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Kubernetes lets you inject configuration data into Pods via environment variables. This declarative approach mirrors Docker’s -e flag but offers tighter integration with Kubernetes primitives like ConfigMaps and Secrets.

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Why Use Environment Variables?

  • Decouple configuration from container images
  • Simplify application customization across environments
  • Secure sensitive data using Secrets

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Prerequisites

  • A running Kubernetes cluster
  • kubectl configured against your cluster
  • Basic knowledge of Pods and YAML manifests

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1. Direct Assignment with value

To set an environment variable directly in your Pod spec, use the env array under the container definition: 
apiVersion: v1
kind: Pod
metadata:
  name: simple-webapp-color
spec:
  containers:
    - name: simple-webapp-color
      image: simple-webapp-color
      ports:
        - containerPort: 8080
      env:
        - name: APP_COLOR
          value: pink
This makes APP_COLOR=pink available inside the container.
Direct assignment is ideal for non-sensitive, static configuration values.
You can replicate this behavior in Docker with: 
docker run -e APP_COLOR=pink simple-webapp-color

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2. Decoupling Config with valueFrom

Rather than embedding values in your Pod spec, you can reference external sources: 
env:
  # From a ConfigMap
  - name: APP_COLOR
    valueFrom:
      configMapKeyRef:
        name: my-configmap
        key: color.value

  # From a Secret
  - name: DB_PASSWORD
    valueFrom:
      secretKeyRef:
        name: my-secret
        key: db.password
Secrets are only base64-encoded by default and not encrypted at rest. Use encryption providers or external secret stores for stronger security.

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3. Comparison at a Glance

Configuration SourceFieldWhen to Use
DirectvalueStatic, non-sensitive values
ConfigMapvalueFrom.configMapKeyRefApplication settings shared across Pods
SecretvalueFrom.secretKeyRefSensitive data (passwords, tokens, certificates)

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4. References

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