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5.5 ncdump

The ncdump tool generates the CDL text representation of a netCDF dataset on standard output, optionally excluding some or all of the variable data in the output. The output from ncdump is intended to be acceptable as input to ncgen. Thus ncdump and ncgen can be used as inverses to transform data representation between binary and text representations.

As of netCDF version 4.1, ncdump can also access DAP data sources if DAP support is enabled in the underlying netCDF library. Instead of specifying a file name as argument to ncdump, the user specifies a URL to a DAP source.

ncdump may also be used as a simple browser for netCDF datasets, to display the dimension names and lengths; variable names, types, and shapes; attribute names and values; and optionally, the values of data for all variables or selected variables in a netCDF dataset.

ncdump defines a default format used for each type of netCDF variable data, but this can be overridden if a C_format attribute is defined for a netCDF variable. In this case, ncdump will use the C_format attribute to format values for that variable. For example, if floating-point data for the netCDF variable Z is known to be accurate to only three significant digits, it might be appropriate to use this variable attribute:

     Z:C_format = "%.3g"

Ncdump uses '_' to represent data values that are equal to the _FillValue attribute for a variable, intended to represent data that has not yet been written. If a variable has no _FillValue attribute, the default fill value for the variable type is used unless the variable is of byte type.

UNIX syntax for invoking ncdump:

     ncdump  [ -c | -h]  [-v var1,...]  [-b lang]  [-f lang]
     [-l len]  [ -p fdig[,ddig]] [ -s ] [ -n name]  [input-file]


Show the values of coordinate variables (variables that are also dimensions) as well as the declarations of all dimensions, variables, and attribute values. Data values of non-coordinate variables are not included in the output. This is often the most suitable option to use for a brief look at the structure and contents of a netCDF dataset.
Show only the header information in the output, that is, output only the declarations for the netCDF dimensions, variables, and attributes of the input file, but no data values for any variables. The output is identical to using the '-c' option except that the values of coordinate variables are not included. (At most one of '-c' or '-h' options may be present.)
-v var1,...
The output will include data values for the specified variables, in addition to the declarations of all dimensions, variables, and attributes. One or more variables must be specified by name in the comma-delimited list following this option. The list must be a single argument to the command, hence cannot contain blanks or other white space characters. The named variables must be valid netCDF variables in the input-file. The default, without this option and in the absence of the '-c' or '-h' options, is to include data values for all variables in the output.
-b lang
A brief annotation in the form of a CDL comment (text beginning with the characters '//') will be included in the data section of the output for each 'row' of data, to help identify data values for multidimensional variables. If lang begins with 'C' or 'c', then C language conventions will be used (zero-based indices, last dimension varying fastest). If lang begins with 'F' or 'f', then FORTRAN language conventions will be used (one-based indices, first dimension varying fastest). In either case, the data will be presented in the same order; only the annotations will differ. This option may be useful for browsing through large volumes of multidimensional data.
-f lang
Full annotations in the form of trailing CDL comments (text beginning with the characters '//') for every data value (except individual characters in character arrays) will be included in the data section. If lang begins with 'C' or 'c', then C language conventions will be used (zero-based indices, last dimension varying fastest). If lang begins with 'F' or 'f', then FORTRAN language conventions will be used (one-based indices, first dimension varying fastest). In either case, the data will be presented in the same order; only the annotations will differ. This option may be useful for piping data into other filters, since each data value appears on a separate line, fully identified. (At most one of '-b' or '-f' options may be present.)
-l len
Changes the default maximum line length (80) used in formatting lists of non-character data values.
-p float_digits[,double_digits]
Specifies default precision (number of significant digits) to use in displaying floating-point or double precision data values for attributes and variables. If specified, this value overrides the value of the C_format attribute, if any, for a variable. Floating-point data will be displayed with float_digits significant digits. If double_digits is also specified, double-precision values will be displayed with that many significant digits. In the absence of any '-p' specifications, floating-point and double-precision data are displayed with 7 and 15 significant digits respectively. CDL files can be made smaller if less precision is required. If both floating-point and double precisions are specified, the two values must appear separated by a comma (no blanks) as a single argument to the command.
-n name
CDL requires a name for a netCDF dataset, for use by 'ncgen -b' in generating a default netCDF dataset name. By default, ncdump constructs this name from the last component of the file name of the input netCDF dataset by stripping off any extension it has. Use the '-n' option to specify a different name. Although the output file name used by 'ncgen -b' can be specified, it may be wise to have ncdump change the default name to avoid inadvertently overwriting a valuable netCDF dataset when using ncdump, editing the resulting CDL file, and using 'ncgen -b' to generate a new netCDF dataset from the edited CDL file.
Specifies that special virtual attributes should be output for the file format variant and for variable properties such as compression, chunking, and other properties specific to the format implementation that are primarily related to performance rather than the logical schema of the data. All the special virtual attributes begin with '_' followed by an upper-case letter. Currently they include the global attribute “_Format” and the variable attributes “_Fletcher32”, “_ChunkSizes”, “_Endianness”, “_DeflateLevel”, “_Shuffle”, “_Storage”, and “_NoFill”. The ncgen utility recognizes these attributes and supports them appropriately.
Controls display of time data, if stored in a variable that uses a udunits compliant time representation such as “days since 1970-01-01” or “seconds since 2009-03-15 12:01:17”. If this option is specified, time values are displayed as human-readable date-time strings rather than numerical values, interpreted in terms of a “calendar” variable attribute, if specified. Calendar attribute values interpreted with this option include the CF Conventions values “gregorian” or “standard”, “proleptic_gregorian”, “noleap” or “365_day”, “all_leap” or “366_day”, “360_day”, and “julian”.


Look at the structure of the data in the netCDF dataset

ncdump -c

Produce an annotated CDL version of the structure and data in the netCDF dataset, using C-style indexing for the annotations:

ncdump -b c > foo.cdl

Output data for only the variables uwind and vwind from the netCDF dataset, and show the floating-point data with only three significant digits of precision:

ncdump -v uwind,vwind -p 3

Produce a fully-annotated (one data value per line) listing of the data for the variable omega, using FORTRAN conventions for indices, and changing the netCDF dataset name in the resulting CDL file to omega:

ncdump -v omega -f fortran -n omega > Z.cdl

Examine the translated DDS for the DAP source from the specified URL.

ncdump -h