Incorporate ShellCheck into GitHub Action

[dtrodrigues]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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This will run ShellCheck on bin/brew and scripts ending in .sh not in a tap or in vendor.

[MikeMcQuaid]

This is (or at least should be) already run by brew style:

brew/Library/Homebrew/style.rb

Lines 105 to 115 in 081025c

	shellcheck = which("shellcheck")
	shellcheck ||= which("shellcheck", ENV["HOMEBREW_PATH"])
	shellcheck ||= begin
	ohai "Installing `shellcheck` for shell style checks..."
	system HOMEBREW_BREW_FILE, "install", "shellcheck"
	which("shellcheck") || which("shellcheck", ENV["HOMEBREW_PATH"])
	end
	unless shellcheck
	opoo "Could not find or install `shellcheck`! Not checking shell style."
	return rubocop_success
	end

[dtrodrigues]

It looks like 892d0ac from April 2019 may have prevented shellcheck from running since then since the current GitHub action runs brew style --display-cop-names. The 3 detected errors were all introduced after that point (19fd564 99e3087 5d3456f).

Also, I think it would be better to add a #!/bin/bash shebang to the .sh files missing them and remove the --shell=bash argument so the files are more explicit.

[MikeMcQuaid]

It looks like 892d0ac from April 2019 may have prevented shellcheck from running since then since the current GitHub action runs brew style --display-cop-names. The 3 detected errors were all introduced after that point (19fd564 99e3087 5d3456f).

Good catch, I've re-enabled in #7930.

Also, I think it would be better to add a #!/bin/bash shebang to the .sh files missing them and remove the --shell=bash argument so the files are more explicit.

I'd rather not; these files are not designed to be executed as shell scripts but sourced from other files.

[MikeMcQuaid]

Thanks for the PR @dtrodrigues, wouldn't have caught this without you ❤️