README.md

Dynamic Templatefile Rendering

The templatefile() 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:

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:

#!/bin/bash

echo ${VAR_1}
echo ${VAR_2}

DRYing Templates Out

Instead, you can use a string template directive 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, and see how it passes the ENV_VARS = {} map into the function. The for directive inside of user-data.sh.tmpl unwraps that map, renders each line into an export, and the resulting script sets all of variables in the shell.

Using This Example

To start, init and apply this terraform code:

git clone https://github.com/RulerOf/terraform-code-examples.git
cd terraform-code-examples/dynamic-template-output
terraform init
terraform apply -auto-approve

Then observe the output:

23:22 $ terraform apply -auto-approve

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

rendered = #!/bin/bash

export var1="test1"
export var2="test2"

# Do stuff
echo "The rendered version of this template will declare all of the keys in ENV_VARS as variables."

Now, in main.tf, you can add more inputs to the ENV_VARS map and re-apply to see this in action:

23:22 $ terraform apply -auto-approve

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

rendered = #!/bin/bash

export var1="test1"
export var2="test2"
export var_new="This var was added to the map in the function input, but I did not modify the template file"

# Do stuff
echo "The rendered version of this template will declare all of the keys in ENV_VARS as variables."

Discussion

Relates back to this reddit thread