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Comprehensive Guide to Passing Environment Variables to Docker Containers


Environment variables play a crucial role in configuring applications within Docker containers. They provide a convenient way to customize container behavior without modifying the underlying code. In this tutorial, we’ll explore different techniques for passing environment variables to Docker containers, along with code examples illustrating each method.


Before we begin, ensure you have the following prerequisites:

  1. Docker installed on your system
  2. Basic understanding of Docker concepts

Method 1: Using the -e Flag

The simplest way to pass environment variables to a Docker container is by using the -e flag followed by the variable name and value. Here’s how to do it:

docker run -e VAR_NAME=var_value <image_name>

Replace VAR_NAME with the name of the environment variable and var_value with its corresponding value.

Method 2: Using an Environment File

For managing multiple environment variables, it’s more convenient to use an environment file. Create a file (e.g., .env) containing variable-value pairs:


Then, pass this file to Docker using the --env-file flag:

docker run --env-file .env <image_name>

Method 3: Defining Environment Variables in Dockerfile

Another approach is to define environment variables directly within the Dockerfile using the ENV instruction. Here’s an example:

FROM alpine


CMD ["echo", "$VAR_NAME"]

Best Practices

  • Avoid Hardcoding: Instead of hardcoding values in Dockerfiles, use environment variables for flexibility.
  • Sensitive Information: Be cautious when passing sensitive information via environment variables. Consider using Docker secrets or external vaults.
  • Clear Documentation: Document all required environment variables and their purpose for better maintainability.


Passing environment variables to Docker containers is essential for configuring applications across different environments. In this tutorial, we explored various methods, including using flags, environment files, and Dockerfile instructions. By following best practices, you can effectively manage and utilize environment variables in your Docker workflows.

Now, you’re equipped with the knowledge to seamlessly integrate environment variables into your Docker containers.

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