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Maven / GitHub Actions Interview Questions

How do you debug failing GitHub Actions workflows — enabling debug logging and using tmate?

When a workflow fails and the log output is not enough to diagnose the problem, GitHub Actions provides two main debugging mechanisms: enhanced log verbosity via repository secrets, and live interactive SSH access to the runner via the tmate action.

1. Enable debug logging by adding two repository secrets (or re-running the workflow with debug enabled in the UI):

  • ACTIONS_RUNNER_DEBUG = true — enables verbose runner-level diagnostics (why jobs were queued, runner setup details)
  • ACTIONS_STEP_DEBUG = true — enables verbose step-level logs, including inputs/outputs of actions and shell-expanded commands

You can also re-run a failed job with debug logging enabled from the GitHub UI via "Re-run jobs" → "Enable debug logging".

2. Interactive SSH debugging with tmate — the mxschmitt/action-tmate action pauses the runner and opens an SSH tunnel so you can connect to the live runner and explore the filesystem, environment, and run commands manually:

steps:
  - uses: actions/checkout@v4

  - name: Run tests (may fail)
    run: npm test
    continue-on-error: true      # don't abort before tmate step

  - name: Setup tmate debugging
    if: failure()                # only open SSH if something failed
    uses: mxschmitt/action-tmate@v3
    timeout-minutes: 15          # auto-close after 15 min
    with:
      limit-access-to-actor: true  # only the workflow triggerer can connect

Other useful debugging techniques:

  • run: env | sort — print all environment variables at a step to verify injected values.
  • run: cat $GITHUB_EVENT_PATH | python3 -m json.tool — inspect the raw event payload.
  • Use actions/upload-artifact to save log files or test-result directories for inspection after the run.
Which repository secret enables verbose step-level debug logs in GitHub Actions?
What does the mxschmitt/action-tmate action do when added to a workflow step?

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What is GitHub Actions and what problems does it solve? What are the key components of GitHub Actions — workflows, jobs, steps, actions, and runners? How is a GitHub Actions workflow file structured, and where must it be placed? What are workflow triggers (on:) and which event types does GitHub Actions support? What is the difference between push, pull_request, and workflow_dispatch triggers? What are jobs in GitHub Actions, and how do they run in parallel by default? What are steps, and what is the difference between run: and uses: in a step? What are runners, and what is the difference between GitHub-hosted and self-hosted runners? What is the GitHub Actions Marketplace and how do you find and use actions from it? How do you use the actions/checkout action and what does it do? How do you pass environment variables and secrets to a GitHub Actions workflow? What is the difference between the env:, secrets:, and vars: contexts in GitHub Actions? How do you cache dependencies in GitHub Actions using actions/cache? How do you use matrix builds in GitHub Actions to test across multiple environments? How do you control job execution order in GitHub Actions using needs:? How do you share data between steps within a job using step outputs? How do you share build artifacts between jobs using actions/upload-artifact and actions/download-artifact? What are reusable workflows in GitHub Actions and how do you call them? What are composite actions and when should you choose them over reusable workflows? How do you set up a Docker container service for integration tests using services: in GitHub Actions? How do you use conditional steps with if: in GitHub Actions? What are the key GitHub Actions expression contexts and what information does each provide? How do you use concurrency groups to cancel outdated workflow runs in GitHub Actions? What is the GITHUB_TOKEN and what permissions does it have? How do you trigger one GitHub Actions workflow from another using workflow_run? How do you write a custom JavaScript action for GitHub Actions? How do you write a custom Docker container action for GitHub Actions? How do you implement a complete CI/CD pipeline for a container image in GitHub Actions — build, push to a registry, and deploy? How do you implement path filtering so a workflow only runs when specific files change? How do you debug failing GitHub Actions workflows — enabling debug logging and using tmate? How do you implement branch protection rules with required GitHub Actions status checks? How do you handle large monorepos with multiple services in GitHub Actions? What are OpenID Connect (OIDC) tokens in GitHub Actions and how do they replace long-lived cloud credentials? How do you prevent secret exposure and follow security hardening best practices in GitHub Actions? What are the key differences between GitHub Actions, Jenkins, and GitLab CI?
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