How to check jq result is null or not?

Question

I'm using jq to parse some JSON. I want to check whether a property exists or not. If it exists I always want to get the first element in the array. Based on this I want to use if then ... else.

My code looks like this:

JSON_INPUT='{"var1":[{"foo":"bar"}],"var2":[{"fooooo":"baaaaar"}]}'
VAR2=$(echo $JSON_INPUT | jq  '.["var2"] | .[0]')
if [ -z "${VAR2}" ]
then
    echo "does not exist"
    # do some stuff...
else
    echo "exist"
    # do some stuff...
fi

The JSON_INPUT may contain var2 but must not. If it does not exist VAR2 will be null. But I'm not able to check for this null value. Where is the error?

Answer 1

Where is the error?

Replace

[ -z "${VAR2}" ]

with

[ "${VAR2}" = "null" ]

because jq returns string null if var2 is not found in JSON file.

---

Or use --exit-status:

if echo "$JSON_INPUT" | jq --exit-status '.var2' >/dev/null; then 
  echo "exists"
else
  echo "does not exist"
fi