Mastering Red Hat Linux 9 [Electronic resources] نسخه متنی

Exploring the Basic Boot Process

Before getting into the nitty-gritty of Red Hat Linux configuration files, it’s important to have a “big picture” overview of the process. While small changes can keep Red Hat Linux from booting, an understanding of the big picture can help you identify the problem quickly.

When you start your Linux computer, three basic steps are involved in the process. Hardware is initialized through your Basic Input/Output System (BIOS) and is then detected by Linux. The BIOS points to the Linux bootloader. Once the bootloader starts, it opens the kernel. Next, it starts init, the so-called "first program," which then loads your kernel, and moves to initialize other startup programs. Finally, Linux finds the default runlevel and starts all associated processes.

We provide detailed information on each of these processes later in this chapter.

Initializing Hardware

While this is not a book on computer hardware, it’s helpful to know some basics. Then it’s easy to determine if you have a hardware problem or a Linux problem.

Everything on a standard PC starts with the BIOS. The first step, associated with a series of beeps, is known as the power-on self-test (POST), which checks connections to basic hardware. It looks for other BIOSes related to IDE and SCSI hard drives. It may also detect other basic hardware on your system.

After Linux initiates the loading process through the bootloader, it begins to detect hardware using the kudzu utility. Then it adds modules related to your hardware, using settings stored in /etc/modules.conf. You can analyze the results with the dmesg command. If you’re having a hardware problem, a little detective work with dmesg output can help you identify the trouble.

Bootloaders

There are two basic Linux bootloaders, the Grand Unified Bootloader (GRUB) and the Linux Loader (LILO). GRUB is the default for Red Hat Linux. LILO is now obsolete, and will probably be removed in some future release of Red Hat Linux.

In either case, the bootloader is used for four purposes:

To select an operating system (if more than one is installed on your computer)

To identify the partition with the appropriate boot files

To locate the kernel

To run the Initial RAM disk to set up the kernel and associated modules

Runlevels

A runlevel is a specific way to organize initialized software in Linux. Different services are started and stopped at different runlevels. When you start Red Hat Linux, it looks to /etc/inittab to determine the default runlevel, which then points to an associated subdirectory of /etc/rc.d to identify the services to kill and start.