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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
The image is an infographic titled "run_cmd" that outlines three benefits: adapting based on custom command execution, integrating workflows within Terragrunt, and allowing custom script execution during Terragrunt runs.

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Best Practices for run_cmd

Use CaseExample
Inject current OS userrun_cmd("whoami")
Fetch latest Git commit SHArun_cmd("git rev-parse HEAD")
Read environment variablesrun_cmd("echo $MY_ENV_VAR")
  • 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.
Executing arbitrary shell commands can introduce security vulnerabilities. Ensure you trust and sanitize any external inputs or scripts invoked via run_cmd.

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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.

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