Let's start with a more basic question. What is a shell? To make it simple, a shell is an interface between the user and the underlying operating system. It can be a command-line interpreter or a graphic interface, but the purpose of a shell is to be an intermediary between the user and the core of the system, allowing the former to access the services offered by the latter. So, for instance, a bash shell gives us a command-line interface access through a Terminal and a series of commands that allows us to communicate with the operating system and make use of its services to perform tasks.

In a shell, each command is usually executed after the former has concluded its task, but we can change to some extent this behavior leveraging some key concepts: background process, signals, and subshells.