How to append json array using jq+shell in a loop?

Question

I can create a json object with an array using jq like this

echo '{"input":{"names":[]}}' | jq --arg val "alice" '.input.names[0] += $val'| jq --arg val "bob" '.input.names[1] += $val'

which gives

{
  "input": {
    "names": [
      "alice",
      "bob"
    ]
  }
}

Now I want to recreate this in a shell script where "names" come from a shell array

#!/bin/sh

names=( alice bob )
                 
ITER=0
for i in "${names[@]}"
do                  
    echo '{"input":{"names":[]}}' |  jq --arg val "$i" '.input.names[$ITER] += $val'
    echo ${I} ${ITER}                                                               
    ITER=$(expr $ITER + 1)                                                          
done

but I run into

jq: error: $ITER is not defined at <top-level>, line 1:
.input.names[$ITER] += $val             
jq: 1 compile error
0
jq: error: $ITER is not defined at <top-level>, line 1:
.input.names[$ITER] += $val             
jq: 1 compile error
1

Either assign (=, not +=) to an array index jq --arg val "alice" '.input.names[0] = $val', or append (+=) to an array using no index and the array brackets on the RHS: jq --arg val "alice" '.input.names += [$val]'. — pmf
– pmf, Commented May 12, 2022 at 12:52
i tried echo '{"input":{"names":[]}}' | jq --arg val "$i" '.input.names += [$val]' but it just creates two different objects — SAS
– SAS, Commented May 12, 2022 at 12:58
Because you don't capture the output of an iteration step, so you lose the output from one call and start all over again in the next. I have added a bash solution implementing your for loop while capturing the outputs. — pmf
– pmf, Commented May 12, 2022 at 13:09

Fravadona · Accepted Answer · 2022-05-17 18:28:12Z

3

You could get rid of the shell loop and use one jq call:

names=( alice bob )
jq -n --arg names "${names[*]}" '{"input": {"names": ($names / " ") } }'

or

names=( alice bob )
printf '%s\0' "${names[@]}" |
jq -sR '(. / "\u0000") as $names | { "input": { "names": $names } }'

edited May 17, 2022 at 18:28

answered May 12, 2022 at 13:00

Fravadona

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2 Comments

A.H. Over a year ago

If I replace \0 in the printf variant with | (and of course \u0000 too) then the array is ["alice","bob",""]. So why is this not the case with \0?

Fravadona Over a year ago

I don't know the reason. It might be a long standing bug that \u0000 is treated this way, it's convenient though

pmf · Accepted Answer · 2022-05-12 13:03:53Z

2

You don't capture the output of an iteration step, so you lose the output from the jq call and start all over again in the next iteration step.

Here's a bash solution which captures the output using $(...) and provides it for the next using <<<. (Note that there are better ways to solve this problem, e.g. without looping but by proving jq all elements to be added at once.)

json='{"input":{"names":[]}}'

for i in alice bob
do json="$(jq --arg val "$i" '.input.names += [$val]' <<< "$json")"
done

echo "$json"

{
  "input": {
    "names": [
      "alice",
      "bob"
    ]
  }
}

answered May 12, 2022 at 13:03

pmf

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1 Comment

SAS Over a year ago

yep...that worked also... thanks

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