Using Environment Variables in Jenkinsfile

Environment variables in Jenkinsfiles help manage configurations dynamically across pipelines. Below are different ways to define and use them, along with key differences:

---

1. Environment Variable Block (Global Scope)

Defined at the pipeline top level, these variables are available across all stages.

Example:

pipeline {
    agent any
    
    environment {
        GLOBAL_VAR = "value"          // Static value
        BUILD_NUM = "${env.BUILD_ID}" // Jenkins built-in variable
        CREDS = credentials('aws-key') // Secret credential (masked)
    }
    
    stages {
        stage('Example') {
            steps {
                echo "Global var: ${env.GLOBAL_VAR}"
                echo "Build ID: ${env.BUILD_NUM}"
            }
        }
    }
}

Key Points:

✅ Available in all stages.
✅ Supports credentials masking (e.g., credentials()).
❌ Cannot be modified after declaration (immutable in environment block).

---

2. Environment Variable per Stage (Stage-Specific)

Variables defined inside a stage are only available in that stage.

Example:

pipeline {
    agent any
    
    stages {
        stage('Stage 1') {
            environment {
                STAGE_VAR = "stage1_value"
            }
            steps {
                echo "Stage var: ${env.STAGE_VAR}"
            }
        }
        
        stage('Stage 2') {
            steps {
                // This will FAIL! STAGE_VAR is not accessible here.
                echo "Trying to access: ${env.STAGE_VAR}"
            }
        }
    }
}

Key Points:

✅ Useful for stage-specific configs (e.g., test env vs. prod env).
❌ Not shared across stages.

---

3. Reusing a Variable Across Stages

To pass a variable from one stage to another, you must:

• Use script { } block + env.VAR_NAME (for Jenkins environment variables).
• Or use def (temporary variable) but it won’t persist across stages.

Example:

pipeline {
    agent any
    
    stages {
        stage('Stage A') {
            steps {
                script {
                    env.SHARED_VAR = "data"  // Available in all future stages
                    def tempVar = "temp"     // Only available in this script block
                }
            }
        }
        
        stage('Stage B') {
            steps {
                echo "Shared var: ${env.SHARED_VAR}"  // Works!
                echo "Temp var: ${tempVar}"          // Fails (not defined)
            }
        }
    }
}

Key Points:

✅ env.VAR_NAME persists globally.
❌ def variables are local to the script block.

---

4. def vs environment Block Variables

Feature	environment { VAR = ... }	env.VAR = ... (script)	def var = ...
Scope	Global (all stages)	Global (if using env.)	Local (script block only)
Modifiable	❌ No (immutable)	✅ Yes	✅ Yes
Credentials	✅ Supports credentials()	✅ Yes	❌ No
Built-in Jenkins vars	✅ ${env.BUILD_ID}	✅ ${env.BUILD_ID}	❌ No

When to Use Which?

• environment { } → Best for secrets & global constants.
• env.VAR = ... → Best for dynamic values shared across stages.
• def → Best for temporary calculations (not reusable later).

---

Summary Cheat Sheet

Use Case	Recommended Approach
Global config (all stages)	environment { } block
Stage-specific config	environment { } inside stage
Dynamic vars across stages	script { env.VAR = ... }
Temporary variables (math, loops)	def var = ...