Problem scenario
You run "hadoop version" but you receive this message "Error: Could not find or load main class org.apache.hadoop.util.VersionInfo". What do you do?
Possible Solution #1
Use "sudo " before the "hadoop version" command.
Possible Solution #2
Use "sudo -i " before the "hadoop version" command.
Possible Solution #3
Use a different user. Sometimes Hadoop is set up to be run under a different user (e.g., hduser).
Possible Solution #4
Re-install Hadoop. If you need assistance, see this posting.