How to understand the environment variable which is set via "--param"

Drone Support
1

I run the deploy command with --param "TF_VAR_app=app-1" in below pipeline

  terraform_plan:
    image: hashicorp/terraform
    commands:
      - env |grep "TF_VAR_"
      - echo ${TF_VAR_app}

Why I can grep the variable by env, but can’t echo it directly?

+ env |grep "TF_VAR_"
TF_VAR_app=app-1
+ echo

+
2

I find the answer with this document:

http://docs.drone.io/environment/