title: find and replace with sed
body: <p>I have the following problem.</p>
  
  <p>I have a file with a lot of words inside what I have to do is find these words in another file and replace them with just one letter.</p>
  
  <p>I do not know the words that I have to delete (too many!) so I don't know how to use the following sed command</p>
  
  <pre><code>$ sed -i 's/words_old/word_new/g' /home/user/test.txt
  </code></pre>
  
  <p>however I think I have to use also the cat command:</p>
  
  <pre><code>$ cat filewithwordstobedeleted.txt
  </code></pre>
  
  <p>but I don't know how to combine them.</p>
  
  <p>Thank you for helping me! :)</p>
  
  <p>Fabio</p>