$OperatingSystem

$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 The Mathematica Book: Section 2.13.5.

See also: $MachineType, $System, $ProcessorType.