Environment variables can be used to set variables.

Environment variables can be used to set variables. The environment variables must be in the format TF_VAR_name and this will be checked last for a value. For example:

export TF_VAR_region=us-west-1

export TF_VAR_ami=ami-049d8641

export TF_VAR_alist='[1,2,3]'

export TF_VAR_amap='{ foo = "bar", baz = "qux" }'

https://www.terraform.io/docs/commands/environment-variables.html