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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
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.
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.
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 into 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.
/fastboot /etc/inittab /etc/init.d/halt /etc/init.d/reboot /etc/shutdown.allow
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).
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See also fsck(8), init(8), halt(8), poweroff(8), reboot(8)
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