add terraform variable lesson

terraform-aws/lesson5-terraform-variables/Readme.MD

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+## This file contains some details on how terraform variables work.
+
+
+If you've used AWS cloudformation before , the variables are a lot like parameters in AWS Cloudformation.  
+
+To get started quickly, hard coding values on your configuration file is okay. But eventually you will want to move to using variables, because variables make things sharable, modular and reusable.
+
+Let's say that you want to create a terraform template that will deploy a VPC with few EC2 instances over and over again.\
+
+To log in to the EC2 instance you need to have a key file described. So you could do something like this on the configuration file
+
+```HCL
+
+resource "aws_instance" "web-server" {
+  ami = "ami-abcdef"
+  instance_type = "t2.micro"
+  key_name = "my-key"
+  security_groups = ["${aws_security_group.allow_ssh.name}"]
+
+
+  tags {
+    Name = "terraform-web-server"
+  }
+}
+
+resource "aws_security_group" "allow_ssh" {
+  name        = "allow_ssh"
+  description = "Allow ssh traffic"
+
+
+  ingress {
+
+    from_port   = 22 #
+    to_port     = 22
+    protocol =   "tcp"
+
+    cidr_blocks =  ["0.0.0.0/0"]
+  }
+}
+```
+
+The problem here is that when you share the file with others, they have to manually edit the key every time.
+
+This problem can be solved by using variables in terraform.
+
+Check the block of code below and notice the difference when trying to achieve the same thing as above this time by using variables.
+
+```HCL
+variable "ec2-key" {} # this is how you declare a variable.
+
+variable "ami" {default='ami-0080e4c5bc078760e'} # You can have default values for variables
+
+
+resource "aws_instance" "web-server" {
+  ami = "${var.ami}"
+  instance_type = "t2.micro"
+  key_name = "${var.ec2-key}"
+  vpc_security_group_ids = ["${aws_security_group.allow_ssh.id}"]
+
+  tags {
+    Name = "terraform-web-server"
+  }
+}
+
+resource "aws_security_group" "allow_ssh" {
+  name        = "allow_ssh"
+  description = "Allow ssh traffic"
+
+
+  ingress {
+
+    from_port   = 22 #
+    to_port     = 22
+    protocol =   "tcp"
+
+    cidr_blocks =  ["0.0.0.0/0"]
+  }
+}
+
+
+```
+
+Notice the difference? We referred to the variable to get the key-name instead of hardcoding it. Now when we run `terraform apply`, we will be asked for a value for the key-name.
+
+This way we can change the value of the variable at run time giving us ability to choose different SSH key pairs as needed without modifying the configuration file.
+
+
+--------
+There are multiple ways to pass in variables when running the terraform apply command.
+In the order of preference
+1) as a command line flag
+```console
+$ terraform apply -var 'key-name=my-key'
+
+```
+
+2) A seperate variable file which you reference like this while running the script.
+```console
+terraform apply  -var-file="vars.tfvars"
+```
+> For all files which match `terraform.tfvars` or `*.auto.tfvars` present in the current directory, Terraform  automatically loads them to populate variables. If the file is named something else, you can use the -var-file flag directly to specify a file. These files are the same syntax as Terraform configuration files.
+
+
+3) Environment variables:
+Terraform will read environment variables in the form of TF_VAR_name to find the value for a variable. For example, the TF_VAR_access_key variable can be set to set the access_key variable.
+
+4) If none of the first 3 steps are used to set variables, you are asked for variables when you run terraform apply.
+```console
+$ terraform apply
+var.ec2-key
+  Enter a value:
+```
+
+
+
+5) Default variable values. You could set a default value when declaring the variable.
+```HCL
+variable "ec2-key" { default = "my-key"}
+
+```
+
+## Homework: It's your turn now
+Convert Instance type and EC2 instance tag into variables too.
+
+
+
+
+For more Reference:https://www.terraform.io/intro/getting-started/variables.html

terraform-aws/lesson5-terraform-variables/ec2-with-variables.tf

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+variable "ec2-key" {} # this is how you declare a variable.
+
+variable "ami" {
+  default = "ami-0080e4c5bc078760e"
+} # You can have default values for variables. Variables with default value won't be prompted when you run terraform init.
+
+resource "aws_instance" "web-server" {
+  ami                    = "${var.ami}"
+  instance_type          = "t2.micro"
+  key_name               = "${var.ec2-key}"
+  vpc_security_group_ids = ["${aws_security_group.allow_ssh.id}"]
+
+  tags {
+    Name = "terraform-web-server"
+  }
+}
+
+resource "aws_security_group" "allow_ssh" {
+  name        = "allow_ssh"
+  description = "Allow ssh traffic"
+
+  ingress {
+    from_port = 22    #
+    to_port   = 22
+    protocol  = "tcp"
+
+    cidr_blocks = ["0.0.0.0/0"]
+  }
+}