How do I copy a selected number of messages from a GRIB file - ecCodes GRIB FAQ

Step-by-step guide

Say you want to copy the first 3 messages. This can be done with:

% grib_copy -w count=1/2/3 in.grib out.grib

But it is better to write a rules file and use grib_filter:

my.filter

if (count < 4) {
  print "Copying message number [count]";
  write;
}

Then run this as follows:

% grib_filter -o out.grib my.filter in.grib

Of course you can make the IF condition more complicated:

if (count == 3 || count == 13) {
  write;  # Write out message 3 and 13 only
}

if (count % 2 == 0) {
  write;  # Write out even-numbered messages, e.g., 2, 4, 6 etc
}