
      ( EXPRESSION )	Returns the value of EXPRESSION
      ! EXPRESSION		True if EXPRESSION is false; else false
      EXPR1 && EXPR2	True if both EXPR1 and EXPR2 are true; else false
      EXPR1 || EXPR2	True if either EXPR1 or EXPR2 is true; else false
    
    When the `==' and `!=' operators are used, the string to the right of
    the operator is used as a pattern and pattern matching is performed.
    When the `=~' operator is used, the string to the right of the operator
    is matched as a regular expression.
    
    The && and || operators do not evaluate EXPR2 if EXPR1 is sufficient to
    determine the expression's value.
    
    Exit Status:
    0 or 1 depending on value of EXPRESSION.