Script blocks for Groovy in Jenkins - Mini Project: Build & Apply

Script blocks for Groovy in Jenkins

📖 Scenario: You are automating a Jenkins pipeline. You want to use Groovy script blocks to run custom code inside the pipeline.

🎯 Goal: Build a Jenkins pipeline script that uses Groovy script blocks to define variables, add configuration, and print output.

📋 What You'll Learn

Create a Groovy script block with a variable

Add a configuration variable inside the script block

Use a script block to run a loop

Print the final output from the script block

💡 Why This Matters

🌍 Real World

Jenkins pipelines often use Groovy <code>script</code> blocks to run custom code for build, test, and deployment automation.

💼 Career

Knowing how to write Groovy <code>script</code> blocks in Jenkins is essential for DevOps engineers and automation specialists to customize CI/CD pipelines.

1

Create a Groovy script block with a variable

Write a Jenkins pipeline script block that defines a variable called message with the value "Hello from Groovy".

Jenkins

pipeline {
    agent any
    stages {
        stage('Setup') {
            steps {
                script {
                    // Your code here
                }
            }
        }
    }
}

Hint

Use def message = "Hello from Groovy" inside the script block.

2

Add a configuration variable inside the script block

Inside the existing script block, add a variable called count and set it to 3.

Jenkins

pipeline {
    agent any
    stages {
        stage('Setup') {
            steps {
                script {
                    def message = "Hello from Groovy"
                    // Your code here
                }
            }
        }
    }
}

Hint

Define def count = 3 inside the script block.

3

Use a script block to run a loop

Inside the script block, write a for loop that runs from 1 to count and appends " Run " plus the loop number to a variable called result. Initialize result as an empty string before the loop.

Jenkins

pipeline {
    agent any
    stages {
        stage('Setup') {
            steps {
                script {
                    def message = "Hello from Groovy"
                    def count = 3
                    // Your code here
                }
            }
        }
    }
}

Hint

Initialize result as an empty string, then use a for loop with i from 1 to count. Append to result inside the loop.

4

Print the final output from the script block

Add a println statement inside the script block to print the combined message: the message variable followed by the result variable.

Jenkins

pipeline {
    agent any
    stages {
        stage('Setup') {
            steps {
                script {
                    def message = "Hello from Groovy"
                    def count = 3
                    def result = ""
                    for (int i = 1; i <= count; i++) {
                        result += " Run ${i}"
                    }
                    // Your code here
                }
            }
        }
    }
}

Hint

Use println(message + result) to print the combined string.

Practice

1. What is the main purpose of using script blocks in Jenkins pipelines?

easy

A. To define environment variables

B. To specify the agent where the pipeline runs

C. To run Groovy code for custom logic inside the pipeline

D. To declare pipeline stages

Solution

1. Step 1: Understand Jenkins pipeline structure

   Jenkins pipelines use declarative syntax but allow Groovy code inside script blocks for flexibility.

2. Step 2: Identify the role of script blocks

   script blocks let you write Groovy code for loops, conditions, and other logic not directly supported in declarative syntax.

3. Final Answer:

   To run Groovy code for custom logic inside the pipeline -> Option C

4. Quick Check:

   Script blocks = Groovy code inside pipeline [OK]

Hint: Script blocks run Groovy code inside steps [OK]

Common Mistakes:

• Confusing script blocks with environment or agent declarations
• Thinking script blocks define stages
• Assuming script blocks are optional for all logic

2. Which of the following is the correct way to start a script block inside a declarative Jenkins pipeline?

easy

A. script { echo 'Hello' }

B. script: { echo 'Hello' }

C. script() { echo 'Hello' }

D. script => { echo 'Hello' }

Solution

1. Step 1: Recall correct syntax for script blocks

   In declarative pipelines, script is a block keyword followed by curly braces containing Groovy code.

2. Step 2: Compare options

   Only script { ... } is valid Groovy block syntax. Others use invalid punctuation or keywords.

