title: tar and tee command, md5sum on the fly
body: <p>Why is creating an archive like :</p>
  
  <p><code>tar -p -c -f - --acls file1 | tee mybackup.tar | md5sum</code></p>
  
  <p>give a different checksum than doing md5sum on this same archive, doing :</p>
  
  <p><code>tar --to-command=md5sum -x -f mybackup.tar</code> (This is the same as <code>tar -x -f mybackup.tar -O | md5sum</code>)</p>
  
  <p>The objective here is to catch md5sum on stdout while doing the archiving. Avoiding me to "dry-run" a tar only for md5sum.</p>