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# Global System Information

In order to write the most general Mathematica programs you will sometimes need to find out global information about the setup under which your program is being run.
Thus, for example, to tell whether your program should be calling functions like NotebookWrite, you need to find out whether the program is being run in a Mathematica session that is using the notebook front end. You can do this by testing the global variable \$Notebooks.
 \$Notebooks whether a notebook front end is being used

Determining whether a notebook front end is being used.

Mathematica is usually used interactively, but it can also operate in a batch mode—say taking input from a file and writing output to a file. In such a case, a program cannot for example expect to get interactive input from the user.
 \$BatchInput whether input is being given in batch mode \$BatchOutput whether output should be given in batch mode, without labeling, etc.

Variables specifying batch mode operation.

The Mathematica kernel is a process that runs under the operating system on your computer. Within Mathematica there are several global variables that allow you to find the characteristics of this process and its environment.
 \$CommandLine the original command line used to invoke the Mathematica kernel \$ParentLink the MathLink LinkObject specifying the program that invoked the kernel (or Null if the kernel was invoked directly) \$ProcessID the ID assigned to the Mathematica kernel process by the operating system \$ParentProcessID the ID of the process that invoked the Mathematica kernel \$UserName the login name of the user running the Mathematica kernel Environment["var"] the value of a variable defined by the operating system

Variables associated with the Mathematica kernel process.

If you have a variable such as x in a particular Mathematica session, you may or may not want that variable to be the same as an x in another Mathematica session. In order to make it possible to maintain distinct objects in different sessions, Mathematica supports the variable \$SessionID, which uses information such as starting time, process ID and machine ID to try to give a different value for every single Mathematica session, whether it is run on the same computer or a different one.
 \$SessionID a number set up to be different for every Mathematica session

A unique number different for every Mathematica session.

Mathematica provides various global variables that allow you to tell which version of the kernel you are running. This is important if you write programs that make use of features that are, say, new in Version 6. You can then check \$VersionNumber to find out if these features will be available.
 \$Version a string giving the complete version of Mathematica in use \$VersionNumber the Mathematica kernel version number (e.g. 6.0) \$ReleaseNumber the release number for your version of the Mathematica kernel on your particular computer system \$CreationDate the date, in DateList format, on which your particular Mathematica release was created

Variables specifying the version of Mathematica used.

Mathematica itself is set up to be as independent of the details of the particular computer system on which it is run as possible. However, if you want to access external aspects of your computer system, then you will often need to find out its characteristics.
 \$System a full string describing the computer system in use \$SystemID a short string specifying the computer system in use \$ProcessorType the architecture of the processor in your computer system \$MachineType the general type of your computer system \$ByteOrdering the native byte ordering convention on your computer system \$OperatingSystem the basic operating system in use \$SystemCharacterEncoding the default raw character encoding used by your operating system

Variables specifying the characteristics of your computer system.

Mathematica uses the values of \$SystemID to label directories that contain versions of files for different computer systems, as discussed in "Reading and Writing Mathematica Files: Files and Streams" and "Portability of MathLink Programs". Computer systems for which \$SystemID is the same will normally be binary compatible.
\$OperatingSystem has values such as "Windows" or "Unix". By testing \$OperatingSystem you can determine whether a particular external program is likely to be available on your computer system.
This gives some characteristics of the computer system on which the input is evaluated.
 Out[1]=
 \$MachineAddresses the list of current IP addresses \$MachineName the name of the computer on which Mathematica is running \$MachineDomains the current network domains for the computer \$MachineID the unique ID assigned by Mathematica to the computer

Variables identifying the computer on which Mathematica is running.