Define Environment Variables for a Container

This page shows how to define environment variables for a container in a Kubernetes Pod.

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:

Define an environment variable for a container

When you create a Pod, you can set environment variables for the containers that run in the Pod. To set environment variables, include the env or envFrom field in the configuration file.

The env and envFrom fields have different effects.

env

allows you to set environment variables for a container, specifying a value directly for each variable that you name.

envFrom

allows you to set environment variables for a container by referencing either a ConfigMap or a Secret. When you use envFrom, all the key-value pairs in the referenced ConfigMap or Secret are set as environment variables for the container. You can also specify a common prefix string.

You can read more about ConfigMap and Secret.

This page explains how to use env.

In this exercise, you create a Pod that runs one container. The configuration file for the Pod defines an environment variable with name DEMO_GREETING and value "Hello from the environment". Here is the configuration manifest for the Pod:

  1. pods/inject/envars.yaml
  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   name: envar-demo
  5.   labels:
  6.     purpose: demonstrate-envars
  7. spec:
  8.   containers:
  9.   - name: envar-demo-container
  10.     image: gcr.io/google-samples/hello-app:2.0
  11.     env:
  12.     - name: DEMO_GREETING
  13.       value: "Hello from the environment"
  14.     - name: DEMO_FAREWELL
  15.       value: "Such a sweet sorrow"

  1. Create a Pod based on that manifest:

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

  2. List the running Pods:

    1. kubectl get pods -l purpose=demonstrate-envars

    The output is similar to:

    1. NAME            READY     STATUS    RESTARTS   AGE
    2. envar-demo      1/1       Running   0          9s

  3. List the Pod’s container environment variables:

    1. kubectl exec envar-demo -- printenv

    The output is similar to this:

    1. NODE_VERSION=4.4.2
    2. EXAMPLE_SERVICE_PORT_8080_TCP_ADDR=10.3.245.237
    3. HOSTNAME=envar-demo
    4. ...
    5. DEMO_GREETING=Hello from the environment
    6. DEMO_FAREWELL=Such a sweet sorrow

Note:

The environment variables set using the env or envFrom field override any environment variables specified in the container image.

Note:

Environment variables may reference each other, however ordering is important. Variables making use of others defined in the same context must come later in the list. Similarly, avoid circular references.

Using environment variables inside of your config

Environment variables that you define in a Pod’s configuration under .spec.containers[*].env[*] can be used elsewhere in the configuration, for example in commands and arguments that you set for the Pod’s containers. In the example configuration below, the GREETING, HONORIFIC, and NAME environment variables are set to Warm greetings to, The Most Honorable, and Kubernetes, respectively. The environment variable MESSAGE combines the set of all these environment variables and then uses it as a CLI argument passed to the env-print-demo container.

Environment variable names consist of letters, numbers, underscores, dots, or hyphens, but the first character cannot be a digit. If the RelaxedEnvironmentVariableValidation feature gate is enabled, all printable ASCII characters except “=” may be used for environment variable names.

  1. apiVersion: v1
  2. kind: Pod
  3. metadata:
  4.   name: print-greeting
  5. spec:
  6.   containers:
  7.   - name: env-print-demo
  8.     image: bash
  9.     env:
  10.     - name: GREETING
  11.       value: "Warm greetings to"
  12.     - name: HONORIFIC
  13.       value: "The Most Honorable"
  14.     - name: NAME
  15.       value: "Kubernetes"
  16.     - name: MESSAGE
  17.       value: "$(GREETING) $(HONORIFIC) $(NAME)"
  18.     command: ["echo"]
  19.     args: ["$(MESSAGE)"]

Upon creation, the command echo Warm greetings to The Most Honorable Kubernetes is run on the container.

What’s next