$OperatingSystem (Built-in Mathematica Symbol)

$OperatingSystem is a string giving the type of operating system under which Mathematica is being run.

$Output (Built-in Mathematica Symbol)

$Output gives the list of files and pipes to which standard output from Mathematica is sent.

$Packages (Built-in Mathematica Symbol)

$Packages gives a list of the contexts corresponding to all packages which have been loaded in your current Mathematica session.

$ParentLink (Built-in Mathematica Symbol)

$ParentLink is the MathLink LinkObject currently used for input and output by the Mathematica kernel in a particular session.

$ParentProcessID (Built-in Mathematica Symbol)

$ParentProcessID gives the ID assigned to the process which invokes the Mathematica kernel by the operating system under which it is run.

$PerformanceGoal (Built-in Mathematica Symbol)

$PerformanceGoal gives the default setting for the option PerformanceGoal for graphics and other algorithmic functions.

$ProcessID (Built-in Mathematica Symbol)

$ProcessID gives the ID assigned to the Mathematica kernel process by the operating system under which it is run.

$ProcessorType (Built-in Mathematica Symbol)

$ProcessorType is a string giving the architecture of the processor on which Mathematica is being run.

$ReleaseNumber (Built-in Mathematica Symbol)

$ReleaseNumber is an integer which gives the current Mathematica kernel release number, and increases in successive releases.

$SessionID (Built-in Mathematica Symbol)

$SessionID is a number set up to be unique to a particular Mathematica session.