Terraform template variables from other Terraform resources

Question

I have Terraform that is using a templated bash script to set my user data section for an AWS launch configuration.

data "template_file" "user_data" {
  template = "${file("${path.module}/user-data.tpl")}"

  vars {
    file_system_id = "${aws_efs_mount_target.my_efs_alpha.dns_name}"
  }
}

The file_system_id variable then needs to be used in my template:

sudo mount -t nfs -o nfsvers=4.1,rsize=1048576,wsize=1048576,hard,timeo=600,retrans=2 $$$${file_system_id}:/ /mnt/efs

Bash will interpret a single dollar sign as a bash variable. As I understand it, Terraform will interpret a double-dollar-sign as a Terraform variable. For added fun, the dollar signs in the template need to be escaped with another dollar sign -- hence the 4 dollar signs in front of file_system_id.

Looking at the user data in my Launch Config over in AWS Console, Terraform does not appear to be making any effort to replace my $$$${file_system_id) with the variable value from my template_file definition. Rather, it just shows up in the user data section as literally $${file_system_id}.

So, the question is, how do I get my EFS DNS name (or whatever other value I want) to replace the file_system_id variable in my template? What have I missed?

so what's wrong to use ${file_system_id} directly in user_data.tpl? — BMW
Well, that's some humble pie for me. I figured that since it was an escape character, and since I've found cases where I need to escape the '$' with double dollar signs to get Terraform variable replacement to work, that this would be a similar case. Indeed, it looks like the single dollar sign works here. Feel free to submit your comment as an answer and I'll approve it. — REW

Preston Robertson Preston Robertson · Accepted Answer · 2018-09-04T17:27:56

As BMW mentioned, you don't need to escape the dollar signs. ${file_system_id} works just fine.

Terraform's variable-replacement in templates will run first so you don't need to worry about how Bash will parse it until after the variables are replaced.

Terraform template variables from other Terraform resources

1 Answers