INIT(8) BSD System Manager's Manual INIT(8)
init -- process control initialization
init [0 | 1 | 6 | c | q]
The init utility is the last stage of the boot process. It normally runs
the automatic reboot sequence as described in rc(8), and if this suc-
ceeds, begins multi-user operation. If the reboot scripts fail, init
commences single-user operation by giving the super-user a shell on the
console. The init utility may be passed parameters from the boot program
to prevent the system from going multi-user and to instead execute a sin-
gle-user shell without starting the normal daemons. The system is then
quiescent for maintenance work and may later be made to go to multi-user
by exiting the single-user shell (with ^D). This causes init to run the
/etc/rc start up command file in fastboot mode (skipping disk checks).
If the console entry in the ttys(5) file is marked ``insecure'', then
init will require that the super-user password be entered before the sys-
tem will start a single-user shell. The password check is skipped if the
console is marked as ``secure''.
The kernel runs with five different levels of security. Any super-user
process can raise the security level, but no process can lower it. The
security levels are:
-1 Permanently insecure mode - always run the system in level 0 mode.
This is the default initial value.
0 Insecure mode - immutable and append-only flags may be turned off.
All devices may be read or written subject to their permissions.
1 Secure mode - the system immutable and system append-only flags may
not be turned off; disks for mounted file systems, /dev/mem,
/dev/kmem and /dev/io (if your platform has it) may not be opened
for writing; kernel modules (see kld(4)) may not be loaded or
2 Highly secure mode - same as secure mode, plus disks may not be
opened for writing (except by mount(2)) whether mounted or not.
This level precludes tampering with file systems by unmounting
them, but also inhibits running newfs(8) while the system is multi-
In addition, kernel time changes are restricted to less than or
equal to one second. Attempts to change the time by more than this
will log the message ``Time adjustment clamped to +1 second''.
3 Network secure mode - same as highly secure mode, plus IP packet
filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be
changed and dummynet(4) or pf(4) configuration cannot be adjusted.
If the security level is initially nonzero, then init leaves it
unchanged. Otherwise, init raises the level to 1 before going multi-user
for the first time. Since the level cannot be reduced, it will be at
least 1 for subsequent operation, even on return to single-user. If a
level higher than 1 is desired while running multi-user, it can be set
before going multi-user, e.g., by the startup script rc(8), using
sysctl(8) to set the kern.securelevel variable to the required security
If init is run in a jail, the security level of the ``host system'' will
not be effected. Part of the information set up in the kernel to support
a jail is a per-jail ``securelevel'' setting. This allows running a
higher security level inside of a jail than that of the host system. See
jail(8) for more information about jails.
In multi-user operation, init maintains processes for the terminal ports
found in the file ttys(5). The init utility reads this file and executes
the command found in the second field, unless the first field refers to a
device in /dev which is not configured. The first field is supplied as
the final argument to the command. This command is usually getty(8);
getty opens and initializes the tty line and executes the login(1) pro-
gram. The login program, when a valid user logs in, executes a shell for
that user. When this shell dies, either because the user logged out or
an abnormal termination occurred (a signal), the init utility wakes up,
deletes the user from the utmp(5) file of current users and records the
logout in the wtmp(5) file. The cycle is then restarted by init execut-
ing a new getty for the line.
The init utility can also be used to keep arbitrary daemons running,
automatically restarting them if they die. In this case, the first field
in the ttys(5) file must not reference the path to a configured device
node and will be passed to the daemon as the final argument on its com-
mand line. This is similar to the facility offered in the AT&T System V
Line status (on, off, secure, getty, or window information) may be
changed in the ttys(5) file without a reboot by sending the signal SIGHUP
to init with the command ``kill -HUP 1''. On receipt of this signal,
init re-reads the ttys(5) file. When a line is turned off in ttys(5),
init will send a SIGHUP signal to the controlling process for the session
associated with the line. For any lines that were previously turned off
in the ttys(5) file and are now on, init executes the command specified
in the second field. If the command or window field for a line is
changed, the change takes effect at the end of the current login session
(e.g., the next time init starts a process on the line). If a line is
commented out or deleted from ttys(5), init will not do anything at all
to that line. However, it will complain that the relationship between
lines in the ttys(5) file and records in the utmp(5) file is out of sync,
so this practice is not recommended.
The init utility will terminate multi-user operations and resume single-
user mode if sent a terminate (TERM) signal, for example, ``kill -TERM
1''. If there are processes outstanding that are deadlocked (because of
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.
The init utility will cease creating new processes and allow the system
to slowly die away, if it is sent a terminal stop (TSTP) signal, i.e.
``kill -TSTP 1''. A later hangup will resume full multi-user operations,
or a terminate will start a single-user shell. This hook is used by
reboot(8) and halt(8).
The init utility will terminate all possible processes (again, it will
not wait for deadlocked processes) and reboot the machine if sent the
interrupt (INT) signal, i.e. ``kill -INT 1''. This is useful for shut-
ting the machine down cleanly from inside the kernel or from X when the
machine appears to be hung.
The init utility will do the same, except it will halt the machine if
sent the user defined signal 1 (USR1), or will halt and turn the power
off (if hardware permits) if sent the user defined signal 2 (USR2).
When shutting down the machine, init will try to run the /etc/rc.shutdown
script. This script can be used to cleanly terminate specific programs
such as innd (the InterNetNews server).
The role of init 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 panic with the message ``panic: init died
(signal %d, exit %d)''.
If run as a user process as shown in the second synopsis line, init will
emulate AT&T System V UNIX behavior, i.e., super-user can specify the
desired run-level on a command line, and init will signal the original
(PID 1) init as follows:
Run-level Signal Action
0 SIGUSR2 Halt and turn the power off
1 SIGTERM Go to single-user mode
6 SIGINT Reboot the machine
c SIGTSTP Block further logins
q SIGHUP Rescan the ttys(5) file
getty repeating too quickly on port %s, sleeping. A process being
started to service a line is exiting quickly each time it is started.
This is often caused by a ringing or noisy terminal line. Init will
sleep for 30 seconds, then continue trying to start the process.
some processes would not die; ps axl advised. A process is hung and
could not be killed when the system was shutting down. This condition is
usually caused by a process that is stuck in a device driver because of a
persistent device error condition.
/dev/console system console device
/dev/tty* terminal ports found in ttys(5)
/var/run/utmp record of current users on the system
/var/log/wtmp record of all logins and logouts
/etc/ttys the terminal initialization information file
/etc/rc system startup commands
/etc/rc.shutdown system shutdown commands
kill(1), login(1), sh(1), dummynet(4), ipfirewall(4), kld(4), pf(4),
ttys(5), crash(8), getty(8), halt(8), ipfw(8), jail(8), pfctl(8), rc(8),
reboot(8), shutdown(8), sysctl(8)
An init utility appeared in Version 6 AT&T UNIX.
Systems without sysctl(8) behave as though they have security level -1.
Setting the security level above 1 too early in the boot sequence can
prevent fsck(8) from repairing inconsistent file systems. The preferred
location to set the security level is at the end of /etc/rc after all
multi-user startup actions are complete.
BSD April 18, 1994 BSD