 Base command
All on one line - try this as root (which has been hanging) and then run it as the zimbra user, see if there's a difference.
/opt/zimbra/java/bin/java -client -Dzimbra.home=/opt/zimbra -Djava.library.path=/opt/zimbra/lib -Djava.ext.dirs=/opt/zimbra/java/jre/lib/ext:/opt/zimbra/lib com.zimbra.cs.util.RandomPassword 8 10 |