What does the 'terraform apply' command do?

The command 'terraform apply' is used to execute the changes specified in your Terraform configuration files to your infrastructure. When you run this command, Terraform reviews your current infrastructure state and compares it with the desired state outlined in your configuration. It then makes the necessary changes to reach that desired state, which may involve creating, updating, or deleting resources in your cloud environment.

This process is essential for managing infrastructure as code because it ensures that the actual cloud resources accurately reflect the specifications defined in your Terraform files. The command takes any changes that might have been proposed in a previous 'terraform plan' step and applies those changes to update the actual environment. This immediate application of changes is central to how Terraform manages infrastructure over time.

The other options address different aspects of Terraform's functionality but do not describe the specific role of 'terraform apply.' Showing planned changes without making them refers to 'terraform plan,' while checking the integrity of configuration files is not a function of any single command; resources can be evaluated for those aspects in different ways throughout the workflow. Generating reports of unused resources is not a standard output of 'terraform apply', rather it could be done via separate reporting tools or commands when looking to optimize resource usage.