Install the Sentinel CLI

HCP Terraform uses Sentinel to enable granular policy control for your infrastructure. Sentinel is a language and policy framework, which restricts Terraform actions to defined, allowed behaviors. Policy authors manage Sentinel policies in HCP Terraform with policy sets, which are groups of policies. Organization owners control the scope of policy sets by applying certain policy sets to the entire organization or to select workspaces.

The Policy-as-Code framework lets you manage governance requirements as you would your applications: written by operators, controlled in VCS, reviewed, and automated during your deployment process.

The Sentinel CLI (command-line interface) validates and tests rules so you can develop Sentinel policies.

After you install the Sentinel CLI, try the other Sentinel tutorials to practice writing, testing, and importing policies for HCP Terraform.

Install Sentinel CLI

To install the Sentinel CLI, find the appropriate package for your system and download it. The CLI is packaged as a zip archive.

After downloading Sentinel, unzip the package. The CLI runs as a single binary named sentinel. Any other files in the package can be safely removed and Sentinel will still function.

Finally, make sure that the sentinel binary is available on your PATH. This process will differ depending on your operating system.

$ echo $PATH

$ mv ~/Downloads/sentinel /usr/local/bin/sentinel

Verify Sentinel installation

After installing Sentinel CLI, verify the installation was successful by opening a new terminal session and checking that the sentinel binary is available. By executing sentinel, you should see help output similar to the following:

$ sentinel
Usage: sentinel [--version] [--help] <command> [<args>]

Available commands are:
    apply      Execute a policy and output the result
    fmt        Format Sentinel policy to a canonical format
    test       Test policies
    version    Prints the Sentinel runtime version

If you get an error that the binary could not be found, then your PATH environment variable was not set-up properly. Please go back and ensure you set your PATH correctly.

Run a policy

In order for Sentinel to run a policy, it needs data to test the policy against. Open the embedded terminal session and follow the instructions on the left side of the window to test a policy against pre-populated HCP Terraform plan data. The example policy ensures that S3 buckets have tags attached.

Launch Terminal

This tutorial includes a free interactive command-line lab that lets you follow along on actual cloud infrastructure.

Start interactive lab

This is the policy you will test:

import "tfplan/v2" as tfplan

s3_buckets = filter tfplan.resource_changes as _, rc {
    rc.type is "aws_s3_bucket" and
    (rc.change.actions contains "create" or rc.change.actions is ["update"])
}

bucket_tags = rule {
    all s3_buckets as _, instances {
        instances.change.after.tags is not null
    }
}

main = rule {
    bucket_tags
}

Next Steps

Complete the following tutorials to learn how to use the Sentinel CLI and enforce Sentinel policies:

• Learn how to Generate Mock Policy Data
• Review how to Write a Sentinel Policy
• Review how to Test a Sentinel Policy
• Learn how to Upload a Sentinel Policy Set to HCP Terraform