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When I run this in my sh file 
engine_public_version="$(/rdsdbbin/oscar/bin/mysql -u rdsbmsperf -p auroraperf -e "SELECT aurora_version();" -prdsbmsperf | grep -P '[\d.]+')"

I get the output 
but when I do this in my python file
cmds.append(
            "engine_public_version=$(mysql -u rdsbmsperf -p auroraperf -e \"SELECT aurora_version();\" -prdsbmsperf | grep -oP '[\\d.]+' | tr -d '\\n')"
        )

I get empty output