# java -XshowSettings:properties -version 2>&1 > /dev/null | grep 'java.home'
    java.home = /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-0.b14.el7_4.x86_64/jre
	
set JAVA_HOME in ./etc/hadoop(hadoop-env.sh

From hadoop installation directory 
$ ./bin/hadoop jar share/hadoop/mapreduce/hadoop-mapreduce-examples-3.0.0.jar wordcount input output

output directory should not exist before running above

-- output directories should not exist
./bin/hadoop jar ./share/hadoop/mapreduce/hadoop-mapreduce-examples-3.0.0.jar grep input output2 'dfs[a-z.]+'

./bin/hadoop jar ./share/hadoop/mapreduce/hadoop-mapreduce-examples-3.0.0.jar grep input output3 'yar[a-z.]+'