Archana, I've never used the SshWrapper class before so I can't comment on the error you are having but I am curious as to why you don't just use an SSH client. There are several freely available ones out there. I personally use PuTTy which has telnet, ssh and more.
I did some searching on this class and found a link to the SOURCE. I noticed that the class is using a basic socket to connect. As I mentioned before I'm not familiar with the API other than just reading the source but to me that generic socket connection may be your problem if it isn't taking permission into account. If the SshWrapper class does not account for permissions on the machine you are trying to access then that may explain why you are getting NULL when you send a command. In other words, the Unix machine may permit you to do a remote SSH connection but may forbid you from changing directories or running other commands.
I am guessing that you want to just issue a
to the server and you can't even change directories (remote access permission block?).
My recommendations: 1) I would try to create my own custom socket server/client application that would read the files or even transfer them locally for reading 2) use an SSH client.