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Learn how to effectively use `environment variable substitution` in Docker with practical examples and solutions to common problems.
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This video is based on the question https://stackoverflow.com/q/63314995/ asked by the user 'Vijay Gharge' ( https://stackoverflow.com/u/332841/ ) and on the answer https://stackoverflow.com/a/63315176/ provided by the user 'Alejandro Galera' ( https://stackoverflow.com/u/3351141/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Achieving Environment Variable Substitution in Docker Run

Managing environment variables is crucial when working with Docker containers, especially when you want to maintain consistency and avoid redundancy. However, many users find themselves asking, "How do I define one environment variable and refer to it in others when using docker run?" This guide will address this issue, guiding you through effective methods for environment variable substitution in Docker.

Understanding the Problem

When trying to pass multiple environment variables to a Docker container, it's common to want to reference one variable within another. This not only saves time but also prevents errors from hardcoding values. However, when using docker run, many users face limitations on how to achieve this.

For example, you might want to set version variables like so:

[[See Video to Reveal this Text or Code Snippet]]

Here, you intend to set ES_VERSION relative to ELK_VERSION, but this results in ES_VERSION being empty because it hasn't been defined in the current shell environment.

Solution for Environment Variable Substitution

Step 1: Export the Parent Variable

Before invoking your docker run command, you need to ensure that the parent variable (ELK_VERSION in this case) is defined in the shell. You can achieve this using the export command. Here's how:

[[See Video to Reveal this Text or Code Snippet]]

Explanation:

export ELK_VERSION=7.8.0: This command sets ELK_VERSION as an environment variable in your shell session.

The docker run command then uses ${ELK_VERSION} for both ELK_VERSION and ES_VERSION, ensuring both are set properly within the container.

Step 2: Using a .env File with Docker Compose

If you plan to use docker-compose instead of docker run, another viable method is to define your environment variables in a .env file. Here’s how you can set it up:

Create a .env file in the same directory as your docker-compose.yml.

Add your variables to the .env file like so:

[[See Video to Reveal this Text or Code Snippet]]

In your docker-compose.yml, reference these variables as follows:

[[See Video to Reveal this Text or Code Snippet]]

Important Notes

Variables defined in the shell via export or in a .env file must be accessible when the Docker commands are executed.

If you reference a variable that hasn't been defined, it will simply result in an empty string, which can lead to unexpected behavior in your application.

Conclusion

To effectively use environment variable substitution in Docker, it’s crucial to define your parent variable beforehand. By using the methods outlined in this post, you can ensure a smoother experience with Docker and a more manageable codebase. Whether you're using docker run or docker-compose, managing your environment variables effectively helps maintain clarity and reduce potential errors.

Now that you know how to achieve variable substitution in Docker, you can streamline your container configurations and make your deployments much easier! Happy Dockerizing!