Ubuntu Manpage: ncdump - Convert netCDF file to text form (CDL)

NAME

       ncdump - Convert netCDF file to text form (CDL)

SYNOPSIS


       ncdump  [-chistxw]  [-v  var1,...]   [-b  lang] [-f lang] [-l len] [-n name] [-p f_digits[,d_digits]] [-g
              grp1,...]  file

       ncdump -k file

DESCRIPTION

The ncdump utility generates a text representation of a specified netCDF file on standard output, option‐
ally 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 da‐
ta 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, in‐
tended 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' at‐
tribute 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"

OPTIONS

-c 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.

-h Show only the header information in the output, that is, output only the declarations for the di‐
mensions, 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 com‐
mand, hence cannot contain unescaped blanks or other white space characters. 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 de‐
fault, 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 iden‐
tified. (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 val‐
ues.

-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 in‐
put 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 nc‐
dump 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 respec‐
tively. 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. (To represent every last bit of precision in a CDL file for all
possible floating-point values would require -p 9,17.)

-k Show 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.

-s Output special virtual attributes that provide performance-related information about the file for‐
mat 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 de‐
fined. `_Shuffle' is `true' if use of the shuffle filter was specified for the variable. `_Stor‐
age' 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 repre‐
sentation such as `days since 1970-01-01' or `seconds since 2009-03-15 12:01:17', a variable iden‐
tified in a "bounds" attribute of such a time variable, or a numeric attribute of a time variable.
If this option is specified, time data values are displayed as human-readable date-time strings
rather than numerical values, interpreted in terms of a `calendar' variable attribute, if speci‐
fied. 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'.

-i 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,...
For netCDF-4 files, 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 in‐
put-file. A group in a netCDF-4 file may be specified with an absolute or relative path name.
Use of a relative path name specifies all matching group 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 groups
in the output.

-w For file names that request remote access using DAP URLs, access data with client-side caching of
entire variables.

-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.

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 in‐
       dexing 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 float‐
       ing-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

BUGS