Simplified to its core, a shell really doesn’t do a whole lot: it loops forever, reads commands from the user, then executes them. When a program finishes executing, it displays that program’s output. — a. The shell reads the input, from the getline function STDIN (Standard Input), parses the input and its arguments (if supplied any). The shell also checks if the input entered is an alias of a sort, and expands the relevant value.