$ gcloud auth login
$ gcloud config set project {{プロジェクト名}}
$ gsutil mb gs://{{バケット名}}

$  tree .
.
├── README.md
├── ci
│   ├── terraform_apply.sh
│   └── terraform_plan.sh
├── cloudbuild.yaml
└── terraform
    └── backend.tf

＃!/bin/bash

cd terraform

WORKSPACE=prd

terraform init -backend-config="prefix=${WORKSPACE}" -reconfigure

if [ ! $(terraform workspace list | grep ${WORKSPACE}) ]; then
  terraform workspace new ${WORKSPACE}
fi
terraform workspace select ${WORKSPACE}

echo 'terraform apply'
terraform apply -auto-approve -parallelism=50

＃!/bin/bash

cd terraform

WORKSPACE=prd

terraform init -backend-config="prefix=${WORKSPACE}" -reconfigure

if [ ! $(terraform workspace list | grep ${WORKSPACE}) ]; then
  terraform workspace new ${WORKSPACE}
fi
terraform workspace select ${WORKSPACE}

echo 'terraform plan'
terraform plan -parallelism=50

substitutions:
  _TERRAFORM_VERSION_: 0.12.10
steps:
  - id: 'tf plan'
    name: 'hashicorp/terraform:${_TERRAFORM_VERSION_}'
    entrypoint: 'sh'
    args: ['./ci/terraform_plan.sh']
  - id: 'tf apply'
    name: 'hashicorp/terraform:${_TERRAFORM_VERSION_}'
    entrypoint: 'sh'
    args: ['./ci/terraform_apply.sh']

terraform {
  backend "gcs" {
    bucket  = "{{バケット名}}"
    prefix  = "terraform/state"
  }
}