How linux boots

22 Dec

Linux is open source Kernel . That with several other GNU software is the basis of so called Linux Operation systems like Ubuntu, red hat , fedora etc.
It has a simple 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.

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.

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. 

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