terraform plan is one of Terraform’s most valuable features — it shows you exactly what will change before anything is applied. This is the key workflow difference from Bicep/ARM’s --what-if.

How It Works

Terraform compares three sources to produce a plan:

  1. Configuration — what your .tf files declare
  2. State file — what Terraform last deployed (.tfstate)
  3. Live infrastructure — the actual current state in Azure (via a refresh)

The diff between these produces the plan output.

Running a Plan 

# Basic plan (prints to console)
terraform plan

# Save the plan to a file for later apply
terraform plan -out=tfplan

# Apply the saved plan exactly (no re-prompting)
terraform apply tfplan

# Plan with variable overrides
terraform plan -var="location=westeurope" -var-file="prod.tfvars"

Reading Plan Output 

Terraform will perform the following actions:

  # azurerm_storage_account.main will be created
  + resource "azurerm_storage_account" "main" {
      + account_kind             = "StorageV2"
      + account_replication_type = "LRS"
      + account_tier             = "Standard"
      + location                 = "westeurope"
      + min_tls_version          = "TLS1_2"
      + name                     = "mystorageaccount123"
      + resource_group_name      = "my-rg"
    }

Plan: 1 to add, 0 to change, 0 to destroy.
Symbol Meaning
+ Resource will be created
~ Resource will be modified in-place
- Resource will be destroyed
-/+ Resource will be replaced (destroy + create)

Plan vs What-If (Bicep/ARM)

  terraform plan az deployment ... --what-if
Diff source State file + live refresh Azure Resource Manager
Accuracy High — based on provider knowledge Moderate — known noisy diffs
Ordering Shows dependency order No ordering
Saveable Yes (-out=tfplan) No
Apply from saved plan Yes (terraform apply tfplan) No
Cross-resource impacts Shown via dependencies Not shown

Targeted Plans

Plan (or apply) only specific resources:

terraform plan -target=azurerm_storage_account.main

Useful for large configurations when you only want to change one resource.

Terraform Plan Docs