Isn't that only a GNU and FreeBSD thing? I think other Unixes will still let you rm -rf /.

On FreeBSD you can do

    sudo dd if=/dev/random of=/dev/mem

and it will do exactly that, write random crap into your memory without any sort of safeguard, causing a spectacular crash and a console that looks like it's having a seizure. Linux won't let you do that unless it's been compiled with a flag to enable full access to /dev/mem and /dev/kmem.

I want to try that dd command. For fun. Just to see what happens on OpenBSD.

No lasting effects are there? Besides a crash, afterwards I shouldn't have any memory issues, right?

I tried this in a FreeBSD VM and it booted fine afterwards. OpenBSD wouldn't let me do it at all for some reason.

Obviously no warranty on bare metal/systems that are actually used for anything.

worth noting that it can be trivially workarounded by adding an asterisk at the end: `rm -rf /*` still works.

Fair enough, but on the other hand there's nothing preventing you from just putting --no-preserve-root in the command either. I see the feature as something to prevent accidents, not as a way to secure the rm command.

That version will helpfully leave behind any .files lingering in /