In order to properly manage any computing platform, you must understand the boot process. Many systems administration tasks cannot be done while the system is running. OpenBSD specifically requires that certain tasks be done before the boot process has completed. And, of course, on any operating system, sometimes a process starting up prevents the system from completing its boot. The only way to fix these problems is to interrupt the boot partway through.

First, we’ll look at the key to OpenBSD’s booting process: the boot loader. Then we’ll move on to single-user mode, and finally multiuser startup. You can perform useful work at any of these stages.

I recommend playing with the OpenBSD boot process on a test machine before one of your machines won’t boot. That way, when something breaks in the wee hours of the morning, you can spend your time fixing the problem instead of fumbling around with unfamiliar commands.