Getting user agent and delivering separate executable based on OS

Let's say I want to send someone a link, and depending on their operating system, they will be prompted to download a different executable (one for macOS, one for Linux, one for Windows). Any ideas for how to get the user agent and then choose which file to deliver based on that info?

How the fuck is this more difficult than googling and using a prepared stack over flow answer in FIVE minutes
Fuqin angered