Terraform Advanced — Splat Expressions

Splat expressions extract attributes from lists and resource collections in a single expression.

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The Splat Operator [*]

# Create 3 EC2 instances
resource "aws_instance" "web" {
  count         = 3
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t3.micro"
}

# Get all instance IDs
output "instance_ids" {
  value = aws_instance.web[*].id
}

# Equivalent for expression:
output "instance_ids_alt" {
  value = [for inst in aws_instance.web : inst.id]
}

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Splat with for_each Resources

Splat [*] only works with count. For for_each, use values():

resource "aws_s3_bucket" "this" {
  for_each = toset(["logs", "assets", "backups"])
  bucket   = "${var.project}-${each.key}"
}

# ❌ Won't work:
# aws_s3_bucket.this[*].arn

# ✅ Correct:
output "bucket_arns" {
  value = values(aws_s3_bucket.this)[*].arn
}

# ✅ Also correct — for expression:
output "bucket_arns_alt" {
  value = [for b in aws_s3_bucket.this : b.arn]
}

---

Nested Splat

# Get all private IPs from all network interfaces
output "all_private_ips" {
  value = aws_instance.web[*].network_interface[*].private_ip
}

💡 Production Tip: Prefer for expressions over splat for complex transformations. Splat is a shorthand — for gives you filtering and restructuring.

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What’s Next?

Learn try(), can(), and one() for safe attribute access.

← Advanced for_each try, can, one →