Help - zabbix calculated items

[Spectator]

I want to display a calculated value.
One value is a modbus query value.
This is what this item looks like in Zabbix:

Name: Active energy
Type: Zabbix agent
Key: modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,24576,,float]
Type of information: Numeric (float)
Units: !kWh


The current value of this measurement is 1410.27002

I want to subtract the other value from this value.
The other value is in a file:
zabbix_get -s 127.0.0.1 -p 10050 -k vfs.file.contents[/tmp/powermeter_daily_beginning.txt]
1400.310059

I don’t want to measure this other value continuously in Zabbix, it’s just needed for subtraction.

I created a new item:

Name: Daily active energy
Type: Calculated
Key: daily.active.energy
Type of information: Numeric (float)

But the "Formula" .... has a problem with that.
I tried this:

(//modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float])-(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])

Result:
Cannot parse formula: item query must be first argument of a historical function at "//modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float])-(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])"

last(//modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float])-(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])

Result:
Cannot parse formula: item query must be first argument of a historical function at "//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])"


last(//modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float])-last(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])

Result:
Cannot evaluate function: item "/Pro1-mod_native/modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float]" does not exist at "last(//modbus.get[rtu://{$MBPORT}:{$MBBAUDRATE}:{$MBDATABITS}{$MBPARITY}{$ MBSTOPBIT},{$MBID},3,20514,,float])-last(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])".


How could I implement this subtraction?

[LenR]

I think both must be items, so load your value from the file into Zabbix as an item, then subtract one item from the other.

[Spectator]

Dear LenR,

Thanks for your response!

I added the value in the text file to Zabbix as a new item:

Name: AEBD
Type: Zabbix agent
Key:vfs.file.contents[/tmp/powermeter_daily_beginning.txt]
Type of information: text (but I tried Numeric (float) also)

The contents of this txt file currently: 1413.040039 and the zabbix shows it correctly.

And to get started in a simpler way, subtracting the AEBD value from a fixed static number, I wanted to get the new calculated item.
That is, 5000 minus the contents of the /tmp/powermeter_daily_beginning.txt file. So I want to subtract 1413.040039 from 5000.

I created a new calculated item with this paramters:
Name: Active energy - daily calculated
Type: Calculated
Key: daily.active.energy
Type of information: Numeric (float)
Formula... I tried these with the following results:

(5000)-last(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])
Cannot evaluate function: item "/Pro1-mod_native/vfs.file.contents[/tmp/powermeter_daily_beginning.txt]" does not exist at "last(//vfs.file.contents[/tmp/powermeter_daily_beginning.txt])"

(5000)-last(/vfs.file.contents[/tmp/powermeter_daily_beginning.txt])
Cannot parse formula: invalid item query starting at "/vfs.file.contents[/tmp/powermeter_daily_beginning.txt])"

(5000)-last(vfs.file.contents[/tmp/powermeter_daily_beginning.txt])
Cannot parse formula: invalid token starting with "vfs.file.contents[/tmp/powermeter_daily_beginning.txt])"

(5000)-last(AEBD)
Cannot parse formula: invalid token starting with "AEBD)"

(5000)-last(/AEBD)
Cannot parse formula: invalid item query starting at "/AEBD)"

(5000)-last(//AEBD)
Cannot evaluate function: item "/Pro1-mod_native/AEBD" does not exist at "last(//AEBD)"

I don't think there are any more variations ...
How should a numeric value read from a txt file be subtracted from a fixed number? I'm clueless...