---
name: "ncmpidiff"
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/ncmpidiff"
---

# ncmpidiff(1)

compares two netCDF files in parallel

## NAME

ncmpidiff - compares two netCDF files in parallel

## SYNOPSIS

**mpiexec -n np ncmpidiff** **\[-b\]** **\[-q\]** **\[-h\]** **\[-v var1,...,varn\]** **\[-t diff,ratio\]** *file1 file2*

## DESCRIPTION

**ncmpidiff** *runs in parallel on np number of MPI processes to compare the* *contents of the two files and reports the first difference to the standard* *output.*

*For variables and attributes, it reports the array indices of the first* *element found different when option* **-t** *is not used. When option* **-t** *is used, it reports the element with the largest difference that fails to meet* *the tolerance requirements.*

*If neither argument -v nor -h is given besides the two file names, the entire* *files are compared.*

*When comparing two files entirely, the difference between* **ncmpidiff** *and the* *Unix command* **diff** *is that* **ncmpidiff** *skips the gaps between variables. The* *gaps may occur when the alignment feature is used to create a new file. This* *alignment allows to allocate a larger space for the file header and align the* *starting file offsets of fixed-size variables (see API* **ncmpi\_\_enddef** *and* **PnetCDF hints***). Oftentimes, the contents of gaps are non-zero arbitrary* *bytes. Thus, two netCDF files (of same or different sizes) can be reported* *identical by* **ncmpidiff** *but not by* **diff***.*

## OPTIONS

- **-b**

  *Verbose mode - print results (same or different) for all components (file,* *header, or variables) in comparison*

- **-q**

  *Quiet mode - print nothing on the command-line output. This also disables* *verbose mode. When in quiet mode, users should check exit status. See below in* *"EXIT STATUS".*

- **-h**

  *Compare file header only*

- **-v** var1,...,varn

  *Compare only the given list of variables (names separated by comma without* *space).*

- **-t** diff,ratio

  *Compare variables element-wisely with tolerance (diff and ratio separated by* *comma without space).  diff is the absolute value of element-wise* *difference of  two variables with the same name but stored in the two input* *files.  ratio is the relative element-wise difference ratio defined as* *\|x - y\|/max(\|x\|, \|y\|), where x is an array element from a variable in the first* *file and y is the corresponding array element of the same variable in the* *second file. \|x\| represents the absolute value of x. Note when this option is* *used, the output reports only the first array element that fails to meet both* *tolerance requirements.*

## EXIT STATUS

*An exit status of 0 means no differences were found, and 1 means some* *differences were found.  Note on VMS-based system, the exit status values are* *reversed.*

## SEE ALSO

[**ncmpidump**(1)](/ncmpidump/1.md), [**cdfdiff**(1)](/cdfdiff/1.md), [**diff**(1)](/diff/1.md), [**pnetcdf**(3)](/pnetcdf/3.md)

## DATE

*September 18, 2026*