gd1 reads its information from the standard input unit. You will normally use two or more xterms to operate gd1. In one of the xterms you edit an inputfile that describes your geometry, in the other xterm you iteratively start gd1 and try out how gd1 reacts to your input.
gd1 reads the information about the geometry that you are interested in
from stdin or from a file that you specify via include(filename).
gd1 generates the mesh and computes the resonant fields or time dependent
fields, depending on the input you give him.
The results are written to a database that can be read by gd1.pp.