What happens first when Linux server is started?

The Linux boot process is a meticulously orchestrated sequence of events that transforms a powered-off machine into a fully operational system. For system administrators, developers, and enthusiasts, understanding this process is critical for troubleshooting, optimizing performance, and securing infrastructure. In this comprehensive guide, we’ll dissect every stage of the Linux boot process, from the moment you press the power button to the user login prompt. We’ll explore modern components like UEFI, initramfs, and systemd, and provide practical examples to solidify your understanding.

Table of Contents

1. Introduction: Why Understanding the Boot Process Matters
2. Stage 1: Power-On Self-Test (POST)
3. Stage 2: UEFI/BIOS Initialization
4. Stage 3: Bootloader (GRUB2) Execution
5. Stage 4: Kernel Initialization and initramfs
6. Stage 5: systemd – The Modern Init System
7. Stage 6: Targets, Services, and Dependency Management
8. Stage 7: User Login and Session Management
9. Troubleshooting Boot Issues

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