docs: Promote Terraform distribution agnosticism in Terragrunt example

[lachlankidson]

what

Updates the terragrunt example documentation to be terraform distribution agnostic.
(i.e. no longer hardcode terraform or tofu).

why

This allows for Atlantis + terragrunt to manage repositories where there are both terraform and tofu projects present, e.g. an atlantis.yaml similar to:

projects:
  - dir: projects/a
    autoplan:
      when_modified: ["**/*.hcl", "**/*.tf"]
    terraform_distribution: terraform

  - dir: projects/b
    autoplan:
      when_modified: ["**/*.hcl", "**/*.tofu"]
    terraform_distribution: opentofu

tests

We're using this approach successfully in our own repositories.

references

Environment variables set here:

atlantis/server/core/runtime/run_step_runner.go

Lines 59 to 61 in ac39085

	customEnvVars := map[string]string{
	"ATLANTIS_TERRAFORM_DISTRIBUTION": tfDistribution.BinName(),
	"ATLANTIS_TERRAFORM_VERSION": tfVersion.String(),

Resolves to either tofu or terraform, confirmed by tests:

atlantis/server/core/terraform/distribution_test.go

Lines 14 to 17 in ac39085

	func TestOpenTofuBinName(t *testing.T) {
	d := terraform.NewDistributionOpenTofu()
	Equals(t, d.BinName(), "tofu")
	}

atlantis/server/core/terraform/distribution_test.go

Lines 26 to 29 in ac39085

	func TestTerraformBinName(t *testing.T) {
	d := terraform.NewDistributionTerraform()
	Equals(t, d.BinName(), "terraform")
	}

DEFAULT_TERRAFORM_VERSION removed as it refers to terraform specifically, the tofu equivalent is DEFAULT_OPENTOFU_VERSION:

atlantis/Dockerfile

Lines 9 to 11 in ac39085

	ARG DEFAULT_TERRAFORM_VERSION=1.14.9
	# renovate: datasource=github-releases depName=opentofu/opentofu versioning=hashicorp
	ARG DEFAULT_OPENTOFU_VERSION=1.12.3

which is not required as both tofu and terraform's default versions are symlinked:

atlantis/scripts/download-release.sh

Line 33 in ac39085

ln -s "${COMMAND_DIR}/${DEFAULT_VERSION}/${COMMAND_NAME}" "${COMMAND_DIR}/${COMMAND_NAME}"

ATLANTIS_TERRAFORM_VERSION can either be specified by projects/pins or derived if not present:

atlantis/cmd/server.go

Lines 490 to 493 in ac39085

	DefaultTFVersionFlag: {
	description: "Terraform version to default to (ex. v0.12.0). Will download if not yet on disk." +
	" If not set, Atlantis uses the terraform binary in its PATH.",
	},

[Copilot]

Pull request overview

Updates the Terragrunt custom workflow documentation to avoid hardcoding terraform/tofu, enabling mixed Terraform/OpenTofu repositories to work cleanly with Atlantis’ per-project distribution selection.

Changes:

• Replace terraform${...} hardcoding in Terragrunt TG_TF_PATH examples with ${ATLANTIS_TERRAFORM_DISTRIBUTION}${ATLANTIS_TERRAFORM_VERSION}.
• Remove usage of DEFAULT_TERRAFORM_VERSION in the example workflow.

[lachlankidson]

See DEFAULT_TERRAFORM_VERSION / DEFAULT_OPENTOFU_VERSION notes in references section of the PR description. This suggestion might be a good one, but its a divergence from the previous behaviour, similarly we'd want to make the same change to L334.