With quotes:
cat /etc/*-release | grep "PRETTY_NAME" | sed 's/PRETTY_NAME=//g'
gives output as:
"CentOS Linux 7 (Core)"
Without quotes:
cat /etc/*-release | grep "PRETTY_NAME" | sed 's/PRETTY_NAME=//g' | sed 's/"//g'
gives output as:
CentOS Linux 7 (Core)