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Rescuing a Non-booting GRUB on Linux

Rescuing a Non-booting GRUB on Linux

Booting into system

We’re going to learn how to fix two of the more common failures. When you boot up your system and it stops at the grub> prompt, that is the full GRUB 2 command shell. That means GRUB 2 started normally and loaded the normal.mod module (and other modules which are located in /boot/grub/[arch]/), but it didn't find your grub.cfg file. If you see grub rescue> that means it couldn't find normal.mod, so it probably couldn't find any of your boot files.

How does this happen? The kernel might have changed drive assignments or you moved your hard drives, you changed some partitions, or installed a new operating system and moved things around. In these scenarios your boot files are still there, but GRUB can’t find them. So you can look for your boot files at the GRUB prompt, set their locations, and then boot your system and fix your GRUB configuration.

GRUB 2 Command Shell

When you’re at the grub> prompt, you have a lot of functionality similar to any command shell such as history and tab-completion. The grub rescue> mode is more limited, with no history and no tab-completion.

There must be no spaces on either side of the equals sign. Now let’s do a little exploring. Type ls to list all partitions that GRUB sees:

grub> ls
(hd0) (hd0,msdos2) (hd0,msdos1)

What’s all this msdos stuff? That means this system has the old-style MS-DOS partition table, rather than the shiny new Globally Unique Identifiers partition table (GPT).
If you're running GPT it will say (hd0,gpt1).

Now let’s snoop. Use the ls command to see what files are on your system:

grub> ls (hd0,1)/
lost+found/ bin/ boot/ cdrom/ dev/ etc/ home/ lib/
lib64/ media/ mnt/ opt/ proc/ root/ run/ sbin/
srv/ sys/ tmp/ usr/ var/ vmlinuz vmlinuz.old
initrd.img initrd.img.old

Hurrah, we have found the root filesystem. You can omit the msdos and gpt labels. If you leave off the slash it will print information about the partition. You can read any file on the system with the cat command:

grub> cat (hd0,1)/etc/issue
Ubuntu 14.04 LTS \n \l

Booting From grub>

This is how to set the boot files and boot the system from the grub> prompt. We know from running the ls command that there is a Linux root filesystem on (hd0,1), and you can keep searching until you verify where /boot/grub is. Then run these commands, using your own root partition, kernel, and initrd image:

grub> set root=(hd0,1)
grub> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
grub> initrd /boot/initrd.img-3.13.0-29-generic
grub> boot

The first line sets the partition that the root filesystem is on.
The second line tells GRUB the location of the kernel you want to use. Start typing,/boot/vmli and then use tab-completion to fill in the rest. Type root=/dev/sdX to set the location of the root filesystem. Yes, this seems redundant, but if you leave this out you'll get a kernel panic.
How do you know the correct partition? hd0,1 = /dev/sda1. hd1,1 = /dev/sdb1. hd3,2 = /dev/sdd2. I think you can extrapolate the rest.

The third line sets the initrd file, which must be the same version number as the kernel.

The fourth line boots your system.

Booting From grub-rescue>

If you’re in the GRUB rescue shell the commands are different, and you have to load the normal.mod andlinux.mod odules:

grub rescue> set prefix=(hd0,1)/boot/grub
grub rescue> set root=(hd0,1)
grub rescue> insmod normal
grub rescue> normal
grub rescue> insmod linux
grub rescue> linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
grub rescue> initrd /boot/initrd.img-3.13.0-29-generic
grub rescue> boot

Tab-completion should start working after you load both modules.

Making Permanent Repairs

So far we have made temporary changes on the terminal, so they will work only once and when we reboot again we have to do the same thing.
To get rid of this hassle we have to make settings permanent and as now We have successfully booted your system, run these commands to fix GRUB permanently:

$ sudo update-grub
Generating grub configuration file ...
Found background: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
Found background image: /usr/share/images/grub/Apollo_17_The_Last_Moon_Shot_Edit1.tga
Found linux image: /boot/vmlinuz-3.13.0-29-generic
Found initrd image: /boot/initrd.img-3.13.0-29-generic
Found linux image: /boot/vmlinuz-3.13.0-27-generic
Found initrd image: /boot/initrd.img-3.13.0-27-generic
Found linux image: /boot/vmlinuz-3.13.0-24-generic
Found initrd image: /boot/initrd.img-3.13.0-24-generic
Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
done
$ sudo grub-install /dev/sda
Installing for i386-pc platform.
Installation finished. No error reported.

When you run grub-install remember you're installing it to the boot sector of your hard drive and not to a partition, so do not use a partition number like /dev/sda1.

But It Still Doesn’t Work

If your system is so messed up that none of this works, try the Super GRUB2 live rescue disk. The official GNU GRUB Manual 2.00 should also be helpful.

If you find something wrong or have any question about the solution then comment down below, you can also contact us using contact form.

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Ashish Patel

Ashish Patel

Big Data Engineer at Skyscanner , loves Competitive programming, Big Data.