Functions & Services of an OS

2. User Operating-System Interface: Allows the user to communicate with the operating system, such as by issuing command (instructions) to the OS or by obtaining information from the OS.
   We divide user interface types into 3 general categories:
   1. Command-line interface (CLI): When a computer user logs into the computer, a program called command interpreter (also referred to as a shell) launches.
      • The command interpreter awaits the user to enter commands, and then runs the programs that perform these commands.
      • The interpreter might be a part of the kernel itself or, more commonly today, a separate program.
      • Some operating systems, like Mac and Linux, might have several shells installed on it, and users can choose what shell to launch by default.
      • The command-line interface is a wrapper program through which a user interacts with the shell. On Windows, this interface is called Command Prompt, and on Linux or Mac, it is called the Terminal.