What is the Shell in Linux?

The shell is a program that takes keyboard commands and passes them to the operating system to carry out. It is named a shell because it the outermost layer around the operating system. Almost all Linux distributions have a shell program from the GNU project called bash which is an acronym for "Bourne Again SHell". It was founded by Steve Bourne.

To gain access to the shell, we use terminal emulators or as commonly known the terminal. We use them to interact with the shell when using a grahical user interface (GUI). Examples are gnome-terminal which is used in GNOME.

Starting a Terminal Session
There are various ways to launch the terminal emulator or terminal.
1. Use Keyboard shortcut - Ctrl + Alt + T 
2. Launch it from applications menu. Type Terminal in the search bar and hit enter.

What appears is called a shell prompt and it appears whenever the shell is ready to accept input. It includes your username@computername followed by your current working directory and a dollar sign.

Some prompts may show a # sign instead of a $ sign and this means the terminal session has superuser or administrative privileges. Either you logged in as the root user or the terminal emulator you selected has superuser or administrative privileges.

Typing Commands
We'll start by typing basic commands:
cal - displays the calendar of the current month.

date - displays current date and time.

free -displays amount of free memory.

df - displays amount of free space on your disk drives. 

Ending A Terminal Session
You can end a terminal session by entering the exit command or closing the terminal emulator window. 

To learn more about the shell, visit this wiki page.