def sort_csv(in_file):
    out_file = "%s.sort" % in_file
    if not (os.path.exists(out_file) and os.path.getsize(out_file) > 0):
        cl = ["sort", "-k", "1,1", in_file]
        with open(out_file, "w") as out_handle:
            child = subprocess.Popen(cl, stdout=out_handle)
            child.wait()
    return out_file