title: How do I join pairs of consecutive lines in a large file (1 million lines) using vim, sed, or another similar tool?
body: <p>I need to move the contents of every second line up to the line above such that line2's data is alongside line1's, either comma or space separated works.</p>
  
  <p>Input:</p>
  
  <pre><code>line1
  line2
  line3
  line4
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>line1 line2
  line3 line4
  </code></pre>
  
  <p>I've been doing it in vim with a simple recording but vim seems to crash when I tell it to do it 100 000 times...  I'm thinking maybe sed would be a good alternative but not sure how to do what I want or maybe there's a better option?  </p>
  
  <p>Each line only contains 1 numerical value, I just have a million lines...</p>