Linux man pages : shutdown (8)
SHUTDOWN(8) Linux System Administrator's Manual SHUTDOWN(8)
shutdown - bring the system down
/sbin/shutdown [-t sec] [-arkhncfF] time [warning-message]
shutdown brings the system down in a secure way. All logged-in users
are notified that the system is going down, and login(1) is blocked.
It is possible to shut the system down immediately or after a specified
delay. All processes are first notified that the system is going down
by the signal SIGTERM. This gives programs like vi(1) the time to save
the file being edited, mail and news processing programs a chance to
exit cleanly, etc. shutdown does its job by signalling the init pro-
cess, asking it to change the runlevel. Runlevel 0 is used to halt the
system, runlevel 6 is used to reboot the system, and runlevel 1 is used
to put to system into a state where administrative tasks can be per-
formed; this is the default if neither the -h or -r flag is given to
shutdown. To see which actions are taken on halt or reboot see the
appropriate entries for these runlevels in the file /etc/inittab.
-a Use /etc/shutdown.allow.
-t sec Tell init(8) to wait sec seconds between sending processes the
warning and the kill signal, before changing to another run-
-k Don't really shutdown; only send the warning messages to every-
-r Reboot after shutdown.
-h Halt after shutdown.
-n [DEPRECATED] Don't call init(8) to do the shutdown but do it
ourself. The use of this option is discouraged, and its results
are not always what you'd expect.
-f Skip fsck on reboot.
-F Force fsck on reboot.
-c Cancel an already running shutdown. With this option it is of
course not possible to give the time argument, but you can enter
a explanatory message on the command line that will be sent to
time When to shutdown.
Message to send to all users.
The time argument can have different formats. First, it can be an
absolute time in the format hh:mm, in which hh is the hour (1 or 2 dig-
its) and mm is the minute of the hour (in two digits). Second, it can
be in the format +m, in which m is the number of minutes to wait. The
word now is an alias for +0.
If shutdown is called with a delay, it creates the advisory file
/etc/nologin which causes programs such as login(1) to not allow new
user logins. Shutdown removes this file if it is stopped before it can
signal init (i.e. it is cancelled or something goes wrong). It also
removes it before calling init to change the runlevel.
The -f flag means `reboot fast'. This only creates an advisory file
/fastboot which can be tested by the system when it comes up again.
The boot rc file can test if this file is present, and decide not to
run fsck(1) since the system has been shut down in the proper way.
After that, the boot process should remove /fastboot.
The -F flag means `force fsck'. This only creates an advisory file
/forcefsck which can be tested by the system when it comes up again.
The boot rc file can test if this file is present, and decide to run
fsck(1) with a special `force' flag so that even properly unmounted
filesystems get checked. After that, the boot process should remove
The -n flag causes shutdown not to call init, but to kill all running
processes itself. shutdown will then turn off quota, accounting, and
swapping and unmount all filesystems.
shutdown can be called from init(8) when the magic keys CTRL-ALT-DEL
are pressed, by creating an appropriate entry in /etc/inittab. This
means that everyone who has physical access to the console keyboard can
shut the system down. To prevent this, shutdown can check to see if an
authorized user is logged in on one of the virtual consoles. If shut-
down is called with the -a argument (add this to the invocation of
shutdown in /etc/inittab), it checks to see if the file /etc/shut-
down.allow is present. It then compares the login names in that file
with the list of people that are logged in on a virtual console (from
/var/run/utmp). Only if one of those authorized users or root is logged
in, it will proceed. Otherwise it will write the message
shutdown: no authorized users logged in
to the (physical) system console. The format of /etc/shutdown.allow is
one user name per line. Empty lines and comment lines (prefixed by a #)
are allowed. Currently there is a limit of 32 users in this file.
Note that if /etc/shutdown.allow is not present, the -a argument is
A lot of users forget to give the time argument and are then puzzled by
the error message shutdown produces. The time argument is mandatory; in
90 percent of all cases this argument will be the word now.
Init can only capture CTRL-ALT-DEL and start shutdown in console mode.
If the system is running the X window System, the X server processes
all key strokes. Some X11 environments make it possible to capture
CTRL-ALT-DEL, but what exactly is done with that event depends on that
Shutdown wasn't designed to be run setuid. /etc/shutdown.allow is not
used to find out who is executing shutdown, it ONLY checks who is cur-
rently logged in on (one of the) console(s).
Miquel van Smoorenburg, firstname.lastname@example.org
fsck(8), init(8), halt(8), poweroff(8), reboot(8)
Juli 31, 2001 SHUTDOWN(8)