brew/Library/Homebrew/utils/ruby.sh

test-ruby () {
  "$1" --enable-frozen-string-literal --disable=gems,did_you_mean,rubyopt -rrubygems -e \
    "puts Gem::Version.new(RUBY_VERSION.to_s.dup).to_s.split('.').first(2) == \
          Gem::Version.new('$required_ruby_version').to_s.split('.').first(2)" 2>/dev/null
}

setup-ruby-path() {
  local vendor_dir
  local vendor_ruby_current_version
  local vendor_ruby_path
  local usable_ruby_version
  # When bumping check if HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH (in brew.sh)
  # also needs to be changed.
  local required_ruby_version="2.6"
  local old_ruby_path
  local old_ruby_usable
  local advice="
If there's no Homebrew Portable Ruby available for your processor:
- install Ruby $required_ruby_version with your system package manager (or rbenv/ruby-build)
- make it first in your PATH
- try again
"

  vendor_dir="$HOMEBREW_LIBRARY/Homebrew/vendor"
  vendor_ruby_current_version="$vendor_dir/portable-ruby/current"
  vendor_ruby_path="$vendor_ruby_current_version/bin/ruby"

  if [[ -n $HOMEBREW_RUBY_PATH ]]
  then
    old_ruby_path="$HOMEBREW_RUBY_PATH"
    old_ruby_usable=$(test-ruby "$HOMEBREW_RUBY_PATH")
  fi

  unset HOMEBREW_RUBY_PATH

  if [[ "$HOMEBREW_COMMAND" != "vendor-install" ]]
  then
    if [[ -x "$vendor_ruby_path" ]]
    then
      HOMEBREW_RUBY_PATH="$vendor_ruby_path"

      if [[ $(readlink "$vendor_ruby_current_version") != "$(<"$vendor_dir/portable-ruby-version")" ]]
      then
        if ! brew vendor-install ruby
        then
          if [[ -n "$HOMEBREW_MACOS" ]]
          then
            odie "Failed to upgrade Homebrew Portable Ruby!"
          else
            odie "Failed to upgrade Homebrew Portable Ruby!$advice"
          fi
        fi
      fi
    else
      if [[ -n "$HOMEBREW_MACOS" ]]
      then
        HOMEBREW_RUBY_PATH="/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby"
      else
        HOMEBREW_RUBY_PATH=$(type -P ruby)
        if [[ $(test-ruby $HOMEBREW_RUBY_PATH) != "true" ]]
        then
          HOMEBREW_RUBY_PATH=$(PATH="$HOMEBREW_PATH" type -P ruby)
          if [[ $(test-ruby "$HOMEBREW_RUBY_PATH") != "true" ]]
          then
            if [[ $old_ruby_usable != true ]]
            then
              onoe "Failed to find usable Ruby $required_ruby_version!"
              unset HOMEBREW_RUBY_PATH
            else
              HOMEBREW_RUBY_PATH="$old_ruby_path"
            fi
          fi
        fi
      fi

      if [[ -n "$HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH" ]]
      then
        usable_ruby_version="true"
      elif [[ -n "$HOMEBREW_RUBY_PATH" && -z "$HOMEBREW_FORCE_VENDOR_RUBY" ]]
      then
        usable_ruby_version=$(test-ruby "$HOMEBREW_RUBY_PATH")
      fi

      if [[ -z "$HOMEBREW_RUBY_PATH" || -n "$HOMEBREW_FORCE_VENDOR_RUBY" || "$usable_ruby_version" != "true" ]]
      then
        brew vendor-install ruby
        if [[ ! -x "$vendor_ruby_path" ]]
        then
          if [[ -n "$HOMEBREW_MACOS" ]]
          then
            odie "Failed to install Homebrew Portable Ruby (and your system version is too old)!"
          else
            odie "Failed to install Homebrew Portable Ruby and cannot find another Ruby $required_ruby_version!$advice"
          fi
        fi
        HOMEBREW_RUBY_PATH="$vendor_ruby_path"
      fi
    fi
  fi

  export HOMEBREW_RUBY_PATH
}