The sad thing is that this is happening so frequently in France that I'm starting to get desensitized to it. Like this is just normal life for the French these days, they're getting a handful of these terror attacks every year now.
What a heck can anyone do about it?
Invade privacy, near-constant surveillance of your own people, foster a national sense of paranoia and distrust in anyone and anything unusual or weird. Basically, become America. I have noticed that we haven't had quite the level of international terrorist attacks that many European nations seem to have. That said, we have a
lot of domestic violence. Which I wonder if it isn't a by-product of this "trust no one" mentality we all seem to have, here. Us and our automatic guns under our pillows or under our beds as we sleep, because we all believe that someone will break into our house in the middle of the night and we will have to shoot them... which what that says about a society's collective worldview is yours to judge.
Any measure taken to prevent terrorism will have side-effects. In the USA we just decided that if anyone is going to kill Americans in mass numbers, it was going to be Americans.
Honestly, if it gets any worse, and these people keep doing the things they are doing in the name of zeal, there may just come a point where a real, honest-to-goodness war ensues where America (and England will be there, of course) and co. have to draft young men and go deal with these countries. We haven't had a military beyond volunteer level since I have been alive.
Anyway, I doubt France has any interest in becoming America, but we have managed to prevent a lot of international terrorism... at a cost.