title: what is the difference between 'sort -k1 file.txt' and 'sort -k1,1 file.txt'?
body: <p>if a file contains multiple columns, seperated by comma, like this:</p>
  
  <pre><code>aaa,1,4,4,5,7
  bbb,1,4,9,1,2
  </code></pre>
  
  <p>Is there difference between 'sort -t, -k1 file.txt' and 'sort -t, -k1,1 file.txt'? 
  though with the example above, there is no difference, but in some of my project case, it
  does has difference, but the difference is reflected in the case that I use the sorted file to join, and the join command throws out exception that 'join: file 2 is not in sorted order' (at that time , I use 'sort -t, -k1 file.txt') . Later I use 'sort -t, -k1,1 file.txt', and join command works well then. Can anybody tell me why?</p>