title: Using variables in a vim shell command
body: <p>How would I go about using a variable in a vim shell command (one done with <code>!</code>) in a vimscript? For instance, something kind of like this: (just an example, not what I'm really trying to do)</p>
  
  <pre><code>function Ls(dir)
      !ls a:dir
  endfunction
  </code></pre>