Running containers in detached mode in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of running a container with detached mode?

You run the command docker run -d nginx. What will the output be?

Docker

docker run -d nginx

A A long container ID string is printed

B The nginx welcome page HTML is printed

C An error message about missing image is shown

D The container logs start streaming in the terminal

💡 Hint

Detached mode runs the container in the background and prints the container ID.

When you run a container with the -d flag, Docker starts it in the background and prints the container ID. It does not show logs or HTML output.

🧠 Conceptual

intermediate

Why use detached mode when running containers?

Which reason best explains why detached mode (-d) is used when running Docker containers?

A To run the container in the background so the terminal is free for other commands

B To automatically remove the container after it stops

C To limit the container's CPU usage

D To run the container with interactive shell access

💡 Hint

Detached mode frees up your terminal by running the container in the background.

Detached mode allows the container to run in the background, so you can continue using the terminal without being attached to the container's output.

❓ Troubleshoot

advanced

Why does docker run -d fail with 'no tty present' error?

You run docker run -d -it ubuntu but get an error: 'the input device is not a TTY'. What is the cause?

A The Ubuntu image is missing from local cache

B Using both -d (detached) and -t (allocate tty) together causes conflict

C The Docker daemon is not running

D The container port is already in use

💡 Hint

Detached mode means no terminal attached, but -t requests a terminal.

The -d flag runs the container detached (no terminal), but -t requests a terminal allocation. These conflict and cause the error.

🔀 Workflow

advanced

What is the correct sequence to run a container detached and then view its logs?

Arrange the commands in the correct order to run an nginx container detached and then see its logs.

A 3,1,2,4

B 2,1,3,4

C 1,2,3,4

D 1,3,2,4

💡 Hint

You must run the container first, check it's running, then view logs, then stop it.

First, run the container detached. Then list running containers to confirm. Next, view logs. Finally, stop the container.

✅ Best Practice

expert

Which command correctly runs a container detached with port mapping and environment variable?

You want to run a container detached, map port 8080 on host to 80 in container, and set ENV VAR=prod. Which command is correct?

A docker run --detach -p 80:8080 -e ENV=prod nginx

B docker run -p 8080:80 -d -env ENV=prod nginx

C docker run -d -p 8080:80 -e ENV=prod nginx

D docker run -d -p 80:8080 --env ENV prod nginx

💡 Hint

Check the order and syntax of flags for port and environment variables.

Option C uses correct flags: -d for detached, -p hostPort:containerPort, and -e for environment variable with correct syntax.