Welcome to this article about Dockerfile ARG! If you are familiar with Docker, you know that Dockerfiles are used to describe the build process for Docker images. In this article, we will be exploring Dockerfile ARG, a feature that allows users to pass arguments to the Docker build process.
1. Introduction
Docker is a popular tool for containerization, which allows developers to package their applications and dependencies in a portable way. Dockerfiles are used to describe the build process for Docker images. They are essentially scripts that specify the steps needed to create a Docker image. Dockerfile ARG is a feature that enables users to pass arguments to the Docker build process.
2. Dockerfile ARG
A Dockerfile ARG is a way to pass a variable to the Docker build process. This variable can be used in the Dockerfile to control how the image is built. ARG is used to specify values that are used during the build time of the image. ARG values can be specified using the –build-arg flag when running the docker build command.
The syntax for defining an ARG in a Dockerfile is:
ARG <name>=<default value>
Here, <name> is the name of the argument, and <default value> is the default value that is used if the argument is not provided when building the image. The default value is optional.
ARGs can be used in the Dockerfile using the ${<name>} syntax. For example, the following Dockerfile uses an ARG to set the version of an application:
FROM ubuntu
ARG APP_VERSION=1.0
RUN echo "The application version is ${APP_VERSION}"
In this example, the ARG instruction defines the APP_VERSION argument, and the RUN instruction uses it to print the application version.
3. Using ARG in the build process
When you build a Docker image, you can pass the value of an ARG using the --build-arg flag. For example:
docker build --build-arg APP_VERSION=2.0 .
Here, the APP_VERSION argument is set to 2.0. The . at the end specifies the build context.
You can also specify multiple ARGs using the --build-arg flag:
docker build --build-arg APP_VERSION=2.0 --build-arg APP_NAME=myapp .
Here, two arguments are passed: APP_VERSION and APP_NAME.
When an argument is passed to the build process, it overrides the default value specified in the Dockerfile. If no default value is specified and no value is passed during the build process, the argument is not defined.
4. ARG vs ENV
It is worth noting that Dockerfile ARGs are not persisted in the final image. They only exist during the build process and are used to customize the build. ENV, on the other hand, is used to set environment variables in the image that can be used by the application at runtime. ENV values are persisted in the final image and can be accessed by the application.
Here’s an example that illustrates the difference between ARG and ENV:
FROM ubuntu
ARG APP_VERSION=1.0
ENV APP_VERSION=${APP_VERSION}
RUN echo "The application version is ${APP_VERSION}"
In this example, the ARG is used to set the APP_VERSION variable during the build process, and the ENV is used to set the same variable as an environment variable in the image.
When the image is built with the default value of APP_VERSION, the output of the RUN instruction is:
If the image is built with a different value of APP_VERSION, for example:
docker build --build-arg APP_VERSION=2.0 .
The output of the RUN instruction is:
In this case, the ARG value is used during the build process and the ENV value is persisted in the final image.
5. Conclusion
Dockerfile ARG is a powerful feature that enables users to customize the build process of their Docker images. By passing arguments to the build process, users can change the behavior of the Dockerfile and control how the image is built. ARGs are particularly useful when building images for multiple environments or when building images that need to be customized for different use cases.
It is important to note that ARGs are not persisted in the final image. They only exist during the build process and are used to customize the build. If you need to set environment variables in your image that can be accessed by the application at runtime, you should use ENV instead of ARG.
When using ARG in your Dockerfiles, you should also be careful to avoid exposing sensitive information. Since ARG values can be passed using the –build-arg flag, they can potentially be viewed by anyone with access to the build logs. You should never pass sensitive information like passwords or API keys using ARGs.
In conclusion, Dockerfile ARG is a powerful feature that enables users to customize the build process of their Docker images. By passing arguments to the build process, users can change the behavior of the Dockerfile and control how the image is built. However, it is important to be careful when using ARGs and to avoid exposing sensitive information.
6. Download the Source Code
You can download the full source code of this example here: Dockerfile ARG
