8.6.35 chroot
The chroot program, chroot(8), enables us to run
different instances of the GNU/Linux environment on a single system
simultaneously without rebooting.
One may also run a resource hungry program such as apt-get or
dselect under the chroot of a fast host machine while NFS-mounting
a slow satellite machine to the host as r/w and the chroot point being the
mount point of the satellite machine.
8.6.35.1 Run a different Debian distribution with chroot
A chroot Debian environment can easily be created by the
debootstrap command in Sarge. For post-Sarge distributions, you
may use cdebootstrap command instead with appropriate option. For
example, to create a Sid chroot on /sid-root while having fast
Internet access:
main # cd / ; mkdir /sid-root
main # debootstrap sid /sid-root https://ftp.debian.org/debian/
... watch it download the whole system
main # echo "proc-sid /sid-root/proc proc none 0 0" >> /etc/fstab
main # mount proc-sid /sid-root/proc -t proc
main # cp /etc/hosts /sid-root/etc/hosts
main # chroot /sid-root /bin/bash
chroot # cd /dev; /sbin/MAKEDEV generic ; cd -
chroot # apt-setup # set-up /etc/apt/sources.list
chroot # vi /etc/apt/sources.list # point the source to unstable
chroot # dselect # you may use aptitude, install mc and vim :-)
At this point you should have a fully working Debian system, where you can play
around without fear of affecting your main Debian installation.
This debootstrap trick can also be used to install Debian to a
system without using a Debian install disk, but instead one for another
GNU/Linux distribution. See https://www.debian.org/releases/stable/i386/apcs04.html.en.
8.6.35.2 Setting up login for chroot
Typing chroot /sid-root /bin/bash is easy, but it
retains all sorts of environment variables that you may not want, and has other
issues. A much better approach is to run another login process on a separate
virtual terminal where you can log in to the chroot directly.
Since on default Debian systems tty1 to tty6 run
Linux consoles and tty7 runs the X Window System, let's set up
tty8 for a chrooted console as an example. After creating a
chroot system as described in
Run a different Debian
distribution with chroot, Section 8.6.35.1, type from the root
shell of the main system:
main # echo "8:23:respawn:/usr/sbin/chroot /sid-root "\
"/sbin/getty 38400 tty8" >> /etc/inittab
main # init q # reload init
8.6.35.3 Setting up X for chroot
You want to run the latest X and GNOME safely in your chroot? That's entirely
possible! The following example will make GDM run on virtual terminal
vt9.
First install a chroot system using the method described in Run a different Debian distribution with
chroot, Section 8.6.35.1. From the root of the main system,
copy key configuration files to the chroot system.
main # cp /etc/X11/XF86Config-4 /sid-root/etc/X11/XF86Config-4
main # chroot /sid-root # or use chroot console
chroot # cd /dev; /sbin/MAKEDEV generic ; cd -
chroot # apt-get install gdm gnome x-window-system
chroot # vi /etc/gdm/gdm.conf # do s/vt7/vt9/ in [servers] section
chroot # /etc/init.d/gdm start
Here, /etc/gdm/gdm.conf was edited to change the first virtual
console from vt7 to vt9.
Now you can easily switch back and forth between full X environments in your
chroot and your main system just by switching between Linux virtual terminals;
e.g. by using Ctrl-Alt-F7 and Ctrl-Alt-F9. Have fun!
[FIXME] Add a comment and link to the init script of the chrooted
gdm.
8.6.35.4 Run other distributions with chroot
A chroot environment for another Linux distribution can easily be created. You
install a system into separate partitions using the installer of the other
distribution. If its root partition is in /dev/hda9:
main # cd / ; mkdir /other-dist
main # mount -t ext3 /dev/hda9 /other-dist
main # chroot /other-dist /bin/bash
Then proceed as in
Run a different Debian
distribution with chroot, Section 8.6.35.1, Setting up login for chroot, Section
8.6.35.2, and
Setting up X for chroot,
Section 8.6.35.3.
8.6.35.5 Build a package with chroot
There is a more specialized chroot package, pbuilder, which
constructs a chroot system and builds a package inside the chroot. It is an
ideal system to use to check that a package's build-dependencies are correct,
and to be sure that unnecessary and wrong build dependencies will not exist in
the resulting package.
