Using BC And Command Substitution In OpenHAB’s Exec Binding

My husband has been setting up OpenHAB to control our home automation. Our dimmers are very direct – there’s a z-Wave binding that you set to 100 if you want it at 100%, set it to 18 if you want it at 18%, and so on. We have a handful of Zigbee bulbs, though, which are not so direct. We are controlling these bulbs through a Wink hub by running a curl command with the exec binding.

The OpenHAB exec binding runs a shell with a command string passed in from the -c parameter. Thus far, I have not found anything that runs within a shell not work in the exec binding. This includes command substitution {I personally use the backtick format instead of the $(command) format, but I expect the later to be equally functional}.

What is command substitution (without having to read the Open Group Base Specifications linked above)? If you run

kill `pidof java`

the shell takes the component within the backticks, evaluates it, and then takes the standard output and places that into the command. When “pidof java” returns “938 984 1038”, the command above becomes “kill 938 984 1038”.

We want to set the value to the OpenHab value (0-100) scaled to the Wink value (0-255 for GE Link bulbs) using command substitution with bc (an arbitrary precision calculator language). To evaluate a mathematical expression, echo the expression text and pipe it to bc. To set a bulb to 75% of its maximum brightness, our post data is “nodeId=a&attrId=aprontest -u -m9 -t2 -v`echo 2.55*75/1|bc`”.

Notice the divide by 1 at the end — that’s to turn a decimal value into an integer. If you use just 2.55*75, you post a value of 191.25 which throws an error. In bc’s language, / returns the quotient — this isn’t *rounding* but rather truncating the decimal portion( i.e. bc 9.99999/1 = 9).

We configure the OpenHAB item to take the selected value (the %2$s below), scale the value with bc, and insert the result into the curl command. We use a similar technique to read the data from Wink and present the scaled value through OpenHAB.

The item entry in our sitemap.items file:

Dimmer  DS_Pantry_Bulb_Level                                            “Bulb (Pantry Downstairs) [%d]”                                 <slider>        (gZigbeeBulb,gDS_Pantry,gLight)                                                                                 { exec=”<[/bin/sh@@-c@@echo `/usr/bin/curl \”http://wink.hub.address/set_dev_value.php\” -s -d \”nodeId=a&attrId=aprontest -l -m9;\”|grep Level|grep -oP \”\\d+\\D+\\K\\d+\\D+\\K\\d+\”` /2.55|bc:3600000:] >[*:/bin/sh@@-c@@/usr/bin/curl \”http://wink.hub.address/set_dev_value.php\” -s -d \”nodeId=a&attrId=aprontest -u -m9 -t2 -v`echo 2.55*%2$s/1|bc`;\”]”}

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