I use Ubuntu 17.10 with i3 as window manager. I would like to backup my home directory to an external HDD. How could I do such a thing? With the terminal?
Best tool for this is Aptik. It encrypts and backs up and then restores with all the permissions, with everything going back into its place, rather than just copying.
It has a simple GUI. See this for complete instructions on installation and usage.
Linux distributions come with their own backup tool - RSync. (CLI utility). It is a very reliable backup tool and a Graphical Frontend can also be installed - GRsync. You can install the application using this command:
sudo apt install grsync
Grsync has the same useful options like the CLI utility. See this video for more detailed overview. You can backup to any location.