GRUB Bootloader

# GRUB configuration files
/boot/grub/grub.cfg          # Main config (generated, do not edit)
/etc/default/grub            # User-editable settings
/etc/grub.d/                 # Scripts that generate grub.cfg

# GRUB modules
/boot/grub/x86_64-efi/       # UEFI modules
/boot/grub/i386-pc/          # BIOS modules

# GRUB environment
/boot/grub/grubenv            # Environment block (saved entry, timeout)

# Key settings in /etc/default/grub
GRUB_DEFAULT=0                    # Default menu entry (0=first)
GRUB_TIMEOUT=5                    # Seconds to wait before booting
GRUB_TIMEOUT_STYLE=menu           # menu, hidden, or countdown
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"  # Kernel params for default
GRUB_CMDLINE_LINUX=""             # Kernel params for all entries
GRUB_DISABLE_OS_PROBER=false      # Enable OS probing (dual boot)

# Regenerate grub.cfg
sudo grub-mkconfig -o /boot/grub/grub.cfg

# For UEFI systems
sudo grub-mkconfig -o /boot/efi/EFI/ubuntu/grub.cfg

# View current kernel command line
cat /proc/cmdline
# BOOT_IMAGE=/boot/vmlinuz-6.1.0-15-generic root=/dev/sda2 ro quiet splash

# Common kernel parameters
# root=/dev/sda2           — Root filesystem device
# ro                       — Mount root read-only initially
# quiet                    — Suppress boot messages
# splash                   — Show splash screen
# init=/bin/bash           — Emergency shell (bypass init)
# single / 1 / systemd.unit=rescue.target  — Boot to single-user
# nomodeset                — Disable kernel mode setting (GPU issues)
# net.ifnames=0            — Legacy network interface names
# mitigations=off          — Disable CPU vulnerability mitigations

# Edit GRUB at boot (press 'e' in GRUB menu)
# Add parameters to the linux line
# Press Ctrl+X to boot

# Boot from live USB, then:

# Mount root and boot partitions
sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi

# Chroot into the system
for i in /dev /proc /sys /run; do
  sudo mount --bind $i /mnt$i
done
sudo chroot /mnt

# Reinstall GRUB
grub-install /dev/sda
update-grub
exit

# Or for UEFI
grub-install --target=x86_64-efi --efi-directory=/boot/efi
update-grub

# Add custom menu entries
cat > /etc/grub.d/40_custom << 'EOF'
#!/bin/sh
exec tail -n +3 $0

menuentry "Ubuntu Recovery" {
    set root='(hd0,1)'
    linux /boot/vmlinuz-$(uname -r) root=/dev/sda2 ro single
    initrd /boot/initrd.img-$(uname -r)
}

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

sudo chmod +x /etc/grub.d/40_custom
sudo grub-mkconfig -o /boot/grub/grub.cfg

# Set default entry
GRUB_DEFAULT="Ubuntu Recovery"
# or use index
GRUB_DEFAULT=2

# Set timeout
GRUB_TIMEOUT=10

# Hide GRUB menu (show on Shift key)
GRUB_TIMEOUT_STYLE=hidden

# GRUB rescue shell (when GRUB cannot find config)
grub> set root=(hd0,1)
grub> set prefix=(hd0,1)/boot/grub
grub> insmod normal
grub> normal

# Emergency boot from GRUB menu
# Press 'e' to edit, add to kernel line:
# init=/bin/bash   — Bypasses init, drops to root shell

# After emergency boot, fix filesystem
fsck -y /dev/sda2
mount -o remount,rw /dev/sda2

# Rebuild initramfs
update-initramfs -u

# Reinstall GRUB
grub-install /dev/sda
update-grub