GRIB tools examples

grib_compare examples

The default behaviour for grib_compare without any option is to perform a bit by bit comparison of the two messages. If the messages are found to be bitwise different then grib_compare switches to a "key based" mode to find out which coded keys are different. To see how grib_compare works we first set the shortName=2d (2 metre dew point temperature) in the file regular_latlon_surface.grib1
```
 
> grib_set -s shortName=2d regular_latlon_surface.grib1 2d.grib1
```

grib_copy examples

To copy only the pressure levels from a file

>  grib_copy -w levtype=pl ../data/tigge_pf_ecmwf.grib2 out.grib

To copy only the fields that are not on pressure levels from a file

>  grib_copy -w levtype!=pl ../data/tigge_pf_ecmwf.grib2 out.grib

To copy only the first three fields from a file

>  grib_copy -w count=1/2/3 ../data/tigge_pf_ecmwf.grib2 out.grib

A grib_file with multi field messages can be converted in single field messages with a simple grib_copy.
```
>  grib_copy multi.grib simple.grib
```

Use the square brackets to insert the value of a key in the name of the output file (This is a good way to split a large GRIB file)
```
>  grib_copy in.grib 'out_[shortName].grib'
```
Note: we need to quote the name of the output so the shell does not interpret the square brackets

To copy fields whose typeOfLevel is either 'surface' or 'meanSea'
```
>  grib_copy -w typeOfLevel=surface/meanSea orig.grib out.grib
```

To copy selected fields and apply sorting (sorted by level in ascending order)
```
>  grib_copy -w typeOfLevel=heightAboveGround -B'level:i asc' tigge_af_ecmwf.grib2 out.grib
```
Note: we need to specify the ':i' to get a numerical sort. By default values are sorted as strings so a level of 100 would come before 20!

grib_dump examples

To dump in a WMO documentation style with hexadecimal octet values (-H).
```
> grib_dump -OH ../data/reduced_gaussian_model_level.grib1
```

To add key aliases and type information.

> grib_dump -OtaH ../data/reduced_gaussian_model_level.grib1

To obtain all the key names (computed keys included) available in a grib file.
```
 
>  grib_dump -D ../data/regular_latlon_surface.grib1
```

grib_filter examples

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
```

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
```

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
missing(keyname) to check if the value of the key is set to MISSING
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 &amp;&amp; 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;
}

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" &amp;&amp; !(gridType is "reduced_gg" )) {
    set step = 72;
}
```

The switch statement is an enhanced version of the if statement. Its syntax is the following:
```
switch (key1) {
    case val1:
        # block of rules;
    case val2:
        # block of rules;
    default:
        # block of rules
}
```
Each value of each 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 block or rules corresponding to the matching case statement is 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". Following is an example showing the use of the switch statement:
```
processing paramId=[paramId] [shortName] [stepType]
switch (shortName) {
    case tp :
        set stepType=accum;
    case 10u :
        set typeOfLevel=surface;
    default:
}
```

grib_get examples

grib_get fails if a key is not found.

 
> grib_get -p gribname ../data/tigge_pf_ecmwf.grib2

To get the step of the first GRIB message in a file:

 
> grib_get -w count=1 -p step ../data/tigge_pf_ecmwf.grib2

grib_get_data examples

To get a latitude, longitude, value list, skipping the missing values(=9999)
```
> grib_get_data ../data/reduced_gaussian_model_level.grib2
```

If you want to define your missing value=1111 and to print the string 'missing' in place of it
```
> grib_get_data -m 1111:missing ../data/reduced_gaussian_model_level.grib2
```

If you want to print the value of other keys with the data value list

> grib_get_data -p centre,level,step ../data/reduced_gaussian_model_level.grib2

grib_index_build examples

By default grib_index_build will index on the MARS keys.

 
>  grib_index_build ../data/reduced*.grib1 ../data/regular*.grib1 ../data/reduced*.grib2

To specify a custom list of keys to index on, use the -k option.

 
>  grib_index_build -k paramId,dataDate ../data/reduced*.grib1 ../data/regular*.grib1 ../data/reduced*.grib2

grib_ls examples

Without options a default list of keys is printed. The default list is different depending on the type of grib message.
```
 
>  grib_ls ../data/reduced*.grib1 ../data/regular*.grib1 ../data/reduced*.grib2 
```

To print offset and count number in file use the keys offset and count Also the total count in a set of files is available as countTotal
```
 
>  grib_ls -p offset,count,countTotal ../data/reduced*.grib1
```

To list only a subset of messages use the -w (where option). Only the pressure levels are listed with the following line.
```
 
>  grib_ls -w levelType=pl ../tigge_pf_ecmwf.grib2 
```

All the grib messages not on pressure levels are listed as follows:
```
 
