I want to do nightly backups of the whole system and be able to rollback to any of last 7 days. And, it must be incremental backups. What tool should I use? The tutorials I've read about rsync tell only how to store latest incremental backup and I need last 7.
If you want to back up the whole system including the system folders like /var, /usr, and /etc, you'll need to do more than just installing some backup software. Backing up a running system is problematic unless you do the right things to enable it to happen reliably.
Either you'll need to reboot using a CD or external drive or another OS on the same machine so that the partitions and files you want to back up aren't in use during the backup process, or you'll need to use a filesystem setup that has snapshot capabilities, such as LVM and/or btrfs. A true full system backup would also include backing up the MBR, not just the files.
If you really want a full system backup, it won't be quick and easy. Do you really need that level of backup? Maybe all you really need is to back up specific folders in your home directory?
I am using duplicity. It is simple as soon as you read the manpage and figure out the command line options. I do backups for a single directory, not for the whole system though.
I think duplicity also has some GUI frontends, but interacting directly with the CLI app is not scary.