How-to: Terraform Destroy-time Provisioners

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Julie
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Prerequisites

 

How to

  • Create an EC2 instance in AWS with a key pair that you have access to on your local machine. Make sure you launch this instance with a security group that allows tcp on port 22 from your IP address. Once launched, make note of your instance's Public IP (XX.XX.XXX.XXX).

 

  • From your terminal, connect to this instance: ssh -i "private-key-used-to-make-ec2.cer" ec2-user@XX.XX.XXX.XXX. Once connected, make a random directory: mkdir /hello.

 

  • In a different terminal window on your local machine, create a new directory and touch a Terraform configuration file: mkdir test && touch test/main.tf. Configure a null_resource and remote-exec provisioner (see main.tf below). You will use this resource to run commands on your EC2 instance. 
# /test/main.tf --------------------
resource "null_resource" "foo" {
  count      = 1

  connection {
    type     = "ssh"
    user     = "ec2-user"
    private_key = file("${path.module}/private-key-used-to-make-ec2.cer")
    host     = "XX.XX.XXX.XXX"
  }

  provisioner "remote-exec" {
    inline = ["ls"]
  }

  provisioner "remote-exec" {
    when        = destroy
    on_failure  = continue
    inline = ["touch bar.txt"]
  }
}
  • In your /test directory, execute terraform init. Next, execute terraform apply. You should see the following output: 
null_resource.foo[0]: Creating...
null_resource.foo[0]: Provisioning with 'remote-exec'...
null_resource.foo[0] (remote-exec): Connecting to remote host via SSH...
null_resource.foo[0] (remote-exec):   Host: XX.XX.XXX.XXX
null_resource.foo[0] (remote-exec):   User: ec2-user
null_resource.foo[0] (remote-exec):   Password: false
null_resource.foo[0] (remote-exec):   Private key: true
null_resource.foo[0] (remote-exec):   Certificate: false
null_resource.foo[0] (remote-exec):   SSH Agent: true
null_resource.foo[0] (remote-exec):   Checking Host Key: false
null_resource.foo[0] (remote-exec):   Target Platform: unix
null_resource.foo[0] (remote-exec): Connected!
null_resource.foo[0] (remote-exec): hello
null_resource.foo[0]: Creation complete after 0s [id=xxxxxxxxxxxxxxxx]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.
  • Notice null_resource.foo[0] (remote-exec): hello is the output from ls.

 

  • Go back to your terminal connected to the EC2 instance and execute ls. You will only see 1 directory: /hello. As expected, the Destroy-time Provisioner did not run.

 

  • Now, back in directory /test, execute terraform destroy. You will see the following output:
null_resource.foo[0]: Destroying... [id=xxxxxxxxxxxxxxxx]
null_resource.foo[0]: Provisioning with 'remote-exec'...
null_resource.foo[0] (remote-exec): Connecting to remote host via SSH...
null_resource.foo[0] (remote-exec):   Host: XX.XX.XXX.XXX
null_resource.foo[0] (remote-exec):   User: ec2-user
null_resource.foo[0] (remote-exec):   Password: false
null_resource.foo[0] (remote-exec):   Private key: true
null_resource.foo[0] (remote-exec):   Certificate: false
null_resource.foo[0] (remote-exec):   SSH Agent: true
null_resource.foo[0] (remote-exec):   Checking Host Key: false
null_resource.foo[0] (remote-exec):   Target Platform: unix
null_resource.foo[0] (remote-exec): Connected!
null_resource.foo[0]: Destruction complete after 1s

Destroy complete! Resources: 1 destroyed.
  • Go to your terminal connected to the EC2 instance and execute ls. Now, you see "bar.txt". This time, the Destroy-time Provisioner executed. For more information, see Destroy-time Provisioners [b].
  • End your ssh session with your EC2 instance. Clean-up/destroy the instance and security group.

 

Links

[a] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connection-prereqs.html#connection-prereqs-enable-inbound-traffic 

[b] https://developer.hashicorp.com/terraform/language/resources/provisioners/syntax

 

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