Environment variables in Unix-like operating systems determine the behavior and access in the system. Some of the settings are contained within configuration settings, other are determined by user input. The shell keeps track of all the settings in the environment, that builds every time it starts a session with variables, defining system properties. Environment variables by convention are defined using capital letters.
This variable is defined only for the shell.
PATH – a list of directory paths. When the user types a command without providing the full path, this list is checked to see whether it contains a path that leads to the command.
HOME (Unix-like) - dicate where a user's home directory is located in the file system.
TERM – specifies the type of computer terminal or terminal emulator being used (e.g., vt100).
MAIL – used to indicate where a user's mail is to be found.