title: filter with grep to print when text is NOT present in another file
body: <p>I have a linux filter to extract all lines from an xcode project that contain localized strings and produce a sorted list of unique entries. The filter works fine and is shown below. </p>
  
  <pre><code>grep NSLocalized *.m | perl -pe 's/.*NSLocalizedString\((.+?)\,.*/$1/' | sort | uniq 
  </code></pre>
  
  <p>The result is a list of strings looking like this</p>
  
  <pre><code>@"string1"
  @"string2"
  etc
  </code></pre>
  
  <p>What I now need to do is identify the entries that do not exist within another textfile. So imagine I have a text file containing;</p>
  
  <pre><code>@"string1"
  @"string3"
  etc
  </code></pre>
  
  <p>The result would be <code>@"string2"</code> as it is not present in the file</p>
  
  <p>For the sake of argument, the file is named list.txt</p>
  
  <p>What do I need to add to my filter? I'm sure I can do this with grep but my brain has failed!</p>