Removing images in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command is used to remove a Docker image by its image ID or name?

Use docker rmi <image_id_or_name> to remove a Docker image.

beginner

What happens if you try to remove a Docker image that is currently used by a running container?

Docker will not allow removing an image if it is used by a running container. You must stop or remove the container first.

intermediate

How can you force remove a Docker image even if it is used by stopped containers?

Use docker rmi -f <image_id_or_name> to force remove the image even if it is used by stopped containers.

intermediate

How do you remove all unused Docker images (dangling images) at once?

Use docker image prune to remove all dangling images that are not tagged or used by containers.

beginner

What is the difference between docker rmi and docker image rm?

Both commands do the same thing: remove Docker images. docker rmi is a shortcut for docker image rm.

Which command removes a Docker image by its name or ID?

A docker rmi <image>

B docker rm <image>

C docker stop <image>

D docker run <image>

docker rmi is used to remove images. docker rm removes containers, not images.

What must you do before removing an image used by a running container?

A Stop or remove the container first

B Restart the container

C Nothing, just remove the image

D Rename the image

You must stop or remove containers using the image before removing it.

How do you force remove a Docker image even if it is used by stopped containers?

A docker image prune

B docker rm -f <image>

C docker rmi -f <image>

D docker stop -f <image>

docker rmi -f forces removal of the image even if it is used by stopped containers.

Which command removes all dangling (unused) Docker images?

A docker rmi all

B docker image prune

C docker rm dangling

D docker image rm -a

docker image prune removes all dangling images that are not tagged or used.

What is the shortcut command to remove Docker images?

A docker remove

B docker rm

C docker image rm

D docker rmi

docker rmi is a shortcut for docker image rm.

Explain how to safely remove a Docker image that is currently used by containers.

Think about container dependencies before deleting images.

Describe the difference between removing images normally and forcing their removal in Docker.

Consider what happens when containers depend on the image.

Practice

1. What is the basic Docker command to remove an image from your system?

easy

A. docker delete <image_name_or_id>

B. docker rmi <image_name_or_id>

C. docker remove image <image_name_or_id>

D. docker rm <image_name_or_id>

Solution

1. Step 1: Understand Docker image removal command

   The correct command to remove Docker images is docker rmi. It stands for 'remove image'.

2. Step 2: Identify the correct syntax

   The command requires the image name or ID after docker rmi to specify which image to remove.

3. Final Answer:

   docker rmi <image_name_or_id> -> Option B

4. Quick Check:

   Remove image = docker rmi [OK]

Hint: Use 'docker rmi' to remove images quickly [OK]

Common Mistakes:

• Using 'docker rm' which removes containers, not images
• Typing 'docker delete' which is not a valid Docker command
• Confusing image removal with container removal commands

2. Which of the following is the correct syntax to force remove a Docker image named myapp:latest?

easy

A. docker rmi -f myapp:latest

B. docker rmi -force myapp:latest

C. docker rmi -rf myapp:latest

D. docker rmi --f myapp:latest

Solution

1. Step 1: Identify the correct flag for force removal

   The short flag -f is used to force remove images in Docker.

2. Step 2: Confirm the correct command syntax

   The correct command is docker rmi -f myapp:latest. Other options use invalid flags like -rf, -force, or --f.

3. Final Answer:

   docker rmi -f myapp:latest -> Option A

4. Quick Check:

   Force remove = docker rmi -f [OK]

Hint: Use '-f' to force remove images safely [OK]

Common Mistakes:

• Using incorrect flag like '-rf', '-force' or '--f'
• Confusing force flag with other options
• Misspelling the flag causing command failure

3. What will be the output of the command docker rmi 123abc456def if the image ID 123abc456def exists and is not used by any container?

medium

A. Image removed but container still running

B. Error: image is being used by a running container

C. No such image: 123abc456def

D. Untagged: 123abc456def
Deleted: sha256:123abc456def...

Solution

1. Step 1: Check if image exists and is unused

   If the image ID exists and no container uses it, Docker removes it cleanly.

2. Step 2: Understand Docker rmi output

   The command outputs lines showing the image is untagged and deleted with its sha256 hash.

3. Final Answer:

   Untagged: 123abc456def Deleted: sha256:123abc456def... -> Option D

4. Quick Check:

   Image removed cleanly = Untagged: 123abc456def Deleted: sha256:123abc456def... [OK]

Hint: If image unused, 'docker rmi' shows untagged and deleted [OK]

Common Mistakes:

• Assuming image removal works if container uses it
• Expecting no output on successful removal
• Confusing image ID with container ID

4. You run docker rmi myimage but get the error: Error response from daemon: conflict: unable to delete image (must be forced) - image is being used by running container. What should you do to fix this?

medium

A. Use docker rm myimage to remove the image

B. Run docker rmi myimage again without changes

C. Stop the running container using the image, then run docker rmi myimage again

D. Restart Docker daemon and try again

Solution

1. Step 1: Understand the error meaning

   The error says the image is used by a running container, so Docker won't remove it without force.

2. Step 2: Stop the container using the image

   You must stop or remove the container using the image before removing the image itself.

3. Final Answer:

   Stop the running container using the image, then run docker rmi myimage again -> Option C

4. Quick Check:

   Stop container before removing image [OK]

Hint: Stop containers using image before removal [OK]

Common Mistakes:

• Trying to remove image without stopping containers
• Using 'docker rm' which removes containers, not images
• Restarting Docker daemon unnecessarily

5. You want to remove all Docker images that are not used by any container to free up space. Which command will safely do this?

hard

A. docker image prune -a

B. docker rmi $(docker images -q)

C. docker rmi -f $(docker ps -aq)

D. docker container prune

Solution

1. Step 1: Understand the goal

   You want to remove all unused images, but keep images used by containers.

2. Step 2: Identify the correct command

   docker image prune -a removes all unused images safely without affecting running containers.

3. Step 3: Analyze other options

   docker rmi $(docker images -q) tries to remove all images, including used ones, causing errors.
   docker rmi -f $(docker ps -aq) is invalid because docker ps -aq lists containers, not images.
   docker container prune removes stopped containers, not images.

4. Final Answer:

   docker image prune -a -> Option A

5. Quick Check:

   Remove unused images safely = docker image prune -a [OK]

Hint: Use 'docker image prune -a' to clean unused images [OK]

Common Mistakes:

• Trying to remove all images forcibly causing errors
• Confusing container prune with image prune
• Using container IDs instead of image IDs in commands