❓ Explaining the boot process is often one of the most frequently asked questions during DevOps engineer interviews. If an interviewer omits this question, it might come as a surprise as it is typically expected. Here is my attempt to provide a straightforward explanation of the boot process:
🔌 Power On and POST: When you power on your machine, it performs a Power-On Self-Test (POST) to ensure all hardware necessary for the system to operate is functioning correctly.
🧠 BIOS/UEFI: The BIOS/UEFI (Basic Input/Output System or Unified Extensible Firmware Interface) is then loaded. This low-level software checks the hardware and then scans the system's storage devices for bootable media.
📚 MBR and GRUB: Once a bootable device (like your hard drive) is found, it reads the first 512 bytes known as the Master Boot Record (MBR) or, in newer systems, it uses the GUID Partition Table (GPT). The MBR/GPT contains the bootloader (often GRUB in Linux systems), which is then executed.
⚙️ Kernel Loading: The GRUB menu allows you to select an operating system (in a dual boot scenario) or different kernel versions. GRUB/GRUB2 then loads the selected Linux kernel into memory.
🗃️ Initramfs: The bootloader also loads a special filesystem called the initramfs (Initial RAM FileSystem) into memory alongside the kernel. This temporary root filesystem includes the drivers and tools needed to mount the actual root filesystem.
🚀 Kernel Initialization: Once loaded, the kernel decompresses and initializes itself, sets up system functions (like hardware interfaces, memory management, process control, etc.), and then extracts the initramfs archive in memory.
🔁 Switch Root: The kernel then executes the /init program located in the initramfs, which continues the boot process, typically by locating and mounting the actual root filesystem. The kernel then 'switches' from the initramfs to the newly mounted root filesystem.
🚦 Init and Runlevel/Targets: After the root filesystem is mounted, the kernel starts the first process init (or systemd in most modern systems). This process continues the boot process by moving the system into a specific runlevel, which dictates what processes or services to start.
🖥️ User-Space: Init or systemd starts the necessary user-space processes according to the chosen runlevel.
🛂 Login: Finally, a login prompt or graphical user interface (GUI) appears, and the system is ready for user interaction.
📚 If you're interested in more in-depth explanation of these topics, please check out my new book "Cracking the DevOps Interview"
https://pratimuniyal.gumroad.com/l/cracking-the-devops-interview
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