Pull request builds in Jenkins - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pull request build in Jenkins?

A pull request build is a Jenkins job that automatically runs tests and checks on code changes submitted via a pull request before merging into the main branch.

beginner

Why are pull request builds important?

They help catch errors early by testing code changes before merging, ensuring the main codebase stays stable and reducing bugs in production.

intermediate

Which Jenkins plugin is commonly used to trigger builds on pull requests?

The 'GitHub Branch Source' plugin is commonly used to detect pull requests and trigger Jenkins builds automatically.

intermediate

How does Jenkins know which pull request to build?

Jenkins listens to webhook events from the source code repository (like GitHub) that notify it when a pull request is created or updated.

beginner

What is a common step in a pull request build pipeline?

Common steps include checking out the pull request code, running automated tests, performing static code analysis, and reporting results back to the pull request.

What triggers a pull request build in Jenkins?

A Scheduled daily at midnight

B Manual start by a developer only

C A webhook event from the source code repository

D When the main branch is updated

Jenkins uses webhook events from the repository to automatically trigger builds when pull requests are created or updated.

Which plugin helps Jenkins detect pull requests from GitHub?

A Email Extension

B GitHub Branch Source

C Docker Plugin

D Pipeline Utility Steps

The GitHub Branch Source plugin enables Jenkins to detect branches and pull requests from GitHub repositories.

What is the main benefit of running pull request builds?

A To test code changes before merging

B To deploy code to production

C To delete old branches

D To increase build time

Pull request builds test code changes early to prevent broken code from entering the main branch.

What does Jenkins typically do after a pull request build finishes?

A Reports the build status back to the pull request

B Deletes the pull request automatically

C Merges the pull request without review

D Sends an email to all users

Jenkins reports the build results back to the pull request so developers can see if tests passed or failed.

Which of these is NOT usually part of a pull request build pipeline?

A Running automated tests

B Checking out pull request code

C Static code analysis

D Manual code review by Jenkins

Manual code review is done by humans, not Jenkins. Jenkins automates testing and analysis.

Explain how Jenkins integrates with GitHub to run pull request builds.

Think about how Jenkins knows when a pull request is created or updated.

Describe the key steps in a typical pull request build pipeline in Jenkins.

Focus on what Jenkins does from start to finish during the build.

Practice

1. What is the main purpose of pull request builds in Jenkins?

easy

A. To automatically test code changes before merging

B. To deploy code directly to production

C. To delete old branches after merging

D. To manually review code without testing

Solution

1. Step 1: Understand pull request builds concept

   Pull request builds run tests automatically on code changes before merging to ensure quality.

2. Step 2: Compare options with purpose

   Only To automatically test code changes before merging describes automatic testing before merging, which is the main goal.

3. Final Answer:

   To automatically test code changes before merging -> Option A

4. Quick Check:

   Pull request builds = automatic pre-merge tests [OK]

Hint: Pull request builds test code before merging [OK]

Common Mistakes:

• Confusing deployment with testing
• Thinking pull request builds delete branches
• Assuming manual review replaces automated tests

2. Which Jenkins pipeline syntax correctly triggers a build on a GitHub pull request?

easy

A. triggers { githubPush() }

B. triggers { cron('H/5 * * * *') }

C. triggers { githubPullRequest() }

D. triggers { pollSCM('H/15 * * * *') }

Solution

1. Step 1: Identify trigger for pull request builds

   The correct trigger for GitHub pull request builds in Jenkins is githubPullRequest().

2. Step 2: Eliminate other triggers

   githubPush() triggers on push events, cron and pollSCM are time-based triggers, not pull request triggers.

3. Final Answer:

   triggers { githubPullRequest() } -> Option C

4. Quick Check:

   Pull request trigger = githubPullRequest() [OK]

Hint: Use githubPullRequest() for PR triggers [OK]

Common Mistakes:

• Using githubPush() instead of githubPullRequest()
• Confusing time-based triggers with PR triggers
• Omitting triggers block entirely

3. Given this Jenkinsfile snippet, what happens when a pull request is opened?

pipeline {
  agent any
  triggers {
    githubPullRequest()
  }
  stages {
    stage('Build') {
      steps {
        echo 'Building...'
      }
    }
  }
}

medium

A. The pipeline waits for manual approval before running

B. The pipeline runs automatically and prints 'Building...'

C. The pipeline runs only on push events, not pull requests

D. The pipeline fails due to syntax error

Solution

1. Step 1: Analyze trigger block

   The trigger githubPullRequest() starts the pipeline automatically on pull request events.

2. Step 2: Check pipeline steps

   The pipeline has a 'Build' stage that echoes 'Building...'. No manual approval or syntax errors are present.

3. Final Answer:

   The pipeline runs automatically and prints 'Building...' -> Option B

4. Quick Check:

   githubPullRequest() triggers automatic build [OK]

Hint: githubPullRequest() triggers auto build on PR [OK]

Common Mistakes:

• Assuming manual approval is needed
• Confusing push and pull request triggers
• Thinking syntax error exists in snippet

4. A Jenkins pipeline with githubPullRequest() trigger does not start when a pull request is created. What is the most likely cause?

medium

A. The GitHub webhook for pull requests is not configured

B. The Jenkinsfile has a syntax error in the stages block

C. The pipeline uses cron trigger instead

D. The pull request was merged before the build started

Solution

1. Step 1: Understand trigger requirements

   The githubPullRequest() trigger requires GitHub webhooks to notify Jenkins of PR events.

2. Step 2: Identify common failure reason

   If the webhook is missing or misconfigured, Jenkins won't start the build on PR creation.

3. Final Answer:

   The GitHub webhook for pull requests is not configured -> Option A

4. Quick Check:

   Missing webhook = no PR build trigger [OK]

Hint: Check GitHub webhook setup if PR builds don't start [OK]

Common Mistakes:

• Blaming syntax errors without checking webhooks
• Ignoring webhook configuration
• Assuming cron triggers handle PR events

5. You want Jenkins to build pull requests only from specific branches (e.g., 'feature/*') and skip others. Which configuration snippet achieves this?

hard

A. triggers { githubPullRequest { branchFilter { exclude 'feature/*' } } }

B. triggers { pollSCM { ignoreBranches 'feature/*' } }

C. triggers { githubPush { branch 'feature/*' } }

D. triggers { githubPullRequest { branchFilter { include 'feature/*' } } }

Solution

1. Step 1: Identify correct trigger and filter syntax

   The githubPullRequest trigger supports branchFilter with include to specify branches to build.

2. Step 2: Compare options for filtering

   triggers { githubPullRequest { branchFilter { include 'feature/*' } } } includes only 'feature/*' branches, which matches the requirement. triggers { githubPullRequest { branchFilter { exclude 'feature/*' } } } excludes them, which is opposite. Options C and D use wrong triggers or syntax.

3. Final Answer:

   triggers { githubPullRequest { branchFilter { include 'feature/*' } } } -> Option D

4. Quick Check:

   Use branchFilter include to limit PR builds [OK]

Hint: Use branchFilter include to build specific PR branches [OK]

Common Mistakes:

• Using exclude instead of include
• Using githubPush instead of githubPullRequest
• Trying to filter branches with pollSCM