Starting and stopping containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of stopping a non-running container?

You run the command docker stop mycontainer but the container mycontainer is already stopped. What will Docker output?

A Nothing, the command silently succeeds

B Error: No such container: mycontainer

C Container mycontainer is already stopped

D mycontainer

💡 Hint

Docker stop returns the container name if it exists, even if already stopped.

When you run docker stop on a container that exists but is not running, Docker returns the container name. It does not error or silently succeed.

💻 Command Output

intermediate

What happens when you start a container that is already running?

You run docker start mycontainer but mycontainer is already running. What is the output?

A mycontainer

B Error: Container already running

C Nothing, command silently succeeds

D Container mycontainer restarted

💡 Hint

Docker start returns the container name regardless of state if container exists.

Starting a container that is already running returns the container name. Docker does not error or restart it.

❓ Troubleshoot

advanced

Why does docker stop hang on a container?

You run docker stop mycontainer but the command hangs and does not return. What is a likely cause?

A Docker daemon is not running

B The container image is corrupted

C The container's main process ignores SIGTERM and does not exit

D The container is already stopped

💡 Hint

Stopping sends SIGTERM then SIGKILL after timeout if needed.

If the container's main process ignores the stop signal (SIGTERM), Docker waits until timeout before forcing kill. This causes the stop command to hang until timeout.

🔀 Workflow

advanced

What is the correct sequence to restart a container safely?

You want to restart a container named webapp safely, ensuring it stops before starting again. Which sequence of commands is best?

A docker restart webapp

B docker stop webapp && docker start webapp

C docker kill webapp && docker start webapp

D docker start webapp && docker stop webapp

💡 Hint

Restart command stops then starts but may not allow custom wait times.

Using docker stop then docker start ensures the container stops gracefully before starting again. docker restart is a shortcut but less flexible.

✅ Best Practice

expert

Which command safely stops all running containers?

You want to stop all running Docker containers safely with one command. Which command achieves this?

A docker stop $(docker ps -q)

B docker kill $(docker ps -q)

C docker rm $(docker ps -q)

D docker pause $(docker ps -q)

💡 Hint

Stopping sends SIGTERM to allow graceful shutdown.

docker stop $(docker ps -q) sends stop signals to all running containers, allowing them to exit cleanly. docker kill forces immediate stop, docker rm removes containers (fails if running), and docker pause only suspends processes.