Difference Between Running and Starting a Docker Container

TLDR

docker run creates and starts a new container from an image, while docker start restarts an existing, stopped container. Use run for new containers, and start to bring back a container you previously stopped.

What Does docker run Do?

docker run is the main command to create and launch a new container. It does several things at once:

• Creates a new container from the specified image
• Assigns it a unique ID and (optionally) a name
• Sets up networking, storage, and environment variables
• Starts the container's main process

Example:

docker run -d --name my-nginx -p 8080:80 nginx

This creates a new container named my-nginx from the nginx image and starts it in the background.

What Does docker start Do?

docker start is used to restart a container that was previously created (and stopped). It does not create a new container or change its configuration.

Example:

docker stop my-nginx  # Stop the container
# ... do something ...
docker start my-nginx # Start it again

• The container keeps its data, configuration, and name.
• Any changes made to the container's filesystem persist.

Key Differences

• docker run creates a new container every time you use it.
• docker start only works on containers that already exist (but are stopped).
• You can only use docker run with an image; docker start uses a container name or ID.
• docker run lets you set options (ports, env vars, volumes) at creation; docker start does not.

When to Use Each Command

• Use docker run when you want a fresh container, possibly with new options.
• Use docker start to restart a stopped container with the same settings and data.
• For stateless or short-lived containers, docker run is common.
• For persistent services or debugging, docker start is handy.

Best Practices

• Name your containers with --name for easier management.
• Use docker ps -a to see all containers (running and stopped).
• Remove containers you no longer need with docker rm to avoid clutter.

Conclusion

docker run and docker start serve different purposes: one creates and starts new containers, the other restarts existing ones. Use the right command for your workflow to keep your Docker environment organized and efficient.