The Boot Process#

Prerequisite — Firmware: Every computer has firmware, low-level software stored on a chip on the motherboard. It initializes hardware and hands control to the operating system. There are two types: BIOS (legacy, from the 1980s) and UEFI (modern replacement, supports larger disks, secure boot, faster startup). Most systems sold after 2012 use UEFI.

Concepts#

Boot Sequence Overview#

Power on
  │
  ▼
Firmware (BIOS/UEFI)       ← initializes hardware, finds bootloader
  │
  ▼
Bootloader (GRUB)           ← loads the kernel
  │
  ▼
Linux Kernel                ← takes control, detects hardware
  │
  ▼
initramfs                   ← temporary root filesystem (loads drivers)
  │
  ▼
systemd (PID 1)             ← init system, starts services
  │
  ▼
Login prompt / Desktop      ← system ready

Stage 1: Firmware (BIOS/UEFI)#

The firmware runs a POST (Power-On Self-Test), initializes hardware (CPU, RAM, display), and looks for a bootable device.

On UEFI systems, the boot files live in the EFI System Partition:

ls /boot/efi/EFI/
# ubuntu/  debian/  (bootloader files)

Stage 2: GRUB (Bootloader)#

GRUB (GRand Unified Bootloader) is the default bootloader on Ubuntu and Debian. It:

• Displays a boot menu (choose OS or kernel version)
• Loads the Linux kernel and initramfs into memory
• Passes boot parameters to the kernel

GRUB Configuration#

# Main config (auto-generated — do not edit directly)
cat /boot/grub/grub.cfg

# Edit settings here instead:
sudo nano /etc/default/grub

# Key settings:
# GRUB_TIMEOUT=5                  # seconds to show menu (0 = skip)
# GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"   # kernel parameters
# GRUB_CMDLINE_LINUX=""           # additional kernel parameters

# After editing, regenerate grub.cfg:
sudo update-grub

Ubuntu: update-grub is available as a convenience script. Debian: Also available, but you can also use sudo grub-mkconfig -o /boot/grub/grub.cfg.

Useful GRUB Operations#

# View installed kernels
ls /boot/vmlinuz-*

# View GRUB menu entries
grep menuentry /boot/grub/grub.cfg

# Boot into recovery mode: at GRUB menu, select "Advanced options" → recovery mode

Stage 3: The Linux Kernel#

Once GRUB loads the kernel (vmlinuz-*) into memory, the kernel:

1. Decompresses itself
2. Initializes core hardware (CPU, memory manager)
3. Mounts the initramfs as a temporary root filesystem

Stage 4: initramfs#

The initramfs (initial RAM filesystem) is a small, temporary root filesystem loaded into RAM. It contains:

• Essential drivers (disk, filesystem, LVM, RAID)
• Scripts to find and mount the real root filesystem

Once the real root filesystem is mounted, initramfs hands control to the init system.

# View initramfs files
ls /boot/initrd.img-*

# Rebuild initramfs (after driver or module changes)
sudo update-initramfs -u

Stage 5: systemd (PID 1)#

systemd is the first process the kernel starts (PID 1). It:

1. Reads its configuration and target (like a runlevel)
2. Starts services in parallel based on dependencies
3. Mounts filesystems from /etc/fstab
4. Sets up networking, logging, and user sessions

systemd Targets (Runlevels)#

Targets define what state the system boots into:

# Check current target
systemctl get-default

# Change default target
sudo systemctl set-default multi-user.target    # boot to terminal
sudo systemctl set-default graphical.target     # boot to desktop

# Switch target right now (without rebooting)
sudo systemctl isolate multi-user.target

Viewing Boot Information#

# Boot log (current boot)
journalctl -b

# Previous boot
journalctl -b -1

# Boot time breakdown
systemd-analyze

# Service startup times
systemd-analyze blame | head -15

# Visual boot chart (generates SVG)
systemd-analyze plot > /tmp/boot.svg

# Kernel messages
dmesg | head -30
dmesg | tail -20

Troubleshooting Boot Problems#

# Boot into recovery mode:
# 1. At GRUB menu, select "Advanced options" → recovery mode
# 2. This boots to rescue.target (single-user)

# If GRUB is broken, boot from a live USB and:
sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi      # UEFI systems
sudo grub-install --root-directory=/mnt /dev/sda
sudo chroot /mnt update-grub

# Check why a service failed to start during boot
systemctl --failed
journalctl -b -p err

Lab#

Exercise 1: Examine Your Boot Setup#

# Check firmware type
[ -d /sys/firmware/efi ] && echo "UEFI" || echo "BIOS/Legacy"

# Check EFI partition (UEFI only)
ls /boot/efi/EFI/ 2>/dev/null

# List installed kernels
ls /boot/vmlinuz-*

# Current kernel
uname -r

Exercise 2: Explore GRUB#

# View GRUB settings
cat /etc/default/grub

# List menu entries
grep -E "^menuentry" /boot/grub/grub.cfg 2>/dev/null | head -5

Exercise 3: Boot Analysis#

# How long did boot take?
systemd-analyze

# Slowest services
systemd-analyze blame | head -10

# Current target
systemctl get-default

Exercise 4: Boot Logs#

# Kernel messages from this boot
dmesg | head -30

# systemd boot log
journalctl -b --no-pager | head -30

# Any boot errors?
journalctl -b -p err --no-pager

# Failed services
systemctl --failed

Review#

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