Usage: gdal_contour [--help] [--long-usage] [--help-general]
                    [-b <name>] [-a <name>] [-amin <name>] [-amax <name>] [-3d]
                    [-inodata] [-snodata <value>]
                    [[-i <interval>]|[-e <base>]]
                    [-fl <level>] [-off <offset>] [-nln <name>] [-p]
                    [-gt <n>|unlimited] [--quiet] [-oo <NAME>=<VALUE>]...
                    [-dsco <NAME>=<VALUE>]... [-of <output_format>]
                    [-co <NAME>=<VALUE>]... [-lco <NAME>=<VALUE>]...
                    src_filename dst_filename

One of -i, -fl or -e must be specified.

Note: gdal_contour --long-usage for full help.