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Function join()

📝 Description

The join function produces a string by concatenating together all elements of a list, using a specific separator between each element.

Use Case: Creating comma-separated lists for Security Group rules, building environment paths, or generating human-readable labels from a list of strings.

💻 Syntax

join(separator, list)

💡 Examples

1. Creating a Comma-Separated List:

Useful for policy documents or metadata.

# Result: "admin, developer, guest"
output "users" {
  value = join(", ", ["admin", "developer", "guest"])
}

2. Building a Custom Path:

# Result: "opt/myapp/config"
output "path" {
  value = join("/", ["opt", "myapp", "config"])
}

3. Practical Scenario: Security Group Rules

Imagine you have a list of CIDR blocks and you need to pass them as a single string (though many resources accept lists directly, some legacy modules or scripts might require a string):

variable "whitelist_ips" {
  default = ["10.0.1.0/24", "1.1.1.1/32", "8.8.8.8/32"]
}

locals {
  # Result: "10.0.1.0/24,1.1.1.1/32,8.8.8.8/32"
  authorized_ranges = join(",", var.whitelist_ips)
}

⚠️ Common Errors

Input Result Why?
join(",", "string") Error Second argument must be a list, not a single string.
join(",", [1, 2]) Success Terraform will automatically convert numbers to strings inside the list.

  • [[split]] — The opposite: turns a string into a list.

  • [[format]] — Better for complex templates where logic is more than just "glueing".

  • Official HashiCorp Docs