Add dynamic template output

Author: RulerOf

README.md

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 # terraform-code-examples
 Solving weird problems with Terraform
+
+
+* [Dynamic Templatefile Rendering](./dynamic-template-output/README.md)
+  * Keeping `templatefile()` function calls and the files themselves DRY — avoids writing variables twice, as input and output.

dynamic-template-output/.terraform-version

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+0.13.4

dynamic-template-output/README.md

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+# Dynamic Templatefile Rendering
+
+The [`templatefile()`](https://www.terraform.io/docs/configuration/functions/templatefile.html) function is very useful, but using it as-written in the documentation samples encourages very repetitive coding practices. For example, you might be supplying instance user_data from a rendered script like this:
+
+```hcl
+resource "aws_instance" "my_server" {
+  user_data = templatefile("${path.module}/files/user-data.sh.tmpl",
+    {
+      VAR_1 = "my first variable"
+      VAR_2 = "my second variable"
+    }
+  )
+}
+```
+
+Then, over in the actual template, you have to re-write all of the inputs you supplied to the `templatefile()` function like this:
+
+```shell
+#!/bin/bash
+
+echo ${VAR_1}
+echo ${VAR_2}
+```
+
+## DRYing Templates Out
+
+Instead, you can use a [string template directive](https://www.terraform.io/docs/configuration/expressions.html#string-templates) in the template file, combined with a map input to the `templatefile()` function to DRY this out.
+
+To see this in action, review [main.tf](./main.tf), and see how it passes the `ENV_VARS = {}` map into the function. The `for` directive inside of [user-data.sh.tmpl](./user-data.sh.tmpl) unwraps that map, renders each line into an `export`, and the resulting script sets all of variables in the shell.
+
+### Discussion
+
+Relates back to [this reddit thread](https://www.reddit.com/r/Terraform/comments/jy05en/terraform_variables_in_bash_scripting/gd0dfhv/)

dynamic-template-output/main.tf

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+output "rendered" {
+  value = templatefile("${path.module}/user-data.sh.tmpl",
+  {
+    ENV_VARS = {
+      var1 = "test1"
+      var2 = "test2"
+    }
+  })
+}

dynamic-template-output/user-data.sh.tmpl

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+#!/bin/bash
+
+%{ for k, v in ENV_VARS ~}
+export ${k}="${v}"
+%{ endfor ~}
+
+# Do stuff
+echo "The rendered version of this template will declare all of the keys in ENV_VARS as variables."