I am trying to install a compressed package ending in .tar.gz and I am getting an error saying that it is not in the right format,

I am new to Ubuntu and after doing some research I still can't figure what I am doing wrong. I am downloading the a package ending in .tar.gz,then I when I try to use:

tar -xvf matlab_r2011b.tar.gz

I get the following error:

gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now

What am I doing wrong?

Try checking the file type with the following command:

file matlab_r2011b.tar.gz

If it tells you that it is a tar archive rather than gzip compressed data, then it is possible that the file got decompressed while being downloaded (possibly due to a faulty web server configuration on the other end). If that is the case, just rename the archive and try again:

mv matlab_r2011b.tar.gz matlab_r2011b.tar
tar xvf matlab_r2011b.tar