How to Load Environment Variables from a .env File in CMake

Learn how to easily load environment variables from a `.env` file in CMake without using complex bash commands. Discover a straightforward CMake implementation right here!
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This video is based on the question https://stackoverflow.com/q/68824042/ asked by the user 'Wang' ( https://stackoverflow.com/u/1492613/ ) and on the answer https://stackoverflow.com/a/68824838/ provided by the user 'Alex Reinking' ( https://stackoverflow.com/u/2137996/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Load Environment Variables from a .env File in CMake

When working with CMake, you might find yourself in a situation where you need to load environment variables from a .env file. This is particularly useful for managing configurations and secrets without hardcoding them into your project. However, CMake does not provide built-in support for .env files, which may lead you to wonder: How can I load environment variables from a .env file when running the cmake command?

In this guide, we will explore a complete solution to this problem using CMake scripting. The following sections will provide a step-by-step guide to implement a solution that reads key-value pairs from a .env file and loads them as environment variables.

The Problem

You want to load multiple environment variables efficiently from a .env file within your CMake project to avoid extensive command-line input. While it is possible to set environment variables using the -E env key=value option, it quickly becomes impractical when dealing with numerous variables. You also want to ensure that the solution works across platforms without leveraging complex bash scripting tricks.

The Solution

Step 1: Create Your .env File

First, you need to create a .env file that contains your environment variables. Here's a simple example:

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You can add as many variables as you need in this file, with each variable on a new line in the format key=value.

Step 2: Modify Your CMakeLists.txt

Next, you will need to set up your CMakeLists.txt file to read from the .env file. Here's an example CMake setup:

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Step 3: Create Your env.cmake Script

The real magic happens in the env.cmake script, which you will create to handle the loading of environment variables. See the sample implementation below:

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Step 4: Run Your CMake and Build Commands

After setting everything up, you can run your CMake command and build the project as usual. Here’s how to do it:

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Upon executing the command, you should see the output reflecting the environment variables loaded from your .env file:

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Conclusion

In summary, while CMake does not natively support .env files, the above implementation allows you to easily load environment variables from a .env file without dealing with cumbersome bash scripts or manual inputs. This solution not only simplifies the process but also makes it cross-platform, catering to a broader range of projects and developers.

Implement this approach in your CMake projects, and enjoy a more efficient way to manage your environment variables!