The Terraform Book

Chapter 3: Building an application stack with Terraform
We can then populate this file with variables—here a string, map, and list respectively.
Listing 3.19: Adding variable assignments
access_key = "abc123"
secret_key = "abc123"
ami = {
us-east-1 = "ami-0d729a60"
us-west-1 = "ami-7c4b331c"
}
security_group_ids = [
"sg-4f713c35",
"sg-4f713c35",
"sg-4f713c35"
]
When Terraform runs it will automatically load the terraform.tfvars file and
assign any variable values in it. The file can contain Terraform configuration
syntax or JSON, just like normal Terraform configuration files.
Any variable for which you define a value needs to exist. In our case, the variables
access_key, secret_key, and security_group_ids need to be defined with
variable blocks in our variables.tf file. If they do not exist you’ll get an error
like so:
Listing 3.20: Variable doesn’t exist error
module root: 1 error(s) occurred:
* provider config 'aws': unknown variable referenced: '
access_key'. define it with 'variable' blocks
You can also name the terraform.tfvars file something else—for example, we
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