Docker Dockfile: ARG vs ENV

Docker is an open-source platform that automates the deployment, scaling, and management of applications. It allows developers to package their applications into containers, providing an isolated and consistent environment for running their software.

In Docker, a Dockerfile is used to define the instructions for building a Docker image. It specifies the base image, the dependencies, the environment variables, and other configurations required to run the application.

Two commonly used directives in a Dockerfile are ARG and ENV. Both are used to set environment variables, but they have some key differences. In this article, we will explore the differences between ARG and ENV and how to use them effectively in your Dockerfile.

ARG

ARG is used to define build-time variables that are passed to the builder with the docker build command using the --build-arg flag. These variables are accessible during the image build process but are not persisted in the final image.

Here's an example of using ARG in a Dockerfile:

# Define the build-time argument
ARG MY_VAR

# Use the argument in the build process
RUN echo "My variable is: ${MY_VAR}"

To build an image with the value for MY_VAR, you can use the --build-arg flag:

docker build --build-arg MY_VAR=my_value -t my_image .

ARG is useful when you want to pass dynamic values during the build process, such as version numbers or authentication tokens. However, it is important to note that ARG values are only available during the build stage and are not available at runtime.

ENV

ENV is used to set environment variables that are available both during the build process and at runtime in the final image. These variables can be accessed by the running application inside the container.

Here's an example of using ENV in a Dockerfile:

# Set the environment variable
ENV MY_VAR=my_value

# Use the environment variable at runtime
CMD echo "My variable is: $MY_VAR"

ENV variables are set globally, meaning they will be available to all subsequent instructions in the Dockerfile. They can also be overridden during the build process by using the --build-arg flag:

docker build --build-arg MY_VAR=my_new_value -t my_image .

ENV is commonly used to provide configuration values to the running application inside the container, such as database URLs, API keys, or other settings that may vary between environments.

Best Practices

When working with ARG and ENV in your Dockerfile, it is important to follow these best practices:

  1. Use ARG for values that are only required during the build process and not needed at runtime.
  2. Use ENV for values that need to be accessible by the running application inside the container.
  3. Provide default values for ARG and ENV variables to ensure the Dockerfile can be built without specifying them.
  4. Document the required ARG and ENV variables in your Dockerfile or in the project's documentation to make it easier for users to build and run the image.

By following these best practices, you can effectively use ARG and ENV in your Dockerfile to provide dynamic configuration values and ensure a consistent and reproducible build process.

Conclusion

In this article, we have explored the differences between ARG and ENV in a Dockerfile. We have seen how ARG is used to define build-time variables that are accessible only during the build process, while ENV is used to set environment variables available both during the build process and at runtime.

Understanding the differences between ARG and ENV is crucial for building efficient and flexible Docker images. By using them effectively, you can provide dynamic configuration values and create portable and scalable applications.

Remember to follow the best practices mentioned in this article to ensure a smooth and reliable Docker build process. Happy Dockerizing!