<s>[INST] def shift_cipher(text, shift):
    """
    This function performs a Caesar cipher (shift cipher) on the input text.
    
    Parameters:
    text (str): The text to be encrypted or decrypted.
    shift (int): The number of positions to shift the letters. A positive shift 
                 shifts to the right, a negative shift shifts to the left.
    
    Returns:
    str: The encrypted or decrypted text.
    """[/INST]