Running grib_filter with the print.filter rules file and tigge.grib as the input GRIB file produces the following:

      
	%  grib_filter print.filter tigge.grib   
	Hello ecCodes!
	Hello ecCodes!
	...

This is produced by the first line in the rules file:

print "Hello ecCodes!";

Next, comment the first line with a ‘#’ and uncomment the second line so that the active rule is:

print "- [count] - shortName=[shortName] date=[dataDate] step=[step][stepUnits:s]";

Running grib_filter with the new rules file produces the following output:

	%  grib_filter print.filter tigge.grib
   	- 1 - shortName=gh date=20170224 step=24h
   	- 2 - shortName=q date=20170224 step=24h
   	...
   	- 239 - shortName=tcw date=20170224 step=24h
   	- 240 - shortName=tp date=20170224 step=24h

Using the third rule:

print ("out.txt") "- [count] - shortName=[shortName] date=[dataDate] step=[step][stepUnits:s]";

does not produce any output to the screen but instead writes the output to a file called out.txt.

The fourth line is:

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

Running with this rule prints the distinct latitudes array for each message as a single column with the numbers in decimal format with 3 decimal places:

      
	%  grib_filter print.filter tigge.grib  
  	90.000
  	88.500
  	87.000
  	85.500
  	...

The final rule is:

print "[latLonValues!3%.5f',']";

Running grib_filter with this rule prints the latitudes, longitudes and values for each message formatted as 3 comma-separated columns with the numbers in decimal format with 5 decimal places:

	%  grib_filter print.filter tigge.grib
   	90.00000,0.00000,1.00000,
   	90.00000,1.50000,1.00000,
   	90.00000,3.00000,1.00000,
   	90.00000,4.50000,1.00000,
   	...

Running grib_filter with tigge.grib as the input GRIB file with the write.filter rules file:

write "[dataDate]_[centre].grib[edition]";

splits the messages in the input file into a separate files for each date, centre and GRIB edition combination:

	%  grib_filter write.filter tigge.grib  

	%  ls *_*.grib*
	  20170224_ammc.grib2  20170224_cwao.grib2  20170224_egrr.grib2  20170224_lfpw.grib2  20170224_rksl.grib2
	  20170224_babj.grib2  20170224_ecmf.grib2  20170224_kwbc.grib2  20170224_rjtd.grib2  20170224_sbsj.grib2

Running with the second line of the rule uncommented:

write;

simply writes each message to the default output GRIB file, “filter.out”.  This is identical to the input tigge.grib file:

	% grib_compare tigge.grib filter.out 
	
	% echo $?
	0

Running grib filter with the transient.filter rule:

transient mykey1 = (step == 4 ) * step;  
transient mykey2 = (step == 24 ) * step;
print "-- [count] -- step=[step] mykey1=[mykey1] mykey2=[mykey2]";

Prints the following output to the screen:

	-- 1 -- step=24 mykey1=0 mykey2=24 
	-- 2 -- step=24 mykey1=0 mykey2=24
	-- 3 -- step=24 mykey1=0 mykey2=24
	-- 4 -- step=24 mykey1=0 mykey2=24
	-- 5 -- step=24 mykey1=0 mykey2=24
	...

In the output seen here, mykey1 is set to 0 because step=24 and the first expression is not satisfied so (step == 4 ) = 0. But mykey2 is set to 24 because the second expression is satisfied and (step == 24 ) = 1.  Effectively, " (step == 4 )" acts a little like an "if":  "If (step == 4) {mykey1 = step}".

The filter rule file is:

#
# Question 1 solution
#
# $> grib_filter question1.filter tigge.grib
#

if (centre is "ecmf" ) {
  set dataDate = 20160301;
  set step = step + 48;
}

write "question1.grib";

Note that the test on centre as a string uses "is" - if (centre is "ecmf").

The filter rule file is:

#
# Question 2 solution
#
# $> grib_filter question2.filter tigge.grib
#

if (count == 1) {
    set step = 72;
    set values = {1.2,3.4,5.6,3.7};
    write "question2.grib";
}

Note that we identify the first message as count == 1.

We can check the result of running the filter by using grib_get_data:

	% grib_filter question2.filter tigge.grib
	
	% grib_get_data -P step -F"%.2f" question2.grib
	Latitude, Longitude, Value, step
	   90.000    0.000 1.20 72
	   90.000    1.500 3.40 72
	   90.000    3.000 5.60 72
	   90.000    4.500 3.70 72

If you choose to set stepRange instead then note that this key is a 'string' so you need use:

         set stepRange = "72" ;
      

The filter rule file is:

#
# Question 3 solution
#
# $> grib_filter question3.filter tigge.grib
#

if (count != 1 && 
   shortName is "gh" && 
   !(gridType is "reduced_gg" )) {
    set step = 72;
    append "question2.grib";
}

Note the construct for "NOT IS".  The "is" test is negated with the "!":

!(gridType is "reduced_gg" )

Check the output with grib_ls

	% grib_ls question2.grib
	question2.grib
	edition      centre       date         dataType     gridType     stepRange    typeOfLevel  level        shortName    packingType  
	2            ammc         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            babj         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            cwao         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            egrr         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            kwbc         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            lfpw         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            rjtd         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            rksl         20170224     pf           regular_ll   72           isobaricInhPa  1000         gh           grid_simple 
	2            sbsj         20170224     pf           regular_gg   72           isobaricInhPa  1000         gh           grid_simple 
	9 of 9 grib messages in question2.grib
	
	9 of 9 total grib messages in 1 files

The filter rule is:

#
# Question 4 solution
#
# $> grib_filter question4.filter tigge.grib
#

if ((typeOfLevel is "surface") || ((typeOfLevel is "heightAboveGround") && (level == 10))) {

    switch (typeOfLevel) {

        case "surface":
            set changeDecimalPrecision=2;
        case "heightAboveGround":
            set changeDecimalPrecision=3;
        default:
            print "Unknown level type!";
    }

    print "Centre [centre] parameter [shortName] written to question4-[centre].grib";
    write "question4-[centre].grib";

} else {
 
   print "Centre [centre] parameter [shortName] not written";

}

The filter rule is:

#
# Question 5 solution
#
# $> grib_filter question5.filter question4-*.grib
#

if ((gridType is "regular_ll") && !(shortName is "10u") && !(shortName is "10v") ) {
    write "question5.grib";
}