Jenkins CI/CD: Automate Your Builds Without Losing Your Mind

// io.thecodeforge — DevOps tutorial

pipeline {
    agent any  // Run on any available agent
    stages {
        stage('Build') {
            steps {
                echo 'Compiling code...'
                // In a real project, you'd run: sh 'mvn compile'
            }
        }
        stage('Test') {
            steps {
                echo 'Running unit tests...'
                // sh 'mvn test'
            }
        }
        stage('Deploy') {
            steps {
                echo 'Deploying to staging...'
                // sh 'scp target/app.jar user@staging:/opt/app/'
            }
        }
    }
}

// io.thecodeforge — DevOps tutorial

# On the agent machine, download and run the agent JAR
# Replace JENKINS_URL and SECRET with actual values
wget http://JENKINS_URL/jnlpJars/agent.jar
java -jar agent.jar -jnlpUrl http://JENKINS_URL/computer/agent-name/slave-agent.jnlp \
    -secret YOUR_SECRET \
    -workDir /home/jenkins/agent

# To run as a service, use systemd:
# /etc/systemd/system/jenkins-agent.service
[Unit]
Description=Jenkins Agent
After=network.target

[Service]
User=jenkins
ExecStart=/usr/bin/java -Xmx2g -jar /home/jenkins/agent.jar -jnlpUrl http://JENKINS_URL/computer/agent-name/slave-agent.jnlp -secret YOUR_SECRET -workDir /home/jenkins/agent
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

// io.thecodeforge — DevOps tutorial

# Option 1: Docker (quick start)
docker run -d --name jenkins \
    -p 8080:8080 -p 50000:50000 \
    -v jenkins_home:/var/jenkins_home \
    jenkins/jenkins:lts-jdk11

# Get initial admin password
docker exec jenkins cat /var/jenkins_home/secrets/initialAdminPassword

# Option 2: Native install on Ubuntu
wget -q -O - https://pkg.jenkins.io/debian-stable/jenkins.io.key | sudo apt-key add -
sudo sh -c 'echo deb https://pkg.jenkins.io/debian-stable binary/ > /etc/apt/sources.list.d/jenkins.list'
sudo apt-get update
sudo apt-get install -y jenkins
sudo systemctl start jenkins
sudo systemctl enable jenkins

# Initial password
sudo cat /var/lib/jenkins/secrets/initialAdminPassword

// io.thecodeforge — DevOps tutorial

pipeline {
    agent { label 'linux' }  // Only run on Linux agents
    tools {
        maven 'Maven-3.8'  // Use a pre-configured Maven installation
    }
    environment {
        APP_NAME = 'checkout-service'
        STAGING_SERVER = 'staging.example.com'
    }
    stages {
        stage('Checkout') {
            steps {
                checkout scm  // Checkout from Git (SCM configured in job)
            }
        }
        stage('Build') {
            steps {
                sh 'mvn clean compile'  // Compile source code
            }
        }
        stage('Test') {
            steps {
                sh 'mvn test'  // Run unit tests
            }
            post {
                failure {
                    // If tests fail, send email and stop pipeline
                    emailext(
                        subject: "${env.JOB_NAME} - Build #${env.BUILD_NUMBER} - Test Failure",
                        body: "Tests failed. Check ${env.BUILD_URL}",
                        to: 'team@example.com'
                    )
                }
            }
        }
        stage('Package') {
            steps {
                sh 'mvn package -DskipTests'  // Package JAR (tests already run)
                archiveArtifacts artifacts: 'target/*.jar', fingerprint: true
            }
        }
        stage('Deploy to Staging') {
            steps {
                sh "scp target/${APP_NAME}-*.jar jenkins@${STAGING_SERVER}:/opt/app/"
                sh "ssh jenkins@${STAGING_SERVER} 'sudo systemctl restart ${APP_NAME}'"
            }
        }
    }
    post {
        always {
            cleanWs()  // Clean workspace after build
        }
        success {
            emailext(
                subject: "${env.JOB_NAME} - Build #${env.BUILD_NUMBER} - Success",
                body: "Deployed successfully to staging.",
                to: 'team@example.com'
            )
        }
        failure {
            emailext(
                subject: "${env.JOB_NAME} - Build #${env.BUILD_NUMBER} - Failed",
                body: "Build failed. Check ${env.BUILD_URL}",
                to: 'team@example.com'
            )
        }
    }
}

