Tagged: JSON

Day 8: Processing JSON data from webservices

08_librecatDuring the last two days we got an introduction into Catmandu and learned how to transform structured JSON data. The JSON data in these examples was first fetched from a an URL with command curl. Today we will learn how to simplify fetching more data from web services.

In short, a web service is a server that can be queried by HTTP requests. Most web services return JSON data if queried with an URL. For instance the weather web service used during the last two days is documented at openweathermap.org/api. To retrieve current weather data from selected cities, we used commands and URLs like this:

$ curl http://api.openweathermap.org/data/2.5/weather?q=Gent,be
$ curl http://api.openweathermap.org/data/2.5/weather?q=Tokyo,jp

The URLs only differ in its query parameter q, so we can construct a so called URL template. The form of an URL template is defined in RFC 6570, so our template is:


Catmandu supports URL templates to retrieve JSON data with its getJSON Importer. Let’s use it to fetch weather data for Toyko:

$ echo '{"q":"Tokyo,jp"}' | catmandu convert getJSON --url 'http://api.openweathermap.org/data/2.5/weather{?q}'

URL templates make most sense, if applied with multiple values, so let’s create a list of cities. We could use a text editor, such as learned at day 5 but here is an alternative way to learn something new:

$ echo q > cities.csv
$ echo Ghent,be >> cities.csv
$ echo Tokyo,jp >> cities.csv
$ echo Berlin,de >> cities.csv
$ catmandu convert CSV --sep_char _ to JSON < cities.csv > cities.json

We first created the CSV file cities.csv by appending one line after another. The > character is used to pipe output to a file and >> can be used to append to a file instead of overwriting it. You will learn more about processing CSV files in a later article. The last command converts the CSV file to line-separated JSON. Have a look at both files with cat:

$ cat cities.csv

$ cat cities.json

Now we can finally use this list of cities to retrieve weather data in one call:

$ cat cities.json | catmandu convert getJSON --url 'http://api.openweathermap.org/data/2.5/weather{?q}'

Try to append to YAML or to JSON --pretty 1 to this command to get a better view of the data, as described in introduction into catmandu (day 6)!

To better see what’s going on we can skip retrieving data and just get the full URLs instead. This is done by setting the option --dry to 1:

$ catmandu convert getJSON --dry 1 --url 'http://api.openweathermap.org/data/2.5/weather{?q}' < cities.json

With the knowledge from previous days we can extract some information. Here is an improved fix to get both name, and temperature:


Save this fix as file weather2.fix and get temperate of cities of your choice:

$ cat cities.json | catmandu convert getJSON --url 'http://api.openweathermap.org/data/2.5/weather{?q}' --fix weather2.fix

The getJSON Importer get be used to retrieve JSON data from various web services. Catmandu further includes specialized importers for selected web services, for instance:

Continue to Day 9: Processing MARC with Catmandu >>

Day 7: Catmandu JSON paths

07_librecatrojectYesterday we learned the command catmandu and how it can be used to parse structured information. Today we will go deeper into catmandu and describe how to pluck data out of structured information. As always, you need to startup your Virtual Catmandu (hint: see our day 1 tutorial) and start up the UNIX prompt (hint: see our day 2 tutorial).

Today will we fetch a new weather report and store it in a new file weather2.json. Lets try to download Tokyo:

$ curl http://api.openweathermap.org/data/2.5/weather?q=Tokyo,jp > weather2.json

From the previous tutorials we know many commands how to examine this data set. For instance, to get a quick overview of the content of weather2.json we can use the cat command:

$ cat weather2.json

Or, we could use the less command:

$ less weather2.json

Remember to type the ‘q’ key to exit less.

We could also use nano to inspect the data, but we skip that for now. Nano is a text editor and is not particularly suited for data.

To count the number of lines, words and characters in weather2.json we can use the wc command:

$ wc weather2.json
1 3 463

This output shows that weather2.json contains 1 line , 3 words and 463 characters. The 1 line is indeed correct: the file contains one big line of JSON. The 463 characters is also correct: when you count every character including spaces you get to 463. But 3 words is obviously wrong. Generic UNIX programs like wc have trouble with counting words in structured information. The command doesn’t know this file is in the JSON format which contains fields and values. You need to use specialized tools like catmandu to make sense of this output.

We also saw in the previous post how you can use catmandu to transform the JSON format into the YAML format which is easier to read and contains the same information:

$ catmandu convert JSON to YAML < weather2.json



We also learned some fixes to retrieve information out of the JSON file like retain_field(main.temp).

In this post we delve a bit deeper into ways how to point to fields in a JSON file.

This main.temp is called a JSON Path and points to a part of the JSON data you are interested in. The data, as shown above, is structured like a tree. There are top level simple fields like: base,cod,dt,id which contain only text values or numbers. There are also fields like coord that contain a deeper structure like lat and lon.

Using a JSON path you can point to every part of the JSON file using a dot-notation. For simple top level fields the path is just the name of the field:

  • base
  • cod
  • dt
  • id
  • name

For the fields with deeper structure you add a dot ‘.’ to point to the leaves:

  • clouds.all
  • coord.lat
  • coord.lon
  • main.temp
  • etc…

So for example. If you would have a deeply nested structure like:


Then you would point to the c field with the JSON Path x.y.z.a.b.c.

