Linux Praktikumstermin 2025_05: Processes / Files

To show that you have successfully finished this lab, please upload a log file to ILIAS. The log file will contain all commands that you typed in during your session and all system outputs.

It is not a problem if there are commands in the logfile that did not work on first try or were typed in wrong, we just want to see that at some point all the required commands were used correctly.

Open a terminal in the virtual machine and start the logging by using the script command (option -a means: append to file in case the file is non-empty when the logging to file is started):

The following output should appear:

From now on, please only use the console in which the logger is running, so that everything gets logged correctly!

If you want to interrupt the lab and shut down the virtual machine, e.g. to take a break, or if you get interrupted somehow, you have to restart the logging when you continue. Choose a new / different file name. Finally, use a text editor to merge the resulting log files into a single file in the correct order.

After completing the lab, upload the complete result files to the Ilias course.

Start a Python Interpreter in the background. The interpreter shall solely execute a continuous loop:

Find out via the /proc subdirectory related to the Python process which working directory the Python process was started with. That means if the Python process tries to read a file from his "current directory", which directory would be searched forthe file? (Such questions should only be answered for yourself, you do not have to answer "submit" …​)

Find out via the /proc subdirectory related to the Python process which command this process was started with. You should see the full command, but all parts of the command are "glued together". Why? Because the driver for the /proc filesystem separates the individual parts (parameters) of the command with \0 (NULL), which means it is a list of C-strings. The following command at least shows that there are still "separators" between the parts of the command:

And by means of the following command, all \0 characters are replaced by a line break:

Let the system output the environment (i.e., the list of environment variables with their values) of the python process.

Kill the running Python Interpreter:

Download the tar archive (= "packed" archive file) of a basic Linux filesystem:

Unzip the archive as user itsadmin:

Take a look at the archive contents:

Create a blank file in-rootfs.txt as follows. This serves as a "marking" to recognize the directory /home/itsadmin/rootfs based on the existence of this file:

Check that you are currently working as user itsadmin:

Now execute the chroot command with administrator rights. The command should start a bash shell (= command prompt, console session) with the directory /home/itsadmin/rootfs as a root directory:

Check that this shell really has the directory above as its root directory by looking for the file in-rootfs.txt:

Check that due to the chroot you are now working as administrator user root:

Now check that even the Administrator user root can not leave the "restricted root directory":

But by leaving the shell by means of exit you get out of the chroot "filesystem jail". chroot was only applicable for the bash shell program that was started by it …​

Also applications can run in a chroot. We will try this via a Python interpreter that will run a mini web server. First, we check that Python interpreter executable exists within the filesystem under rootfs:

For Python to work, the following workaround is necessary (otherwise /dev/urandom will b emissing, which is needed by Python):

Now we can start Python in the chroot "jail":

Via a new, other console, call a web page served by this web server:

Note: The file in-rootfs.txt is also part of the listing on this web page!

Switch to the console in which the Python program is running. Exit the program via Ctrl-C.

Leave the chroot "filesystem jail":

End the above workaround:

"::h1 Simple Container"

Now combine PID namespaces and chroot using the following command:

Take a look at the root filesystem as well as the current user of the "container".

To view the processes in the "container", a command must be executed that instructs the procfs driver to make the needed pseudo files available within the /proc directory of the "container".

Now the processes visible in the "container" can be viewed via the ps command.

End the container via exit.

Use the dd command to create a file /tmp/disk1.img. The file shall have a size of 512 MB and shall be filled with zeros:

Within the file, create a File system of ext3 type.

Create a directory /mnt/disk1.

Mount the file into the just created directory (mount).

Create a new file in the directory. You may have to do so as root, because standard users do not have write permissions in the directory. Alternatively, you can also grant write permissions to the directory to all users and then create the file as the default user.

Check that the file is created successfully.

Check as well that a subdirectory lost+found also exists in that directory. This automatically created directory is a good indicator that the directory is the root directory of a filesystem of a separate disk: it is a kind of "wastebasket of the filesystem", where the filesystem possibly stores orphaned files when filesystem operations should run into errors.

