How to use jq when the response is not a list

My json response is like this.

{
  "level": "info",
  "timestamp": "2022-03-21T16:10:27.060Z",
  "msg": "starting test",
}
{
  "level": "warn",
  "timestamp": "2022-03-21T16:10:27.060Z",
  "msg": "message 2"
}
{
  "level": "error",
  "timestamp": "2022-03-21T16:10:27.060Z",
  "msg": "message 3"
}

I want to extract.

Either 3rd message (or nth message) or the message associated with error

How can I do this with jq?

>Solution :

Use select to filter by condition

jq 'select(.level == "error")'

{
  "level": "error",
  "timestamp": "2022-03-21T16:10:27.060Z",
  "msg": "message 3"
}

Demo

To extract the message, use the -r option

jq -r 'select(.level == "error").msg'

message 3

Demo

If you want to access the nth object, you may want to use a structure whose items you can count. Using the -s option would turn the input stream into an array. Using the --argjson option enables you using an external (JSON) value for reference:

jq -sr --arg n 2 '.[$n].msg'

message 3

Demo

To directly address the nth element of a stream, use the nth function, in combination with inputs and the -n option to address the entire stream at once:

jq -nr --arg n 2 'nth($n;inputs).msg'

message 3

Demo