INIT(8) INIT(8)
NAME
init - process control initialization
SYNOPSIS
/etc/init
DESCRIPTION
Init is invoked inside Unix as the last step in the boot
procedure. It normally then runs the automatic reboot
sequence as described in reboot(8), and if this succeeds,
begins multi-user operation. If the reboot fails, it com-
mences single user operation by giving the super-user a
shell on the console. It is possible to pass parameters
from the boot program to 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 hit-
ting ^D), init runs /etc/rc without the reboot parameter.
This command file performs housekeeping operations such as
removing temporary files, mounting file systems, and start-
ing daemons.
In multi-user operation, 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 /etc/ttys and forks to
create a process for each terminal specified in the file.
Each of these processes opens the appropriate terminal for
reading and writing on file descriptors 0, 1, 2, and 3 (the
standard input and output, the diagnostic output and
/dev/tty). Opening the terminal will usually involve a
delay, since the open is not completed until someone dials
and carrier is established on the channel. Then /etc/getty
is called with argument as specified by the second character
of the ttys file line. Getty reads the user's name and
invokes login to log in the user and execute the shell.
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 init, which has been waiting
for such an event, wakes up and removes the appropriate
entry from the file utmp, which records current users, and
makes an entry in /usr/adm/wtmp, which maintains a history
of logins and logouts. Then the appropriate terminal is
reopened and getty is reinvoked.
Init catches the hangup signal (signal SIGHUP) and inter-
prets it to mean that the file /etc/ttys should be read
again. The shell process on each line which used to be
active in 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
INIT(8) INIT(8)
add terminal lines without rebooting the system by changing
the ttys file and sending a hangup signal to the init pro-
cess: use `kill -1 1.'
Init will terminate multi-user operations and resume
single-user mode if sent a terminate (SIGTERM, the default)
signal, i.e. `kill 1'. If there are processes outstanding
which are deadlocked (due to hardware or software failure),
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.
Init's role is so critical that if it dies, the system will
reboot itself automatically. If, at bootstrap time, the
init process cannot be located, the system will loop in user
mode at location 0x13.
FILES
/dev/console, /dev/tty?, /etc/utmp, /usr/adm/wtmp,
/etc/ttys, /etc/rc
SEE ALSO
login(8), kill(1), sh(1), ttys(5), getty(8), rc(8),
reboot(8)