How to show progress when converting documents using libreoffice

When I convert documents using libreoffice on server. It takes a long time to process. I need to know progress to response to front end. But I don't find any info about progress of this conversion. I also try as below when converting doc to pdf:

exec("soffice --headless --convert-to pdf $oriFile --outdir $outputDirectory 1> block.txt 2>&1");


But in file block.txt is blank. It was not written anything about processing.

Anyone please help. Thanks!

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