CodeBlanks Logo Icon CodeBlanks
Login Register

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.

Practice 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.

Starter code

terminal
# buraya yaz

Sample solution

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

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 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 Correct answer
  • They are the same

Explanation: To carry an ARG value to run time, ENV NAME=$NAME is written.

2 Give the build argument.

Question 
docker build ____ SURUM=1.4.0 -t api .
Answer 
docker build --build-arg SURUM=1.4.0 -t api .

Explanation: --build-arg NAME=value fills an ARG.

3 Match.

Term Meaning
ARG Build variable
--build-arg Gives the ARG value
ARG before FROM Base image choice
--secret Secret build value

All lessons in Beginner

1 What is Docker?

  1. What is Docker?
  2. Containers and virtual machines
  3. Images and containers
  4. Docker architecture
  5. Use cases
  6. Summary - What is Docker

2 Installation and first commands

  1. Installing Docker
  2. docker version
  3. docker info
  4. The hello-world container
  5. Docker Desktop
  6. Summary - Installation

3 First container

  1. docker run basics
  2. Running in the background
  3. docker ps
  4. docker stop and rm
  5. docker logs
  6. Summary - First container

4 Dockerfile basics

  1. What is a Dockerfile?
  2. The FROM instruction
  3. The RUN instruction
  4. COPY and ADD
  5. CMD and ENTRYPOINT
  6. Summary - Dockerfile

5 Build and images

  1. docker build
  2. Tagging images
  3. Build context
  4. Layers and cache
  5. .dockerignore
  6. Summary - Build

6 docker run options

  1. Interactive mode
  2. Naming containers
  3. Restart policy
  4. Working directory
  5. Switching users
  6. Summary - docker run

7 Image management

  1. docker images
  2. docker rmi
  3. docker pull
  4. docker push
  5. docker tag
  6. Summary - Image management

8 Port mapping

  1. The port concept
  2. The -p flag
  3. Multiple ports
  4. Random ports
  5. IP binding
  6. Summary - Port mapping

9 Volumes and data

  1. Data persistence
  2. Named volumes
  3. The -v flag
  4. Bind mounts
  5. Volume management
  6. Summary - Volumes

10 Environment variables

  1. The -e flag
  2. The ENV instruction
  3. The ARG instruction
  4. env-file
  5. The WORKDIR instruction
  6. Summary - Environment

11 Project: docker-compose

  1. Compose design
  2. Service definition
  3. Database service
  4. Volume and network
  5. Putting it together - compose

12 Docker Hub and registry

  1. What is Docker Hub?
  2. docker login
  3. Searching images
  4. Official images
  5. Private registry
  6. Summary - Registry

13 Network basics

  1. Network drivers
  2. docker network ls
  3. Bridge network
  4. Container connectivity
  5. Publishing ports
  6. Summary - Network

14 Beginner final

  1. Challenge - Dockerfile
  2. Challenge - Build and run
  3. Challenge - Volume
  4. Challenge - Compose
  5. Challenge - Beginner project