Query mosquitto MQTT broker on one specific topic for what ever is retained there, then disconnect

[K Smith]

Does anyone know how to query the mosquitto MQTT broker for one specific topic for what ever is retained there, then disconnect?
I have had a fairly extensive look through mosquito_sub man file and couldn't see anything, unless I missed it.

[Andy Stanford-Clark]

assuming you want to use mosquito_sub and not write a program to do it, then.

-C 1

… wait for one message to be received.

This only works if there’s definitely a retained topic there… otherwise it will wait until the next message comes along (which might be never ;))

If you are happy to write some code, you can add a watchdog timer to make it return “nothing” if the retained message doesn’t come through within a couple of seconds.

Andy

On 9 Jul 2019, at 11:25, K Smith <kes...@gmail.com> wrote:

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[K Smith]

Thank you.

I was querying the retained status of a sonoff with Tasmota flashed onto it (that is very sucessfully switching my printer in the living room on and off as needed). 

For no particular reason I though retained messages were set. They were'nt.

I had been going in circles and had tried -C 1 a number of times and thought it should work. 

It does work now that the topic is set to be retained by the broker

    mosquitto_sub -C 1 -h 192.168.1.120 -t home/livingroom/printer

Setting the topic to be retained by the broker on the Tasmota can be done two ways

- in the console type PowerRetain on, or 

 - enter into the web browser
http://192.168.1.245/cm?cmnd=PowerRetain%20on
where 192.168.1.245 is the IP address of the tasmota switch.

On Tuesday, 9 July 2019 11:32:27 UTC+1, andysc wrote:

assuming you want to use mosquito_sub and not write a program to do it, then.

-C 1

… wait for one message to be received.

This only works if there’s definitely a retained topic there… otherwise it will wait until the next message comes along (which might be never ;))

If you are happy to write some code, you can add a watchdog timer to make it return “nothing” if the retained message doesn’t come through within a couple of seconds.

Andy

On 9 Jul 2019, at 11:25, K Smith <kes...@gmail.com> wrote:

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[Andy Stanford-Clark]

That’s weird - a message from 2019 popped into my inbox!

On 8 Nov 2022, at 08:41, KC help <he...@kettlecompanion.com> wrote:

Great tips - thank you :)

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