Why are other folder paths also added to system PATH with SetX and not only the specified folder path?

Windows creates a copy of the entire environment table of the process starting a new process for the new process. Therefore on start of your C++ application, your application gets the environment table including PATH from parent process, Windows Explorer or in your case Visual Studio. And this PATH is copied for cmd.exe on start of the batch file.

Taking the entire process tree into account from Windows desktop to the batch file, there have been many copies made for PATH and some processes perhaps appended something to their local copy of PATH like Visual Studio has done, or have even removed paths from PATH.

What you do now with SETX PATH "%PATH% is appending the local copy of PATH modified already by the parent processes in process tree completely to system PATH without checking for duplicate paths.

Much better would be to throw away all code using local copy of PATH and instead read the value of system PATH, check if the path you want to add is not already in system PATH and if this is not the case, append the path you want to add to system PATH using setx.

And this should be done without expanding the environment variables in system PATH like %SystemRoot%\System32 to C:\Windows\System32.

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UPDATE

Here is the batch code required for your task tested on Windows 7 x64 and Windows XP x86.

@echo off
setlocal EnableExtensions DisableDelayedExpansion
set "KeyName=HKLM\SOFTWARE\Ansoft\Designer\2014.0\Desktop"
set "ValueName=InstallationDirectory"
for /F "skip=2 tokens=1,2*" %%N in ('%SystemRoot%\System32\reg.exe query "%KeyName%" /v "%ValueName%" 2^>nul') do (
    if /I "%%N" == "%ValueName%" (
        set "PathToAdd=%%P"
        if defined PathToAdd goto GetSystemPath
    )
)
echo Error: Could not find non-empty value "%ValueName%" under key
echo        %KeyName%
echo/
endlocal
pause
goto :EOF

:GetSystemPath
for /F "skip=2 tokens=1,2*" %%N in ('%SystemRoot%\System32\reg.exe query "HKLM\System\CurrentControlSet\Control\Session Manager\Environment" /v "Path" 2^>nul') do (
    if /I "%%N" == "Path" (
        set "SystemPath=%%P"
        if defined SystemPath goto CheckPath
    )
)
echo Error: System environment variable PATH not found with a non-empty value.
echo/
endlocal
pause
goto :EOF

:CheckPath
setlocal EnableDelayedExpansion
rem The folder path to add must contain \ (backslash) as directory
rem separator and not / (slash) and should not end with a backslash.
set "PathToAdd=%PathToAdd:/=\%"
if "%PathToAdd:~-1%" == "\" set "PathToAdd=%PathToAdd:~0,-1%"
set "Separator="
if not "!SystemPath:~-1!" == ";" set "Separator=;"
set "PathCheck=!SystemPath!%Separator%"
if "!PathCheck:%PathToAdd%;=!" == "!PathCheck!" (
    set "PathToSet=!SystemPath!%Separator%!PathToAdd!"
    set "UseSetx=1"
    if not "!PathToSet:~1024,1!" == "" set "UseSetx="
    if not exist %SystemRoot%\System32\setx.exe set "UseSetx="
    if defined UseSetx (
        %SystemRoot%\System32\setx.exe Path "!PathToSet!" /M >nul
    ) else (
        set "ValueType=REG_EXPAND_SZ"
        if "!PathToSet:%%=!" == "!PathToSet!" set "ValueType=REG_SZ"
        %SystemRoot%\System32\reg.exe ADD "HKLM\System\CurrentControlSet\Control\Session Manager\Environment" /f /v Path /t !ValueType! /d "!PathToSet!" >nul
    )
)
endlocal
endlocal

The batch code above uses a simple case-insensitive string substitution and a case-sensitive string comparison to check if the folder path to append is present already in system PATH. This works only if it is well known how the folder path was added before and the user has not modified this folder path in PATH in the meantime. For a safer method of checking if PATH contains a folder path see the answer on How to check if directory exists in %PATH%? written by Dave Benham.

Note 1: Command setx is by default not available on Windows XP.

Note 2: Command setx truncates values longer than 1024 characters to 1024 characters.

For that reason the batch file uses command reg to replace system PATH in Windows registry if either setx is not available or new path value is too long for setx. The disadvantage on using reg is that WM_SETTINGCHANGE message is not sent to all top-level windows informing Windows Explorer running as Windows desktop and other applications about this change of system environment variable. So the user must restart Windows which is best done always on changing something on persistent stored Windows system environment variables.

The batch script was tested with PATH containing currently a folder path with an exclamation mark and with a folder path being enclosed in double quotes which is necessary only if the folder path contains a semicolon.

For understanding the used commands and how they work, open a command prompt window, execute there the following commands, and read entirely all help pages displayed for each command very carefully.

• echo /?
• endlocal /?
• for /?
• goto /?
• if /?
• pause /?
• reg /? and reg add /? and reg query /?
• set /?
• setlocal /?
• setx /?