How to Extract Data from a JSON File

jsontext processing

I have bin searching a solution for my question but didn't find a or better said I did not get it with what I found.
So lets talk about what my problem is about.
I am using a Smart Home Control Software on a Raspberry Pi and as I found out this weekend using pilight-receive I can catch the data from my outdoor temperature sensor. The output of pilight-receive looks like that:

{
        "message": {
                "id": 4095,
                "temperature": 409.5
        },
        "origin": "receiver",
        "protocol": "alecto_wsd17",
        "uuid": "0000-b8-27-eb-0f3db7",
        "repeats": 3
}
{
        "message": {
                "id": 1490,
                "temperature": 25.1,
                "humidity": 40.0,
                "battery": 1
        },
        "origin": "receiver",
        "protocol": "alecto_ws1700",
        "uuid": "0000-b8-27-eb-0f3db7",
        "repeats": 3
}
{
        "message": {
                "id": 2039,
                "temperature": 409.5
        },
        "origin": "receiver",
        "protocol": "alecto_wsd17",
        "uuid": "0000-b8-27-eb-0f3db7",
        "repeats": 4
}

Now my question to you:
How the heck can I extract the temperature and humidity from where the id is 1490. And how would you recommend me to do check this frequently? By a cron job that runs every 10 minutes, creates an output of the pilight-receive, extracts the data of the output and pushes it to the Smart Home Control Api.

Someone having an idea – thanks a lot

Best Answer

You can use jq to process json files in shell.

For example, I saved your sample json file as raul.json and then ran:

$ jq .message.temperature raul.json 
409.5
25.1
409.5
$ jq .message.humidity raul.json 
null
40
null

jq is available pre-packaged for most linux distros.

There's probably a way to do it in jq itself, but the simplest way I found to get both the wanted values on one line is to use xargs. For example:

$ jq 'select(.message.id == 1490) | .message.temperature, .message.humidity' raul.json | xargs
25.1 40

or, if you want to loop through each .message.id instance, we can add .message.id to the output and use xargs -n 3 as we know that there will be three fields (id, temperature, humidity):

jq '.message.id, .message.temperature, .message.humidity' raul.json | xargs -n 3
4095 409.5 null
1490 25.1 40
2039 409.5 null

You could then post-process that output with awk or whatever.

Finally, both python and perl have excellent libraries for parsing and manipulating json data. As do several other languages, including php and java.

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