Based on the information read in the command file, gstat decides what to do: variogram modelling, prediction or simulation, and, in case of prediction, which prediction method to use. The decision tree for the default program action is shown in the tree for the default program action:
When observations, variogram, and prediction locations are specified, the default action is ordinary kriging on mask map locations. When, in the final example in section 2.1, the line
variogram(zinc):... ;
is left out, inverse distance weighted interpolation is the default action (kriging demands specification of the variogram). When the command
mask: ... ;
is left out, the default action is variogram modelling because prediction demands specification of prediction locations.
Sometimes it is necessary to override the default program action (e.g. to calculate variograms non-interactively, or to do simulation instead of prediction). Information about overriding the default program action is found in sections 2.4, 2.6 and 4.5.