jq - Access array index in a loop

If your json contains a list of objects, jq allows you to access such elements using an array index filter jq '.[index]' someData.json. But the value index is static, say jq '.[2]' someData.json.

How can I access the element at position index when the value of index is dynamic, such as when in loops?

What I want to do is, when in a loop, call jq to access an element at some index that I specify.

In pseudocode, it’d look something like this :

array=[ {"name": "a"}, {"name": "b"}, {"name": "c"} ]
for (( i = 0; i < $array.length; i++ ))
do
    element = jq '[$i]' $array
    echo "element = $element"
done

What worked - using --argjson

For that to work, we need to be in the context of jq, where:

1. we declare a variable and assign it a value;
2. and then use that variable as an argument for the array index.

That means:

• Using --argjson to assign a numeric variable that’s passed to jq as the index of the array. It has the form: --argjson myIndex $i '.[$myIndex]'
  • The first part, --argjson myIndex $i, defines variable myIndex and assigns the value of $i to it.
  • The second part, '.[$myIndex]', passes the argument $myIndex to the array index filter.
• Using --raw-output to get the result that’s not “formatted as a JSON string with quotes”.

Let the variable jsonData hold example json. I came across storing json in a bash variable on Stackoverflow

# load json data
jsonData=$(cat <<EOF
  [
    {
      "id": 1,
      "name": "One"
    },
    {
      "id": 2,
      "name": "Two" 
    },
    {
      "id": 3,
      "name": "Three" 
    }
  ]
  EOF
)

numberOfItems=$(echo $jsonData | jq --raw-output length)

# loop over the list in json
for (( i=0; i<numberOfItems; i++ ))
do
    echo "At index, $i"
    # assign a numerical variable that's passed to jq as the array index
    name=$( echo $jsonData | jq --raw-output --argjson myIndex $i '.[$myIndex].name' )
    echo "Name=$name"
done

What didn’t work - using variables not in jq context

Using the i in the loop directly as the index of the array didn’t work.

jsonData=$(cat <<EOF
  [
    {
      "id": 1,
      "name": "One"
    },
    {
      "id": 2,
      "name": "Two" 
    },
    {
      "id": 3,
      "name": "Three" 
    }
  ]
  EOF
)

numberOfItems = $(echo $jsonData | jq --raw-output length)

# loop over the list in json
for (( i=0; i<numberOfItems; i++ ))
do
    echo "At index, $i"
    # passing i directly doesn't work
    name=$( echo $jsonData | jq --raw-output '.[$i].name' )
    echo "name=$name"
done

That returned error

jq: error: $i is not defined at <top-level>, line 1:
.[$i].name  
jq: 1 compile error

Why? Although i is defined in the shell script, it’s not defined in the jq’s context. So jq has no idea about ì.

What didn’t work - using --arg

Using --arg name value also didn’t work.

jsonData=$(cat <<EOF
  [
    {
      "id": 1,
      "name": "One"
    },
    {
      "id": 2,
      "name": "Two" 
    },
    {
      "id": 3,
      "name": "Three" 
    }
  ]
  EOF
)

numberOfItems = $(echo $jsonData | jq --raw-output length)

# loop over the list in json
for (( i=0; i<numberOfItems; i++ ))
do
    echo "At index, $i"
    # using --arg doesn't work
    name=$( echo $jsonData | jq --raw-output --arg myIndex $i '.[$myIndex].name' )
    echo "name=$name"
done

I’m assigning i to variable myIndex so as to pass it to jq. But that fails with

jq: error (at jsonData.json:2): Cannot index array with string "0"

Why? Because --arg assigns a variable as a String, but jq’s array index must be a number.

References

• jq documentation: https://stedolan.github.io/jq/manual/#Invokingjq

• Stackoverflow: https://stackoverflow.com/questions/52609226/how-i-pass-argument-as-index-to-jq