Unmount the "disk implemented via a file" at the location /mnt/disk1 again.

Delete the directory /mnt/disk1 as well as the data file /tmp/disk1.img.

Download the following file into the directory /home/itsadmin using wget:

Unzip with administrator rights (important) the downloaded file:

Switch to the rootfs subdirectory just created and as root user create four subdirectories there: container1_subdir_1, container1_subdir_2, container2_subdir_1, container2_subdir_2

Let the system output the contents of directory /home/`datei1.txt, datei2.txt. /rootfs` and check that the four directories are present.

As root user, create two files in each of the four directories: datei1.txt, datei2.txt. The content of the files does not matter and can also be empty.

Back as user itsadmin, in the /home/itsadmin directory, create the following subdirectories: upper1, upper2, work1, work2, overlay1, overlay2.

Now create two overlay filesystem structures:

Let the system list the directory contents of the directories /home/itsadmin/overlay1 and /home/itsadmin/overlay2. You should both times see the files from rootfs.

Enter the command mount and watch the last two lines of the output of the command. There you should find the mounted overlay structures.

Open a new command line or a new command line tab and start a "simple container" there:

Check your user identity in the container and have the system output the contents of the root directory of the container using the command ls /. You should see the files from rootfs there.

In this first container, delete the directory /srv and all of its content. Check that the directory is indeed deleted.

In the root directory of the container, create a file container1_added.txt. The file contents do not matter and can also be empty. Insert a directory /container1_added_subdir_1 and in this directory a file /container1_added_subdir_1/container1_added.txt.

In the container, recursively delete the directory /container1_subdir_1 with all its contents. Create a new file /container1_subdir_2/container1_added.txt and delete the file /container1_subdir_2/file1.txt.

After all these steps in the container, check that the required changes have all been executed correctly.

Now open another new command line or a new command line tab and start a second "simple container" there:

Check your user identity in the container and have the system output the contents of the root directory of the container using the command ls /. You should see the files from rootfs there.

In this second container, delete the directory /media including all its contents. Check that the directory is indeed deleted. Also, check that the /srv directory still exists, i.e. the deletion of this directory in container1 obviously has had no effect on container2.

In the root directory of the container, create a file container2_added.txt. The file contents do not matter and can also be empty.

Create a directory /container2_added_subdir_1 and in this directory a file /container2_added_subdir_1/container2_added.txt.

In the container, recursively delete the directory /container2_subdir_1 with all its contents.

Create a new file /container2_subdir_2/container2_added.txt and delete the file /container2_subdir_2/file1.txt.

Switch to each of the command lines of Container1 and Container2 and check that the changes made in each container did not have an impact on each other container.

Switch to the command line of Container2. Use the command ps aux to view the list of processes in the container and terminate this command line and thus the container via exit.

Also leave the command line or the command line tab that was opened for the container by means of exit.

Switch to the command line of Container1. Use ps aux to get the list of processes in the container and terminate this command line and thus the container via exit.

Also leave the command line or the command line tab that was opened for this container by means of exit.

Stop the mounting and end the existence of the two overlay filesystem structures:

Now look at the contents of the directories upper1 and upper2 (including their subdirectories) and analyze the results for yourself.

Take a look at the content of the rootfs directory and observe that all these activities have no changed the original directory contents. The container…​ subdirectories should also be completely present; these were created before the containers were started and thus belong to the "initial state". Also observe that in rootfs, the directories srv and media are still available with their contents.

With administrator permissions, recursively delete the directories upper1, upper2, work1, work2, overlay1, overlay2 und rootfs with all their contents.

When you have finalized your lab work, you need to end the logging by entering exit into your console. The following output should appear:

Start a webbrowser inside the virtual machine and upload the log file to ILIAS, using this web browser.

Shut down the VM, remove the VM completely (delete all files) and end your session.