linux boot-process

GRUB (GRUB)

grub bootloader linux boot-process recovery
Plain English

GRUB is the program that starts Linux on most computers. After the motherboard hands off control, GRUB finds the Linux kernel on the disk and launches it. It can show a menu so you can pick an older kernel, start recovery mode, or boot a different operating system. When Linux will not start, GRUB is often where you begin fixing it.

Technical Definition

GRUB 2 (the current version; GRUB Legacy is long retired) is a GNU project bootloader that understands filesystems (ext4, XFS, Btrfs, LVM, some RAID), so it can read the kernel straight from /boot.

Configuration flow on Ubuntu:

  • Edit /etc/default/grub (timeout, default entry, GRUB_CMDLINE_LINUX_DEFAULT)
  • Scripts in /etc/grub.d/ (such as 10_linux, 30_os-prober, 40_custom) generate entries
  • update-grub (a wrapper for grub-mkconfig -o /boot/grub/grub.cfg) writes the final config

At boot:

  • Hold Shift (BIOS) or tap Esc (UEFI) to show a hidden menu
  • e edits an entry for one boot; Ctrl+X boots it
  • c opens the GRUB command line
  • “Advanced options” lists older kernels and recovery mode

Useful one-time kernel parameters: systemd.unit=rescue.target, systemd.unit=emergency.target, nomodeset (skip graphics drivers), removing quiet splash to see boot messages.

The grub> prompt means GRUB loaded but found no config. grub rescue> means it could not even find its own modules, usually a missing or moved partition.

Source: ArchWiki: GRUB

Configuring and repairing GRUB 

# Current defaults
$ grep -E "^GRUB_(DEFAULT|TIMEOUT|CMDLINE)" /etc/default/grub
GRUB_DEFAULT=0
GRUB_TIMEOUT=0
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

# Apply changes
$ sudo update-grub

# Boot by hand from the grub> prompt
grub> ls
grub> set root=(hd0,gpt2)
grub> linux /boot/vmlinuz root=/dev/sda2 ro
grub> initrd /boot/initrd.img
grub> boot
In the Wild

Knowing GRUB turns “the server will not boot” from a reinstall into a ten-minute fix. Typical jobs: booting the previous kernel after a bad update, adding a kernel parameter to get past a graphics hang, recovering a forgotten password through recovery mode, and reinstalling GRUB after a dual-boot accident. Set a visible timeout on lab and server machines so you can reach the menu without fighting the keyboard timing.

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