terraform version 0.12.29

main.tf

 variable "availability_zones" {
   description = "List of availability zone in which the ECS cluster should reside"
-  type        = "list"
+  type        = list(string)
 }
 
 variable "vpc_id" {
@@ -9,7 +9,7 @@ variable "vpc_id" {
 
 variable "subnet_ids" {
   description = "List of subnets in which the ECS cluster should reside"
-  type        = "list"
+  type        = list(string)
 }
 
 variable "image_id" {
@@ -34,7 +34,7 @@ variable "bastion_instances" {
 
 resource "aws_security_group" "bastion" {
   name        = "Bastion"
-  vpc_id      = "${var.vpc_id}"
+  vpc_id      = var.vpc_id
   description = "Allows traffic from and to the Bastion EC2 instance"
 
   ingress {
@@ -51,7 +51,6 @@ resource "aws_security_group" "bastion" {
     cidr_blocks = ["0.0.0.0/0"]
   }
 
-
   egress {
     from_port   = 0
     to_port     = 0
@@ -59,61 +58,70 @@ resource "aws_security_group" "bastion" {
     cidr_blocks = ["0.0.0.0/0"]
   }
 
-  tags = {
-    Name        = "Bastion"
-  }
+  tags = [
+    {
+    key = "Name"
+    value = "Bastion"
+    propagate_at_launch = true
+    }
+  ]
 
-  lifecycle { create_before_destroy = true }
+  lifecycle { 
+    create_before_destroy = true 
+    }
 }
 
 data "template_file" "userdata" {
-  template = "${file("${path.module}/userdata.yml")}"
-  vars {}
+  template = file("${path.module}/userdata.yml")
+  vars = {}
 }
 
 resource "aws_launch_configuration" "bastion" {
-  name_prefix = "${format("%s-", "Bastion")}"
-
-  image_id             = "${var.image_id}"
-  instance_type        = "${var.instance_type}"
-  key_name             = "${var.key_name}"
-  security_groups      = ["${aws_security_group.bastion.id}"]
-  user_data            = "${data.template_file.userdata.rendered}"
+  name_prefix          = format("%s-", "Bastion")
+  image_id             = var.image_id
+  instance_type        = var.instance_type
+  key_name             = var.key_name
+  security_groups      = [aws_security_group.bastion.id]
+  user_data            = data.template_file.userdata.rendered
 
   lifecycle {
     create_before_destroy = true
     ignore_changes = [
-      "image_id",
+      image_id,
     ]
   }
 }
 
 resource "aws_autoscaling_group" "bastion" {
   name                 = "Bastion - ${aws_launch_configuration.bastion.name}"
-  availability_zones   = ["${var.availability_zones}"]
-  vpc_zone_identifier  = ["${var.subnet_ids}"]
-  launch_configuration = "${aws_launch_configuration.bastion.id}"
-  min_size             = "${var.bastion_instances}"
-  max_size             = "${var.bastion_instances}"
-  desired_capacity     = "${var.bastion_instances}"
-
-  tag {
+  availability_zones   = var.availability_zones
+  vpc_zone_identifier  = flatten([var.subnet_ids])
+  launch_configuration = aws_launch_configuration.bastion.id
+  min_size             = var.bastion_instances
+  max_size             = var.bastion_instances
+  desired_capacity     = var.bastion_instances
+
+  tags = [
+    {
     key = "Name"
     value = "Bastion"
     propagate_at_launch = true
-  }
-  tag {
+    },
+    {
     key = "service"
     value = "bastion"
     propagate_at_launch = true
-  }
+    }
+  ]
 
-  lifecycle { create_before_destroy = true }
+  lifecycle { 
+    create_before_destroy = true 
+  }
 }
 
 #############################################################################################################
 # Outputs
 #############################################################################################################
 output "security_group" {
-    value = "${aws_security_group.bastion.id}"
+    value = aws_security_group.bastion.id
 }