Customizing Your Computing Environment with Unix Commands

3. Environment Variables

In Unix, there are variables, called environment variables, that store system values which affect your user environment. These values are available in every shell you initialize during a session and are used by the shell and programs you execute on the shell command line. Many UNIX commands and tools such as compilers, debuggers, profilers, and editors look in your environment variable's list to find the specific information they need. 

Previously, you used the command 'echo $SHELL' to find which shell was set as your default. By doing this, you asked the system to display the value of the environment variable named SHELL. The variable name is preceded with a dollar ($) sign to tell the shell you are entering a variable name rather than issuing a command. 

By convention, environment variable names are in uppercase and consist of only letters, numbers, or the underscore character (_).