🔹HCL syntax used in terraform

Terraform is an Infrastructure as Code (IaC) tool that allows you to define and provision infrastructure resources in a declarative manner. The configuration files in Terraform use a language called HashiCorp Configuration Language (HCL). In this response, I'll provide an overview of the HCL syntax used in Terraform.

Blocks and Arguments: HCL code is organized into blocks and arguments. Blocks define different configuration sections, while arguments define the properties within a block.

resource "aws_instance" "example" {
  ami           = "ami-0123456789abcdef0"
  instance_type = "t2.micro"
  tags = {
    Name        = "my-instance"
    Environment = "production"
  }
}

- In this example, the resource block is used to define an AWS EC2 instance resource.
  - The block has a type of aws_instance and a name of example.
  - Within the block, there are several arguments assigned using the = symbol, such as ami, instance_type, and tags.
  - The tags argument is assigned a nested block using curly braces {} to define multiple key-value pairs.
Comments: You can add comments in your HCL code using the # symbol. Anything after the # on a line is considered a comment.
```
 # This is a comment in HCL
```
Comments are used to provide explanations or annotate the code, but they are ignored by Terraform during execution.
Strings: String values are typically assigned using double quotes ("").
```
 variable "region" {
   description = "AWS region"
   type        = string
   default     = "us-west-2"
 }
```
In this example, the string value "AWS region" is assigned to the description argument of the variable block.
Integers and Floating-Point Numbers: Numeric values can be assigned as integers or floating-point numbers.
```
 resource "aws_instance" "example" {
   ami              = "ami-0123456789abcdef0"
   instance_count   = 3
   instance_type    = "t2.micro"
   price_per_hour   = 0.25
   cpu_utilization  = 80.5
 }
```
In this example, the instance_count argument is assigned an integer value of 3, while price_per_hour and cpu_utilization are assigned floating-point numbers.

Booleans: Boolean values can be assigned using true or false.

 resource "aws_instance" "example" {
   ami           = "ami-0123456789abcdef0"
   instance_type = "t2.micro"
   monitoring    = true
 }

In this example, the monitoring argument is assigned a boolean value of true.

Lists: Lists can be used to represent collections of values. They are defined using square brackets [].
```
 resource "aws_instance" "example" {
   ami               = "ami-0123456789abcdef0"
   instance_type     = "t2.micro"
   security_group_ids = ["sg-0123456789abcdef0", "sg-9876543210fedcba"]
 }
```
In this example, the security_group_ids argument is assigned a list of two strings representing security group IDs.

Maps: Maps are used to represent key-value pairs. They are defined using curly braces {}.

 variable "content_map" {
 type=map
 default={
 "content1"="this is content1"
 "content2"="this is content2"
  }
 }

🔹Variable, Datatype Expressions in HCL

Variables: Variables allow you to parameterize your configurations and make them more flexible. You can declare variables using the variable block in your Terraform configuration files.

create a file called variable.tf :-

variable "region" {
  description = "AWS region"
  type        = string
  default     = "us-west-2"
}

In this example, a variable named "region" is defined.
The description argument provides a description for the variable.
The type argument specifies the data type of the variable. In this case, it is string.
The default argument sets a default value for the variable.

Data Types: HCL supports various data types that you can use in your Terraform configurations:

String: A sequence of characters, represented using double quotes.
Number: Numeric values, which can be integers or floating-point numbers.
Boolean: Represents true or false.
List: An ordered collection of values, enclosed in square brackets [].
Map: A collection of key-value pairs, enclosed in curly braces {}.
Object: A complex data structure that represents an object with properties.

Expressions: Expressions in HCL allow you to dynamically compute and manipulate values within your Terraform configurations. Expressions can include variables, functions, operators, and references to other resources or data sources.

resource "aws_instance" "example" {
  ami           = var.ami_id
  instance_type = "${var.instance_type}-small"
  key_name      = "key-${aws_region}"
}

In this example, the ami argument is assigned the value of the variable var.ami_id.
The instance_type argument uses string interpolation ${} to dynamically concatenate the value of the variable var.instance_type with the string -small.
The key_name argument concatenates the string "key-" with the value of the aws_region resource, which is a reference to another resource.

Expressions in HCL support various operators, including arithmetic, logical, comparison, and string manipulation operators. You can use parentheses () to group expressions and control the order of operations.

Additionally, HCL provides built-in functions that you can use in expressions to perform operations, transformations, and validations. These functions cover a wide range of functionalities, such as string manipulation, mathematical calculations, encoding, decoding, and more.

It's important to consult the Terraform documentation and the documentation of the specific provider you're working with to explore the available data types, variables, and functions specific to that provider.

🔹Writing Terraform Configurations

Install AWS CLI: Follow the steps mentioned earlier to install the AWS CLI on your machine. Make sure it is successfully installed and configured with your AWS credentials.
```
 # On Ubuntu or Debian
 sudo apt-get install awscli

 # On CentOS or RHEL
 sudo yum install awscli

 #configure
 aws configure
```
Create a directory for your Terraform project: Open a terminal or command prompt and create a directory for your Terraform project. For example:
```
 mkdir my-terraform-project
 cd my-terraform-project
```

Create the Terraform configuration file: Create a file named main.tf in the project directory and add the following content:

 terraform {
   required_providers {
     aws = {
       source  = "hashicorp/aws"
       version = "~> 4.16"
     }
   }
   required_version = ">~ 1.2.0"
 }

 provider "aws" {
   region = "us-east-1"
 }

 resource "aws_instance" "my_ec2_instance" {
   ami           = "ami-00103374621"
   instance_type = "t2.micro"

   tags = {
     name = "first_instance"
   }
 }

Save the file.

Initialize the Terraform project: Run the following command to initialize the Terraform project:
```
 terraform init
```
This command will download the required provider plugins specified in your configuration.
Validate the Terraform configuration: Run the following command to validate the syntax and configuration of your Terraform files:
```
 terraform validate
```
This command checks for any errors or warnings in your configuration.
Review the execution plan: Run the following command to see what changes Terraform will make before actually creating resources:
```
 terraform plan
```
Review the plan output to ensure it matches your expectations.
Apply the Terraform configuration: If the plan looks good, you can proceed with creating the resources. Run the following command:
```
 terraform apply
```
Terraform will prompt for confirmation. Type yes and press Enter to proceed. Terraform will create the EC2 instance based on the specified configuration.

Note: Make sure you have the necessary permissions and valid AWS credentials configured on your machine for Terraform to interact with AWS.
Verify the created resources: After the terraform apply command completes successfully, you can verify the created EC2 instance in the AWS Management Console or by using the AWS CLI.

The EC2 instance will have the specified AMI, instance type, and the tag "name" with the value "first_instance".

To destroy the created resources and remove the EC2 instance, you can run the following command:
```
 terraform destroy
```
This will prompt for confirmation. Type yes and press Enter to proceed with destroying the resources.

Refrence:-https://www.youtube.com/live/VyB_vETjvT0?feature=share

Day 2-Terraform Configuration Language(HCL)

Table of contents

🔹HCL syntax used in terraform

🔹Variable, Datatype Expressions in HCL

🔹Writing Terraform Configurations