Environment variables are specially named aliases or placeholders for certain basic system properties that are present for convenience in programming and in system administration.
Some more common predefined environment variables
| Variable | Typical value (May vary, depending on system) |
| %ALLUSERSPROFILE% |
C:\Documents and Settings\All Users |
| %APPDATA% |
C:\Documents and Settings\{username}\Application Data |
| %COMPUTERNAME% |
{computername} |
| %COMSPEC% |
C:\Windows\System32\cmd.exe |
| %HOMEDRIVE% |
C: |
| %HOMEPATH% |
\Documents and Settings\{username} |
| %PATH% |
C:\Windows\System32\;C:\Windows\;C:\Windows\System32\Wbem |
| %PATHEXT% |
.COM; .EXE; .BAT; .CMD; .VBS; .VBE; .JS ; .WSF; .WSH |
| %PROGRAMFILES% |
Directory containing program files, usually C:\Program Files |
| %PROMPT% |
Code for current command prompt format. Code is usually $P$G |
| %SYSTEMDRIVE% |
The drive containing the Windows XP root directory, usually C: |
| %SYSTEMROOT% |
The Windows XP root directory, usually C:\Windows |
| %TEMP% and %TMP% |
C:\DOCUME~1\{username}\LOCALS~1\Temp |
| %USERNAME% |
{username} |
| %USERPROFILE% |
C:\Documents and Settings\{username} |
| %WINDIR% |
C:\Windows |
The so-called predefined variables are generally unchanged during a login session but there are also some dynamic variables whose value may change. Some of these are listed in the next table.
Table II. Some dynamic environment variables
| Variable | Value |
| %DATE% |
Current date in the format determined by the Date command |
| %TIME% |
Current time in the format determined by the Time command |
| %CD% |
Current directory with its full path |
| %ERRORLEVEL% |
Number defining exit status of a previous command or program |
| %RANDOM% |
Random number between 0 and 32767 |
Managing Environment Variables in Windows XP
System Variables
You must be an administrator to modify a system environment variable. System environment variables are defined by Windows and apply to all computer users. Changes to the system environment are written to the registry, and usually require a restart to become effective.
User Variables for User Name
Any user can add, modify, or remove a user environment variable. These variables are established by Windows XP Setup, by some programs, and by users. The changes are written to the registry, and are usually effective immediately. However, after a change to user environment variables is made, any open software programs should be restarted to force them to read the new registry values. The common reason to add variables is to provide data that is required for variables that you want to use in scripts.
To view or change environment variables:
- Right-click My Computer, and then click Properties.
- Click the Advanced tab.
- Click Environment variables.
- Click one the following options, for either a user or a system variable:
- Click New to add a new variable name and value.
- Click an existing variable, and then click Edit to change its name or value.
- Click an existing variable, and then click Delete to remove it.
