BSD versionWindows
Bring the system down in a orderly way, immediately or later.

Linux version:

/sbin/shutdown [-t sec] [-arkhncfFHP] time† [message]

  1. Terminal users are sent message and logins are disabled.
  2. processes are notified by the signal SIGTERM. This gives programs like editors the time to save the file being edited, mail and news processing programs a chance to exit cleanly, etc.
  3. report SIGKILL to all processes
  4. shutdown signals the init process, requset a change in runlevel.
    Runlevel 0 is used to halt the system,
    runlevel 6 is used to reboot the system, and
    runlevel 1, the default, is used to put to system into a state where administrative tasks can be performed;if neither the -h or -r flag is given to shutdown.
    /etc/inittab defines what action is taken for halt or reboot.

-c Cancel a running shutdown now
-a Use /etc/shutdown.allow.
-t sec Tell init to wait sec seconds after sending processes the SIGTERM until
the kill signal and changing to another runlevel.
-k Don't really shutdown; only send the message to everybody.
-r Reboot after shutdown.
-h Halt or poweroff after shutdown.
-H Halt action is to halt or drop into boot monitor on systems that support it.
-P Halt action is to turn off the power.
-f Skip fsck on reboot, 'reboot fast'. Creates /fastboot The boot /etc/rc can test if this file is present, and decide not to run fsck(1) since the system was shut down in the proper way. The boot process should remove /fastboot.
-F Force fsck on reboot. Creates /forcefsck . The boot rc file can test if this file is present, decide to run fsck with a special 'force' flag so that even properly unmounted filesystems get checked. The boot process should remove /forcefsck.
-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.
kill all processes, turn off quota, accounting, and swapping and unmount all filesystems.
time When to shutdown. The time argument can be of the form: hh:mm, +minutes , now

message sent to all users.

/etc/nologin is created if shutdown is called with a delay. This causes programs such as login to deny logins. Shutdown removes this file if it is stopped before it signals init (i.e. it is cancelled or something goes wrong). Shutdown removes /etc/nologin before calling init to change the runlevel.

ACCESS CONTROL

Shutdown can be called from init 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 shutdown is called with -a (add this to the invocation of shutdown in /etc/inittab), it checks to see if the file /etc/shutdown.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. There is a limit of 32 users in this file. Note that if /etc/shutdown.allow is not present, the -a is ignored.

HALT OR POWEROFF

The -H option just sets the init environment variable INIT_HALT to HALT, and the -P just sets that variable to POWEROFF. The shutdown script that calls halt as the last thing in the shutdown sequence should check these environment variables and call halt with the right options for these options to actually have any effect. Debian 3.1 (sarge) supports this.

FILES

/fastboot /etc/inittab /etc/init.d/halt /etc/init.d/reboot /etc/shutdown.allow

The time argument is mandatory

Pressing [CTRL]-[ALT]-[DEL] will start shutdown from a console in text mode.
If the console is running a window System, the windows server processes key strokes and processing of [CTRL]-[ALT]-[DEL] is server dependent.

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 currently logged in on (one of the) console(s).

SEE ALSO

fsck, init, halt, poweroff, reboot November 12, 2003

BSD

shutdown [-] [-h | -r | -k] [-o [-n]] time [message]

-h halt
-r reboot
-k Kick off non-super users, leaves the system multi-user with logins disabled (for all but super-user).
-o If -h or -r is specified, shutdown executes halt or reboot instead of sending signal to init.

-n If -o is specified, prevent the file system cache from being flushed by passing -n option to halt or reboot. This option should not be used.

time when shutdown will commence.
     now indicates an immediate shutdown or
    +minutes or
     [yymmdd]hhmm

message broadcast to users using wall

*** System shutdown message from username@host.domainName ***
System going down at 21:06

- message is read from the standard input.

Starting at no more than 10 hours before shutdown, at intervals becoming more frequent
messages are sent to logged in users .

5 minutes before shutdown, or immediately if shutdown is in less than 5 minutes, the message is copied to /var/run/nologin. When a user attempts to login, its contents are displayed and login exits. The file is removed just before shutdown exits.

At time a message is written to the system log, which includes the user who initiated the shutdown and the reason.
A signal is then sent to init(8) to halt, reboot or bring the system down to single-user state.

A scheduled shutdown can be canceled by reporting SIGTERM.

{
echo "";     #show shutdown process information
echo kill -TERM `/bin/ps -A -o pid -o command| grep shutdown | grep -v grep` ;
echo "";     # Ask if this is the right process
echo -n " [return] to cancel shutdown, else ^C "; read resp;
export shutdownPS=`/bin/ps -A -o pid -o command| grep shutdown | grep -v grep `;
sudo kill -TERM `echo $shutdownPS | sed "s/^ \{1,3\}/0/" |cut -f1-1 -d " "`;
}

/var/run/nologin will be removed automatically.

FILES /var/run/nologin tells login not to let anyone log in

SEE ALSO kill(1), login(1), wall(1), nologin(5), halt(8), init(8), reboot(8)

The hours and minutes may be separated by a :

BSD December 11, 1998 BSD

Windows

shutdown [/i ]

          [ /l | /s | /r | /g | /a | /p | /h ]

          [ /e] [/d [p|u:]MM:mm [/c "comment"]]

           [/f] [/m \\computer][/t sss]
/i Display the graphical user interface( must be the first option.)
/l Log off. cannot be used with /m (other system) or /d (reason)
/s Shutdown
/r restart
/g restart; then restart any registered applications. This can only be used during the time-out period.
/m \\computer Specify the target computer.
/t sss time, in seconds, before shutdown (default 30) Force (/f) implied it not 0.
/f Force running applications to close without forewarning users implied if /t is not 0
/p with /d (reason) and /f (force) NOW!
/h Hibernate with /f
/e explain
/c "comment" the reason for the restart or shutdown.
/d [p|u:]MM:mm reason p planned u user definedi. default is unplanned. MM major reason < 255 ; mm minor reason < 655
/a Abort a system shutdown.
/? Display help. This is the same as not typing any options. 
Reasons on this computer:
(E = Expected U = Unexpected P = planned, C = customer defined)
Type    Major   Minor   Title 
EU      0       0       Other (Unplanned)
E P     0       0       Other (Planned)
 U      0       5       Other Failure: System Unresponsive
E P     1       1       Hardware: Maintenance (Unplanned)
E P     1       2       Hardware: Installation (Planned)
E P     2       2       Operating System: Recovery (Planned)
  P     2       3       Operating System: Upgrade (Planned)
E P     2       4       Operating System: Reconfiguration (Planned)
  P     2       16      Operating System: Service pack (Planned)
  P     2       17      Operating System: Hot fix (Planned)
  P     2       18      Operating System: Security fix (Planned)
E P     4       1       Application: Maintenance (Planned)
E P     4       2       Application: Installation (Planned)
E       4       5       Application: Unresponsive
E       4       6       Application: Unstable
 U      5       15      System Failure: Stop error
EUP     5       19      Security issue
E       5       20      Loss of network connectivity (Unplanned)
 U      6       11      Power Failure: Cord Unplugged
 U      6       12      Power Failure: Environment
  P     7       0       Legacy API shutdown