This article relates to GRIB edition 2

Step-by-step guide

To convert multi-field GRIB messages into single-field, you can use the grib_copy command.
For example here we have a multi-field GRIB2 file with 30 messages some of which contain several fields. The grib_copy command creates a new file in which every message has a single field:

% grib_copy multi.grib2 single.grib2

% grib_count multi.grib2
% grib_count single.grib2

The use of the multi-field feature is not recommended.