spark-shell intends to set MASTER automatically if we do not provide the option when we start the shell , but there's a problem. 
The condition is "if [[ "x" != "x$SPARK_MASTER_IP" && "y" != "y$SPARK_MASTER_PORT" ]];" we sure will set SPARK_MASTER_IP explicitly, the SPARK_MASTER_PORT option, however, we probably do not set just using spark default port 7077. So if we do not set SPARK_MASTER_PORT, the condition will never be true. We should just use default port if users do not set port explicitly I think.