How do you do simple string concatenation in Terraform? - terraform

Question

I must be being incredibly stupid but I can't figure out how to do simple string concatenation in Terraform.
I have the following data null_data_source:
data "null_data_source" "api_gw_url" {
inputs = {
main_api_gw = "app.api.${var.env_name == "prod" ? "" : var.env_name}mydomain.com"
}
}
So when env_name="prod" I want the output app.api.mydomain.com and for anything else - let's say env_name="staging" I want app.api.staging.mydomain.com.
But the above will output app.api.stagingmydomain.com <-- notice the missing dot after staging.
I tried concating the "." if the env_name was anything but "prod" but Terraform errors:
data "null_data_source" "api_gw_url" {
inputs = {
main_api_gw = "app.api.${var.env_name == "prod" ? "" : var.env_name + "."}mydomain.com"
}
}
The error is __builtin_StringToInt: strconv.ParseInt: parsing ""
The concat() function in TF appears to be for lists not strings.
So as the title says: How do you do simple string concatenation in Terraform?
I can't believe I'm asking how to concat 2 strings together XD
Update:
For anyone that has a similar issue I did this horrific workaround for the time being:
main_api_gw = "app.api.${var.env_name == "prod" ? "" : var.env_name}${var.env_name == "prod" ? "" : "."}mydomain.com"

Answer 1

I know this was already answered, but I wanted to share my favorite:
format("%s/%s",var.string,"string2")
Real world example:
locals {
documents_path = "${var.documents_path == "" ? format("%s/%s",path.module,"documents") : var.documents_path}"
}
More info:
https://www.terraform.io/docs/configuration/functions/format.html

Answer 2

so to add a simple answer to a simple question:
enclose all strings you want to concatenate into one pair of ""
reference variables inside the quotes with ${var.name}
Example: var.foo should be concatenated with bar string and separated by a dash
Solution: "${var.foo}-bar"

Answer 3

Try Below data resource :
data "null_data_source" "api_gw_url" {
inputs = {
main_api_gw = "app.api${var.env_name == "prod" ? "." : ".${var.env_name}."}mydomain.com"
}
}

Answer 4

For Terraform 0.12 and later, you can use join() function:
join(separator, list)
Example:
> join(", ", ["foo", "bar", "baz"])
foo, bar, baz
> join(", ", ["foo"])
foo
If you just want to concatenate without a separator like "foo"+"bar" = "foobar", then:
> join("", ["foo", "bar"])
foobar
Reference: https://www.terraform.io/docs/configuration/functions/join.html
Use the Interpolation Syntax for versions < 0.12

Answer 5

Here is a simple example:
output "s3_static_website_endpoint" {
value = "http://${aws_s3_bucket.bucket_tf.website_endpoint}"
}
Reference the Terraform Interpolation docs:
https://developer.hashicorp.com/terraform/language/expressions/strings#string-templates

Answer 6

after lot of research, It finally worked for me. I was trying to follow https://www.hashicorp.com/blog/terraform-0-12-preview-first-class-expressions/, but it did not work. Seems string can't be handled inside the expressions.
data "aws_vpc" "vpc" {
filter {
name = "tag:Name"
values = ["${var.old_cluster_fqdn == "" ? "${var.cluster_fqdn}" : "${var.old_cluster_fqdn}"}-vpc"]
}
}