Variables locals and output

Hi

I am trying to sort out a scenario/module where I want to

a. define a variable in variables.tf file
b. perform some business logic inside main.tf using null provider and null_resources block
c. In the null_resource block I want to store the processed output in the variable which I had defined in step 1
d. output the variable in output.tf which can in turn be called in other modules

my variables.tf contains the definition for the variable called as value, it’s of type string and has no default value

this is how my main.tf looks
provider "null" { }

resource "null_resource" "get-valuefromfile" {var.value= file("/var/result/value.txt")}

In the output.tf file,

    output "valuefromfile" {value = ["${var.value}"]}

Whenever I run this
on main.tf line 15, in resource “null_resource” “get-valuefromfile”:
15: var.value= file("/var/result/value.txt")

An argument or block definition is required here. To set an argument, use the
equals sign “=” to introduce the argument value.

I am not able to figure out why value assignment throws up an error
What are the possible ways of accomplishing this.

Going by the same analogy, can I do this inside the null resource block
var.value1 = local.local_variable_lookup_from_a_map1
var.value2 = local.local_variable_lookup_from_a_map2
var.value3 = “{var.value1}{var.value2}”
output var.value3