Using jq to list keys in a JSON object

Question

I have a hierarchically deep JSON object created by a scientific instrument, so the file is somewhat large (1.3MB) and not readily readable by people. I would like to get a list of keys, up to a certain depth, for the JSON object. For example, given an input object like this

{
    "acquisition_parameters": {
        "laser": {
            "wavelength": {
                "value": 632,
                "units": "nm"
            }
        },
        "date": "02/03/2525",
        "camera": {}
    },
    "software": {
        "repo": "github.com/username/repo",
        "commit": "a7642f",
        "branch": "develop"
    },
    "data": [{},{},{}]
}

I would like an output like such.

{
    "acquisition_parameters": [
        "laser",
        "date",
        "camera"
    ],
    "software": [
        "repo",
        "commit",
        "branch"
    ]
}

This is mainly for the purpose of being able to enumerate what is in a JSON object. After processing the JSON objects from the instrument begin to diverge: for example, some may have a field like .frame.cross_section.stats.fwhm, while others may have .sample.species, so it would be convenient to be able to interrogate the JSON object on the command line.

Answer 1

The following should do exactly what you want

jq '[(keys - ["data"])[] as $key | { ($key): .[$key] | keys }] | add'

This will give the following output, using the input you described above:

{
  "acquisition_parameters": [
    "camera",
    "date",
    "laser"
  ],
  "software": [
    "branch",
    "commit",
    "repo"
  ]
}

Answer 2

Given your purpose you might have an easier time using the paths builtin to list all the paths in the input and then truncate at the desired depth:

$  echo '{"a":{"b":{"c":{"d":true}}}}' | jq -c '[paths|.[0:2]]|unique'
[["a"],["a","b"]]

Answer 3

Here is another variation uing reduce and setpath which assumes you have a specific set of top-level keys you want to examine:

  . as $v
| reduce ("acquisition_parameters", "software") as $k (
     {}; setpath([$k]; $v[$k] | keys)
  )