Removing containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of removing a running container

What will be the output or result of running this command if the container myapp is currently running?

docker rm myapp

Docker

docker rm myapp

A Container myapp stopped and removed automatically.

B Container myapp removed successfully.

C Error: You cannot remove a running container. Stop it first.

D Warning: Container myapp is running but removed forcefully.

💡 Hint

Think about whether Docker allows removing containers that are running without extra flags.

Docker does not allow removing a running container with 'docker rm' unless you use the '-f' (force) option. Without it, you get an error telling you to stop the container first.

💻 Command Output

intermediate

Effect of force removing a container

What happens when you run this command?

docker rm -f myapp

Docker

docker rm -f myapp

A The container myapp is stopped if running and then removed.

B The container myapp is removed only if it is stopped.

C The command fails if the container is running.

D The container myapp is paused but not removed.

💡 Hint

The '-f' flag forces removal even if the container is running.

The '-f' or '--force' option stops the container if it is running and then removes it.

❓ Configuration

advanced

Removing all stopped containers

Which command will remove all stopped containers safely without affecting running ones?

A docker rm -a

B docker rm $(docker ps -a -q)

C docker rm -f $(docker ps -q)

D docker container prune

💡 Hint

Look for a command that cleans up stopped containers only.

The command 'docker container prune' removes all stopped containers safely. Option D tries to remove all containers including running ones, causing errors. Option D forcibly removes running containers which is unsafe. Option D is invalid syntax.

❓ Troubleshoot

advanced

Error when removing container by name

You run docker rm myapp but get the error:

Error: No such container: myapp

What is the most likely cause?

A Docker daemon is not running.

B The container named 'myapp' does not exist or is already removed.

C The container is running and cannot be removed.

D You do not have permission to remove containers.

💡 Hint

Check if the container name is correct and exists.

The error means Docker cannot find a container with the name 'myapp'. It may have been removed or the name is misspelled.

✅ Best Practice

expert

Safe workflow to remove containers and free disk space

Which sequence of commands is the safest and most effective to remove all stopped containers and free disk space without affecting running containers or images?

A 1,2,3,4

B 2,1,4,3

C 4,3,2,1

D 1,3,4,2

💡 Hint

Start by removing stopped containers, then images, volumes, and networks.

The best practice is to first remove stopped containers, then unused images, volumes, and networks in that order to safely free space without affecting running containers.

Practice

1. What does the docker rm command do?

easy

A. Lists all containers

B. Stops running containers

C. Deletes one or more stopped containers

D. Creates a new container

Solution

1. Step 1: Understand the purpose of docker rm

   The docker rm command is used to remove containers from the system.

2. Step 2: Check container state requirement

   Containers must be stopped before removal unless forced. So, docker rm deletes stopped containers.

3. Final Answer:

   Deletes one or more stopped containers -> Option C

4. Quick Check:

   docker rm = Delete stopped containers [OK]

Hint: Remember: rm means remove stopped containers only [OK]

Common Mistakes:

• Thinking it stops containers
• Confusing with container creation
• Assuming it lists containers

2. Which of the following is the correct syntax to force remove a running container named webapp?

easy

A. docker rm -f webapp

B. docker rm --force webapp

C. docker rm webapp -f

D. docker rm webapp --force

Solution

1. Step 1: Identify the correct flag position

   The force flag -f must come before the container name in the command.

2. Step 2: Verify correct syntax

   The correct syntax is docker rm -f webapp. Other placements cause errors or are invalid.

3. Final Answer:

   docker rm -f webapp -> Option A

4. Quick Check:

   Force flag before name = correct syntax [OK]

Hint: Put flags before container names in docker commands [OK]

Common Mistakes:

• Placing flags after container names
• Using long flag incorrectly
• Omitting the force flag for running containers

3. Given these commands run in order:

docker run -d --name test1 nginx
docker run -d --name test2 nginx
docker stop test1
docker rm test1 test2

What will happen?

medium

A. Only test2 is removed; test1 removal fails

B. Both containers test1 and test2 are removed

C. Neither container is removed

D. Only test1 is removed; test2 removal fails

Solution

1. Step 1: Analyze container states before removal

   test1 is stopped before removal, test2 is still running.

2. Step 2: Understand docker rm behavior

   docker rm removes stopped containers only. It will remove test1 but fail on running test2.

3. Final Answer:

   Only test1 is removed; test2 removal fails -> Option D

4. Quick Check:

   Stopped containers removed; running containers not [OK]

Hint: Only stopped containers get removed without -f flag [OK]

Common Mistakes:

• Assuming running containers get removed without force
• Thinking both containers are stopped
• Ignoring the stop command effect

4. You run docker rm mycontainer but get an error: Error response from daemon: You cannot remove a running container. What is the best fix?

medium

A. Use docker rm -v mycontainer to remove volumes

B. Stop the container first using docker stop mycontainer

C. Restart Docker daemon and try again

D. Rename the container and remove it

Solution

1. Step 1: Understand the error cause

   The error says the container is running and cannot be removed directly.

2. Step 2: Apply correct fix

   Stopping the container first with docker stop mycontainer allows safe removal.

3. Final Answer:

   Stop the container first using docker stop mycontainer -> Option B

4. Quick Check:

   Stop container before removal = correct fix [OK]

Hint: Stop running containers before removing them [OK]

Common Mistakes:

• Trying to remove running container without stopping
• Using volume removal flag incorrectly
• Restarting Docker unnecessarily

5. You want to clean up all stopped containers at once. Which command achieves this safely and efficiently?

hard

A. docker rm $(docker ps -a -q -f status=exited)

B. docker rm -f $(docker ps -q)

C. docker stop $(docker ps -q) && docker rm $(docker ps -q)

D. docker rm $(docker ps -q -f status=running)

Solution

1. Step 1: Identify stopped containers

   The filter -f status=exited lists only stopped containers.

2. Step 2: Remove only stopped containers safely

   Using docker rm $(docker ps -a -q -f status=exited) removes all stopped containers without affecting running ones.

3. Final Answer:

   docker rm $(docker ps -a -q -f status=exited) -> Option A

4. Quick Check:

   Remove stopped containers only = docker rm $(docker ps -a -q -f status=exited) [OK]

Hint: Use filter exited to remove only stopped containers [OK]

Common Mistakes:

• Removing running containers forcibly without need
• Stopping and removing all containers blindly
• Using wrong filters causing errors