CEL Expressions for Variables

This page covers how to use CEL expressions to access webhook payload data and construct dynamic values in your PipelineRuns. CEL (Common Expression Language) is a lightweight expression language designed for evaluating structured data. Pipelines-as-Code uses it to let you build conditions and extract values from webhook payloads at runtime.

Using the body and headers in a Pipelines-as-Code parameter

Pipelines-as-Code gives you access to the full body and headers of the webhook request as CEL expressions. This lets you go beyond the standard dynamic variables and combine multiple conditions and values from the raw payload.

For example, to get the title of a pull request in your PipelineRun, access it like this:

{{ body.pull_request.title }}

You can also embed these variables inside a script to evaluate JSON data at runtime.

The following task uses Python to check the labels on a pull request. It exits with 0 if the pull request has the label bug, or 1 if it does not:

taskSpec:
  steps:
    - name: check-label
      image: registry.access.redhat.com/ubi10/ubi
      script: |
        #!/usr/bin/env python3
        import json
        labels=json.loads("""{{ body.pull_request.labels }}""")
        for label in labels:
            if label['name'] == 'bug':
              print('This is a PR targeting a BUG')
              exit(0)
        print('This is not a PR targeting a BUG :(')
        exit(1)

Because Pipelines-as-Code evaluates expressions as CEL, you can use conditionals directly:

- name: bash
  image: registry.access.redhat.com/ubi10/ubi
  script: |
    if {{ body.pull_request.state == "open" }}; then
      echo "PR is Open"
    fi

If the pull request is open, the condition returns true, which the shell script interprets as a valid boolean.

You can access headers from the webhook payload using the headers keyword. Headers are case-sensitive. For example, the following retrieves the GitHub event type:

{{ headers['X-Github-Event'] }}

You can apply the same conditional logic and access patterns described above for the body keyword to headers as well.

Using the cel: prefix for advanced CEL expressions

When you need more than simple property access, use the cel: prefix to write arbitrary CEL expressions with access to all available data sources. This is useful for conditional logic, string manipulation, and checking changed files.

The cel: prefix provides access to:

• body - The full webhook payload
• headers - HTTP request headers
• files - Changed files information (files.all, files.added, files.deleted, files.modified, files.renamed)
• pac - Standard Pipelines-as-Code parameters (pac.revision, pac.target_branch, pac.source_branch, etc.)

The following examples demonstrate common patterns.

Examples

Conditional values based on event action:

params:
  - name: pr-status
    value: "{{ cel: body.action == \"opened\" ? \"new-pr\" : \"updated-pr\" }}"

Environment selection based on target branch:

params:
  - name: environment
    value: "{{ cel: pac.target_branch == \"main\" ? \"production\" : \"staging\" }}"

Safe field access with has() function:

Use the has() function to safely check if a field exists before accessing it:

params:
  - name: commit-type
    value: "{{ cel: has(body.head_commit) && body.head_commit.message.startsWith(\"Merge\") ? \"merge\" : \"regular\" }}"

Check if Go files were modified:

params:
  - name: run-go-tests
    value: "{{ cel: files.all.exists(f, f.endsWith(\".go\")) ? \"true\" : \"false\" }}"

String concatenation:

params:
  - name: greeting
    value: "{{ cel: \"Build for \" + pac.repo_name + \" on \" + pac.target_branch }}"

Count changed files:

params:
  - name: file-count
    value: "{{ cel: files.all.size() }}"