title: Using ./update.sh instead of update.sh to execute it
body: <p>Suppose I have a command update.sh.
  Since . refers to the current directory (on Linux/UNIX systems), I think that ./update.sh and update.sh should be the same.
  But it turns out that until I use</p>
  
  <pre><code>    chmod +x update.sh
  </code></pre>
  
  <p>./update.sh doesn't work.
  Why is it so?
  Why do we sometimes run a command like this - ./update.sh instead of update.sh?</p>