JSON JQ if without else

Question

I use the following JQ command to filter out the JSON. My requirement is to filter out the JSON message if the expected node is present. Or else, do nothing. Hence, I use if, elif, ....

sed -n "s/.*Service - //p" $1/response.log* |
  jq "if (.requests | length) != 0 then .requests |= map(select(.id == \"123\"))
      elif (.result | length ) != 0  then .result |= map(select(.id== \"123\")) 
      else " "  end" > ~/result.log

Looks like else is mandatory here. I dont want to do anything inside the else block. Is there anyway, I can ignore else or just print some whitespce inside else.

In the above case, it prints double quotes " " in the result file.

peak · Accepted Answer · 2019-05-16T20:12:21.283

87

You may want to use the idiom:

if CONDITION then WHATEVER else empty end

empty is a filter that outputs nothing at all -- not even null, which is after all something (namely a JSON value). It's a bit like a black hole, only blacker -- it will consume whatever it's offered, but unlike a black hole, it does not even emit Hawking radiation.

In your case, you have an "elif" so using "else empty" is probably what you want, but for reference, the above is exactly equivalent to:

select(CONDITION) | WHATEVER

P.S. My guess is that whatever the goal of the sed command, it could be done more reliably as part of the jq program, perhaps using walk/1.

UPDATE

After the release of jq 1.6, a change was made so that "if without else" has the semantics of "if _ then _ else . end", that is:

if P then Q end === if P then Q else . end

edited May 16 '19 at 20:12

answered Apr 30 '15 at 08:03

peak

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how to use the if statement for for array like values. For instance: .[]|{result": if (.foo == null) then [ ] end} [{"foo": null , "bar": "less interesting data"},{"foo": "null" , "bar": "less interesting data"}] – devanathan Oct 03 '16 at 08:03
Please ask a new SO question. – peak Oct 03 '16 at 15:28
Erasing any kind of memory does cause some waste heat. This is true for black holes and for RAM. – Daniel H Apr 14 '18 at 22:45
1

The documentation says about empty: It’s useful on occasion. You’ll know if you need it :) – Eddy Hernandez Aug 27 '19 at 20:37
So, in jq 1.7 then we can skip the `else .` ? – luckman212 Nov 19 '22 at 18:42
I have jq v1.6 and leaving the `else .` does not work... get error "jq: error: Possibly unterminated 'if' statement at" – user1822391 Jun 02 '23 at 18:42

score 10 · Answer 2 · answered Apr 29 '15 at 21:15

Since you're not making any further changes to the object, just use the "identity" filter ..

if (.requests | length) then ... else . end

On the other hand, you're just updating the requests or result property if non-empty using map. The check isn't necessary. If it's empty, then it will return empty.

You can simplify your filter to just:

.requests |= map(select(.id == \"123\")) | .result |= map(select(.id== \"123\"))

JSON JQ if without else

2 Answers2

UPDATE

Linked