An environment variable is a named value supplied to a running program. It lets you change configuration without editing application code. For example, a web server can read PORT to decide where to listen; changing the port changes how you reach the app, not its application logic. section 2.2.1 section 6.4.3
Try a different port
An Express.js app can use `const port = process.env.PORT || 3000;`. Run `PORT=3011 node main.js` on macOS or Linux, or `set PORT=3011 && node main.js` on Windows. Visit `http://127.0.0.1:3011` instead of the default port 3000. section 2.2.1 section 2.2.2
The same idea works inside a container. A Python entrypoint can set `RUNTIME_PORT=${PORT:-8080}` and bind the server to that value. The fallback keeps the app runnable when PORT is absent; supplying PORT at runtime changes its configuration without rebuilding the image. section 5.3.4 section 5.6
As deployment grows, choose where the values come from:
- For a local Docker run, pass a value with `-e` or load an environment file with `--env-file`. Docker Compose can declare `environment` values and load a `.env` file through `env_file`. section 6.0 section 6.4.3
- For a production Compose deployment, a workflow can create a `.env` file from GitHub Actions secrets and copy it to the virtual machine. The shown transfer does not include encryption. section 7.3.3
- For Kubernetes, a Deployment can set values directly or obtain them from a ConfigMap or Secret. Shared resources avoid repeating values across manifests; a Secret is intended for sensitive values but is only Base64-encoded. section 8.3.5
Keep passwords and API keys out of application code, the Dockerfile, and container builds. A development `.env` file can keep them out of the Git repository, but must not be committed. Production secret-management tools are suggested rather than configured here. section 5.3.4 section 6.4.3
Listing 8.12 ConfigMap-based environment variables in a Deployment Manifest
kind: Deployment
...
spec:
...
template:
...
spec:
containers:
- name: nginx
image: nginx:latest
env:
- name: MY_ENV_VAR
value: "This is simple"
- name: PY_PORT
valueFrom:
configMapKeyRef:
name: project-ports-cm
key: PY_PORT
- name: JS_PORT
valueFrom:
configMapKeyRef:
name: project-ports-cm
key: JS_PORT
envFrom:
- configMapRef:
name: project-ports-cm
Compare `configMapKeyRef`, which selects a named key, with `envFrom`, which loads all keys from a ConfigMap. section 8.3.5
