What does double colon :: stand for in YAML for GitHub Actions?

Question

I'm trying to write a file in my GitHub repo with GitHub Actions. When reading the docs, I stumbled across this:

Actions can communicate with the runner machine to set environment variables, output values used by other actions, add debug messages to the output logs, and other tasks.

Most workflow commands use the echo command in a specific format, while others are invoked by writing to a file. For more information, see "Environment files".

echo "::workflow-command parameter1={data},parameter2={data}::{command value}"

I don't know Ansible so I don't understand if this is YAML syntax or Ansible syntax. I've tried to search Google and Stack Overflow but no results for double colon or ::

Can someone give me the link to the appropriate doc for :: or explain what this command does?

in other words, what does the example in my post throws in the shell? where are data and parameter1 and parameter2 defined if they are (in the yml, in the shell/env)? is command value a value i can reuse in the yml or in the shell?

Answer 1

The ::command can be logged to the console by any script or executable. They are special strings the GitHub runner will detect, interpret and then take the appropriate action on.

They are essentially the communication mechanism between the runner and the thing it's currently running. Anything that can write to the console can issue these strings.

It's totally up to you to build these stings, to inject any parameters these 'magic strings' require to function.

The docs you've found are the right docs on these to understand how to log there strings and what commands there are available to you.

If you're building a GitHub action using the JavaScript/Typescript toolkit, then it provides nice wrapper functions for these commands. The JavaScript SDK also gives you a sneak peak into how to composekthese strings.

If you're building a composite action, container task or are directly issueing commands from a script block in the workflow, then it's up to you to build the correct strings and log these to the console.

More details:

• https://github.com/actions/toolkit/blob/main/packages/core/README.md
• https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions (you had found that already)

Communicating through the console is the lowest common denominator between any tools running on just about any platform and requires no interprocess communication if any kind. It's the simplest way to communicate from a child process to it's parent.

You'd use the command to set an output variable.

echo "::set-output name=name::value"

To be able to reference the value cross at you'd reference any output variable from any action.

Or set an environment variable which will be set for the next job: echo "action_state=yellow" >> $GITHUB_ENV

See: https://stackoverflow.com/a/57989070/736079