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Terraform - Function

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• Terraform - Function
  • Function
  • Common Functions

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Function

• Built-in function

• Syntax

  • ${name(arg1, arg2, ...)}

• Example

  • ${file("mykey.pub")}: read the file content

• doc:

  • https://developer.hashicorp.com/terraform/language/functions

• can use terraform console to test function

  • terraform console

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Common Functions

• Numeric Functions

Function	Description
abs(number)	Returns the absolute value.
ceil(number)	Rounds a number up to the nearest whole number.
floor(number)	Rounds a number down to the nearest whole number.
max(a, b, …)	Returns the largest value from the given arguments.
min(a, b, …)	Returns the smallest value from the given arguments.

• String Manipulation

Function	Description
lower(string)	Converts to lowercase.
upper(string)	Converts to uppercase.
trimspace(string)	Removes leading and trailing whitespace.
replace(target_str, sub_str, replace_str)	Replaces substring with another string.
split(delimiter, string)	Splits a string into a list using a delimiter.
join(delimiter, list)	Joins a list of strings into one string using a delimiter.
format(format, values…)	Formats a string using placeholders like printf.
coalesce(str1, str2, ...)	returns the first one that isn’t null or an empty string.
substr(str, offset, length)	Extract substring from string

• Collection (List/Map) Operations

Function	Description
length(list/map/string)	Returns the number of elements in a list/map or characters in a string.
contains(list, value)	Checks if a list contains a given value.
element(list, index)	Returns the element at a given index (negative index counts from end).
lookup(map, key, default)	Gets a value from a map, or returns a default if missing.
keys(map)	Returns a list of keys from a map.
values(map)	Returns a list of values from a map.
coalesce(list1, list2, ...)	returns the first one that isn’t null.
index(list,element)	returns the index of a given element in a list
list(item1, item2)	Create a new list
map(key1, val1, key2, val2)	Create a map
merge(map1, map2, …)	Combines multiple maps into one.
slice(list, start_index,length)	Slice a list

• Example:

  • lookup(map("k", "v"), "k", "Not found"): returns “v”

• File & Encoding

Function	Description
basename(path)	Get the filename (only the filename in a path)
file(path)	Reads the contents of a file.
filebase64(path)	Reads a file and returns Base64-encoded text.
base64encode(string)	Encodes a string to Base64.
base64decode(string)	Decodes a Base64 string.

• Time & Date

Function	Description
timestamp()	Returns the current UTC timestamp in RFC 3339 format.
timeadd(timestamp, duration)	Adds a duration (e.g., "1h", "30m") to a timestamp.

• Networking

Function	Description
cidrsubnet(prefix, newbits, netnum)	Calculates a subnet address within a CIDR range.
cidrhost(prefix, hostnum)	Returns a specific host IP address within a CIDR range.

• Miscellaneous

Function	Description
uuid()	Generates a random UUID each time it’s evaluated.

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• Example
  • terraform console

"hello world"
# "hello world"
replace("hello world", "o", "@")
# "hell@ w@rld"
"the server launched at ${timestamp()}"
# "the server launched at 2025-08-14T16:34:12Z"
tolist(["subnet-1","subnet-2","subnet-3"])
# tolist([
#   "subnet-1",
#   "subnet-2",
#   "subnet-3",
# ])
split(",","subnet-1,subnet-2,subnet-3")
# tolist([
#   "subnet-1",
#   "subnet-2",
#   "subnet-3",
# ])
element(tolist(["subnet-1","subnet-2","subnet-3"]),0)
# "subnet-1"
element(tolist(["subnet-1","subnet-2","subnet-3"]),3)
# "subnet-1"
element(tolist(["subnet-1","subnet-2","subnet-3"]),4)
# "subnet-2"
element(tolist(["subnet-1","subnet-2","subnet-3"]),5)
# "subnet-3"

index(tolist(["subnet-1","subnet-2","subnet-3"]),"subnet-1")
# 0

slice(tolist(["subnet-1","subnet-2","subnet-3"]),0,2)
# tolist([
#   "subnet-1",
#   "subnet-2",
# ])

join(":", tolist(["subnet-1","subnet-2","subnet-3"]))
# "subnet-1:subnet-2:subnet-3"

tomap({"us-east-1"="ami-1", "ca-central-1"="ami-2"})
# tomap({
#   "ca-central-1" = "ami-2"
#   "us-east-1" = "ami-1"
# })

lookup(tomap({"us-east-1"="ami-1", "ca-central-1"="ami-2"}), "ca-central-1")
# "ami-2"

substr("abcd", 0, 1)
# "a"

substr("abcd", 0, 4)
# "abcd"

substr("abcd", 0, 2)
# "ab"