Label-based filtering with kubectl in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Filter pods by label with kubectl

You have several pods with labels: app=web and env=prod. Which command lists only pods with label app=web?

A kubectl get pods -l env=web

B kubectl get pods --filter app=web

C kubectl get pods -l app=web

D kubectl get pods --label-selector env=prod

💡 Hint

Use the -l or --selector flag to filter by label.

The -l flag filters pods by label. Option C correctly filters pods with label app=web. Option C uses an invalid flag. Option C filters by a different label. Option C filters by a label that does not exist.

💻 Command Output

intermediate

Filter pods with multiple labels

Which command lists pods with labels app=web AND env=prod?

A kubectl get pods -l app=web,env=prod

B kubectl get pods -l app=web; env=prod

C kubectl get pods --selector app=web env=prod

D kubectl get pods -l app=web env=prod

💡 Hint

Use commas to combine multiple labels in the selector.

Option A correctly uses commas to combine labels for AND filtering. Options B and C have invalid syntax. Option A uses a semicolon which is invalid.

❓ Troubleshoot

advanced

Why does this label filter return no pods?

You run kubectl get pods -l app=web env=prod but get no pods listed. Why?

A There are no pods with label <code>app=web</code>.

B The command syntax is incorrect; labels must be comma-separated with no spaces.

C The <code>kubectl</code> version does not support label filtering.

D The pods are in a different namespace.

💡 Hint

Check the label selector syntax carefully.

The command incorrectly separates labels with a space instead of a comma. The correct syntax is -l app=web,env=prod. Options A, C, and D may be possible but the question focuses on syntax error.

🧠 Conceptual

advanced

Label selector operators in kubectl

Which label selector operator filters pods that have label env but its value is NOT prod?

A kubectl get pods -l 'env!=prod'

B kubectl get pods -l 'env=prod'

C kubectl get pods -l 'env in (prod)'

D kubectl get pods -l 'env notin (prod)'

💡 Hint

Use the != operator to exclude a specific label value.

Option A uses the != operator to select pods where env label is not prod. Option A uses notin which requires parentheses and is valid but the question asks for the operator that filters pods with label env but value not prod using simple syntax. Option A selects only prod. Option A selects only prod.

✅ Best Practice

expert

Efficiently watch pods with label changes

You want to watch pods with label app=web and get notified immediately when pods are added or removed. Which command is best?

A kubectl logs -l app=web --follow

B kubectl describe pods -l app=web

C kubectl get pods -l app=web --watch-only

D kubectl get pods -l app=web --watch

💡 Hint

Use the watch flag to continuously monitor resource changes.

Option D uses --watch to continuously show pod changes matching the label. Option D only shows current details once. Option D is invalid flag. Option D follows logs from matching pods but does not watch for pods being added or removed.

Practice

1. What does the -l option do in the kubectl get command?

easy

A. Filters resources based on their labels

B. Lists all namespaces in the cluster

C. Deletes resources matching a label

D. Updates the labels of a resource

Solution

1. Step 1: Understand the purpose of -l in kubectl get

   The -l option is used to specify a label selector to filter resources.

2. Step 2: Identify what filtering means in this context

   Filtering means showing only resources that have the specified label(s).

3. Final Answer:

   Filters resources based on their labels -> Option A

4. Quick Check:

   -l filters by labels [OK]

Hint: Remember: -l means label filter in kubectl [OK]

Common Mistakes:

• Confusing -l with namespace filtering
• Thinking -l deletes resources
• Assuming -l updates labels

2. Which of the following is the correct syntax to list pods with the label app=frontend?

easy

A. kubectl get pods -l app=frontend

B. kubectl get pods --label app=frontend

C. kubectl get pods -label app=frontend

D. kubectl get pods -filter app=frontend

Solution

1. Step 1: Recall the correct flag for label filtering

   The correct flag is -l or --selector, not --label or -filter.

2. Step 2: Check the syntax for label selector

   The syntax is -l key=value, so -l app=frontend is correct.

3. Final Answer:

   kubectl get pods -l app=frontend -> Option A

4. Quick Check:

   Use -l key=value for label filtering [OK]

Hint: Use -l key=value to filter by label [OK]

Common Mistakes:

• Using incorrect flags like --label or -filter
• Missing the equals sign between key and value
• Using spaces instead of equals sign

3. Given these pods with labels:
pod1: app=frontend, env=prod
pod2: app=frontend, env=dev
pod3: app=backend, env=prod
What is the output of kubectl get pods -l 'app=frontend,env=prod' -o name?

medium

A. pod/pod3

B. pod/pod1

C. pod/pod2

D. pod/pod1
pod/pod2

Solution

1. Step 1: Understand label selector with multiple labels

   The selector app=frontend,env=prod means pods must have both labels with these exact values.

2. Step 2: Match pods with both labels

   pod1 has app=frontend and env=prod, so it matches. pod2 has env=dev, so no. pod3 has app=backend, so no.

3. Final Answer:

   pod/pod1 -> Option B

4. Quick Check:

   Multiple labels require all to match [OK]

Hint: Multiple labels with commas mean AND condition [OK]

Common Mistakes:

• Assuming comma means OR instead of AND
• Ignoring one of the labels in the selector
• Expecting multiple pods when only one matches all labels

4. You run kubectl get pods -l app=frontend env=prod but get an error. What is the mistake?

medium

A. Labels must be quoted

B. Using wrong resource type

C. Missing comma between labels in the selector

D. kubectl does not support multiple labels

Solution

1. Step 1: Analyze the label selector syntax

   The command uses -l app=frontend env=prod without a comma between labels, which is invalid syntax.

2. Step 2: Correct syntax requires comma separation

   Labels must be separated by commas like -l app=frontend,env=prod to work properly.

3. Final Answer:

   Missing comma between labels in the selector -> Option C

4. Quick Check:

   Separate multiple labels with commas [OK]

Hint: Separate multiple labels with commas, no spaces [OK]

Common Mistakes:

• Forgetting commas between multiple labels
• Adding spaces instead of commas
• Quoting labels unnecessarily

5. You want to list all pods that have label app=frontend but exclude those with label env=dev. Which command achieves this?

hard

A. kubectl get pods -l 'app=frontend' -l 'env!=dev'

B. kubectl get pods -l 'app=frontend' --exclude-label env=dev

C. kubectl get pods -l 'app=frontend' | grep -v env=dev

D. kubectl get pods -l 'app=frontend,env!=dev'

Solution

1. Step 1: Understand label selector for exclusion

   To exclude pods with a label, use key!=value syntax inside the selector.

2. Step 2: Combine inclusion and exclusion in one selector

   Use -l 'app=frontend,env!=dev' to include pods with app=frontend and exclude those with env=dev.

3. Final Answer:

   kubectl get pods -l 'app=frontend,env!=dev' -> Option D

4. Quick Check:

   Use key!=value to exclude labels [OK]

Hint: Use key!=value to exclude labels in selector [OK]

Common Mistakes:

• Trying to use non-existent flags like --exclude-label
• Using multiple -l flags instead of one combined selector
• Using grep which is less efficient and error-prone