Can Terraform use bash environment variables?

You could just use bare provider like so:

provider "aws" {}

And make sure env vars available in your shell session:

$ export AWS_ACCESS_KEY_ID="your-key-id"
$ export AWS_SECRET_ACCESS_KEY="your-secret-key"
$ export AWS_DEFAULT_REGION="your-region"

Then, check if the above works:

terraform plan

If you satisfy the the plan, you might perform terraform apply to make the change.

For more details: https://www.terraform.io/docs/providers/aws/#environment-variables

Yes, can read environment variables in Terraform. There is a very specific way that this has to be done. You will need to make the environment variable a variable in terraform.

For example I want to pass in a super_secret_variable to terraform. I will need to create a variable for it in my terraform file.

variable "super_secret_variable" {
    type = "string
}

Then based on convention I will have to prefix my environment variable with TF_VAR_ like this:

TF_VAR_super_secret_variable

Then terraform will automatically detect it and use it. Terraform processors variables based on a specific order that order is -var option, -var-file option, environment variable, then default values if defined in your tf file.

Alternative you can pass environment variables in through the CLI to set variables in terraform like so.

> terraform apply -var super_secret_variable=$super_secret_variable

This doesn't require that you prefix it so if they are something you can't change that may be your best course of action.

You can read more here in the docs.