The Unix OS, and all its applications, utilities, and tools, were always built to be used from a command-line-interface (CLI), typically, the shell. From the 1980s onward, the need for a Graphical User Interface (GUI) became apparent.
Robert Scheifler of MIT, considered the chief design architect behind the X Window System, built an exceedingly clean and elegant architecture, a key component of which is this: the GUI forms a layer (well, actually, several layers) above the OS, providing libraries for GUI clients, that is, applications.
This architecture still holds up today. Having said that, especially on embedded Linux, performance reasons are seeing the advent of newer architectures, such as the frame buffer and Wayland. Also, though Android, which uses the Linux kernel, necessitates a GUI for the end user, the system developer’s interface to Android, ADB, is a CLI.
A huge number of production-embedded and server Linux systems run purely on CLI interfaces. The GUI is almost like an add-on feature, for the end user’s ease of operation.
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