Boot and recovery

Between the power button and the login prompt runs a chain of handoffs. The firmware starts the bootloader, the bootloader starts the kernel, the kernel brings up the first process, and that process drives the system to a working state. Once you know which link does what, you can tell from where the boot stopped which command will fix it.

This chapter is about where each link lives and how to step in when the system never reaches login. It also covers the classic admin task: you lost the root password and you need to reset it with nothing but a console.

The first link is the firmware on the motherboard. The old one is BIOS, the new one is UEFI. You can tell which you have by whether the directory of UEFI variables is present:

$ ls /sys/firmware/efi    # directo