System Repair: HowTo chroot

Many times, especially when messing around, we have found ourselves in the predicament of not being able to access the system to repair it, but the solution is easy: use the command chroot from any LiveCD / RepairCD.

In many places (forums, blogs ...) this command is mentioned and a "copy / paste" of the code is given, but my intention with this post is to explain those steps a little, in order to make good use of this tool, with knowledge of cause.

Introduction

The command chroot It is known as CHangeROOT, that is, a command that allows you to change the root of the system on which you are working. In other words: if you are from a LiveCD and you want everything you are working on the console to have an effect on the installed system, you must first make use of chroot.

The problem is that it is not enough to use chroot As such, before we must properly mount certain partitions.

HOW TO

First we need to start a terminal, either from another installed system (on another partition / disk) or from a LiveCD. IMPORTANT: the architecture of the LiveCD must match that of the system to be repaired (32 or 64 bits).
Once we are in the terminal we will start by identifying our partitions:
fdisk -l

• With this command we will list all our partitions / disks. We must identify which is the target partition, where our system to repair is installed, from now on we will call it broken system.

For this example we will consider that our broken system is in / Dev / sda1 .

We go on to assemble the system. First we will create the folder where we are going to work and later we mount the partition where our broken system is located in that folder
mkdir /mnt/my_linux
mount /dev/sda1 /mnt/my_linux
If you have the folder / Home o / var or any other on another partition, you should mount it / s as follows:
mount /dev/sda2 /mnt/my_linux/var

• NOTE: I have taken the / dev / sda2 partition for the / var folder as an example, let each one adjust the code to their characteristics.

Normally this would suffice if you simply need to edit files by hand, but if we want to execute some commands that configure the system, we need to mount certain special system folders: / Dev, / Proc y  / sys.
mount -t proc proc /mnt/my_linux/proc
mount -t sysfs sys /mnt/my_linux/sys
mount -o bind /dev /mnt/my_linux/dev

• With option -t we tell mount the type of "filesystem" we want to mount. It is necessary to specify it because of the special nature of the folders / Proc y / sys.

• With option -o we specify the options of mount. The option connective it serves to "link". In UNIX all hardware devices are accessible through the folder / Dev, that's why we must mount our current / Dev in the folder where our broken system is now. As this folder is already mounted, it is only necessary to tell mount where it is originally mounted.

It is done this way so that chroot have access to these folders as if they were the broken system, although they must be from the current system (ex: the LiveCD session) since they are related to the state of the system, processes and hardware.

Now is the time to use chroot:
chroot /mnt/my_linux/ /bin/bash

• The command is passed as arguments the path of the new root «/» (which in our case is / mnt / my_linux) and the console you want to use (in this case we have opted for the well-known bash, found in / bin / bash). If we do not specify the console, we will find ourselves before a somewhat archaic command interpreter (it does not fill when pressing the tab, etc).

Now we can use the console as if we had the root session started on our broken system (edit files, check scripts, install / uninstall packages ...). WARNING! For the changes made to take effect, you must unmount the filesystem after exiting chrootLook at the example below.

More information in https://wiki.archlinux.org/index.php/Change_Root (more than recommended reading).

Usage example: restore GRUB2

One of the most widespread uses of chroot It is as a tool to repair the GRUB. Since if the grub breaks, it is practically impossible to boot our system to fix it.

NOTICE: this little tutorial is a mere example, it works on various distributions derived from Debian, Ubuntu and openSUSE among others. Even so, check the documentation of your distribution, since in many the command is not found update-grub.
# NOTE: these commands run once inside chroot.update-grub
grub-install /dev/sda

• With update-grub We update GRUB2's input menu, thus adding any missing inputs. Later we reinstall GRUB on our disk, since it has been damaged.

In this case I have taken / Dev / sda like the album where we have our system, this must be adapted to your case.

Our GRUB should already be repaired, so we must exit chroot, unmount the filesystem (IMPORTANT) and reboot for the changes to take effect. If we forget to unmount the filesystem, it is possible that rebooting the files will not unmount correctly and therefore some changes will not take effect.
# we leave chrootexit
# unmount the filesystem and rebootumount /mnt/my_linux/dev
umount /mnt/my_linux/sys
umount /mnt/my_linux/proc
umount /mnt/my_linux
reboot
And that's all. I hope you enjoy it and that it helps you. A greeting!