run cmd

Terragrunt’s run_cmd is a powerful interpolation function that lets you execute shell commands during a run and return their standard output. By integrating run_cmd into your configurations, you can:

• Dynamically adapt module inputs based on external context
• Incorporate existing workflows or scripts
• Feed custom data into Terraform resources at plan/apply time

Best Practices for run_cmd

• Always validate and sanitize any external scripts or commands to mitigate security risks.
• Prefer native Terraform/Terragrunt functions (like timestamp() and file()) when possible.
• Reserve run_cmd for scenarios where built-in functions cannot produce the needed output.

Example: Tagging AWS VPC Resources with the Current User

In this example, we’ll consume the Terraform AWS VPC module and automatically tag every resource with the username running Terragrunt.

terraform {
  source = "tfr://terraform-aws-modules/vpc/aws//?version=5.8.1"
}

inputs = {
  name = "Kodekloud-VPC"
  tags = {
    CreatedBy = run_cmd("whoami")
  }
}

When you run the command locally:

$ whoami
abc

Terragrunt will interpolate the result into your Terraform plan:

$ terragrunt plan
...
Plan: 4 to add, 0 to change, 0 to destroy.

Changes to Outputs:
+ tags = {
    + CreatedBy = "abc"
  }
...

Every resource provisioned by this Terragrunt configuration will now carry the tag CreatedBy = "abc".

With run_cmd, you can extend this pattern to pull data from any script, API call, or toolchain, giving you a highly flexible Terragrunt workflow.

Links and References

• Terraform AWS VPC Module
• Terragrunt Documentation
• Terraform Interpolation Functions