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.

Practice

1. What is the primary purpose of the docker logs command?

easy

A. To start a new Docker container

B. To remove a Docker container

C. To list all Docker images on the system

D. To view the output and error messages of a running or stopped container

Solution

1. Step 1: Understand the role of docker logs

   This command is designed to show the logs generated by a container, including output and error messages.

2. Step 2: Compare with other Docker commands

   Starting containers uses docker run, listing images uses docker images, and removing containers uses docker rm.

3. Final Answer:

   To view the output and error messages of a running or stopped container -> Option D

4. Quick Check:

   docker logs = view container output [OK]

Hint: Logs show container output and errors, not start or remove [OK]

Common Mistakes:

• Confusing logs with starting or removing containers
• Thinking logs list images
• Assuming logs modify container state

2. Which of the following is the correct syntax to show the last 10 lines of logs from a container named webapp?

easy

A. docker logs --tail 10 webapp

B. docker logs -n 10 webapp

C. docker logs --last 10 webapp

D. docker logs -lines 10 webapp

Solution

1. Step 1: Recall the correct option for tailing logs

   The --tail option followed by a number shows that many lines from the end of the logs.

2. Step 2: Verify other options

   -n, --last, and -lines are not valid options for docker logs.

3. Final Answer:

   docker logs --tail 10 webapp -> Option A

4. Quick Check:

   Use --tail to limit log lines [OK]

Hint: Use --tail to show last lines of logs [OK]

Common Mistakes:

• Using -n or --last which are invalid
• Confusing tail with head
• Forgetting container name at the end

3. Given the command docker logs --since 2024-06-01T12:00:00 webapp, what will be the output?

medium

A. Logs from the container webapp before June 1, 2024, 12:00 PM

B. Logs from the container webapp starting from June 1, 2024, 12:00 PM

C. All logs from the container webapp without any time filter

D. An error because the date format is incorrect

Solution

1. Step 1: Understand the --since option

   This option filters logs to show only those generated after the specified date and time.

2. Step 2: Check the date format

   The ISO 8601 format YYYY-MM-DDTHH:MM:SS is valid for this option, so no error occurs.

3. Final Answer:

   Logs from the container webapp starting from June 1, 2024, 12:00 PM -> Option B

4. Quick Check:

   --since filters logs after given time [OK]

Hint: Use --since with ISO 8601 date to filter logs [OK]

Common Mistakes:

• Thinking --since shows logs before the date
• Assuming date format is invalid
• Expecting all logs without filter

4. You run docker logs -f mycontainer but see no new logs even though the app is running. What is a likely cause?

medium

A. The container mycontainer is not producing any new output

B. The -f option is invalid and causes no output

C. You forgot to specify the container ID instead of the name

D. The Docker daemon is stopped

Solution

1. Step 1: Understand the -f option

   The -f or --follow option streams new logs as they appear.

2. Step 2: Analyze why no new logs appear

   If the app inside the container does not produce new output, the log stream stays empty despite the container running.

3. Final Answer:

   The container mycontainer is not producing any new output -> Option A

4. Quick Check:

   -f streams logs; no output means no new logs [OK]

Hint: No new logs means app inside container is silent [OK]

Common Mistakes:

• Thinking -f is invalid syntax
• Confusing container name and ID
• Assuming Docker daemon is stopped without checking

5. You want to troubleshoot a container named dbserver that crashed. You need to see the last 50 lines of logs and follow new logs as they appear. Which command should you use?

hard

A. docker logs -n 50 -f dbserver

B. docker logs -f --since 50 dbserver

C. docker logs --tail 50 -f dbserver

D. docker logs --follow --last 50 dbserver

Solution

1. Step 1: Identify correct options for tail and follow

   --tail 50 shows last 50 lines, and -f streams new logs.

2. Step 2: Check option validity

   --since 50 is invalid because --since expects a timestamp, not a number. --last and -n are not valid options for docker logs.

3. Final Answer:

   docker logs --tail 50 -f dbserver -> Option C

4. Quick Check:

   Use --tail and -f together for last lines and live logs [OK]

Hint: Combine --tail and -f to see recent and live logs [OK]

Common Mistakes:

• Using --since with a number instead of timestamp
• Using invalid options like --last or -n
• Mixing option order incorrectly