Red Hat Enterprise Linux Installation Guide 5.2 - linux.meuhobby.com

Chapter 30.
Boot Process, Init, and Shutdown
An important and powerful aspect of Red Hat Enterprise Linux is the open, user-configurable
method it uses for starting the operating system. Users are free to configure many aspects of
the boot process, including specifying the programs launched 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 not only allows customization, but
also makes it easier to troubleshoot problems related to starting or shutting down the system.
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 configuration 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.
2. A Detailed Look at the Boot Process
The 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 is
identical across all architectures. This chapter focuses primarily on the x86 architecture.
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.
337