NCDUMP(1) UNIDATA UTILITIES NCDUMP(1)NAMEncdump - Convert netCDF file to text form (CDL)
SYNOPSISncdump [-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, 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 gener‐
ate 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 out‐
put.
ncdump uses `_' to represent data values that are equal to the `_Fill‐
Value' 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' 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 signifi‐
cant 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 di‐
mensions, 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 val‐
ues for multidimensional variables. If lang begins with `C' or
`c', then C language conventions will be used (zero-based in‐
dices, 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 an‐
notations 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 be‐
ginning 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 lan‐
guage 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 annota‐
tions 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
-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 con‐
structs 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 inadver‐
tently 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 over‐
rides the value of the C_format attribute, if any, for a vari‐
able. 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 signifi‐
cant digits respectively. CDL files can be made smaller if less
precision is required. If both floating-point and double preci‐
sions are specified, the two values must appear separated by a
comma (no blanks) as a single argument to the command. (To rep‐
resent every last bit of precision in a CDL file for all possi‐
ble floating-point values would require -p 9,17.)
-k Show kind of netCDF file the pathname references, one of `clas‐
sic', `64-bit offset',`netCDF-4', or `netCDF-4 classic model'.
Before version 3.6, there was only one kind of netCDF file, des‐
ignated as `classic' (also know as format variant 1). Large
file support introduced another variant of the format, designat‐
ed 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-re‐
lated information about the file format and variable properties
for netCDF-4 data. These special virtual attributes are not ac‐
tually part of the data, they are merely a convenient way to
display miscellaneous properties of the data in CDL (and eventu‐
ally NcML). They include `_ChunkSizes', `_DeflateLevel', `_En‐
dianness', `_Fletcher32', `_Format', `_NoFill', `_Shuffle', and
`_Storage'. `_ChunkSizes' is a list of chunk sizes for each di‐
mension 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. `_Fletch‐
er32' is `true' if the checksum property was set for the vari‐
able. `_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 persis‐
tent 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. `_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', a variable
identified in a "bounds" attribute of such a time variable, or a
numeric attribute of a time variable. If this option is speci‐
fied, 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 at‐
tributes 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', `prolep‐
tic_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 ab‐
sence 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 da‐
ta 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 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 signif‐
icant 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 ALSOncgen(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 short‐
er 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)
