What defines an Operating System?
Let us check  and :
 An operating system is a software that manages the computer hardware.
 [...] Computers are equipped with a layer of software called the operating system, whose job is to provide user programs with a better, simpler, cleaner model of the computer and handle managing all the resources just mentioned.
Whatever, however, an Operating System is just a piece of software, a whole piece of software, and nothing but software!
What is Linux?
Let us GSGS (Google Here, There, and Everywhere):
 Linux is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds.
What is Free Software?
According to the Free Software Foundation (FSF):
 (Free Software) means that the users have the freedom to run, copy, distribute, study, change and improve the software.
- Free Software is free only if you have a lot of free time.
- Free Software is (sometimes) developed with whatever (non-standard, ancient) development tools.
- Free Software is sometimes maintained, and sometimes not.
The Three Layers Model
- An Operating System is between your Application and your Hardware (or Hypervisor).
- OS API: Application Programming Interface
- OS Resources Managers and Utilities:
- Process, Scheduler, Dispatcher, (Virtual) Memory, Disk, I/O, Network, Security, Protection, etc.
- OS Device Drivers: controls devices
- Without an Operating System, your Application needs to handle the hardware on its own.
 Silberschatz, A., Galvin, P. B., & Gagne, G. (2018). Operating System Concepts, 10th ed. Wiley.
 Tanenbaum, A.S. & Bos, H. (2015). Modern Operating Systems, 4th ed. Pearson.
 Linux. Retrieved July 5, 2021, In Wikipedia. https://en.wikipedia.org/wiki/Linux/
 What is Free Software? Retrieved July 5, 2021, from https://www.gnu.org/philosophy/free-sw.html
This is HOW Me Do IT! Grrr... this blog memo is mainly written for OWN PURPOSES. This post is based on "Google There, Google Here, Try That, Try This, Then Ask". Whether this is PLAGIARY or RESEARCH, there has never been a claim that this is an original work, nor is it necessarily the best solution, and not for Scopus consumption :). Please provide feedback, especially if you have alternative explanations. Hopefully, this note will be helpful in the future when you have forgotten how to solve this trivia problem.
INIlah yang KUlakukan!Grrr... memo blog ini terutama ditulis untuk KEPERLUAN SENDIRI. Tulisan ini berbasis "Google Sana, Google Sini, Coba Itu, Coba Ini, Lalu Tanya-tanyi". Entah ini PLAGIAT, entah ini RISET, yang jelas tidak pernah ada klaim bahwa ini merupakan karya asli, serta belum tentu pula merupakan solusi terbaik, serta bukan untuk konsumsi Scopus :). Mohon kiranya memberikan tanggapan, terutama jika memiliki solusi alternatif. Semoga catatan ini akan bermanfaat di masa mendatang, saat sudah lupa cara menyelesaikan masalah trivia ini.