Unix, Unix-like, or Linux-based OS provide a lot of powerful features to work upon. Among them, the most powerful and important feature is executing a wide rangeof commands to perform a task quickly and easily; for example, ls, cat, sort,grep, and so on. We will come to know about a subset of commands and usages throughout this book. In order to run a command, we need an interface that is widely known as shell.Shell is a program that acts as an interface between the users (we) and the OS kernel (Linux, Unix, and so on). Understanding in terms of Windows OS, shell serves a similar purpose DOS does. Different shells are available for Unix, Unix-like, or Linux OS. Some of the popular shells are Bourne shell (sh), C shell (csh), Korn shell (ksh), Bourne Again shell (bash), and Z shell (zsh).In this book, we will be using Linux OS and Bourne Again shell, popularly known by its acronym bash. Linux-based systems generally have bash already installed.In case bash is not installed, try installing the bash package from your distribution's package manager. In order to know which shell currently your Linux console is using, run the following command in terminal: