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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.

21 thoughts on “Using chroot”

  6. binance

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