Terrorism is the weapon of the weak.
Terrorists use carefully scripted acts of violence to try and achieve a political aim that they could not achieve by main force alone. That's the point. They can't achieve their goals unless we let let them; they're not strong enough.
So by all means attend the candle-lit vigils and share the Je suis Paris memes, but at the end of the day all that stuff is meaningless. The correct reaction to any terrorist incident is to stiffen the upper lip and hold the fucking line.