Concat strings and list variable in terraform

Question

I need to pass a list of cidr blocks to a module in the form of ["0.0.0.0/23","0.0.0.1/23",...] etc.

I have two cidr blocks which I'm getting from a subnet data source, which I can reference as an example as:

cidr_blocks = [data.aws_subnet.subnet1.id,data.aws_subnet.subnet2.id]

I also have a variable, depending on the environment, has a specific list of cidr_blocks to pass in, set as an example:

custom_cidrs = [""0.0.0.0/23","0.0.0.1/23","0.0.0.2/23"]

What I want to do is in the module, concat all these cidrs together to create one list. I have tried a few different things, including concat and list(). I just can't get the syntax right.

For some contexts, they'll be no custom cidr blocks to set, only the subnet cidrs, so in that instance my custom cidrs will be set to:

custom_cidrs = []

Any help on how to concat the two outputs and the custom list into one will be appreciated.

I'm thinking I may have to create a list from the two outputs first, then concat the two lists together, but not entirely sure. Can I do it in one command?

I am using terraform version 0.12.28

Please can you edit the question to show what you tried and the errors you got at each attempt? You should be fine with concat but you mentioned it didn't work and it's hard to see why without seeing what you tried and the error you got. — ydaetskcoR
– ydaetskcoR, Commented Jul 2, 2020 at 17:39

ekeeling · Accepted Answer · 2020-07-02 20:10:29Z

10

If I'm understanding you correctly, you would like to concatenate the CIDR blocks from your data sources and the custom list. Here is an example of how you can do that.

        variable "custom_cidrs" {
          default = [
            "0.0.0.0/23",
            "0.0.0.1/23",
            "0.0.0.2/22"
          ]
        }
        
        data "aws_subnet" "private" {
          id = "subnet-000000qq"
        }
        
        data "aws_subnet" "public" {
          id = "subnet-000001qq"
        }
        
        locals {
          all_cidrs = setunion(
            [data.aws_subnet.private.cidr_block, data.aws_subnet.public.cidr_block],
            var.custom_cidrs
          )
        }
        
        output "all_cidrs" {
          value = local.all_cidrs
        }

The output looks like this.

        [PS][7.0.2] tst > terraform apply --auto-approve -var "region=us-west-2"
        data.aws_subnet.public: Refreshing state...
        data.aws_subnet.private: Refreshing state...
        
        Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
        
        Outputs:
        
        all_cidrs = [
          "0.0.0.0/23",    # Custom CIDR block from var
          "0.0.0.1/23",    # Custom CIDR block from var
          "0.0.0.2/22",    # Custom CIDR block from var
          "xxx.xx.x.x/24", # Private subnet CIDR block
          "xxx.xx.x.x/24", # Public subnet CIDR block
        ]

To reference the concatenated list, reference local.all_cidrs.

edited Jul 2, 2020 at 20:10

answered Jul 2, 2020 at 19:58

ekeeling

3912 silver badges8 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Molenpad Over a year ago

This worked for me and I learned something new. Thanks a lot :)

Collectives™ on Stack Overflow

Concat strings and list variable in terraform

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related