JQ can be used to create JSON objects of a certain shape, with the help of a few command line options.

You can use --argjson to load JSON objects into variables that can then be referenced by $name. Here’s an example:

> jq -n --argjson vars '["a", "b", "c"]' '{"vars": $vars}'
{
  "vars": [
    "a",
    "b",
    "c"
  ]
}

(The -n option is to avoid jq attempting to read files as input)

Similarly, you can use the option --slurpfile to load a json file in a variable (arrays only):

> cat vars.json 
["a", "b", "c"]

> jq -n --slurpfile vars vars.json '{"vars": $vars}'
{
  "vars": [
    [
      "a",
      "b",
      "c"
    ]
  ]
}

All this can be extended to manipulate the keys too. Here’s an example (written in fish shell)

> cat vars.json
{
  "key1": "val1"
}

> set vars (cat vars.json)
> jq -n --argjson vars "$vars" '{"key2": "val2"} + $vars'
{
  "key2": "val2",
  "key1": "val1"
}

Or create a simple key override system:

> cat vars.json
{
  "key2": "val1"
}

> set vars (cat vars.json)
> jq -n --argjson vars "$vars" '{"key2": "val2"} + $vars'
{
  "key2": "val1"
}

> jq -n --argjson vars "$vars" '$vars + {"key2": "val2"}'
{
  "key2": "val2"
}