FitNesse. UserGuide.
CustomizingTestExecution [add child]
You may customize the way tests are executed to suit your needs. You can even change the language of execution!
The is done by defining any of a few special variables in the scope (see VariableScope) of the tests you wish to execute.
The special variables are:

TEST_SYSTEM Defines the interface that FitNesse will use to execute tests. Right now there are two such interfaces: {fit | slim}
default: <undefined> which implies fit.

COMMAND_PATTERN is the basis for every execution performed in FitNesse. It specifies the command that will be executed.
default: java -cp %p %m
There are two special symbols used that will get replaced before execution.

TEST_RUNNER is the name of the class or executable that will be used when the Test button is clicked.
default: fit.FitServer
If you're running tests in Java you probably don't want to change this.

PATH_SEPARATOR is the charater that is used to separate all the path elements in the calculated path.
default: system dependant.... either ; or :

REMOTE_DEBUG_COMMAND is the command pattern used to execute tests in debug mode. See DebugingFixtureCode for more details.
default: java -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000 -cp %p %m
It has the same format as the standand COMMAND_PATTERN

CLASSPATH_PROPERTY specifies the name of the environment variable into which the classpath (gathered from !path statements) will be placed before running the test. This is most commonly used when the size of the classpath is too large for certain inept operating systems to deal with. Instead of specifying the %p argument of the COMMAND_PATTERN, you can instead !define CLASSPATH_PROPERTY {CLASSPATH}. This is most useful for Java. For other languages it may not have much meaning.