Is there any command / program via which I may shutdown my system instantly (1 to 5 secs)?

Update (2015): Thank you to those who have suggested to turn off using the power socket. :D

Update (2016): @technophile suggested that I should add some information to the question describing nature of the problem - mainly which scenario was I trying to solve. Here is the scenario for which this question was asked some years ago.

We were building a system to be deployed in public spaces like (airports, hotels etc). People can use that system for internet surfing, checking their email or even bookings etc. using their credit cards. System built was making sure that a second user can in no way get information (email, credit cards) of the previous user of that system even if keyloggers kind of softwares somehow get installed. How was the system making sure of it? Well, details cannot be shared because that's not public info. but we developed a way to always have fresh Operating System (as if it was newly installed) for a new user of the system. Only problem was that we had to shutdown the machine and bring it up again as soon as first user leaves the machine.

And I wanted that shutdown to be instantly fast ( - that's why the question) and the next boot up to be very fast too for which I researched and tried large number of Linux distributions from Damn Small Linux to Slackware to Debian etc. but that's a separate story.

  • 1
    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. Nov 27, 2015 at 13:38
  • 2
    @Sam152 which wall?
    – Uthman
    Nov 28, 2015 at 18:19
  • 2
    Did you want to also specify "without damaging my system", or is that not a concern? Dec 31, 2015 at 18:53
  • 1
    @Technophile not a permanent damage to my system of course. But what do you have in mind?
    – Uthman
    Jan 2, 2016 at 2:06
  • 1
    My purpose is to clarify what you are asking and what you are trying to do. I work in electronics; does "instantly" mean 1 second? 1ms? 1us? 1ns? Are you simply impatient, or is there some specific need? What are the consequences of a (for example) 6 second shutdown time? I am working with an embedded system which corrupts its SD card if shutdown exceeds 5 seconds when the power button is held down. What are you doing? Jan 3, 2016 at 16:23

9 Answers 9

sudo shutdown -h now
  • 15
    That wouldn't shutdown a system instantly. ;) Feb 14, 2010 at 11:57
  • 18
    @Marcel Korpel: As instantly as one should do without damaging his system ;)
    – Felix
    Feb 14, 2010 at 12:05
  • 3
    @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, 2010 at 11:03
  • 3
    This answer is not correct. It does not answer the question, which was asking for an instant power-down. The correct command for this is halt -f.
    – Bachsau
    Aug 15, 2020 at 11:37
  • 3
    For halt, passing one -f will still perform clean shutdown by system manager, passing -f twice will perform immediate unclean shutdown without going through any sequence at all. E.g., halt -f -f -p will instant power-off the machine, and halt -f -f --reboot will instantly reboot the machine.
    – xuancong84
    May 30, 2023 at 17:05

You can power off the machine immediately without syncing disks by writing "o" to /proc/sysrq-trigger as root, or as any user with write permissions on that file.

As a shell script it would look something like

echo o >/proc/sysrq-trigger

There is a sysctl option about sysrq, but option only affects triggering by keyboard.

If you would like to restart immediately instead of powering off, you can send "b" instead of "o"; but keep in mind that while sysrq-trigger is the fastest way to shut down, there may be faster ways to restart. Board firmware (BIOS/UEFI) can take a lot of time to reinitialize hardware, so it is usually faster to use kexec to effect the restart, because the hardware remains initialized.

If you're on a systemd-based system, loaded with systemd-boot, you can run

systemctl kexec --force

and it will boot into your current kernel without shutting down services. It will unmount writable filesystems, but it will not do anything to read-only filesystems. This is likely to be faster than using sysrq, since it doesn't involve a POST or any other time spent in the board firmware.

If you are on a non-systemd system, or used a bootloader other than systemd-boot, you may want to use kexec more directly. You will need to find the paths to your your desired initramfs and kernel image. It will look something like

kexec -l /boot/vmlinuz-linux --initrd=/boot/initramfs-linux.img --reuse-cmdline
kexec -e

but in your case you will substitute those paths for the desired initramfs and kernel paths. These happen to be the default ones on Arch Linux.

On my system, with 128GiB of memory in eight modules, 16 cores and 32 hardware threads, two GPUs, and a lot of on-board peripherals to initialize. systemctl kexec (without --force) takes less than a quarter as much time as rebooting with sysrq-trigger.

As you probably know, this is not recommended in any case where you have a filesystem that is mounted as writable, even if you don't think you've written anything important to that filesystem while it was mounted. Even if you do not care about the integrity of data on that filesystem, shutting down or restarting like this can make booting up slower.

Make sure that none of your persistent filesystems are mounted as writable.

