title: Problem with commande "Convert" on Ubuntu
body: <p>I'm trying to convert a PDF to TIFF </p>
  
  <pre><code>$ convert -density 300 mydoc.pdf[0] -depth 8 -strip -background white -alpha off mydoc.tiff
  </code></pre>
  
  <p>But I am getting this error : </p>
  
  <pre><code>convert-im6.q16: FailedToExecuteCommand `'gs' -sstdout=%stderr -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=2 '-sDEVICE=pngalpha' -dTextAlphaBits=4 -dGraphicsAlphaBits=4 '-r300x300' -dFirstPage=1 -dLastPage=1 '-sOutputFile=/tmp/magick-7634jYcQFgAWFuMP%d' '-f/tmp/magick-7634mHbM1fZhBVNJ' '-f/tmp/magick-7634ZhGvd52RltPD'' (-1) @ error/delegate.c/ExternalDelegateCommand/462.
  convert-im6.q16: no images defined `mydoc.tiff' @ error/convert.c/ConvertImageCommand/3258.
  </code></pre>
  
  <p>I am using this version of gs and ImageMagick: </p>
  
  <pre><code>gs -v
  GPL Ghostscript 9.26 (2018-11-20)
  Copyright (C) 2018 Artifex Software, Inc.  All rights reserved.
  
  convert --version
  Version: ImageMagick 6.9.7-4 Q16 x86_64 20170114 
  http://www.imagemagick.org
  Copyright: © 1999-2017 ImageMagick Studio LLC
  License: http://www.imagemagick.org/script/license.php
  Features: Cipher DPC Modules OpenMP 
  Delegates (built-in): bzlib djvu fftw fontconfig freetype jbig jng jpeg lcms lqr ltdl lzma openexr pangocairo png tiff wmf x xml zlib
  </code></pre>
  
  <p>And I am on Ubuntu 18.04.1.</p>
  
  <p>Can someone help me ?</p>