Viewing container logs in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command shows the logs of a running Docker container?

Use docker logs <container_name_or_id> to see the output and messages from a running container.

beginner

How do you follow the logs in real-time for a Docker container?

Add the -f or --follow option: docker logs -f <container_name_or_id> to watch new log entries as they appear.

intermediate

How can you limit the number of log lines shown from a Docker container?

Use the --tail <number> option, for example: docker logs --tail 10 <container_name_or_id> shows only the last 10 lines.

beginner

What does the docker logs command show by default?

It shows all the logs that the container has produced since it started, including standard output and standard error.

intermediate

Can you view logs of a stopped Docker container?

Yes, docker logs works for stopped containers too, showing the logs they produced before stopping.

Which command shows the logs of a Docker container named 'webapp'?

A docker logs webapp

B docker show logs webapp

C docker container logs webapp

D docker view logs webapp

The correct command is docker logs webapp to view logs.

How do you watch logs live as they are generated?

A docker logs --tail 10

B docker logs --start

C docker logs -f

D docker logs --stop

Use docker logs -f <container> to follow logs live.

What option limits the number of log lines shown?

A --limit

B --tail

C --head

D --lines

The --tail option limits log lines shown.

Can you view logs of a container that is not running?

A Only if you restart the container first

B No, logs are only available for running containers

C Only if the container was stopped less than 5 minutes ago

D Yes, logs are saved and can be viewed anytime

Docker saves logs and you can view them even after the container stops.

What does the docker logs command show by default?

A All output since container start

B Only the last 10 lines

C Only error messages

D Only logs from the last hour

By default, it shows all logs since the container started.

Explain how to view and follow logs of a Docker container in real-time.

Think about how you watch a live video feed of logs.

Describe how to limit the number of log lines shown when viewing Docker container logs.

Imagine you only want to see the last few messages, not everything.

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