terraform setting hostname on multiple aws instances - terraform

Question

I'm wanting to understand how user data can be used to set hostnames for 2 or more ec2 instances that terrafrom creates. Below is my instance.tf which creates 2 instances.
resource "aws_instance" "example" {
count = 2
ami = "${lookup(var.AMIS, var.aws_region)}"
instance_type = "t2.micro"
tags = {Name = "rb-${count.index}"}
# the VPC subnet
subnet_id = "${aws_subnet.dee-main-public-1.id}"
# the security group
vpc_security_group_ids = ["${aws_security_group.allow-ssh.id}"]
# the public SSH key
key_name = "${aws_key_pair.mykeypair.key_name}"
}
resource "aws_key_pair" "mykeypair" {
key_name = "mykeypair"
public_key = "${file("${var.PATH_TO_PUBLIC_KEY}")}"
}
How do I set the hostnames for those 2 instances. i.e web1.example.com, web2.example.com
I understand cloudinit or remote-exec can be used for this but struggling to come up with the code as I'm still a beginner. Really appreciate If I can get some help to come up to speed. Many thanks in advance.
-B

Answer 1

You can do it with data resource for template file which allows you to use count function. The below syntax will allow you use the hostname variable with the count function.
data "template_file" "init" {
count = 2
template = file("$path")
vars = {
hostname = "web${count.index}.example.com"
}
}
resource "aws_instance" "master" {
ami = "$ami"
count = 2
instance_type = t2.medium
user_data = data.template_file.init[count.index].rendered
}