After starting roo, I get an exception as shown below:
Welcome to Spring Roo. For assistance press TAB or type "hint" then hit ENTER.
NullPointerException at org.springframework.roo.project.AbstractProjectOperation
s.addModuleDependency(AbstractProjectOperations.java:293)
Its an embarrassing known bug. Just ignore it.
https://jira.springsource.org/browse/ROO-3111
Also make sure your Roo installation directory is writable for the user is using Roo (ie chmod -R a+rw roo-directory).