---
name: "ncmpidump"
section: "1"
platform: "linux"
package: "pnetcdf-openmpi"
version: "1.15.1-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/ncmpidump"
---

# ncmpidump(1)

Convert netCDF files to ASCII form (CDL)

## NAME

ncmpidump - Convert netCDF files to ASCII form (CDL)

## SYNOPSIS

**ncmpidump** **\[-ch\]** **\[-v** *var1,...***\]** **\[-b** *lang***\]** **\[-f** *lang***\]** **\[-l** *len***\]** **\[-n** *name***\]** **\[-p** *f\_digits\[,d\_digits\]***\]** *file*\

**ncmpidump** **-k** *file*

## DESCRIPTION

**ncmpidump** *generates an ASCII representation of a specified netCDF file on* *standard output.  The ASCII representation is in a form called CDL* *(\`\`network Common Data form Language'') that can be viewed, edited, or serve* *as input to* **ncmpigen***.*  **ncmpigen** *is a companion program that can* *generate a binary netCDF file from a CDL file.  Hence* **ncmpigen** *and* **ncmpidump** *can be used as inverses to transform the data representation* *between binary and ASCII representations.  See* **ncmpigen** *for a description* *of CDL and netCDF representations.*

**ncmpidump** *may also be used to determine what kind of netCDF file is used* *(which variant of the netCDF file format) with the -k option.*

**ncmpidump** *defines a default 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,* **ncmpidump** *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"*

**ncmpidump** *may also be used as a simple browser for netCDF data* *files, to display the dimension names and sizes; 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.*

**ncmpidump** *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 if the variable is not of* *byte type.*

## OPTIONS

- **-c**

  *Show the values of coordinate variables (variables that are also* *dimensions) as well as the declarations of all dimensions, variables, and* *attribute values.  Data values of non-coordinate variables are not included* *in the output.  This is 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 the* *declarations of dimensions, variables, and attributes 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,...,varn*

  *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 blanks or other white space characters.  The named* *variables must be valid netCDF variables in the input-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** *lang*

  *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 is useful for browsing* *through large volumes of multidimensional data.*

- **-f** *lang*

  *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 (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 piping data into other filters, since each data value* *appears on a separate line, fully identified.*

- **-l** *len*

  *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 data set, for use by* **ncmpigen -b** *in* *generating a default netCDF file name.  By default, ncmpidump constructs* *this name from the last component of the pathname 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* **ncmpigen -b** *can be* *specified, it may be wise to have ncmpidump change the default name to* *avoid inadvertantly overwriting a valuable netCDF file when using* **ncmpidump***, editing the resulting CDL file, and using* **ncmpigen -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 for* *any variable that has such an attribute.* *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-precision precisions are specified, the two values must appear* *separated by a comma (no blanks) as a single argument to the command.* *If you really want every last bit of precision from the netCDF file* *represented in the CDL file for all possible floating-point values, you will* *have to specify this with* **-p 9,17** *(according to Theorem 15 of the* *paper listed under REFERENCES).*

- **-k**

  *Reports the kind of netCDF file: classic, 64-bit offset, or 64-bit data.* *Before netCDF version 3.6, there was only one kind of netCDF file,* *designated as \`classic' (also know as format variant 1 or CDF-1).* *Large file support introduced another variant of the format, designated as* *\`64-bit offset' (known as format variant 2 or CDF-2).* *Large data support introduced another variant of the format, designated as* *\`64-bit data' (known as format variant 5 or CDF-5).*

## EXAMPLES

*Look at the structure of the data in the netCDF file \`***foo.nc***':*

*ncmpidump -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:*

*ncmpidump -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:*

*ncmpidump -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':*

*ncmpidump -v omega -f fortran -n omega foo.nc \> Z.cdl*

## REFERENCES

 *What* *Every Computer Scientist should Know About Floating-Point Arithmetic, D.* *Goldberg,* **ACM Computing Surveys, Vol. 23, No. 1***, March 1991, pp. 5-48.*

## SEE ALSO

[**ncmpigen**(1)](/ncmpigen/1.md), [**pnetcdf**(3)](/pnetcdf/3.md)

## DATE

*September 18, 2026*

## 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.*