  • 13
    This is the best answer to this question. OP clearly states he want to shutdown the system in a similar way as if unplugged from power, they don't care about damage or "doing it properly". I don't understand why nobody can understand that.
    – Petr
    Dec 22, 2017 at 11:55
  • 2
    b instead of o to reboot (relevant since the question was updated). Oct 18, 2018 at 17:15
  • 2
    i honestly wish I could upvote this one more than once because it does exactly what the OP is requesting, does it in a second, and worked for me when all the other solutions didn't (my system was hung up on something and I couldn't get anything else to work)
    – JDS
    Apr 10, 2020 at 16:59
  • 1
    I had a situation where systemd/init was frozen and this was the only answer here that worked. Aug 11, 2022 at 20:35

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 cord...

The correct way is shutdown -h now with sudo before when needed.
Maybe I should say the lawful way; see below or better tl;dr.

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 handle 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 will it 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 lawful (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 poweroff  # the most close to kill -9 1 
systemctl --force poweroff          # rough but still safe
sudo halt -f                                       # rough
sudo telinit 0        # or 6                       # safe
kill -SIGINT 1        # cause reboot   as the reboot command
kill -SIGRTMIN+4 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.

  1. 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?

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

     systemctl --force --force poweroff  

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].

  • 3
    This answer is brilliant!
    – cst1992
    Apr 16, 2021 at 5:28
  • @cst1992 Thank you for appreciation. Sometimes, while trying to remain precise, one can also play in giving an answer...
    – Hastur
    Apr 16, 2021 at 7:54
  • So the answer is systemctl --force --force poweroff. I'm not sure why you don't mention it in the top, it wouldn't generate any core dumps or anything alike compared to the kills that you mentioned. It works for me to quickly shutdown a VM whose disk content I don't care about with Packer.
    – Hi-Angel
    Dec 8, 2023 at 12:37
  • @Hi-Angel Hi, Angel... No the correct way is sudo shutdown -h now. The others are prone to create problems especially with --force specified 2 times (you do not unmount drives, sync, flush etc etc). At most you can use systemctl --force poweroff, but remember that every time you use --force, you run the risk of losing data with processes that don't respond in time... For example, if you were writing a db you could lose not only the changes but also all the data or, even worse, it could write part of a record --but not all of it-- and you might not realize it.
    – Hastur
    Dec 8, 2023 at 15:47
  • ...Imagine if it was writing the new password that encrypts the whole SSD...
    – Hastur
    Dec 8, 2023 at 15:55

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

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

  • 5
    No. REISUB will restart it. REISUO will shutdown. Or simply O.
    – Benoit
    Nov 22, 2010 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:


I used this small script to turn off the system. Actually it's used this for a prank on a "prototyping" machine: "Hey dude, let's see what that awesome .sh do!"

echo 1 > /proc/sys/kernel/sysrq-trigger; #enables sysrq triggers
echo u > /proc/sysrq-trigger # remount the system as ready only
#edited# echo s > /proc/sysrq-trigger # sync all write-pending data to disk
echo o > /proc/sysrq-trigger # shut off the system power.

(This script imitates the same as hitting Alt + SysRq + u/s/o as previously explained.) Of course i't cause an abnormal system termination (mysql tables, etc may crash), so use only on dummy machines. Note: Sync may take some time. I've seen that if the system has a lot of free memory (for cache) and you copy a bunch of files to a pen drive, sync-ing takes more time, but I recommend it even for a prank.

Edit: It's better to use sync instead of writing 's' to sysrq-trigger: writing 's' will not block the script execution, so after requesting cache to be written into disk, the machine will shut off immediately. In contrast with sync command, sync will return after the writing process get done. So the script execution will be blocked, until the cache flushing get done.

  • 1
    Shouldn't you 's' before 'u'? Also, as a loop: SEQUENCE=(s u o); for i in "${SEQUENCE[@]}"; do sleep 3; printf "${i}" > /proc/sysrq-trigger; done May 28, 2017 at 2:53
  • 1
    The question specifically talks about shutting down in a way similar to loss of power. Every symbol and action in that sequence is not called for, except for o. If you want to sync the disks and everything, there are other cleaner ways to shut down (even if you don't want to wait for processes to be killed properly). Apr 11, 2021 at 1:00

Taken from another SE site:

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

Reference: Here

  • 2
    seems you have missed the reference, and quote your answer, then i'll reverse my vote Nov 26, 2015 at 14:28
  • This is the original answer: askubuntu.com/a/56072
    – cst1992
    Nov 27, 2015 at 5:39

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.


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

  • 1
    If doing so, flipping the power switch is actually faster ;)
    – Gnoupi
    Feb 16, 2010 at 9:58
  • That's the same as any other shutdown method, assuming that the OS does power management.
    – harrymc
    Feb 16, 2010 at 11:04
  • 1
    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, 2010 at 11:33
  • Not looking for a non-programmatic / non-software way of performing the shutdown otherwise @Gnoupi suggestion is way faster. ;)
    – Uthman
    Nov 28, 2015 at 18:17

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