title: Auto suggestion in Commands in command line
body: <p>I typed <strong>git s</strong> an pressed tab in ubuntu terminal and It shows git's sub-commands given below:</p>
  
  <pre><code>test: git s &lt;tab&gt;
  shortlog  show  show-branch   stage stash status submodule
  </code></pre>
  
  <p>How is It happening? Auto suggestion for files is usual one. How can implement it for a command with subcommands? </p>