DevOps / GitHub Actions Interview Questions

How do you implement path filtering so a workflow only runs when specific files change?

GitHub Actions supports built-in path filtering on push and pull_request triggers via the paths: and paths-ignore: filters. When set, the workflow only fires if at least one file in the commit diff matches the given glob pattern.

on:
  push:
    branches: [main]
    paths:
      - 'backend/**'         # any file under backend/
      - 'Dockerfile'
      - '.github/workflows/backend-ci.yml'

  pull_request:
    paths-ignore:
      - '**.md'              # skip when only docs changed
      - 'frontend/**'

You can use both paths and paths-ignore but not on the same trigger event simultaneously. Use paths when you want an allow-list and paths-ignore when you want to exclude certain patterns.

Monorepo scenario — for finer-grained per-step filtering (e.g., only run specific jobs when specific subdirectories changed), the community action dorny/paths-filter is widely used:

jobs:
  changes:
    runs-on: ubuntu-latest
    outputs:
      backend: ${{ steps.filter.outputs.backend }}
      frontend: ${{ steps.filter.outputs.frontend }}
    steps:
      - uses: actions/checkout@v4
      - uses: dorny/paths-filter@v3
        id: filter
        with:
          filters: |
            backend:
              - 'backend/**'
            frontend:
              - 'frontend/**'

  test-backend:
    needs: changes
    if: needs.changes.outputs.backend == 'true'
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: ./gradlew :backend:test

The limitation of the built-in paths: filter is that it applies to the entire workflow; you cannot skip only certain jobs within it. dorny/paths-filter solves this by producing per-path boolean outputs that individual job conditions can check.

Take quiz

What is the limitation of the built-in paths: trigger filter compared to dorny/paths-filter? paths: only supports exact filenames, not globs

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paths: applies to the entire workflow — you cannot skip individual jobs within it

✓ Well done — that is correct!

paths: is not supported on pull_request events

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paths: requires a separate configuration file

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Can you use both paths: and paths-ignore: on the same trigger event in the same workflow? Yes, they are additive filters

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No, you can only use one per trigger event

✓ Well done — that is correct!

Yes, but only on push events

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Yes, paths: takes precedence over paths-ignore:

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