In college, many aeons ago, I bought a used Sun workstation (mainly for the copy of FrameMaker on the machine). At some point I was trying to clean up the root filesystem and deleted the root shell (I can't remember the details but I think there were two shells, one used to boot the system, and one for the user shell).
That's how I learned that SunOS machines at the time had an extensive set of features for booting the machine (like, a built in EFI shell) and providing kernel params, so I was able to finally boot into the machine by telling the kernel to launch the user shell, which was enough to boot the machine to user space. The SunOS machines also could do pretty much everything over serial- including installing an entire OS.
That's how I learned that SunOS machines at the time had an extensive set of features for booting the machine (like, a built in EFI shell) and providing kernel params, so I was able to finally boot into the machine by telling the kernel to launch the user shell, which was enough to boot the machine to user space. The SunOS machines also could do pretty much everything over serial- including installing an entire OS.