Create a comma separated list of quoted strings on terraform

2 min read | by Jordi Prats

Some times we need to generate a quoted comma separated list of strings out of a variable that is list of strings, for example, to generate an IAM policy like this one:

{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": [ "secretsmanager:GetResourcePolicy", "secretsmanager:GetSecretValue", "secretsmanager:DescribeSecret", "secretsmanager:ListSecretVersionIds" ],  "Resource": [ "arn:aws:secretsmanager:...", "arn:aws:secretsmanager:.." ]  } ] }

To do so we can take advantage of the jsonencode function that produces compact JSON that we will push into a template. So first we will have to create a template like this, on this example the ARN_LIST variable will hold the JSON object

{  "Version": "2012-10-17",  "Statement": [  {  "Effect": "Allow",  "Action": [  "secretsmanager:GetResourcePolicy",  "secretsmanager:GetSecretValue",  "secretsmanager:DescribeSecret",  "secretsmanager:ListSecretVersionIds"  ],  "Resource": ${ARN_LIST}  }  ] }

So now we'll have to create the policy like this:

resource "aws_iam_policy" "policy" {  name = "policy"  path = "/example/jsonencode/"  policy = templatefile("${path.module}/iam_policies/external-secrets.json",  {  ARN_LIST = jsonencode(var.secretsmanager_arns)  }) }

Thus, the ARN_LIST will hold a JSON object that will be translated to a string that will match what the policy expects, rendering the policy we wanted to generate in the first place

Posted on 03/11/2021

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