```export PROJECT_ID="your-project-id"```

Terraform offers various methods to provide input variables, making it flexible and adaptable to different use cases. These methods include command-line flags, environment variables, variable definition files (including auto-loaded files), and default values in variable declarations.

Here is a summary of the variable input methods in order of priority, from highest to lowest:

Command-line flags (-var):
These flags have the highest priority, meaning that values provided using -var will override any other method.
Example:

```terraform apply -var="project_id=$PROJECT_ID" -var="region=us-central1" -var="zone=us-central1-f" -var="image_name=debian-cloud/debian-10" -var="instance_type=n1-standard-4"```

Environment variables (TF_VAR_):
This method takes precedence over auto-loaded variable definition files, variable definition files specified with the -var-file flag, and default values in variable declarations.

Example:

```
export TF_VAR_credentials_file="credentials.json"
export TF_VAR_project_id="$PROJECT_ID"
export TF_VAR_region="us-central1"
export TF_VAR_zone="us-central1-a"
export TF_VAR_image_name="debian-cloud/debian-11"
export TF_VAR_instance_type="n1-standard-1"
terraform apply
```

Auto-loaded variable definition files (.auto.tfvars or .auto.tfvars.json):
Auto-loaded files have priority over variable definition files specified with the -var-file flag and default values in variable declarations.
Example: Two .auto.tfvars files (a.auto.tfvars and b.auto.tfvars) contain different values for project_id. When you run terraform apply, Terraform loads both files, and the final project_id value is taken from b.auto.tfvars.

Variable definition files specified with the -var-file flag (.tfvars or .tfvars.json):
This method has a lower priority than auto-loaded variable definition files but takes precedence over default values in variable declarations.
Example (terraform.tfvars.json):

```
{
  "credentials_file": "credentials.json",
  "project_id": "$PROJECT_ID",
  "region": "us-central1",
  "zone": "us-central1-a",
  "image_name": "debian-cloud/debian-11",
  "instance_type": "n1-standard-1"
}
```

Example (terraform.tfvars):

```
credentials_file = "credentials.json"
project_id       = "$PROJECT_ID"
region           = "us-central1"
zone             = "us-central1-a"
image_name       = "debian-cloud/debian-11"
instance_type    = "n1-standard-1"
```
Default values in variable declarations (within .tf files):
These values have the lowest priority and will only be used if no value is provided through any other method.
Example (variables.tf):

```
variable "project_id" {
  default = "$PROJECT_ID"
}

variable "region" {
  default = "us-central1"
}
```
In summary, Terraform supports various methods to input variables, each with its own use case. You can choose the method that best suits your needs and preferences.