If you are confronted with an assailant with a decent knife, the best thing to do if possible, is to have on a good pair of running shoes and run like hell. Even Paul Vunak recommended that defensive maneuver. LOL
This is a true story. A friend of mine was walking through Kapiolani Park one night when a guy with a little 1.5" blade pen knife demanded his money. This friend had next to no martial arts skills but was taught how to deliver a sidekick to the knee, which he never practiced. He did know that a good kick to the knee will usually drop someone if they have their leg planted. He gave the guy a straight kick to the knee and the guy surprisingly enough dropped to the ground holding his knee in pain. Instead of running as he should, he asked the guy, "Now, was that a smart thing to do?" The guy swore at him, he laughed and walked away.
Another time we were trimming the coconut trees on the beach in Waikiki (this is done between 2200-0400 when the beach is empty) when two guys got into a fist fight. One of my guys ran over there to break up the fight when one of the guys pulled a knife on the other. The guy with the knife then threatened my guy. My guy held up his cane knife and told him my knife is bigger than yours so you better get out of here. A few seconds later you could hear police sirens and the guy with the knife ran. He would have been stupid if he tried to attack my guy with a 3" folder. Common sense will tell you that you don't attack a coconut tree trimmer who swings that cane knife hundreds of times accurately and hard everyday with a 3" blade. LOL