the effect is a system shutdown.
.IP \fBSIGUSR1\fR
Causes init to try to receive a message (see
.BR procrecv (2))
and act on it. The message is typically sent by
.BR init (1)
and contains status inquiries or run level change requests.
.IP \fBSIGUSR2\fR
Causes 
.BR init
to exit its
.BR sigpause (2)
and check for any pending events (which is useful if a timing bug has caused 
.BR init
to go to sleep while it had pending events).
.SH NOTES
Under real UNIX, there are fewer run levels.
.LP
When switching to some run levels, 
.BR init 
automatically issues a 
.I runl
command to carry out the apropriate action for that run level. 
This action can be overridden with another 
.I runl 
command, possibly 
.BR n
to cancel any special action at all.
.LP
If 
.BR init 
becomes idle and has no children, it switch to the idle run level,
which is 
.BR 5
by default.
.LP
If a LOG_EMERG message is received by
.BR syslogd (8),
.BR init
will switch to the panic run level, 
.BR 6
by default.
.LP
When a child of 
.BR init 
dies, any entry it has in the
.BR utmp (5)
database will be removed before any other action is taken. Also, stray bogus
entries are removed whenever they are noticed.
.LP
At startup time,
.BR init
tries to read the file
.BR /etc/rchost
and set certain system globals according to the information in that file.
.SH FILES
.BR /etc/inittab
.br
.BR /usr/sbin/initd
.br
.BR /var/adm/utmp
.br
.BR /var/adm/wtmp
.br
.BR /etc/rchost
.SH "SEE ALSO"
.LP
.BR init (1),
.BR inittab (5),
.BR rchost (5),
.BR utmp (5),
.BR getty (8),
.BR syslogd (8)
.SH AUTHOR
Phillip Vandry, <vandry@cam.org>