Pull request builds in Jenkins - Commands & Configuration

Introduction

When developers create a pull request, it is important to automatically test the changes before merging. Pull request builds run tests on the new code to catch errors early and keep the main project stable.

When you want to check if new code changes break the build before merging them.

When multiple developers work on the same project and you want to ensure quality.

When you want to automate testing for every pull request to save manual effort.

When you want to prevent broken code from entering the main branch.

When you want to provide feedback to developers quickly about their code changes.

Config File - Jenkinsfile

Jenkinsfile

pipeline {
  agent any
  triggers {
    githubPullRequest()
  }
  stages {
    stage('Build') {
      steps {
        echo 'Building the project...'
        // Add build commands here
      }
    }
    stage('Test') {
      steps {
        echo 'Running tests...'
        // Add test commands here
      }
    }
  }
}

This Jenkinsfile defines a pipeline that runs on any available agent.

The triggers section uses githubPullRequest() to start the build automatically when a pull request is created or updated.

There are two stages: Build and Test. Each stage runs commands to build and test the code changes.

Commands

This command updates or creates the Jenkins job that will run pull request builds based on the job configuration file.

Terminal

jenkins-jobs --conf jenkins.ini update my-pull-request-job.yaml

Expected Output

Job 'my-pull-request-job' updated successfully.

This command triggers the pull request build manually using a token for authentication.

Terminal

curl -X POST http://jenkins.example.com/job/my-pull-request-job/build?token=pr-build-token

Expected Output

Started build #15

This command fetches the console output of build number 15 to check the build and test results.

Terminal

jenkins-cli -s http://jenkins.example.com console my-pull-request-job 15

Expected Output

[Pipeline] Start of Pipeline

[Pipeline] echo

Building the project...

[Pipeline] echo

Running tests...

[Pipeline] End of Pipeline

Finished: SUCCESS

Key Concept

If you remember nothing else from this pattern, remember: pull request builds automatically test code changes before merging to keep the main project stable.

Common Mistakes

Not setting up the trigger for pull request events in Jenkins.

Without the trigger, builds won't start automatically when a pull request is created or updated.

Add the appropriate trigger like githubPullRequest() in the Jenkinsfile or configure webhook triggers in Jenkins.

Forgetting to add test steps in the pipeline stages.

If tests are not run, errors in the new code can go unnoticed and break the main branch.

Include test commands in the Test stage to verify code correctness.

Triggering builds without authentication tokens when required.

Build triggers may fail if Jenkins requires tokens or credentials for security.

Use the correct token or authentication method when triggering builds via API.

Summary

Create a Jenkins pipeline with triggers to run on pull request events.

Use stages to build and test the code changes automatically.

Trigger and monitor builds to ensure code quality before merging.

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