Example of a systemd unit file and the script that it runs as a daemon. The script is from https://github.com/puppetlabs/razor-el-mk though I edited it a little to make the point a little clearer.

mk

#!/usr/bin/env ruby

# This is the /usr/local/bin/mk referenced in mk.service, with slight edits for readability
                                                              
require 'mk/script'

def usage(outcome, message = nil)
  ...
end

if ARGV[0] == "--daemon"
  daemon = true
  ARGV.shift
else
  daemon = false
end

# Figure out, and dispatch, to our command.
command = ARGV.shift or usage(false, 'No command was supplied')
MK::Script.respond_to?(command) or usage(false, "Unknown command #{command.inspect}")

result = false

begin
  loop do
    result = MK::Script.send(command, *ARGV)
    break unless daemon
    sleep 15
    puts "\nRunning at #{Time.now}"
  end
rescue Exception => e
  # FIXME: Be more discriminating about errors here; in particular,
  # do not die in daemon mode if we have network trouble
  # This would otherwise catch SystemExit, which we kind of don't want to do.
  usage(false, "error running #{command}: #{e}")
end

exit result

mk.service

[Unit]
Description=Microkernel agent

[Service]
Type=simple
WorkingDirectory=/var/lib/mk
StandardInput=null
StandardOutput=syslog
StandardError=inherit
SyslogIdentifier=microkernel
ExecStart=/usr/local/bin/mk --daemon register
# This is the killer: systemd will automatically restart the script
# if it dies (e.g., because of a SEGV) as long as that doesn't
# happen too often
Restart=always

[Install]
WantedBy=multi-user.target