Chroot

"Changing root" or "chrooting" is a method for zooming in on part of your filesystem, so that, for example, will refer to what was formerly accessible at . The "root" in the expression "chroot" refers to the root filesystem , not to the root user. (Though typically you will need root user privileges in order to chroot.)
A common reason for chrooting is to perform maintenance on existing systems where booting and/or logging in no longer works. One has to boot the hardware somehow, such as with an Installation or Rescue CD or USB; then one mounts the broken system and chroots into it and performs the repairs. Common examples are:
* reinstalling the bootloader
* rebuilding the initramfs image
* upgrading or downgrading packages
* resetting a forgotten password
Chrooting can also be used to create and host a separate virtualized installation of a system. This can be useful for:
* Testing and development, with software that's too risky to deploy on a production system.
* Software can be developed, built and tested in a chroot populated only with its expected dependencies.
* Can run legacy software or other software whose supporting libraries or data files can't be installed in the host system.
* Open file descriptors (for files, pipelines and network connections) can be "carried into" the chroot by running programs, making it unnecessary for those programs' working files to be visible inside the chroot directory. This can help separate privileges or design sandboxes. Note that chroot is not a secure way to contain a hostile process with root privileges. See Wikipedia's discussion of some of chroot's limits.
Scripts for installing Alpine Linux that use chroot:
* alpine-chroot-install
* alpine-make-vm-image
* alpine-make-rootfs
== Preparation ==
All the steps in this guide will have to be performed as root user.
If you are using chroot to repair an existing Linux system, it will need to be mounted first. If you don't know the location and/or filetype of the disk(s) where it resides, read the output of fdisk -l or lsblk. We will suppose the broken system is at and its filetype is ext3. Create a directory for mounting this disk, and mount it:
where $CHROOT evaluates to the path you want to chroot into (for example, ).
If the system directory is encrypted or in a LVM group or anything like that, we'll leave it to you to figure out what steps to take to prepare it and mount it.
If there are separate filesystems for other directories (such as , , , ), mount them too:
For many repair tasks, you may not need to mount the broken system's directory.
If you do have on a separate partition, it's essential to have it mounted before working with GRUB, performing a kernel upgrade, or anything of that sort. It's also possible to mount some of these filesystems after you've chrooted, and in some cases mounting in particular after chrooting has worked better with GRUB. If you can, though, it's smarter to mount these beforehand. The reason is that if you do it from inside the chroot, the outside/host environment won't know about the mounted filesystems, so if you forget to umount them before exiting the chroot, the system won't know to umount them when it shuts down, either. That could damage those filesystems.
The script presented below will take care of generating or remounting other system directories like , , , and so on.
The architecture of the host system you're booted into (e.g., if it's a 32bit LiveCD, that's x86) and the system you want to chroot into must match. You can determine what architecture you're booted into using uname -m.
Before chrooting, be sure any kernel modules you'll need when working inside the chroot have been loaded. For example:
Set up your network if you'll need it inside the chroot, e.g., to install updated packages. The script presented below will copy the from the host system, but will prompt before overwriting an existing inside the chroot. If you like, you can generate one from scratch by doing:
== Performing the chroot ==
The most reliable way to chroot is to use the following script:
Copy that script to some location on your system, such as , run chmod +x on it, and then invoke it as:
It is certainly possible to perform a chroot by hand, without using this script. For some purposes, you can get away with a simpler procedure than the script follows. But it's hard to catalogue in advance when taking shortcuts will get you in trouble, so it's best to just get in the habit of using this script (or doing the same things it does). As you read around in various wikis (perhaps including this one), you will encounter a variety of instructions for how to chroot by hand. These instructions often differ in small details from each other and are usually not as thorough as the above script.
=== Troubleshooting ===
If you see the error:
* chroot: cannot run command '/bin/sh': Exec format error, it is likely that the architectures of the host environment and the chroot environment do not match. For example, you booted with a 32bit Live CD and are trying to chroot into an x86_64 system.
* chroot: '/bin/sh': Permission denied, try remounting the system with exec permissions: mount -o remount,exec $CHROOT
== Inside the chroot ==
At this point, you're still running the kernel you booted with, but all paths will refer to what used to be .
If you have multiple terminals open on or connected to the running system, the chroot will only be effective in the terminal that you performed it in.
Now you can perform whatever troubleshooting (or do whatever development work) you need to do:
* resintall GRUB to your disk's MBR
* rebuild your initramfs image
* fix your 
* perform a kernel upgrade (or downgrade)
* (re)install other packages using your package manager
* reset a forgotten password
or whatever.
If you'll be doing anything with GRUB, you should first make sure that sure your file is up-to-date:
=== Run graphical applications from the chroot ===
If you have an X server running on your outside/host system, you can start graphical applications from inside the chroot. To allow the chroot environment to connect to the X server, you have to do this in the host system (either before chrooting, or in a terminal other than the one you performed the chroot in):
Then, to direct applications from the chroot to the X server, set the DISPLAY environment variable inside the chroot to match its value in the host environment (for the user who owns the X server). So for example, inside the chroot you might say:
== Cleaning up ==
When you're finished, ensure that all running programs have stopped. Then exit the chroot:
If you started the chroot using the script, it will take care of umounting , , , , and so on. Else you should do this by hand. In any case, you should umount any partitions like that you mounted yourself, and then umount the system partition at $CHROOT if appropriate.
If you get an error saying that $CHROOT (or any partition inside it) is busy, this can mean one of two things:
* A program was left running inside of the chroot.
* Or more frequently, a mountpoint still exists on this mount. For example, is still mounted when trying to umount .
In the latter case, simply unmount the offending mountpoint first. To get a reminder of all the current mountpoints, run mount with no parameters.
== Other Resources ==
* What's the proper way to prepare chroot to recover a broken Linux installation?
* ArchLinux wiki on "Change Root"
* Gentoo Linux Wiki: Chroot from a livecd (via archive.org)
* Ubuntu wiki on "Basic chroot"
Category:System Administration