This chapter looks at what happens during a Linux boot. It examines the processes that take place and the configuration files that are read. Booting is a critical part of the operation of a server. The boot process brings all of the network hardware online and starts all of the network daemon processes when the system is powered-up. If the server will not boot, it is unavailable to all of the users and computers that depend on it. For this reason, it is essential that the administrator of a network server understand the boot process and the configuration files involved in that process. After all, you're the person who maintains those configuration files and who is responsible for recovering the system when it won't boot.
The term boot comes from bootstrap loader, which in turn comes from the old saying "pull yourself up by your bootstraps." The meaning of this expression is that you must accomplish everything on your own without any outside help. This is an apt term for a system that must start from nothing and finish running a full operating system. When the boot process starts, there is nothing in RAM—no program to load the system. The loader that begins the process resides in non-volatile memory. On PC systems, this means that the loader is part of the ROM BIOS.
Booting a Linux PC is a multistep procedure. It involves basic PC functions as well as Linux processes. This complex process begins in the PC ROM BIOS; it starts with the ROM BIOS program that loads the boot sector from the boot device. The boot sector either contains or loads a Linux boot loader, which then loads the Linux kernel. Finally, the kernel starts the init process, which loads all of the Linux services. The next few sections discuss this process in detail.
Note Two Linux loaders, LILO and GRUB, are covered in this chapter. LILO is given the bulk of the coverage because it is the default for most Linux distributions. GRUB is covered because it is the default loader for Red Hat Linux 7.2.
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