Jenkins

Pronunciation: JENG-kinz

Updated October 3, 20262 min read

In short

Jenkins is an open-source automation server that builds, tests and deploys software through pipelines, and one of the oldest and most widely used CI/CD tools.

What is Jenkins?

Jenkins grew out of Hudson, a build server created by Kohsuke Kawaguchi at Sun Microsystems, and took its current name in 2011 after a dispute over the project's direction. It runs on your own servers, watches source repositories, and runs jobs whenever code changes, on a schedule or when someone presses a button.

Modern Jenkins jobs are pipelines defined in a Jenkinsfile stored in the repository, written in a Groovy-based syntax with stages such as build, test and deploy. A controller schedules the work and agents, which can be machines, containers or Kubernetes pods, run the steps, so many builds can run in parallel.

Its strength is flexibility: plugins integrate it with almost any version control system, build tool, test framework, cloud and chat app, and it can automate tasks far beyond CI. That flexibility comes at a cost, because teams must host, secure, upgrade and back up Jenkins themselves and keep its plugins compatible.

A common misconception is that Jenkins is the only serious CI/CD option. Hosted services such as GitHub Actions, GitLab CI and CircleCI need no servers and keep pipeline files next to the code, so many new projects choose them. Jenkins remains common in large companies with existing setups or special on-premises requirements.

Key takeaways

Jenkins is a self-hosted, open-source automation server for CI/CD.
It forked from Hudson and took its name in 2011.
Pipelines are defined in a Jenkinsfile with build, test and deploy stages.
A controller schedules jobs and agents run them, often in parallel.
Plugins make it flexible, but teams must maintain and secure it.

Example

A declarative Jenkinsfilegroovy

pipeline {
    agent { docker { image 'node:22' } }

    stages {
        stage('Install') {
            steps { sh 'npm ci' }
        }
        stage('Test') {
            steps { sh 'npm test' }
        }
        stage('Deploy') {
            when { branch 'main' }
            steps { sh './deploy.sh production' }
        }
    }

    post {
        failure { echo 'Build failed: notify the team' }
    }
}

Readers ask

What is the difference between Jenkins and GitHub Actions?

Jenkins is a server you install and maintain, configured through Jenkinsfiles and plugins. GitHub Actions is a hosted service built into GitHub, with workflows in YAML and runners provided for you. Jenkins offers more control; Actions offers less maintenance.

What is a Jenkinsfile?

A text file in the repository that defines a Jenkins pipeline as code, with its stages and steps. Keeping it with the code means the pipeline is reviewed and versioned like everything else.

Is Jenkins free?

Yes. Jenkins is open source under the MIT license. The costs are the servers it runs on and the time spent operating it.