ncmpidiff(1)

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), cdfdiff(1), diff(1), pnetcdf(3)

DATE
July 31, 2025