Reviewing Filesystem Changes

Reviewing filesystem changes
In this lab, we will learn how to review filesystem changes made inside a Docker container. Docker provides a command to list all changes to the filesystem, including added, changed, or deleted files and directories.

When you read a file from a union filesystem, that file will be read from the topmost layer where it exists. If a file was not created or changed on the top layer, the read will fall through the layers until it reaches a layer where that file does exist. Here is a simple example:

The layer functionality is hidden by the UFS. No special actions are required by the software running in a container to utilize these features. The UFS manages the complexity of handling files across multiple layers.

Most union filesystems use something called copy-on-write, which is easier to under stand if you think of it as copy-on-change. When a file in a read-only layer (not the top layer) is modified, the whole file is first copied from the read-only layer into the writable layer before the change is made.

In this illustration, files are added, changed, deleted, and added again over a range of three layers.

Task

• Create a container and add a new file and review changes
• Create another container and delete an existing file and review changes
• Create yet another container and change an existing file and review changes
• Clean up our workspace Steps Follow these steps to complete the task:

Create a container and add a new file:

Command:

docker container run --name tweak-a busybox:latest touch /HelloWorld

This command runs a new container named tweak-a from the busybox:latest image and creates an empty file named /HelloWorld inside the container.

Review the filesystem changes:

docker container diff tweak-a

This command lists all the changes made to the filesystem of the tweak-a container. You should see:

Expected Output:

A /HelloWorld
This indicates that the file /HelloWorld was added.

Create another container and delete an existing file:

Command:

docker container run --name tweak-d busybox:latest rm /bin/vi

This command runs a new container named tweak-d from the busybox:latest image and deletes the file /bin/vi.

Review the filesystem changes:

docker container diff tweak-d

This command lists all the changes made to the filesystem of the tweak-d container. You should see:

Expected Output:

This indicates that the directory /bin was changed and the file /bin/vi was deleted.

Create yet another container and change an existing file:

Command:

docker container run --name tweak-c busybox:latest touch /bin/vi

This command runs a new container named tweak-c from the busybox:latest image and updates the modification time of the file /bin/vi.

Review the filesystem changes:

docker container diff tweak-c
This command lists all the changes made to the filesystem of the tweak-c container. You should see:

This indicates that the directories /bin and /bin/busybox were changed.

Clean up our workspace by removing the containers:

docker container rm -vf tweak-a
docker container rm -vf tweak-d
docker container rm -vf tweak-c

These commands forcefully remove the tweak-a, tweak-d, and tweak-c containers, cleaning up our workspace.

Explanation of docker container diff Output
Lines that start with an A indicate files that were added.
Lines that start with a C indicate files that were changed.
Lines that start with a D indicate files that were deleted.
By following these steps, you'll be able to track and understand filesystem changes within Docker containers. This is particularly useful for debugging and for ensuring that your containerized applications behave as expected.

#/Commands Summary
Create a container and add a new file:
docker container run --name tweak-a busybox:latest touch /HelloWorld

#Review the filesystem changes:
docker container diff tweak-a

#Create another container and delete an existing file:
docker container run --name tweak-d busybox:latest rm /bin/vi

#Review the filesystem changes:
docker container diff tweak-d

#Create yet another container and change an existing file:
docker container run --name tweak-c busybox:latest touch /bin/vi

#Review the filesystem changes:
docker container diff tweak-c

#Clean up your workspace:
docker container rm -vf tweak-a
docker container rm -vf tweak-d
docker container rm -vf tweak-c