title: "Unprocessed input" error using `cdo` in loop with variable
body: <p>I am trying to embed a <code>cdo</code> line in a shell script with variables. As a command line it works, but not in a script.  This is the <code>cdo</code> line:</p>
  <pre><code>cdo -expr,'T_mask=((T &gt; 200 ))' file_in.nc file_out.nc
  </code></pre>
  <p>In the shell script I want to run it for three different temperature thresholds, <em>e.g.</em> 200, 250, 300.</p>
  <p>This is the code:</p>
  <pre><code>T_string='T_mask=((T &gt; '
  fileA='file_in.nc'
  fileB='file_out.nc'
  for T_var in 200 250 300; do
      cdo_string=&quot;'$T_string$T_var))'&quot;
      cdo  -expr,$cdo_string  $fileA $fileB
  done
  </code></pre>
  <p>I get the following error:</p>
  <pre><code>cdo (Abort): Unprocessed Input, could not process all Operators/Files
  </code></pre>
  <p>What did I miss?
  Are the hyphens set right?</p>