ref-DigitalOcean

Install

64 bit:

cd ~
wget -O dropbox-linux-x86_64.tar.gz https://www.dropbox.com/download?plat=lnx.x86_64 && \
sudo mkdir -p /opt/dropbox && \
sudo tar xzfv dropbox-linux-x86_64.tar.gz --strip 1 -C /opt/dropbox && \
sudo ln -s /opt/dropbox/dropboxd /usr/bin/dropboxd && \
dropboxd &

for 32 bit OS the url is: xxxx?plat=lnx.x86

Install Command Line Interface (CLI) [Optional]

cd ~
sudo wget -O /usr/bin/dropbox.py https://www.dropbox.com/download?dl=packages/dropbox.py && \
sudo chmod +x /usr/bin/dropbox.py && \
ln -s /opt/dropbox ~/.dropbox-dist && \
dropbox.py status # run CLI

see CLI usage in ref

Auto Start (as root) in Ubuntu

sudo wget -O /etc/init.d/dropbox https://gist.githubusercontent.com/thisismitch/d0133d91452585ae2adc/raw/699e7909bdae922201b8069fde3011bbf2062048/dropbox && \
sudo chmod +x /etc/init.d/dropbox && \
echo 'DROPBOX_USERS="root"' > /etc/default/dropbox && \
sudo service dropbox start && \
sudo update-rc.d dropbox defaults

OBS: space-separated sys usernames for multi users, see ref-DigitalOcean for details.
Tip: for other systems, see here