Example of a systemd service file to start a Sinatra app.

foo.service

# /etc/systemd/system/foo.service
[Unit]
Description=Foo service
Wants=nginx.service
Requires=postgresql.service
After=postgresql.service

[Service]
Type=simple
User=foo
Group=foo
ExecStart=/srv/foo/app/serve
Restart=always
TimeoutSec=10

[Install]
WantedBy=multi-user.target

nginx-config

# /etc/nginx/sites-enabled/foo
# example nginx config, which proxies /foo to this app
server {
  # ...basic stuff omitted...
  location / {
    try_files $uri $uri/ =404;
  }
  
  location /foo {
    # this nginxroot directory has "foo" symlinked to /srv/foo/app/public
    # to avoid having to rewrite that prefix out here :)
    root /srv/foo/nginxroot;
    try_files $uri @foosinatra;
  }
  
  location @foosinatra {
    include /etc/nginx/sinatra_params;
    proxy_pass http://localhost:1234;
  }
}

serve

#!/bin/sh

cd $(dirname $(readlink -f "$0"))
rackup --env production -o 127.0.0.1 -p 1234 >>log/production.log 2>&1

sinatra_params

# /etc/nginx/sinatra_params
# extra headers that seem to be needed for Sinatra apps, especially when nginx uses https
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Forwarded-Host $host;
proxy_set_header Host $http_host;
proxy_set_header Connection "";
proxy_http_version 1.1;

And I guess I'll have to put extra info here. foo.service goes to /etc/systemd/system/foo.service (at least in Ubuntu 16.04). Run systemctl daemon-reload, systemctl start foo. systemctl enable foo to make it autostart on boot.