The ENV instruction

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.

Exercise 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.

Show example solution

FROM node:20-alpine
ENV NODE_ENV=production PORT=3000
WORKDIR /app
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

Test yourself

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
Show answer

9000 — The run-time value replaces the default in the image.

Make Python output unbuffered.

____ PYTHONUNBUFFERED=1

Show answer

ENV PYTHONUNBUFFERED=1

Match.

ENV

Permanent variable in the image
-e

Overrides at run time
$APP_HOME

Using the variable
PYTHONUNBUFFERED

Immediate logs

Next lesson: The ARG instruction

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