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Is there any command / program by which I may shutdown my system instantly?

Update: Thank you to those who have suggested to turn off using the power socket. :D

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migrated from Feb 14 '10 at 11:52

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Flipping the power switch at the wall. – Sam152 Feb 14 '10 at 11:51
Since the fastest code is the one that doesn't run, I'd say the fastest way to shutdown a system is not to power it on. Explain what you're trying to achieve if you need a specific answer. – Dmitry Grigoryev Nov 27 at 13:38
@Sam152 which wall? – usajbalt 2 days ago

8 Answers 8

up vote 11 down vote accepted
sudo shutdown -h now
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That wouldn't shutdown a system instantly. ;) – Marcel Korpel Feb 14 '10 at 11:57
@Marcel Korpel: As instantly as one should do without damaging his system ;) – Felix Feb 14 '10 at 12:05
@Felix - I keep hearing that Linux is a very robust system, and it can't handle the same "hard reboots" as my Windows endured? Would be surprising ;) – Gnoupi Feb 16 '10 at 9:57
@Gnoupi: No OS can be built without some vulnerability to abrupt shutdown. Otherwise, the answer to the fastest shutdown method would be to yank the power cord. – harrymc Feb 16 '10 at 11:03
Some versions the syntax is different. "sudo shutdown -g0 -y -i5" (Solaris). And of course, just to make it difficult, the exact value following "i" can be different for shutdown on different systems. I believe it's 0 on a lot of other systems for shutdown. Sigh. Gotta love standards, so many to choose from! – Brian Knoblauch Feb 16 '10 at 13:34

You've got to use the Magic SysRq key combination REISUB.

Repeat after me: "Raising Elephants Is So Utterly Boring..."

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No. REISUB will restart it. REISUO will shutdown. Or simply O. – Benoit Nov 22 '10 at 20:19

This should work on most common distros. Unlike with shutdown, on many systems you can execute this as a normal user (no sudo needed).

Edit: Further reading:

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On certain linux distributions, shutdown offers the option -n. (See man shutdown). Certain distributions (such as ubuntu which makes use of upstart instead of sysvinit) don't support his flag however, and this flags is (as can be learned from the manpage) not without risk. Though I've been using this for years (on my laptop i don't care that all daemons are terminated properly, it just want everything killed and my diskcache flushed) (it is faster than a regular shutdown, and more safe than pulling the plug (and friends ) ). You could see if this solves your problem.

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Taken from another SE site:

Alt + SysRq + B is instant reboot, and Alt + SysRq + O is instant shutdown.

Reference: Here

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seems you have missed the reference, and quote your answer, then i'll reverse my vote – Francisco Tapia Nov 26 at 14:28
This is the original answer: – cst1992 Nov 27 at 5:39

Hold the power key for about 4 seconds, it's quite fast.

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If doing so, flipping the power switch is actually faster ;) – Gnoupi Feb 16 '10 at 9:58
That's the same as any other shutdown method, assuming that the OS does power management. – harrymc Feb 16 '10 at 11:04
If you actually can't wait a few seconds, and are going to kill power, use a journaling filesystem like ext3/ext4 so you don't lose as much data due to unwritten writes. – CarlF Feb 16 '10 at 11:33
Not looking for a non-programmatic / non-software way of performing the shutdown otherwise @Gnoupi suggestion is way faster. ;) – usajbalt 2 days ago

I don't suggest to do the following if you are not forced by really special reasons:

kill -SEGV 1   # should generate a core dumps and kernel panic
kill -ABRT 1   # should generate a core dumps and kernel panic
kill -9 1      # On old systems worked nowadays not

It is rough, brutal and it can be considered a close equivalent of unplugging the power chord...

Some words more, aka The story, Chapter I
In the beginning was the init and it will be till the very end.

The whole Linux depends from the loving care of init [1] [2]. Nonetheless and not without a certain amount of ungratefulness, there was a time in which the good Lord root user can betray this love and suddenly kill init with an incontrovertible (-9) order.
(The Book of Etiquette prescribes for counts, dukes and marquises users to invoke before a sudo).

Then some wizards made a charm to protect init (from the Book of man 2 init)

The only signals that can be sent to process ID 1, the init process, are those for which init has explicitly installed signal handlers. This is done to assure the system is not brought down accidentally.

