DevOps / 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.

Take quiz

Which repository secret enables verbose step-level debug logs in GitHub Actions? ACTIONS_RUNNER_DEBUG

✗ Try again — not quite right.

ACTIONS_STEP_DEBUG

✓ Well done — that is correct!

GITHUB_DEBUG

✗ Try again — not quite right.

RUNNER_VERBOSE

✗ Try again — not quite right.

What does the mxschmitt/action-tmate action do when added to a workflow step? Replays the previous run in a sandbox environment

✗ Try again — not quite right.

Opens an SSH tunnel to the live runner so you can interactively debug it

✓ Well done — that is correct!

Attaches a remote debugger to a running JVM process

✗ Try again — not quite right.

Sends step logs to a Slack channel in real time

✗ Try again — not quite right.

More Related questions...

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