How to use "for_each" to iterate over a list in Terraform

In Terraform, the for_each expression allows you to iterate over a list or map and create multiple instances of a resource or module based on the elements of that list or map. This is useful when you want to manage multiple similar resources or configurations in a more dynamic way.

The for_each value must be a map or a set of strings, which creates an instance for each item in that map or set. Each instance has a distinct infrastructure object associated with it, and each is separately created, updated, or destroyed when the configuration is applied.

Using for_each on a list of strings:

Code explanation

Line 1:

• This line defines a Terraform provider named null. The null provider is a built-in provider that does nothing but can be useful in certain scenarios, like generating data or for testing purposes.

Line 3–6:

• This block defines a variable named user_names.

• The variable has a type of list(string), meaning it is a list of strings.

• The default value of the variable is set to ["user1", "user2", "user3"], so if no value is provided when running Terraform, this default list will be used.

Line 8–14:

• This block defines a null_resource named users.

• The for_each argument is set to toset(var.user_names), which means that this resource will be created for each unique value in the user_names list variable.

• Inside the triggers block, a trigger is set for each instance of the null_resource using the name from each value in the user_names list.

Line 16–18:

• This block defines an output named user_names_output.

• The value of the output is set using a list comprehension ([for … : …]) to loop over each instance of the null_resource resource and extract the name trigger value from each one.

Using for_each on a list of strings:

Code explanation

Line 3–13:

• This block declares a variable named users.

• The variable has a type of list(object({ name = string, age = number })), meaning it is a list of objects with name as a string and age as a number.

• The default value of the variable is set to a list of three objects, each containing a name and an age field for three users.

Line 15–22:

• This block defines a null_resource named users.

• The for_each argument uses a for-loop with the for expression to create a map, where the keys are the name fields from the objects in the users list and the values are the full objects.

• Inside the triggers block, a trigger is set for each instance of the null_resource using the name and age fields from each corresponding object.

Line 24–26:

• This block defines an output named users_with_tags.

• The value of the output is set using a map comprehension ({ for … : … }) to loop over each instance of the null_resource resource and extract the name as the key and age as the value from each trigger.