There is one extra path structure I would like to explain and that is the when a field can have more than one value. This is called an array and looks like this in YAML:


In the example above you see a field my which contains a deeper field colors which has 3 values. To point to one of the colors you need to use an index. The first index in a array has value 0, the second the value 1, the third the value 2. So, the JSON path of the color red would be:

  • my.color.2

In almost all programming languages things get counted starting with 0. An old programming joke is:

There are 10 types of people in the world:
Those who understand binary,
Those who don’t,
And those who count from zero.

(hint: this is a double joke, 10 in binary == 2 if you count from 0, or 3 when you count from 1).

There is one array type in our JSON report and that is the weather field. To point to the description of the weather you need the JSON Path weather.0.description.

In this post we learned the JSON Path syntax and how it can be used to point to parts of a JSON data set want to manipulate. We explained the JSON path using a YAML transformation as example, because this is easier to read. YAML and JSON are two formats that contain the same informational content (and thus both can work with JSON Path) but look different when written into a file.

Continue to Day 8: Processing JSON data from webservices >>

Day 6: Introduction into Catmandu

06_librecatprojectIn the previous days we learned the UNIX commands grep, nano, ls and less. Today we will introduce you to a UNIX command we have created in the LibreCat project called catmandu. The catmandu command is used to process structured information.  To demo this command, as always, you need to startup your Virtual Catmandu (hint: see our day 1 tutorial) and start up the UNIX prompt (hint: see our day 2 tutorial).

In this tutorial we are going to process structured information. We call data structured when it organised in such a way is that it easy processable by computers. Previously we processed text documents like War and Peace which is structured only in words and sentences, but a computer doesn’t know which words are part of the title or which words contain names. We had to tell the computer that. Today we will download a weather report in a structured format called JSON and inspect it with the command catmandu.

At the UNIX prompt type in this command:

$ curl http://api.openweathermap.org/data/2.5/weather?q=Gent,be

[Update: as of end 2015 the OpenWeatherMap API requires an API key. Use this link to download a copy of the Ghent weather report :

$ curl https://gist.githubusercontent.com/phochste/7673781b19690f66cada/raw/67050da98a7e04b3c56bb4a8bc8261839af57e35/weather.json


You will see a JSON output like:

"sunrise":1417159365,"sunset":1417189422},"weather":[{"id":500,"main":"Rain","description":"light rain",
"icon":"10d"}],"base":"cmc stations","main":{"temp":281.15,"pressure":1006,"humidity":87,"temp_min":281.15,

All these fields tell something about the current weather in Gent, Belgium. You can recognise that there is a light rain and the temperature is 281.15 degrees Kelvin (about 8 degrees Celsius).  Write the output of this command to a file weather.json (using the ‘>’ sign we learned in the day 5 tutorial) so that we can use it in the next examples.

$ curl https://gist.githubusercontent.com/phochste/7673781b19690f66cada/raw/67050da98a7e04b3c56bb4a8bc8261839af57e35/weather.json > weather.json

When you type the ls command you should see the new file name weather.json appearing.

With the catmandu command you can process this file to make it a bit easier readable. For instance type:

$ catmandu convert JSON to YAML < weather.json

YAML is another format for structured information which is a bit easier to read for human eyes. Our weather report should now look like this:


Catmandu can be used to process structured information like the UNIX grep command can process unstructured information. For instance lets try to filter out the name of this report. Type in this command:

$ catmandu convert JSON --fix 'retain_field(name)' to YAML < weather.json

You should end up with something like:

name: Gent

The –fix option in Catmandu is used to ‘massage’ the input weather.json filtering fields we would like to see. Only one fix was used ‘retain_field’, which throws away all the data from the input except the ‘name’ field. By the way, the file weather.json wasn’t changed! We only read the file and displayed the output of catmandu command.

The temperature in Gent is the in ‘temp’ part of the ‘main’ section in weather.json. To filter this out we need two retain_field fixes: one for the main section, one for the temp section:

$ catmandu convert JSON --fix 'retain_field(main); retain_field(main.temp)' to YAML < weather.json

You should now see something like this:

  temp: 281.15

When massaging data you often need to create many fixes to process a data file in the format you need. With the nano command you can write all the fixes in a file. Start the nano editor with the command:

$ nano weather.fix

In nano type now the two fixes above:


To exit nano type Ctrl-X, press Y to confirm the changes and press Enter to confirm the file name.

With this file it will be a bit easier to create many fixes. The name of the fix file can be used to repeat the commands above:

$ catmandu convert JSON --fix weather.fix to YAML < weather.json

To add more fixes we can again edit the weather.fix file. Type:

$ nano weather.fix

And add these lines after the two previous lines:

prepend(main.temp,"The temperature is")
append(main.temp," degrees Kelvin")

Save the changes with Ctrl-X, Y, Enter and execute catmandu  again:

$ catmandu convert JSON --fix weather.fix to YAML < weather.json

You should now see as ouput:

  temp: The weather is 281.15 degrees Kelvin

Catmandu contains many fixes to manipulate data. Check the documentation to get a complete list. This post only presented a short introduction into catmandu. In the next posts we will go deeper into its capabilities.

Continue to Day 7: Catmandu JSON paths >>