Why GitHub Actions environment variables come back empty in later steps

A GitHub Actions environment variable that fails to expand in a step usually stems from one of several distinct mechanisms rather than a single bug. The most common cause is a variable exported in one step's shell, which disappears when that step's process ends, so later steps never see it. Values written to $GITHUB_ENV only become available after the writing step finishes, not within that same step. Confusion between ${{ env.NAME }} expressions, which resolve before the script runs, and $NAME shell expansion, which resolves during execution, can also yield empty or literal output. Scope problems, such as step-level env maps or job-level variables not reaching reusable workflows, add a further source of failure. The suggested fix is to write the value to $GITHUB_ENV in one step and read it in a subsequent step, or to use step outputs for an explicit, visible dependency.