This info is from http://www.noobslab.com/2014/02/set-nemo-as-default-file-manager-in.html Show archive.org snapshot . It applies to GNOME based desktops.
> xdg-mime default nemo.desktop inode/directory application/x-gnome-saved-search
> gsettings set org.gnome.desktop.background show-desktop-icons false
> gsettings set org.nemo.desktop show-desktop-icons true
Check:
> xdg-mime query default inode/directory
...should return nemo.desktop.
To revert back to Nautilus:
> xdg-mime default nautilus*.desktop inode/directory application/x-gnome-saved-search
> gsettings set org.nemo.desktop show-desktop-icons false
> gsettings set org.gnome.desktop.background show-desktop-icons true
