An environment variable is a named string value available to a running process. In a Kubernetes pod, it gives an application configuration without requiring that value to be built into the container image. Variables are set for each container, not once for the whole pod. A Dockerfile’s ENV directive can provide a default; the pod manifest can supply a different value. section 8.1 section 8.1.2
Set a value in a pod
Add an env entry beneath the container definition: env: - name: INITIAL_STATUS_MESSAGE value: This status message is set in the pod spec. The application can read INITIAL_STATUS_MESSAGE and display it in its response. Values must be strings: quote values such as numbers or true so YAML does not interpret them as another type. To inspect the running container, use kubectl exec kiada -- env. By contrast, kubectl set env pod kiada --list shows only variables defined in the pod manifest. section 8.1.2
A value can also refer to another environment variable. Kubernetes examples use $(POD_NAME) in a value or $(LISTEN_PORT) in a container argument. This lets a value supplied through env become a command-line argument when the application expects one. section 8.1.2 section 8.2.1
Instead of writing a literal value in the pod manifest, choose a source that fits the data:
- ConfigMap: use env.valueFrom.configMapKeyRef to select one key, or envFrom.configMapRef to expose all entries. A separate ConfigMap can supply different configuration in each environment while the pod manifest stays the same. An optional missing reference leaves the variable unset; a required missing key blocks the affected container from starting. section 8.2.1 section 8.2.3
- Secret: use valueFrom.secretKeyRef for one entry, or envFrom.secretRef for the entire Secret. Take care: logs or error reports may reveal environment variables, and child processes inherit them. Secret files are the suggested safer alternative. section 8.3.3
- Downward API: use valueFrom.fieldRef for pod or node information, such as the pod name and IP address; use valueFrom.resourceFieldRef for container CPU or memory limits. section 8.4.2
Listing 8.10 Setting an environment variable from a ConfigMap entry
kind: Pod
...
spec:
containers:
- name: kiada
env: #1
- name: INITIAL_STATUS_MESSAGE #1
valueFrom: #2
configMapKeyRef: #2
name: kiada-config #3
key: status-message #4
optional: true #5
volumeMounts:
- ...
The manifest shows the exact valueFrom, ConfigMap name, and key fields for injecting one entry. section 8.2.3
Environment variables are one way to deliver configuration to a container; configuration files are another. ConfigMaps and Secrets can also be mounted as files, an approach especially worth considering for sensitive values. section 8.2.1 section 8.3.3
