Provided by: netcdf-bin_4.1.3-7ubuntu2_amd64 
NAME
ncdump - Convert netCDF file to text form (CDL)
SYNOPSIS
ncdump [-c] [-h] [-v var1,...] [-b lang] [-f lang] [-l len] [-n name] [-p f_digits[,d_digits]] [-k] [-x]
[-s] [-t] file
DESCRIPTION
ncdump generates a text representation of a specified netCDF file on standard 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. ncgen is 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 for a description of CDL and netCDF representations.
As of NetCDF version 4.1, and if DAP support was enabled when ncdump was built, the file name may specify
a DAP URL. This allows ncdump to print out data sources from DAP servers. When used with the -h option,
ncdump can be used to show the translation from the DAP DDS data model to the NetCDF data model.
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
Z:C_format = "%.3g"
ncdump may also be used as a simple browser for netCDF data files, to display the dimension names and
sizes; 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.
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 if the variable is not of byte type.
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.
OPTIONS
-c 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 the most suitable option to use for a brief
look at the structure and contents of a netCDF file.
-h Show only the header information in the output, that is the declarations of dimensions, variables,
and attributes 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,...,varn
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 unless escaped. The named
variables must be valid netCDF variables in the input-file. A variable within a group in a
netCDF-4 file may be specified with an absolute path name, such as `/GroupA/GroupA2/var'. Use of
a relative path name such as `var' or `grp/var' specifies all matching variable names in the 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 is 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.
-l len 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 data set, for use by ncgen -b in generating a default netCDF file
name. By default, ncdump constructs this name from the last component of the path 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 for any variable that has such an attribute. 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-precision precisions are specified, the two values must appear
separated by a comma (no blanks) as a single argument to the command. If you really want every
last bit of precision from the netCDF file represented in the CDL file for all possible floating-
point values, you will have to specify this with -p 9,17 (according to Theorem 15 of the paper
listed under REFERENCES).
-k Show what kind of netCDF file the pathname references, one of `classic', `64-bit
offset',`netCDF-4', or `netCDF-4 classic model'. Before version 3.6, there was only one kind of
netCDF file, designated as `classic' (also know as format variant 1). Large file support
introduced another variant of the format, designated as `64-bit offset' (known as format variant
2). NetCDF-4, uses a third variant of the format, `netCDF-4' (format variant 3). Another format
variant, designated `netCDF-4 classic model' (format variant 4), is restricted to features
supported by the netCDF-3 data model but represented using the HDF5 format, so that an unmodified
netCDF-3 program can read or write the file just by relinking with the netCDF-4 library. The
string output by using the `-k' option may be provided as the value of the `-k' option to ncgen(1)
to specify exactly what kind of netCDF file to generate, when you want to override the default
inferred from the CDL.
-x 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.
-s Output special virtual attributes that provide performance-related information about the file
format and variable properties for netCDF-4 data. These special virtual attributes are not
actually part of the data, they are merely a convenient way to display miscellaneous properties of
the data in CDL (and eventually NcML). They include `_ChunkSizes', `_DeflateLevel',
`_Endianness', `_Fletcher32', `_Format', `_NoFill', `_Shuffle', and `_Storage'. `_ChunkSizes' is
a list of chunk sizes for each dimension of the variable. `_DeflateLevel' is an integer between 0
and 9 inclusive if compression has been specified for the variable. `_Endianness' is either
`little' or `big', depending on how the variable was stored when first written. `_Fletcher32' is
`true' if the checksum property was set for the variable. `_Format' is a global attribute
specifying the netCDF format variant, one of `classic', `64-bit offset', `netCDF-4', or `netCDF-4
classic model'. `_NoFill' is `true' if the persistent NoFill property was set for the variable
when it was defined. `_Shuffle' is `true' if use of the shuffle filter was specified for the
variable. `_Storage' is `contiguous' or `chunked', depending on how the variable's data is
stored.
-t 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'.
EXAMPLES
Look at the structure of the data in the netCDF file `foo.nc':
ncdump -c 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 dataset name in the resulting CDL file to
`omega':
ncdump -v omega -f fortran -n omega foo.nc > Z.cdl
REFERENCES
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.
SEE ALSO
ncgen(1), netcdf(3)
BUGS
Character arrays that contain a null-byte are treated like C strings, so no characters after the null
byte appear in the output.
Multidimensional character string arrays are not handled well, since the CDL syntax for breaking a long
character string into several shorter lines is weak.
There should be a way to specify that the data should be displayed in `record' order, that is with the
all the values for `record' variables together that have the same value of the record dimension.