Assembly (x86/x64, Linux, as), 22 21 19 bytes

mov  $0x58,       %al  # 2 bytes: b0 58
mov  $0xfee1dead, %ebx # 5 bytes: bb ad de e1 fe
mov  $0x28121969, %ecx # 5 bytes: b9 69 19 12 28
mov  $0x4321fedc, %edx # 5 bytes: ba dc fe 21 43
int  $0x80             # 2 bytes: cd 80

Must be run as root.

This is equivalent to pressing the power button and not a safe way to power off your PC. Make sure you close all open applications and execute sync to flush all file system buffers before executing this program, to at least minimize the risk of file corruption.

Test run

$ as -o poweroff.o poweroff.s
$ ld -o poweroff   poweroff.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000000400078
$ sudo sh -c 'sync && ./poweroff'
root's password:

Followed by darkness.

How it works

int $0x80 invokes a software interrupt. It works on both x86 and x64, but has been deprecated for over a decade now and should not be used in production code. x64 code should use a syscall (syscall instead. x86 should use sysenter, but is is too cumbersome for code golf.

The resulting action from the syscall depends on registers EAX - EDX, ESI, and EDI. The Linux Syscall Reference shows all syscalls that are available via int $0x80.

When EAX holds 0x58 (88), reboot is called, which can also be used to power off, put to sleep, or hibernate the computer, as well as switching kernels and disabling or enabling the Ctrl - Alt - Del key combo.

At the start of the program – and by compiling with as or gcc -nostdlib, we can make sure that we're actually at the start of the program – most registers are set to 0. This includes EAX, so we can use mov $0x58, %al to set the lower 8 bits of EAX to 0x58, thus setting EAX itself to 0x58. This saves two bytes over manually zeroing the register with xor %eax, %eax and one more over the straighforward mov $0x58, %eax which encodes 0x58 in 32 bits.

The first two arguments to reboot are magic numbers, presumably to prevent accidental reboots, and are read from registers EBX and ECX. Unless these numbers are equal to certain constants, reboot refuses to perform any action.

• The first magic number must equal 0xfee1dead (feel dead), probably referring to the power off / death of the PC.

• The second magic number can be equal to four different constants, although the latter three did not work in ancient versions of Linux. All of them seem to refer to the subsequent power on / birth of the PC. Patricia, Daniela, and Celeste Torvalds are Linus Torvalds's three daughters.

  • 0x28121969 represents Linus Torvalds's birthday (December 28, 1969).

  • 0x05121996 represents Patricia Torvalds's birthday (December 5, 1996).

  • 0x16041998 represents Daniela Torvalds's birthday (April 16, 1998).

  • 0x20112000 represents Celeste Torvalds's birthday (November 20, 2000).

• The EDX register selects the type of "reboot" we want. 0x4321fedc is RB_POWER_OFF, shutting the PC down and powering it off.

• Finally, the value of the ESI register is ignored for RB_POWER_OFF; the value of the EDI register is ignored entirely by reboot.

Alternate version, x64-only, 19 bytes

On x64, we can use a proper syscall for the same byte count.

mov     $0xa9,       %al  # 2 bytes: b0 a9
mov     $0xfee1dead, %edi # 5 bytes: bf ad de e1 fe
mov     $0x28121969, %esi # 5 bytes: be 69 19 12 28
mov     $0x4321fedc, %edx # 5 bytes: ba dc fe 21 43
syscall                   # 2 bytes: 0f 05

The only differences lie in the instruction (syscall vs int $0x80), the value of __NR_REBOOT (0xa9 vs 0x58), and the involved registers.

PowerShell, 13 bytes

Stop-Computer

Pretty self-explanatory? Doesn't work on PowerShell core, so it won't work on PowerShell for Linux/MacOS (already tried it on TIO :-D ).

Bonus Garbage Submission, 12 bytes

gcm s*r|iex

In my testing, this works on Windows 2003 with PowerShell 2.0. It won't work on most of the newer versions, and it may not work depending on which modules are installed. It's quite terrible!

This searches the list of commands for any with the pattern s*r and then runs all of them! To be clear, this is dangerous, don't do this!

The trick of course, is getting a list of commands that don't have any mandatory parameters, otherwise PowerShell will prompt for a value.

On my 2003 machine, the 3 commands it returns are:

CommandType     Name                                                Definition                                         
-----------     ----                                                ----------                                         
Application     scrnsave.scr                                        C:\WINDOWS\system32\scrnsave.scr                   
Application     ssmarque.scr                                        C:\WINDOWS\system32\ssmarque.scr                   
Cmdlet          Stop-Computer                                       Stop-Computer [[-ComputerName] <String[]>] [[-Cr...

Yes, it does launch the marquee screensaver, but no, it does not wait for it to complete before launching Stop-Computer.

