Why is my Terraform output not working in module? - terraform

Question

I have the following simple setup:
~$ tree
.
├── main.tf
└── modules
└── world
└── main.tf
~$ cat main.tf
output "root_module_says" {
value = "hello from root module"
}
module "world" {
source = "modules/world"
}
~$ cat modules/world/main.tf
output "world_module_says" {
value = "hello from world module"
}
I then run:
~$ terraform get
~$ terraform apply
I expect to see world_module_says in the outputs, but I do not, I only see root_module_says.
This is really confusing as to why?
If it helps:
~$ terraform --version
v0.10.8

Answer 1

Terraform only shows the output from root (by default pre v0.12)
https://www.terraform.io/docs/commands/output.html
Prior to Terraform 0.12 you can get the output from the world module with:
terraform output -module=world
I think the logic here is that the output from the module would be consumed by root and if you actually needed the output then you'd output it in root too so main.tf might contain this:
output "root_module_says" {
value = "hello from root module"
}
output "world_module_says" {
value = "${module.world.world_module_says}"
}
module "world" {
source = "modules/world"
}
Beginning with Terraform 0.12 this is the only way to get the output from within a module.

Answer 2

Before Terraform 0.12
The below command shows the output for a specific module
terraform output -module=example_module
After Terraform 0.12
The below command fails in Terraform 0.12 and above with an error:
terraform output -module=example_module
Error: Unsupported option
The -module option is no longer supported since Terraform 0.12, because now
only root outputs are persisted in the state.
To get outputs from a module in Terraform 0.12 and above, you must export them from the root (e.g example_module) module using an output block in the caller module. This can be now done simply by adding one line in the caller module like below:
output "example_module_outputs" {
value = module.example_module
}
Now you can see the output running the following command:
terraform output

Answer 3

Expansion to the answers above when using multiple output varialbes:
When accessing a module output group (using the 'terraform output' command from a console or a script) then Terraform will printout all output variables within this group in a formatted way, or in json format using '-json' flag.
When extracting output values, we can use the '-raw' flag only if there is a single variable which has a string format.
If we have more than 1 outout in the output group within the module:
Lets say we declared in the 'root' module the following outout group in a module:
output "world_world_module_says" {
value = module.world.world_module_says
}
and in the module main.tf we have 2 vars:
output "world_module_says" {
value = "hello from world module"
}
output "world_module_says_again" {
value = "hi again from world module"
}
Two ways to extract a single output variable from the group:
using -json flag and manipulating the output using the jq command:
terraform output -json world_world_module_says |jq '.'"world_module_says"
The other way is to declare each output individually in the 'root' module to allow easy access using '-raw':
output "world_world_module_says" {
value = module.world.world_module_says
}
output "world_world_module_says_again" {
value = module.world.world_module_says_again
}