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, 11 Feb 2008
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A free tool for setting/updating/deleting System Environment Variables.
Screenshot - SetEnv.gif


I recently used the excellent Inno Setup utility to create some install scripts for my own applications. I discovered that there was no way to set a system-wide environment variable except by using an external application. SetEnv supports User and System environment variables.

I began by searching for the Microsoft setex tool, but couldn't find it. I didn't really try that hard though. I thought I'd write my own, hence this article.
You can find a proper setup kit for SetEnv on my website.

System Environment Variables

The method used to create a system-wide environment is dependent upon the operating system in use. SetEnv will automatically detect this and will use one of the following techniques to create/modify or delete environment variables.

Windows 95/98/ME

Under Windows 9x, creating an environment variable requires modifying the user's autoexec.bat file and then executing it or rebooting before the variable is recognized by the operating system. SetEnv will automatically locate the autoexec.bat file itself -- as long as the file is on the C:\ or D:\ drive -- and then add or modify the selected system variable. The one thing SetEnv will not do is reboot the PC, as it should be left up to the user or the Setup Kit -- from Inno Setup, for example -- to choose when to perform the reboot.

Windows XP/2000/2K3/Vista

Under more modern (i.e. proper) operating systems such as Windows 2000, XP, and Windows 2003 Server, environment variables are stored in the registry under the following key:

    Control\Session Manager\Environment

Variables are added by creating a new value under this key or by modifying a value if it already exists. To delete a variable, we simply delete its registry value. That is, unless we are removing part of an expanded value such as PATH, in which case we only remove the part we want.

At this point, Windows will not be aware of our changes unless we log off or reboot. To get around this, SetEnv will broadcast a WM_SETTINGCHANGE to all of the Windows in the system. This allows other running applications such as Explorer.exe to be notified of our change. If you run SetEnv from a command prompt, then this will not update the environment variable for the current DOS window. This is mainly due to the fact that a process (SetEnv) cannot change the environment of its parent, the command prompt. However, any new DOS/command prompt that you open will show the new variable/value.

Broadcasting this message results in a slight delay of around 2-3 seconds, whilst the open Windows process it. So it may appear that SetEnv has hung, but this is not the case.

Using SetEnv

SetEnv is very easy to use and has only a few command line arguments. This section will describe how to use them. Typing SetEnv at a command prompt and then pressing the Return key will make SetEnv display its usage information. This can be seen in the screenshot at the top of this page.

User Environment Variables

As of version 1.04, SetEnv also supports User environment variables. If you want SetEnv to either add or delete a User variable, then add the -u option to the command line.

Creating a Variable

SetEnv can create two types of variables, a simple one that has only a single value such as:

InstallPath = C:\Program Files\Xanya\_Bin

... or more complex variables with multiple values. A good example of this is the PATH variable:

PATH = C:\WINDOWS\system32;C:\WINDOWS;C:\Program Files\Xanya\_Bin

To create a simple variable, just enter in the following command line. Obviously, substitute your own variable name and value:

SetEnv -a InstallPath "C:\Program Files\Xanya\_Bin"

Similarly, to add a variable with multiple values, you need to type the following command line for each value. Ensure that you prefix the value with the % character:

SetEnv -a name %value


SetEnv -a PATH %"C:\Program Files\Xanya\_Bin
SetEnv -a PATH %"C:\Bin

To add a User variable instead of a System one, add the -u option as in the following example:

SetEnv -ua PATH %"C:\Program Files\Xanya\_Bin

Modifying an Existing Variable

You modify variables in exactly the same way in which you would create them. If you are modifying a multi-value variable and you forget the % prefix to the value, then SetEnv will automatically detect that the destination has multiple values and will modify it correctly.

Dynamic Variable Expansion

This allows you to set a variable to always equal the value of an existing environment variable, even if that variable changes. Let me show you an example of when this would be very useful. This example was inspired by Synetech; if you mind me stealing it just let me know.

Say that you have an environment variable called MEDIADIR (set to C:\Media) and you want to create two additional variables called MUSICDIR and PICDIR (set to %MEDIADIR%\Music and %MEDIADIR%\Pics, respectively). These would be expanded by the operating system to the following:


Now, say you decided to move the MEDIADIR folder to drive D (D:\Media) using dynamic variable expansion. SetEnv will ensure that your MUSICDIR and PICDIR variables are correctly relocated to the following automatically.


