The ENV instruction
- Docker & Kubernetes course
- Beginner
- Lesson 56/82
- 4 min
Learn to define variables in a Dockerfile with ENV.
The ENV instruction
ENV adds a permanent environment variable to the image in the Dockerfile. It is valid both in later build steps and in the running container; it can be overridden with docker run -e:
FROM python:3.12-slim
ENV PYTHONUNBUFFERED=1 \
PORT=8000
WORKDIR /app
CMD ["python", "app.py"]PYTHONUNBUFFERED=1 makes Python write output without buffering; that way logs appear immediately in docker logs.
Paths and versions with ENV
ENV variables can be used in later instructions with $NAME. This lets you define a version number or install path in one place:
ENV APP_HOME=/opt/app
WORKDIR $APP_HOME
ENV PATH="$APP_HOME/bin:$PATH"
COPY bin/ $APP_HOME/bin/Secret values like passwords should not be written into the image with ENV: docker history and docker inspect show them.
Practice Defaults with ENV
Write a Dockerfile based on node:20-alpine: define the variables NODE_ENV=production and PORT=3000 with ENV; WORKDIR /app; COPY . .; EXPOSE 3000; CMD is node server.js in exec form.
Starter code
# buraya yazSample solution
FROM node:20-alpine
ENV NODE_ENV=production PORT=3000
WORKDIR /app
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]Expected output
[+] Building 2.9s (8/8) FINISHED => naming to docker.io/library/web:latest done
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 With ENV PORT=8000 in the image, what is PORT in the container with docker run -e PORT=9000 img?
- 8000
- Empty
- 8000,9000
- 9000 Correct answer
Explanation: The run-time value replaces the default in the image.
2 Make Python output unbuffered.
____ PYTHONUNBUFFERED=1ENV PYTHONUNBUFFERED=1Explanation: ENV is valid both in the build and at run time.
3 Match.
| Term | Meaning |
|---|---|
ENV |
Permanent variable in the image |
-e |
Overrides at run time |
$APP_HOME |
Using the variable |
PYTHONUNBUFFERED |
Immediate logs |