The ARG instruction

Learn build-time variables with ARG.

The ARG instruction

ARG defines a variable valid only during the build; it doesn't exist in the running container. Its value is given with docker build --build-arg; if not given, the default is used:

ARG PYTHON_VERSION=3.12
FROM python:${PYTHON_VERSION}-slim
ARG UYGULAMA_SURUMU=dev
RUN echo "$UYGULAMA_SURUMU" > /surum.txt

$ docker build --build-arg PYTHON_VERSION=3.11 --build-arg UYGULAMA_SURUMU=1.4.0 -t api .

ARG before FROM

An ARG defined before FROM can only be used in FROM lines. If you want to use it after FROM too, it is declared again without a value:

ARG SURUM=3.12
FROM python:${SURUM}-slim
ARG SURUM                 # declare again: the value comes from above
RUN echo "Python $SURUM"

ARG values can also appear in docker history; for secret values BuildKit's --secret feature is used.

Exercise Choose the version from outside

Write a Dockerfile: before FROM, ARG NODE_VERSION=20; base image node:${NODE_VERSION}-alpine; after FROM, ARG SURUM=dev and ENV APP_SURUM=$SURUM carrying it to run time.

Show example solution

ARG NODE_VERSION=20
FROM node:${NODE_VERSION}-alpine
ARG SURUM=dev
ENV APP_SURUM=$SURUM

Test yourself

What is the difference between ARG and ENV?
- ARG is valid only in the container
- ENV can't be used in the build
- ARG is valid only in the build, ENV in the build and the running container
- They are the same
Show answer

ARG is valid only in the build, ENV in the build and the running container — To carry an ARG value to run time, ENV NAME=$NAME is written.

Give the build argument.

docker build ____ SURUM=1.4.0 -t api .

Show answer

docker build --build-arg SURUM=1.4.0 -t api .

Match.

ARG

Build variable
--build-arg

Gives the ARG value
ARG before FROM

Base image choice
--secret

Secret build value

Next lesson: env-file

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