Is there a modified Alt + Sysrq + R + E + I + S + U + B which only shuts down the computer (without rebooting)?

There are days when the system is frozen and I'm too tired to debug the problem and might make it even worse if I tried to fix it in my drowsy state...

I tried Alt + Sysrq + R + E + I + S + O, but there is no effect after typing the sequence slowly (I repeated the sequence after a few seconds to confirm I did not mis-type).

After trying the above combination, I used the regular Alt + Sysrq + R + E + I + S + U + B which did work (rebooted).

I'm currently using force shutdown (ie. press and hold power button)

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    how about 'o' after the 's'? (or alt+sysrg+r,e,i,s,o), reference: en.wikipedia.org/wiki/Magic_SysRq_key – Theophrastus Apr 21 '17 at 1:19
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    @Theophrastus why not just make your comment an answer? It looks like that would be exactly what OP is looking for. – domsson Apr 23 '17 at 12:25

Unfortunately, the 'o' sysrq key doesn't always work. The reason for this is quite complicated, and often system-specific, but it boils down to the fact that it's pretty easy to trigger a system reset because doing so just involves pulling a single input on the CPU low (or triggering some internal reset circuitry in the CPU, for example by triple faulting on an x86 system), but actually powering off the system is often somewhat complicated, and usually requires calling into the system firmware, and isn't even supported by the kernel on all systems to begin with (many embedded systems may not have any way to actually power off completely, the Raspberry Pi is a good example).

As a result of this, there isn't really any portable way to power off a hung Linux system short of just manually cutting power like you are already.

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