Restarting containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of restarting a stopped container?

You have a Docker container named webapp that is currently stopped. You run the command docker restart webapp. What will be the output?

Docker

docker restart webapp

A webapp restarted

B Error: No such container: webapp

C Container webapp restarted successfully

D webapp

💡 Hint

The restart command returns the container name if successful.

The docker restart command outputs the container name when it successfully restarts the container, even if it was stopped before.

❓ Troubleshoot

intermediate

Why does docker restart fail with 'No such container'?

You run docker restart mycontainer but get the error Error: No such container: mycontainer. What is the most likely cause?

Docker

docker restart mycontainer

A The container name or ID is incorrect or does not exist

B Docker daemon is not running

C The container is paused

D The container is already running

💡 Hint

Check if the container name matches exactly.

The error means Docker cannot find a container with the given name or ID. It usually means a typo or the container was removed.

❓ Configuration

advanced

How to configure automatic restart of a container on failure?

You want a container to restart automatically only if it crashes (exits with error). Which Docker run option achieves this?

Docker

docker run --restart=? myimage

A --restart=unless-stopped

B --restart=on-failure

C --restart=no

D --restart=always

💡 Hint

This option restarts only on non-zero exit codes.

The --restart=on-failure option restarts the container only if it exits with a failure code, not on normal exit.

🔀 Workflow

advanced

What is the correct sequence to restart all running containers?

You want to restart all currently running Docker containers with a single command sequence. Which option correctly does this?

A docker restart $(docker ps -q)

B docker ps -q | xargs docker restart

C docker ps -a | grep Up | awk '{print $1}' | xargs docker restart

D docker restart $(docker ps -a -q)

💡 Hint

Use only running containers' IDs.

The command docker ps -q lists only running containers. Passing these IDs to docker restart restarts all running containers.

✅ Best Practice

expert

What is the best practice to ensure a container restarts after Docker daemon reboot?

You want your container to restart automatically after the Docker service or host machine restarts. Which restart policy is best suited for this?

A --restart=on-failure

B --restart=unless-stopped

C --restart=always

D --restart=no

💡 Hint

This policy restarts containers except when explicitly stopped.

The --restart=always policy restarts containers after daemon or host reboot regardless of how the container was stopped.