Linux Boot Process

Overview

Overview Understanding the Linux boot process helps diagnose startup failures and tune boot performance.

Stage 1 — POST and firmware: Power-On Self Test runs, then BIOS or UEFI firmware takes over. BIOS uses MBR (Master Boot Record, first 512 bytes of disk), limited to 4 primary partitions and 2TB disks. UEFI supports GPT (GUID Partition Table), Secure Boot (cryptographic signature verification of bootloader), and reads the EFI System Partition (ESP, typically /boot/efi, FAT32 formatted).

Stage 2 — GRUB2: The bootloader. In BIOS mode, GRUB stage 1 lives in the MBR and loads stage 1.5 from the gap after MBR, which loads stage 2 from /boot/grub/. In UEFI mode, GRUB lives on the ESP as a .efi file. GRUB reads /boot/grub/grub.cfg (generated by update-grub or grub2-mkconfig, never edited directly) to display the boot menu. Kernel parameters are passed here (root=UUID=..., ro, quiet, splash, init=/path for alternative init).

Stage 3 — Kernel initialization: GRUB decompresses the kernel image (vmlinuz) into RAM and passes control. The kernel initializes CPU, memory management, and devices. It mounts the initramfs (initial RAM filesystem) as a temporary root.

Stage 4 — initramfs: A small, compressed cpio archive containing just enough tools and drivers to mount the real root filesystem (drivers for disk controllers, LVM, RAID, crypto). After mounting real root, it performs pivot_root to switch over.

Stage 5 — systemd: PID 1 takes over, reads unit files, starts services in dependency order to reach the default target (usually multi-user.target or graphical.target). systemd-analyze blame shows which services are slowest to start.

Key concepts

UEFI vs BIOS

BIOS: MBR, max 2TB, 4 primary partitions, no Secure Boot. UEFI: GPT, no practical size limit, 128 partitions, Secure Boot support, reads ESP partition.

GRUB2

Bootloader that loads the kernel. Config in /boot/grub/grub.cfg (auto-generated). Kernel parameters passed here. GRUB rescue shell for recovery when config is broken.

initramfs

Compressed cpio archive containing minimal drivers and tools needed to mount the real root filesystem. Lives at /boot/initrd.img-VERSION. Rebuilt with update-initramfs -u.

systemd targets vs runlevels

multi-user.target = runlevel 3 (text multi-user), graphical.target = runlevel 5 (GUI), rescue.target = runlevel 1 (single user), emergency.target (minimal). systemctl get-default shows current.

When to use it

• Diagnosing servers that fail to boot
• Adding kernel parameters for debugging (single user mode, no module loading)
• Understanding initramfs rebuild requirements after kernel module changes
• Performance tuning boot time with systemd-analyze

Interview questions

• Describe every step from pressing the power button to the login prompt appearing.
• What is initramfs and why does Linux need it?

More Linux topics

• iptables & nftables
• Job Control & Process Management
• journalctl & Log Management
• Kernel & Driver Build Environments
• LDAP & sssd Integration
• Linux Audit System
• Linux Capabilities
• Linux Filesystem
• Linux Kernel & /proc
• Linux Networking L2 and L3
• Linux Networking Tools
• Linux Processes