Many assaults and beatings could be avoided if the victim can effectively defend themselves. There are a whole range of "Equalizer opportunities", but there are few such use of which may poorly trained and not very athletic user. The cheapest, most reliable - and thus - the most popular solution is pepper spray.
Now is the time. You noted that you must obtain the knife. It is not a simple matter ... Another type of knife will need a student, another office worker, and yet another engineer, policeman, soldier or lover bushcraftu / survival.
You have to face the truth. Knife, albeit very useful - not always enough. As a rule, does not serve to undermine subjects, screwing or removing screws from the walls. I mean, it can be done even a pen, but why? For this kind of work better to give up - although more dainty than a regular knife - Swiss army knife. Possibly his nemesis - multipurpose tool called less multitool.