This approach can work on newer machines, again depending on which modules are installed, but the best I could do on Windows 2012 and 2016 was 14 bytes:

gcm sto*er|iex

(and yeah, sure, the winbatch submission will work in PowerShell and it's shorter, but.. that's no fun)

Batch, 10 bytes

On windows, this would suffice

shutdown/s

This will set up a shutdown in one minute.

Alternatively, as @flashbang points out:

shutdown/p

This shuts down the computer immediately.

GRUB shell, 4 bytes

Golfed

halt

Command: halt --no-apm

The command halts the computer. If the --no-apm option is specified, no APM BIOS call is performed. Otherwise, the computer is shut down using APM.

Bonus

Here is a proof that GRUB shell is indeed Turning-complete (see comments):

65C02 machine code, 1 byte

STP instruction (0xDB):

STP stops the clock input of the 65C02, effectively shutting down the 65C02 until a hardware reset occurs (i.e. the RES pin goes low). This puts the 65C02 into a low power state. This is useful for applications (circuits) that require low power consumption, but STP is rarely seen otherwise.

(Source: http://www.6502.org/tutorials/65c02opcodes.html)

xxd dump:

00000000: db                                       .

Matlab, 21 Bytes

system('shutdown -s')

I haven't tried this (closing all programs to do a restart in the middle of the day isn't really tempting), but it should work.. This works...

I'll add a short explanation later, my laptop is currently off.

x86 assembly, Lenovo Z510, DOS COM or Bootloader, 7 bytes

BITS 16

mov ax, (1 << 13) | (7 << 10)
mov dx, 1804h
out dx, ax

Assembled into B8 00 3C BA 04 18 EF

I'm exploiting "You are allowed to write code that only works in a specific environment/OS, if you wish" to the ultimate level: this will only work on Lenovo Z510¹.

This writes SLP_TYPa << 10 | SLP_EN to the PM1a_CNT ACPI register, I dumped this values from the ACPI tables months ago for an answer about shutdown (where I give a bit of context to the code above).

This can be run either under DOS as a COM (just write the binary data into a file and change extension to "com") or as a bootloader.
For the latter write the standard signature and beware of the issues with modern firmware.

¹ Not strictly true, any system that happens to have the same ACPI configuration will also shutdown.

MATLAB, 11 bytes

!shutdown/p

Somewhat similar to Stewie Griffin's approach. However, MATLAB has way shorter ways of invoking system commands; in this case, ! is used. Windows does not need .exe for command names, so that's left out as well. The / option is still supported (I'm running Windows 10), and negates the need for the space. Alternatives are:

system shutdown/p % 17 bytes; uses so-called 'command' syntax.
dos shutdown/p    % 14 bytes; the 'dos' command can be used just as well.

SmileBASIC (3.3.2), 14 bytes

READ VAR(A)[0]

Triggers a crash causing the 3DS to restart. I hope this counts.

Bash, 7 6 bytes

init 0

Assuming that the program is run as root.

Java, 101 98 62 42 bytes

()->{Runtime.getRuntime().exec("init 0");}

As @Snowman pointed out, the question didn't specify if a full program was needed, so this should still be acceptable.

Commodore 64, 11 bytes

0 sys64738

This is from the BASIC prompt. Or...

Commodore 64, 6502 assembly, 3 bytes (assembled), 16 bytes (source)

*=8000
JMP $FCE2

Python 2, 29 bytes

import os;os.system('init 0')

Works on Linux systems and needs root privileges.

For Windows, 33 bytes

import os;os.system('shutdown/p')

This just might be the least interesting or helpful code I have ever written.

C++, 51 40 bytes

-11 bytes thanks to Ingve!

#include<os>
int main(){OS::shutdown();}

Only works in IncludeOS (v0.10.0-rc.1)

Test:

$ pwd
/home/simon/IncludeOS/examples/demo_service

$ cat service.cpp
#include<os>
int main(){OS::shutdown();}

$ mkdir build && cd build && cmake .. && make && cd ..
[...]

$ boot build/IncludeOS_example
[...]
================================================================================
================================================================================
 IncludeOS v0.10.0-rc.1-9-g151a2b9
 +--> Running [ IncludeOS minimal example ]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

$

TI-Nspire CX CAS, 3 bytes

" "+" "

summing two empty strings.

On an older version of the calculator, this would crash the device and cause it to reboot (there's no way to shutdown a device without reboot apart from physical button combinations).

Video by Adriweb

AutoHotkey, 11 10 Bytes

Saved a byte thanks to Gurupad Mamadapur

Shutdown,9

AHK Supports a Shutdown command by default. The 9 is a flag, consisting of Shutdown=1 + PowerDown=8.

C#, 79 54 53 bytes

This works on Windows.

