Request: Custom Outputs from null_resource/local-exec

[qivers]

Hi

I had a quick search but I couldn't find a feature request for this so apologies if it's a duplicate.

I was wondering if it would be possible to create custom outputs, from a null_resource, running a script in a local-exec?

Our current use case is that we are building custom AMIs. We would like to pass the AMI ID to our Terraform aws_instance and aws_launch_configuration resources without manually updating a variable in tfvars.

So the idea is, we'd have a null_resource with a local-exec which would pull in the latest AMI IDs using a script, and then we could assign the AMI ID outputted from that script to a terraform output;

resource "null_resource" "debian-ami" {
  provisioner "local-exec" {
    command = "python ./ami-finder.py -o debian"
  }
}

output "debiabn-ami-id" {
  value = "${null_resource.debian-ami.local-exec-output}"
}

This script would return the latest debian AMI ID which could then be ingested like any other module output;

ami_id = "${module.debiain-ami-finder.debian-ami-id}"

I realise this might be difficult but wondered if it was something worth considering?

Cheers.

[jen20]

Hi @qivers! It's not possible to do this using null-resource, but the development branch of Terraform 0.7 (on master) now has data driven configuration, and a data source for finding AMIs is on the roadmap for that release!

[alexraju91]

Hi, @jen20, I'm not able to see how v0.7.0 can fix this issue. This is required essentially for me as I'm using a script to get the private_ips of resources created by auto-scaling group. I need to pass this information to another TF module. So, I need to output this. Is there any way, I can acheive this?

[mrwilby]

Could @jen20 @apparentlymart explain how to use the new "data sources" feature to run a local script and get the output as a terraform variable? TIA!

[apparentlymart]

We don't (yet) have a data source for running a local command, but for the specific use-case given here (finding an existing AMI) we have aws_ami.

[alexraju91]

@apparentlymart Yup, like I thought. As far as I think, there is no direct way for this except some hack with null_resource trigger

[matti]

External data sources get run every time - if you just want to get output once, I made a module for this https://github.com/matti/terraform-shell-resource

[matti]

Update, now the module works also with remote state setups with an insane trick where I store the outputs in null_resource triggers and then ignore lifecycle changes for the triggers (!) https://github.com/matti/terraform-shell-resource/blob/master/main.tf#L58-L67

[matti]

Just revisited the solution mentioned above - managed to drop the requirement for external data source (ruby) now.

[mmack]

Will this module find it's way into the core someday?

[matti]

https://github.com/matti/terraform-shell-resource has been updated so that it updates the outputs if command changes and does not run on every apply

[ghost]

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