🚀 Day 10 of 30 Days of DevOps Interview Prep – Jenkins Git Trigger + Build/Test/Deploy Pipeline

Series: 30 Days DevOps Interview Preparation

Today’s topic is one of the most important real-world DevOps skills:
✅ Triggering Jenkins builds from Git
✅ Creating a multi-stage CI/CD pipeline (Build → Test → Deploy)
✅ Running everything on AWS EC2

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📖 Understanding Jenkins Git Triggers

Jenkins can automatically start a build when you push code to a Git repository.
This is achieved using SCM Polling or Webhooks.

• SCM Polling → Jenkins checks your repo periodically for changes.

• Webhook → GitHub/GitLab sends a push notification to Jenkins instantly (recommended for faster pipelines).

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🛠 AWS Practical – Setting up Jenkins with Git Trigger and Build/Test/Deploy

Step 1 – Launch AWS EC2 Instance

• AMI: Ubuntu 22.04 (or Amazon Linux 2)

• Instance Type: t2.micro (for testing)

• Security Group: Allow:

  • Port 8080 → Jenkins UI

  • Port 22 → SSH

  • App port (e.g., 8081 for deployment)

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Step 2 – Install Jenkins

sudo apt update
sudo apt install openjdk-17-jdk -y

curl -fsSL https://pkg.jenkins.io/debian/jenkins.io.key | sudo tee \
    /usr/share/keyrings/jenkins-keyring.asc > /dev/null

echo deb [signed-by=/usr/share/keyrings/jenkins-keyring.asc] \
    https://pkg.jenkins.io/debian binary/ | sudo tee \
    /etc/apt/sources.list.d/jenkins.list > /dev/null

sudo apt update
sudo apt install jenkins -y
sudo systemctl enable jenkins
sudo systemctl start jenkins

Unlock Jenkins:

sudo cat /var/lib/jenkins/secrets/initialAdminPassword

Login at: http://<EC2-Public-IP>:8080

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Step 3 – Install Maven & Git

sudo apt install maven git -y

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Step 4 – Prepare a Sample Java Application

git clone https://github.com/spring-projects/spring-petclinic.git
cd spring-petclinic
mvn clean package

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Step 5 – Configure Jenkins Job

1. Go to Jenkins Dashboard → New Item

2. Select Pipeline → Name it GitTriggeredPipeline

3. Under Build Triggers → Select:

   • ✅ GitHub hook trigger for GITScm polling

4. Under Pipeline → Select Pipeline script from SCM

5. SCM → Git → Enter repo URL

6. Script Path → Jenkinsfile

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Step 6 – Add Webhook in GitHub

• Go to your GitHub repo → Settings → Webhooks → Add webhook

• Payload URL:

    http://<EC2-Public-IP>:8080/github-webhook/
  

• Content type: application/json

• Select: Just the push event

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📜 Tested Runnable Jenkinsfile

pipeline {
    agent any

    stages {
        stage('Build') {
            steps {
                sh 'mvn clean package'
            }
        }
        stage('Test') {
            steps {
                sh 'mvn test'
            }
        }
        stage('Deploy') {
            steps {
                sh '''
                echo "Deploying to AWS EC2..."
                scp -o StrictHostKeyChecking=no -i /home/ubuntu/my-key.pem \
                target/*.jar ubuntu@<DEPLOY_SERVER_IP>:/home/ubuntu/app.jar
                ssh -o StrictHostKeyChecking=no -i /home/ubuntu/my-key.pem \
                ubuntu@<DEPLOY_SERVER_IP> "nohup java -jar /home/ubuntu/app.jar > app.log 2>&1 &"
                '''
            }
        }
    }
}

Make sure:

• Replace /home/ubuntu/my-key.pem with your SSH key path.

• Replace <DEPLOY_SERVER_IP> with the server where you want to deploy.

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💡 Common Interview Questions

1️⃣ How do you configure a Git webhook in Jenkins?
→ Set up a webhook in your Git repo pointing to http://<JENKINS_URL>/github-webhook/ and enable the GitHub trigger in the job.

2️⃣ Difference between Poll SCM and Webhook?
→ Poll SCM checks periodically → slower. Webhook pushes changes instantly → faster.

3️⃣ How do you pass artifacts between stages in Jenkins?
→ Use stash/unstash or archive artifacts with archiveArtifacts.

4️⃣ How do you deploy securely from Jenkins?
→ Store credentials in Jenkins Credentials Manager, never hardcode keys in Jenkinsfile.

5️⃣ How do you rollback a deployment in Jenkins?
→ Keep previous build artifacts stored and redeploy them on rollback.

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✅ Key Takeaways

• Webhooks = instant builds, faster feedback.

• Separate Build, Test, Deploy stages = cleaner pipelines.

• AWS EC2 is a perfect low-cost environment to test Jenkins CI/CD.

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#DevOps #AWS #Jenkins #CI_CD #GitHub #Automation #LearningInPublic #InterviewPreparation #30DaysOfDevOps