Installation guide

Chapter 33. 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.
Important — Boot Time Message Logging
In previous versions of Red Hat Enterprise Linux, boot time messages from init scripts were
logged to /var/log/boot.log. This functionality is not available in Red Hat Enterprise
Linux 5.
33.1. 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.
33.2. A Det ailed Look at t he 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.
33.2.1. T he BIOS
⁠Chapt er 33. Boot Process, Init , and Shut down
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