System prune for cleanup in Docker - Step-by-Step Execution

Process Flow - System prune for cleanup

Start: Docker system prune command

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Check for unused containers

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Check for unused images

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Check for unused networks

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Check for unused build cache

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Remove all unused data

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Display freed space

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End

The system prune command checks for unused Docker data like containers, images, networks, and cache, then removes them to free space.

Execution Sample

Docker

docker system prune -f

This command removes all unused containers, images, networks, and build cache without asking for confirmation.

Process Table

Step	Action	Evaluation	Result
1	Run 'docker system prune -f'	Identify unused containers	List of stopped containers found
2	Remove unused containers	Containers removed	Space freed from containers
3	Identify unused images	Dangling and unused images found	List of images to remove
4	Remove unused images	Images deleted	Space freed from images
5	Identify unused networks	Networks not used by any container found	List of networks to remove
6	Remove unused networks	Networks deleted	Space freed from networks
7	Identify build cache	Build cache found	Cache to remove
8	Remove build cache	Cache deleted	Space freed from cache
9	Display total space freed	Summary shown	User sees total disk space reclaimed
10	End	No more unused data	Cleanup complete

💡 All unused containers, images, networks, and build cache removed; system cleaned.

Status Tracker

Resource Type	Start	After Prune
Containers (stopped)	3 stopped containers	0 stopped containers
Images (unused)	5 dangling images	0 dangling images
Networks (unused)	2 unused networks	0 unused networks
Build Cache	Some cache data	No cache data

Key Moments - 3 Insights

Why does the command remove stopped containers but not running ones?

The prune command only targets unused resources. Running containers are in use, so they are not removed. See execution_table rows 1 and 2 where only stopped containers are identified and removed.

What does the '-f' flag do in 'docker system prune -f'?

The '-f' flag forces the prune without asking for confirmation. This is why the command proceeds directly to removal steps in the execution_table without pause.

Does 'docker system prune' remove volumes?

By default, volumes are not removed. The command targets containers, images, networks, and build cache only. This is why volumes are not listed in the variable_tracker or execution_table.

Visual Quiz - 3 Questions

Look at the execution table, at which step are unused images removed?

A Step 5

B Step 3

C Step 4

D Step 2

💡 Hint

Check the 'Action' column for image removal in the execution_table.

According to the variable tracker, how many stopped containers remain after prune?

A 3

B 0

C 1

D 5

💡 Hint

Look at the 'Containers (stopped)' row under 'After Prune' in variable_tracker.

If the '-f' flag is removed, what changes in the execution flow?

A The command will ask for confirmation before removal

B It will remove volumes as well

C It will skip removing networks

D It will remove running containers

💡 Hint

Recall the explanation about the '-f' flag in key_moments and how it affects confirmation.

Concept Snapshot

docker system prune -f
Removes all unused containers, images, networks, and build cache
Does not remove volumes by default
-f skips confirmation prompt
Frees disk space by cleaning unused Docker data

Full Transcript

The 'docker system prune -f' command cleans up unused Docker resources. It first finds stopped containers, unused images, networks, and build cache. Then it removes them all without asking for confirmation because of the '-f' flag. Running containers and volumes are not removed. After completion, it shows how much disk space was freed. This helps keep your Docker environment clean and saves storage space.

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