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What a GitHub Actions workflow contains
Workflow files are checked into your repository under .github/workflows. Each is a YAML definition that connects an event to one or more jobs, specifies the runner machine for each job, and lays out the steps to perform. A step can run a shell command or use an action.
Triggers can include repository events, a manual run, or a schedule. A workflow can also use path filters to decide whether a change should trigger it. Before testing locally, identify the workflow file, the event it is meant to handle, and the changes or paths that should qualify. GitHub documents these building blocks in its workflow overview and workflow syntax reference.
How act creates a local feedback loop
The act project describes its purpose as “Run your GitHub Actions locally” and sums up its approach with “Think globally, act locally”. It reads workflow definitions in your repository and uses the Docker API to fetch or build images and run containers for actions. That gives you a way to exercise workflow steps while editing, rather than pushing every change just to see whether the local execution path works.
Start in the repository that contains the workflow. Inspect the workflow’s triggers and jobs, then use act to run the workflow or the relevant event you intend to check. The important question is not simply whether a local run is green: it is whether that run represents the workflow path and change you want to validate. If there are multiple triggers or path filters, be explicit about which one your local test is intended to represent.
Choose a runner image with the tradeoff in mind
GitHub’s runner label describes the machine environment a job requests. In act’s documented setup, that runner definition maps to a container image. The act runner guide lists micro, medium, and large image choices. Smaller images reduce image size and setup/resource overhead; larger images may include more environment contents. Neither choice guarantees an exact match for a GitHub-hosted runner.
The guide’s examples include these mappings. They are version-sensitive: check the current act runner guide before relying on a particular image mapping.
| Workflow runner label | Micro example | Medium example | Large example |
|---|---|---|---|
ubuntu-latest |
node:16-buster-slim |
catthehacker/ubuntu:act-latest |
catthehacker/ubuntu:full-latest |
ubuntu-22.04 |
Corresponding Bullseye image; exact tag not stated in the cited guide examples | Corresponding act image; exact tag not stated in the cited guide examples | Corresponding full image; exact tag not stated in the cited guide examples |
Image contents affect how closely the local environment resembles the one you need to test, while image size and setup affect local resource use. Treat the listed names as guide examples, not a promise of parity with GitHub. The workflow model and act’s container-based approach are documented separately by GitHub and the act project.
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A successful act run is useful evidence that the path exercised in its local container worked. It is not proof that every hosted run will work. Before treating a local result as sufficient, compare the conditions your workflow depends on:
- Runner environment: Is the local image a close enough match for the runner OS and software your GitHub job uses?
- Docker and containers: Does local execution depend on Docker being available, and does that reflect the environment relevant to the workflow?
- Event context: Does the local run represent the trigger, event data, and change set you need to check? Do not assume an arbitrary local invocation recreates every GitHub webhook payload or platform integration.
- Permissions and secrets: Does the workflow rely on GitHub token permissions or secrets that differ from what is available or appropriate locally?
- Network and services: Does the job depend on access to external services or network conditions not established by the local run?
- Required final check: Will the workflow still run on GitHub, where the repository’s actual event, permissions, and runner environment can be verified?
Use local execution as one step in development. Confirm behavior on GitHub when the result depends on hosted-runner behavior, event context, permissions, secrets, or services the local test has not established.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep tokens and secrets safe while testing
Local testing does not make credentials harmless. GitHub recommends giving GITHUB_TOKEN only the permissions required, using read-only repository contents access by default where possible, and elevating permissions for individual jobs only as needed. Its security hardening guidance also advises against putting sensitive values in workflow files and recommends auditing how actions use secrets.
- Do not commit plaintext secret values in workflow YAML or other repository files.
- Use appropriately scoped test credentials when local testing genuinely needs credentials; do not casually pass production credentials into a local run.
- Review how each action handles any secret it receives, and grant only the access the job needs.
- Inspect logs after testing with both valid and invalid inputs. Command output can expose sensitive values.
- If a secret appears in a log without redaction, GitHub advises deleting the log and rotating the exposed secret.
These precautions apply to the workflow and its logs, not just to where the test runs. Follow the repository’s secret-management policy whenever you configure local tests.
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