>  grib_ls -w levelType!=pl ../tigge_pf_ecmwf.grib2 
```

To get the closest grid point to a latitude/longitude.

 
> grib_ls -l 51.46,-1.33,1 -p paramId,name ../data/reduced_gaussian_surface.grib2
../data/reduced_gaussian_surface.grib2
paramId     shortName    value 
167         2t          282.002     
1 of 1 messages in ../data/reduced_gaussian_surface.grib2

1 of 1 total messages in 1 files
Input Point: latitude=51.46  longitude=-1.33
Grid Point chosen #3 index=749 latitude=51.63 longitude=0.00 distance=93.81 (Km)
Other grid Points
<ul><li>1 - index=845 latitude=48.84 longitude=0.00 distance=306.86 (Km)
</li><li>2 - index=944 latitude=48.84 longitude=356.40 distance=333.66 (Km)
</li><li>3 - index=749 latitude=51.63 longitude=0.00 distance=93.81 (Km)
</li><li>4 - index=844 latitude=51.63 longitude=356.25 distance=168.37 (Km)

To get a list ordered by the 'level' key (ascending order).
```
 
>  grib_ls -B 'level:i asc' tigge_af_ecmwf.grib2
```
Note: we need to specify the ':i' to get a numerical sort. By default values are sorted as strings so a level of 100 would come before 20!

grib_set examples

To set productDefinitionTemplateNumber=2 only for the fields with productDefinitionTemplateNumber=11

> grib_set -s productDefinitionTemplateNumber=2 -w productDefinitionTemplateNumber=11 ../data/tigge_pf_ecmwf.grib2 out.grib2

To set productDefinitionTemplateNumber=2 only for the fields for which productDefinitionTemplateNumber is not equal to 11
```
> grib_set -s productDefinitionTemplateNumber=2 -w productDefinitionTemplateNumber!=11 tigge_pf_ecmwf.grib2 out.grib2
```

When a key is not used all the bits of its value should be set to 1 to indicate that it is missing. Since the length (number of octet) is different from a key to another, the value that we have to code for missing keys is not unique. To give an easy way to set a key to missing a string "missing" or "MISSING" is accepted by grib_set as follows:
```
> grib_set -s scaleFactorOfFirstFixedSurface=missing,scaledValueOfFirstFixedSurface=MISSING ../data/regular_latlon_surface.grib2 out.grib2
```
Since some values can not be set to missing you can get an error for those keys.

To set scaleFactorOfSecondFixedSurface to missing only for the fields for which scaleFactorOfSecondFixedSurface is not missing:

 
> grib_set -s scaleFactorOfSecondFixedSurface=missing -w scaleFactorOfSecondFixedSurface!=missing tigge_pf_ecmwf.grib2 out.grib2

It's possible to produce a grib edition 2 file from a grib edition 1 just changing the edition number with grib_set. At this stage of development all the geography parameters, level and time information is correctly translated, for the product definition extra set calls must be done. To do this properly, \ref grib_filter is suggested.
```
 
grib_set -s edition=2 ../data/reduced_gaussian_pressure_level.grib1
```

With grib edition 2 is possible to compress data using the jpeg algorithm. To change packing algorithm from grid_simple (simple packing) to grid_jpeg (jpeg2000 packing):
```
 
> grib_set -s packingType=grid_jpeg ../data/regular_gaussian_model_level.grib2 out.grib2
```

It's possible to ask ecCodes to calculate the number of bits per value needed to pack a given field with a fixed number of decimal digits of precision. For example if we want to pack a temperature expressed in Kelvin with 1 digits of precision after the decimal point we can set changeDecimalPrecision=1
```
 
> grib_set -s changeDecimalPrecision=1 ../data/regular_latlon_surface.grib2 ../data/out.grib2
```

grib_to_netcdf examples

Produce a NetCDF file from grib edition 1

> grib_to_netcdf -o output.nc input.grib1

If your grib file has analysis and 6-hour forecast, then ignore keys 'type' and 'step'. Thus type=an/fc and step=00/06 will not be considered as netcdf dimensions.
```
> grib_to_netcdf -I type,step -o output.nc input.grib
```

Do not use time of validity. If time of validity is used, it means the 1D time coordinate is considered as date+time+step, otherwise 3 different dimensions are created. The default behaviour is to use the time of validity.
```
> grib_to_netcdf -T -o output.nc input.grib
```

Produce NetCDF with data type of FLOAT (32bit floating point, for higher precision). Note these types were chosen to provide a reasonably wide range of trade-offs between data precision and number of bits required for each value
```
> grib_to_netcdf -D NC_FLOAT -o output.nc input.grib
```

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