[an error occurred while processing this directive]
$OperatingSystem is a string giving the type of operating system under which Mathematica is being run.
Typical values for $OperatingSystem are "MSWindows", "MSDOS", "MacOS", "Unix" and "VMS".
You can use $OperatingSystem to get an idea of what external commands will be available from within Mathematica.
$OperatingSystem typically has the same value for different versions or variants of a particular operating system.
See the Mathematica book: Section 2.13.5.
See also: $MachineType, $System, $ProcessorType.