title: os.system() execute command under which linux shell?
body: <p>I am using <code>/bin/tcsh</code> as my default shell.</p>
  <p>However, the tcsh style command <code>os.system('setenv VAR val')</code> doesn't work for me. But <code>os.system('export VAR=val')</code> works.</p>
  <p>So my question is how can I know the <code>os.system()</code> run command under which shell?</p>