Restarting containers in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command is used to restart a running Docker container?

Use docker restart <container_name_or_id> to stop and then start a container again.

beginner

What happens when you run docker restart on a container?

Docker stops the container if it is running, then starts it again. It does not apply any changes made to the container's configuration or environment.

intermediate

How can you restart all running Docker containers at once?

You can use docker restart $(docker ps -q) to restart all running containers by passing their IDs to the restart command.

intermediate

What is the difference between docker restart and docker stop followed by docker start?

docker restart combines stopping and starting in one command, while docker stop and docker start are two separate commands that achieve the same result.

intermediate

Can you specify a timeout when restarting a Docker container? How?

Yes, use docker restart -t <seconds> <container> to set how long Docker waits before forcibly stopping the container.

Which command restarts a Docker container named 'webapp'?

A docker reload webapp

B docker restart webapp

C docker reset webapp

D docker refresh webapp

The correct command to restart a container is docker restart <container_name>.

What does the -t option do in docker restart -t 5 mycontainer?

A Tests the container before restarting

B Specifies the total restart attempts

C Tags the container with a label

D Sets the timeout in seconds before forcefully stopping the container

The -t option sets the timeout in seconds Docker waits before killing the container.

How can you restart all running containers with one command?

A docker restart $(docker ps -q)

B docker restart --all

C docker restart -a

D docker restart *

Using docker restart $(docker ps -q) restarts all running containers by passing their IDs.

What is the effect of running docker restart on a stopped container?

A It removes the container

B It does nothing

C It starts the container

D It pauses the container

Restarting a stopped container will start it.

Which command sequence is equivalent to docker restart mycontainer?

A docker stop mycontainer && docker start mycontainer

B docker pause mycontainer && docker unpause mycontainer

C docker kill mycontainer && docker run mycontainer

D docker remove mycontainer && docker create mycontainer

Stopping and then starting a container manually is the same as restarting it.

Explain how to restart a Docker container and what happens during the restart process.

Think about what stopping and starting means for a container.

Describe a method to restart all running Docker containers at once and why this might be useful.

Consider how to get all container IDs and use them in one command.

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