code: def _get_file_list(orig_files, out_file, regions, ref_file, config):
      
      sorted_files = _sort_by_region(orig_files, regions, ref_file, config)
      exist_files = [(c, x) for c, x in sorted_files if os.path.exists(x) and vcf_has_variants(x)]
      if len(exist_files) == 0:  # no non-empty inputs, merge the empty ones
          exist_files = [x for c, x in sorted_files if os.path.exists(x)]
      elif len(exist_files) > 1:
          exist_files = _fix_gatk_header(exist_files, out_file, config)
      else:
          exist_files = [x for c, x in exist_files]
      ready_files = run_multicore(p_bgzip_and_index, [[x, config] for x in exist_files], config)
      input_file_list = "%s-files.list" % utils.splitext_plus(out_file)[0]
      with open(input_file_list, "w") as out_handle:
          for fname in ready_files:
              out_handle.write(fname + "\n")
      return input_file_list
docs: Create file with region sorted list of non-empty VCFs for concatenating.