Small script that allows to read/set a value for a specific path in a JSON or YAML file.

The main motivation behind this is to quickly be able to read a specific property from a JSON/YAML file, do something with that value and then change the original value with a new one in the document. And being able to do that transparently regardless of the source format.

To accomplish this, YAML input is converted behind the scenes to JSON using Python, a jq filter is applied on the JSON and the result is converted back into YAML.

Like I said before, the goal is to get/set a specific value associated with a key in the document. If you are looking to run arbitrary jq-like filters on YAML I'd recommend you to take a look at y2j.

The script's entry point accepts 2 operations: get and set. The script can take its inputs from stdin or read from a file if this is passed as the first parameter. We'll see some examples below.

The get command takes an arbitrary jq filter. If the result is a number, string or boolean then that value is returned. Otherwise, the resulting JSON or YAML is returned (depending on what the input was).

Given the following JSON file:

$ cat test.json

{"menu": {
  "id": "file",
  "value": "File",
  "popup": {
    "menuitem": [
      {"value": "New", "onclick": "CreateNewDoc()"},
      {"value": "Open", "onclick": "OpenDoc()"},
      {"value": "Close", "onclick": "CloseDoc()"}
    ]
  }
}}

If you wanted to get the value of the id property you could use:

$ cat test.json | docker run -i --rm jlordiales/jyparser get ".menu.id"

"file"

The json is passed via stdin, which is useful if you get that from something like curl. If you have an actual file that you want to use as input then you can pass it directly as the first parameter to the script:

$ docker run -i --rm -v `pwd`:/jyparser:ro jlordiales/jyparser test.json get ".menu.id"

"file"

The example above mounts the current dir with the file into /jyparser (which is the default WORKDIR for the docker image) and then uses that file as input.

Exactly the same command works for YAML as well. Given the YAML file:

$ cat test.yml
menu:
  id: file
  value: File
  popup:
    menuitem:
    - onclick: CreateNewDoc()
      value: New
    - onclick: OpenDoc()
      value: Open
    - onclick: CloseDoc()
      value: Close

We can get the id property with:

$ cat test.yml | docker run -i --rm jlordiales/jyparser get ".menu.id"

"file"

If the result from running the jq filter is not a simple value, then the corresponding JSON or YAML is returned:

$ cat test.json | docker run -i --rm jlordiales/jyparser get ".menu.popup.menuitem[1]"

{
  "value": "Open",
  "onclick": "OpenDoc()"
}

$ cat test.yml | docker run -i --rm jlordiales/jyparser get ".menu.popup.menuitem[1]"

onclick: OpenDoc()
value: Open

Similarly to the get operation, there's a set one. This operation takes 2 parameters: a jq filter to select a specific element of the input and a new value to update that element to. The result is the original input with the value updated.

$ cat test.json | docker run -i --rm jlordiales/jyparser set ".menu.id" \"new_id\"
{
  "menu": {
    "id": "new_id",
    "value": "File",
    "popup": {
      "menuitem": [
        {
          "value": "New",
          "onclick": "CreateNewDoc()"
        },
        {
          "value": "Open",
          "onclick": "OpenDoc()"
        },
        {
          "value": "Close",
          "onclick": "CloseDoc()"
        }
      ]
    }
  }
}

Important: given the way bash scripts handle quotes on parameters passed to them, if the new value you want to set for the property is a string you need to explicitly escape the quotes as in the example. Otherwise, jq will complain that the value is not valid (rightfully so). This is not needed for numbers or booleans. So the following works as expected:

$ cat test.json | docker run -i --rm jlordiales/jyparser set ".menu.id" 15
{
  "menu": {
    "id": 15,
    "value": "File",
    "popup": {
      "menuitem": [
        {
          "value": "New",
          "onclick": "CreateNewDoc()"
        },
        {
          "value": "Open",
          "onclick": "OpenDoc()"
        },
        {
          "value": "Close",
          "onclick": "CloseDoc()"
        }
      ]
    }
  }
}

The same works for YAML:

$ cat test.yml | docker run -i --rm jlordiales/jyparser set ".menu.id" \"new_id\"

menu:
  id: new_id
  popup:
    menuitem:
    - onclick: CreateNewDoc()
      value: New
    - onclick: OpenDoc()
      value: Open
    - onclick: CloseDoc()
      value: Close
  value: File

Since YAML is actually a superset of JSON this will only work for those YAML files that can be correctly converted to JSON.

The JSON <=> YAML conversion to be able to apply jq filters seamlessly is heavily inspired by y2j.

You can find more info on the use case and motivation for this project on this blog post.

The automated build for the docker image can be found here.

MIT.
Full license text is available in LICENSE.