How to set a global environment variable from Inno Setup installer?

Question

How do I set a global environment variable in Inno Setup?

^{Background: I am using the Inno install utility and need to set a global environment variable before I}

Answer 1

The solutions in @Adrian's answer (actually copied from @TLama's answer to similar question) are correct for many situations.

But it won't work for [Run] tasks with runasoriginaluser flag (what is implied by postinstall flag). I.e. the variable won't be propagated to an application run with common "Run My Program" check box on the "Finished" page.

The reason is that the tasks with runasoriginaluser are executed by a un-elevated hidden parent process of the Inno Setup installer. The SetEnvironmentVariable will change environment for the installer, but not for its parent process. Unfortunately, the parent process of the installer cannot be controlled (imo).

As a workaround, to set the variable for the runasoriginaluser tasks, you have to inject an intermediate process between the installer parent process and the task, and have the intermediate process set the variable.

Such an intermediate process can easily be the cmd.exe with its set command:

[Run]
Filename: "{cmd}"; Parameters: "/C set MYVAR=MyValue & ""{app}\MyProg.exe"""; \
    Description: "Run My Program"; Flags: postinstall runhidden

The runhidden flag hides the cmd.exe console window, not the application (assuming it's a GUI application). If it's a console application, use start to start it in its own (visible) console window.