title: curl: how to not overwrite existing file?
body: <p>Given a +350MB file online <a href="http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/data/ice_surface/grid_registered/georeferenced_tiff/ETOPO1_Ice_g_geotiff.zip" rel="nofollow">ETOPO1_Ice_g_geotiff.zip</a></p>
  
  <p>Within a script, the following <code>curl</code> command is currently used for downloading :</p>
  
  <pre><code>curl  -o ../data/ETOPO1/ETOPO1.zip \
    'http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/data/ice_surface/grid_registered/georeferenced_tiff/ETOPO1_Ice_g_geotiff.zip'
  </code></pre>
  
  <p>Given the target size and downloading time (~20min), I want to download it the first time only. Then, when I restart the whole script, the command should notice the file as already there, and <strong>NOT</strong> redownloading it.</p>
  
  <p>Also, when the output <code>-o</code> already exist, <strong>how to NOT restart the download/overwrite the file</strong> ?</p>
  
  <p>Note: preference for <code>curl</code>, but other utilities welcome (<code>wget</code>, ...)</p>