Container filesystem is ephemeral in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to start a container that prints a message.

Docker

docker run --rm alpine echo [1]

Drag options to blanks, or click blank then click option'

AHello, World!

B"Hello, World!"

C'Hello, World!'

DHello World

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes that cause the shell to treat the message as a single argument with quotes included.

Forgetting to include the message after echo.

The command docker run --rm alpine echo Hello, World! runs a container that prints the message. Quotes are not needed here.

2fill in blank

medium

Complete the command to create a file inside a container.

Docker

docker run --rm alpine sh -c "echo 'data' > [1]"

Drag options to blanks, or click blank then click option'

A/data/file.txt

B/tmp/file.txt

C/var/file.txt

D/file.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a directory that might not exist or be writable inside the container.

Using root directory which is not recommended for temporary files.

The /tmp directory is commonly used for temporary files inside containers. This file will be created there.

3fill in blank

hard

Fix the error in the command to keep a file after the container stops.

Docker

docker run --rm -v [1]:/data alpine sh -c "echo 'save me' > /data/file.txt"

Drag options to blanks, or click blank then click option'

A/var/lib/docker

B/data

C/tmp

D/host/path

Attempts:

3 left

💡 Hint

Common Mistakes

Using a container path on the left side of the volume mount.

Using a system folder that is not intended for mounting.

Mounting a host directory (like /host/path) to /data inside the container saves files outside the ephemeral container filesystem.

4fill in blank

hard

Fill both blanks to create a volume and mount it to persist data.

Docker

docker volume [1] mydata && docker run -v mydata:[2] alpine sh -c "echo 'persist' > /data/file.txt"

Drag options to blanks, or click blank then click option'

Acreate

Bdata

C/data

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' instead of 'create' for the volume command.

Mounting the volume to a wrong or non-existent container path.

First, create a volume with docker volume create mydata. Then mount it inside the container at /data to persist files.

5fill in blank

hard

Fill all three blanks to explain ephemeral filesystem behavior.

Docker

When a container stops, all files in [1] are lost unless [2] or [3] are used.

Drag options to blanks, or click blank then click option'

Athe container's writable layer

Ba volume

Ca bind mount

Dthe image

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the image with the writable layer.

Not knowing the difference between volumes and bind mounts.

The container's writable layer is ephemeral. To keep data, use volumes or bind mounts which store data outside the container.

Practice

1. What happens to files created inside a Docker container when the container is stopped and restarted?

easy

A. The files are deleted because the container filesystem is ephemeral.

B. The files are saved permanently inside the container.

C. The files are automatically copied to the host machine.

D. The files are backed up to Docker Hub.

Solution

1. Step 1: Understand container filesystem behavior

   Docker containers have an ephemeral filesystem, meaning changes inside the container do not persist after restart.

2. Step 2: Apply this to file creation

   Files created inside the container are lost when the container stops or restarts unless saved externally.

3. Final Answer:

   The files are deleted because the container filesystem is ephemeral. -> Option A

4. Quick Check:

   Container filesystem ephemeral = files lost on restart [OK]

Hint: Remember: container files reset on restart unless saved outside [OK]

Common Mistakes:

• Thinking files inside container persist after restart
• Assuming Docker automatically saves files externally
• Confusing container filesystem with host filesystem

2. Which Docker command correctly mounts a host directory /data to a container directory /app/data to persist data?

easy

A. docker run --env /data:/app/data myimage

B. docker run -p /data:/app/data myimage

C. docker run --link /data:/app/data myimage

D. docker run -v /data:/app/data myimage

Solution

1. Step 1: Identify volume mount syntax

   The correct syntax to mount a host directory is using -v host_path:container_path.

2. Step 2: Match the correct option

   docker run -v /data:/app/data myimage uses -v /data:/app/data, which is the correct way to mount the host directory.

3. Final Answer:

   docker run -v /data:/app/data myimage -> Option D

4. Quick Check:

   Volume mount uses -v host:container [OK]

Hint: Use -v host_path:container_path to mount volumes [OK]

Common Mistakes:

• Using -p instead of -v for volumes
• Confusing --link or --env with volume mounts
• Forgetting to specify both host and container paths

3. Consider this Dockerfile snippet:

FROM alpine
RUN echo "Hello" > /greeting.txt
CMD cat /greeting.txt

If you run this container, then modify /greeting.txt inside the running container to "Hi", then restart the container, what will cat /greeting.txt output?

medium

A. Hi

B. Hello

C. File not found error

D. Empty output

Solution

1. Step 1: Understand Dockerfile build vs container runtime

   The file /greeting.txt is created during image build with content "Hello".

2. Step 2: Recognize ephemeral container filesystem

   Changes made inside a running container (changing to "Hi") are lost after restart because the container filesystem resets.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Container restart resets filesystem, original file restored [OK]

Hint: Changes inside container lost on restart; image files stay [OK]

Common Mistakes:

• Assuming changes inside container persist after restart
• Confusing image build files with runtime changes
• Expecting file to be missing after restart

4. You ran a container and created a file inside it. After restarting, the file disappeared. You want to keep the file persistent. Which of these fixes the problem?

medium

A. Restart the container with --restart=always flag.

B. Increase container memory limit.

C. Use a Docker volume to store the file outside the container.

D. Run the container with --rm option.

Solution

1. Step 1: Identify cause of file loss

   Files inside container are lost on restart because filesystem is ephemeral.

2. Step 2: Choose method to persist data

   Using a Docker volume stores data outside container, preserving files across restarts.

3. Final Answer:

   Use a Docker volume to store the file outside the container. -> Option C

4. Quick Check:

   Volumes keep data persistent beyond container life [OK]

Hint: Persist files with volumes, not restart flags or memory [OK]

Common Mistakes:

• Thinking restart flags keep files
• Confusing memory limits with filesystem persistence
• Using --rm removes container on exit, losing files

5. You want to run a container that writes logs to /var/log/app.log inside the container. You want these logs to persist even if the container is removed or recreated. Which Docker run command achieves this?

hard

A. docker run -v /host/logs/app.log:/var/log/app.log myapp

B. docker run --rm myapp

C. docker run -v /host/logs:/var/log/app.log myapp

D. docker run -v /host/logs/app.log:/var/log myapp

Solution

1. Step 1: Understand volume mount for specific file

   To persist a single log file, mount the host file to the container file path.

2. Step 2: Analyze options

   docker run -v /host/logs/app.log:/var/log/app.log myapp mounts /host/logs/app.log to /var/log/app.log inside container, persisting logs exactly.

3. Step 3: Why other options fail

   docker run -v /host/logs/app.log:/var/log myapp tries to mount file to directory path (invalid). docker run -v /host/logs:/var/log/app.log myapp tries to mount directory to file path (invalid). docker run --rm myapp removes container on exit, losing logs.

4. Final Answer:

   docker run -v /host/logs/app.log:/var/log/app.log myapp -> Option A

5. Quick Check:

   Mount host file to container file for persistent logs [OK]

Hint: Mount host file to container file path to persist logs [OK]

Common Mistakes:

• Mounting directory to file path (invalid)
• Using --rm which deletes container and files
• Mounting directory when only file persistence needed