System prune for cleanup in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of the command docker system prune -f?

You run docker system prune -f on a system with unused containers, networks, images, and build cache. What will this command do?

Docker

docker system prune -f

A Only shows what would be removed without actually deleting anything.

B Removes only stopped containers and unused networks, but keeps images and build cache.

C Removes all containers, including running ones, and all images.

D Removes all stopped containers, unused networks, dangling images, and build cache without asking for confirmation.

💡 Hint

The -f flag forces the prune without confirmation.

The docker system prune -f command cleans up all unused data including stopped containers, unused networks, dangling images, and build cache without asking for confirmation.

✅ Best Practice

intermediate

Which option is the safest way to clean up unused Docker resources without affecting running containers?

You want to free disk space by removing unused Docker resources but must not stop or remove any running containers. Which command should you use?

A docker system prune -f

B docker system prune -a -f

C docker container prune -f

D docker image prune -a -f

💡 Hint

Consider what resources each prune command affects.

docker system prune -f removes stopped containers, unused networks, dangling images, and build cache but does not affect running containers.

❓ Troubleshoot

advanced

Why does docker system prune not remove some unused images?

You ran docker system prune -f but noticed some unused images remain. What is the most likely reason?

A The command requires the <code>--all</code> flag to remove any images.

B The images are corrupted and cannot be removed.

C The images are tagged and not dangling, so they are not removed by default.

D Docker daemon is not running, so prune has no effect.

💡 Hint

Think about what 'dangling' means for images.

By default, docker system prune removes only dangling images (untagged). Tagged images are kept unless --all is specified.

🔀 Workflow

advanced

What is the correct sequence to safely clean up all unused Docker images, including untagged and tagged, without affecting running containers?

Arrange the following commands in the correct order to clean up all unused Docker images and other unused resources safely.

A 1,3,2,4

B 4,1,3,2

C 1,2,3,4

D 3,1,4,2

💡 Hint

Start by pruning containers and networks before images and system prune.

First remove stopped containers, then unused networks, then all unused images including tagged ones, and finally run system prune to clean build cache and any leftovers.

🧠 Conceptual

expert

What is the effect of running docker system prune --volumes -f?

You execute docker system prune --volumes -f. What additional cleanup does this command perform compared to docker system prune -f?

A It removes only volumes and leaves containers, networks, and images untouched.

B It removes all unused volumes in addition to stopped containers, unused networks, dangling images, and build cache.

C It removes volumes but requires confirmation even with <code>-f</code> flag.

D It removes volumes only if they are attached to running containers.

💡 Hint

Volumes are not removed by default with system prune.

The --volumes flag adds removal of all unused volumes to the standard system prune cleanup.

Practice

1. What does the Docker command docker system prune do?

easy

A. Stops all running containers immediately

B. Removes all unused containers, networks, images, and optionally volumes to free disk space

C. Updates Docker to the latest version

D. Creates a backup of all Docker images

Solution

1. Step 1: Understand the purpose of system prune

   The docker system prune command is designed to clean up unused Docker data like stopped containers, unused networks, dangling images, and optionally volumes.

2. Step 2: Identify what the command removes

   It removes unused resources to free disk space but does not stop running containers or update Docker.

3. Final Answer:

   Removes all unused containers, networks, images, and optionally volumes to free disk space -> Option B

4. Quick Check:

   System prune cleans unused Docker data = A [OK]

Hint: System prune cleans unused Docker data to free space [OK]

Common Mistakes:

• Thinking it stops running containers
• Confusing it with Docker update commands
• Assuming it creates backups

2. Which of the following is the correct syntax to remove all unused images, containers, networks, and volumes with Docker system prune?

easy

A. docker system prune --all --volume

B. docker prune system -a --volumes

C. docker system prune -a --volumes

D. docker system prune -all -volumes

Solution

1. Step 1: Recall correct Docker system prune syntax

   The correct command to remove all unused images, containers, networks, and volumes is docker system prune -a --volumes. The -a flag removes all unused images, and --volumes removes unused volumes.

2. Step 2: Identify syntax errors in other options

   Options A, B, and D have incorrect command order or wrong flags like prune system or misspelled flags.

3. Final Answer:

   docker system prune -a --volumes -> Option C

4. Quick Check:

   Correct flags and order = C [OK]

Hint: Use '-a' and '--volumes' flags with 'docker system prune' [OK]

Common Mistakes:

• Swapping 'system' and 'prune' keywords
• Using incorrect flag names like '--volume'
• Omitting the dash in flags

3. What will be the output after running docker system prune -a --volumes if you have 2 stopped containers, 3 unused images, 1 unused volume, and 1 unused network?

medium

A. No resources removed because containers are stopped

B. Removed 2 containers and 3 images only

C. Removed only unused volumes

D. Removed 2 containers, 3 images, 1 volume, and 1 network

Solution

1. Step 1: Understand what the command removes

   The command docker system prune -a --volumes removes all unused containers (including stopped), all unused images, unused volumes, and unused networks.

2. Step 2: Apply to given resources

   Given 2 stopped containers, 3 unused images, 1 unused volume, and 1 unused network, all will be removed by this command.

3. Final Answer:

   Removed 2 containers, 3 images, 1 volume, and 1 network -> Option D

4. Quick Check:

   All unused resources removed = A [OK]

Hint: System prune with '-a' and '--volumes' removes all unused resources [OK]

Common Mistakes:

• Thinking stopped containers are not removed
• Assuming volumes are not removed without extra flag
• Confusing networks with containers

4. You ran docker system prune -a --volumes but received an error: unknown flag: --volumes. What is the likely cause?

medium

A. Docker version is outdated and does not support the --volumes flag

B. You need to run the command as root user

C. The flag should be --volume without 's'

D. You must stop all containers before pruning

Solution

1. Step 1: Analyze the error message

   The error unknown flag: --volumes indicates Docker does not recognize the flag, likely due to an older Docker version.

2. Step 2: Check other options

   Running as root is not required for this flag error. The correct flag is --volumes (with 's'), and stopping containers is not mandatory for the flag to work.

3. Final Answer:

   Docker version is outdated and does not support the --volumes flag -> Option A

4. Quick Check:

   Flag error means outdated Docker = D [OK]

Hint: Update Docker if flags are unrecognized [OK]

Common Mistakes:

• Mistaking flag spelling as the cause
• Assuming root user is required
• Thinking containers must be stopped first

5. You want to free up disk space but keep volumes because they contain important data. Which command should you run?

hard

A. docker system prune

B. docker system prune -a --volumes

C. docker system prune --volumes

D. docker system prune -all -volumes

Solution

1. Step 1: Understand volume preservation

   Volumes store important data and are not removed by default with docker system prune. The --volumes flag explicitly removes unused volumes.

2. Step 2: Choose command that keeps volumes

   Running docker system prune without -a or --volumes removes stopped containers, unused networks, and dangling images but keeps volumes and unused images.

3. Final Answer:

   docker system prune -> Option A

4. Quick Check:

   Default prune keeps volumes = B [OK]

Hint: Avoid '--volumes' to keep volumes safe [OK]

Common Mistakes:

• Using '-a' removes all unused images
• Adding '--volumes' deletes volumes
• Assuming default prune removes volumes