Gem::Commands::SetupCommand#install_executables

# File tmp/rubies/ruby-3.3.0/lib/rubygems/commands/setup_command.rb, line 245
  def install_executables(bin_dir)
    prog_mode = options[:prog_mode] || 0o755

    executables = { "gem" => "exe" }
    executables.each do |tool, path|
      say "Installing #{tool} executable" if @verbose

      Dir.chdir path do
        bin_file = "gem"

        require "tmpdir"

        dest_file = target_bin_path(bin_dir, bin_file)
        bin_tmp_file = File.join Dir.tmpdir, "#{bin_file}.#{$$}"

        begin
          bin = File.readlines bin_file
          bin[0] = shebang

          File.open bin_tmp_file, "w" do |fp|
            fp.puts bin.join
          end

          install bin_tmp_file, dest_file, mode: prog_mode
          bin_file_names << dest_file
        ensure
          rm bin_tmp_file
        end

        next unless Gem.win_platform?

        begin
          bin_cmd_file = File.join Dir.tmpdir, "#{bin_file}.bat"

          File.open bin_cmd_file, "w" do |file|
            file.puts <<-TEXT
  @ECHO OFF
  IF NOT "%~f0" == "~f0" GOTO :WinNT
  @"#{File.basename(Gem.ruby).chomp('"')}" "#{dest_file}" %1 %2 %3 %4 %5 %6 %7 %8 %9
  GOTO :EOF
  :WinNT
  @"#{File.basename(Gem.ruby).chomp('"')}" "%~dpn0" %*
  TEXT
          end

          install bin_cmd_file, "#{dest_file}.bat", mode: prog_mode
        ensure
          rm bin_cmd_file
        end
      end
    end
  end