Clearing customs in Chicago
 

So I am taking a flight from Nashville to Chicago and then Chicago to Toronto.

Do I have to clear customs? I won't be taking any checked baggage.

I ask because I have a super tight connection of 50 mins, and I have to change terminals.

I am flying United, booked all one ticket.

I am Canadian.

Thanks in advanced.

The U.S. doesn't have exit immigration/customs, so there will be no formalities in Chicago. You can just walk from one gate to the next.

The connection between Terminal 2 and Terminal 1 won't be a problem with 50 minutes if your flight is on time. They are connected airside, which is to say you won't have to re-clear security.

If your departing flight is in the "C" gates, and you aren't a fast walker, you may want to consider taking the shuttle bus that departs for near gate E4 and ends up at C9. It cuts down the distance, but if you move briskly you will likely be able to get there on foot faster.

Thanks for the replay, in fairly fit guy so I don't have a problem giving it a sprint if I have to. Sounds like it is doable if everything is on time. Thanks for your replay!

You may not need to change terminals, it all depends on your flights that day. Make sure you check the monitors before you go walking to a different terminal. As noted, T1 and T2 are connected airside, make sure you don't leave airside.

No US customs (you may have to deal with Canadian customs in Toronto).
Try to sit as far in front of the plane as possible in Nashville-Chicago flight.
Explain your tight connection to the crew and try to leave as early as possible.
Pay attention if United has any staff waiting to help pax on a tight connection.
Otherwise find out your gate and walk fast. :)
Do not leave secure area by mistake.

This is true enough, but 50 minutes isn't an especially tight connection in ORD. There will likely be others with similar or even shorter connections. If your arrival is more or less on time, I wouldn't bother drawing attention to yourself unless you are just completely disoriented and truly need the help.