What is chroot ?
As its man page says, “it is used to run commands or an interactive shell with special root directory“. It provides an environment to test new packages in a secured way without touching an actual system. It can be called as a virtual system with a new as an root(/) directory.
Why chroot environment ?
Suppose I have a new package to test and compile with many dependencies. Also I may have to compile each and every dependent package till my requirement for the test-package is met. This process can make my development machine highly unstable or sometime unusable, this is certainly not I want. The best way I can deal with this is to create a virtual machine, I can use Qemu or Virtual Box for that or I can make a chroot environment in a separate directory and start working. chroot environments are also used to host web-servers, so if at all the web-server is compromised, not all the services are hampered and the physical is still safe.
An advantage of having a chroot environment is the file-system is totally isolated from the physical host. chroot has a separate file-system inside the file-system, the difference is its uses a newly created root(/) as its root directory.
Building a chroot environment
For chroot, we need to create a file-system. The file hierarchy within the directory is same as any other Linux file-system such as /root, /usr, /etc, /bin, /opt etc. We can make a Debian chroot environment using debootstrap or rootstock, both are available for Ubuntu systems. In this post I will use debootstrap to create a chroot environment.
Install debootstrap using,
sudo apt-get install debootstrap
We can specify a system architecture, a suite(release name) and a mirror to download from in the debootstrap parameter.
The syntax is as follows,
debootstrap --arch ARCHITECTURE SUITE YOUR-ROOT-DIRECTORY MIRROR
for example, if I want arch to be i686 of Ubuntu 12.04(precise) and my root directory is precise-chroot/ with mirror as http://archive.ubuntu.com/ubuntu, then create directory for chroot
mkdir precise-chroot
and create a chroot environment using debootstrap
debootstrap --arch i686 precise precise-chroot http://archive.ubuntu.com/ubuntu
this will create a chroot environment for Ubuntu 12.04, from the mirror.
Chrooting
Once all the file are downloaded, we can chroot into precise-chroot/ directory using
sudo chroot precise-chroot /bin/bash
where precise-chroot is the root directory, and the shell is /bin/bash. You will be landed with the root prompt. Now you can setup the package manager and update it. This will work same as any other Linux environment.