[vidarh]

Pid 1 is special - it can't be killed.

It can crash due to bugs it can't handle, or it can voluntarily shut down.

Try it:

  vidarh@opus:~$ sudo kill -9 1
  [sudo] password for vidarh: 
  vidarh@opus:~$ 

No effect.

In the case of systemd, if it runs into a non-recoverable situation, crash() in core/main.c gets called, which then proceeds to try to create a core dump and spawn a shell as an absolute last resort to give an admin a chance to take corrective action, which is already a step up from your typical init assuming the manage to get the part of systemd that runs as pid 1 (by no means all of systemd runs as pid 1) as stable/bug-free as your usual init.

Of course there's an uncertainty there, and they'll have to prove they can keep that part rock solid or it'll be useless.

[throwaway092834]

There are plenty of ways to kill pid 1. The major concern is simply software error, which is another reason why the current systemd design is poor. But if you'd like a concrete example, go ahead and attach gdb and use your imagination.