How Linux Boots Part-1

Hello friends in this post and in the next two post i will tell you how Linux operating system boots and also tell you the whole processes that are connected during its booting.

As it turns out, there isn't much to the boot process:

1. A boot loader finds the kernel image on the disk, loads it into memory, and starts it.
2. The kernel initializes the devices and its drivers.
3. The kernel mounts the root filesystem.
4. The kernel starts a program called init.
5. init sets the rest of the processes in motion.
6. The last processes that init starts as part of the boot sequence allow you to log in.

Identifying each stage of the boot process is invaluable in fixing boot problems and understanding the system as a whole. To start, zero in on the boot loader, which is the initial screen or prompt you get after the computer does its power-on self-test, asking which operating system to run. After you make a choice, the boot loader runs the Linux kernel, handing control of the system to the kernel.

There is a detailed discussion of the kernel elsewhere in this book from which this article is excerpted. This article covers the kernel initialization stage, the stage when the kernel prints a bunch of messages about the hardware present on the system. The kernel starts init just after it displays a message proclaiming that the kernel has mounted the root filesystem:

VFS: Mounted root (ext2 filesystem) readonly.

Soon after, you will see a message about init starting, followed by system service startup messages, and finally you get a login prompt of some sort.

NOTE On Red Hat Linux, the init note is especially obvious, because it "welcomes" you to "Red Hat Linux." All messages thereafter show success or failure in brackets at the right-hand side of the screen.

Most of this chapter deals with init, because it is the part of the boot sequence where you have the most control.
init

There is nothing special about init. It is a program just like any other on the Linux system, and you'll find it in /sbin along with other system binaries. The main purpose of init is to start and stop other programs in a particular sequence. All you have to know is how this sequence works.

There are a few different variations, but most Linux distributions use the System V style discussed here. Some distributions use a simpler version that resembles the BSD init, but you are unlikely to encounter this.

Runlevels

At any given time on a Linux system, a certain base set of processes is running. This state of the machine is called its runlevel, and it is denoted with a number from 0 through 6. The system spends most of its time in a single runlevel. However, when you shut the machine down, init switches to a different runlevel in order to terminate the system services in an orderly fashion and to tell the kernel to stop. Yet another runlevel is for single-user mode, discussed later.

The easiest way to get a handle on runlevels is to examine the init configuration file, /etc/inittab. Look for a line like the following:

id:5:initdefault:
This line means that the default runlevel on the system is 5. All lines in the inittab file take this form, with four fields separated by colons occurring in the following order:
# A unique identifier (a short string, such as id in the preceding example)
# The applicable runlevel number(s)
# The action that init should take (in the preceding example, the action is to set the default runlevel to 5)
# A command to execute (optional)

There is no command to execute in the preceding initdefault example because a command doesn't make sense in the context of setting the default runlevel. Look a little further down in inittab, until you see a line like this:

l5:5:wait:/etc/rc.d/rc 5

This line triggers most of the system configuration and services through the rc*.d and init.d directories. You can see that init is set to execute a command called /etc/rc.d/rc 5 when in runlevel 5. The wait action tells when and how init runs the command: run rc 5 once when entering runlevel 5, and then wait for this command to finish before doing anything else.