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shutdown

shutdown - bring the system down

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The command shutdown  can reboot the system or brings the system down in a secure way. Time of reboot or shutdown is recoded in /var/log/wtmp and can be viewed via last command, for example

last reboot

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  process, 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 performed; 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.

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

The most important options are:

You can use now instead of time for immediate reboot. for example

/sbin/shutdown -r now 

This is equivalent to command reboot, but sends message to all users.

All options:

The time argument

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 digits) 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.

Access Control

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 shutdown  is called with the -a  argument (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. Currently there is a limit of 32 users in this file.

Note that if /etc/shutdown.allow is not present, the -a  argument is ignored.

Halt or Poweroff

The -H  option just sets the init environment variable INIT_HALT to HALT, and the -P  option just sets that variable to POWEROFF. The shutdown script that calls halt(8) as the last thing in the shutdown sequence should check these environment variables and call halt(8) 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

Notes

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 environment.

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).

Author

Miquel van Smoorenburg, miquels@cistron.nl

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shutdown (8)

See also fsck(8), init(8), halt(8), poweroff(8), reboot(8)



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