Collected info from a few threads and corrected a few things that were similar but not exactly the same on Debian stretch.
Here goes:
sudo apt-get update
sudo apt-get install apt-transport-https sudo nano git-core python software-properties-common dirmngr -y
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
sudo echo "deb http://download.mono-project.com/repo/debian stretch main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
sudo apt-get update
sudo apt-get install mono-devel -y
Download Debian *.deb file from Duplicati/download
(edit the url below to be exactly as the one found as the Debian link/download button on the web page above)
wget https://updates.duplicati.com/beta/duplicati_2.0.3.3-1_all.deb
Edit this line too before you run it with the correct deb file filename you just downloaded:
sudo apt-get install ./duplicati_2.0.3.3-1_all.deb -y
Create and edit the file /etc/systemd/system/duplicati.service
sudo nano /etc/systemd/system/duplicati.service
Should look like this:
[Unit]
Description=Duplicati web-server
After=network.target
[Service]
Nice=19
IOSchedulingClass=idle
EnvironmentFile=-/etc/default/duplicati
ExecStart=/usr/bin/duplicati-server $DAEMON_OPTS
Restart=always
[Install]
WantedBy=multi-user.target
Edit the file /etc/default/duplicati and add DAEMON_OPTS options to your liking:
sudo nano /etc/default/duplicati
# Defaults for duplicati initscript
# sourced by /etc/init.d/duplicati
# installed at /etc/default/duplicati by the maintainer scripts
#
# This is a POSIX shell fragment
#
# Additional options that are passed to the Daemon.
DAEMON_OPTS="--webservice-interface=any --webservice-port=8200 --portable-mode"
Enable, start and check running status of the duplicati service:
sudo systemctl enable duplicati.service
sudo systemctl daemon-reload
sudo systemctl start duplicati.service
sudo systemctl status duplicati.service
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Threads I picked up info from: