How to set a variable for the current OS session only

Question

setx permanently modifies environment variables.
set only makes variables available during batch script duration.
Is there any way to set a

Answer 1

You may do that via a Batch-JScript hybrid script that use JScript's WshShell.Environment method. The documentation specify that there are four types of environments: System, User, Volatile and Process, and that Volatile type "Applies to current logon session and is not saved between logoffs and restarts", that is exactly what you want:

@if (@CodeSection == @Batch) @then

@echo off

rem Define a persistent variable for the current OS session only via JScript
Cscript //nologo //E:JScript "%~F0" MYVAR "This is the value"
goto :EOF

@end

var colEnvVars = WScript.CreateObject("WScript.Shell").Environment("Volatile");
colEnvVars(WScript.Arguments(0)) = WScript.Arguments(1);

You must save previous code in a .bat file and execute it as any Batch file. Note that the variable defined this way is not available in the current cmd.exe window, but only in future cmd.exe windows opened in the same OS session.

Tested on Windows 8.1.