howto use tar to untar (unzip) tar.bz tar.gz tar.xz files
howto use tar to untar (unzip) tar.bz tar.gz tar.xz files
apparently the latest (xubuntu) version of tar now AUTOMATICALLY recognises the compression that has been used
The only case when you have to specify a decompression option while reading the archive is when reading from a pipe or from a tape drive that does not support random access. However, in this case GNU tar will indicate which option you should use.