pg_isready is utility to check whether postgres is running or not

-bash-4.2$ ./pg_isready 
/tmp:5432 - accepting connections

can write shell which will check and give alert if database down

vi alert.sh

while true; do
 if ! /opt/PostgreSQL/9.4/bin/pg_isready &>/dev/null; then
 echo 'database down';
 fi;
 sleep 3;
 done;