One brief note
A lot of people were mentioning Linux here.
I'd just like to interject for a moment. What you’re referring to as Linux, is in fact, SystemD/Linux, or as I’ve recently taken to calling it, SystemD plus Linux. Linux is not an operating system unto itself, but rather another free component of a fully functioning SystemD system made useful by Redhead Enterprises, Inc., some shell utilities and vital system components comprising a full OS as defined by POSIX. All the so-called "Linux" distributions are really distributions of SystemD/Linux.
Thanks for your attention.