Check if string is a valid JSON with jq

Question

I need to catch an error when lifting a service. The response can be null, a string error message like

error services-migration/foobar: Not found: services-migration/foobar

or a valid JSON when everything is fine. I was wondering if there is a way with jq to simply check if the provided string is a valid JSON. I could ofc check the string for some keywords like error f.e., but I'm looking for a more robust option, where eg. I get a true/false or 1/0 from jq. I was looking through the docs of jq and also some questions here on SO but everything was about finding and picking out key-values from a JSON, but nothing about simply validating a string.

UPDATE:

I've got this:

 result=$(some command) 

from which the result is the string error services-migration/foobar: Not found: services-migration/foobar

And then the if statement:

 if jq -e . >/dev/null 2>&1 <<<"$result"; then     echo "it catches it"  else     echo "it doesn't catch it"  fi 

And it always ends up in the else clause.

Answer 1

From the manual:

-e / --exit-status:

Sets the exit status of jq to 0 if the last output values was neither false nor null, 1 if the last output value was either false or null, or 4 if no valid result was ever produced. Normally jq exits with 2 if there was any usage problem or system error, 3 if there was a jq program compile error, or 0 if the jq program ran.

So you can use:

if jq -e . >/dev/null 2>&1 <<<"$json_string"; then     echo "Parsed JSON successfully and got something other than false/null" else     echo "Failed to parse JSON, or got false/null" fi 

In fact, if you don't care about distinguishing between the different types of error, then you can just lose the -e switch. In this case, anything considered to be valid JSON (including false/null) will be parsed successfully by the filter . and the program will terminate successfully, so the if branch will be followed.

Answer 2

This is working for me

echo $json_string | jq -e . >/dev/null 2>&1  | echo ${PIPESTATUS[1]} 

that returns return code:

• 0 - Success
• 1 - Failed
• 4 - Invalid

Then you can evaluate the return code by further code.