GRUB Command Line Boot Repair Guide

GRUB (GRand Unified Bootloader) is the first program that runs when you power on a Linux machine. It loads the kernel and hands control to your operating system. When GRUB fails, you typically see error messages like "error: no such partition" or "GRUB rescue>" prompt. Common causes include accidental partition deletion, corrupted boot sector, misconfigured boot files, or hardware changes. Understanding what went wrong helps you apply the right fix. Boot repair from the command line requires access to a live Linux environment, which you can create using a USB drive or DVD with a Linux distribution.

If your system won't boot normally, you'll likely see the GRUB rescue prompt. This minimal environment lets you run basic commands to diagnose and fix problems. From the rescue prompt, you can list partitions using the ls command, which shows available disks and their contents. Type 'ls' to see all partitions, then 'ls (hd0,gpt1)/' to examine a specific partition's contents. Look for the /boot directory to confirm you've found your Linux installation. If you see familiar directories like bin, lib, or etc, you're on the right partition. This reconnaissance step is crucial before attempting any boot repair work.

Once you've identified your Linux partition, you can reinstall GRUB to fix boot issues. Boot from a live USB or DVD, then open a terminal. Use fdisk or lsblk to identify your disk (usually /dev/sda or /dev/nvme0n1). Mount your root partition to a temporary directory: mount /dev/sdaX /mnt (replace X with your partition number). Mount the boot partition if it's separate: mount /dev/sdaY /mnt/boot. Then reinstall GRUB with: grub-install --root-directory=/mnt /dev/sda. This command writes GRUB files to your disk's boot sector. Update the GRUB configuration afterward using grub-mkconfig to ensure all boot entries are recognized properly.

After reinstalling GRUB, you need to regenerate its configuration file to recognize your installed kernels and operating systems. While still in the live environment with your partitions mounted, run: grub-mkconfig -o /mnt/boot/grub/grub.cfg. This command scans your system for installed kernels and creates a new configuration file. If you see output listing detected Linux installations, the process worked correctly. The configuration file tells GRUB which kernels to load and in what order. Without updating it, GRUB won't know how to boot your system even though the bootloader itself is installed. This step is essential for completing any boot repair process.

Different error messages point to different problems. "Error: no such partition" means GRUB can't find the partition it's configured to use, often from disk changes or partition table corruption. "Error: unknown filesystem" indicates GRUB doesn't recognize the filesystem type, sometimes fixable by reinstalling GRUB. "GRUB rescue>" prompt means GRUB couldn't load its normal environment, requiring the reinstallation steps above. If you see "error: file not found" when booting, your kernel or initramfs might be corrupted or missing. In this case, boot from live media, mount your partitions, and check if /boot/vmlinuz and /boot/initrd files exist. If missing, you may need to reinstall your kernel package using your distribution's package manager.

After completing boot repair steps, reboot your system without the live media to test if GRUB now works. Remove the USB drive or DVD and restart. If GRUB menu appears with your operating system listed, the repair was successful. Select your Linux entry and verify the system boots normally. If you still see errors, you may need to repeat the process or investigate further. Check that you mounted the correct partitions and used the right disk identifier. Some systems have multiple disks, so confirming /dev/sda versus /dev/sdb is critical. If problems persist, consider checking your BIOS settings to ensure the correct disk is set as the boot device.

Once your system boots reliably, take steps to prevent future problems. Keep your system updated, as kernel and bootloader updates are usually tested thoroughly. Avoid manual changes to partition tables unless absolutely necessary. If you must modify partitions, back up your boot sector first using: dd if=/dev/sda of=boot_backup.img bs=512 count=1. Document your partition layout and boot configuration for reference. Consider maintaining a bootable live USB with your preferred Linux distribution for emergency access. Regular backups of your /boot directory ensure you can restore kernel files if corruption occurs. These preventive measures significantly reduce the likelihood of encountering boot problems.