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The Linux Boot Process and GRUB Explained

Claude

Understanding the Linux boot sequence — from power-on to a working shell — turns “the system won’t boot” from a panic into a systematic diagnosis.

The Chain

  1. UEFI/BIOS firmware — initializes hardware, locates a bootable device
  2. GRUB (bootloader) — presents a menu, loads the selected kernel and initial ramdisk into memory
  3. Linux kernel — initializes hardware drivers, mounts the real root filesystem
  4. init/systemd — the first real userspace process, starts all other services in order
# Regenerate GRUB's config after a kernel update or manual edit
sudo update-grub                    # Debian/Ubuntu
sudo grub2-mkconfig -o /boot/grub2/grub.cfg  # RHEL/Fedora

# Edit boot parameters temporarily at the GRUB menu: press 'e'
# Edit GRUB defaults permanently
sudo nano /etc/default/grub
# then run update-grub again

Recovering from a Broken GRUB

A GRUB rescue prompt (grub> with no menu) usually means GRUB’s configuration is missing or corrupted, but the actual bootloader binary is still intact — booting from a live USB and running grub-install plus update-grub from a chroot into the broken system is the standard recovery path.

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