chroot for a custom environment

Using a single computer, I found myself wanting multiple times to be able to switch environment based on what I was doing.

For example, when developping, I would like a git-aware prompt in my fish shell. When playing games, I don't want all the 32-bit libraries accumulating in my os. And I don't want to load too much packages in emacs when I am not developping.

In order to achieve this, I will present here a method based on chroot, that enables :

to have multiple environments integrated into one : no need to launch a VM, no need to swap sessions
to have the environments perform under the same X session
to have the users of the main OS enabled on the other environments, simplying system administration
to easily setup shared folders across environments

1. Installation

Install a bare linux system in a folder
Setup schroot
Setup the new environment

1.1 Install a bare linux system in a folder

This step is highly dependant on the distribution you want to install. I advise you to look for ways to install a bootstrap of the distribution of your choice. For example

debian provides the debootstrap utility
archlinux provides the pacstrap utility : # pacstrap /envs/myenv base package1 ... package n
and so on...

I will assume in the rest of this post that an distribution has been installed in the /envs/myenv directory of your main OS.

Installion of

1.2 Setup schroot

schroot is a tool that simplifies the use of chroot. Install the package in your main OS

1.2.1 schroot.conf

The first thing to do is to write a configuration file for schroot. Examples of configuration can be found in the file /etc/schroot/schroot.conf. You can either add a configuration to this file, or write a configuration in a new file, which we will do.

I will here assume that we want a chroot to an os with the same architecture, with a shared X session.

Create the file /etc/schroot/chroot.d/myenv.conf and add these lines :

[my-environment]
description=custom environment
aliases=myenv
type=directory
directory=/envs/myenv
users=me
root-groups=root
preserve-environment=true
profile=desktop-custom

See man schroot.conf to describe the provided options.

The line profile=desktop-custom points schroot to look for specific configuration files in the folder /etc/schroot/desktop-custom that we will discuss in the next session.
The line preserve-environment=true is here to easily enable applications run in the chroot to access the X server, although it could be enabled by other means.

1.2.2 desktop-custom directory

First, create your user directory in the chroot /home, then change owner and group so that your user can access and modify it.

You can then copy the desktop directory in /etc/schroot/ to desktop-custom and modify it according to our needs. This folder contains three files :

nssdatabases that we will not modify.

copyfiles :

# network
/etc/resolv.conf

# shared X session
/home/sapristi/.Xauthority

# locale
/etc/locale.gen
/etc/locale.conf

# time
/etc/localtime

fstab : a custom fstab file. I did some changes to this file :

/proc           /proc           none    rw,bind         0       0
/sys            /sys            none    rw,bind         0       0
/dev            /dev            none    rw,bind         0       0
/dev/pts        /dev/pts        none    rw,bind         0       0
# /home is commented out to keep the same user without sessions poluting each other
# /home           /home           none    rw,bind         0       0
/tmp            /tmp            none    rw,bind         0       0

### shared documents
/home/Documents  /home/Documents none   rw,bind         0       0

### fonts & themes
/usr/share/fonts /usr/share/fonts none rw,bind          0       0
/usr/share/themes /usr/share/themes none rw,bind          0       0

# If you use gdm3, uncomment this line to allow Xauth to work
#/var/run/gdm3  /var/run/gdm3   none    rw,bind         0       0
# For PulseAudio and other desktop-related things
/var/lib/dbus    /var/lib/dbus  none    rw,bind         0       0

1.3 Setup the new environment

Once the previous session is set, you can enter the chroot with the command schroot -c myenv

For easier access, you can put a script in your path that launches the terminal emulator of your choice (terminator here).

:::bash
#!/bin/bash

# options for terminator : 
# -u : no systemd (prevents some warning)
# -T myenv : sets the title of the terminal so that you can
# differentiate it from non-chroot terminals 
schroot -c dev -- sh -c "terminator -u -T myenv"

It is important to differentiate the terminal launched in the chroot from the one of you main OS. You can either use another terminal emulator, use different color scheme, etc. The same applies to any other program that you could launch in both environments, like emacs.

Depending on you window manager, it might be possible to change the window decorations based on the environments they were launched on, but this is not possible in XFCE. It is possible though to change the GTK theme.

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