CentOS Reference Guide

Peter Kitson

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Sample Chapter From CentOS Reference Guide
     Copyright © Red Hat Inc.


Welcome to the Red Hat Enterprise Linux Reference Guide.

The Red Hat Enterprise Linux Reference Guide contains useful information about the Red Hat Enterprise Linux system. From fundamental concepts, such as the structure of the file system, to the finer points of system security and authentication control, we hope you nd this book to be a valuable resource.

This guide is for you if you want to learn a bit more about how the Red Hat Enterprise Linux system works. Topics that you can explore within this manual include the following:

- The boot process

-  The le system structure

-  The X Window System

-  Network services

-  Security tools


Boot Process, Init, and Shutdown

An important and powerful aspect of Red Hat Enterprise Linux is the open, user-congurable method it uses for starting the operating system. Users are free to congure many aspects of the boot process, including specifying the programs launched at boot-time. Similarly, system shutdown gracefully terminates processes in an organized and congurable way, although customization of this process is rarely required.

Understanding how the boot and shutdown processes work not only allows customization, but also makes it easier to troubleshoot problems related to starting or shutting down the system.

1.1. 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 second stage boot loader loads the kernel into memory, which in turn loads any necessary modules and mounts the root partition read-only.

4. The kernel transfers 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 screen for the freshly booted Linux system.

Because conguration of the boot process is more common than the customization of the shutdown process, the remainder of this chapter discusses in detail how the boot process works and how it can be customized to suite specific needs.