What Is The Difference Between A Unit And A Stack

Unit

A unit is a single instance of infrastructure that Terragrunt manages: one VPC, one database, one cluster. A directory containing a terragrunt.hcl file is a unit, and each unit gets its own state file. Units are the target of a single run, so terragrunt apply inside a unit directory applies exactly that unit.

Stack

A stack is a collection of units managed together, such as every unit that makes up one environment or one region. Units in a stack can depend on each other, and Terragrunt orders them using the DAG built from their dependency blocks.

Stacks come in two forms:

• Implicit stacks are just a directory tree of units. Nothing declares the stack; it is the set of units Terragrunt discovers under the directory you run from.
• Explicit stacks are declared in a terragrunt.stack.hcl file, which generates units into a .terragrunt-stack directory from sources you point at. This lets you define a pattern once and instantiate it repeatedly with different values.

A single directory cannot be both. Putting a terragrunt.hcl and a terragrunt.stack.hcl side by side in the same directory is an error.

Which commands apply to which

Command	Operates on
terragrunt plan, terragrunt apply	The unit in the current directory
terragrunt run --all <cmd>	Every unit discovered under the working directory
terragrunt stack generate	The terragrunt.stack.hcl in the current directory
terragrunt stack run <cmd>	The units generated by that terragrunt.stack.hcl
terragrunt stack output	Outputs of the units in the generated stack

Both run --all and stack run generate explicit stacks before they run, so you do not need a separate stack generate step. Pass --no-stack-generate to skip it.