PATHEXT is an environment variable of MS Windows NT/2000/XP.
The function is to determine which file extensions mark files, that are executable from every commandline!
The default value is on my MS Windows 2000 system:
.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;
I added to be able to execute tcl script files from every commandline:
.TCL;.TK;.TCLSH;