Attaching to running containers in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of attaching to a running container

You have a running Docker container named webapp that outputs logs continuously. What will be the output of the command docker attach webapp?

Docker

docker attach webapp

A You will get an error saying the container <code>webapp</code> does not exist.

B You will see the live output stream (logs and console) of the <code>webapp</code> container.

C The command will detach immediately without showing any output.

D The command will start a new container named <code>webapp</code> and show its logs.

💡 Hint

Think about what docker attach does to a running container.

The docker attach command connects your terminal to the standard input, output, and error streams of a running container. This means you see the live output and can interact if the container accepts input.

💻 Command Output

intermediate

Effect of detaching from a container

You attached to a running container using docker attach mycontainer. Which key sequence will safely detach your terminal without stopping the container?

A Press <code>Ctrl+P</code> then <code>Ctrl+Q</code>

B Press <code>Ctrl+Z</code>

C Press <code>Ctrl+C</code>

D Press <code>Ctrl+D</code>

💡 Hint

Detaching requires a special key sequence to avoid stopping the container.

The key sequence Ctrl+P followed by Ctrl+Q detaches your terminal from the container without stopping it. Other sequences may stop or suspend the container.

❓ Troubleshoot

advanced

Troubleshooting attach failure

You run docker attach myapp but get the error: you cannot attach to a stopped container. What is the most likely cause?

A You do not have permission to attach to the container.

B The container <code>myapp</code> is running but has no output streams.

C The Docker daemon is not running on your machine.

D The container <code>myapp</code> is not running; it is stopped.

💡 Hint

Check the container status before attaching.

You cannot attach to a container that is stopped. You must start the container first with docker start myapp.

✅ Best Practice

advanced

Best practice for interactive container sessions

Which command is best to start a new container and attach to it interactively with a terminal session?

A docker run -it myimage

B docker run -d myimage

C docker attach myimage

D docker exec myimage

💡 Hint

Think about starting and attaching in one step with a terminal.

The -it flags start a container interactively with a terminal attached. The -d flag runs detached (in background).

🔀 Workflow

expert

Correct workflow to attach and detach safely

Arrange the steps in the correct order to attach to a running container named db, interact with it, and then detach safely without stopping it.

A 2,1,3,4

B 1,3,2,4

C 1,2,3,4

D 1,2,4,3

💡 Hint

Think about the natural order of attaching, using, detaching, and returning.

First attach to the container, then interact, then detach safely with the key sequence, and finally return to your shell.