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Charly's Column – XMLStarlet

Article from Issue 199/2017

Charly and XML have never been best friends. However, because it was vital for him to have an excellent climate indoors, he plucked up his courage and considered XMLStarlet.

A few smart home components from the HomeMatic system are permanent guests in my bathroom – they are for controlling the temperature in the room. A radiator thermostat and three sensors detect whether doors and windows are open. I use a central unit called CCU2 to set the climate I want and in which scenario. Their web interface seriously tests my patience. If, for example, I name a window contact Bathroom_Window_North instead of NEQ1651969 – because that's what it's called – I can't rely on the web interface to use the new name in all views (Figure 1).

Figure 1: Window with a poor view: CCU2 cannot remember the name of window sensors.

The issue of security is another low point: The user interface is not password protected by default. I may be able to set one, but because CCU2 does not speak HTTPS, the password would fall into the clutches of the next sniffer anyway. Access to the script engine on port 8181 is also completely insecure. Nevertheless, the benefit is great because it allows me to use HTTP requests to read out the status of HomeMatic components and even set values.

Given this information, however, it should be clear that nobody is allowed to simply put the CCU2 onto the Internet via port forwarding, at least not anyone who doesn't want to invite all hackers to play poltergeist in their own home. Anyone wanting to view their home heating system from Jamaica will need a good VPN.

XML Instead of a GUI

So, I have a tool at hand for operating the CCU2 without its web interface. To receive the status of a window contact, I type the following into the console:


The CCU2 responds very quickly with (brrr!) XML:

    User Agent: wget

I will suppress my XML aversion to help machines communicate with other machines – because that's just what XML is designed for. The true between the Response tags means that the window is closed. I can also retrieve the thermostat using similar commands and then set new temperature values. This means I can set up my own CCU2 GUI-free control using a few Bash scripts. Now I'm still missing an XML parser for the command line – with which I can finally angle for the XMLStarlet [1].

I previously had the output of the wget command above written to the bathwindow.xml file. In this way,

xmlstarlet sel -T -t -m xml -v Response $WDIR/bathwindow.xml

I now get the value between the Response tags. Fancy a ride through the hell of parameters? Just read the 17-page (!) PDF document [2].

Charly Kühnast

Charly Kühnast manages Unix systems in the data center in the Lower Rhine region of Germany. His responsibilities include ensuring the security and availability of firewalls and the DMZ.

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