Pull request builds in Jenkins - Step-by-Step Execution

Process Flow - Pull request builds

Developer creates PR

↓

PR triggers Jenkins build

↓

Jenkins checks out PR code

↓

Run build and tests

↓

Build success?

No→Report failure to PR

Yes↓

Report success to PR

↓

PR can be merged

This flow shows how a pull request triggers a Jenkins build, runs tests, and reports results back to the PR.

Execution Sample

Jenkins

pipeline {
  agent any
  triggers {
    githubPullRequest()
  }
  stages {
    stage('Build and Test') {
      steps {
        sh 'make test'
      }
    }
  }
}

A Jenkins pipeline triggered by pull requests that runs tests on the PR code.

Process Table

Step	Action	Evaluation	Result
1	PR created in GitHub	PR event detected	Trigger Jenkins build
2	Jenkins checks out PR branch	Checkout success	Ready to build
3	Run 'make test'	Tests pass?	Yes
4	Report build status	Send success to GitHub	PR marked as successful
5	PR merge allowed	No conflicts, tests passed	PR can be merged
6	If tests fail	Report failure to GitHub	PR marked as failed, merge blocked

💡 Build ends after reporting status to GitHub and allowing or blocking merge.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
PR Status	Open	Checked out	Tests passed	Success reported	Merge allowed
Build Status	Not started	Running	Passed	Reported	Complete

Key Moments - 3 Insights

Why does Jenkins need to check out the PR branch instead of the main branch?

Jenkins checks out the PR branch to test the exact code changes in the pull request, as shown in step 2 of the execution table.

What happens if the tests fail during the build?

If tests fail (step 6), Jenkins reports failure back to GitHub and blocks the PR from merging, preventing broken code from entering main.

How does Jenkins know when to trigger a build for a pull request?

Jenkins listens for pull request events from GitHub (step 1), which triggers the build automatically.

Visual Quiz - 3 Questions

Look at the execution table, what is the build status after step 3?

A Failed

B Passed

C Not started

D Reported

💡 Hint

Check the 'Build Status' variable after step 3 in the variable_tracker.

At which step does Jenkins report the build result back to GitHub?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for the action 'Report build status' in the execution_table.

If the tests fail, what is the effect on the pull request according to the execution flow?

A PR is marked as failed and merge is blocked

B Build is retried automatically

C PR is merged anyway

D Nothing happens

💡 Hint

Refer to step 6 in the execution_table and key_moments about test failure.

Concept Snapshot

Pull request builds trigger Jenkins to test PR code.
Jenkins checks out the PR branch, runs tests.
Build results are reported back to GitHub.
Success allows merge; failure blocks it.
Automates quality checks before merging code.

Full Transcript

When a developer creates a pull request, Jenkins detects this event and triggers a build. Jenkins then checks out the pull request branch to get the exact code changes. It runs the build and tests, such as 'make test'. If the tests pass, Jenkins reports success back to GitHub, marking the PR as successful and allowing it to be merged. If tests fail, Jenkins reports failure and blocks the merge to keep the main branch stable. This process automates quality checks on code changes before they are merged.