$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.
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