Here’s a complete tutorial on docker commit, covering what it does, examples, and use cases.

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What is docker commit?

docker commit is a Docker command used to create a new image from an existing container’s state. This allows you to save the current state of a container, including its file system and configuration, as a reusable image.

Key Features:

• Creates a new Docker image from a modified or running container.
• Useful for snapshotting the container state.
• Allows for quick prototyping or manual image creation without a Dockerfile.
• Supports adding custom metadata (author, message).

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Basic Syntax

docker commit [OPTIONS] CONTAINER IMAGE[:TAG]

Common Options:

• -a, --author: Set the image author.
• -m, --message: Add a commit message.
• -p, --pause: Pause the container during commit (default: true).

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Examples of docker commit

1. Create an Image from a Running Container

docker commit my_container my_new_image:latest

This creates a new image named my_new_image:latest from my_container.

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2. Add an Author and Message

docker commit -a "John Doe" -m "Updated with new configurations" my_container my_image:v1

This sets John Doe as the image author and adds a commit message.

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3. Create an Image Without Pausing the Container

docker commit --pause=false my_container my_image:latest

This commits the container without pausing it, which is useful for real-time services.

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4. Create an Image from a Stopped Container

docker commit stopped_container my_backup_image

You can commit a stopped container to create a backup image.

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5. Use the Container ID Instead of the Name

docker commit a1b2c3d4e5f6 my_custom_image:dev

This commits the container with ID a1b2c3d4e5f6 to my_custom_image:dev.

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6. Automate Commit with a Script

#!/bin/bash
docker commit -a "DevOps Team" -m "Nightly snapshot" my_app nightly_snapshot:$(date +%Y%m%d)

This script creates a snapshot image with the current date as the tag.

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7. Create a Base Image with Pre-Installed Dependencies

If you have a container with installed software or dependencies:

docker commit my_python_container python_base:latest

This saves the container state as a base image for future Python projects.

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8. Export Configuration Changes

After changing configuration files inside a running container:

docker commit my_nginx_container nginx_custom:1.0

This creates an image with your customized NGINX configuration.

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Use Cases for docker commit

1. Snapshotting and Backups

• Create images from containers to snapshot the current state for backup purposes.
• Example: Backup a database container with all its data.

2. Quick Prototyping and Manual Changes

• Save an image after manual configuration or software installation in a container.
• Example: Install a library in a running Python container, then commit it for future use.

3. Debugging and Hotfixes

• Modify a running container to apply a quick fix and commit it as a temporary image.
• Example: Fix a misconfiguration in a running NGINX container and save the new state.

4. Custom Base Images

• Create custom base images with pre-installed dependencies or configurations.
• Example: Build a Python base image with specific libraries installed.

5. Preserving Legacy Containers

• Preserve old containers by committing them as images for future use.

6. CI/CD Snapshots

• Use docker commit to snapshot application state during continuous integration and testing.

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List of Common docker commit Commands

Command	Description
docker commit my_container my_image:latest	Create an image from a container
docker commit -a "John Doe" -m "Update configs"	Add author and commit message to the image
docker commit --pause=false my_container my_image	Commit without pausing the container
docker commit stopped_container my_backup	Create an image from a stopped container
docker commit a1b2c3d4e5f6 my_custom_image	Commit using container ID
docker commit my_app nightly_snapshot:20230207	Automate commit with a timestamp in the tag
docker commit my_nginx_container nginx_custom	Save a container with custom configurations

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Best Practices for Using docker commit:

1. Avoid using docker commit in production—prefer Dockerfiles for reproducibility and version control.
2. Add descriptive tags and messages to track changes.
3. Pause the container during commit for consistent snapshots (--pause=true).
4. Use docker commit for debugging or quick experiments, then convert to a Dockerfile for long-term use.
5. Combine with docker save to create portable images for backup or transfer.

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Common Errors and Solutions

1. “No such container”
   → Ensure the container exists and is running/stopped. Use docker ps -a to verify.
2. “Image is too large”
   → Remove unnecessary files or use a multi-stage build to reduce the image size.
3. Changes not reflected in the new image
   → Ensure you’re modifying the correct container and committing it after the changes.
4. “Permission denied”
   → Run the command with elevated privileges (sudo) or ensure your user has Docker permissions.

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Combining docker commit with Other Commands

Backup and Transfer the Committed Image

docker commit my_container my_backup_image
docker save my_backup_image > my_backup_image.tar

Restore the Image on Another Machine

docker load < my_backup_image.tar

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