Flatten JSON into greppable paths

Goal: Turn a nested JSON document into one path = value; line per value, so grep can find where a value lives, and rebuild JSON from the lines you keep.

Prerequisites: None for the command line, because -F gron writes gron and -I gron reads it. The built-in gron module does the same conversion from inside a query.

Query

Find where vitest and react appear in a package.json:

$ mq -I json -F gron '.' package.json | grep -i 'vitest\|react'

Input (`package.json`)

{
  "name": "demo",
  "version": "1.0.0",
  "scripts": { "build": "tsc", "test": "vitest" },
  "dependencies": { "react": "^18.2.0", "zod": "^3.22.0" },
  "keywords": ["a", "b"]
}

Output

json.scripts.test = "vitest";
json.dependencies.react = "^18.2.0";

Without the grep, every value gets its own line, rooted at json:

json = {};
json.name = "demo";
json.version = "1.0.0";
json.scripts = {};
json.scripts.build = "tsc";
json.scripts.test = "vitest";
json.dependencies = {};
json.dependencies.react = "^18.2.0";
json.dependencies.zod = "^3.22.0";
json.keywords = [];
json.keywords[0] = "a";
json.keywords[1] = "b";

Rebuild JSON from the lines you keep

Pipe the filtered lines back through -I gron to get JSON again. This keeps only the dependencies:

$ mq -I json -F gron '.' package.json | grep '^json.dependencies' | mq -I gron -F json '.'

{
  "dependencies": {
    "react": "^18.2.0",
    "zod": "^3.22.0"
  }
}

And this drops the scripts object instead:

$ mq -I json -F gron '.' package.json | grep -v '^json.scripts' | mq -I gron -F json '.'

{
  "name": "demo",
  "version": "1.0.0",
  "dependencies": {
    "react": "^18.2.0",
    "zod": "^3.22.0"
  },
  "keywords": [
    "a",
    "b"
  ]
}

Notes

-I json is only the source format. YAML, TOML, XML and CSV work the same way, for example mq -I yaml -F gron '.' config.yaml.
Keys that are not plain identifiers use bracket notation, so {"a-b": {"c d": 1}} becomes json["a-b"]["c d"] = 1;.
Inside a query, import "gron" | gron::gron_stringify produces the same lines, and gron::gron_parse reads them back. Unlike -F gron, gron_stringify sorts object keys alphabetically, so use -F gron when you want the lines in the file’s own key order.
You do not need the json.dependencies = {}; line to rebuild. -I gron creates parent objects from the paths, so grep '^json.dependencies\.', which matches only the leaf lines, gives the same result as the example above.
To convert the whole file rather than search it, see Convert between YAML and JSON.

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