12

Given a bash array, how to convert it to a JSON array in order to output to a file with jq?

Additionnally: is there a way to keep the server_nohup array unchanged instead of re-writing the whole json file each time?

newArray=(100 200 300)
jq -n --arg newArray $newArray '{
    client_nohup: [ 
        $newArray
    ],
    server_nohup: [

    ]
}' > $projectDir/.watch.json

Current output:

{
"client_nohup": [
    "100"
],
"server_nohup": []
}

Desired output:

{
"client_nohup": [
    100,
    200,
    300
],
"server_nohup": []
}
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2 Answers 2

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8

Given a bash array, how to convert it to a JSON array

Assuming your jq supports the --args command-line option, you can use it in conjunction with the built-in $ARGS pseudo-variable, as illustrated by this example:

$ declare -a output=($'abc\ndef' "x y z")
$ jq -c -n '$ARGS.positional' --args "${output[@]}"
["abc\ndef","x y z"]

Here, all the values of the bash array are converted to JSON strings. Notice that the embedded new-line in the first value of the bash array is correctly handled.

Additionally ...

(1) If all the values in newArray are valid as JSON values without spaces, then you could get away with piping the values as a stream, e.g.

newArray=(100 200 300)
echo "${newArray[@]}" |
  jq -s '{client_nohup: ., server_nohup: []}'

(2) Now let's suppose you merely wish to update the "nohup" object in a file, say nohup.json:

{ "client_nohup": [], "server_nohup": [ "keep me" ] }

Since you are using bash, you can then write:

echo "${newArray[@]}" |
  jq -s --argjson nohup "$(cat nohup.json)" '
    . as $newArray | $nohup | .client_nohup = $newArray
  '

Output

(1)

{
  "client_nohup": [
    100,
    200,
    300
   ],
  "server_nohup": []
}

(2)

{
  "client_nohup": [
    100,
    200,
    300
  ],
  "server_nohup": [
    "keep me"
  ]
}

Other cases

Where there's a will, there's a jq way :-)

See for example the accepted answer at How to format a bash array as a JSON array (though this is not a completely generic solution).

Further thoughts

You might wish to convert numeric strings to JSON numbers, e.g. using the jq idiom: (tonumber? // .).

You might also wish to exploit the fact that bash strings cannot contain NUL characters.

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12 Comments

Thanks for a thorough answer, but does case 1 need the value for key client_nohup to be an array nested in another array?
It seems I copied-and-pasted the wrong output. Fixed.
This would work for simple values, but in general will fail because the array elements could contain newlines or other invalid JSON characters.
@chepner -please note that your point was made in the answer, both at the end and in the final section, which points to techniques which can be applied in other cases.
The answer in the FAQ fails for the array x=(1 $'a\nb' 2).
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0

In general, the only truly safe way to do this is with multiple invocations of jq, adding each element to the output of the previous command. 

arr='[]'  # Empty JSON array
for x in "${newArray[@]}"; do
  arr=$(jq -n --arg x "$x" --argjson arr "$arr" '$arr + [$x]')
done

This ensures that each element x of your bash array is properly encoded prior to adding it to the JSON array.

This is complicated, though, by the fact that bash doesn't not distinguish between numbers and strings. This encodes your array as ["100", "200", "300"], not [100, 200, 300]. In the end, you need to have some awareness of what your array contains, and preprocess it accordingly.

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4 Comments

It is not the case that ‘the only truly safe way to do this is with multiple invocations of jq’. The jq FAQ shows how this can be avoided using NUL. Search for NUL in the FAQ github.com/stedolan/jq/wiki/FAQ
One can get around the number/string problem using tonumber? // ., though of course that would convert quoted numbers to numbers ...
@peak The problem here isn't the separator, but that bash strings aren't necessarily valid JSON strings. It's broken for the same reason that a hard-coded value like $'"foo\bar"' isn't valid JSOn.
As mentioned elsewhere, you have evidently overlooked the use of the -R option. Of course this forces everything to be a JSON string, and thus your point about the distinction between numbers and strings being lost in a "generic" solution is perfectly valid.0.

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