This assumes you have installed Terraform, as described at https://learn.hashicorp.com/tutorials/terraform/install-cli?in=terraform/aws-get-started.

Let's say you have a script that returns JSON. As a simple example, let's say you have a Perl script named sample.pl that returns JSON.

#!/usr/bin/perl
use strict;
use warnings;
print '{ "foo": "bar" }';

This Perl script will return the following JSON.

{
  "foo": "bar"
}

Let's say you have the following files on your Terraform server.

├── locals.tf
├── modules.tf
├── outputs.tf
├── provider.tf
├── terraform.tfstate
├── variables.tf
├── child (directory, child module)
│   ├── data.tf
│   ├── outputs.tf
│   ├── resources.tf

In data.tf in the same directory as your main root module (main.tf), external data source can use the JSON. 

data "external" "example" {
  program = [ "perl", "sample.pl" ]
}

The terraform console command can be used to display the map being returned. Notice in this example that the "result" map contains the "foo" key and "bar" value.

~]# terraform console
> data.external.example
{
  "id" = "-"
  "program" = tolist([
    "perl",
    "sample.pl",
  ])
  "query" = tomap(null) /* of string */
  "result" = tomap({
    "foo" = "bar"
  })
  "working_dir" = tostring(null)
}

And here is how you would use the external data source in resources.tf. In this example, data.external.example.result.foo will contain the value "bar".

resource "external" "example" {
  name = "${data.external.example.result.foo}"
}