gui is a macro that starts up the DL_POLY_2 Java GUI. It invokes the following unix commands:
java -jar ../java/GUI.jar
In other words the macro invokes the Java Virtual Machine which executes the instructions in the Java archive file GUI.jar, which is stored in the java subdirectory of DL_POLY_2 . (Note: Java 1.3.0 or a higher version is required to run the GUI.)