GRUB Bootloader

Configuration Files

• /etc/default/grub - Main settings
• /etc/grub.d/ - Menu entry scripts
• /boot/grub/grub.cfg - Generated config (do not edit directly)

Default Configuration

# /etc/default/grub
GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
GRUB_DISABLE_RECOVERY="false"

Common Kernel Parameters

# Add to GRUB_CMDLINE_LINUX_DEFAULT
quiet               # Suppress boot messages
splash              # Show splash screen
nomodeset           # Disable kernel mode setting
acpi=off            # Disable ACPI
noapic              # Disable APIC
init=/bin/bash      # Boot to shell (recovery)
single              # Single user mode
systemd.unit=rescue.target  # Rescue mode

Updating GRUB

# After editing /etc/default/grub
update-grub          # Debian/Ubuntu
grub2-mkconfig -o /boot/grub2/grub.cfg  # RHEL/Fedora

# Install GRUB to disk
grub-install /dev/sda
grub-install --target=x86_64-efi --efi-directory=/boot/efi

Custom Menu Entries

Create /etc/grub.d/40_custom:

#!/bin/sh
exec tail -n +3 $0

menuentry "Custom Linux" {
    set root=(hd0,1)
    linux /vmlinuz root=/dev/sda2 ro
    initrd /initrd.img
}

menuentry "Windows" {
    set root=(hd0,2)
    chainloader +1
}

Boot Menu Navigation

Recovery Mode

At GRUB prompt:

# List partitions
ls
ls (hd0,1)/

# Set root and boot
set root=(hd0,1)
linux /vmlinuz root=/dev/sda2 ro single
initrd /initrd.img
boot

Reinstalling GRUB

From live USB:

# Mount root partition
mount /dev/sda2 /mnt
mount /dev/sda1 /mnt/boot/efi  # EFI systems

# Bind system directories
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys

# Chroot and reinstall
chroot /mnt
grub-install /dev/sda
update-grub
exit

Password Protection

# Generate password hash
grub-mkpasswd-pbkdf2

# Add to /etc/grub.d/40_custom
set superusers="admin"
password_pbkdf2 admin grub.pbkdf2.sha512...

Troubleshooting

# Check installed version
grub-install --version

# Verify configuration
grub-script-check /boot/grub/grub.cfg

# List available kernels
ls /boot/vmlinuz*