79 bytes:

class _{static void Main(){System.Diagnostics.Process.Start("shutdown","/s");}}

54 bytes, thanks to TheLethalCoder:

_=>System.Diagnostics.Process.Start("shutdown","/s");

AppleScript, 30 bytes

tell app "Finder" to shut down

Does exactly what it says: tells Finder to shut the computer off.

Machine code (x86, boot loader) 18 bytes

b8 01 53 33 db cd 15 b8 07 53 bb 01 00 b9 03 00 cd 15

Disassembly:

mov ax, 0x5301 ; connect to real-mode APM services
xor bx, bx     ; device id 0 - APM BIOS
int 0x15       ; call APM
mov ax, 0x5307 ; set power state
mov bx, 0x0001 ; on all devices
mov cx, 0x0003 ; to Off
int 0x15       ; call APM

You can test it in Bochs' debugger with the following commands (after starting simulation, having some disk drive set up):

pb 0x7c00
c
setpmem 0x7c00 4 0x335301b8
setpmem 0x7c04 4 0xb815cddb
setpmem 0x7c08 4 0x01bb5307
setpmem 0x7c0c 4 0x0003b900
setpmem 0x7c10 2 0x15cd
c

This same machine code will also work as a DOS executable (*.COM).

AutoIt, 11 bytes

Shutdown(1)

Shutdown function gets combination of following codes

0 = Logoff
1 = Shutdown
2 = Reboot
4 = Force
8 = Power down
16= Force if hung
32= Standby
64= Hibernate

To shutdown and power down, for example, the code would be 9 (shutdown + power down = 1 + 8 = 9).

Clojure, 40 39 bytes

(.exec(Runtime/getRuntime)"shutdown/s")

-1 byte by changing the flag from -s to /s, which allowed me to get rid of the space.

Basically the Java answer.

Testing this was fun. Windows decided to do a firmware update on my Surface the second it shut down. Of course.

(defn shutdown []
  (.exec (Runtime/getRuntime) "shutdown -s"))

Groovy, 23 bytes

'shutdown /p'.execute()

Windows - NativeAPI, 212 bytes

Ok this is not really the shortest version. But maybe interesting to some. This will not execute on the Win32 subsystem. However it will run when executed within the context of the native api (like autochk.exe does)

Code

#define WIN32_NO_STATUS
#include <windef.h>
#include <winnt.h>
#include <ntndk.h>
NTSTATUS main() {
RtlAdjustPrivilege(SE_SHUTDOWN_PRIVILEGE,TRUE,FALSE,NULL);
return ZwShutdownSystem(ShutdownPowerOff);
}

Build

To build this either use the WinDDK build.exe tool (with an appropriate source file for NMAKE)

Or use these commands to compile and link:

cl /Gd /D_X86_ /showIncludes /I%DDK_INC_PATH% /I%CRT_INC_PATH% /c main.c
link /verbose /nodefaultlib /subsystem:native /machine:X86 /entry:NtProcessStartup@4 C:\WinDDK\7600.16385.1\lib\win7\i386\nt.lib C:\WinDDK\7600.16385.1\lib\win7\i386\ntdllp.lib C:\WinDDK\7600.16385.1\lib\win7\i386\BufferOverflow.lib .\main.obj 

Note: This will build an x86 native executable. Change the appropriate bits if you want to build this for a different architecture.

Test

To test a native executable you have to execute it before the win32 subsystem is loaded. One common way to do it is to append the executable in the following registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\BootExecute

Warning

If you do this however, you need to have an option to edit the registry hive of the target OS from another OS. Since you will not be able to boot the target OS anymore!

I did it on a VM where i could easily revert to a previous snapshot!

J, 14 bytes

2!:0'poweroff'

Works on Linux systems. 2!:0 executes the string in /bin/sh.

C++, 48 47 (Windows) 46 45 (Linux) bytes

Removed 1 byte thanks to Snowman

#include<cstdlib>
main(){system("shutdown/p");}

This should shutdown the system for windows

#include<cstdlib>
main(){system("poweroff");}

This should shutdown the system for Linux

Most Linux shells, 15 bytes

# echo o>/p*/*ger

It could be made to be 11 bytes long, if you happen to be in the /proc directory:

# echo o>*ger

This is very straightforward (has no side effects, like unmounting filesystems or killing processes) and rather brutal.

AppleScript, 44 bytes

tell application "Finder"
shut down
end tell

I don't know how AppleScript syntax works or anything, so if anybody knows how to shorten this, please tell.

Also, I'm doing this on my phone at school ("hacking"), so I can't test it.

At 14 fewer bytes (thanks to kindall) you can say

tell app "Finder" to shut down

Node.js, 39 bytes

Linux

require('child_process').exec('init 0')

AHK, 19 15 bytes

Run,Shutdown -s

Works on windows only.

Mathematica on Windows 16 Bytes

Invoke the shell command with Run.

Run@"Shutdown/p"