Terragrunt is a mature, production-grade Infrastructure as Code orchestration CLI tool that wraps OpenTofu and Terraform. Its core function is to manage the lifecycle of IaC modules across environments: it parses HCL configuration (`terragrunt.hcl`, stacks), discovers units and stacks in a file tree, resolves inter-module dependencies, executes OpenTofu/Terraform commands with automatic remote state backend management (S3, GCS, Azure), and caches provider downloads and module sources via a content-addressable store (CAS). The architecture is a monolith with a rich CLI surface (over 20 subcommands including `run`, `catalog`, `browse`, `scaffold`, `find`, `list`, `render`, `hcl fmt`, `dag graph`, etc.) and a custom implementation of the CLI framework (`internal/clihelper`). It features a Bubbletea-based TUI for catalog browsing and an extensible filter engine for selecting units by path, attributes, or git changes. The codebase is heavily test-covered (unit, integration, fuzz) and actively maintained by Gruntwork. Key reusable patterns include virtual abstractions for filesystem, execution, and HTTP; a generic concurrent cache; a strict controls system for deprecation gating; and a content-addressable git store for deduplicating source downloads.