Parameters and Outputs

Lesson 3 of 10 · 4 minutes

Parameters are a workflow's inputs, set each time it starts. Outputs carry a value from one step to the steps after it.

The workflow

curl -fsSLO https://docs.dagu.sh/tutorial/dags/tutorial-03-params-and-outputs.yaml

# Dagu tutorial, lesson 3: https://docs.dagu.sh/tutorial/params-and-outputs
description: Take inputs and pass a value from one step to the next
type: graph

params:
  - name: environment
    type: string
    enum: [dev, staging, prod]
    default: dev
  - name: version
    type: string
    default: "1.0.0"

steps:
  - id: build
    run: echo "app-${params.version}.tar.gz"
    output:
      bundle:
        from: stdout

  - id: deploy
    run: echo "Deploying ${steps.build.outputs.bundle} to ${params.environment}"
    depends: build

• params declares two inputs. environment accepts only the three listed values. Both have a default, so the workflow also runs with no input.
• ${params.version} inserts a parameter's value into a command.
• output on build publishes the step's printed text as an output named bundle.
• ${steps.build.outputs.bundle} reads that output in a later step. The reading step must depend on build.

Watch it run

0.2s / 0.2sSucceeded

deployrunSucceeded

Deploying app-1.0.0.tar.gz to dev

A recorded run of this lesson's DAG. Select a step to see its log.

Select build to see the bundle output it published.

Run it yourself

Run it with the defaults, then with your own values after --:

dagu start tutorial-03-params-and-outputs.yaml
dagu start tutorial-03-params-and-outputs.yaml -- environment=staging version=2.1.0

deploy prints Deploying app-2.1.0.tar.gz to staging.

In the Web UI, Start opens a dialog with one field per parameter. environment is a list with the three allowed values.

Try it

1. Run with environment=qa. What happens?
2. Add a third step, notify, that prints Shipped <bundle> using the output of build.

Answers

1. Dagu rejects the input before any step runs, because qa is not one of the allowed values.
2. Add this step to steps:

  - id: notify
    run: echo "Shipped ${steps.build.outputs.bundle}"
    depends: build

Go deeper: Parameters →Go deeper: Outputs →