title: Extracting just a phone number from a file
body: <p>I'm sure the answer to this is already online however i don't know what i am looking for. I just started a course in Unix/Linux and my dad asked me to make him something for his work. He has a text file and on every fourth line there is a 10 digit number somewhere. How do i make a list of just the numbers? I assume the file looks something like this:</p>
  
  <pre><code>Random junk
  Random junk fake number 1234567809
  Random junk
  My phone number is 1234567890 and it is here random numbers 32131;1231
  Random junk
  Random junk another fake number 2345432345
  Random junk
  Just kidding my phone number is here 1234567890 the date is mon:1231:31231
  </code></pre>
  
  <p>I assume its something like grep <code>[1-9].\{9\}</code> file but how do i get just lines 4,8,12 etc. Because i tested it and i get all phone numbers on every line. Also how do i get just the number not the whole line?</p>
  
  <p>Any help will be greatly appreciated, even if its pointing me in the right direction so i can research it myself. Thanks.</p>