ncdump [-chistxw] [-v var1,...] [-b lang] [-f lang]
[-l len] [-n name] [-p n[,n]] [-g grp1,...] file
ncdump -k file
The ncdump utility generates a text representation of a specified netCDF file on standard output, optionally excluding some or all of the variable data in the output. The text representation is in a form called CDL (network Common Data form Language) that can be viewed, edited, or serve as input to ncgen, a companion program that can generate a binary netCDF file from a CDL file. Hence ncgen and ncdump can be used as inverses to transform the data representation between binary and text representations. See ncgen documentation for a description of CDL and netCDF representations.
ncdump may also be used to determine what kind of netCDF file is used (which variant of the netCDF file format) with the -k option.
If DAP support was enabled when ncdump was built, the file name may specify a DAP URL. This allows ncdump to access data sources from DAP servers, including data in other formats than netCDF. When used with DAP URLs, ncdump shows the translation from the DAP data model to the netCDF data model.
ncdump may also be used as a simple browser for netCDF data files, 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 file. For netCDF-4 files, groups and user-defined types are also included in ncdump output.
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.
ncdump defines a default display format used for each type of netCDF data, but this can be changed if a `C_format' attribute is defined for a netCDF variable. In this case, ncdump will use the `C_format' attribute to format each value. For example, if floating-point data for the netCDF variable `Z' is known to be accurate to only three significant digits, it would be appropriate to use the variable attribute
- Show the values of coordinate variables (1D variables with the same names as dimensions) as well as the declarations of all dimensions, variables, attribute values, groups, and user-defined types. Data values of non-coordinate variables are not included in the output. This is usually the most suitable option to use for a brief look at the structure and contents of a netCDF file.
- Show only the header information in the output, that is, output only the declarations for the netCDF dimensions, variables, attributes, groups, and user-defined types 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 unescaped 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 [c|f]
- 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 [c|f]
- 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. If lang begins with 'F' or 'f', then FORTRAN language conventions will be used. 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 length
- Changes the default maximum line length (80) used in formatting lists of non-character data values.
- -n name
- CDL requires a name for a netCDF file, for use by 'ncgen -b' in generating a default netCDF file name. By default, ncdump constructs this name from the last component of the file name of the input netCDF file 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 file when using ncdump, editing the resulting CDL file, and using 'ncgen -b' to generate a new netCDF file from the edited CDL file.
- -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.
- Show kind of netCDF file, that is which format variant the file uses. Other options are ignored if this option is specified. Output will be one of 'classic'. '64-bit offset', 'netCDF-4', or 'netCDF-4 classic model'.
- 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 '_ChunkSizes', '_DeflateLevel', '_Endianness', '_Fletcher32', '_NoFill', '_Shuffle', and '_Storage'. 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 a human-readable date-time strings rather than numerical values, interpreted in terms of a 'calendar' variable attribute, if specified. For numeric attributes of time variables, the human-readable time value is displayed after the actual value, in an associated CDL comment. 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'.
- Same as the '-t' option, except output time data as date-time strings with ISO-8601 standard 'T' separator, instead of a blank.
- -g grp1,...
- The output will include data values only for the specified groups. One or more groups must be specified by name in the comma-delimited list following this option. The list must be a single argument to the command. The named groups must be valid netCDF groups 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 groups in the output.
- For file names that request remote access using DAP URLs, access data with client-side caching of entire variables.
- Output XML (NcML) instead of CDL. The NcML does not include data values. The NcML output option currently only works for netCDF classic model data.
Look at the structure of the data in the netCDF file foo.nc:
Produce an annotated CDL version of the structure and data in the netCDF file foo.nc, using C-style indexing for the annotations:
ncdump -b c foo.nc > foo.cdl
Output data for only the variables uwind and vwind from the netCDF file foo.nc, and show the floating-point data with only three significant digits of precision:
ncdump -v uwind,vwind -p 3 foo.nc
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 file name in the resulting CDL file to omega:
ncdump -v omega -f fortran -n omega foo.nc > Z.cdl
Examine the translated DDS for the DAP source from the specified URL:
Without dumping all the data, show the special virtual attributes that indicate performance-related characterisitics of a netCDF-4 file:
NOTE ON STRING OUTPUT
For classic, 64-bit offset or netCDF-4 classic model data, ncdump generates line breaks after embedded newlines in displaying character data. This is not done for netCDF-4 files, because netCDF-4 supports arrays of real strings of varying length.
What Every Computer Scientist should Know About Floating-Point Arithmetic, D. Goldberg, ACM Computing Surveys, Vol. 23, No. 1, March 1991, pp. 5-48.
[Climate and Forecast Metadata Conventions] (http://www.cfconventions.org/)