Editing files in Docker containers: practical approaches

The problem

Docker containers are designed to be immutable, following the principle that any changes should be made to the Dockerfile and then rebuilt. However, during development or debugging, you might need to quickly edit files inside a running container. This becomes challenging when working with slim Docker images that don't include text editors like Vim or nano.

Why it happens

Modern containerisation practices emphasise minimal images to reduce attack surface, improve security, and decrease overall size. As a result, many popular Docker images are built using slim or alpine variants that exclude common utilities, including text editors. When you need to modify files in these environments, you'll often find yourself without the familiar tools.

Solution approaches

Approach 1: Copy, edit locally, and return

A straightforward approach is to use Docker's file copy capabilities to extract the file, edit it locally, and copy it back into the container:

# Copy the file from container to host
docker cp container_id:/path/to/file/in/container ./local_filename

# Edit the file locally using your preferred editor
vim local_filename

# Copy the modified file back to the container
docker cp local_filename container_id:/path/to/file/in/container

Note

This technique works well for quick changes but remember it doesn't persist after the container is removed. For permanent changes, you should update your Dockerfile or mount volumes.

Approach 2: Install an editor inside the container

For more extensive editing sessions, you might prefer installing an editor directly in the container:

# For Debian/Ubuntu-based containers
docker exec container_id apt-get update && apt-get install -y vim

# For Alpine-based containers
docker exec container_id apk add --no-cache vim

Warning

Installing packages in a running container is not recommended for production environments. This approach should be limited to development and debugging scenarios.

Approach 3: Use volume mounts

For ongoing development, the most efficient approach is to mount local directories as volumes:

docker run -v $(pwd)/local_dir:/container_dir image_name

This creates a shared directory between your host and the container, allowing you to edit files using your local tools while the container sees the changes in real-time.

Best practices

While editing files directly in containers can be necessary during development, consider these best practices:

Document any changes made directly to containers
Transfer important changes to your Dockerfile or infrastructure as code
Use volume mounts for development workflows
Consider using development-specific images with additional tools
For configuration files, leverage environment variables or config management

Conclusion

While Docker's philosophy encourages immutable infrastructure, practical development often requires flexible approaches. The techniques outlined above provide pragmatic solutions when you need to modify files in containers lacking editing tools.

For more permanent solutions, consider implementing proper volume mounting strategies or creating development-specific Docker images that include your preferred editing tools.