GitHub Actions: Setting env vars with other env vars

GitHub Actions provides an env configuration that allows a build to inject environmental variables into a full workflow or an individual step:

jobs:
  build:
    env:
      PG_DATA_DIR: /home/runner/data

String literals are fine, but users will find out the hard way that using environment variables as inputs to other environment variables does not work:

jobs:
  build:
    env:
      PG_DATA_DIR: $HOME/data

The reason is that the values are slurped up when a workflow’s YAML is being parsed, and never interpreted through a shell that would enable variable expansion. $HOME/data above comes out as literally $HOME/data instead of the intended /home/runner/data.

The fix

The workaround is to use GitHub Actions environment files ¹. Values written to $GITHUB_ENV are available in subsequent steps:

jobs:
  build:
    steps:
      - name: "Set environmental variables"
        run: |
          echo "PG_DATA_DIR=$HOME/data" >> $GITHUB_ENV

      - name: "Can use environment variables"
        run: |
          echo "Working variable from variable: $PG_DATA_DIR"