Docker logs for troubleshooting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to view logs of a Docker container named webapp.

Docker

docker logs [1]

Drag options to blanks, or click blank then click option'

Arun

Bstart

Cwebapp

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using commands like start or stop instead of the container name.

Forgetting to specify any container name.

The docker logs command followed by the container name webapp shows the logs of that container.

2fill in blank

medium

Complete the command to follow the live logs of a container named db.

Docker

docker logs [1] db

Drag options to blanks, or click blank then click option'

A-f

B--tail

C--help

Drun

Attempts:

3 left

💡 Hint

Common Mistakes

Using --tail which only shows last lines but does not follow.

Using run which is not a valid option here.

The -f option means 'follow' and shows live logs as they come.

3fill in blank

hard

Fix the error in the command to show only the last 50 lines of logs for container api.

Docker

docker logs --tail=[1] api

Drag options to blanks, or click blank then click option'

A100

B50

C0

Dall

Attempts:

3 left

💡 Hint

Common Mistakes

Using 100 which shows more lines than needed.

Using 'all' which is not a valid number.

The --tail=50 option shows the last 50 lines of logs.

4fill in blank

hard

Fill both blanks to show the last 20 lines and follow live logs of container cache.

Docker

docker logs [1] [2] cache

Drag options to blanks, or click blank then click option'

A-f

B--tail=20

C--follow

D--since=1h

Attempts:

3 left

💡 Hint

Common Mistakes

Using --since=1h which filters by time, not lines.

Mixing order of options incorrectly.

Use --tail=20 to show last 20 lines and -f to follow live logs.

5fill in blank

hard

Fill all three blanks to show logs since 10 minutes ago, follow live logs, and limit output to last 100 lines for container worker.

Docker

docker logs [1] [2] [3] worker

Drag options to blanks, or click blank then click option'

A--since=10m

B-f

C--tail=100

D--timestamps

Attempts:

3 left

💡 Hint

Common Mistakes

Using --timestamps which adds timestamps but does not filter or follow.

Mixing up the order of options.

--since=10m shows logs from last 10 minutes, -f follows live logs, and --tail=100 limits output to last 100 lines.

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