Terraformのvariableにはvalidationがあり、入力値の条件を事前にチェックできます。
この記事では「10文字以内」という文字数制限にフォーカスして、validationの書き方と挙動を紹介します。

この記事でやること

10文字以内の制約をvalidationで実装する
ルール違反時のエラー表示を確認する

validationの書き方

今回のポイントはこの3行です。

validation {
  condition     = length(var.bucket_name) <= 10
  error_message = "bucket_name must be 10 characters or fewer."
}

何をしているか

condition: 真偽値を返す式を書く（今回は長さが10以下かどうか）
error_message: conditionがfalseのときに表示されるメッセージ

このブロックがあるだけで、入力時点で文字数チェックが走ります。

サンプル構成

ファイル配置構成

.
├── main.tf
├── variables.tf
└── outputs.tf

main.tf

main.tf

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = ">= 5.0"
    }
  }
}

provider "aws" {
  region = var.aws_region
}

resource "aws_s3_bucket" "this" {
  bucket = var.bucket_name
  tags   = var.tags
}

variables.tf（validationの実装例）

variables.tf

variable "aws_region" {
  type        = string
  description = "AWS region."
  default     = "ap-northeast-1"
}

variable "bucket_name" {
  type        = string
  description = "Name of the S3 bucket (max 10 characters)."

  validation {
    condition     = length(var.bucket_name) <= 10
    error_message = "bucket_name must be 10 characters or fewer."
  }
}

variable "tags" {
  type        = map(string)
  description = "Tags to apply to the bucket."
  default     = {}
}

outputs.tf

outputs.tf

output "bucket_id" {
  value       = aws_s3_bucket.this.id
  description = "ID of the S3 bucket."
}

output "bucket_arn" {
  value       = aws_s3_bucket.this.arn
  description = "ARN of the S3 bucket."
}

使い方（確認方法）

terraform

% terraform init
% terraform plan -var="bucket_name=example10"

実行結果（OK: 10文字以内）

OK

% terraform plan -var="bucket_name=example10"

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated
with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_s3_bucket.this will be created
  + resource "aws_s3_bucket" "this" {
      + acceleration_status         = (known after apply)
      + acl                         = (known after apply)
      + arn                         = (known after apply)
      + bucket                      = "example10"
      + bucket_domain_name          = (known after apply)
      + bucket_prefix               = (known after apply)
      + bucket_region               = (known after apply)
      + bucket_regional_domain_name = (known after apply)
      + force_destroy               = false
      + hosted_zone_id              = (known after apply)
      + id                          = (known after apply)
      + object_lock_enabled         = (known after apply)
      + policy                      = (known after apply)
      + region                      = "ap-northeast-1"
      + request_payer               = (known after apply)
      + tags_all                    = (known after apply)
      + website_domain              = (known after apply)
      + website_endpoint            = (known after apply)

      + cors_rule (known after apply)

      + grant (known after apply)

      + lifecycle_rule (known after apply)

      + logging (known after apply)

      + object_lock_configuration (known after apply)

      + replication_configuration (known after apply)

      + server_side_encryption_configuration (known after apply)

      + versioning (known after apply)

      + website (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + bucket_arn = (known after apply)
  + bucket_id  = (known after apply)

Note: You didn't use the -out option to save this plan, so Terraform can't guarantee to take exactly these
actions if you run "terraform apply" now.

実行結果（NG: 10文字超）

NG

% terraform plan -var="bucket_name=too-long-name"

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Invalid value for variable
│ 
│   on variables.tf line 7:
│    7: variable "bucket_name" {
│     ├────────────────
│     │ var.bucket_name is "too-long-name"
│ 
│ bucket_name must be 10 characters or fewer.
│ 
│ This was checked by the validation rule at variables.tf:11,3-13.
╵

上記のように10文字を超えると、validationのメッセージが表示されます。

補足

実際のS3バケット名の制約はもっと多いです（文字種や長さなど）。
ここではvalidationの書き方に集中するため、あえてシンプルなルールにしています。

まとめ

validationを使うと、入力ミスを早い段階で発見できます。
まずは文字数制限のようなシンプルなルールから導入するのがおすすめです。