Unix, and many other operating systems, including Windows NT/2000/XP/2003 have the concept of environment variables. These are name-to-value mappings that are available to each running program, and constitute the program’s environment. In Unix, environment variables are widely used for simple configuration of programs. http://webtrackker.com/linux-training-center-institute-in-noida-sector-15-16-3-18-63-62-64.php