Linux Boot Process
In this article
we are going to study “Linux Boot Process”. During booting time of Linux
operating system, boot process have to go through six basic stages.
Boot Stage 1:
BIOS Checks: When a Linux server or desktop or laptop
booted, The processor goes for Basic Input/Output System or BIOS program and
runs it.The BIOS controls does not controll only the first step of the boot
process, but BIOS also provides the lowest level interface to peripheral
devices. It means during the BIOS initialization, all hardware peripheral
devices checked out by BIOS. If BIOS found any hardware is faulty, you will get
message on your screen.For this reason BIOS Process is written into read-only
mode.The BIOS then loads into memory whatever program is residing in the first
sector of this device, called the Master Boot Record or MBR. The MBR is only
512 bytes in size and contains machine code instructions for booting the
machine, called a boot loader, along with the partition table. Once the BIOS
finds and loads the boot loader program into memory,it yields control of the
boot process to it.
Note: The default boot
loader for Linux platform is GRUB.
Boot Stage 2:
First stage boot loader: The first stage of Boot Loader is a small
machine code binary on MBR. The first stage boot loader loads itself into
memory.first stage boot loader job is to find out the second stage boot loader
and load the first part of it into memory.
Boot Stage 3:
Second stage boot
loader: The second stage boot loader loads the kernel
into memory, which in turn loads any necessary modules and mounts the root
partition read-only. Once the second stage boot loader is in memory, it
presents the user with a graphical screen showing the different operating
systems or kernels that has been configured to boot. You can use Arrow keys to
select your operating system for boot.
Stage
3.5: the Second Stage Bootloader loads the Linux Kernel and hands control to
it. The Kernel takes control of the hardware. This includes taking control of
the memory management hardware, timers, interrupts, etc. The processor is no
longer in x86 16-bit mode like it was when the BIOS started. The kernel will
start using other processors and/or additional cores within processors. Linux
may have some modules pre-compiled into the kernal, and may also load some
modules from disk as needed, such as device driver modules that apply to the
actual hardware present on this system. Once the kernel is fully running, it
starts a single user space process, which is the ancestor of all processes:
/sbin/init. Up until this point, all Linux systems work very similarly. Once
/sbin/init begins, this is where the major differences between distributions
become highly visible.
Boot Stage 4:
INIT Program: The Linux operating system kernel transfers the
control of the boot process to the /sbin/init program.The /sbin/init
program coordinates the rest of the boot process and configures the
environment for the user.
Boot Stage 5:
User Environment Setup: The init program loads all services and
user-space tools, and mounts all partitions listed in /etc/fstab. When the init
command starts, it becomes the parent or grandparent of all of the processes
that start up automatically on the system. First, it runs the
/etc/rc.d/rc.sysinit script, which sets the environment path, starts swap,
checks the file systems, and executes all other steps required for system
initialization.The init command then runs the /etc/inittab script, which
describes how the system should be set up in each SysV init runlevel.The init program
starts all of the background processes by looking in the appropriate rc
directory for the runlevel specified as the default in /etc/inittab. The rc
directories are numbered to correspond to the runlevel they represent.
Note: By Default, run
level 5 is defined in /ete/inittab file.
Boot Stage 6:
Login Screen: The user will get a login screen for the
freshly booted Linux system.
