Pipe output through jq ONLY if it is JSON - bash

Question

I have the following BASH code:
response=$( curl -Ls $endpoint )
if [ -n "$response" ]; then # nonempty
echo "$response" | jq .
fi
The problem is that sometimes the response can be non-empty but not JSON (if it's not a 200).
Is it possible to pipe the output through jq ONLY if it is valid JSON?
The following works:
echo $x | jq . 2>/dev/null || echo $x
Test:
> x='{"foo":123}'; echo $x | jq . 2>/dev/null || echo "Invalid: $x"
{
"foo": 123
}
> x='}'; echo $x | jq . 2>/dev/null || echo "Invalid: $x"
Invalid: }
However, I don't feel comfortable with it.

Answer 1

If you want to test the response type before submitting it to jq, it is possible if you test the Content-Type header from the server's response.
So you want curl to send you the full response headers and body with curl -i.
Here is an implementation of it:
#!/usr/bin/env sh
endpoint='https://worldtimeapi.org/api/timezone/Europe/Paris.json'
# Headers and body are delimited by an empty line, with CRLF as the line ending.
# See: RFC7230 HTTP/1.1 Message Syntax and Routing / section 3: Message Format
# https://tools.ietf.org/html/rfc7230#section-3
crlf="$(printf '\r\n_')" # add trailing _ to prevent trailing newline trim
crlf="${crlf%_}" # remove trailing _
http_delim="$crlf$crlf" # RFC7230 section 3
full_http_response="$(curl --silent --include --url "$endpoint")"
http_headers="${full_http_response%$http_delim*}"
http_body="${full_http_response#*$http_delim}"
case $http_headers in
'HTTP/1.1 200 OK'*'Content-Type: application/json'*)
# Yes, response body is JSON, so process it with jq.
jq -n "$http_body"
;;
esac

Answer 2

The following works:
echo $x | jq . 2>/dev/null || echo $x
Except for the use of echo here, this is actually a good approach - it has the advantages of both simplicity and efficiency. It is better than using the -e option naively as the return codes produced by -e are more complex.
In other words, there is much to be said for:
printf "%s" "$x" | jq . 2> /dev/null || printf "%s\n" "$x"
Efficiency
The argument for efficiency is as follows:
If $x holds valid JSON, then there is no overhead.
If $x is invalid as JSON, jq will quickly fail; in this case also, the overhead of calling jq will almost surely be no worse or not much worse than checking the Content-Type.
Warning
The official documentation for the return codes produced by jq when invoked without the -e option is not strictly correct, as illustrated by:
$ jq empty <<< 'foo bat' 2> /dev/null ; echo $?
4

Answer 3

This works:
response=$( curl -Ls -H 'Cache-Control: max-age=0' $endpoint )
if [ -n "$response" ]; then # nonempty
echo "Got server response"
# https://stackoverflow.com/questions/46954692/check-if-string-is-a-valid-json-with-jq
if jq --exit-status type >/dev/null 2>&1 <<<"$response"; then
# Parsed JSON successfully and got something other than false/null
echo "$response" | jq .
echo "... after $i seconds"
return 0
else
echo "Response is not valid JSON"
echo "$response"
return 1
fi
fi