jenkins

What it does

How to write it

systemctl {start|status} jenkins
java -jar jenkins-cli.jar -s URL -auth USER:TOKEN COMMAND

Main parts

Jenkinsfile blocks

pipeline

The whole declarative pipeline.

agent

Where it runs (any, label, docker, kubernetes).

stages / stage / steps

The work.

environment

Variables, also credentials('id').

when

Conditions (branch, tag, changes).

post

Steps after the run (always, success, failure).

options

timeout, retries, build history.

parameters

Inputs for a build.

Most used

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$ run as a normal user# run as root (or with sudo)

1. 1Install on RHEL

   #curl -o /etc/yum.repos.d/jenkins.repo https://pkg.jenkins.io/redhat-stable/jenkins.repo
   #rpm --import https://pkg.jenkins.io/redhat-stable/jenkins.io-2023.key
   #dnf install -y java-21-openjdk jenkins
   #systemctl enable --now jenkins
   #cat /var/lib/jenkins/secrets/initialAdminPassword

2. 2Run Jenkins in a container

   $docker run -d --name jenkins -p 127.0.0.1:8080:8080 -v jenkins_home:/var/jenkins_home jenkins/jenkins:lts-jdk21

3. 3A declarative Jenkinsfile

   $cat Jenkinsfile
   pipeline {
     agent { docker { image 'python:3.12-slim' } }
     options { timeout(time: 20, unit: 'MINUTES') }
     stages {
       stage('Test')  { steps { sh 'pip install -r requirements.txt && pytest -q' } }
       stage('Scan')  { steps { sh 'trivy fs --exit-code 1 --severity HIGH,CRITICAL .' } }
       stage('Build') { when { branch 'main' } steps { sh 'docker build -t shop:$BUILD_NUMBER .' } }
     }
     post { failure { echo 'Build failed' } }
   }

4. 4Use stored credentials safely

   $cat Jenkinsfile
       stage('Push') {
         steps {
           withCredentials([usernamePassword(credentialsId: 'ghcr', usernameVariable: 'U', passwordVariable: 'P')]) {
             sh 'echo "$P" | docker login ghcr.io -u "$U" --password-stdin'
           }
         }
       }

5. 5Use the CLI with an API token

   $curl -O http://localhost:8080/jnlp/jenkins-cli.jar
   $java -jar jenkins-cli.jar -s http://localhost:8080 -auth admin:API_TOKEN who-am-i
   $java -jar jenkins-cli.jar -s http://localhost:8080 -auth admin:API_TOKEN build shop/main -s

6. 6Start a build with the REST API

   $curl -X POST -u admin:API_TOKEN http://localhost:8080/job/shop/job/main/build

7. 7Configuration as Code (JCasC)

   $cat jenkins.yaml
   jenkins:
     numExecutors: 0
     securityRealm:
       local:
         allowsSignup: false
   unclassified:
     location:
       url: https://ci.example.com/

8. 8Read the logs

   #journalctl -u jenkins -n 50

9. 9Back up Jenkins

   #tar czf jenkins-backup-$(date +%F).tgz -C /var/lib jenkins --exclude='jenkins/workspace'

10. 10Run agents as Kubernetes pods

    $cat Jenkinsfile
    pipeline {
      agent { kubernetes { yamlFile 'ci/pod.yaml' } }
      stages { stage('Test') { steps { container('python') { sh 'pytest -q' } } } }
    }

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Files

/var/lib/jenkins/

JENKINS_HOME: jobs, plugins, secrets.

/var/lib/jenkins/secrets/initialAdminPassword

First-start password.

Jenkinsfile

The pipeline in the repository.

Exit codes