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.

Practice

1. Which command is used to start a container that has been stopped in Docker?

easy

A. docker run <container_name>

B. docker stop <container_name>

C. docker start <container_name>

D. docker restart <container_name>

Solution

1. Step 1: Understand the purpose of docker start

   This command resumes a container that was previously stopped without creating a new one.

2. Step 2: Differentiate from other commands

   docker run creates a new container, docker stop stops a running container, and docker restart stops then starts a container.

3. Final Answer:

   docker start <container_name> -> Option C

4. Quick Check:

   Start stopped container = docker start [OK]

Hint: Use 'docker start' to resume stopped containers fast [OK]

Common Mistakes:

• Confusing 'docker run' with 'docker start'
• Using 'docker stop' to start containers
• Thinking 'docker restart' only starts containers

2. Which of the following is the correct syntax to stop a running Docker container named webapp?

easy

A. docker terminate webapp

B. docker kill webapp

C. docker pause webapp

D. docker stop webapp

Solution

1. Step 1: Identify the command to safely stop a container

   docker stop sends a signal to gracefully stop the container.

2. Step 2: Check other options

   docker kill forcefully stops, docker pause suspends, and docker terminate is invalid.

3. Final Answer:

   docker stop webapp -> Option D

4. Quick Check:

   Stop container safely = docker stop [OK]

Hint: Use 'docker stop' to safely end container processes [OK]

Common Mistakes:

• Using 'docker kill' instead of 'docker stop'
• Confusing 'docker pause' with stopping
• Using non-existent commands like 'docker terminate'

3. What will be the output of the following commands if the container db is currently stopped?

docker start db
docker stop db

medium

A. Container 'db' starts, then stops successfully

B. Error: container 'db' not found

C. Container 'db' remains stopped

D. Container 'db' restarts automatically

Solution

1. Step 1: Analyze docker start db

   This command starts the stopped container named 'db'.

2. Step 2: Analyze docker stop db

   This command stops the running container 'db' safely.

3. Final Answer:

   Container 'db' starts, then stops successfully -> Option A

4. Quick Check:

   Start then stop container = success [OK]

Hint: Start then stop a container runs commands without error [OK]

Common Mistakes:

• Assuming 'docker start' creates a new container
• Expecting errors if container exists
• Thinking container restarts automatically without command

4. You run docker start myapp but get the error: Error response from daemon: No such container: myapp. What is the most likely cause?

medium

A. Docker daemon is not running

B. The container 'myapp' does not exist

C. You need to use docker run instead

D. The container 'myapp' is already running

Solution

1. Step 1: Understand the error message

   The error says 'No such container', meaning 'myapp' is not found in Docker's container list.

2. Step 2: Check other options

   If it was running, 'docker start' would not error; 'docker run' creates new containers but error is about missing container; daemon not running causes different errors.

3. Final Answer:

   The container 'myapp' does not exist -> Option B

4. Quick Check:

   No such container error = container missing [OK]

Hint: Check container existence before starting [OK]

Common Mistakes:

• Assuming container is running when error says missing
• Trying 'docker run' without checking container list
• Ignoring Docker daemon status

5. You want to stop a container named api-server but ensure it has 10 seconds to finish its work before stopping. Which command should you use?

hard

A. docker stop -t 10 api-server

B. docker kill -t 10 api-server

C. docker pause api-server

D. docker stop api-server -timeout 10

Solution

1. Step 1: Understand the timeout option in docker stop

   The -t or --time flag sets seconds to wait before killing the container.

2. Step 2: Evaluate the options

   docker kill does not support -t, docker pause only suspends, and -timeout is invalid syntax.

3. Final Answer:

   docker stop -t 10 api-server -> Option A

4. Quick Check:

   Stop with timeout = docker stop -t [OK]

Hint: Use 'docker stop -t seconds' to allow graceful shutdown [OK]

Common Mistakes:

• Using 'docker kill' with timeout flag
• Using invalid flags like '-timeout'
• Confusing 'pause' with stopping