INIT(8) BSD System Manager's Manual INIT(8)
init -- process control initialization
The init program is the last stage of the boot process. It normally runs the automatic reboot sequence as described in reboot(8), and if
this succeeds, 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 program may be passed parameters from the boot program to prevent the system from going multi-user and to
instead execute a single 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 superuser password be entered before the
system will start a single-user shell. The password check is skipped if the console is marked as ``secure''.
The kernel runs with four different levels of security. Any superuser process can raise the security level, but only init can lower it.
Security levels are defined as follows:
-1 Permanently insecure mode - always run system in level 0 mode.
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 - immutable and append-only flags may not be changed; disks for mounted filesystems, /dev/mem, and /dev/kmem are read-only.
The settimeofday(2) system call can only advance the time.
2 Highly secure mode - same as secure mode, plus disks are always read-only whether mounted or not. This level precludes tampering with
filesystems by unmounting them, but also inhibits running newfs(8) while the system is multi-user.
Normally, the system runs in level 0 mode while single user and in level 1 mode while multiuser. If the level 2 mode is desired while run-
ning multiuser, it can be set in the startup script /etc/rc using sysctl(8). If it is desired to run the system in level 0 mode while multi-
user, the administrator must build a kernel with the variable securelevel defined in the file /sys/compile/MACHINE/param.c and initialize it
In multi-user operation, init maintains processes for the terminal ports found in the file ttys(5). Init reads this file, and executes the
command found in the second field. This command is usually getty(8); getty opens and initializes the tty line and executes the login 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 program wakes up, deletes the user from the utmp(5) file of current users and
records the logout in the wtmp file. The cycle is then restarted by init executing a new getty for the line.
Line status (on, off, secure, getty, or window information) may be changed in the ttys file without a reboot by sending the signal SIGHUP to
init with the command ``kill -s HUP 1''. On receipt of this signal, init re-reads the ttys file. When a line is turned off in ttys, 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 file and are now on, init executes a new getty to enable a new login. If the getty 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, init will not do anything at all to that line. However, it will complain that the relationship between lines in
the ttys file and records in the utmp file is out of sync, so this practice is not recommended.
Init will terminate multi-user operations and resume single-user mode if sent a terminate (TERM) signal, for example, ``kill -s 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.
Init will cease creating new getty's and allow the system to slowly die away, if it is sent a terminal stop (TSTP) signal, i.e. ``kill -s
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 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)''.
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 10 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.
/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.
login(1), kill(1), sh(1), ttys(5), crash(8), getty(8), rc(8), reboot(8), halt(8), shutdown(8)
A init command appeared in Version 6 AT&T UNIX.
Systems without sysctl behave as though they have security level -1.
4th Berkeley Distribution May 26, 1995 4th Berkeley Distribution