Thanks! script works when put the string into a pair of single quote. But I found this may not be the true cause to make the problem. actually, user put that string in double quote, and eval remove it in silence. Anyway, I realize eval input from users is very danger, it treat them with some extra actions background, and even it would be made use of by some ppl, though it looks a easy way to implement. I've changed the design and thus the question is going away .