Boot Process, Init, and ShutdownAn important and powerful aspect of Red Hat Enterprise Linux is the open, user-configurablemethod it uses for starting the operating system. Users are free to configure many aspects ofthe boot process, including specifying the programs launched at boot-time. Similarly, systemshutdown gracefully terminates processes in an organized and configurable way, althoughcustomization of this process is rarely required.Understanding how the boot and shutdown processes work not only allows customization, butalso makes it easier to troubleshoot problems related to starting or shutting down the system.1. The Boot ProcessBelow 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 ofthe primary hard disk.2. The first stage boot loader loads itself into memory and launches the second stage bootloader from the /boot/ partition.3. The second stage boot loader loads the kernel into memory, which in turn loads anynecessary 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 partitionslisted in /etc/fstab.6. The user is presented with a login screen for the freshly booted Linux system.Because configuration of the boot process is more common than the customization of theshutdown process, the remainder of this chapter discusses in detail how the boot process worksand how it can be customized to suite specific needs.2. A Detailed Look at the Boot ProcessThe beginning of the boot process varies depending on the hardware platform being used.However, once the kernel is found and loaded by the boot loader, the default boot process isidentical across all architectures. This chapter focuses primarily on the x86 architecture.2.1. The BIOSWhen an x86 computer is booted, the processor looks at the end of system memory for theBasic Input/Output System or BIOS program and runs it. The BIOS controls not only the firststep of the boot process, but also provides the lowest level interface to peripheral devices. Forthis reason it is written into read-only, permanent memory and is always available for use.Chapter 1.3
