How to check for presence of 'key' in jq before iterating over the values

Question

I get Cannot iterate over null (null) from the below query because .property_history is not present in result object.

How do i check for the presence of .property_history key before proceeding with map(...) ?

I tried using something like sold_year= `echo "$content" | jq 'if has("property_history") then map(select(.event_name == "Sold"))[0].date' else null end

Original Query:

sold_year=`echo "$content" | jq '.result.property_history | map(select(.event_name == "Sold"))[0].date'`

JSON:

{  
   "result":{  
      "property_history":[  
         {  
            "date":"01/27/2016",
            "price_changed":0,
            "price":899750,
            "event_name":"Listed",
            "sqft":0
         },
         {  
            "date":"12/15/2015",
            "price_changed":0,
            "price":899750,
            "event_name":"Listed",
            "sqft":2357
         },
         {  
            "date":"08/30/2004",
            "price_changed":0,
            "price":739000,
            "event_name":"Sold",
            "sqft":2357
         }
      ]
   }
}

Inian · Accepted Answer · 2021-03-01T20:36:10.377

106

You can use the select-expression in jq to do what you intend to achieve, something as,

jq '.result 
  | select(.property_history != null) 
  | .property_history 
  | map(select(.event_name == "Sold"))[0].date'

edited Mar 01 '21 at 20:36

answered Feb 07 '17 at 20:22

Inian

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select(.property_history != null) is redundant, you can write it as simply: select(.property_history) – icenac Oct 09 '18 at 13:06
5

@icenac - The two expressions are not equivalent. Beware! – peak Jul 31 '20 at 18:09
@peak I know, but for this particular example they are – icenac Aug 02 '20 at 12:59

peak · Answer 2 · 2023-04-14T14:26:07.567

38

Technically, to test for the presence of a property, you should use has/1, but in the present context, it would probably be better to use the postfix ? operator, e.g.:

$ jq '.result 
  | .property_history[]?
  | select(.event_name == "Sold") 
  | .date'
"08/30/2004"

If there is a possibility that the value of .result is not a JSON object, then you could replace the second line above by:

try(.property_history[])

edited Apr 14 '23 at 14:26

answered Feb 07 '17 at 20:36

peak

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cool. i tried that... but it was giving `boolean`. Looks like trick is `.[]` – Rahul Dess Feb 08 '17 at 01:20
3

You may shorten this to `jq '.result.property_history[]? | select(.event_name == "Sold") | .date'` – Olaf Dietsche Jan 15 '19 at 09:23
3

This is far more compact, than the accepted answer. – z2s8 Jul 31 '20 at 15:19
This answer results in error when `property_history` does not exist. The shortened version in the comment by Olaf does work. – aff Apr 19 '21 at 03:08

score 28 · Answer 3 · edited Jul 19 '23 at 22:44

28

use has("mykey1") (for objects) or has(0) (for arrays):

jq 'has("name")' <<< '{"name": "hello"}'

output:

true

edited Jul 19 '23 at 22:44

Ivelin

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answered Jul 15 '21 at 22:03

123

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`if [ "$( jq 'has("alert")' <<< $this_alert_json )" == "true" ]; then` – 123 Jul 15 '21 at 22:09
2

Neat. Clear and concise. +1 – Neeraj Aug 20 '21 at 09:50
2

for POSIX shells (no redirection feature): `echo "{\"name\": \"hello\"}" | jq 'has("name")'` – 123 Oct 13 '21 at 05:54
if you aren't sure the incoming string is valid JSON: `echo "{\"name\": \"hello\"}" | jq -R 'fromjson? | has("name")'` this will not throw an error for bad JSON, if that's what you want – 123 Oct 13 '21 at 05:55
1

Can I give the json file here ???? – Srinivasu Sep 28 '22 at 10:04
@Srinivasu https://stackoverflow.com/a/36273303/5563327 – 123 Sep 28 '22 at 19:30

score 5 · Answer 4 · edited Apr 19 '21 at 11:48

5

The trick is to use // together with empty:

jq '.result.property_history // empty | map(select(.event_name == "Sold"))[0:1][].date'

Another alternative is to use an additional select:

jq '.result.property_history | select(.) | map(select(.event_name == "Sold"))[0:1][].date'

edited Apr 19 '21 at 11:48

aff

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answered Jan 29 '21 at 23:38

reegnz

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score 5 · Answer 5 · answered May 25 '21 at 22:39

General pattern:

try (...) // "default_value"

With your logic:

jq 'try (.result.property_history | map(select(.event_name == "Sold"))[0].date) // "default_value"'

try (without a catch) returns empty if the expression fails. // provides a default value if the value is empty.

score 2 · Answer 6 · answered May 18 '23 at 00:02

The shortest you can use (with the bonus that the exit code will be set):

# Example that exits with code 0
jq -e 'has("result")'

# Example that exists with code 0
jq -e '.result[].property_history | has("price")'

# Example that exists with code 1
jq -e '.result[].property_history | has("dummyKey")'

In these examples, because of setting the flag -e, if has returned true, the exit code will be set to 0, otherwise 1. Which saves unnecessary additional steps.

How to check for presence of 'key' in jq before iterating over the values

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