services/system: print warnings to stderr instead of stdout

[stepbrobd]

---

• 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 (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

---

• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

---

in #21830

ac06ea7 seem to break my nix-darwin configuration as nix-darwin is executing the bundle command with root

https://github.com/nix-darwin/nix-darwin/blob/da529ac9e46f25ed5616fd634079a5f3c579135f/modules/homebrew.nix#L951

thus these two warning prints lines to stdout mess with json parsing

brew/Library/Homebrew/services/system.rb

Line 101 in 86fe878

puts "Hide this warning by setting `HOMEBREW_SERVICES_NO_DOMAIN_WARNING=1`."

and im getting confusing errors like this:

Homebrew bundle...
Using stepbrobd/tap
Using resonative/proaudio
Using lyraphase/av-casks
Using nextfire/tap
Using apple-music-discord-rpc
Error: unexpected character: 'Hide' at line 1 column 1
Warning: Removed Sorbet lines from backtrace!
/opt/homebrew/Library/Homebrew/vendor/portable-ruby/4.0.2/lib/ruby/4.0.0/json/common.rb:353:in 'JSON::Ext::Parser.parse'
/opt/homebrew/Library/Homebrew/vendor/portable-ruby/4.0.2/lib/ruby/4.0.0/json/common.rb:353:in 'JSON.parse'
/opt/homebrew/Library/Homebrew/bundle/brew_services.rb:68:in 'Homebrew::Bundle::Brew::Services.started_services'
/opt/homebrew/Library/Homebrew/bundle/brew_services.rb:58:in 'Homebrew::Bundle::Brew::Services.started?'
/opt/homebrew/Library/Homebrew/bundle/brew.rb:417:in 'Homebrew::Bundle::Brew#start_service_needed?'
/opt/homebrew/Library/Homebrew/bundle/brew.rb:443:in 'Homebrew::Bundle::Brew#service_change_state!'
/opt/homebrew/Library/Homebrew/bundle/brew.rb:369:in 'Homebrew::Bundle::Brew#install!'
/opt/homebrew/Library/Homebrew/bundle/brew.rb:43:in 'Homebrew::Bundle::Brew.install!'
/opt/homebrew/Library/Homebrew/bundle/installer.rb:70:in 'block in Homebrew::Bundle::Installer.install!'
/opt/homebrew/Library/Homebrew/bundle/installer.rb:56:in 'Array#each'
/opt/homebrew/Library/Homebrew/bundle/installer.rb:56:in 'Homebrew::Bundle::Installer.install!'
/opt/homebrew/Library/Homebrew/bundle/commands/install.rb:25:in 'Homebrew::Bundle::Commands::Install.run'
/opt/homebrew/Library/Homebrew/cmd/bundle.rb:235:in 'Homebrew::Cmd::Bundle#run'
/opt/homebrew/Library/Homebrew/brew.rb:115:in '<main>'

this can be reproduced with (remember to change the username, and e> syntax is for nushell only)

$ sudo --preserve-env=PATH --user=ysun --set-home env brew services list --json
Warning: running through sudo, using user/* instead of gui/* domain!
Hide this warning by setting `HOMEBREW_SERVICES_NO_DOMAIN_WARNING=1`.
Hide these hints with `HOMEBREW_NO_ENV_HINTS=1` (see `man brew`).
[
  {
    "name": "apple-music-discord-rpc",
    "status": "started",
    "user": "ysun",
    "file": "/Users/ysun/Library/LaunchAgents/homebrew.mxcl.apple-music-discord-rpc.plist",
    "exit_code": 0
  }
]
$ sudo --preserve-env=PATH --user=ysun --set-home env brew services list --json e> /dev/null
Hide this warning by setting `HOMEBREW_SERVICES_NO_DOMAIN_WARNING=1`.
Hide these hints with `HOMEBREW_NO_ENV_HINTS=1` (see `man brew`).
[
  {
    "name": "apple-music-discord-rpc",
    "status": "started",
    "user": "ysun",
    "file": "/Users/ysun/Library/LaunchAgents/homebrew.mxcl.apple-music-discord-rpc.plist",
    "exit_code": 0
  }
]

[stepbrobd]

cc @carlocab

[Copilot]

Pull request overview

Adjusts brew services domain-selection messaging so warnings/hints don’t pollute machine-readable stdout (e.g., --json output), addressing downstream JSON parsing failures (notably when invoked via sudo in nix-darwin/Homebrew Bundle flows).

Changes:

• Replace two puts hint lines with opoo so they emit on stderr instead of stdout.
• Keep the “hide warning/hints” guidance gated behind HOMEBREW_NO_ENV_HINTS.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[carlocab]

Thanks; I think at least the first, if not both, copilot review comments need addressing.

[stepbrobd]

thanks for getting back so quickly! i've addressed the first one but i'm not very familiar with the code base so i'm not sure how to get started on the second one

maybe that could be done in another pr by someone else?

[carlocab]

Thanks!