Function format()

📝 Description

The format function produces a string by formatting a number of other values according to a specification string. It works similarly to the printf function in many programming languages.

Use Case: Perfect for generating standardized resource names, like web-server-001, web-server-002, or building complex ARNs and connection strings.

💻 Syntax

format(spec, values...)

spec: A string containing text and "verbs" (placeholders like %s, %d).
values: The data to plug into the placeholders.

💡 Common Verbs (Placeholders)

Verb	Description
`%s`	Default format for strings.
`%d`	Default format for decimal integers.
`%t`	Default format for booleans (`true`/`false`).
`%03d`	Integer padded with zeros to 3 digits (e.g., `001`).
`%%`	A literal percent sign.

💡 Examples

1. Basic String Injection:

# Result: "Hello, Ivan!"
output "greeting" {
  value = format("Hello, %s!", "Ivan")
}

2. Zero-Padding for Resource Names (Very common):

Useful when you want your server names to be sorted correctly in the UI.

# Result: "server-001", "server-002", etc.
resource "aws_instance" "web" {
  count = 3
  tags = {
    Name = format("server-%03d", count.index + 1)
  }
}

3. Multiple Values:

# Result: "Env: production | Region: us-east-1"
output "info" {
  value = format("Env: %s | Region: %s", var.env, var.region)
}

⚠️ Pro Tip

If you need to format a list of strings into a single string (like "a, b, c"), use the join() function instead. format() is better for structured, single-line templates.

[[join]] — Concatenate a list of strings.
[[upper]] / [[lower]] — Change case before formatting.
Official HashiCorp Docs