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 env file format used by Kubernetes is a well-defined subset of the environment variable semantics for POSIX-compliant bash. Any env file supported by Kubernetes will produce the same environment variables as when interpreted by a POSIX-compliant bash. However, POSIX-compliant bash supports some additional formats that Kubernetes does not accept.

Example:

MY_VAR='my-literal-value'

Rules

• Variable declaration: Use the form VAR='value'. Spaces surrounding = are ignored; leading spaces on a line are ignored; blank lines are ignored.
• Quoted values: Values must be enclosed in single quotes (').
  • The content inside single quotes is preserved literally. No escape-sequence processing, whitespace folding, or character interpretation is applied.
  • Newlines inside single quotes are preserved (multi-line values are supported).
• Comments: Lines that begin with # are treated as comments and ignored. A # character inside a single-quoted value is not a comment.

Examples:

# comment
DB_ADDRESS='address'

MULTI='line1
line2'

Unsupported forms

• Unquoted values are prohibited:
  • VAR=value — not supported.
• Double-quoted values are prohibited:
  • VAR="value" — not supported.
• Multiple adjacent quoted strings are not supported:
  • VAR='val1''val2' — not supported.
• Any form of interpolation, expansion, or concatenation is not supported:
  • VAR='a'$OTHER or VAR=${OTHER} — not supported.

The strict single-quote requirement ensures the value is taken literally by the kubelet when loading environment variables from files.

What's next

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