1.1.2 Linux Boot Process Explained (BIOS vs. UEFI, GRUB2, and Commands)

1.1.2 Linux Boot Process Explained (BIOS vs. UEFI, GRUB2, and Commands) #

The Linux boot process is a structured sequence that loads the operating system into memory. Below is a detailed breakdown of the key components, including BIOS/UEFI, GRUB2, boot files, and essential commands.

---

1. Firmware Initialization: BIOS vs. UEFI #

1) Basic Input/Output System (BIOS)

• Legacy firmware (older systems).
• Process:
  1. POST (Power-On Self-Test) → Checks hardware.
  2. Loads MBR (Master Boot Record) → First 512 bytes of the boot disk.
  3. Starts the bootloader (e.g., GRUB2).
• Limitations:
  • No secure boot.
  • Limited to MBR partitioning (max 2TB disks).

2) Unified Extensible Firmware Interface (UEFI)

• Modern replacement for BIOS.
• Process:
  1. Runs POST.
  2. Looks for an EFI System Partition (ESP) (/boot/efi).
  3. Loads bootloader (e.g., GRUB2) from ESP.
• Advantages:
  • Supports Secure Boot (prevents malware).
  • GPT partitioning (>2TB disks).
  • Faster boot times.

---

2. Bootloader: GRUB2 (Grand Unified Bootloader 2) #

• Default bootloader for most Linux distros.
• Config File: /boot/grub2/grub.cfg (auto-generated).
• Key Files:
  • vmlinuz → Compressed Linux kernel.
  • initrd.img or initramfs → Temporary root filesystem (loads drivers before real root is mounted).

GRUB2 Commands #

Command	Purpose
grub2-install	Installs GRUB2 to a disk (/dev/sda).
grub2-mkconfig	Generates a new grub.cfg file.
grub2-update	Updates GRUB2 (some distros alias this to update-grub).
mkinitrd	Creates an initrd (initial RAM disk).
dracut	Modern alternative to mkinitrd (used in RHEL/Fedora).

---

3. Kernel & Initramfs Loading #

1. GRUB2 loads vmlinuz (Linux kernel).
2. initrd.img or initramfs is loaded → Contains drivers/modules needed to mount the real root (/).
3. Kernel initializes hardware and switches to the real root filesystem.

---

4. Boot Sources #

a) Preboot eXecution Environment (PXE) #

• Network booting (used in servers/data centers).
• Requires:
  • DHCP server (assigns IP).
  • TFTP server (serves boot files).

b) Booting from USB #

• Requires a bootable USB (created with dd or tools like Rufus).
• Example:bashCopyDownloaddd if=ubuntu.iso of=/dev/sdX bs=4M status=progress

c) Booting from ISO (Directly Mounted) #

• Some UEFI systems allow booting directly from ISO files (without USB).

---

5. System Initialization (systemd / SysVinit) #

After the kernel loads:

• systemd (modern init system, PID 1) takes over.
• Targets (runlevels) determine what services start:
  • multi-user.target → CLI mode.
  • graphical.target → GUI mode.

---

Summary: Linux Boot Process Flow #

1. Firmware (BIOS/UEFI) → POST → Finds boot device.
2. Bootloader (GRUB2) → Loads vmlinuz + initramfs.
3. Kernel → Initializes hardware, mounts root (/).
4. Init system (systemd) → Starts services, reaches target.

---

Key Troubleshooting Commands #

• Fix GRUB2:bashCopyDownloadgrub2-install /dev/sdX # Reinstall GRUB grub2-mkconfig -o /boot/grub2/grub.cfg # Regenerate config
• Rebuild initramfs:bashCopyDownloaddracut -fv # For RHEL/Fedora mkinitrd # For Debian/Ubuntu (older)

---

Final Notes #

• UEFI + GPT is the modern standard (faster, more secure).
• GRUB2 is highly configurable (edit /etc/default/grub before running grub2-mkconfig).
• initramfs is critical for loading drivers (e.g., encrypted LVM, RAID).