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.

Practice

1. What does the docker restart <container_name> command do?

easy

A. Only starts the specified container

B. Only stops the specified container

C. Deletes the specified container

D. Stops and then starts the specified container

Solution

1. Step 1: Understand the restart command

   The docker restart command is designed to stop a running container and then start it again.

2. Step 2: Compare with other commands

   Stopping only halts the container, starting only launches it, and deleting removes it. Restart does both stop and start.

3. Final Answer:

   Stops and then starts the specified container -> Option D

4. Quick Check:

   Restart = Stop + Start [OK]

Hint: Restart means stop then start container [OK]

Common Mistakes:

• Thinking restart only stops or only starts
• Confusing restart with remove or delete
• Assuming restart creates a new container

2. Which of the following is the correct syntax to restart a container named webapp?

easy

A. docker restart -c webapp

B. docker restart webapp

C. docker restart --container webapp

D. docker restart container webapp

Solution

1. Step 1: Recall the basic restart syntax

   The correct syntax is docker restart <container_name> without extra flags for container name.

2. Step 2: Evaluate the options

   Options B, C, and D add invalid flags or keywords not recognized by Docker CLI for restart.

3. Final Answer:

   docker restart webapp -> Option B

4. Quick Check:

   Simple restart command = docker restart container_name [OK]

Hint: Use 'docker restart' plus container name only [OK]

Common Mistakes:

• Adding unnecessary flags like -c or --container
• Using extra keywords after restart
• Confusing restart syntax with other commands

3. What will be the output of the command docker restart myapp if the container myapp is currently stopped?

medium

A. myapp

B. Error: Container is not running

C. Container myapp restarted successfully

D. No output

Solution

1. Step 1: Understand restart behavior on stopped container

   If the container is stopped, docker restart will start it and print the container name.

2. Step 2: Check Docker CLI output format

   The command outputs the container name on success, not a success message or error if stopped.

3. Final Answer:

   myapp -> Option A

4. Quick Check:

   Restart prints container name on success [OK]

Hint: Restart prints container name even if stopped [OK]

Common Mistakes:

• Expecting an error if container is stopped
• Expecting a success message instead of container name
• Thinking restart does nothing if container stopped

4. You run docker restart 123abc but get an error: Error: No such container: 123abc. What is the likely cause?

medium

A. Docker daemon is not running

B. The container is already running

C. The container ID or name is incorrect or does not exist

D. You need to add --force flag to restart

Solution

1. Step 1: Analyze the error message

   The error says no such container, meaning Docker cannot find a container with ID or name '123abc'.

2. Step 2: Check other options

   Running container does not cause this error, Docker daemon not running causes different errors, and --force is not a valid restart flag.

3. Final Answer:

   The container ID or name is incorrect or does not exist -> Option C

4. Quick Check:

   No such container = wrong ID/name [OK]

Hint: Check container ID/name spelling if 'No such container' error [OK]

Common Mistakes:

• Assuming container must be stopped to restart
• Trying to force restart with invalid flags
• Ignoring Docker daemon status

5. You want a container to automatically restart if it crashes or Docker restarts. Which command correctly starts a container named db with this behavior?

hard

A. docker run --restart=always db

B. docker run --restart=never db

C. docker run --restart=on-failure db

D. docker run --restart=unless-stopped db

Solution

1. Step 1: Understand restart policies

   The --restart=always policy restarts the container whenever it stops, including crashes and Docker daemon restarts.

2. Step 2: Compare other policies

   never disables restart, on-failure restarts only on errors, and unless-stopped restarts except when manually stopped.

3. Final Answer:

   docker run --restart=always db -> Option A

4. Quick Check:

   Always restart = auto restart on crash and daemon restart [OK]

Hint: Use --restart=always for auto restart on crash and reboot [OK]

Common Mistakes:

• Using --restart=never which disables restart
• Confusing on-failure with always
• Not setting restart policy at container start