Below is some tips when running Hadoop on Ubuntu. If you find some errors running Hadoop on Ubuntu please comment the problem and how you solved it.

When you get this Warning: $HADOOP_HOME is deprecated

Solution: add “export HADOOP_HOME_WARN_SUPPRESS=”TRUE”"  in the hadoop-env.sh.

Cannot create directory `/usr/local/hadoop/libexec/../logs

Solution: sudo chown -R hduser:hadoop /usr/local/hadoop/

Enter passphrase when running ./start-all.sh

Solution: ssh-keygen -t rsa -P “”     Create a ssh-key without a password.

Warning: <property>/<configuration> not set

Solution: make sure <property> and <configuration> tags are populated in core.site.xml, mapred.site.xml, hdfs.site.xml

Send or retrieve file to and from HDFS

Solution:

Send file to HDFS > bin/hadoop dfs -put /home/someone/interestingFile.txt /user/yourUserName/

Get file from HDFS > bin/hadoop dfs -get foo localFile

ssh: connect to host localhost port 22: Connection refused
Solution: By default Ubuntu will not have ssh installed so you will have to install and start it.

Install > sudo apt-get install ssh

Start > sudo service ssh start

hadoop Incompatible namespaceIDs in /app/hadoop/tmp/*

Solution: 

1. Stop the cluster: ./stop-dfs.sh
2. Delete the directory specified on the DataNode: rm -r /app/hadoop/tmp/*
3. Reformat the NameNode: hadoop namenode -format

OR

1. Stop the DataNode: ./stop.dfs.sh
2. Edit the value of namespaceID in /current/VERSION to match the value of the current NameNode.
3. Restart the DataNode: ./start.dfs.sh

hadoop java.net.UnknownHostException: ubuntu: ubuntu

Solution: 

1. Add ubuntu as your localhost IP to your /etc/hosts file: sudo vi /etc/hosts

2. Restart your network: sudo /etc/init.d/networking restart

So your /etc/hosts file on your master machine will look something like this:

172.16.62.152      master 172.16.62.151 slave 172.16.62.152 ubuntu

On your slave machine

172.16.62.152      master 172.16.62.151 slave 172.16.62.151 ubuntu

If none of it works then you can go and change the master/localhost hostname to the ipaddress in core-site.xml, mapred-site.xml