TU Dubuntu – a customised live USB version of Xubuntu Linux 19.04 for TU Dublin

This post documents the steps I followed to create a customised version of the Xubuntu 19.04 live CD. I called the modified image “TU Dubuntu” (after my university, TU Dublin), but it’s really just Xubuntu with some additional software packages installed that aren’t included in the default image. The steps I followed are based on the “LiveCDCustomization” article on the Ubuntu Community Wiki.

https://help.ubuntu.com/community/LiveCDCustomization

That article isn’t specific to this version of Xubuntu and it includes a lot of optional customisations that I didn’t use, so I wrote this post to capture the specific sequence of steps that produced my working image. Also, I added a step at the end to turn the iso file into a “hybrid” image. That step wasn’t included in the article on the Ubuntu Community Image, but my image wouldn’t boot without it.

Preparing to Create the Custom Image

First, we install some required packages:

sudo apt install squashfs-tools genisoimage syslinux-utils

First, create a directory and download the original Xubuntu iso image into it:

cd ~
mkdir livecdtmp
cd livecdtmp
wget http://cdimages.ubuntu.com/xubuntu/releases/19.04/release/xubuntu-19.04-desktop-amd64.iso

Create a sub-directory “mnt” and mount the downloaded iso image as a loopback filesystem.

mkdir mnt
sudo mount -o loop xubuntu-19.04-desktop-amd64.iso mnt

Extract the contents of the installation CD from the downloaded iso image:

mkdir extract-cd
sudo rsync --exclude=/casper/filesystem.squashfs -a mnt/ extract-cd

Extract the SquashFS filesystem into the current directory, then rename the root folder as “edit”:

sudo unsquashfs mnt/casper/filesystem.squashfs
sudo mv squashfs-root edit

The “edit” folder will be the root directory of the chroot environment. Before chrooting into it, we copy some network configuration files into it and clone a couple of special directories as sub-directories of “edit”.

sudo cp /etc/resolv.conf edit/etc/
sudo mount -o bind /run/ edit/run
sudo cp /etc/hosts edit/etc/
sudo mount --bind /dev/ edit/dev

Now, we chroot into the edit folder, mount some special filesystems, and set a couple of environment variables.

sudo chroot edit
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devpts none /dev/pts
export HOME=/root
export LC_ALL=C

I actually don’t really understand what the following commands do, but I ran than because they were in the original instructions and everything worked out ok!

dbus-uuidgen > /var/lib/dbus/machine-id
dpkg-divert --local --rename --add /sbin/initctl
ln -s /bin/true /sbin/initctl

Customising the Image

The following command runs the timezone configuration. I selected “Europe” and “Dublin”.

dpkg-reconfigure tzdata

Now install whatever packages are to be added using apt-get.

apt-get -y install inkscape octave geany hexedit mplayer ffmpeg blender python-scipy python-matplotlib python-serial obs-studio php texlive texstudio audacity

Note: I intended to add the Arduino IDE to the chroot filesystem at this point, but I forgot so I had to add it in later. The required commands are included later in the process.

Cleaning Up

The following commands basically undo those commands above that I didn’t understand.

rm /var/lib/dbus/machine-id
rm /sbin/initctl
dpkg-divert --rename --remove /sbin/initctl

The following commands unmount the special filesystems and exit chroot.

umount /proc
umount /sys
umount /dev/pts
exit
sudo umount edit/dev
sudo umount edit/run
sudo umount mnt

Assembling the Modified Filesystem

Regenerate manifest:

sudo chmod +w extract-cd/casper/filesystem.manifest
sudo su
chroot edit dpkg-query -W --showformat='${Package} ${Version}\n' > extract-cd/casper/filesystem.manifest
exit
sudo cp extract-cd/casper/filesystem.manifest extract-cd/casper/filesystem.manifest-desktop
sudo sed -i '/ubiquity/d' extract-cd/casper/filesystem.manifest-desktop
sudo sed -i '/casper/d' extract-cd/casper/filesystem.manifest-desktop

When I was installing pacakges earlier in the process using apt-get, I forgot to also download the Arduino IDE and unpack it into the /opt directory of the chroot environment so I did it at this point instead:

wget https://downloads.arduino.cc/arduino-1.8.9-linux64.tar.xz
tar xf arduino-1.8.9-linux64.tar.xz
rm arduino-1.8.9-linux64.tar.xz
sudo mv arduino-1.8.9 edit/opt/

The following command compresses the modified filesystem:

sudo mksquashfs edit extract-cd/casper/filesystem.squashfs

This recalculates the filesystem size:

sudo su
printf $(du -sx --block-size=1 edit | cut -f1) > extract-cd/casper/filesystem.size
exit

To change the image name to 'TUDubuntu 19.04 "Learned Lecturer" - Release amd64', we edit the first line of the file “extract-cd/README.diskdefines” using nano:

sudo nano extract-cd/README.diskdefines

The following command recalculates the md5sum:

cd extract-cd
sudo rm md5sum.txt
find -type f -print0 | sudo xargs -0 md5sum | grep -v isolinux/boot.cat | sudo tee md5sum.txt

Create the Bootable Image

This creates the iso image:

sudo mkisofs -D -r -V "$IMAGE_NAME" -cache-inodes -J -l -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -o ../TUDubuntu-19.04-desktop-amd64.iso .

This command runs isohybrid on the iso file to turn it into a “hybrid” image that will boot from a USB drive:

cd ..
sudo isohybrid TUDubuntu-19.04-desktop-amd64.iso

Finally, the image needs to be copied onto a USB drive. In my case, when I plugged in my USB key it appeared as “/dev/sda”, but you must check the correct device before running the following command!!! The entire contents of the device you specify (hopefully your USB key) will be erased.

WARNING: RUNNING THE NEXT COMMAND WITHOUT CHECKING WHICH DEVICE FILE IS THE USB DRIVE MAY WIPE YOUR ENTIRE OPERATING SYSTEM. MINE IS /dev/sda BUT THERE IS NO GUARANTEE THAT WILL BE THE CASE ON OTHER SYSTEMS.

sudo dd if=TUDubuntu-19.04-desktop-amd64.iso of=/dev/sda

The previous command may take a few minutes to complete because it has to copy approximately 2.7 GB of data to the USB drive.

That’s it! The USB drive should now be ready to boot.

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