To enable dynamic variable expansion, simply create your variables as in the following example. Use the ~ symbol to identify the existing variable name.

setenv -a MEDIADIR C:\Media
// Create the original variable

// Create our dynamic variable, when MEDIADIR changes, so will MUSICDIR
setenv -a MUSICDIR ~MEDIADIR~/Music
// Our second dynamic variable, PICDIR will change if MEDIADIR changes
setenv -a PICDIR ~MEDIADIR~/Pics

Deleting a Variable

To delete a variable, specify the -d option instead of the -a option, as in the following example:

SetEnv -d InstallPath

To delete a value from a multi-value variable, you simply enter the following, specifying the value to remove:

SetEnv -d PATH %"C:\Program Files\Xanya\_Bin

As with modifying a multi-value variable, if you forget to specify the % prefix, then SetEnv will automatically work this out and delete the specified value only. To delete a User environment variable, simply add the -u option as follows:

SetEnv -ud PATH %"C:\Program Files\Xanya\_Bin

Running SetEnv from a Batch File

SetEnv can be run successfully from a batch file. However, a problem was found by David Langford, which occurs when you attempt to specify the expanded variable prefix % to a value which contains a drive letter, as in the following example. Note that this is only an issue when running SetEnv from a batch file:

SetEnv -d PATH %C:\Test

The Windows batch file interpreter will interpret the % character as a prefix to a variable. It will replace the character with what it believes is the variable %C: with the contents of that variable, which of course is nothing. This results in SetEnv being passed a modified value, as shown here:

SetEnv -d PATH \Test

This is obviously wrong, and SetEnv will not be able to match the value to be removed in the expanded variable. It will thus fail to delete the value from the variable. The workaround is to escape the % character with another % character. The following example will correctly remove the C:\Test value from the expanded variable, PATH.

SetEnv -d PATH %%C:\Test


There are no known bugs, but if you do find any, then please let me know.


  • 1.09 (Feb 9, 2008) - Fixed a problem on Windows 98 where it sometimes failed to open the Autoexec.bat file (Found by mulinux).
  • 1.08 (May 31, 2007) - Updated SetEnv's usage information on how to delete a user environment variable using SetEnv. The code has not changed.
  • 1.07 (Jan 25, 2007) - Fixed a bug found by depaolim
  • 1.06 (Jan 14, 2007) - Added dynamic expansion support (same as using ~ with setx), originally requested by Andre Amaral, additional request by Synetech.
  • 1.05 (Sep 06, 2006) - Added support to prepend (rather than append) a value to an expanded string, requested by Masuia.
  • 1.04 (May 30, 2006) - Added support for User environment variables.
  • 1.03 (May 17, 2006) - Updated the article to explain how to use SetEnv from a batch file, after a problem was found by David Langford.
  • 1.03 (Apr 20, 2006) - Bug fix in ProcessWinXP() discovered by attiasr.
  • 1.01 (Nov 15, 2005) - Bug fix in IsWinME() discovered by frankd.
  • 1.00 (Oct 29, 2005) - Initial public release


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Jonathan [Darka]

United Kingdom United Kingdom
No Biography provided

Comments and Discussions

QuestionPossible to delete several strings from an expanded var at once? Pinmemberkj_easy26-Dec-10 6:06 
AnswerRe: Possible to delete several strings from an expanded var at once? PinmemberJonathan [Darka]7-Dec-10 0:57 
GeneralSetEnv not setting envs PinmemberDennis Merritt17-Jul-10 14:29 
GeneralRe: SetEnv not setting envs PinmemberJonathan [Darka]18-Jul-10 22:57 
GeneralRe: SetEnv not setting envs PinmemberDennis Merritt19-Jul-10 5:44 
GeneralRe: SetEnv not setting envs PinmemberJonathan [Darka]22-Jul-10 0:27 
GeneralRe: SetEnv not setting envs PinmemberDennis Merritt22-Jul-10 6:16 
GeneralDynamic variable expansion in a multiple valued variable PinmembervawdEngineer7-May-10 15:00 
GeneralRe: Dynamic variable expansion in a multiple valued variable PinmemberJonathan [Darka]7-May-10 21:35 
GeneralPercent sign problems... [modified] PinmemberSir Athos2-Feb-10 7:30 
Hi and thanks for this great utility!
I have a suggestion: it would be easier if the special meaning of a trailing "%" would be replaced by an option, such as "-e". The problem is, besides needing to escape it in batch scripts, that it creates confusion when trying to add a variable which is not an expanded variable, and its content starts with "%".
What do you think?

Edit: On second thought, I was confused. I was thinking of multi string registry entries instead of expanded variables. If your variable contains "%", it's very likely you'll want it to be an expanded variable. So, to put the above in a script, you'd write:
SetEnv -a MYSUBDIR=%%%%MYMAINDIR%%\sub
modified on Wednesday, February 3, 2010 6:43 PM

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