Using a chroot to manipulate another installation.

[edbarx]

Using a chroot in Debian is not as difficult as it sounds, and the power inherent in chroot is amazing to the point of restoring bootability to an otherwise unbootable installation. One such instance can arise if the kernel is uninstalled leaving an installation without it.


The Procedure:

1. For conformity, create a directory under /mnt say sda2.

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   mkdir /mnt/sda2

2. Mount the root partition holding the installation to be modified. I will assume it is: /dev/sda2

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   mount /dev/sda2 /mnt/sda2

3. I found that for the chroot to work properly I had to bind the /sys, /proc and /dev directories as follows:

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   mount -o bind /sys /mnt/sda2/sys
   mount -o bind /proc /mnt/sda2/proc
   mount -o bind /dev /mnt/sda2/dev

4. Do the actual chroot as follows:

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   chroot /mnt/sda2 /bin/sh

   If you are going to install packages containing daemons, you may need to prevent them from starting. The Debian Chroot Wiki contains this information.
5. After finishing with the chroot, exit.

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   exit

6. Unmount the bound file systems.

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   umount /mnt/sda2/dev
   umount /mnt/sda2/proc
   umount /mnt/sda2/sys

7. Finally unmount the partition. If applicable to your case, unmount the other partitions that make part of the repaired installation. Finally, unmount the partition (in our case, sda2).

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   umount /mnt/sda2

I use this same procedure to configure a new installation after installing it with debootstrap.


Using chroot to repair grub-pc from within a LiveCD session or from within another running session.
The first thing I found about repairing an otherwise badly behaving grub-pc, is by purging it completely together with its configuration files. This means, when I was asked to remove the grub files I replied in the affirmative.


The Procedure

1. Purge grub-pc together with all its files under /boot.

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   # apt-get remove --purge grub-pc

   Needless to repeat, answer that you want to delete all traces of grub-pc.
2. Reinstall grub-pc.

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   # apt-get install grub-pc

   When asked where to install the first stage select the MBR or the protective MBR in case of GPT.

[zilleplus]

nice guide, for those who want to do this in a image from a qemu/kvm use this line:

add module:

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modprobe nbd max_part=16

link image to network block device:

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kvm-nbd -c /dev/nbd1 debian.raw

all the rest is the same use /dev/nbd1p1 instead of /dev/sda1.