How to Negate a Condition in an if in GitHub Actions

Use ! to negate, and quote the whole if value when it starts with ! so YAML parses it.

The ! operator inverts a boolean. Because YAML treats a leading ! as a tag, wrap the entire expression in quotes when it begins with !.

Steps

Prefix the expression with ! to invert it.
Quote the whole if: value if it starts with !.
Combine with contains, startsWith, or comparisons as needed.

Workflow

.github/workflows/ci.yml

steps:
  - name: Deploy unless skipped
    if: "!contains(github.event.head_commit.message, '[skip-deploy]')"
    run: ./deploy.sh
  - name: Non-fork only
    if: "!github.event.pull_request.head.repo.fork"
    run: ./privileged.sh

Gotchas

An unquoted leading ! triggers a YAML error about an unexpected tag.
Negating a non-boolean uses truthiness: empty string is false, non-empty is true.

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