Make your self at home
Environmental variables give users the ability to set up the command line to suit their own needs.
If you enter a command like ls in the shell, the shell knows that you want to call the /bin/ls program. Bash knows where on the filesystem to look for executables, so there is no need to type in the full path. This path is defined in what is known as an environmental variable. Variables of this kind are also used to modify the appearance of the shell prompt, set the time zone and the user’s home directory, and other things.