feat: add appimage support for linux

[SMillerDev]

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This adds support for the first linux-only cask stanza: app_image.
We should probably add CI for Linux casks, but haven't been able to get that working yet.

[MikeMcQuaid]

We should probably add CI for Linux casks, but haven't been able to get that working yet.

Need CI before this PR is merged.

Where are the app images installed and is that an expected/sensible location?

Maybe we should reuse app for this on Linux (or is that too confusing)?

[SMillerDev]

Maybe we should reuse app for this on Linux

I considered it, but deemed it too confusing. Especially with it not working on macOS.

Where are the app images installed and is that an expected/sensible location?

In ~/Applications, their docs recommend it.
https://docs.appimage.org/user-guide/faq.html#question-where-do-i-store-my-appimages

[MikeMcQuaid]

@SMillerDev sounds good 👍🏻

[SMillerDev]

I'm also considering adding a desktop file for these so the various Linux systems can pick it up easily.

Ref: https://specifications.freedesktop.org/desktop-entry-spec/latest/

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[SMillerDev]

This is waiting for the following pieces:

• Adding integration tests to testbot so I don't break it again
• Add cask change detection to testbot
• Adding Linux CI to homebrew/cask

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[SMillerDev]

Not stale, but it is waiting for Linux CI for casks, and that's not going as fast as I'd like. So help would be much appreciated.

[SMillerDev]

Okay, now that Linux CI for casks is merged I'm planning to:

• make a PR that stops requiring --cask in Linux
• Prepare this PR for review

[Copilot]

Pull Request Overview

This PR adds AppImage support as a Linux-only cask stanza, extending Homebrew's cross-platform capabilities. It implements a new artifact type app_image that allows Linux users to install AppImage applications through Homebrew casks.

• Implements AppImage artifact class with symlink functionality
• Updates OS-specific installer logic to validate platform compatibility
• Enhances CI matrix generation to support Linux cask testing

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Library/Homebrew/cask/artifact/appimage.rb	New AppImage artifact class that extends symlinked artifacts
Library/Homebrew/cask/artifact.rb	Registers AppImage in artifact system and defines Linux-only artifacts
Library/Homebrew/cask/config.rb	Adds appimagedir configuration option with default path
Library/Homebrew/cli/parser.rb	Adds --appimagedir CLI flag for configuring AppImage installation directory
Library/Homebrew/cask/dsl.rb	Registers AppImage as an ordinary artifact class in DSL
Library/Homebrew/cask/cask.rb	Updates platform support detection logic for macOS and Linux
Library/Homebrew/extend/os/mac/cask/installer.rb	New macOS-specific installer validation
Library/Homebrew/extend/os/linux/cask/installer.rb	Updates Linux installer validation logic
Library/Homebrew/extend/os/cask/installer.rb	Includes macOS-specific installer extensions
Library/Homebrew/dev-cmd/generate-cask-ci-matrix.rb	Enhances CI matrix generation for Linux cask support

Comments suppressed due to low confidence (1)

Library/Homebrew/dev-cmd/generate-cask-ci-matrix.rb:1

• Empty comment should either be removed or expanded to explain why the exception is being silently ignored, unlike the macOS case which logs the error message.

# typed: strict

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[Copilot]

This hardcoded repository override for testing should not be committed to production code. Consider using a command-line flag or environment variable for testing instead of commenting out the original code.

Suggested change

	# repository = ENV.fetch("GITHUB_REPOSITORY", nil)
	repository = "homebrew/homebrew-cask" # For testing outside of GitHub Actions
	repository = ENV["HOMEBREW_TEST_REPOSITORY"] || ENV["GITHUB_REPOSITORY"]

[Copilot]

[nitpick] Use return false unless any_loaded instead of return false if !any_loaded for better Ruby idiom consistency.

Suggested change

	return false if !any_loaded
	return false unless any_loaded

[Copilot]

[nitpick] Use return false unless any_loaded instead of return false if !any_loaded for better Ruby idiom consistency.

Suggested change

	return false if !any_loaded
	return false unless any_loaded