How to use jq when the response is not a list - jq

Question

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?

Answer 1

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 --argjson n 2 '.[$n].msg'
message 3
Demo
To directly address the nth element of a stream, use the nth function, in combination with inputs and the -n option to address the entire stream at once:
jq -nr --argjson n 2 'nth($n;inputs).msg'
message 3
Demo