Jenkinsfile & Pipeline Structure

Freestyle vs Pipeline

Freestyle jobs are great for learning. In real-world projects you use Pipelines — CI/CD logic in a Jenkinsfile at the root of (or path inside) your repository.

What is a Jenkinsfile?

A Jenkinsfile is a text file written in a Groovy-based DSL that defines your entire pipeline. Because it lives in the repo, pipeline changes are tracked in Git like any other change.

Two syntax styles:

• Declarative (recommended) — structured, easier to read
• Scripted — more flexible, raw Groovy

This course uses Declarative.

Jenkinsfile structure

pipeline {
  agent any
 
  environment {
    APP_NAME = "my-app"
  }
 
  stages {
    stage('Build') {
      steps {
        sh 'npm install'
      }
    }
  }
 
  post {
    always {
      echo 'Cleanup or notify'
    }
  }
}

pipeline

Top-level block for a Declarative pipeline. Everything else nests inside it.

agent

Where the pipeline (or a stage) runs:

agent any                          // any available node
agent none                         // each stage picks its own agent
agent { label 'linux' }            // node with label
agent {
  docker { image 'node:18-alpine' } // clean Node environment per build
}

environment

Variables available globally or per stage — including credentials bindings (Day 3–4).

stages / stage

Logical phases: Checkout, Install, Test, Build, Deploy. Stages run sequentially by default. If one fails, Jenkins stops and marks the build failed.

Parallel example:

stage('Parallel Tests') {
  parallel {
    stage('Unit') {
      steps { sh 'npm run test:unit' }
    }
    stage('Integration') {
      steps { sh 'npm run test:integration' }
    }
  }
}

steps

Commands that actually run:

steps {
  sh 'npm install'
  echo 'Installing...'
  checkout scm
  script {
    def out = sh(script: 'npm test', returnStdout: true)
    echo out.trim()
  }
}

sh '''
  npm install
  npm test
'''

post

Runs after the pipeline (or stage) finishes:

post {
  always  { echo 'Always runs' }
  success { echo 'Only on success' }
  failure { echo 'Only on failure' }
  unstable { echo 'Tests unstable' }
  changed { echo 'Result changed vs previous build' }
}

Next: Project 2 puts this structure into a real Node.js Express pipeline with PM2 deploy and email.