# apt install lsyncd

# lsyncd --version

output:
Version: 2.2.3

settings {
        logident        = "lsyncd",
        logfacility     = "user",
        logfile         = "/var/log/lsyncd/lsyncd.log",
        statusFile      = "/var/log/lsyncd/status.log",
        statusInterval  = 1
}
initSync = {
        delay = 1,
        maxProcesses = 1,
        action = function(inlet)
                local config = inlet.getConfig()
                local elist = inlet.getEvents(function(event)
                        return event.etype ~= "Init"
                end)
                local directory = string.sub(config.source, 1, -2)
                local paths = elist.getPaths(function(etype, path)
                        return "\t" .. config.syncid .. ":" .. directory .. path
                end)
                log("Normal", "Processing syncing list:\n", table.concat(paths, "\n"))
                spawn(elist, "/usr/sbin/csync2", "-C", config.syncid, "-x")
        end,
        collect = function(agent, exitcode)
                local config = agent.config
                if not agent.isList and agent.etype == "Init" then
                        if exitcode == 0 then
                                log("Normal", "Startup of '", config.syncid, "' instance finished.")
                        elseif config.exitcodes and config.exitcodes[exitcode] == "again" then
                                log("Normal", "Retrying startup of '", config.syncid, "' instance.")
                                return "again"
                        else
                                log("Error", "Failure on startup of '", config.syncid, "' instance.")
                                terminate(-1)
                        end
                        return
                end
                local rc = config.exitcodes and config.exitcodes[exitcode]
                if rc == "die" then
                        return rc
                end
                if agent.isList then
                        if rc == "again" then
                                log("Normal", "Retrying events list on exitcode = ", exitcode)
                        else
                                log("Normal", "Finished events list = ", exitcode)
                        end
                else
                        if rc == "again" then
                                log("Normal", "Retrying ", agent.etype, " on ", agent.sourcePath, " = ", exitcode)
                        else
                                log("Normal", "Finished ", agent.etype, " on ", agent.sourcePath, " = ", exitcode)
                        end
                end
                return rc
        end,
        init = function(event)
                local inlet = event.inlet;
                local config = inlet.getConfig();
                log("Normal", "Recursive startup sync: ", config.syncid, ":", config.source)
                spawn(event, "/usr/sbin/csync2", "-C", config.syncid, "-x")
        end,
        prepare = function(config)
                if not config.syncid then
                        error("Missing 'syncid' parameter.", 4)
                end
                local c = "csync2_" .. config.syncid .. ".cfg"
                local f, err = io.open("/etc/" .. c, "r")
                if not f then
                        error("Invalid 'syncid' parameter: " .. err, 4)
                end
                f:close()
        end
}
local sources = {
        ["/opt/testdir"] = "csync2"
}
for key, value in pairs(sources) do
        sync {initSync, source=key, syncid=value}
end

# systemctl start lsyncd.service

output:
● lsyncd.service - LSB: lsyncd daemon init script
     Loaded: loaded (/etc/init.d/lsyncd; generated)
     Active: active (running) since Thu 2024-05-30 02:09:42 UTC; 15min ago
       Docs: man:systemd-sysv-generator(8)
    Process: 7389 ExecStart=/etc/init.d/lsyncd start (code=exited, status=0/SUCCESS)
      Tasks: 1 (limit: 2177)
     Memory: 612.0K
        CPU: 32ms
     CGroup: /system.slice/lsyncd.service
             └─7395 /usr/bin/lsyncd -pidfile /var/run/lsyncd.pid /etc/lsyncd/lsyncd.conf.lua

May 30 02:09:42 csync3 systemd[1]: Starting LSB: lsyncd daemon init script...
May 30 02:09:42 csync3 lsyncd[7389]:  * Starting synchronization daemon lsyncd
May 30 02:09:42 csync3 lsyncd[7394]: 02:09:42 Normal: --- Startup, daemonizing ---
May 30 02:09:42 csync3 lsyncd[7389]:    ...done.
May 30 02:09:42 csync3 systemd[1]: Started LSB: lsyncd daemon init script.

# touch /opt/testdir/testing.txt

# tail -f /var/log/lsyncd/lsyncd.log

Thu May 30 03:23:01 2024 Normal: Processing syncing list:
  csync2:/opt/testdir/.testing.txt.pZCAyj
  csync2:/opt/testdir/.testing.txt.pZCAyj
  csync2:/opt/testdir/.testing.txt.pZCAyj
  csync2:/opt/testdir/.testing.txt.pZCAyj
Thu May 30 03:23:01 2024 Normal: Finished events list = 0

Setup CSync Server