Linux Interview Q&A #58 - Explain Linux Boot Process ? #linux #viral

Summary

This video script offers an insightful guide to the Linux boot process, essential for troubleshooting and system repair. It begins with the server power-up, followed by the BIOS's POST hardware check and bootable device search. The MBR's role in loading the bootloader, GRUB, is explained, detailing its stages and configuration. The script then delves into the kernel's responsibilities, including initializing hardware, loading drivers, and preparing the root file system. Finally, it touches on systemd's role in managing system services and reaching the operational state, providing a comprehensive overview of Linux's startup sequence.

Takeaways

• 🔌 The Linux boot process begins with powering up the server, which triggers the BIOS to perform POST (Power-On Self-Test) to check hardware integrity.
• 📂 BIOS, located in ROM, scans for bootable devices and looks for the MBR (Master Boot Record), the first sector of a bootable disk, to locate the bootloader.
• 🔍 The MBR consists of a small program (446 bytes), a partition table (64 bytes), and a magic number (2 bytes) for error detection.
• 🛠️ The bootloader, historically Lilo and now commonly GRUB or GRUB2, is responsible for loading the operating system.
• 🔁 GRUB has multiple stages: stage 1 is in the MBR, stage 1.5 acts as an intermediary for filesystem interpretation, and stage 2 is the main GRUB image.
• 📝 The bootloader reads configuration from files like '/boot/grub/grub.conf' or '/boot/grub2/grub.cfg' to determine how to load the kernel and initrd images.
• 💾 The kernel initializes hardware, locates and decompresses the initrd image, loads necessary drivers, and initializes virtual devices like LVM or RAID.
• 🌐 Once the kernel has set up the system, it creates a root device, mounts the root partition, and starts the first process (pid 1), which is now systemd in modern systems.
• 🔄 Systemd, known as the 'mother of all processes,' manages the boot process by mounting filesystems, starting services, and bringing the system to a productive state.
• 🔖 The boot process concludes with systemd reading '/etc/systemd/system/default.target' to determine the system's target state, such as graphical for desktops.

Q & A

What is the first step in the Linux boot process?

The first step in the Linux boot process is powering up the server, after which the hardware transfers control to the BIOS.

What are the two important tasks performed by BIOS during the boot process?

BIOS performs a Power-On Self-Test (POST) to check the health of all hardware components and then scans for bootable devices.

What is the significance of the Master Boot Record (MBR) in the boot process?

The MBR is the first sector of a bootable storage device, containing a small program that points to a bootloader, a partition table, and a magic number for error detection.

What is the role of the bootloader such as GRUB or GRUB2 in the Linux boot process?

The bootloader, like GRUB or GRUB2, is responsible for loading the operating system by reading configuration settings and identifying the kernel and initrd images.

What are the different stages of GRUB and what do they do?

GRUB has stages 1, 1.5, and 2. Stage 1 is in the MBR, stage 1.5 is an intermediate stage for file system interpretation, and stage 2 is the main GRUB image that loads the operating system.

What does the kernel do after it is loaded during the boot process?

Once the kernel is loaded, it initializes and configures the computer's memory and hardware, decompresses itself, locates and loads the initrd image, and initializes drivers for file systems like LVM or RAID.

What is the purpose of the initrd image in the boot process?

The initrd image contains drivers and system information needed during the boot process before the main system's root file system is available.

How does the kernel prepare the system for the final boot steps?

The kernel creates a root device, mounts the root partition in read-only mode, freezes unused memory, and starts the first process (pid 1), which is managed by init or systemd in modern systems.

What is the role of systemd in the boot process of a Linux system?

Systemd is responsible for managing the boot process by starting services, mounting file systems, and bringing the Linux host to a state where productive work can be done.

How does systemd determine the system's boot target?

Systemd reads the /etc/systemd/system/default.target file to identify the default target, which could be a graphical target for desktop workstations, equivalent to run level 5 in older systems.