title: Git command to display HEAD commit id?
body: <p>What command can I use to print out the commit id of HEAD?</p>
  
  <p>This is what I'm doing by hand:</p>
  
  <pre><code>$ cat .git/HEAD
  ref: refs/heads/v3.3
  $ cat .git/refs/heads/v3.3
  6050732e725c68b83c35c873ff8808dff1c406e1
  </code></pre>
  
  <p>But I need a script that can reliably pipe the output of some command to a text file such that the text file contains exactly the commit id of HEAD (nothing more or less, and not just a ref).  Can anyone help?</p>