Chapter 3.

Boot Process, Init, and Shutdown

An important and powerful aspect of Red Hat Linux is the open method it uses for starting and stop 

ping the operating system. During boot time, Red Hat Linux loads programs in a specific and config 

urable order. Once booted, you are free to change the configuration files controlling the boot process

as well as the configuration files for the programs started at boot time. Similarly, system shutdown

gracefully terminates processes in an organized and configurable way, although customization of this

process is rarely required.

Understanding how the boot and shutdown processes work will not only allow you to easily customize

Red Hat Linux to your needs, but will also make it easier to troubleshoot problems related to starting

or shutting down your system.

3.1. The Boot Process

Below are the basic stages of the boot process for an x86 system:

1. The system BIOS checks the system and launches the first stage boot loader on the MBR of the

primary hard disk.

2. The first stage boot loader loads itself into memory and launches the second stage boot loader

from the

/boot/

partition.

3. The kernel is loaded into memory, which in turn loads any necessary modules and mounts the

root partition read only.

4. The kernel hands control of the boot process to the

/sbin/init

program.

5. The

/sbin/init

program loads all services and user space tools, and mounts all partitions

listed in

/etc/fstab

.

6. The user is presented with a login prompt for the freshly booted Linux system.

Because configuration of the boot process is more common than the customization of the shutdown

process, the remainder of this chapter will discuss in detail how the boot process works and how it

can be customized to suite your needs.

3.2. A Detailed Look at the Boot Process

The very beginning of the boot process varies depending the platform on which you are booting Red

Hat Linux. However, once the kernel is found and loaded by the system, the default boot process is

identical across all architectures. In the example outlined below, the system will be an x86 computer.

3.2.1. The BIOS

When an x86 computer is booted, the processor looks at the end of system memory for the Basic

Input/Output System or BIOS program and runs it. The BIOS controls not only the first step of the

boot process, but also provides the lowest level interface to peripheral devices. For this reason it is

written into read only, permanent memory and is always available for use.

Other platforms use different programs to perform low level tasks roughly equivalent to those of the

BIOS on an x86 system. For instance, Itanium based computers use the Extensible Firmware Interface

(EFI) Shell, while Alpha systems use the SRM console.