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Environment variables in Windows NT

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Article ID:100843
Last Review:February 20, 2007
Revision:3.3
This article was previously published under Q100843
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INTRODUCTION

There are three levels of environment variables in Microsoft Windows NT; the system environment variables, the user environment variables, and the environment variables that are set in the AUTOEXEC.BAT file. There are also some predefined environment variables that are set when the user logs on. This article discusses the following topics:

•System environment variables
•User environment variables
•AUTOEXEC.BAT environment variables
•How environment variables are set
•How the path is built
•Changing user environment variables by using Control Panel

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MORE INFORMATION

System environment variables

System environment variables can be viewed from Control Panel by choosing the System icon. These variables are always set no matter who logs on and they cannot be changed by any user.

There are a few additional predefined environment variables that are set when the user logs on that do not appear in the System dialog box: 

   USERNAME
   USERDOMAIN
   NTVERSION (not available in Windows NT 4.0 anymore)
   WINDIR
   OS
   PROCESSOR_ARCHITECTURE
values: x86, MIPS or ALPHA 
   PROCESSOR_LEVEL
values for x86: 3, 4, 5 values for MIPS: 3000, 4000 values for ALPHA: 21064 
   HOMEPATH
   HOMEDRIVE
   HOMESHARE
These three environment variables are set based on the value of the home directory. The user's home directory is specified in User Manager (Choose Profile and Properties). If the home directory uses universal naming conventions (UNC), then they will have the following values:

HOMESHARE=\\<server name>\<share name> HOMEPATH=\<path> HOMEDRIVE=<drive letter>:


If the home directory is a local path such as c:\nt then they will look like this:

HOMEDRIVE=c: HOMESHARE= HOMEPATH=\nt


All above environment variables are always present and therefore may be used in log on scripts.

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User environment variables

User environment variables can be viewed from Control Panel as well. The user may add, delete or modify the environment variables in the User Environment Variables for User field. These variables take precedence over system environment variables. The user path is appended to the system path.

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AUTOEXEC.BAT environment variables

All environment variables and the paths set in the AUTOEXEC.BAT file are used to create the Windows NT environment. Any paths in the AUTOEXEC.BAT file are append to the system path.

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How environment variables are set

Environment variables are set in the following order:

•System variables
•AUTOEXEC.BAT variables
•User variables

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How the path is built

The Path is constructed from the system path, which can be viewed in the System Environment Variables field in the System dialog box. The User path is appended to the system path. Then the path from the AUTOEXEC.BAT file is appended.

Note: The environment variables LibPath and Os2LibPath are built the same way (system path + user path + AUTOEXEC.BAT path).

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Changing user environment variables by using Control Panel

User environment variables can be added, changed or deleted from Control Panel. There is no need to reboot after making any of these changes. Changes take effect immediately after exiting the System dialog box.

The next application you start will use the new settings. All other applications running before the changes were made will not recognize the changes.

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APPLIES TO
•Microsoft Windows 2000 Server
•Microsoft Windows 2000 Advanced Server
•Microsoft Windows 2000 Professional Edition
•Microsoft Windows NT Advanced Server 3.1
•Microsoft Windows NT Server 4.0 Standard Edition
•Microsoft Windows NT Workstation 3.1
•Microsoft Windows NT Workstation 4.0 Developer Edition

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Keywords: 
kbenv KB100843

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