Demo Sharing a file between containers

In Kubernetes, all containers in the same Pod share the same filesystem volume. This demo shows how one Jenkins pipeline stage writes a file in an Ubuntu container, and a subsequent stage reads it in a Node.js container—both running side by side in the same Pod.

Jenkinsfile Overview

The following Jenkinsfile defines two stages: one running on Ubuntu, the other in a Node.js sidecar. They share the workspace via an emptyDir volume.

pipeline {
  agent any
  stages {
    stage('Write File in Ubuntu') {
      steps {
        // On the default Ubuntu container
        sh 'hostname'
        sh 'echo important_UBUNTU_data > ubuntu-${BUILD_ID}.txt'
        sh 'ls -ltr ubuntu-${BUILD_ID}.txt'
        sh 'sleep 60s' // keep the Pod alive briefly
      }
    }
    stage('Read File in Node.js') {
      steps {
        container('node-container') {
          sh 'node -v'
          sh 'npm -v'
          // List and display the same file
          sh 'ls -ltr ubuntu-${BUILD_ID}.txt'
          sh 'cat ubuntu-${BUILD_ID}.txt'
        }
      }
    }
  }
}

Executing the Pipeline

When this pipeline runs, Jenkins dynamically provisions a Pod with two containers—ubuntu-container (default) and node-container (sidecar)—sharing an emptyDir workspace volume.

Under the hood, the Pod specification looks like this:

apiVersion: v1
kind: Pod
metadata:
  name: k8s-cloud-agent-demo
spec:
  restartPolicy: Never
  volumes:
    - name: workspace-volume
      emptyDir: {}
  containers:
    - name: ubuntu-container
      image: ubuntu:latest
      volumeMounts:
        - name: workspace-volume
          mountPath: /workspace
    - name: node-container
      image: node:18
      volumeMounts:
        - name: workspace-volume
          mountPath: /workspace

Pipeline Log Example

The log below confirms the file created in the Ubuntu container is accessible in the Node.js container:

[Pipeline] stage
[Pipeline] { (Write File in Ubuntu)
+ hostname
k8s-cloud-agent-demo-5-rv7b8-8frt0-n41pw
+ echo important_UBUNTU_data > ubuntu-5.txt
+ ls -ltr ubuntu-5.txt
-rw-r--r-- 1 root root 22 Nov 10 09:47 ubuntu-5.txt
[Pipeline] // stage
[Pipeline] stage
[Pipeline] { (Read File in Node.js)
+ node -v
v18.20.4
+ npm -v
6.14.17
+ ls -ltr ubuntu-5.txt
-rw-r--r-- 1 root root 22 Nov 10 09:47 ubuntu-5.txt
+ cat ubuntu-5.txt
important_UBUNTU_data
[Pipeline] // stage

Static Nodes vs. Dynamic Agents

You can run Jenkins jobs on:

Further Reading

• Jenkins Kubernetes Plugin Documentation
• Kubernetes Basics
• Jenkins Blue Ocean