ENVIRONMENT VARIABLES

In MS-DOS, there are such variables called environment 
variables. These variables can be used in almost any program, 
and they share the same storage space, so usually environment 
variables can be used to store common information that will be 
shared by most programs. There is something special that batch 
programs can use environment variables, so these variables 
are also applied to batch programs for necessary information 
exchange when needed.

For example, you can access the "PATH" environment variable 
using the format "%PATH%" in a batch program. Also, you can 
do it using "ENVIRON" function in BASIC, or using "getenv" 
function in C.

Note: In Windows 9x/NT/2000, Windows has a common environment 
space. Besides, each program has its own environment space. 
The programs can share and change common environment variables, 
but cannot change those in running programs. For one program, 
the private environment space is independent to other programs. 
A program can make changes to its own environment space, and all 
code of the program can share it, but changes made in one program 
(such as a Command Prompt) don't affect other programs. Changes 
made to the common environment space can take effect in programs
started after that action, but not in running programs. This is 
because a program usually has its own new copy of the common 
environment space when it is started.

In Windows 9x, common environment variables are defined in the
AUTOEXEC.BAT file.

In Windows NT/2000, common environment variables are defined at "System 
Properties -> Advanced -> Environment Variables". They are composed of 
two parts: The System Variable Setting and the User Defined Variables. 
The latter will often override the previous. The System Variable Setting 
applies to any new user in Windows NT/2000. Still, there is a bug found 
when I was using MS-DOS command prompt in Windows NT/2000: In a batch 
file, it was all right. But in an old MS-DOS program (QBasic), the User 
Defined Variables don't lay over the System Variable Setting. This was 
discovered when I was using QBASIC to access the temporary folder. The 
temporary folder under an NT system should be inside the user profile, 
but the result of checking Environ$("TEMP") (note that TEMP is in upper 
case) in qbasic was C:\WINNT\TEMP.

Below is a list of commonly seen environment variables:
1. COMSPEC
   This variable specifies the location of the command intepreter (the 
   file name is COMMAND.COM in MS-DOS or in Window 9x, CMD.EXE in 
   Windows NT).
2. PROMPT
   Specifies the prompt (usually $P$G - path name plus the "greater 
   than" character).
3. TEMP
   Specifies the temporary directory for programs to store temporary 
   files. To keep compatibility, it should better be specified in short 
   file names.
4. PATH
   Specifies paths that MS-DOS or Windows should search for executable 
   files.
5. windir
   Stores the Windows installation path (not available under MS-DOS).

For more information, please refer to MS-DOS Help or Windows NT 
command line command help.