4BSD/usr/man/man8/init.8

.TH INIT 8
.UC 4
.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.
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 single user operation by giving
the super-user a shell on the console.  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.
Then
.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.
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'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 loop in user mode at location
0x13.
.SH FILES
/dev/console, /dev/tty?, /etc/utmp, /usr/adm/wtmp, /etc/ttys, /etc/rc
.SH "SEE ALSO"
login(1), kill(1), sh(1), ttys(5), crash(8), getty(8), rc(8), reboot(8)