How to keep one specific key from a nested json object?

Question

I have the following config.json file:

{
        "auths": {
                "reg1.io": {
                        "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
                },
                "reg2.io": {
                        "auth": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy"
                },
                "reg3.io": {
                        "auth": "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
                }
        }
}

I want to keep only the reg2.io object to get:

{
        "auths": {
                "reg2.io": {
                        "auth": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy"
                }
        }
}

I can delete one specific object with e.g.:

→ jq 'del(.. | ."reg2.io"? )' ./config.json
{
  "auths": {
    "reg1.io": {
      "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    },
    "reg3.io": {
      "auth": "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
    }
  }
}

but what if i want to keep one specific object? Thanks.

Answer 1

Using select() with with_entries() to select only the key that you want:

$ jq '.auths |= with_entries(select(.key == "reg2.io"))' file
{
  "auths": {
    "reg2.io": {
      "auth": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy"
    }
  }
}

Parametrised so that it takes the wanted key from the command line:

$ jq --arg key "reg2.io" '.auths |= with_entries(select(.key == $key))' file
{
  "auths": {
    "reg2.io": {
      "auth": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy"
    }
  }
}

Change the == to != to delete the named key.

Note that your own attempt does not distinguish where the key is found in the document, which could cause some confusion in the general case.

---

Just to add a bit more explanation as to what's happening in the jq expression.

Within the with_entries() command, what's visible is the following set of objects, each with a key and a value key:

{
   "key": "reg1.io",
   "value": { "auth": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" }
}
{
   "key": "reg2.io",
   "value": { "auth": "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy" }
}
{
   "key": "reg3.io",
   "value": { "auth": "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz" }
}

The select() call extracts each object for which the given test is true, and discards the rest. Our test is simply "Is the value of the key key the string reg2.io?".

A call to with_entries(expr) is equivalent to to_entries | map(expr) | from_entries.

Answer 2

You can keep one (or a few) entries in an object using the syntax {"reg2.io"}. (The double quotes are not needed if you key is just a word.) With the operator |= you can replace the original object with the constructed one.

jq '.auths|={"reg2.io"}' config.json

If you have more than one key on the top level and you want to keep just the one, you can combine this with one more similar filter, on either side:

jq '{auths}|.auths|={"reg2.io"}' config.json

jq '.auths|={"reg2.io"}|{auths}' config.json