Can hackers gain access to a webcam without turning the webcam on? Can this be done on Ubuntu?
Yes, this can happen. This attack is typically targeted at MS Windows users.
The counter measure is, as usual, to have all your components up to date.
Note that the light of your webcam will be on, which is an indication of the attack (
it is not possible to disable this functionality it may be possible to disable the light on older machines or specific configurations - thanks @silverpenguin for the comment).
Is it silly to cover my webcam?
It's not silly to cover your webcam if you don't want the "hacker" to know what you look like. But you'd better not have any pictures of yourself on your computer, or anything personal for that matter. They could probably find your name by keylogging/watching you trough a RAT, and if you have a facebook, they could look you up and find out what you look like anyway.
It's also not silly if you're engaging in, er, relations. Paranoia.
And you never know who could be watching, especially if they're able to bypass the webcam light icon. I personally keep a dud webcam on as part of an intrusion detection mechanism. If it's turned on, someone tried to access it. It's gone off a few times, and has helped me track down the culprit.
Can hackers and access to a webcam without turning webcam on
Not if there's a hardware on/off switch. The webcam must be physically turned on, unless you can turn on the webcam with software, but then it would have to be running to receive that signal anyway.
They can, however, activate the webcam and start viewing you, and anything within line of sight.
can they attack on Ubuntu(a GNU/Linux system)
If there are drivers for it, and their RAT supports this, then yes. But they probably can't easily turn it on if there's a hardware on/off switch.