How to build an Ubuntu chroot environment

chroot allows us have a separate and temporary system for development and development without high overhead such as with virtual machines.

An Ubuntu chroot environment could easily be created using debootstrap tool. This is basically a similar process to building a chroot environment for Debian.

1. Launch terminal application.
2. Update apt's package repository.

   $ sudo apt update
   [sudo] password for user:
   Hit:1 http://jp.archive.ubuntu.com/ubuntu focal InRelease
   Hit:2 http://jp.archive.ubuntu.com/ubuntu focal-updates InRelease
   Hit:3 http://jp.archive.ubuntu.com/ubuntu focal-backports InRelease
   Hit:4 http://jp.archive.ubuntu.com/ubuntu focal-security InRelease
   Reading package lists... Done
   Building dependency tree
   Reading state information... Done
   All packages are up to date.

3. Install debootstrap using apt.

   $ sudo apt install --assume-yes debootstrap
   Reading package lists... Done
   Building dependency tree
   Reading state information... Done
   Suggested packages:
     arch-test squid-deb-proxy-client
   The following NEW packages will be installed:
     debootstrap
   0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
   Need to get 39.4 kB of archives.
   After this operation, 299 kB of additional disk space will be used.
   Get:1 http://jp.archive.ubuntu.com/ubuntu focal-updates/main amd64 debootstrap all 1.0.118ubuntu1.1 [39.4 kB]
   Fetched 39.4 kB in 1s (66.1 kB/s)
   Selecting previously unselected package debootstrap.
   (Reading database ... 107133 files and directories currently installed.)
   Preparing to unpack .../debootstrap_1.0.118ubuntu1.1_all.deb ...
   Unpacking debootstrap (1.0.118ubuntu1.1) ...
   Setting up debootstrap (1.0.118ubuntu1.1) ...
   Processing triggers for man-db (2.9.1-1) ...

4. Create a chroot folder.

   $ mkdir chroot-ubuntu

5. Create a base Ubuntu system using debootstrap on the newly-created folder.

    $ sudo debootstrap --variant=buildd focal chroot-ubuntu
   I: Retrieving InRelease
   I: Checking Release signature
   I: Valid Release signature (key id F6ECB3762474EDA9D21B7022871920D1991BC93C)
   I: Retrieving Packages
   I: Validating Packages
   I: Resolving dependencies of required packages...
   I: Resolving dependencies of base packages...
   I: Checking component main on http://archive.ubuntu.com/ubuntu...
   I: Retrieving adduser 3.118ubuntu2
   I: Validating adduser 3.118ubuntu2
   I: Retrieving apt 2.0.2
   I: Validating apt 2.0.2
   I: Retrieving base-files 11ubuntu5
   I: Validating base-files 11ubuntu5
   ##### snipped

   Change focal to any Ubuntu release code name that's still supported.
   List of Ubuntu releases

   More options for debootstrap.

   $ debootstrap --help
   Usage: debootstrap [OPTION]... <suite> <target> [<mirror> [<script>]]
   Bootstrap a Debian base system into a target directory.
   
         --help                 display this help and exit
         --version              display version information and exit
         --verbose              don't turn off the output of wget
   
         --download-only        download packages, but don't perform installation
         --print-debs           print the packages to be installed, and exit
   
         --arch=A               set the architecture to install (use if no dpkg)
                                  [ --arch=powerpc ]
   
         --include=A,B,C        adds specified names to the list of base packages
         --exclude=A,B,C        removes specified packages from the list
         --extra-suites=A,B,C   also use packages from the listed suites of the
                                archive
         --components=A,B,C     use packages from the listed components of the
                                archive
         --variant=X            use variant X of the bootstrap scripts
                                (currently supported variants: buildd, fakechroot,
                                 minbase)
         --no-merged-usr        do not make /{bin,sbin,lib}/ symlinks to /usr/
         --keyring=K            check Release files against keyring K
         --no-check-gpg         avoid checking Release file signatures
         --force-check-gpg      force checking Release file signatures
                                (also disables automatic fallback to HTTPS in case
                                of a missing keyring), aborting otherwise
         --no-resolve-deps      don't try to resolve dependencies automatically
         --log-extra-deps       record extra dependency info in debootstrap.log
         --cache-dir=DIR        Use specified directory as package cache directory
   
         --unpack-tarball=T     acquire .debs from a tarball instead of http
         --make-tarball=T       download .debs and create a gzipped tarball
         --second-stage-target=DIR
                                Run second stage in a subdirectory instead of root
                                  (can be used to create a foreign chroot)
                                  (requires --second-stage)
         --extractor=TYPE       override automatic .deb extractor selection
                                  (supported: dpkg-deb ar)
         --debian-installer     used for internal purposes by debian-installer
         --private-key=file     read the private key from file
         --certificate=file     use the client certificate stored in file (PEM)
         --no-check-certificate do not check certificate against certificate authorities

6. Mount proc, sys and dev filesystem on to the base system.

   $ sudo mount -t proc /proc chroot-ubuntu/proc
   $ sudo mount --rbind /sys chroot-ubuntu/sys
   $ sudo mount --rbind /dev chroot-ubuntu/dev

7. chroot to the folder .

   $ sudo chroot chroot-ubuntu /bin/bash
   bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
   [email protected]:/#

8. Use the chroot environment as required.

   [email protected]:/# ls
   bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
   [email protected]:/# uname -a
   Linux host 5.4.0-33-generic #37-Ubuntu SMP Thu May 21 12:53:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
   [email protected]:/# df -h
   Filesystem      Size  Used Avail Use% Mounted on
   tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup
   udev            1.9G     0  1.9G   0% /dev
   tmpfs           2.0G     0  2.0G   0% /dev/shm

9. Unmount the mounted proc, sys and dev filesystem once exiting the chroot environment.

   $ sudo umount chroot-ubuntu/proc chroot-ubuntu/sys chroot-ubuntu/dev 

10. Remove the chroot folder if necessary.

    $ sudo rm -rf chroot-folder