When people train with me they learn knife sparring first. There are many reasons for this: unlike firearms knives are found in every home that needs defending, knife sparring teaches evasive footwork that translates well to many situations outside of employing a knife, knife sparring makes you understand the various ways you can be attacked by a knife, etc.
I very often see YouTubers suggesting that if someone has a knife, there's nothing you can do but run. However it turns out that may not a be a reliable solution.
The reality is having a knife pulled on you is a very common self defense situation, and one of the few that justifies spending a lot of time studying self defense in the first place. Every martial art has to be considered in this context: what if they have a Knife?
People think of sparring oriented stick fighting styles like Arnis and Kendo as bladed fighting arts. However were they really shine is for using sticks to knock a knife out of the hand of someone causing problems. This is true of virtually all sparring oriented stick, cane and staff fighting systems: they are great for dealing with a knife attacker.
Brazillian Jiu Jitsu compared to stick fighting does not fair well. If you don't spar with any strikes, defending against a force multiplier like a knife is nearly impossible. If you pull guard on a knife attacker, you just made his job of killing you much easier as now you can not evade him with footwork.
Wrestling does much better than BJJ in the context of a knife, because Wrestling has tools used in sparring for getting back to their feet. Wrestling's raw explosive aggression is more appropriate for a situation with a knife than BJJ's chilled laid back mentality. Wrestling is more focused on takedowns that will make a knife attacker far less dangerous. However not handling strikes in wrestling makes them generally unfamiliar with how to handle a weapon swinging or stabbing at them.
Some think Western Boxing changed drastically when the USA got involved with the Philippines. Filipino Boxing is more or less that square style of boxing with your hands up and forearms facing your opponent like you see with Manny Pacquiao or Mike Tyson. If you are going to be struck with a knife, you want it to be on the outside of your forearms. Having clever head movement and defensive footwork focused on KO strikes is a viable option for defending against a knife.
Tae Kwon Do and Capoiera have been mocked for being impractical. However in the context of a knife, on the street someone has shoes on, the bottom of those shoes is probably the best striking weapon they have against a blade. Jumping back and managing distance while looking for a chance to KO the attacker with a kick is not a bad option to have compared to grappling.
This is another place Muay Thai shines. With the aggression of wrestling and most of the same tools as both boxing and Tae Kwon Do, if someone must take on a knife wielder unarmed, Muay Thai is one of your better training backgrounds to have. Muay Thai has footwork and potent kicks to keep the enemy at bay, while also having practical sweeps and takedowns with which they may be able to get the knife attacker on the ground.
As Christopher Hein has pointed out, Aikido theory makes the most sense when a knife is in play. I would add that Aikido's defensive footwork and courage to use practice knives frequently in Randori actually teach the Aikidoka a lot more about knife fighting than they would like to admit.
There are too many Karate styles to analyze here one by one how effective they are against a knife. However all Karate styles seem to have some kind of clue about where a person could start with knife defense as well as practical kicks.
There are at least 10 times as many Kung Fu systems as Karate styles. In general Kung Fu systems are more comfortable with using bladed weapons for self defense, train in variety of weapons so that they can use any improvised weapons available to them should they have to face someone with a knife, and they have more answers for defending against bladed weapons while unarmed than most other martial arts.
Next time you are evaluating a martial art, don't forget to ask how it would work against a knife.
