Terraform Environment Variable

Terraform searches the environment of its own process for environment variables named TF_VAR_ followed by the name of a declared variable.

This can be useful when running Terraform in automation, or when running a sequence of Terraform commands in succession with the same variables.

On operating systems where environment variable names are case-sensitive, Terraform matches the variable name exactly as given in configuration, and so the required environment variable name will usually have a mix of upper and lower case letters.

let's take an example create a terraform file

variable "username" {
  type = string

output printname {
        value = "Hello, ${var.username}"

now set the value of username variable in the environment using the below command

export TF_VAR_username=Hinal

now run terraform plan command and see the output.

└─$ export TF_VAR_username=Hinal

└─$ terraform plan

Changes to Outputs:
  + printname = "Hello, Hinal"

You can apply this plan to save these new output values to the Terraform state, without changing any real infrastructure.
Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to take exactly these actions if you run "terraform apply" now.