Docker logs for troubleshooting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Identify the output of a Docker logs command

You run the command docker logs myapp on a container named myapp. What will you see if the container has just started and printed Hello World once?

A No output, the command waits indefinitely

B Error: No such container: myapp

C Container myapp is running

D Hello World

💡 Hint

Think about what docker logs shows from the container's output.

The docker logs command shows the standard output and error streams from the container. If the container printed Hello World, that text will appear.

🧠 Conceptual

intermediate

Understanding Docker logs options

Which docker logs option allows you to continuously stream new log output as it is generated?

A --since

B --tail

C --follow

D --timestamps

💡 Hint

This option is like watching a live feed of logs.

The --follow option streams logs live, similar to tail -f in Linux.

❓ Troubleshoot

advanced

Diagnosing missing logs from a container

You run docker logs myservice but see no output, even though the container is running and should produce logs. What is the most likely cause?

A The container was started with <code>--log-driver=none</code>

B The container has crashed and stopped

C You need to restart the Docker daemon

D The container logs are stored in a different directory

💡 Hint

Think about Docker's logging drivers and how they affect log availability.

If the container was started with the none log driver, Docker does not store logs, so docker logs shows nothing.

🔀 Workflow

advanced

Steps to troubleshoot a failing container using logs

Arrange the steps in the correct order to troubleshoot a container that fails immediately after starting using Docker logs.

A 1,3,2,4

B 1,2,3,4

C 3,1,2,4

D 2,1,3,4

💡 Hint

Start by checking if the container is running before looking at logs.

First, verify the container status with docker ps. Then check logs for errors. Next, inspect the exit code to understand failure cause. Finally, fix and restart.

✅ Best Practice

expert

Choosing the best logging driver for troubleshooting

You want to ensure your Docker containers' logs are easily accessible and include timestamps for troubleshooting. Which logging driver and option combination is best?

A Use the <code>json-file</code> logging driver with <code>--log-opt timestamps=true</code>

B Use the <code>none</code> logging driver to avoid log overhead

C Use the <code>syslog</code> logging driver without timestamps

D Use the <code>journald</code> logging driver with no extra options

💡 Hint

Consider default drivers and how to add timestamps.

The json-file driver is Docker's default and supports timestamps=true to add timestamps to logs, making troubleshooting easier.