I must be a little dumb but when it try the -c option, it close effectively the connection but it don't send the datas...(the connection just open and close)

Thank you but -q 0 don't works for me (certainly my version of netcat on Synology)
I've found a temporary solution by adding this at the end of the script: os.execute("sleep 1 && killall netcat");

It work perfectly but, if i have a netcat session open for another usage (not domoticz) it will be ...

Ok Thank you waaren.
I have read the post and i found my ridiculous issue: I've just forgot the path before netcat (/opt/bin/netcat)
Now, the sript works but not completely (not very responsive).
After this, the Domoticz logs wtite an error message on each scene command:
2019-04-24 01:49:13.976 ...

Thank you but it don't works:
The escape caracters are already in the line i've tested before.
When i try yours, i've a syntax error.
With my first version: no syntax error...

Hello.
Here is my configuration:
Domoticz on Synology
Yeelight Bulb
Lua Scene Script:
runcommandoff = "echo -ne '{\"id\":1,\"method\":\"set_power\", \"params\":[\"off\", \"smooth\", 500]}\\r\\n' | netcat -w1 " ..IP.." " ..PORT.."";
os.execute(runcommandoff);
print(runcommandoff)

Here is my ...