Define Environment Variable Values Using An Init Container

This page show how to configure environment variables for containers in a Pod via file.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• iximiuz Labs
• Killercoda
• KodeKloud
• Play with Kubernetes

Your Kubernetes server must be at or later than version v1.34.

To check the version, enter kubectl version.

How the design works

In this exercise, you will create a Pod that sources environment variables from files, projecting these values into the running container.

pods/inject/envars-file-container.yaml

apiVersion: v1
kind: Pod
metadata:
  name: envfile-test-pod
spec:
  initContainers:
    - name: setup-envfile
      image:  nginx
      command: ['sh', '-c', 'echo "DB_ADDRESS=address\nREST_ENDPOINT=endpoint" > /data/config.env']
      volumeMounts:
        - name: config
          mountPath: /data
  containers:
    - name: use-envfile
      image: nginx
      command: [ "/bin/sh", "-c", "env" ]
      env:
        - name: DB_ADDRESS
          valueFrom:
            fileKeyRef:
              path: config.env
              volumeName: config
              key: DB_ADDRESS
              optional: false
  restartPolicy: Never
  volumes:
    - name: config
      emptyDir: {}

In this manifest, you can see the initContainer mounts an emptyDir volume and writes environment variables to a file within it, and the regular containers reference both the file and the environment variable key through the fileKeyRef field without needing to mount the volume. When optional field is set to false, the specified key in fileKeyRef must exist in the environment variables file.

The volume will only be mounted to the container that writes to the file (initContainer), while the consumer container that consumes the environment variable will not have the volume mounted.

The env file format adheres to the kubernetes env file standard.

During container initialization, the kubelet retrieves environment variables from specified files in the emptyDir volume and exposes them to the container.

Create the Pod:

kubectl apply -f https://k8s.io/examples/pods/inject/envars-file-container.yaml

Verify that the container in the Pod is running:

# If the new Pod isn't yet healthy, rerun this command a few times.
kubectl get pods

Check container logs for environment variables:

kubectl logs dapi-test-pod -c use-envfile | grep DB_ADDRESS

The output shows the values of selected environment variables:

DB_ADDRESS=address

Env File Syntax

The format of Kubernetes env files originates from .env files.

In a shell environment, .env files are typically loaded using the source .env command.

For Kubernetes, the defined env file format adheres to stricter syntax rules:

• Blank Lines: Blank lines are ignored.

• Leading Spaces: Leading spaces on all lines are ignored.

• Variable Declaration: Variables must be declared as VAR=VAL. Spaces surrounding = and trailing spaces are ignored.

  VAR=VAL → VAL
  

• Comments: Lines beginning with # are treated as comments and ignored.

  # comment
  VAR=VAL → VAL
  
  VAR=VAL # not a comment → VAL # not a comment
  

• Line Continuation: A backslash (\) at the end of a variable declaration line indicates the value continues on the next line. The lines are joined with a single space.

  VAR=VAL \
  VAL2
  → VAL VAL2
  

What's next

• Learn more about environment variables.
• Read Defining Environment Variables for a Container
• Read Expose Pod Information to Containers Through Environment Variables