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The command shutdown can reboot the system or brings the system down in a secure way. 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.
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 time argument is mandatory; in 90% of all cases this argument is the word now. Which means immediate reboot/shutdown.
/sbin/shutdown -r now
Time of reboot/shutdown is specified as HH:MM. You can use keyword now instead of time for immediate reboot. For example
/sbin/shutdown -r 21:00
With option -t you can additionally specify the number of seconds wait sec seconds between sending processes the warning and the kill signal and changing to another runlevel.
The most important options are:
This is equivalent to command reboot, but in addition to rebooting system it sends message to all users (not very useful with now ;-)
The time argument can have different formats.
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.
Linux shutdown Command Explained with Examples
Q2. How to halt machine using shutdown?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.
This can be achieved using the -H option.
In case you aren't aware, there's a difference between halting and powering off a system. While the former involves stopping all CPUs, the latter also makes sure the main power is disconnected.Q3. How to make shutdown power-off machine?
Although this is by default, you can still use the -P option to explicitly specify that you want shutdown to power off the system.
shutdown -PQ4. How to reboot using shutdown?
For reboot, the option is -r.
shutdown -rQ5. How to make shutdown only write wall message, or vice-versa?
In case you just want shutdown to shoot wall messages, without performing a halt, power-off, or reboot, use the -k command line option.
shutdown -kQ6. How to cancel a pending shutdown?
You can use shutdown to schedule a halt, power off, or reboot. For this, all you have to do is to pass the time at which you want the process to start. For example:
shutdown -r 18:00
However, in case you want to cancel a scheduled shutdown, you can do that as well. This can be done using the -c command line option.
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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Linux shutdown Command Explained with Examples
See also fsck(8), init(8), halt(8), poweroff(8), reboot(8)
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