What you need to backup depends on your particular system*.
So this is going to take a little work on your part to sort out. Start by figuring out what does not need to be backed up. First take a look at your root directory, and then work backwards.
cd /; ls -F gives me:
bin/ initrd.img@ mnt/ snap/ vmlinuz@
boot/ initrd.img.old@ opt/ srv/ vmlinuz.old@
cdrom/ lib/ proc/ sys/
dev/ lib64/ root/ tmp/
etc/ lost+found/ run/ usr/
home/ media/ sbin/ var/
/mnt are mount points so don't need backup.
/tmp get auto re-created on reboot. [I'm guessing the links:
/vmlinuz@, /vmlinuz.old@ get re-created on boot ubuntu reinstall (I'm not sure which).]
On my system
/root is empty (use
sudo -s to open a shell as root user to view it ... be careful to
exit immediately after you inspect
/snap is also empty. Perhaps it a mount point.
/var ONLY if you really want to. (" Contains variable data like system logging files, mail and printer spool directories, and transient and temporary files." ref: http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/var.html)
/sbin presumably will get reloaded via a reinstall of Ubuntu unless you are doing system development work or something like that. You can either back these up or rely on a fresh install to recover them.
/home should be in it's own backup. There will be times when you will want to restore just
That leaves other changes you've made to your system in
/usr which you will want to also backup, either together or individually.
Here are a couple of pages that might help understand these directories:
A related line of thinking is: Say you just installed a fresh Ubuntu. What would you need to back up? Answer: nothing. You haven't changed anything yet, so you can simply reinstall Ubuntu. It restores /bin, /etc, /root, /usr, etc.
So the only reason you might want to backup /bin is because you have changed it or added to it. So part of backing up is understanding what is where and when it is created and modified. Just know that the rest of us struggle with this too.
*****And, although you didn't ask, one can make full disk or partition images. These take a lot of time to backup and restore and may leave your system down while that work is proceeding. And it's how I used to backup my Windows systems using Acronis. The one thing they provide you with is a partition map, and images from non-linux partitions.
(I'm very open to suggestions as to how I could make this better.)