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I had this code in my sh file 
engine_public_version="$(/rdsdbbin/oscar/bin/mysql -u rdsbmsperf -p auroraperf -e "SELECT aurora_version();" -prdsbmsperf | grep -P '[\d.]+')" 
which on execution gives
engine_public_version: 3.999.95

But when I do this
 "engine_public_version=$(mysql -u rdsbmsperf -p auroraperf -e \"SELECT aurora_version();\" -prdsbmsperf | grep -P '[\\d.]+' | tr -d '\\n');" to reterive that in my code I get 

engine_public_version: as empty