gdal_contour -V (return code: 1)
FAILURE: Neither -i nor -fl are specified.
GDAL 2.2.1, released 2017/06/23
Usage: gdal_contour [-b <band>] [-a <attribute_name>] [-3d] [-inodata]
[-snodata n] [-f <formatname>] [-i <interval>]
[-f <formatname>] [[-dsco NAME=VALUE] ...] [[-lco NAME=VALUE] ...]
[-off <offset>] [-fl <level> <level>...]
[-nln <outlayername>] [-q]
<src_filename> <dst_filename>
gdal_contour --help (return code: 1)
Usage: gdal_contour [-b <band>] [-a <attribute_name>] [-3d] [-inodata]
[-snodata n] [-f <formatname>] [-i <interval>]
[-f <formatname>] [[-dsco NAME=VALUE] ...] [[-lco NAME=VALUE] ...]
[-off <offset>] [-fl <level> <level>...]
[-nln <outlayername>] [-q]
<src_filename> <dst_filename>