Environment Variables
    
    Unix & MS-DOS provide a number of variables for passing information about 
    the state of the user's environment, the state in which their shell 
    is currently configured.  
     Under Unix, there may 
      be environment variables (usually named in allcaps) storing the user's: 
      
      
      
    
      - home directory path 
        (HOME) 
      
- default printer (PRINTER) 
        
      
- terminal type (TERM) 
        
      
- current directory 
        path (PWD) 
      
- ...and lots besides. 
        
    
    From within a C program, environment variables may be read by calling 
    
    char* getenv(char*) 
    
    a function whose prototype appears within stdlib.h 
     From the Unix shell, 
      environment variables may be read using printenv and set using setenv. 
    
 Refer to the Unix man 
      pages for further details.