How to fix missing and underreplicated blocks - HDFS

$ su - <$hdfs_user> 

$ hdfs fsck / | grep 'Under replicated' | awk -F':' '{print $1}' >> /tmp/under_replicated_files

$ for hdfsfile in `cat /tmp/under_replicated_files`; do echo "Fixing $hdfsfile :" ; hadoop fs -setrep 3 $hdfsfile; done

In above command, 3 is the replication factor. If you are using single datanode, it must be 1.