why is not possible to convert a file from .zip compression to tar.gz
compression.. using only "unzip -c file.zip | gzip -c > file.tar.gz".
I afirm that is possible with conversion tar.gz to tar.bz2 with
"gunzip -c file.tar.gz | bzip2 -c > tar.bz2". In this example we se
the '-c' parameter used in the comands gunzip and bzip2, it means 'do
what the comand do, not to a file but to the standard output.

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Clarification of Question by seishi-ga on 11 Oct 2006 12:35 PDT

andre@ubuntu:~$ gzip -V
gzip 1.3.5

As an aside, I don't think the proper extension is just .gz or .bz2. 
The .tar refers to a tar archive which this is not.

As for your question, what error do you get? What version of gzip do
you have (gzip -V). I just tried something similar and it worked.