Variables can be defined using the -var command line option. Let's say you have the following files on your Terraform server.

├── locals.tf
├── modules.tf
├── outputs.tf
├── provider.tf
├── terraform.tfstate
├── variables.tf
├── child (directory, child module)
│   ├── data.tf
│   ├── outputs.tf
│   ├── resources.tf

And let's say provider.tf contains the Amazon Web Services (AWS) provider with two variables, var.profile and var.region.

~]$ cat provider.tf 
terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
    }
  }
}

provider "aws" {
  profile = var.profile
  region  = var.region
}

When initializing the provider using the terraform init command, here is how you could define the profile and region variables on the command line.

terraform init -var="profile=my-profile" -var="region=us-east-1"