Pull request builds in Jenkins - Mini Project: Build & Apply

Pull Request Builds with Jenkins

📖 Scenario: You work on a software project where developers submit code changes through pull requests on GitHub. To keep the project stable, you want Jenkins to automatically build and test each pull request before merging.

🎯 Goal: Set up a Jenkins pipeline that triggers a build whenever a pull request is created or updated on GitHub. This helps catch errors early and ensures code quality.

📋 What You'll Learn

Create a Jenkinsfile with a pipeline definition

Configure the pipeline to trigger on pull request events

Add a simple build stage that prints the pull request ID

Print a confirmation message when the build runs

💡 Why This Matters

🌍 Real World

Many teams use Jenkins to automatically build and test code changes submitted via pull requests to catch errors early and maintain code quality.

💼 Career

Understanding pull request builds is essential for DevOps engineers to automate CI/CD pipelines and improve collaboration between developers.

1

Create Jenkinsfile with pipeline block

Create a Jenkinsfile with a pipeline block that defines the pipeline structure.

Jenkins

pipeline {
    // Your code here
}

Hint

The Jenkinsfile must start with the pipeline keyword followed by curly braces.

2

Add triggers for pull request events

Inside the pipeline block, add a triggers section with githubPullRequest() to trigger builds on pull request events.

Jenkins

pipeline {
    triggers {
        // Your code here
    }
}

Hint

Use the triggers block and add githubPullRequest() inside it.

3

Add a build stage to print pull request ID

Inside the pipeline block, add a stages section with one stage named Build. In the steps, add a script block that prints the pull request ID using env.CHANGE_ID.

Jenkins

pipeline {
    triggers {
        githubPullRequest()
    }
    stages {
        stage('Build') {
            steps {
                script {
                    // Your code here
                }
            }
        }
    }
}

Hint

Use echo inside a script block to print the pull request ID from env.CHANGE_ID.

4

Print confirmation message when build runs

Add a post section inside the pipeline block with a always block that prints Build completed for pull request ${env.CHANGE_ID}.

Jenkins

pipeline {
    triggers {
        githubPullRequest()
    }
    stages {
        stage('Build') {
            steps {
                script {
                    echo "Building pull request ID: ${env.CHANGE_ID}"
                }
            }
        }
    }
    post {
        always {
            // Your code here
        }
    }
}

Hint

The post block runs after the pipeline. Use always to print the confirmation message.

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