Writing Jenkinsfile Pipelines

# CHAPTER 6

Writing Jenkinsfile Pipelines #

1. Introduction #

2. Learning Objectives #

• Define and use Global Environment Variables in a Jenkinsfile.

• Execute Linux shell commands (sh) and Windows batch commands (bat).

• Use the post block to handle Success, Failure, and Always conditions.

• Understand how to structure a professional, multi-stage pipeline.

3. Beginner-Friendly Explanation #

• Environment Variables: Before you start, you agree on some rules. CARCOLOR = Red, WHEELSIZE = Large. You write this on a whiteboard. Every builder can look at the whiteboard instead of asking.

• The sh Step: This is the actual physical action. "Snap the red brick onto the base." In Jenkins, this is where you type actual Linux terminal commands.

• The post Block (Cleanup): When the car is finished, regardless of whether it looks good or if you broke a piece, you *always* sweep the floor and put the extra pieces away. In Jenkins, the post block ensures cleanup and notifications happen, no matter what went wrong.

4. Executing Commands (sh and bat) #

steps {
    // Run a single Linux command
    sh 'npm install'
    
    // Run multiple Linux commands
    sh '''
        echo "Building the application"
        zip -r app.zip ./src
    '''
}

5. Environment Variables #

pipeline {
    agent any
    environment {
        // Define a variable
        APP_VERSION = '1.0.4'
        DEPLOY_SERVER = 'staging.example.com'
    }
    stages {
        stage('Deploy') {
            steps {
                // Use the variable using ${} syntax
                echo "Deploying version ${APP_VERSION} to ${DEPLOY_SERVER}"
                sh "scp app.zip user@${DEPLOY_SERVER}:/var/www/html"
            }
        }
    }
}

6. Mini Project: Build a Resilient Multi-Stage Pipeline #

Step-by-Step Walkthrough: Update your Jenkinsfile with the following code. Note the new post section at the very end.

pipeline {
    agent any
    environment {
        PROJECT_NAME = 'MyAwesomePHPApp'
    }
    stages {
        stage('Build') {
            steps {
                echo "Starting build for ${PROJECT_NAME}..."
                sh 'mkdir -p build_output'
                sh 'echo "Simulated compiled code" > build_output/app.bin'
            }
        }
        stage('Test') {
            steps {
                echo 'Running tests...'
                // If this command fails, the pipeline stops here!
                sh 'ls -la build_output/app.bin' 
            }
        }
    }
    // The Post block runs after all stages finish
    post {
        always {
            echo 'Sweeping the floor. Deleting temporary build files.'
            sh 'rm -rf build_output'
        }
        success {
            echo 'Build succeeded! Sending Slack notification...'
        }
        failure {
            echo 'Build failed! Sending angry email to developers...'
        }
    }
}

7. Real-World Scenarios #

• Use Single Quotes vs Double Quotes: In Groovy/Jenkins syntax, use single quotes ('text') for static text. You MUST use double quotes ("text") if you want to inject an environment variable (e.g., "Deploying ${APPVERSION}"). If you use single quotes, it will print the literal dollar sign.

9. Security Recommendations #

• Do not define passwords in the Environment block: Never write DBPASSWORD = 'password123' in the environment block of a Jenkinsfile. This file is saved in Git in plaintext for anyone to read. We will learn how to handle secure credentials in Chapter 14.

• Q: A developer complains that their environment variable $APIKEY is evaluating literally as the string "$API_KEY" instead of the actual key value. What Groovy syntax error is the developer making?

14. Summary #

15. Next Chapter Recommendation #