// io.thecodeforge — DevOps tutorial

# Essential plugins (install via UI or CLI):
- git
- workflow-aggregator (Pipeline)
- blueocean
- credentials-binding
- email-ext
- docker-workflow
- junit (for test reports)

# Plugins to avoid:
- build-pipeline-plugin (use Blue Ocean instead)
- dashboard-view (bloated)
- any plugin that hasn't been updated in 2+ years

# CLI to install plugins:
java -jar jenkins-cli.jar -s http://localhost:8080/ install-plugin git workflow-aggregator blueocean credentials-binding email-ext docker-workflow junit -restart

// io.thecodeforge — DevOps tutorial

// Declarative pipeline (recommended)
pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                echo 'Building...'
            }
        }
    }
}

// Scripted pipeline (use only when needed)
node {
    stage('Build') {
        echo 'Building...'
    }
    // Can use loops, conditionals, etc.
    def branches = ['linux', 'windows']
    for (branch in branches) {
        stage("Build on ${branch}") {
            echo "Building on ${branch}"
        }
    }
}

// io.thecodeforge — DevOps tutorial

pipeline {
    agent any
    triggers {
        // Webhook: Jenkins will listen for POST from GitHub
        // Configure in GitHub: Settings > Webhooks > Add webhook
        // Payload URL: http://jenkins:8080/github-webhook/
        
        // Poll SCM every 5 minutes (fallback)
        pollSCM('H/5 * * * *')
        
        // Scheduled nightly build at 2 AM
        cron('0 2 * * *')
    }
    stages {
        stage('Build') {
            steps {
                echo 'Triggered by: ' + env.BUILD_CAUSE
            }
        }
    }
}

// io.thecodeforge — DevOps tutorial

pipeline {
    agent any
    environment {
        // Reference a credential ID stored in Jenkins
        // The value is masked in logs
        DOCKER_PASSWORD = credentials('docker-hub-password')
    }
    stages {
        stage('Login to Docker Hub') {
            steps {
                withCredentials([string(credentialsId: 'docker-hub-username', variable: 'DOCKER_USER')]) {
                    sh '''
                        echo $DOCKER_PASSWORD | docker login -u $DOCKER_USER --password-stdin
                    '''
                }
            }
        }
        stage('Deploy with SSH Key') {
            steps {
                withCredentials([sshUserPrivateKey(
                    credentialsId: 'staging-server-key',
                    keyFileVariable: 'SSH_KEY',
                    usernameVariable: 'SSH_USER'
                )]) {
                    sh 'ssh -i $SSH_KEY -o StrictHostKeyChecking=no $SSH_USER@staging.example.com "sudo systemctl restart app"'
                }
            }
        }
    }
}

// io.thecodeforge — DevOps tutorial

pipeline {
    agent {
        docker {
            image 'maven:3.8-openjdk-11'  // Use a Maven image
            args '-v /tmp:/tmp'  // Mount host temp directory
            reuseNode true  // Reuse the same workspace across stages
        }
    }
    stages {
        stage('Build') {
            steps {
                sh 'mvn clean compile'
            }
        }
        stage('Test') {
            steps {
                sh 'mvn test'
            }
        }
    }
}

// io.thecodeforge — DevOps tutorial

# Check JVM heap usage via JMX (if enabled)
jstat -gc $(pgrep -f jenkins) 2>/dev/null | awk '{print "Heap usage: " $3+$4+$6+$8 " KB"}'

# Check disk usage for JENKINS_HOME
df -h /var/lib/jenkins

# Check queue length via Jenkins API
curl -s http://jenkins:8080/queue/api/json | jq '.items | length'

# Set log rotation via Jenkins CLI
java -jar jenkins-cli.jar -s http://localhost:8080/ set-build-discarder \
    --job my-job \
    --strategy count-based \
    --days-to-keep 30 \
    --num-to-keep 100