$OperatingSystem is a string giving the type of operating system under which Mathematica is being run.
Typical values for $OperatingSystem are "Windows98", "MacOS" and "Unix".
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 Section 2.14.5.
See also: $MachineType, $System, $ProcessorType.
New in Version 2.