TIL how to use a variable as the default for another variable in GitLab CI

In GitLab CI, the variables block does not expand other variables at definition time. So this does not work as expected:

variables:
  DEPLOY_USER: "${DEPLOY_USER:-$CI_REGISTRY_USER}"
  DEPLOY_TOKEN: "${DEPLOY_TOKEN:-$CI_JOB_TOKEN}"

The $CI_REGISTRY_USER and $CI_JOB_TOKEN references won’t be resolved in the variables block because GitLab CI doesn’t support nested variable expansion there.

The fix is to move the fallback logic into before_script, where normal shell expansion applies:

before_script:
  - export DEPLOY_USER="${DEPLOY_USER:-$CI_REGISTRY_USER}"
  - export DEPLOY_TOKEN="${DEPLOY_TOKEN:-$CI_JOB_TOKEN}"

This is especially relevant when writing reusable CI templates where you want to allow overriding a variable but fall back to a CI-provided default.