Running containers in detached mode in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does running a Docker container in detached mode mean?

Running a Docker container in detached mode means the container runs in the background, allowing you to continue using the terminal without being attached to the container's output.

beginner

Which Docker command option runs a container in detached mode?

The -d option in the docker run command runs the container in detached mode.

beginner

How can you check running containers after starting one in detached mode?

Use the command docker ps to list all running containers, including those started in detached mode.

intermediate

What is the difference between running a container in attached vs detached mode?

Attached mode shows the container's output in the terminal and keeps the terminal busy. Detached mode runs the container in the background, freeing the terminal for other commands.

beginner

How do you stop a container running in detached mode?

You can stop a detached container using docker stop <container_id_or_name>.

Which command runs a Docker container in detached mode?

A docker run --rm nginx

B docker run -it nginx

C docker run -d nginx

D docker run --name nginx

The -d option runs the container in detached mode, so docker run -d nginx is correct.

What does the docker ps command show?

A All running containers

B All stopped containers

C Docker images

D Docker volumes

docker ps lists all currently running containers.

If you want to see the logs of a detached container, which command do you use?

A docker exec <container_id>

B docker attach <container_id>

C docker stop <container_id>

D docker logs <container_id>

Use docker logs <container_id> to view output from a detached container.

What happens if you run a container without -d option?

A Container runs attached to terminal

B Container runs in background

C Container stops immediately

D Container runs with no network

Without -d, the container runs attached to the terminal, showing output and blocking the terminal.

How do you stop a container running in detached mode?

A docker start <container_id>

B docker stop <container_id>

C docker run -d <container_id>

D docker rm <container_id>

Use docker stop <container_id> to stop a running container, including those in detached mode.

Explain what detached mode is in Docker and why you might want to use it.

Think about running a program and still being able to use your computer for other things.

Describe the steps to run a container in detached mode and then check if it is running.

Start with the command to run, then how to verify.

Practice

1. What does running a Docker container with the -d option do?

easy

A. Runs the container in detached mode, in the background

B. Runs the container interactively with a terminal

C. Removes the container after it stops

D. Builds a new Docker image from a Dockerfile

Solution

1. Step 1: Understand the -d option meaning

   The -d flag tells Docker to run the container in detached mode, meaning it runs in the background.

2. Step 2: Compare with other options

   Other options like interactive mode (-it) keep the container attached to the terminal, which is different from detached mode.

3. Final Answer:

   Runs the container in detached mode, in the background -> Option A

4. Quick Check:

   Detached mode = background run [OK]

Hint: Remember: -d means detach and run in background [OK]

Common Mistakes:

• Confusing -d with -it for interactive mode
• Thinking -d removes the container
• Mixing up build and run commands

2. Which of the following is the correct syntax to run a container named myapp in detached mode using the image nginx?

easy

A. docker run nginx -d --name myapp

B. docker run -d --name myapp nginx

C. docker run --detach myapp nginx

D. docker run --name myapp -it nginx

Solution

1. Step 1: Identify correct order and flags

   The correct syntax places options before the image name. The -d flag enables detached mode, and --name myapp names the container.

2. Step 2: Validate each option

   docker run -d --name myapp nginx correctly uses docker run -d --name myapp nginx. docker run --detach myapp nginx misplaces arguments, docker run nginx -d --name myapp places flags after image, and docker run --name myapp -it nginx uses interactive mode instead of detached.

3. Final Answer:

   docker run -d --name myapp nginx -> Option B

4. Quick Check:

   Options before image, -d for detached [OK]

Hint: Put all flags before the image name in docker run [OK]

Common Mistakes:

• Placing flags after the image name
• Using -it instead of -d for detached mode
• Misusing --detach instead of -d

3. What will be the output of the command docker ps after running docker run -d nginx?

medium

A. Shows the logs of the nginx container

B. Shows no containers because detached mode hides them

C. Shows an error because -d is not valid with nginx

D. Shows the running nginx container with its container ID and status

Solution

1. Step 1: Understand docker run -d nginx

   This command runs the nginx container in detached mode, so it runs in the background.

2. Step 2: Understand docker ps output

   The docker ps command lists all running containers, including those running detached. It will show the nginx container with its ID, image, and status.

3. Final Answer:

   Shows the running nginx container with its container ID and status -> Option D

4. Quick Check:

   docker ps lists running containers including detached [OK]

Hint: docker ps always lists running containers, detached or not [OK]

Common Mistakes:

• Thinking detached containers are hidden from docker ps
• Confusing docker ps with docker logs
• Assuming -d causes errors with nginx image

4. You ran docker run -d nginx but the container stopped immediately. What is the most likely reason?

medium

A. The -d flag is invalid and caused the container to stop

B. You forgot to add the -it flag for interactive mode

C. The nginx container exited because it has no foreground process

D. Docker daemon is not running

Solution

1. Step 1: Understand container lifecycle with detached mode

   Detached mode runs containers in background, but if the main process exits immediately, the container stops.

2. Step 2: Analyze nginx container behavior

   Some nginx images may exit if no foreground process runs. This causes the container to stop right after starting.

3. Final Answer:

   The nginx container exited because it has no foreground process -> Option C

4. Quick Check:

   Container stops if main process exits immediately [OK]

Hint: Detached mode won't keep container alive if main process ends [OK]

Common Mistakes:

• Assuming -d flag causes errors
• Confusing interactive mode with container lifecycle
• Not checking if Docker daemon is running

5. You want to run a container in detached mode and later check its logs. Which sequence of commands is correct?

hard

A. docker run -d --name myapp nginx; docker logs myapp

B. docker run --name myapp nginx -d; docker stop myapp

C. docker run -it --name myapp nginx; docker logs myapp

D. docker run -d nginx; docker attach myapp

Solution

1. Step 1: Run container in detached mode with a name

   Use docker run -d --name myapp nginx to start the container in background with a name for easy reference.

2. Step 2: Check logs of the running container

   Use docker logs myapp to view the logs of the named container without attaching to it.

3. Final Answer:

   docker run -d --name myapp nginx; docker logs myapp -> Option A

4. Quick Check:

   Detached run + named container + logs command [OK]

Hint: Name containers to easily check logs after detached run [OK]

Common Mistakes:

• Using -it instead of -d for detached mode
• Trying to attach instead of using logs to view output
• Placing -d after image name