Change Root

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Chroot is the process of changing of the apparent disk root directory (and the current running process and its children) to another root directory. When you change root to another directory you cannot access files and commands outside that directory. This directory is called a chroot jail. Changing root is commonly done for system maintenance, such as reinstalling the bootloader or resetting a forgotten password.

1 Requirements

  • You'll need to boot from another working GNU environment (e.g. from a LiveCD or USB flash media, or from another installed GNU/Linux distribution).
  • Root privileges are required in order to chroot.
  • Be sure that the architecture of the GNU environment you have booted into matches the architecture of the root directory you wish to enter (i.e. i686, x86_64). You can find the architecture of your current environment with:
# uname -m
  • If you need any kernel modules loaded in the chroot environment, load them before chrooting. It may also be useful to initialize your swap (swapon /dev/sdxY) and to establish an internet connection before chrooting.

2 Mount the partitions

The root partition of the GNU/Linux system that you're trying to chroot into needs to be mounted. To find out the device name assigned by the kernel, run:

# lsblk /dev/sda

Then create a directory where you would like to mount this partition and mount it:

# mkdir /mnt/parabola
# mount /dev/sda3 /mnt/parabola

Next, if you have separate partitions for other parts of your system (e.g. /boot, /home, /var, etc), you should mount them, as well:

# mount /dev/sda1 /mnt/parabola/boot/
# mount /dev/sdb5 /mnt/parabola/home/
# mount ...

While it's possible to mount filesystems after you've chrooted, it is more convenient to do so beforehand. The reasoning for this is that you'll have to unmount the temporary filesystems after you exit the chroot, so this lets you umount all the filesystems with a single command. This also allows for a safer shutdown. Because the external GNU environment knows all mounted partitions, it can safely unmount them during shutdown.

3 Change root

Mount the temporary filesystems:

Note: Using a newer (2012) Parabola release, the following mount commands can be replaced with arch-chroot /mnt/parabola (found in the arch-install-scripts package), if the root partition was mounted in that location. Of course, you may still type these, if you want, or if you only have some other "live" GNU/Linux distribution.
# cd /mnt/parabola
# mount -t proc proc proc/
# mount -t sysfs sys sys/
# mount -o bind /dev dev/
# mount -t devpts pts dev/pts/

If you established an internet connection and want to use it in the chroot environment, you may have to copy over your DNS servers so that you will be connected to the network:

# cp -L /etc/resolv.conf etc/resolv.conf

Now chroot into your installed system and define your shell:

# chroot . /bin/bash
Note: If you see the error chroot: cannot run command '/bin/bash': Exec format error, it is likely that the two architectures do not match.

Optionally, to source your Bash configuration (~/.bashrc and /etc/bash.bashrc), run:

# source ~/.bashrc
# source /etc/profile

Optionally, create a unique prompt to be able to differentiate your chroot environment:

# export PS1="(chroot) $PS1"

4 Run graphical chrooted applications

If you have X running on your system, you can start graphical applications from the chroot environment.

To allow the connection to your X server, you have to run the following from a terminal:

# xhost +

Then, to direct the applications to your X server, run:

# export DISPLAY=:0.0

5 Perform system maintenance

At this point you can perform whatever system maintenance you require inside the chroot environment. A few common examples are:

6 Exit the chroot environment

When you're finished with system maintenance, exit the chroot:

# exit

Then unmount the temporary filesystems and any mounted devices:

# umount {proc,sys,dev,boot,[...],}

Finally, attempt to unmount your root partition:

# cd ..
# umount parabola/
Note: If you get an error saying that /mnt (or any other partition) is busy, this can mean one of two things:
  • A program was left running inside of the chroot.
  • Or, more frequently, a sub-mount still exists (e.g. /mnt/parabola/boot within /mnt/parabola). Check with lsblk to see if there are any mountpoints left:
lsblk /dev/sda
If you are still unable to unmount a partition, use the --force option:
# umount -f /mnt

After this, you will be able to safely reboot.