variables-different-ways.txt

#CASE #1 - We can use variables using "locals" 
# vim main.tf
# -----

locals {

 ami  = "ami-0d26eb3972b7f8c96"

type = "t2.micro"

tags = {
   Name = "My Virtual Machine"
   Env  = "Dev"
 }

subnet = "subnet-76a8163a"

nic    = aws_network_interface.my_nic.id

}
 
resource "aws_instance" "myvm" {
 ami           = local.ami
 instance_type = local.type
 tags          = local.tags
 
 network_interface {
   network_interface_id = aws_network_interface.my_nic.id
   device_index         = 0
 }
}
 
resource "aws_network_interface" "my_nic" {
 description = "My NIC"
 subnet_id   = local.subnet
 
 tags = {
   Name = "My NIC"
 }
}

### file END here
### try to keep the number of local variables to their minimum. Using many local variables can make the code hard to read

#CASE - 2 - Using external variables.tf file
# vim variables.tf
####

variable "ami" {
 type        = string
 description = "AMI ID for the EC2 instance"
 default     = "ami-0a23ccb2cdd9286bb"
 
 validation {
   condition     = length(var.ami) > 4 && substr(var.ami, 0, 4) == "ami-"
   error_message = "Please provide a valid value for variable AMI."
 }
}
 
variable "type" {
 type        = string
 description = "Instance type for the EC2 instance"
 default     = "t2.micro"
 sensitive   = true
}
 
variable "tags" {
 type = object({
   name = string
   env  = string
 })
 description = "Tags for the EC2 instance"
 default = {
   name = "test-machine"
   env  = "prod"
 }
}
 
variable "subnet" {
 type        = string
 description = "Subnet ID for network interface"
 default     = "subnet-76da2b1f"
}

### file END here

## NOW create another file - main.tf with the code
# vim main.tf
####
resource "aws_instance" "myvm" {
 ami           = var.ami
 instance_type = var.type
 tags          = var.tags
 
 network_interface {
   network_interface_id = aws_network_interface.my_nic.id
   device_index         = 0
 }
}
 
resource "aws_network_interface" "my_nic" {
 description = "My NIC"
 subnet_id   = var.subnet
 
 tags = {
   Name = "My NIC"
 }
}

### file END here


## CASE - 3 - Substituting variable values at run time using -var
#
terraform plan -var "ami=test" -var "type=t2.nano" 
# These variable will override the values in the variables.tf file

## CASE - 4 - If you have a lot of variables to replace at runtime, then passing as -var can be tedious
# There we use .tfvars file
# vim newvalues.tfvar
###
ami  = "ami-0d26eb3972b7f8c96"
type = "t2.nano"
tags = {
 "name" : "My Virtual Machine"
 "env" : "Dev"
}

### file END here
#
#Now run the terraform - 
terraform plan -var-file values.tfvars

#CASE - 4 - Using environment variables

# input variables values can also be set using Terraform environment variables. 
# To do so, simply set the environment variable in the format TF_VAR_<variable name>

export TF_VAR_ami=ami-0d26eb3972b7f8c96

## REMEMBER
# Terraform uses hierarchical logic to decide which value to consider in the event of a conflict.

#The order is as follows:

# 1. Environment variables
# 2.The terraform.tfvars file, if present
# 3. The terraform.tfvars.json file, if present
# 4. Any *.auto.tfvars or *.auto.tfvars.json files, processed in lexical order of their filenames
# 5. Any -var and -var-file options on the command line, in the order they are provided