$ gemmi cif2mtz -h
Usage:
  gemmi cif2mtz [options] CIF_FILE MTZ_FILE
  gemmi cif2mtz [options] CIF_FILE --dir=DIRECTORY
Options:
  -h, --help               Print usage and exit.
  -V, --version            Print version and exit.
  -v, --verbose            Verbose output.
  -b NAME, --block=NAME    mmCIF block to convert.
  -d DIR, --dir=NAME       Output directory.
  --title                  MTZ title.
  -H LINE, --history=LINE  Add a history line.
  -u, --unmerged           Write unmerged MTZ file(s).

First variant: converts the first block of CIF_FILE, or the block
specified with --block=NAME, to MTZ file with given name.

Second variant: converts each block of CIF_FILE to one MTZ file
(block-name.mtz) in the specified DIRECTORY.

If CIF_FILE is -, the input is read from stdin.