init.8

早期freebsd实现

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.\"     @(#)init.8	8.3 (Berkeley) 4/18/94
.\"
.Dd April 18, 1994
.Dt INIT 8
.Os BSD 4
.Sh NAME
.Nm init
.Nd process control initialization
.Sh SYNOPSIS
.Nm init
.Sh DESCRIPTION
The
.Nm init
program
is the last stage of the boot process.
It normally runs the automatic reboot sequence as described in
.Xr reboot 8 ,
and if this succeeds, begins multi-user operation.
If the reboot scripts fail,
.Nm init
commences single user operation by giving
the super-user a shell on the console.
The
.Nm 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
.Nm init
to run the
.Pa /etc/rc
start up command file in fastboot mode (skipping disk checks).
.Pp
If the
.Nm console
entry in the
.Xr ttys 5
file is marked ``insecure'',
then
.Nm 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 
.Nm console
is marked as ``secure''.
.Pp
The kernel runs with four different levels of security.
Any superuser process can raise the security level, but only 
.Nm init
can lower it.
Security levels are defined as follows:
.Bl -tag -width flag
.It Ic -1
Permanently insecure mode \- always run system in level 0 mode.
.It Ic 0
Insecure mode \- immutable and append-only flags may be turned off.
All devices may be read or written subject to their permissions.
.It Ic 1
Secure mode \- immutable and append-only flags may not be changed;
disks for mounted filesystems,
.Pa /dev/mem ,
and
.Pa /dev/kmem
are read-only.
.It Ic 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
.Xr newfs 8
while the system is multi-user.
.El
.Pp
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 running multiuser,
it can be set in the startup script
.Pa /etc/rc
using
.Xr sysctl 8 .
If it is desired to run the system in level 0 mode while multiuser,
the administrator must build a kernel with the variable
.Nm securelevel
defined in the file
.Pa /sys/compile/MACHINE/param.c
and initialize it to -1.
.Pp
In multi-user operation, 
.Nm init
maintains
processes for the terminal ports found in the file
.Xr ttys 5 .
.Nm Init
reads this file, and executes the command found in the second field.
This command is usually
.Xr getty 8 ;
.Xr getty
opens and initializes the tty line
and
executes the
.Xr login
program.
The
.Xr 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
.Nm init
program wakes up, deletes the user
from the
.Xr utmp 5
file of current users and records the logout in the
.Xr wtmp
file.
The cycle is
then restarted by
.Nm init
executing a new
.Xr getty
for the line.
.Pp
Line status (on, off, secure, getty, or window information)
may be changed in the
.Xr ttys
file without a reboot by sending the signal
.Dv SIGHUP
to
.Nm init
with the command
.Dq Li "kill -HUP 1" .
On receipt of this signal,
.Nm init
re-reads the
.Xr ttys
file.
When a line is turned off in
.Xr ttys ,
.Nm 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
.Xr ttys
file and are now on,
.Nm init
executes a new
.Xr 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 
.Nm init
starts a process on the line).
If a line is commented out or deleted from
.Xr ttys ,
.Nm init
will not do anything at all to that line.
However, it will complain that the relationship between lines
in the
.Xr ttys
file and records in the
.Xr utmp
file is out of sync,
so this practice is not recommended.
.Pp
.Nm Init
will terminate multi-user operations and resume single-user mode
if sent a terminate
.Pq Dv TERM
signal, for example,
.Dq Li "kill \-TERM 1" .
If there are processes outstanding that are deadlocked (because of
hardware or software failure),
.Xr 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
.Nm Init
will cease creating new
.Xr getty Ns 's
and allow the system to slowly die away, if it is sent a terminal stop
.Pq Dv TSTP
signal, i.e.
.Dq Li "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
.Xr reboot 8
and
.Xr halt 8 .
.Pp
The role of
.Nm init
is so critical that if it dies, the system will reboot itself
automatically.
If, at bootstrap time, the
.Xr init
process cannot be located, the system will panic with the message
``panic: "init died (signal %d, exit %d)''.
.Sh DIAGNOSTICS
.Bl -diag
.It "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.
.Em "Init will sleep for 10 seconds" ,
.Em "then continue trying to start the process" .
.Pp
.It "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.
.El
.Sh FILES
.Bl -tag -width /var/log/wtmp -compact
.It Pa /dev/console
System console device.
.It Pa /dev/tty*
Terminal ports found in
.Xr ttys .
.It Pa /var/run/utmp
Record of Current users on the system.
.It Pa /var/log/wtmp
Record of all logins and logouts.
.It Pa /etc/ttys
The terminal initialization information file.
.It Pa /etc/rc
System startup commands.
.El
.Sh SEE ALSO
.Xr login 1 ,
.Xr kill 1 ,
.Xr sh 1 ,
.Xr ttys 5 ,
.Xr crash 8 ,
.Xr getty 8 ,
.Xr rc 8 ,
.Xr reboot 8 ,
.Xr halt 8 ,
.Xr shutdown 8
.Sh HISTORY
A
.Nm
command appeared in
.At v6 .
.Sh BUGS
Systems without
.Xr sysctl
behave as though they have security level \-1.