Hi Vincenzo,
That's a normal and documented behavior. Quoting GNU Tar Manual,
subsection 8.1.1 "Creating and Reading Compressed Archives":
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. For example:
$ cat archive.tar.gz | tar tf -
tar: Archive is compressed. Use -z option
tar: Error is not recoverable: exiting now
Regards,
Sergey
