gdal_merge.py [-o out_filename] [-of out_format] [-co NAME=VALUE]* [-ps pixelsize_x pixelsize_y] [-separate] [-v] [-pct] [-ul_lr ulx uly lrx lry] [-n nodata_value] [-init value] [-ot datatype] [-createonly] input_files
NOTE: gdal_merge.py is a Python script, and will only work if GDAL was built with Python support.