.TH INIT 8
.UC
.SH NAME
init \- process control initialization
.SH SYNOPSIS
.B /etc/init
.SH DESCRIPTION
.I Init
is invoked inside UNIX as the last step in the boot procedure.
If this is a boot from software (automatic or from
.IR reboot (8),
it normally then runs the automatic reboot sequence as described in
.IR reboot (8)),
and if this succeeds, begins multi-user operation.
If the reboot fails, it commences ``special session'' operation on a limited
number of terminals (see below).  It is possible to pass parameters
from the boot program to
.I init
so that single user operation is commenced immediately.
When such single user operation is terminated by killing the single-user
shell (i.e. by hitting ^D),
.I init
runs
.I /etc/rc
without the reboot parameter.
This command file
performs housekeeping operations
such as removing temporary files,
mounting file systems, and starting
daemons.
.PP
In multi-user operation, 
.I init's
role is to create a process for each
terminal port on which a user may log in.
To begin such operations, it reads the file
.I /etc/ttys
and
forks several times to create a process
for each terminal specified in the file.
Each of these processes opens the appropriate terminal
for reading and writing.  These channels thus
receive file descriptors 0, 1 and 2, the standard input and
output and the diagnostic output.
Opening the terminal will usually involve a delay,
since the
.IR open ""
is not completed until someone
is dialed up and carrier established on the channel.
If a terminal exists but an error occurs when trying to open the terminal
.I init
complains by writing a message to the system console;
the message is repeated every 10 minutes for each such terminal
until the terminal is shut off in \fI/etc/ttys\fP and init notified (by
a hangup, as described below), or the terminal becomes accessible
(init checks again every minute).
After an open succeeds,
.I /etc/getty
is called with argument as specified by the second character of
the
.I ttys
file line.
.I Getty
reads the user's name and invokes
.I login
to log in the user and execute the Shell.
.PP
Ultimately the Shell will terminate
because of an end-of-file either
typed explicitly or generated as a result of hanging up.
The main path of
.IR init ,
which has been waiting
for such an event,
wakes up and removes the appropriate entry from the
file
.IR utmp ,
which records current users, and
makes an entry in
.IR /usr/adm/wtmp ,
which maintains a history
of logins and logouts.
The
.I wtmp
entry is made only if a user logged in successfully on the line.
Then the appropriate terminal is reopened and
.I getty
is
reinvoked.
.PP
.I Init
catches the
.I hangup
signal (signal SIGHUP) and interprets it to mean that
the file
.I /etc/ttys
should be read again.
The Shell process on each line which used to be active
in
.I ttys
but is no longer there is terminated;
a new process is created for each added line;
lines unchanged in the file are undisturbed.
Thus it is possible to drop or add phone lines without
rebooting the system by changing the
.I ttys
file and sending a
.I hangup
signal to the
.I init
process: use `kill \-HUP 1.'
.PP
.I Init
will terminate multi-user operations and resume single-user mode
if sent a terminate (TERM) signal, i.e. ``kill \-TERM 1''.
If there are processes outstanding which are deadlocked (due to
hardware or software failure),
.I init
will not wait for them all to die (which might take forever), but
will time out after 30 seconds and print a warning message.
.PP
.I Init
will cease creating new
.IR getty 's
and allow the system to slowly die away, if it is sent a quit signal (QUIT)
signal, i.e. ``kill \-QUIT 1''.  A later hangup will resume full
multi-user operations, or a terminate will initiate a single user shell.
This hook is used by
.IR reboot (8).
.PP
``Special session'' is similar to multi-user operation,
except that only root may log in, and a only a subset of the normal
lines are used (those with `2' or `3' in the first position of their
entries in \fI/etc/ttys\fP).
.I Init
enters this mode if an automatic file system check fails at boot time,
allowing the necessary file system repairs to be made.
When
.I init
then receives an interrupt signal (INT), it shuts down, runs \fI/etc/rc\fP,
then comes up for normal multi-user operations.
An interrupt signal from multi-user mode shuts down the system,
then begins special session.
.PP
.I Init's
role is so critical that if it dies, the system will reboot itself
automatically.
If, at bootstrap time, the
.I init
process cannot be located, the system will print a message and
loop in user mode at location 06.
.SH DIAGNOSTICS
Diagnostic messages from \fIinit\fP are printed on the system console.
.LP
\fBinit: \fP\fItty\fP\|\fB: cannot open\fP.  A terminal which is turned
on in the \fIttys\fP file cannot be opened, likely because the requisite
lines are either not configured into the system or the associated
special files in /dev do not exist.
.LP
\fBWARNING: Something is hung (won't die); ps axl advised\fP.  A process
is hung and could not be killed when the system was shutting down.
This is usually caused by a process
which is stuck in a device driver due to a persistent device error condition.
.SH FILES
.ta 2i
/dev/console	for diagnostic messages
.br
/dev/tty??
.br
/etc/utmp
.br
/usr/adm/wtmp
.br
/etc/ttys
.br
/etc/rc	script run when going multiuser
.SH "SEE ALSO"
kill(1), login(1), sh(1), ttys(5), crash(8), getty(8), rc(8), reboot(8),
shutdown(8)