File functions (file, templatefile) in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Terraform file() function do?

The file() function reads the contents of a file from the local filesystem and returns it as a string. It's useful for including file content directly in Terraform configurations.

beginner

How does templatefile() differ from file() in Terraform?

templatefile() reads a file and processes it as a template, replacing placeholders with provided variables. file() just reads the file content as-is without substitutions.

intermediate

What is the correct way to pass variables to templatefile()?

You pass a map of variable names and values as the second argument to templatefile(). The template file uses ${varname} syntax to insert these values.

beginner

Give an example use case for file() in Terraform.

Use file() to read a script file (like a shell script) and pass its content to a resource's user_data field to run on a virtual machine at startup.

intermediate

What happens if the file path given to file() or templatefile() does not exist?

Terraform will return an error and stop the plan or apply process because it cannot read the required file.

What does the Terraform file() function return?

A A map of variables

B The file content as a string

C A list of file names

D A boolean indicating file existence

file() reads the entire file content and returns it as a string.

Which function allows variable substitution inside a file's content?

A templatefile()

B file()

C jsondecode()

D join()

templatefile() processes templates and replaces placeholders with variables.

How do you provide variables to templatefile()?

A As a map of key-value pairs

B No variables can be passed

C As a single string

D As a list of strings

Variables are passed as a map to templatefile() for substitution.

What syntax is used inside a template file for variable placeholders?

A #varname#

B <varname>

C {{varname}}

D ${varname}

Terraform templates use ${varname} syntax for variables.

What happens if the file path in file() is incorrect?

A Terraform ignores the error

B Terraform creates an empty file

C Terraform returns an error and stops

D Terraform retries reading the file

Terraform fails with an error if the file cannot be found or read.

Explain how file() and templatefile() functions work in Terraform and when to use each.

Think about reading plain files versus files with placeholders.

Describe the process of passing variables to a template file in Terraform and how they are used inside the template.

Focus on the map argument and placeholder syntax.

Practice

1. What does the Terraform function file() do?

easy

A. Writes content to a file

B. Reads a file and replaces variables inside it

C. Reads the entire content of a file as plain text

D. Deletes a file from the system

Solution

1. Step 1: Understand the purpose of file()

   The file() function reads the content of a file and returns it as plain text without any processing.

2. Step 2: Compare with other functions

   templatefile() replaces variables inside a file, but file() just reads raw content.

3. Final Answer:

   Reads the entire content of a file as plain text -> Option C

4. Quick Check:

   file() reads file content [OK]

Hint: file() just reads text, no variable replacement [OK]

Common Mistakes:

• Confusing file() with templatefile()
• Thinking file() modifies file content
• Assuming file() writes to files

2. Which of the following is the correct syntax to use templatefile() in Terraform?

easy

A. templatefile("path/to/file.tpl", { var1 = "value1" })

B. templatefile(path = "file.tpl", vars = { var1 = "value1" })

C. templatefile("file.tpl", var1 = "value1")

D. templatefile("file.tpl")

Solution

1. Step 1: Recall the correct function signature

   templatefile() requires two arguments: the file path as a string and a map of variables.

2. Step 2: Match the syntax

   templatefile("path/to/file.tpl", { var1 = "value1" }) correctly passes the file path and a map with variable assignments. Other options either use wrong parameter names or omit the variables map.

3. Final Answer:

   templatefile("path/to/file.tpl", { var1 = "value1" }) -> Option A

4. Quick Check:

   templatefile(path, vars map) syntax [OK]

Hint: templatefile needs path string and vars map [OK]

Common Mistakes:

• Using named parameters instead of positional
• Passing variables outside a map
• Omitting the variables map argument

3. Given the file message.tpl with content:

Hello, ${name}!

What is the output of this Terraform expression?
templatefile("message.tpl", { name = "Alice" })

medium

A. Hello, Alice!

B. Hello, ${name}!

C. templatefile("message.tpl", { name = "Alice" })

D. Error: variable 'name' not found

Solution

1. Step 1: Understand templatefile variable replacement

   The templatefile() function reads the file and replaces variables like ${name} with the provided value.

2. Step 2: Apply the variable substitution

   Since name is set to "Alice", the output becomes "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option A

4. Quick Check:

   templatefile replaces variables [OK]

Hint: templatefile replaces ${var} with values from map [OK]

Common Mistakes:

• Expecting raw template text output
• Confusing file() with templatefile() output
• Missing variable in map causing error

4. You wrote this Terraform code:

output "config" {
  value = templatefile("config.tpl", { env = "prod" })
}

But Terraform shows an error: File not found: config.tpl. What is the likely cause?

medium

A. The file config.tpl is empty

B. The variable env is missing in the map

C. templatefile() cannot be used in output blocks

D. The file config.tpl does not exist in the working directory

Solution

1. Step 1: Analyze the error message

   The error says "File not found", which means Terraform cannot locate the file config.tpl.

2. Step 2: Check file existence and path

   Most likely, the file is missing or the path is incorrect. Variables missing or empty files do not cause this error.

3. Final Answer:

   The file config.tpl does not exist in the working directory -> Option D

4. Quick Check:

   File not found error means missing file [OK]

Hint: Check file path and existence if 'File not found' error [OK]

Common Mistakes:

• Assuming variable map causes file not found
• Thinking templatefile() is invalid in output
• Ignoring file path correctness

5. You want to create a Terraform output that reads a JSON file data.json and replaces a placeholder ${region} inside it with your variable var.region. Which approach is best?

hard

A. Use file() to read data.json and then use replace() to substitute ${region}

B. Use templatefile() with data.json and pass { region = var.region } as variables

C. Use file() only, since it automatically replaces variables

D. Use templatefile() but pass an empty map for variables

Solution

1. Step 1: Understand the difference between file() and templatefile()

   file() reads raw content without variable substitution. templatefile() reads and replaces variables inside the file.

2. Step 2: Choose the correct function and usage

   Since you want to replace ${region} with var.region, templatefile() with a variables map is the best choice.

3. Final Answer:

   Use templatefile() with data.json and pass { region = var.region } as variables -> Option B

4. Quick Check:

   templatefile() replaces placeholders with variables [OK]

Hint: Use templatefile() to replace placeholders with variables [OK]

Common Mistakes:

• Expecting file() to replace variables
• Passing empty variable map to templatefile()
• Using replace() manually instead of templatefile()