3. Final Answer:

   script { echo 'Hello' } -> Option A

4. Quick Check:

   script block syntax = script { ... } [OK]

Hint: Use script { } with braces, no colons or arrows [OK]

Common Mistakes:

• Using colons or arrows instead of braces
• Adding parentheses after script keyword
• Misplacing quotes or braces

3. What will be the output of this Jenkins pipeline snippet?

pipeline {
  agent any
  stages {
    stage('Example') {
      steps {
        script {
          def sum = 0
          for (i in 1..3) {
            sum += i
          }
          echo "Sum is ${sum}"
        }
      }
    }
  }
}

medium

A. Sum is 6

B. Sum is 3

C. Sum is 0

D. Syntax error

Solution

1. Step 1: Analyze the for loop inside script block

   The loop runs from 1 to 3 inclusive, adding each number to sum: 1 + 2 + 3 = 6.

2. Step 2: Understand echo output

   The echo prints the string with sum value interpolated, so output is "Sum is 6".

3. Final Answer:

   Sum is 6 -> Option A

4. Quick Check:

   1+2+3=6 output = Sum is 6 [OK]

Hint: Sum 1 to 3 = 6; echo prints it [OK]

Common Mistakes:

• Thinking loop ends at 2
• Confusing sum initialization
• Assuming syntax error due to script block

4. Identify the error in this Jenkins pipeline snippet using a script block:

pipeline {
  agent any
  stages {
    stage('Test') {
      steps {
        script {
          for i in 1..3 {
            echo "Number: ${i}"
          }
        }
      }
    }
  }
}

medium

A. Script block must be outside steps

B. Incorrect use of echo inside script block

C. Agent declaration is invalid

D. Missing parentheses around for loop variable and range

Solution

1. Step 1: Check for loop syntax in Groovy

   Groovy requires parentheses around the for loop variable and range: for (i in 1..3).

2. Step 2: Identify the error in given code

   The code uses for i in 1..3 without parentheses, causing syntax error.

3. Final Answer:

   Missing parentheses around for loop variable and range -> Option D

4. Quick Check:

   Groovy for loop needs ( ) [OK]

Hint: Always use parentheses in Groovy for loops [OK]

Common Mistakes:

• Omitting parentheses in for loops
• Misplacing script block outside steps
• Confusing echo usage inside script

5. You want to use a script block in Jenkins to create a map of numbers to their squares for numbers 1 to 4, then print each pair. Which code snippet correctly does this inside a script block?

hard

A. def squares = []
for (i in 1..4) {
squares.add(i * i)
}
squares.each { echo "${it} squared" }

B. def squares = [:]
for (i in 1..4) {
squares[i] = i * i
}
squares.each { k, v -> echo "${k} squared is ${v}" }

C. def squares = [:]
for i in 1..4 {
squares[i] = i * i
}
squares.each { k, v -> echo "${k} squared is ${v}" }

D. def squares = [:]
for (i in 1..4) {
squares[i] = i ^ 2
}
squares.each { k, v -> echo "${k} squared is ${v}" }

Solution

1. Step 1: Check map creation and loop syntax

   def squares = [:] for (i in 1..4) { squares[i] = i * i } squares.each { k, v -> echo "${k} squared is ${v}" } correctly creates an empty map with [:] and uses parentheses in the for loop.

2. Step 2: Verify square calculation and iteration

   def squares = [:] for (i in 1..4) { squares[i] = i * i } squares.each { k, v -> echo "${k} squared is ${v}" } uses i * i for squares and iterates map with each { k, v -> ... } to echo key-value pairs.

3. Final Answer:

   def squares = [:] for (i in 1..4) { squares[i] = i * i } squares.each { k, v -> echo "${k} squared is ${v}" } -> Option B

4. Quick Check:

   Map with for (i in 1..4) and i*i is correct [OK]

Hint: Use [:] for map, for (i in range), i*i for squares [OK]

Common Mistakes:

• Using list [] instead of map [:]
• Omitting parentheses in for loop
• Using ^ for power instead of *
• Incorrect map iteration syntax