---
name: "ncdump"
section: "1"
platform: "linux"
package: "netcdf"
version: "4.10.1-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/ncdump"
---

# ncdump(1)

Convert netCDF file to text form (CDL)

## 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)](/ncgen/1.md) 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**

  *When used with* **-s***, display filter information using the generic* *\_Filter and \_Codecs virtual attributes instead of the* *legacy per-filter attributes \_Fletcher32, \_Shuffle, and* *\_Deflate.  The \_Codecs attribute lists the names of all* *filters applied to a variable (e.g., shuffle, deflate,* *fletcher32, szip); the \_Filter attribute provides* *the corresponding HDF5 filter identifier and any parameters.  Filters* *that may appear include the built-in HDF5 filters (deflate/zlib,* *shuffle, fletcher32, szip) as well as any additional filters provided* *by installed HDF5 plugins, such as bzip2 (filter ID 307) or* *Zstandard (filter ID 32015).*

## 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)](/ncgen/1.md), [**netcdf**(3)](/netcdf/3.md)

## 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.*