In general, carrying a gun requires a permit, but as Vic noted, knife laws vary by State, and sometimes even by county or city. They're generally passed as a knee-jerk reaction to a perceived problem rather than an actual one -- the mafia and teen gangs carry switchblades in the movies, so we'll ban them; Asian gangs carry butterfly knives, so we'll ban them too; Mexican gangs carry Bowie knives, so we'll ban them.

Carrying a knife doesn't usually require a permit, but many of the laws are quite general like, "A knife may not be carried as a weapon." Then, they'll classify certain characteristics that automatically classify a knife as a weapon, such as being a fixed-blade, or the way a folder opens, or blade length. However, even if your knife is otherwise legal, making a statement indicating your intent is to carry a knife as a weapon (the "defensive" distinction doesn't matter) then you've crossed the line. As with most things, enforcement is likely to be very selective, but the safest thing to do is always assert that your knife is a cutting tool.