Constructing a JSON object from a bash associative array

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There are many possibilities, but given that you already have written a bash for loop, you might like to begin with this variation of your script:

#!/bin/bash
# Requires bash with associative arrays
declare -A dict

dict["foo"]=1
dict["bar"]=2
dict["baz"]=3

for i in "${!dict[@]}"
do
    echo "$i" 
    echo "${dict[$i]}"
done |
jq -n -R 'reduce inputs as $i ({}; . + { ($i): (input|(tonumber? // .)) })'

The result reflects the ordering of keys produced by the bash for loop:

{
  "bar": 2,
  "baz": 3,
  "foo": 1
}

In general, the approach based on feeding jq the key-value pairs, with one key on a line followed by the corresponding value on the next line, has much to recommend it. A generic solution following this general scheme, but using NUL as the “line-end” character, is given below.

Keys and Values as JSON Entities

To make the above more generic, it would be better to present the keys and values as JSON entities. In the present case, we could write:

for i in "${!dict[@]}"
do
    echo "\"$i\""
    echo "${dict[$i]}"
done | 
jq -n 'reduce inputs as $i ({}; . + { ($i): input })'

Other Variations

JSON keys must be JSON strings, so it may take some work to ensure that the desired mapping from bash keys to JSON keys is implemented. Similar remarks apply to the mapping from bash array values to JSON values. One way to handle arbitrary bash keys would be to let jq do the conversion: 

printf "%s" "$i" | jq -Rs .

You could of course do the same thing with the bash array values, and let jq check whether the value can be converted to a number or to some other JSON type as desired (e.g. using fromjson? // .).

A Generic Solution

Here is a generic solution along the lines mentioned in the jq FAQ and advocated by @CharlesDuffy. It uses NUL as the delimiter when passing the bash keys and values to jq, and has the advantage of only requiring one call to jq. If desired, the filter fromjson? // . can be omitted or replaced by another one.

declare -A dict=( [$'foo\naha']=$'a\nb' [bar]=2 [baz]=$'{"x":0}' )

for key in "${!dict[@]}"; do
    printf '%s\0%s\0' "$key" "${dict[$key]}"
done |
jq -Rs '
  split("\u0000")
  | . as $a
  | reduce range(0; length/2) as $i 
      ({}; . + {($a[2*$i]): ($a[2*$i + 1]|fromjson? // .)})'

Output:

{
  "foo\naha": "a\nb",
  "bar": 2,
  "baz": {
    "x": 0
  }
}

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