Implement BASH_ENV functionality to share env vars between steps.

[lkysow]

Problem

There is currently no way to share environment variables in between steps (see #369). This is useful for users that want to source secrets from custom scripts and use them in terraform.

Solution

To achieve this, we should implement $BASH_ENV type functionality where users can write export statements to the file described by $BASH_ENV which gets source'd at each step:

- run: "echo export TEST=abc >> $BASH_ENV"
- run: "echo $BASH_ENV"
- run: "echo $TEST"

This would result in:

/tmp/kajsldjflksjdljk.sh
abc

Implementation

• Generate a random file for each set of steps
• source that file in each step before executing the actual step code

Notes

• This is how CircleCI works: https://circleci.com/docs/2.0/env-vars/#using-bash_env-to-set-environment-variables
• Need Write temp script instead of running with sh -c #337 first (unless we do sh -c ". $BASH_ENV && {user command}")

[ttk]

The env step option to setup environment variables doesn't quite solve the use case when you have a single command that generates multiple environment variable exports that you want to source to to next step. The original proposal using $BASH_ENV would have been a more flexible option:

- run: "multiple-exports-creds.sh >> $BASH_ENV"

In my case the multiple-exports-creds.sh creates multiple environment variables in one go, whereas env command step option only creates one environment variable at a time. Any workarounds?

[ricardosilva86]

When I need to run TF apply in different environment, I usually export the GOOGLE_APPLICATION_CREDENTIALS env variable. Something like:

terragrunt-non-production:
    plan:
      steps:
      - env:
          name: GOOGLE_APPLICATION_CREDENTIALS
          value: /home/atlantis/.creds/key-non-prod.json
      - env:
          # Reduce Terraform suggestion output
          name: TF_IN_AUTOMATION
          value: 'true'
      - run: terragrunt plan -input=false -out=$PLANFILE
      - run: terragrunt show -json $PLANFILE > $SHOWFILE

Shouldn't be like this? after reading this, it looks like I should be using terraform workspaces. Can someone comment?