How to pass the output of run_cmd as an input

[RafaelWO]

Hello,

I am trying to fetch a Terraform output of the Terragrunt unit A outside of the current working directory (i.e. not part of the current stack) and pass the output to the unit B as an input variable.

Imagine the output called foo of unit A is the following object:

{
  bar = "baz"
}

I identified 3 ways to fetch the output object and pass it as an input.

1. Fetch Plain Output

In unit B, I could specify the inputs as follows:

inputs = {
  foo_link = run_cmd(
    "terragrunt", "--terragrunt-working-dir",  "/path/to/A", "output", "foo"
  )
}

If I run render in unit B, I get the following result in inputs:

"inputs": {
    "foo_link": "{\n  \"bar\" = \"baz\"\n}"
}

2. Fetch JSON Output

Or I could pass -json to the Terraform output command

inputs = {
  foo_link = run_cmd(
    "terragrunt", "--terragrunt-working-dir",  "/path/to/A", "output", "-json", "foo"
  )
}

Now the rendered input is:

"inputs": {
    "foo_link": "{\n  \"bar\": \"baz\"\n}"
}

(Notice the colon : instead of the equals =)

3. Fetch JSON Output and Decode

I could also pass -json to the output command and call jsondecode on the output of run_cmd:

inputs = {
  foo_link = jsondecode(
    run_cmd(
      "terragrunt", "--terragrunt-working-dir",  "/path/to/A", "output", "-json", "foo"
    )
  )
}

Now the rendered input is decoded:

"inputs": {
    "foo_link": {
        "bar": "baz"
    }
}

Summary

As far as I can tell through testing, all three ways seem to work for Terragrunt, i.e. I get the same changes if I run plan.

My question here is: Is one of the above approaches the recommended one?

Thank you very much for your help!