V4/man/man7/init.7

.th INIT VII 6/15/72
.sh NAME
init  \*-  process control initialization
.sh SYNOPSIS
.bd /etc/init
.sh DESCRIPTION
.it Init
is invoked inside UNIX as the last step in the boot procedure.
Generally its role is to create a process for each
typewriter on which a user may log in.
.s3
First,
.it init
checks
to see if the console switches contain 173030.
(This number is likely to vary between
systems.)
If so, the console typewriter \fItty\fR is opened for reading
and writing and the shell is invoked immediately.
This feature is used to bring up a single-user system.
When the system is brought up in this way, the
.it getty
and
.it login
routines mentioned below and described elsewhere are not needed.
.s3
Otherwise, \fIinit\fR
invokes a Shell, with input taken from the
file
.it /etc/rc.
This command file
performs housekeeping
like removing temporary files,
mounting file systems, and starting the data-phone
daemon.
.s3
Then
.it init
forks several times to create a process
for each typewriter mentioned in an internal table.
Each of these processes opens the appropriate typewriter
for reading and writing.  These channels thus
receive file descriptors 0 and 1, the standard input and
output.
Opening the typewriter will usually involve a delay,
since the \fIopen\fR is not completed until someone
is dialled up and carrier established on the channel.
Then the process executes the program
.it /etc/getty
(q.v.).
.it Getty
will read the user's name and invoke
.it login
(q.v.)
to log in the user and execute the shell.
.s3
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 \fIinit\fR, which has been waiting
for such an event,
wakes up and removes the appropriate entry from the
file \fIutmp\fR, which records current users, and
makes an entry in \fIwtmp\fR, which maintains a history
of logins and logouts.
Then the appropriate typewriter is reopened and
.it getty
is
reinvoked.
.sh FILES
/dev/tty, /dev/tty?, /tmp/utmp, /tmp/wtmp,
.sh "SEE ALSO"
login(I), getty(VII), sh(I)