Skip to main content

Gate custom command steps on their own flags and arguments

· 2 min read
Erik Osterman
Erik Osterman
Founder @ Cloud Posse

A custom command often needs to behave differently depending on how it was called — skip the destructive step on --dry-run, only run a cleanup step for a particular environment argument, or branch on whichever component the caller targeted. Doing that meant wrapping the command in a shell script that inspected $@ itself, because a step's when: condition had no visibility into the command's own --flag or positional argument values.

The Problem​

Custom command steps already supported when: conditions built on CEL — ci, stack, component, and more — but a step could never see the values the user actually passed to the command that's running it:

commands:
  - name: deploy
    flags:
      - name: dry-run
        type: bool
    steps:
      - type: shell
        command: terraform apply
        # No way to reference --dry-run here.

The flag values were already being extracted for Go/gomplate templates as {{ .Flags.dry_run }}, but that data never reached the CEL evaluator, so when: conditions couldn't use it.

The Fix​

when: expressions can now read flags and arguments, mirroring the same data already available to steps in templates. component is also resolved for custom commands now, the same way it already was for component hooks — via a semantic-typed flag or argument.

How to Use It​

commands:
  - name: deploy
    flags:
      - name: dry-run
        type: bool
    steps:
      - type: shell
        command: terraform apply
        when: !cel '!flags["dry-run"]'

See the custom command steps docs for the full list of available when: facts.

Get Involved​

Have feedback on this feature? Open an issue or join the conversation in the Cloud Posse community Slack.