How to check for an optional field in jq?

user_6862601 0
Viewed 1504

I have this jq filter:

some_command | jq -r '.elements[] | select(.state=="LIVE" and .group == "some_text" and .someFlag == false) | .name'

someFlag is an optional field. Hence, when it is absent, the expression doesn't show any result. I want to check for:

  • if someFlag is present, pass the check only if it has the false value
  • if someFlag is not present, treat it as false

How can I do that?

jq
2 Answers

I used the alternative operator, //:

(.someFlag // false) == false)

So, if .someFlag isn't there, it is treated as false.

The whole expression is:

some_command | jq -r '.elements[] | select(.state=="LIVE" and .group == "some_text" and (.someFlag // false) == false)) | .name'

From jq documentation:

Alternative operator //:

A filter of the form a // b produces the same results as a, if a produces results other than false and null. Otherwise, a // b produces the same results as b.

This is useful for providing defaults: .foo // 1 will evaluate to 1 if there’s no .foo element in the input. It’s similar to how or is sometimes used in Python (jq’s or operator is reserved for strictly Boolean operations).

user_68626010

In the given context, the direct translation of:

  • if someFlag is present, pass the check only if it has the false value
  • if someFlag is not present, treat it as false

to jq is:

if has("someflag") then .someflag == false else false end

Adjusted filter

.elements[]
| select(.state == "LIVE"
         and .group == "some_text"
         and (if has("someFlag")
              then .someFlag == false 
              else false
              end))
| .name

[This response has been updated in accordance with the update to the Q.]

user_9973580
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