Docker commands Guide – docker diff with examples

Here’s a complete tutorial on the docker diff command, explaining how it works, its purpose, and providing a comprehensive list of examples.

What is docker diff?

docker diff is a Docker command used to inspect changes made to the filesystem of a container. It shows the differences between the container’s current state and its original image.

Key Features:

• Lists changes to the container’s filesystem.
• Helps in debugging and auditing what files have been added, modified, or deleted in the container.
• Useful for verifying the impact of commands executed inside a running container.

Basic Syntax

docker diff [OPTIONS] CONTAINER
Code language: CSS (css)

Options:

There are no specific options for docker diff. You simply provide the container name or ID.

Output Explanation

docker diff provides three types of changes:

• A → File or directory was added to the container.
• C → File or directory was changed (modified).
• D → File or directory was deleted from the container.

Example Output:

C /etc/hostname
A /new_file.txt
D /tmp/old_log.txt

• C /etc/hostname → /etc/hostname was modified.
• A /new_file.txt → new_file.txt was added to the container.
• D /tmp/old_log.txt → old_log.txt was deleted from /tmp.

Examples of docker diff

1. Check Changes in a Running Container

docker diff my_container

This command shows all changes to the my_container filesystem.

2. Check Changes After Modifying a File

1. Start a container: docker run -dit --name my_ubuntu ubuntu
2. Modify or create a file in the container: docker exec my_ubuntu bash -c "echo 'Hello Docker' > /tmp/hello.txt"
3. Run docker diff to check the change: docker diff my_ubuntu Output: A /tmp/hello.txt

3. Detect Deleted Files

1. Start a container: docker run -dit --name my_nginx nginx
2. Delete a file inside the container: docker exec my_nginx rm /etc/nginx/conf.d/default.conf
3. Run docker diff: docker diff my_nginx Output: D /etc/nginx/conf.d/default.conf

4. Monitor Changes in a Container’s Temporary Files

Containers often generate temporary files in /tmp or /var during operation. You can use docker diff to track these changes:

docker diff my_temp_container

This helps identify logs or cache files added during runtime.

5. Check Changes in a Stopped Container

You can run docker diff on both running and stopped containers:

docker diff stopped_container

6. Debugging File Changes in a Database Container

If you’re running a MySQL container and suspect that files in /var/lib/mysql have been modified:

docker diff my_mysql

This will list all changes in the /var/lib/mysql directory, helping you verify data persistence or accidental modifications.

7. Combine docker diff with Other Commands

Using docker diff with docker exec

To confirm a change detected by docker diff, use docker exec:

docker exec my_container cat /path/to/file

Using docker diff with docker commit

If you want to save a modified container after detecting changes:

docker commit my_container my_new_image

List of Common docker diff Commands

Best Practices for docker diff:

1. Use docker diff for debugging unexpected behavior in containers.
2. Combine with docker commit to create a new image from a modified container.
3. Regularly monitor changes in critical containers (e.g., databases, web servers).
4. Avoid leaving unnecessary files in containers—docker diff helps track them.

Common Errors and Solutions

1. “Error response from daemon: No such container”
   → Ensure the container name or ID is correct and the container exists.
2. “No changes detected”
   → This means no changes have been made since the container was created.
3. Permission Denied Errors
   → Ensure you have the necessary permissions to inspect the container.

Rajesh Kumar

I’m Rajesh Kumar, a DevOps, SRE, DevSecOps, Cloud, and Platform Engineering expert passionate about sharing practical knowledge, real-world experiences, and industry best practices. I have worked at Cotocus and regularly write about technology, travel, investing, health, product reviews, and digital marketing through my various platforms.

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