You now know the physical structure of a Linux system, what the kernel is, and how to work with processes. This chapter teaches you how the system starts (boots) — that is, how the kernel gets into memory and how the regular system processes get started.
As it turns out, there isn't much to the boot process:
A boot loader finds the kernel image on the disk, loads it into memory, and starts it.
The kernel initializes the devices and its drivers.
The kernel mounts the root filesystem.
The kernel starts a program called init.
init sets the rest of the processes in motion.
The last processes that init starts as part of the boot sequence allow you to log in.
Identifying each stage of the boot process is invaluable in fixing boot problems and understanding the system as a whole. To start, zero in on the boot loader, which is the initial screen or prompt you get after the computer does its power-on self-test, asking which operating system to run. After you make a choice, the boot loader runs the Linux kernel, handing control of the system to the kernel.
There is a detailed discussion of the kernel elsewhere in this book (Section 2.2 explains the role of the kernel, and Chapter 10 tells you how to build one yourself), but this chapter covers the kernel initialization stage, the stage when the kernel prints a bunch of messages about the hardware present on the system. The kernel starts init just after it displays a message proclaiming that the kernel has mounted the root filesystem:
VFS: Mounted root (ext2 filesystem) readonly.
Soon after, you will see a message about init starting, followed by system service startup messages, and finally you get a login prompt of some sort.
On Red Hat Linux, the init note is especially obvious, because it "welcomes" you to "Red Hat Linux." All messages thereafter show success or failure in brackets at the right-hand side of the screen.
Most of this chapter deals with init, because it is the part of the boot sequence where you have the most control. Section 3.2 deals with the boot loaders.