How to Use Coalesce Function in Terraform: Simple Guide

In Terraform, the coalesce function returns the first non-null value from its arguments. Use it to provide fallback values when some variables might be null or undefined.

Syntax

The coalesce function takes one or more arguments and returns the first argument that is not null. If all arguments are null, it returns null.

• Arguments: One or more values or expressions.
• Returns: The first non-null argument.

terraform

coalesce(value1, value2, ..., valueN)

Example

This example shows how to use coalesce to set a variable with a fallback value if the first variable is null.

terraform

variable "primary_value" {
  type    = string
  default = null
}

variable "fallback_value" {
  type    = string
  default = "default-string"
}

output "result" {
  value = coalesce(var.primary_value, var.fallback_value)
}

Output

result = "default-string"

Common Pitfalls

Common mistakes when using coalesce include:

• Passing no arguments, which causes an error.
• Expecting coalesce to return empty strings or zero values as non-null (only null is skipped).
• Using coalesce with incompatible types in arguments.

Always ensure at least one argument is provided and that types match.

terraform

/* Wrong: no arguments */
output "wrong" {
  value = coalesce()
}

/* Right: at least one argument */
output "right" {
  value = coalesce(null, "fallback")
}

Quick Reference

Function	Description	Example
coalesce	Returns first non-null value	coalesce(null, "a", "b") → "a"
Arguments	One or more values	coalesce(val1, val2, val3)
Returns	First non-null or null	Returns null if all are null

Key Takeaways

Use coalesce to pick the first non-null value from multiple options.

Always provide at least one argument to coalesce to avoid errors.

coalesce skips only null values, not empty strings or zeros.

Ensure all arguments to coalesce have compatible types.

Use coalesce to set fallback values cleanly in Terraform configurations.