CLI Basics
Objective
Understand the basics of the Terraform CLI and the different phases of the deployment process.
Overview
The Terraform CLI is a fully featured CLI for managing and deploying resources. To get started all that is required it to write your HCL configuration, initialize your project, plan the deployment, and apply it. There are many more features but the majority of the time these 3 commands will be the most used.
The typical flow for development and deployment is:
flowchart LR;
A(terraform init)-->B(terraform plan);
B-->C(terraform apply);
C-->B
terraform init
The init command initializes the current directory for use with Terraform. Several things happen when running the init command. The backend is set up if required (backends are an advanced topic, for now we will be using local state). Modules used within the project are copied locally. Finally, any plugins used are retrieved and stored locally. This includes any providers used. This command needs to be run anytime one of these 3 things changes. So if a backend configuration is updated, modules are added, or new providers are used the project needs to be re-initialized.
terraform plan
The plan command can be used to see what changes are pending between the configuration in the current project and the resources deployed. This command is used constantly when working with Terraform to see the changes being made. The plan command is informational only as it does not make any changes. It can be used with the -out flag to write the changes to disk to be used with the apply command.
terraform apply
The apply command is used to deploy the current configuration. By default the
apply command will run a terraform plan before applying the changes. If a plan
plan output is passed to the apply then it will be used rather than creating a
plan.
Review
In this section we reviewed the 3 most used commands in Terraform: init, plan and apply. We understood their uses and where they fit in the development lifecycle.