kubectl logs for debugging in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Viewing logs of a specific container in a pod

You have a pod named webapp-1234 with two containers: frontend and backend. Which kubectl logs command will show logs only from the backend container?

A kubectl logs webapp-1234

B kubectl logs webapp-1234 --container=frontend

C kubectl logs webapp-1234 -c backend

D kubectl logs backend

💡 Hint

Use the -c or --container flag to specify the container name.

The kubectl logs command with -c backend fetches logs from the backend container inside the pod webapp-1234. Option C will fail because the pod has multiple containers and no container is specified, Option C fetches logs from the frontend container, and C tries to get logs from a pod named backend which does not exist.

💻 Command Output

intermediate

Getting logs from previous container instance

A pod restarted due to a crash. You want to see the logs from the previous container instance to understand what caused the crash. Which command will show those logs?

A kubectl logs pod-name --previous

B kubectl logs pod-name --all-containers

C kubectl logs pod-name --follow

D kubectl logs pod-name --timestamps

💡 Hint

Look for a flag that fetches logs from the last terminated container.

The --previous flag shows logs from the previous container instance before it restarted. --all-containers is not a valid flag for kubectl logs, --follow streams live logs, and --timestamps adds timestamps to logs.

❓ Configuration

advanced

Filtering logs by timestamps using kubectl

You want to fetch logs from a pod named api-server but only want to see logs starting from a specific time, for example, logs after 10 minutes ago. Which approach is correct?

A kubectl logs api-server --from-time=10m

B kubectl logs api-server --since-time='2024-01-01T10:00:00Z'

C kubectl logs api-server --tail=10m

D kubectl logs api-server --since=10m

💡 Hint

Check the flag that filters logs by duration from now.

The --since=10m flag fetches logs from the last 10 minutes. --since-time requires a full timestamp, --tail limits number of lines, and --from-time is not a valid flag.

❓ Troubleshoot

advanced

Error when fetching logs from a pod

You run kubectl logs mypod but get the error: Error from server (BadRequest): a container name must be specified for pod "mypod". What is the most likely cause?

A The pod <code>mypod</code> does not exist in the current namespace.

B You forgot to specify the container name with <code>-c</code> because the pod has multiple containers.

C You need to add <code>--all-containers</code> to get logs from all containers.

D The pod is not running, so logs cannot be fetched.

💡 Hint

Check if the pod has more than one container and if the command specifies which container.

This error happens when the pod has multiple containers and you do not specify which container's logs to fetch. Adding -c container-name fixes this. Option B would give a different "NotFound" error. --all-containers is not a valid flag for kubectl logs. Option B is unrelated as logs can be fetched from terminated pods.

🔀 Workflow

expert

Debugging a failing pod with multiple containers and restarts

You have a pod db-pod with two containers: db and sidecar. The pod keeps restarting. You want to check the logs of the previous instance of the db container only. What is the correct command sequence?

A kubectl logs db-pod -c db --previous

B kubectl logs db-pod -c sidecar --previous

C kubectl logs db-pod --previous

D kubectl logs db-pod -c db

💡 Hint

Combine container selection and previous logs flags.

To get logs from the previous instance of a specific container, you must specify both the container name with -c and the --previous flag. Option A selects the wrong container, Option A misses the container name, and D shows current logs only.

Practice

1. What does the kubectl logs command do in Kubernetes?

easy

A. Deletes a pod from the cluster

B. Lists all pods running in the cluster

C. Creates a new pod with specified configuration

D. Shows the output logs of a container inside a pod

Solution

1. Step 1: Understand the purpose of kubectl logs

   This command is used to view the logs generated by containers running inside pods in Kubernetes.

2. Step 2: Differentiate from other commands

   Deleting pods uses kubectl delete, creating pods uses kubectl create, and listing pods uses kubectl get pods. None of these show logs.

3. Final Answer:

   Shows the output logs of a container inside a pod -> Option D

4. Quick Check:

   Logs = Output from container [OK]

Hint: Logs show container output, not pod creation or deletion [OK]

Common Mistakes:

• Confusing logs with pod creation or deletion
• Thinking logs list pods instead of showing output
• Assuming logs show cluster-wide events

2. Which of the following is the correct syntax to follow logs live from a pod named webapp-123?

easy

A. kubectl logs webapp-123 --follow

B. kubectl logs --tail webapp-123

C. kubectl logs webapp-123 -t

D. kubectl logs -t webapp-123

Solution

1. Step 1: Recall correct flag placement

   The --follow flag comes after the pod name in the kubectl logs command syntax.

2. Step 2: Check distractor flags

   Distractors use --tail (shows last lines, no streaming) or -t (timestamps, no follow). Only --follow streams live.

3. Final Answer:

   kubectl logs webapp-123 --follow -> Option A

4. Quick Check:

   Pod name first, then flags [OK]

Hint: Put pod name before flags like --follow [OK]

Common Mistakes:

• Placing flags before pod name
• Using --tail or -t instead of --follow
• Mixing order of arguments

3. Given the command kubectl logs mypod --tail=5, what will be the output?

medium

A. Shows all logs except the last 5 lines from the pod named mypod

B. Shows the first 5 lines of logs from the pod named mypod

C. Shows the last 5 lines of logs from the pod named mypod

D. Shows logs only if the pod has more than 5 containers

Solution

1. Step 1: Understand the --tail option

   The --tail=5 option limits the output to the last 5 lines of the logs.

2. Step 2: Confirm it applies to the specified pod

   The command targets the pod named mypod, so logs shown are from that pod only.

3. Final Answer:

   Shows the last 5 lines of logs from the pod named mypod -> Option C

4. Quick Check:

   --tail = last lines [OK]

Hint: --tail shows last lines, not first [OK]

Common Mistakes:

• Thinking --tail shows first lines
• Confusing pod logs with container logs
• Assuming it filters by container count

4. You run kubectl logs mypod -c mycontainer but get an error saying container not found. What is the likely cause?

medium

A. The pod mypod is not running

B. The container name mycontainer does not exist in pod mypod

C. You forgot to add the --follow flag

D. The pod mypod has multiple containers but you did not specify one

Solution

1. Step 1: Analyze the error message

   The error says container not found, which means the container name given is incorrect or missing in the pod.

2. Step 2: Check command correctness

   You specified -c mycontainer, so if the container name is wrong, the error appears. Pod running state or missing --follow flag does not cause this error.

3. Final Answer:

   The container name mycontainer does not exist in pod mypod -> Option B

4. Quick Check:

   Container name must match exactly [OK]

Hint: Check container name spelling matches pod's containers [OK]

Common Mistakes:

• Assuming pod state causes container not found error
• Forgetting to specify container when multiple exist
• Using wrong flag for container name

5. You want to debug a pod with two containers named frontend and backend. Which command correctly streams live logs from the backend container only?

hard

A. kubectl logs podname -c backend --follow

B. kubectl logs podname --tail -c backend

C. kubectl logs -c backend podname --tail

D. kubectl logs --tail podname -c backend

Solution

1. Step 1: Recall correct argument order

   The pod name must come immediately after kubectl logs, followed by flags like -c and --follow.

2. Step 2: Confirm container and follow flag usage

   Using -c backend specifies the backend container, and --follow streams logs live.

3. Final Answer:

   kubectl logs podname -c backend --follow -> Option A

4. Quick Check:

   Pod name first, then container and flags [OK]

Hint: Pod name first, then container (-c) and --follow [OK]

Common Mistakes:

• Placing flags before pod name
• Mixing order of container and follow flags
• Using unsupported shorthand flags