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Learn how to dynamically source environment variables from a file and pass them to your Docker container using a script.
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Efficiently Export Variables for Docker Run with Dynamic Sourcing

When working with Docker, you might often need to pass environment variables to the containers you run. One common issue developers face is dealing with multiple variables stored in a file and the inability of the docker run command to substitute their values effectively due to its limitations. This blog will guide you on how to dynamically export these variables and reference them in a Docker command using a simple script.

The Problem: Passing Environment Variables in Docker Run

You may have an environment file, examples.env, containing variable definitions that need to be passed to a Docker container. An example of such a file could look like this:

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The challenge occurs because the docker run command cannot evaluate $B to produce the expected output of /path2:/path1. This limitation leads us to find a different approach—an automated way to load and pass these environment variables for our Docker container.

The Solution: Using a Script to Dynamically Source Variables

To tackle the issue, we can create a launcher script, mydocker.sh, to read variables from the examples.env file and format them for the Docker command. Here’s how to do it step-by-step:

Step 1: Source the Variables

Start by sourcing the variables file within your script. This ensures all variable definitions are available for use in the current script.

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Step 2: Build the Docker Environment Variable String

Next, we define a variable to hold our environment variables formatted for Docker. This is done by concatenating each variable from the examples.env file with the --env flag.

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Step 3: Evaluate the Environment Variable String

To ensure that the sourced variables are interpreted correctly within the Docker command, we need to evaluate this string. This step ensures that the values of the variables are substituted appropriately.

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Step 4: Run the Docker Command

Finally, execute the docker run command with the generated environment variables. We use the long-form --env option to avoid conflicts with command-line arguments.

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Complete mydocker.sh Script

Here is the complete mydocker.sh script for clarity:

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Conclusion: Simplifying Variable Management in Docker

By following the steps outlined above, you can dynamically source environment variables from a file and pass them to your Docker container seamlessly. This approach not only simplifies variable management but also enhances the flexibility of your Docker environment setup.

Feel free to adapt this script to suit your specific needs, and enjoy a hassle-free Docker experience!