def concat_files(*files):
    """
    Concat some files together. Returns out and err to keep parity with shell commands.

    Args:
        *files: src1, src2, ..., srcN, dst.

    Returns:
        out: string
        err: string
    """
    out = ''
    err = ''
    dst_name = files[-1]
    sources = [files[f] for f in range(len(files)) if f < len(files) - 1]
    with open(dst_name, 'w') as dst:
        for f in sources:
            with open(f, 'r') as src:
                for line in src:
                    dst.write(line)
    return out, err