You need the following:
- A number set up at Twilio - This is what is used to actually communicate over SMS
- A VPS - You need this to host a server application to accept the text string from Twilio servers. This will also be where the commands are actually run.
- Ruby with twilio-ruby and sinatra gems installed
Purchase a phone number in Twilio and have the "webhook" line in the SMS section point to your VPS URL, for example: http://myvpshere:8080/smscli or whatever you choose. Once you set that up, whatever SMS messages get sent to the phone number you purchased in Twilio will be sent as an HTTP Post request to the URL you specify.
Now all you need is a server listening at that URL on your VPS to accept the HTTP Post requests from the Twilio servers and do something with the body of the request. In this case, we pass whatever text as a system command and reply to the phone number with the result of the command. The following example Ruby code starts up a sinatra web server to do all that:
Overall, this really isn't that different from any other webshell. The only real difference is you are leveraging Twilio to handle SMS communications.
Now you get to do something stupid like this:
I shouldn't have to mention that taking in arbitrary text from untrusted sources and running them as commands under root is pretty much the worst thing you could do security wise. This is merely an example of how to get the pure task done. Implement some auth or something, I don't care... It's your funeral...