For more confort, our work will be based on a copy of the file system which will be copied block to block on the hard drive instead of the flash drive (better access time Read/Write)

$ sudo dd if=/dev/sda1 of=usb_demo_wsim.raw bs=512
	  

The use of Chroot command allows administration of inactive operating system with your system in order to not increase size of your flash drive system. Work and download directories (for apt-get or aptitude) are on your active system, not on the target system.

Filesystem "filesystem.squash" decompression and its modification with Chroot allow to have an effect on live version executed on USB Key and on installable version. So the installation of your distribution stays identical to your "demo" version.

Below, a summary of how work a live/installable version (Ubuntu) :

Figure III.1. How "casper" works (LiveCD Ubuntu)

2.1. Experiments

First, in order to modify filesystem, you must mount RAW image in a directory :

$ sudo mount -o loop image.raw mnt
	  

Then, you must uncompress filesytem with unsquashfs command

$ sudo unsquashfs mnt/casper/filesystem.squashfs 
	  

Once the filesystem has been unpacked in /squashfs-root directory, we just have to do Chroot without forgetting to copy host network configuration.

$ sudo cp /etc/resolv.conf /mnt/etc/

$ sudo mount /dev/    /mnt/dev      -o bind
$ sudo mount /dev/pts /mnt/dev/pts  -o bind
$ sudo mount /proc    /mnt/proc     -o bind
$ sudo mount /sys     /mnt/sys      -o bind
$ sudo chroot squashfs-root
	      

	Chroot mode
	From now on, all commands executed will be executed under chroot mode, that is to say on your target file system. Make sure you have "aptitude" or "apt-get" to make install/uninstall and don't forget to make aptitude clean at the end.

• Make your updates
• Install new packages
• Verify they are no services running with open files under Chroot mode (command : lsof)
• Exit chroot mode with exit command

 $ sudo umount /mnt/sys
 $ sudo umount /mnt/proc
 $ sudo umount /mnt/dev/pts
 $ sudo umount /mnt/dev
	      

When modifications are finished, we just have to recompress filesystem and copy it in the right place in mounted RAW image on /mnt

$ sudo mksquashfs squashfs-root mnt/casper/filesystem.squashfs
	  

2.2. Summary

Here is a summary diagram of differents steps for filesystem modifications :

Figure III.2. Summary of modification steps

QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures. So, we can boot on RAW image previously created to validate content of customized distribution.

	Remark
	There is a graphical user interface for Qemu named Qemu-launcher

Below, there is an example of Qemu utilisation with RAW image :

Figure III.3. Live USB with Qemu