String functions (join, split, format) in Terraform - Mini Project: Build & Apply

Using String Functions in Terraform

📖 Scenario: You are setting up a Terraform configuration to manage cloud resources. You need to work with strings to create resource names and parse input values.

🎯 Goal: Build a Terraform configuration that uses join, split, and format string functions to manipulate resource names and tags.

📋 What You'll Learn

Create a list of strings representing environment names

Create a variable to hold a delimiter string

Use join to combine environment names into a single string

Use split to break the combined string back into a list

Use format to create a formatted resource name string

💡 Why This Matters

🌍 Real World

Cloud engineers often need to manipulate strings to create resource names, tags, and configuration values dynamically in Terraform.

💼 Career

Understanding string functions in Terraform helps you write flexible and reusable infrastructure code, a key skill for cloud infrastructure roles.

1

Create a list of environment names

Create a Terraform variable called environments with the exact list of strings: ["dev", "test", "prod"].

Terraform

# Create a variable called environments with the list ["dev", "test", "prod"]
# Your code here

Hint

Use the variable block and set default to the list.

2

Create a delimiter variable

Create a Terraform variable called delimiter and set its default value to the string "-".

Terraform

variable "environments" {
  type = list(string)
  default = ["dev", "test", "prod"]
}

# Create a variable called delimiter with default "-"
# Your code here

Hint

Use a variable block with default = "-".

3

Join environment names into a single string

Create a Terraform local variable called env_string that uses the join function with var.delimiter and var.environments to combine the list into a single string.

Terraform

variable "environments" {
  type = list(string)
  default = ["dev", "test", "prod"]
}

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

# Create a local variable env_string that joins var.environments with var.delimiter
# Your code here

Hint

Use locals block and join function.

4

Split the string and format a resource name

Add to the locals block a variable called env_list that uses split with var.delimiter and local.env_string. Then add a variable called resource_name that uses format to create the string "resource-%s-%s" with the first two elements of local.env_list.

Terraform

variable "environments" {
  type = list(string)
  default = ["dev", "test", "prod"]
}

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

locals {
  env_string = join(var.delimiter, var.environments)
  # Add env_list using split
  # Add resource_name using format with first two elements of env_list
  # Your code here
}

Hint

Use split and format inside the locals block.

Practice

1. What does the Terraform function join(",", ["a", "b", "c"]) return?

easy

A. "a,b,c"

B. ["a", "b", "c"]

C. "a b c"

D. "abc"

Solution

1. Step 1: Understand the join function

   The join function combines list elements into a single string using the given separator.

2. Step 2: Apply join with separator ','

   Joining ["a", "b", "c"] with "," results in the string "a,b,c".

3. Final Answer:

   "a,b,c" -> Option A

4. Quick Check:

   join with comma = "a,b,c" [OK]

Hint: Join combines list items with separator into one string [OK]

Common Mistakes:

• Confusing join output with original list
• Using space instead of comma as separator
• Expecting a list instead of a string

2. Which of the following is the correct syntax to split the string "one,two,three" into a list using Terraform?

easy

A. format(",", "one,two,three")

B. split("one,two,three", ",")

C. split(",", "one,two,three")

D. join(",", "one,two,three")

Solution

1. Step 1: Recall split function syntax

   The split function takes the separator first, then the string to split.

2. Step 2: Match correct argument order

   Only split(",", "one,two,three") matches the correct syntax.

3. Final Answer:

   split(",", "one,two,three") -> Option C

4. Quick Check:

   split(separator, string) = split(",", "one,two,three") [OK]

Hint: split(separator, string) order matters in Terraform [OK]

Common Mistakes:

• Swapping arguments order in split
• Using join instead of split
• Using format instead of split

3. What is the output of this Terraform expression?

format("Hello %s, you have %d messages", "Alice", 5)

medium

A. "Hello Alice, you have 5 messages"

B. "Hello Alice, you have %d messages"

C. "Hello %s, you have %d messages"

D. "Hello %s, you have 5 messages"

Solution

1. Step 1: Understand format placeholders

   %s is replaced by string, %d by integer in order.

2. Step 2: Replace placeholders with arguments

   "Alice" replaces %s, 5 replaces %d, producing "Hello Alice, you have 5 messages".

3. Final Answer:

   "Hello Alice, you have 5 messages" -> Option A

4. Quick Check:

   format with %s and %d = formatted string [OK]

Hint: format replaces %s and %d placeholders in order [OK]

Common Mistakes:

• Not replacing placeholders
• Mixing up %s and %d types
• Expecting original string without substitution

4. Identify the error in this Terraform code snippet:

locals {
  parts = split(",", "apple,banana,orange")
  result = join(";", parts)
  message = format("Fruits: %s", result)
}

medium

A. join separator should be a list, not a string

B. split function arguments are reversed

C. format placeholder %s cannot accept a string

D. No error, code is correct

Solution

1. Step 1: Check split function usage

   split separator and string are correctly ordered.

2. Step 2: Check join and format usage

   join uses string separator ";" correctly; format uses %s for string correctly.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   split, join, format used correctly = No error, code is correct [OK]

Hint: Check argument order and placeholder types carefully [OK]

Common Mistakes:

• Swapping split arguments
• Using wrong placeholder in format
• Passing wrong types to join

5. You have a list of IP addresses: ["10.0.0.1", "10.0.0.2", "10.0.0.3"]. You want to create a single string with these IPs separated by commas, but enclosed in square brackets like this:
[10.0.0.1,10.0.0.2,10.0.0.3]
Which Terraform expression achieves this?

hard

A. join(",", format("[%s]", ["10.0.0.1", "10.0.0.2", "10.0.0.3"]))

B. format("[%s]", join(",", ["10.0.0.1", "10.0.0.2", "10.0.0.3"]))

C. split(",", format("[%s]", join(",", ["10.0.0.1", "10.0.0.2", "10.0.0.3"])))

D. format(join(",", ["10.0.0.1", "10.0.0.2", "10.0.0.3"]), "[%s]")

Solution

1. Step 1: Join IP list with commas

   Using join(",", [...]) creates "10.0.0.1,10.0.0.2,10.0.0.3".

2. Step 2: Use format to add square brackets

   Wrapping the joined string with format("[%s]", ...) adds brackets around it.

3. Final Answer:

   format("[%s]", join(",", ["10.0.0.1", "10.0.0.2", "10.0.0.3"])) -> Option B

4. Quick Check:

   join then format = format("[%s]", join(",", ["10.0.0.1", "10.0.0.2", "10.0.0.3"])) [OK]

Hint: Join first, then format to add brackets [OK]

Common Mistakes:

• Trying to format before join
• Using split instead of join
• Incorrect argument order in format