Kubernetes Level 02 Day 04: Print Environment Variables

This document outlines the solution for Kubernetes Level 02 Day 04. The objective was to create a Pod that utilizes environment variables to output a concatenated greeting message to the console.

Table of Contents

1. Task Overview
2. Step-by-Step Solution
   • 1. Create the YAML Manifest
   • 2. Apply the Configuration
   • 3. Verification
3. Deep Dive: Environment Variables in Kubernetes

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Task Overview

Objective: Provision a Pod that prints a greeting message constructed from multiple injected environment variables.

• Pod Name: print-envars-greeting
• Container Name: print-env-container
• Image: bash
• Environment Variables:
  • GREETING="Welcome to"
  • COMPANY="Stratos"
  • GROUP="Industries"
• Command: ["/bin/sh", "-c", 'echo "$(GREETING) $(COMPANY) $(GROUP)"']
• Restart Policy: Never

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Step-by-Step Solution

1. Create the YAML Manifest

We need to create a declarative YAML file to properly define the Pod, its container, the specific command, and the necessary environment variables.

Command:

vi pod.yaml

Content:

apiVersion: v1
kind: Pod
metadata:
  name: print-envars-greeting
spec:
  containers:
  - name: print-env-container
    image: bash
    command: ["/bin/sh", "-c", 'echo "$(GREETING) $(COMPANY) $(GROUP)"']
    env:
    - name: GREETING
      value: "Welcome to"
    - name: COMPANY
      value: "Stratos"
    - name: GROUP
      value: "Industries"
  restartPolicy: Never

2. Apply the Configuration

Use kubectl to create the Pod in the cluster.

Command:

kubectl apply -f pod.yaml

* Expected Output: pod/print-envars-greeting created

3. Verification

Since the Pod runs a simple echo command and the restart policy is set to Never, the container will start, print the message to standard output, and then immediately terminate (entering the Completed state).

Check the Pod Status:

kubectl get pods

* Wait until the status changes from ContainerCreating to Completed.

Check the Logs: To verify the output, retrieve the logs from the Pod.

kubectl logs -f print-envars-greeting

* Expected Output: Welcome to Stratos Industries

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Deep Dive: Environment Variables in Kubernetes

Why Use Environment Variables?

Environment variables allow you to externalize configuration data from your container images. This means you can use the exact same bash image in different environments (Development, Staging, Production) just by changing the injected environment variables in the Pod definition, adhering to the principles of the Twelve-Factor App methodology.

$(VAR) Syntax in Kubernetes

In the command block, the task requested the exact syntax: 'echo "$(GREETING) $(COMPANY) $(GROUP)"'. When using the $(VAR) syntax inside a Kubernetes command array, Kubernetes can evaluate and expand these variables dynamically before the container starts, provided the variables are defined in the env block.

restartPolicy: Never

By default, Pods have a restartPolicy of Always. Since this specific container runs a finite echo command and completes its task immediately, an Always policy would cause Kubernetes to endlessly restart the container in a "CrashLoopBackOff" state. Setting restartPolicy: Never tells Kubernetes that it is acceptable for this Pod to finish execution and terminate successfully.