What are pipelines in Jenkins written in? Have you written any yourself?

Pipelines in Jenkins are usually written in Groovy — a scripting language that allows describing a sequence of build, test, and deploy steps. I have written pipelines using the declarative syntax of Jenkins Pipeline, which makes it easy to maintain and scale CI/CD processes.

Example of a simple pipeline in Groovy:

pipeline {
    agent any
    stages {
        stage('Build') {
            steps {
                sh 'make build'
            }
        }
        stage('Test') {
            steps {
                sh 'make test'
            }
        }
        stage('Deploy') {
            steps {
                sh 'make deploy'
            }
        }
    }
}