jq でキーと値を差し替える

a.json

[
  {
    "id": "1-key",
    "attr": "1-value"
  },
  {
    "id": "2-key",
    "attr": "2-value"
  },
  {
    "id": "3-key",
    "attr": "3-value"
  },
  {
    "id": "4-key",
    "attr": "4-value"
  }
]

a.sh

cat a.json | jq '[.[]|{(.id): .attr}]|add'

stdout

{
  "1-key": "1-value",
  "2-key": "2-value",
  "3-key": "3-value",
  "4-key": "4-value"
}