How to grab mplayer output to file?

I need something like this:
When I’ll redirect command by FIFO to mplayer echo “get_property volume” > /tmp/fifo I want to have this command output in output.txt, e.g ANS_volume=100.

I have tried something like this
mplayer -slave -quiet -idle -input file=/tmp/fifo file.mp3 > /tmp/output.txt
But this doesn’t work – file is empty.
I saw a few people did this in this way but in my case this isn’t working.

Anyone knows?

Thank You!!!

I didn’t find the right solution from the Internet.
References
https://unix.stackexchange.com/questions/336091/how-to-grab-mplayer-output-to-file
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