ncdump - Convert netCDF file to text form (CDL)
ncdump [-chistxwF] [-v var1,...] [-b lang] [-f
lang] [-l len] [-n name] [-p
f_digits[,d_digits]] [-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
- -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.
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 dataset name in the resulting CDL file to `omega':
ncdump -v omega -f fortran -n omega foo.nc > Z.cdl
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.