(Our spies report [U1] that init will hanle 1 HUP 6 ABRT 11 SEGV 15 TERM 30 PWR 2 INT 10 USR1 14 ALRM 17 CHLD 32)
So the good Lord root user learn the news and change the command in kill -ABRT 1 or kill -SEGV 1 that usually generate a kernel panic and a core dump.

It works because init is the first process to run and takes the PID number 1 [2b].

This is unsafe, unwise and you feel it is herald of bad omen and a curse but if you cannot materially put the hands on and unplug it...

The curse: it will not write on the log, it will not kill all the processes & wait for their ending, it will not write on the HDD properly updating the inodes neither ti will unmount the filesystems; don't even mind it will save the options of the graphical windows and the shell histories, and many other beyond our imagination... as we said a close equivalent of unplugging the power chord, or the battery if a laptop.

The correct way
"Non nobis, Domine, non nobis, sed nomini tuo da gloriam.", Templar knights motto.
The correct way is to use shutdown[3]

sudo shutdown -h now

shutdown arranges for the system to be brought down in a safe way. All logged-in users are notified that the system is going down and...

but with -h now they will have no enough time to do so much...

Some words more, aka The story, Chapter II
Once upon a time some logic steps felt from the sky over the unix people:

Once system processes have been killed and filesystems have been unmounted, the system halts/powers off or reboots automatically. This is done using the halt or reboot command, which syncs changes to disks and then performs the actual halt/power off or reboot. [4]

Indeed, nowadays, we do not trust any more in the existence of the three Moirai [5] of the Linux world, reboot,poweroff and halt[6]: the modern science of ls -l $(which poweroff halt reboot) and the one of man reboot, spreads new light on this dark age and reveal us that it exists only one true command that parses all their options so that we are finally free to ask for actions contradicting their commands names! (halt -p or reboot -p for poweroff, shutdown -r for reboot...)

Now that all seemed to be clear and cosy for all, rumours claim [7] that in the underworld of systemd toolset [8] a revolution was performed leaving unaware the whole overworld. Thanks to an army of backwards compatibility shims we didn't notice at all that reboot, poweroff, halt [6] and even telinit [9] and shutdown [3] are all already bounded to the new king systemctl [10]. Please listen the whole story from the original voice of JdeBP The Bard [9] because I have no more breath.

If you are a followers of the Ubuntu cult you may still remain aware for a while of all those claims [11].

The middle Earth of halt -f, init, telinit, systemctl
Searching for solution faster than the correct one but likewise wise.

systemctl --force --force isolate  # the most close to kill -9 1 
systemctl --force isolate          # rough but still safe
sudo halt -f                                       # rough ?safe?!
sudo telinit 0    # or 6                           # safe
kill -SIGINT 1    # cause reboot   as the reboot command
kill -SIGPWR 1    # cause shutdown as the halt command

That you are under systemd or not you should be able to stop the computer without invoking all the correct shudown procedures (and so more fast):

  1. halt -f: specifying the option -f (note that you need -f to avoid the shutdown procedure) with the above command, with sudo poweroff -f or maybe even with sudo reboot -f -h. Indeed we can read from man reboot (and equivalents) about the need to specify the option -f to avoid to call the shutdown:

    When called with --force or when in runlevel 0 or 6, this tool invokes the reboot(2) system call itself (with REBOOTCOMMAND argument passed) and directly reboots the system.

    Otherwise this simply invokes the shutdown(8) tool with the appropriate arguments without passing REBOOTCOMMAND argument.

    -f, --force
    Does not invoke shutdown(8) and instead performs the actual action you would expect from the name.

  2. Moreover you can use telinit [2b] (or init directly)

    sudo telinit 0    # or 6

    to tell init to change runlevel... but if so why do not kill it directly?

  3. Under systemd you can use the unwise double option --force --force

    systemctl --force --force isolate  

    Reading from the systemctl manual [10]

    -f, --force
    When used with enable, overwrite any existing conflicting symlinks.
    When used with halt, poweroff, reboot or kexec, execute the selected operation without shutting down all units. However, all processes will be killed forcibly and all file systems are unmounted or remounted read-only. This is hence a drastic but relatively safe option to request an immediate reboot. If --force is specified twice for these operations, they will be executed immediately without terminating any processes or unmounting any file systems. Warning: specifying --force twice with any of these operations might result in data loss.

Ps> Take inspiration about variants from the tail JdeBP The Bard [7].

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halt is pretty quick.

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On some systems it's known as "haltsys" rather than just "halt". – Brian Knoblauch Feb 16 '10 at 13:35

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