With the following bash script I get error message "./GetGridVoltage.sh: line 3: [: 0.0: integer expression expected"

Please tell me how to fix my error.

The program mosquitto_sub is going to stay running and you won't get just one result in $()

I'd move the work into AWK:

(untested)

You might need to have AWK take buffering into account, you'll have to test.

If it gets too much more complex then consider having a Perl or Python script interact with MQTT directly rather than via the Mosquitto client.

Would the $2 have the value of grid_voltage?

It would depend on what you are getting from Mosquitto.

Please show some sample lines of output from mosquitto_sub as you have it configured and we can guess.

Also port 1883 implies that you are making decisions based on unencrypted messages. If this is for anything serious, you need to consider sending MQTT over TLS.

solar_assistant/inverter_1/pv_voltage/state 64.2
solar_assistant/inverter_1/grid_frequency/state 0.0
solar_assistant/inverter_1/pv_power/state 1216
solar_assistant/inverter_1/battery_voltage/state 53.7
solar_assistant/inverter_1/load_apparent_power/state 437
solar_assistant/inverter_1/temperature/state 53.0
solar_assistant/inverter_1/load_percentage/state 8
solar_assistant/inverter_1/battery_current/state 15.0
solar_assistant/inverter_1/grid_power/state 0
solar_assistant/inverter_1/device_mode/state Solar/Battery
solar_assistant/inverter_1/grid_voltage/state 0.0
solar_assistant/inverter_1/ac_output_frequency/state 50.0
solar_assistant/inverter_1/pv_current/state 18.9
solar_assistant/inverter_1/ac_output_voltage/state 230.0

This is just monitoring my solar system. I via this script wish to inform my tenants of wether we are on the grid or using batteries due to grid being down.
I can extract the value from MQTT but get an error when using it in an if statement.

1 members found this post helpful.

Thanks. $1 will be solar_assistant/inverter_1/ac_output_voltage/state and $2 will be 230.0 because white space is the default field separator (FS) in AWK.

You can narrow the subscription there substantially so that Mosquitto shows only the output from the relevant topic, such as solar_assistant/inverter_1/ac_output_voltage/state or any other item there. See "man mosquitto_sub" and scroll down to -t or see "man 7 mqtt"

If you are watching more than one inverter, then you can subscribe to the topics with a wildcard: solar_assistant/+/ac_output_voltage/state

In that case, top priority ought to be deploying TLS first, before working on the rest. Otherwise, with unencrypted connections anything can happen, including spoofed messages.
That's not what was happening. It's what you wanted but the [url=https://mywiki.wooledge.org/CommandSubstitution]command substitution[/font] was capturing something else.

Then the error message is correct: "0.0" isn't an integer value.

Specifically, AWK can handle floating point to an extent, Bash cannot. Bash itself handles only integers.

Be aware that Turbocapitalist has referenced the wrong field, but I'm sure that'll be put right.

Can you use grid_voltage/state ? - that looks like it'll be a binary 0/1, should make using it easier.

Thank you NevemTeve and Turbocapitalist but I should have seen that 0.0 was not an integer.

grid_voltage/state also returns the voltage as a float

I think you can specify the topic:
Additionally you do not need to calculate that much:
may work (not tested)

Tried pan64 suggestion but val is still a float
#
mosquitto_sub -h 192.168.8.119 -p 1883 -v -t '#' -C 15 | grep grid_voltage | awk 'NF>1{print $2}'

val=$(awk '{($2>200)?a=1:a=0;print a}')
echo $val
gpio mode 6 output
gpio write 6 $val
gpio mode 6 input
#

what you posted is not a working script. Otherwise would be nice to use code tags.
I did not post complete solution, you need to adjust it.

[code]
[/code]
@OP There is an underlined part in it, try to find out what it is good for.

1 members found this post helpful.