ncdump(1) UNIDATA UTILITIES ncdump(1)
NAME
ncdump - Convert netCDF file to text form (CDL)
SYNOPSIS
ncdump [-chistxwF] [-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, 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
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 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. 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 [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. (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 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 `compact' 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', a variable
identified 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 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'.
-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 input-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.
-F Use _Filter and _Codecs attributes in place of _Fletcher32,
_Shuffle, and _Deflate.
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
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.
Release 4.2 2012-03-08 ncdump(1)
netcdf 4.9.0 - Generated Fri Jun 24 05:49:01 CDT 2022