option not recognized

I'm trying a very simple example but I'm having issues:

``` javascript
//my-js-file.js
require('shelljs/global');
ls('-l', '/');
```

and then I run `shjs my-js-file.js` on the command line. I get the following error: `ls: option not recognized: l`.

What am I doing wrong?

Note: Trying this using Node version 4.2.4 on an Ubuntu 14.04 server (DigitalOcean).
