def cat_cmd(last_output, expected_output):
	"""Command to convert or concat files.

	>>> cat_cmd('file1.osm', 'file2.osm.pbf')
	('osmium cat file1.osm -o file2.osm.pbf', None)
	>>> cat_cmd('file1.osm file2.osm.pbf', 'file3.osm.gz')
	('osmium cat file1.osm file2.osm.pbf -o file3.osm.gz', None)
	>>> cat_cmd('file1.osm file2.osm.pbf', 'file3.osm.gz')
	('osmium cat file1.osm file2.osm.pbf -o file3.osm.gz', None)
	"""
	return f'osmium cat {last_output} -o {expected_output}', None