Viewing container logs in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Viewing logs of a running container

You have a Docker container named webapp running. Which command shows the logs of this container?

A docker logs webapp

B docker show logs webapp

C docker container logs webapp

D docker get logs webapp

💡 Hint

The command to see logs starts with docker logs.

The correct command to view logs of a container is docker logs [container_name]. Other options are invalid commands.

💻 Command Output

intermediate

Viewing real-time logs with follow option

You want to see live logs from a container named db as they are generated. Which command will do this?

A docker logs db -r

B docker logs --tail=0 db

C docker logs -f db

D docker logs db --live

💡 Hint

The option to follow logs live is a single letter.

The -f or --follow option streams logs live. Other options are invalid or do not stream logs.

❓ Configuration

advanced

Limiting log output to recent lines

You want to see only the last 50 lines of logs from a container named api-server. Which command achieves this?

A docker logs -n 50 api-server

B docker logs --tail 50 api-server

C docker logs --last 50 api-server

D docker logs api-server --limit 50

💡 Hint

The option to limit lines is --tail.

The --tail option followed by a number shows that many recent lines. Other options are invalid or do not exist.

❓ Troubleshoot

advanced

Error when viewing logs of a stopped container

You run docker logs myapp but get the error: Error: No such container: myapp. What is the most likely cause?

A You must use <code>docker logs -a myapp</code> to see logs of stopped containers

B You need to add <code>-f</code> option to see logs

C Docker daemon is not running

D The container named <code>myapp</code> does not exist or was removed

💡 Hint

Check if the container is listed with docker ps -a.

The error means Docker cannot find a container named myapp. It may have been removed or never created. The -f option or -a do not fix this error.

✅ Best Practice

expert

Efficiently managing large container logs

Your container produces very large logs. Which approach is best to avoid filling disk space while still being able to view recent logs?

A Configure Docker logging driver to use log rotation with size limits

B Manually delete log files in /var/lib/docker/containers regularly

C Use <code>docker logs --tail 10</code> every time to limit output

D Stop the container frequently to clear logs

💡 Hint

Docker supports built-in log rotation settings.

Setting a logging driver with rotation and size limits prevents disk from filling up automatically. Manual deletion or stopping containers is inefficient and risky. Using --tail limits output but does not reduce log file size.

Practice

1. What does the command docker logs CONTAINER do?

easy

A. Starts a new container

B. Deletes the specified container

C. Shows the logs of the specified container

D. Lists all running containers

Solution

1. Step 1: Understand the command purpose

   The command docker logs is used to view the output logs of a container.

2. Step 2: Identify the correct action

   Since it shows logs, it helps to see what the container is doing or any errors it has.

3. Final Answer:

   Shows the logs of the specified container -> Option C

4. Quick Check:

   docker logs = show container logs [OK]

Hint: Logs show container output; use docker logs CONTAINER [OK]

Common Mistakes:

• Confusing logs with starting or deleting containers
• Thinking it lists containers instead of showing logs

2. Which of these is the correct syntax to follow live logs of a container named webapp?

easy

A. docker logs webapp -p

B. docker logs -f webapp

C. docker logs --tail webapp

D. docker logs webapp --start

Solution

1. Step 1: Identify the option to follow logs live

   The -f flag means "follow" and shows logs as they appear in real time.

2. Step 2: Check the syntax correctness

   The correct syntax is docker logs -f webapp. Other options are invalid or incorrect.

3. Final Answer:

   docker logs -f webapp -> Option B

4. Quick Check:

   -f means follow logs live [OK]

Hint: Use -f to follow logs live as they update [OK]

Common Mistakes:

• Using --tail without a number to follow logs
• Adding invalid flags like --start or -p
• Placing flags after container name incorrectly

3. What will be the output of the command docker logs --tail 3 mycontainer if the container's last 5 log lines are:

Line1
Line2
Line3
Line4
Line5

medium

A. Line3
Line4
Line5

B. Line1
Line2
Line3

C. Line4
Line5

D. Line1
Line2
Line3
Line4
Line5

Solution

1. Step 1: Understand the --tail option

   The --tail 3 option shows only the last 3 lines of the logs.

2. Step 2: Select the last 3 lines from the given logs

   The last 3 lines are Line3, Line4, and Line5.

3. Final Answer:

   Line3 Line4 Line5 -> Option A

4. Quick Check:

   --tail 3 shows last 3 lines [OK]

Hint: --tail N shows last N lines of logs [OK]

Common Mistakes:

• Choosing first lines instead of last lines
• Confusing --tail with -f
• Expecting all logs instead of limited lines

4. You run docker logs -f myapp but see no output even though the app is running. What is the likely problem?

medium

A. You need to restart Docker daemon

B. The -f flag is not supported

C. Logs are disabled by default

D. The container name is incorrect or not running

Solution

1. Step 1: Check container status and name

   If the container name is wrong or the container is stopped, no logs will appear.

2. Step 2: Understand the -f flag behavior

   The -f flag follows logs live, but if no logs exist or container is not running, output is empty.

3. Final Answer:

   The container name is incorrect or not running -> Option D

4. Quick Check:

   No logs means container name or status issue [OK]

Hint: Check container name and running status first [OK]

Common Mistakes:

• Assuming -f flag is unsupported
• Thinking logs are disabled by default
• Restarting Docker unnecessarily

5. You want to see only the last 10 lines of logs from a container named dbserver and keep watching new logs as they appear. Which command should you use?

hard

A. All of the above

B. docker logs -f --tail 10 dbserver

C. docker logs --tail=10 -f dbserver

D. docker logs --tail 10 -f dbserver

Solution

1. Step 1: Understand combined options usage

   Both -f (follow) and --tail 10 (last 10 lines) can be combined in any order before the container name.

2. Step 2: Check command syntax flexibility

   Docker accepts options in any order before the container name, so all listed commands are valid.

3. Final Answer:

   All of the above -> Option A

4. Quick Check:

   Options order flexible before container name [OK]

Hint: Options order flexible before container name [OK]

Common Mistakes:

• Thinking options must be in fixed order
• Placing options after container name
• Using invalid flags or syntax