Jenkins guide

What Jenkins does in production

Jenkins is an automation server for continuous integration and continuous delivery. Developers push code; Jenkins pulls (or receives webhooks), runs compile/test/deploy steps, and reports pass/fail. It integrates with Git, Docker, Kubernetes, and notification tools — the hub of many delivery pipelines.

CI vs CD

CI (Continuous Integration) — merge often; each change triggers build + tests
CD (Continuous Delivery/Deployment) — automated promotion to staging/production after gates pass

Jenkins handles both: a pipeline stage runs unit tests (CI), later stages push images to Harbor or deploy to Kubernetes (CD).

Controller and agents

The controller (formerly master) schedules work, serves the UI, stores configuration, and coordinates pipelines. Agents (formerly slaves/executors) run build steps — on the same machine (built-in executor) or remote VMs, Docker containers, or Kubernetes pods. Heavy builds should run on agents, not overload the controller.

Job types

Freestyle job — UI-configured steps; legacy but still common
Pipeline job — Jenkinsfile in Git (Declarative or Scripted syntax)
Multibranch pipeline — one pipeline per branch/PR automatically

Modern teams prefer Pipeline as Code — the Jenkinsfile lives in the repo and is reviewed like application code.

Declarative pipeline sketch

pipeline {
  agent any
  stages {
    stage('Build') {
      steps { sh 'make build' }
    }
    stage('Test') {
      steps { sh 'make test' }
    }
    stage('Deploy') {
      when { branch 'main' }
      steps { sh './deploy.sh' }
    }
  }
  post {
    failure { echo 'Notify team' }
  }
}

How a build runs

Trigger — webhook from Git, poll SCM, manual, or cron
Queue — job waits for a free executor on an agent
Checkout — SCM plugin clones the repository into a workspace
Execute — steps run (shell, Docker, Maven, etc.)
Archive / publish — artifacts, test reports, images
Result — SUCCESS, UNSTABLE, FAILURE, or ABORTED

Credentials and secrets

Store Git SSH keys, API tokens, and registry passwords in Jenkins Credentials — referenced by ID in pipelines (credentials('my-git-key')). Prefer short-lived tokens and integration with Vault or cloud secret managers in mature setups. Never hardcode secrets in Jenkinsfiles.

Plugins

Jenkins is extended via plugins (Git, Pipeline, Docker, Kubernetes, Blue Ocean UI, etc.). Pin plugin versions; test upgrades on a staging controller — incompatible plugins are a top cause of post-upgrade breakage.

Web UI and logs

Default URL: http://host:8080 (or behind reverse proxy with TLS). Each build has a Console Output — the first place to read when a stage fails. Blue Ocean or Pipeline Stage View shows parallel stages graphically.

Key paths and service

Home — JENKINS_HOME (often /var/lib/jenkins)
Jobs config — $JENKINS_HOME/jobs/
Plugins — $JENKINS_HOME/plugins/
Logs — $JENKINS_HOME/logs/ or systemd journal
Service — jenkins (package install) or container

Learning resources

Jenkins documentation — jenkins.io/doc
Pipeline syntax — Declarative pipeline
Installing Jenkins — jenkins.io — installing
Blue Ocean — Pipeline UI

Practice scenarios

Hands-on Jenkins scenarios on live Linux VMs: jenkins

Troubleshooting →