title: tar pre-run to evaluate expected size or amount of files
body: <p>The problem:
  I have a back-end process that at some point he collect and build a big tar file. 
  This tar receive few directories and an exclude files. 
  the process can take up to few minutes and i want to report in my front-end process (GUI) about the progress of the taring process (This is a big issue for a user that press download button and it seems like nothing is happening...). </p>
  
  <p>i know i can use -v -R in the tar command and count files and size progress but i am looking for some kind of tar pre-run mode / dry run to help me evaluate either the expected number of files or the expected tar size. </p>
  
  <p>the command I am using: <code>tar -jcf 'FILE.tgz' 'exclude_files' 'include_dirs_and_files'</code></p>
  
  <p>10x for everyone who is willing to assist.</p>