Docker Prune

Introduction

When working with Docker, you'll eventually accumulate unused containers, images, volumes, and networks. These unused resources consume disk space and can clutter your Docker environment. Docker provides a set of prune commands to help you clean up these resources efficiently.

In this guide, we'll explore how to use Docker's pruning commands to maintain a clean and efficient Docker environment. Whether you're a Docker beginner or looking to optimize your workflow, understanding these commands will help you manage your Docker resources more effectively.

Understanding Docker Resources

Before diving into pruning, let's briefly understand the different Docker resources that can be pruned:

1. Containers: Running or stopped instances of Docker images
2. Images: Read-only templates used to create containers
3. Volumes: Persistent data storage used by containers
4. Networks: Communication channels between containers
5. Build Cache: Intermediate layers used during image building

Over time, these resources can accumulate, especially in development environments where you frequently create and destroy containers or build new images.

Docker Prune Commands

Docker provides several commands to clean up unused resources:

1. Pruning Containers

To remove all stopped containers, use:

bash

docker container prune

Example output:

WARNING! This will remove all stopped containers.
Are you sure you want to continue? [y/N] y
Deleted Containers:
container1
container2
container3

Total reclaimed space: 256MB

2. Pruning Images

To remove all dangling images (unused images without tags), use:

bash

docker image prune

For a more aggressive cleanup that removes all unused images (not just dangling ones), add the -a or --all flag:

bash

docker image prune -a

Example output:

WARNING! This will remove all dangling images.
Are you sure you want to continue? [y/N] y
Deleted Images:
image1
image2
image3

Total reclaimed space: 1.2GB

3. Pruning Volumes

To remove all unused volumes (not used by any containers), use:

bash

docker volume prune

Example output:

WARNING! This will remove all volumes not used by at least one container.
Are you sure you want to continue? [y/N] y
Deleted Volumes:
volume1
volume2
volume3

Total reclaimed space: 2.1GB

4. Pruning Networks

To remove all unused networks, use:

bash

docker network prune

Example output:

WARNING! This will remove all networks not used by at least one container.
Are you sure you want to continue? [y/N] y
Deleted Networks:
network1
network2

5. Pruning Build Cache

To remove unused build cache, use:

bash

docker builder prune

Example output:

WARNING! This will remove all build cache.
Are you sure you want to continue? [y/N] y
Deleted build cache objects:
buildcache1
buildcache2

Total reclaimed space: 1.5GB

System-wide Prune

If you want to clean up everything at once, Docker provides a convenient system-wide prune command:

bash

docker system prune

This command removes:

• All stopped containers
• All networks not used by at least one container
• All dangling images
• All dangling build cache

For an even more thorough cleanup that includes unused volumes, add the --volumes flag:

bash

docker system prune --volumes

To remove all unused images (not just dangling ones), use:

bash

docker system prune -a

Example output:

WARNING! This will remove:
  - all stopped containers
  - all networks not used by at least one container
  - all dangling images
  - all dangling build cache

Are you sure you want to continue? [y/N] y
Deleted Containers:
container1
container2

Deleted Networks:
network1

Deleted Images:
image1
image2

Deleted build cache objects:
buildcache1
buildcache2

Total reclaimed space: 5.6GB

Filtering Prune Operations

You can use filters to be more selective about what gets pruned. The most common filter is until, which allows you to prune resources created before a certain time.

For example, to prune containers created more than 24 hours ago:

bash

docker container prune --filter "until=24h"

Similarly, for images:

bash

docker image prune --filter "until=24h"

Real-world Applications

Automated Cleanup Script

Here's a simple shell script you might use in a production environment to perform regular cleanups:

bash

#!/bin/bash
# Docker cleanup script

echo "Starting Docker cleanup..."

# Remove stopped containers older than 24 hours
echo "Removing old stopped containers..."
docker container prune -f --filter "until=24h"

# Remove dangling images
echo "Removing dangling images..."
docker image prune -f

# Remove unused networks
echo "Removing unused networks..."
docker network prune -f

echo "Docker cleanup completed!"

CI/CD Pipeline Cleanup

In continuous integration environments, you might want to add cleanup steps to your pipeline:

yaml

stages:
  - build
  - test
  - deploy
  - cleanup

cleanup_job:
  stage: cleanup
  script:
    - docker system prune -f
  when: always

Development Environment Optimization

For a development machine that needs periodic but thorough cleaning:

bash

# Deep clean (use with caution!)
docker system prune -a --volumes

Best Practices

1. Regular Maintenance: Set up scheduled pruning to prevent resource buildup
2. Use Filters: Be precise about what you want to remove
3. Be Cautious with Volumes: Always double-check before pruning volumes, as they contain data
4. Understand the Flags: Know the difference between -a and standard prune commands
5. Check Space Before and After: Use docker system df to see the impact of your pruning

Visualizing Docker Disk Usage

Before deciding what to prune, it's often helpful to understand what's using space. The docker system df command provides this information:

bash

docker system df

Example output:

TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              25                  10                  3.5GB               2.1GB (60%)
Containers          10                  5                   156MB               76MB (48%)
Local Volumes       15                  12                  2.2GB               500MB (22%)
Build Cache         125                 0                   1.4GB               1.4GB (100%)

For more detailed information:

bash

docker system df -v

Docker Prune Workflow Diagram

Summary

Docker prune commands are essential tools for maintaining a clean and efficient Docker environment. By regularly removing unused resources, you can:

• Free up disk space
• Improve Docker's performance
• Keep your environment organized
• Reduce clutter and confusion

Remember to use these commands with care, especially when dealing with volumes that may contain important data. It's always a good practice to verify what will be removed before confirming a prune operation.

Additional Resources

• Read the official Docker documentation for more details
• Learn about Docker storage drivers to understand how Docker manages disk space
• Explore Docker volume management for more advanced data persistence strategies

Exercises

1. Check your current Docker disk usage using docker system df.
2. Create a few test containers, stop them, and then prune only the containers.
3. Build several versions of a simple Docker image, then use image prune to clean up.
4. Write a script that performs a scheduled cleanup of your Docker environment.
5. Try using different filters with prune commands to selectively remove resources.