grib_filter

DESCRIPTION

Apply the rules defined in rules_file to each GRIB message in the GRIB files provided as arguments. If you specify '-' (a single dash) for the rules_file, the rules will be read from standard input.

USAGE

grib_filter [options] rules_file grib_file grib_file ...

OPTIONS

-f

Force. Force the execution not to fail on error.

-o output_file

Output is written to output_file. If an output file is required and -o is not used, the output is written to 'filter.out'

-M

Multi-field support off. Turn off support for multiple fields in single GRIB message.

-V

Version.

-g

Copy GTS header.

-G

GRIBEX compatibility mode.

-T T | B | M | A

Message type. T->GTS, B->BUFR, M->METAR (Experimental),A->Any (Experimental). The input file is interpreted according to the message type.

-7

Does not fail when the message has wrong length

-v

Verbose.

grib_filter examples

1. The grib_filter processes sequentially all grib messages contained in the input files and applies the rules to each one of them. Input messages can be written to the output by using the "write" statement. The write statement can be parameterised so that output is sent to multiple files depending on key values used in the output file name. If we write a rules_file containing the only statement:

   write "../data/split/[centre]_[date]_[dataType]_[levelType].grib[editionNumber]";
   

   Applying this rules_file to the "../data/tigge_pf_ecmwf.grib2" grib file we obtain several files in the ../data/split directory containing fields split according to their key values

   > grib_filter rules_file ../data/tigge_pf_ecmwf.grib2
   > ls ../data/split
   ecmf_20060619_pf_sfc.grib2
   ecmf_20060630_pf_sfc.grib2
   ecmf_20070122_pf_pl.grib2
   ecmf_20070122_pf_pt.grib2
   ecmf_20070122_pf_pv.grib2
   ecmf_20070122_pf_sfc.grib2
   

   
   

2. The key values in the file name can also be obtained in a different format by indicating explicitly the type required after a colon.

   • :i for integer
   • :d for double
   • :s for string

   The following statement works in a slightly different way from the previous example, including in the output file name the integer values for centre and dataType.

   write "../data/split/[centre:i]_[date]_[dataType:i]_[levelType].grib[editionNumber]";
   

   Running the same command again we obtain a different list of files.

   > grib_filter rules_file ../data/tigge_pf_ecmwf.grib2
   > ls ../data/split
   98_20060619_4_sfc.grib2
   98_20060630_4_sfc.grib2
   98_20070122_4_pl.grib2
   98_20070122_4_pt.grib2
   98_20070122_4_pv.grib2
   98_20070122_4_sfc.grib2
   

   
   

3. Other statements are allowed in the grib_filter syntax:

   • if ( condition ) { block of rules } else { block of rules } The condition can be made using ==,!= and joining single block conditions with || and &&
     The statement can be any valid statement also another nested condition
   • set keyname = keyvalue;
   • print "string to print also with key values like in the file name"
   • transient keyname1 = keyname2;
   • comments beginning with #
   • defined(keyname) to check if a key is defined in a message
   • To format arrays use "[key!c%F'S']": Print the array with 'c' columns, using the C-style format 'F' and 'S' as separator
   • missing(keyname) to check if the value of the key is set to MISSING (Note: This does not apply to codetable keys)
   • To set a key value to MISSING, use 'set key=MISSING;' (note the case)
   • You can also make an assertion with 'assert(condition)'. If condition is false, it will abort the filter.

   A complex example of grib_filter rules is the following to change temperature in a grib edition 1 file.

   # Temperature
   if ( level == 850 && indicatorOfParameter == 11 ) {
       print "found indicatorOfParameter=[indicatorOfParameter] level=[level] date=[date]";
       transient oldtype = type ;
       set identificationOfOriginatingGeneratingSubCentre=98;
       set gribTablesVersionNo = 128;
       set indicatorOfParameter = 130;
       set localDefinitionNumber=1;
       set marsClass="od";
       set marsStream="kwbc";
       # Negatively/Positively Perturbed Forecast
       if ( oldtype == 2 || oldtype == 3 ) {
         set marsType="pf";
         set experimentVersionNumber="4001";
       }
       # Control Forecast
       if ( oldtype == 1 ) {
         set marsType="cf";
         set experimentVersionNumber="0001";
       }
       set numberOfForecastsInEnsemble=11;
       write;
       print "indicatorOfParameter=[indicatorOfParameter] level=[level] date=[date]";
       print;
   }
   

   
   

4. Here is an example of an IF statement comparing a key with a string. Note you have to use the "is" keyword for strings and not "==", and to negate you add the "!" before the whole condition:

   # Select Geopotential Height messages which are not on a Reduced Gaussian Grid
   if (shortName is "gh" && !(gridType is "reduced_gg" )) {
       set step = 72;
   }
   

5. Print the key distinctLatitudes in one column with 3 decimal digits:

   print "[distinctLatitudes!1%.3f]";

6. The switch statement is an enhanced version of the if statement. Its syntax is the following:

   switch (key1) {
       case val1:
           # statements
       case val2:
           # statements
       default:
           # statements
   }
   

   The value of the key given as argument to the switch statement is matched against the values specified in the case statements. If there is a match, then the statements corresponding to the matching case are executed. Otherwise, the default case is executed. The default case is mandatory if the case statements do not cover all the possibilities. The "~" operator can be used to match "anything". The following is an example of the switch statement:

   print 'Processing paramId=[paramId] [shortName] [stepType]';
   switch (shortName) {
       case 'tp' :
           set stepType='accum';
           print 'Message #[count]: Total Precipitation';
       case '10u' :
           set typeOfLevel='surface';
           print 'Message #[count]: 10m U-Wind';
       default:
   }