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IEN-001 - Understanding the Linux Boot Process: From Power-On to User Space
Introduction Imagine arriving at the office on a Monday morning and discovering that one of your production Linux servers refuses to boot. Instead of presenting a login prompt, the system stops with one of the following messages: GRUB Loading... Error 15: ...
IEN-002 - Stage 1 — Firmware Initialization (BIOS & UEFI)
Before Linux, before GRUB, and even before the operating system exists, the computer must first prepare itself to execute software. This responsibility belongs to the system firmware. For decades this firmware was known as the Basic Input/Output System (BIOS...
IEN-003 - Stage 2 — The Bootloader (GRUB)
At the end of the firmware stage, the computer has completed its hardware initialization and identified a bootable storage device. However, the firmware still faces one fundamental problem: It knows where the operating system is stored, but it does not kno...
IEN-004 - Stage 3 — Linux Kernel Initialization
At the end of the previous stage, GRUB has successfully loaded two critical files into memory: the Linux kernel (vmlinuz) the initial RAM disk (initrd or initramfs) It has also prepared the kernel command line, which may include parameters such as: roo...
IEN-005 - Stage 4 — initrd / initramfs: Preparing the System to Mount the Root Filesystem
By the end of the previous stage, the Linux kernel is fully operational. It has: initialized the processor, taken control of memory management, started its internal subsystems, detected hardware, initialized built-in device drivers. Despite all of t...
IEN-006 - Stage 5 — Mounting the Root Filesystem
After the initrd (or initramfs) has completed its work, the system finally reaches one of the most important milestones in the entire boot process: The real root filesystem can now be mounted. Everything that happened before this point—from firmware init...
IEN-007 - Stage 6 — PID 1: The Beginning of User Space
At the end of the previous stage, the Linux kernel has successfully mounted the real root filesystem. The temporary initrd environment has completed its job and has been discarded through the switch_root operation. For the first time since the system was pow...
IEN-008 - Stage 7 — System Service Initialization
At the end of the previous stage, the Linux kernel has successfully transferred control to PID 1. Whether PID 1 is implemented as SysV init, Upstart, or systemd, the operating system has now entered the userspace initialization phase. For the first time sinc...
IEN-009 - Stage 8 — User Login: Entering User Space
At this point, the Linux system has completed nearly all of its startup sequence. The firmware initialized the hardware. The bootloader loaded the Linux kernel. The kernel initialized the operating system. The initrd environment prepared storage. The root...