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Using chroot

Warning: When using --rbind, some subdirectories of dev/ and sys/ will not be unmountable. Attempting to unmount with umount -l in this situation will break your session, requiring a reboot. If possible, use -o bind instead.

In the following example, /location/of/new/root is the directory where the new root resides.

First, mount the temporary API filesystems:

# cd /location/of/new/root
# mount -t proc /proc proc/
# mount -t sysfs /sys sys/
# mount --rbind /dev dev/

And optionally:

# mount --rbind /run run/

If you are running a UEFI system, you will also need access to EFI variables. Otherwise, when installing GRUB, you will receive a message similar to: UEFI variables not supported on this machine:

# mount --rbind /sys/firmware/efi/efivars sys/firmware/efi/efivars/

Next, in order to use an internet connection in the chroot environment, copy over the DNS details:

# cp /etc/resolv.conf etc/resolv.conf

Finally, to change root into /location/of/new/root using a bash shell:

# chroot /location/of/new/root /bin/bash
Note: If you see the error:

  • chroot: cannot run command '/usr/bin/bash': Exec format error, it is likely that the architectures of the host environment and chroot environment do not match.
  • chroot: '/usr/bin/bash': permission denied, remount with the execute permission: mount -o remount,exec /location/of/new/root.
    • if checking this did not help, then make sure the base components of the new environment are intact (if it is an Arch root, try paccheck --root=/location/of/new/root --files --file-properties --md5sum glibc filesystem, from pacutils)

After chrooting, it may be necessary to load the local bash configuration:

# source /etc/profile
# source ~/.bashrc
Tip: Optionally, create a unique prompt to be able to differentiate your chroot environment:

# export PS1="(chroot) $PS1"

When finished with the chroot, you can exit it via:

# exit

Then unmount the temporary file systems:

# cd /
# umount --recursive /location/of/new/root
Note: If there is an error mentioning something like umount: /path: device is busy, this usually means that either: a program (even a shell) was left running in the chroot or that a sub-mount still exists. Quit the program and use findmnt -R /location/of/new/root to find and then umount sub-mounts. It may be tricky to umount some things and one can hopefully have umount --force work. As a last resort, use umount --lazy which just releases them. In either case to be safe, reboot as soon as possible if these are unresolved to avoid possible future conflicts.

23 thoughts on “Using chroot”

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