This seems to work from my brief testing:

#+AUTHOR:
#+OPTIONS: toc:nil
#+PROPERTY: header-args:emacs-lisp :var tar-file="bar.tar.gz" :exports none

#+NAME: tar-eg
#+BEGIN_SRC emacs-lisp
  (let ((contents (format "> tar xzvf %s" tar-file)))
    (with-temp-buffer
      (insert contents)
      (write-file "results.txt"))
    contents)
#+END_SRC

#+CALL: tar-eg() :wrap example :exports results