title: Bash provide default argument if no arguments passed
body: <p>I have a program installed which named <code>zeus</code>. It's allow me to run commands like this:</p>
  
  <pre><code>zeus parallel_rspec spec/
  </code></pre>
  
  <p>Where <code>parallel_rspec</code> is command which is runned by zeus and <code>spec/</code> is the directory, which is required by <code>parallel_rspec</code> command.</p>
  
  <p>I've made an alias in my <code>.profile</code>:</p>
  
  <pre><code>alias rsp='zeus parallel_rspec'
  </code></pre>
  
  <p>So, I can run commands like this:</p>
  
  <pre><code>rsp spec/ # =&gt; equal to `zeus parallel_rspec spec/`
  </code></pre>
  
  <p>But <code>spec/</code> directory is common (95% cases). How can I make my <code>rsp</code> alias pass <code>spec/</code> folder argument by default (if I'm not passed something another, like <code>spec/blablabla</code>)?</p>