INIT(8) | System Manager's Manual | INIT(8) |
init
—
init |
[-s ] |
init
program is the last stage of the boot process.
It normally begins multi-user operation.
init
is executed automatically by the
kernel, after the kernel has initialised all devices and mounted the root
file system. The kernel may try multiple possible paths for
init
, including /sbin/init,
/sbin/oinit, /sbin/init.bak,
and /rescue/init.
The following table describes the state machine used by
init
:
-s
option to
init
to prevent the system from going multi-user
and to instead execute a single user shell without starting the normal
daemons. If the kernel is in a secure mode, init
will downgrade it to securelevel 0 (insecure mode). The system is then
quiescent for maintenance work and may later be made to go to state 2
(multi-user) by exiting the single-user shell (with ^D).If value of the “init.root” sysctl node is not equal to / at this point, the /etc/rc process will be run inside a chroot(2) indicated by sysctl with the same error handling as above.
If the administrator has not set the security level to -1 to
indicate that the kernel should not run multiuser in secure mode, and
the /etc/rc script has not set a higher level of
security than level 1, then init
will put the
kernel into securelevel mode 1. See
rc.conf(5) and
secmodel_securelevel(9)
for more information.
SIGTERM
, go to state 7; on
SIGHUP
, go to state 5; on
SIGTSTP
, go to state 6.SIGHUP
to all controlling
processes, reap the processes for 30 seconds, and then go to state 1
(single user); warning if not all the processes died.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”.
It should be noted that while init
has the
ability to start multi-user operation inside a
chroot(2) environment, the
init
process itself will always run in the
“original root directory”. This also implies that single-user
mode is always started in the original root, giving the possibility to
create multi-user sessions in different root directories over time. The
“init.root” sysctl node is fabricated by
init
at startup and re-created any time it is found
to be missing. Type of the node is string capable of holding full pathname,
and is only accessible by the superuser (unless explicitly destroyed and
re-created with different specification).
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); it opens and
initializes the tty line and executes the
login(1) program. The
login(1) 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) and
utmpx(5) files of current users
and records the logout in the
wtmp(5) and
wtmpx(5) files. The cycle is
then restarted by init
executing a new
getty(8) for the line.
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
-s HUP 1
”. This is referenced in the table above as state 5.
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 a new
getty(8) 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(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.
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(8)'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, or exits during
its initialisation, the system will panic with the message “panic:
init died (signal %d, exit %d)”.
If /dev/console does not exist,
init
will cd to /dev and run
“MAKEDEV -MM init
”.
MAKEDEV(8) will use
mount_tmpfs(8) or
mount_mfs(8) to create a
memory file system mounted over /dev that contains
the standard devices considered necessary to boot the system.
init
command appeared in
Version 1 AT&T UNIX.
September 1, 2019 | NetBSD 9.4 |