GRUB — the bootloader

GRUB has two config layers: the source and the generated file. The file GRUB actually reads at boot is /boot/grub/grub.cfg — a generated file. You should never edit it directly; it is overwritten by update-grub. The source of truth is /etc/default/grub (user-facing options) and hook scripts in /etc/grub.d/ (which generate menu entries). The workflow is:

# Edit the source
sudoedit /etc/default/grub

# Regenerate /boot/grub/grub.cfg from the source
sudo update-grub

If your changes do not survive a kernel update, it means you edited grub.cfg directly instead of the source. Always edit /etc/default/grub.

Key options in /etc/default/grub. The file is a shell-sourced key=value store. The most important variables:

# How long GRUB waits before auto-booting the default entry (seconds)
GRUB_TIMEOUT=5

# The default menu entry to boot (0 = first, or a title string)
GRUB_DEFAULT=0

# Extra parameters appended to every kernel command line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

# Parameters for all kernels including recovery entries
GRUB_CMDLINE_LINUX=""

quiet suppresses most kernel log output to the console. splash shows a graphical boot screen. Remove both to see every kernel log line — extremely useful when debugging a boot that hangs silently.

The kernel command line is the most powerful recovery lever. When GRUB launches the kernel, it passes a string of space-separated parameters. The kernel parses them early, before any filesystem is mounted. Key parameters you will use in practice:

root=/dev/sda2        # which device is the root filesystem
ro                    # mount root read-only initially (normal)
quiet                 # suppress kernel messages
init=/bin/bash        # replace PID 1 with a shell (emergency recovery)
single                # boot into single-user mode (runlevel 1)
systemd.unit=rescue.target   # boot to rescue target (systemd equivalent)

The init=/bin/bash trick gives you a root shell before systemd ever starts. The filesystem is mounted read-only at first; you remount it read-write with mount -o remount,rw / before making any changes.

How to edit the kernel command line at boot (without persistence). This is the most valuable skill in this lesson — it requires physical or console access but no prior setup.

1. Reboot (or power cycle) the machine.
2. At the GRUB menu, press e to edit the selected entry.
3. Find the line beginning with linux — that is the kernel command line.
4. Move to the end of that line (End key) and add your parameter, e.g. systemd.unit=rescue.target.
5. Press Ctrl+X or F10 to boot with the modified line.

The edit is not saved — it applies only to this one boot. The next boot uses the normal config. This is exactly what you want for recovery: make a change, boot, fix the system, reboot normally.

Where GRUB lives on disk and how it is installed. GRUB consists of two parts: a small first-stage loader written to the MBR or EFI partition, and modules/config under /boot/grub/. On UEFI systems the EFI executable is at /boot/efi/EFI/ubuntu/grubx64.efi. If you replace a disk, GRUB must be reinstalled to the new disk’s boot sector:

# Install GRUB to a specific disk (MBR/BIOS systems)
sudo grub-install /dev/sda

# Install GRUB for UEFI systems (usually done automatically by update-grub)
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi

# Then regenerate config
sudo update-grub

A common mistake after dd-cloning a disk: forgetting to reinstall GRUB on the new disk. The data is there, but the new disk has no bootloader in its MBR/EFI partition.