How to combine update with function result in jq?

Question

Given data like this:

cat << EOF > xyz.json
[
  {
    "batch_id": 526,
    "aCods": [
      "IBDD879"
    ]
  },
  {
    "batch_id": 357,
    "aCods": [
      "IBDD212"
    ]
  }
]
EOF

What is the correct way to get this result?

[
  {
    "batch_id": "00000526",
    "aCods": [
      "IBDD879"
    ]
  },
  {
    "batch_id": "00000357",
    "aCods": [
      "IBDD212"
    ]
  }
]

I have tried three different commands hoping to be able to update a object element in an array with the result of a function on that element.

I just cannot find the right syntax.

jq -r '.[] | .batch_id |= 9999999' xyz.json;

{
  "batch_id": 9999999,
  "aCods": [
    "IBDD879"
  ]
}
{
  "batch_id": 9999999,
  "aCods": [
    "IBDD212"
  ]
}

jq -r '.[] | lpad("\(.batch_id)";8;"0")' xyz.json;
00000526
00000357

jq -r '.[] | .batch_id |= lpad("\(.batch_id)";8;"0")' xyz.json;
jq: error (at /dev/shm/xyz.json:14): Cannot index number with string "batch_id"

Answer 1

Assuming you are trying to use the lpad/2 from this peak’s comment, you can do

def lpad($len; $fill): tostring | ($len - length) as $l | ($fill * $l)[:$l] + .;
map(.batch_id |= lpad(8; "0"))

The key here is when using the update assignment operator |= the field being modified is passed on internally, so that you don’t have to call it out explicitly in the RHS