The installer installs the files contained in the .gem into the Gem.home.
Gem::Installer does the work of putting files in all the right places on the filesystem including unpacking the gem into its gem dir, installing the gemspec in the specifications dir, storing the cached gem in the cache dir, and installing either wrappers or symlinks for executables.
The installer invokes pre and post install hooks.  Hooks can be added either through a rubygems_plugin.rb file in an installed gem or via a rubygems/defaults/#{RUBY_ENGINE}.rb or rubygems/defaults/operating_system.rb file.  See Gem.pre_install and Gem.post_install for details.
Paths where env(1) might live. Some systems are broken and have it in /bin
Construct an installer object for the gem file located at path
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 111
def self.at(path, options = {})
  security_policy = options[:security_policy]
  package = Gem::Package.new path, security_policy
  new package, options
end
             
            Defaults to use Ruby's program prefix and suffix.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 103
def exec_format
  @exec_format ||= Gem.default_exec_format
end
             
            Construct an installer object for an ephemeral gem (one where we don't actually have a .gem file, just a spec)
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 149
def self.for_spec(spec, options = {})
  # FIXME: we should have a real Package class for this
  new FakePackage.new(spec), options
end
             
            Changes in rubygems to lazily loading `rubygems/command` (in order to lazily load `optparse` as a side effect) affect bundler's custom installer which uses `Gem::Command` without requiring it (up until bundler 2.2.29). This hook is to compensate for that missing require.
TODO: Remove when rubygems no longer supports running on bundler older than 2.2.29.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 80
def inherited(klass)
  if klass.name == "Bundler::RubyGemsGemInstaller"
    require "rubygems/command"
  end
  super(klass)
end
             
            Constructs an Installer instance that will install the gem at package which can either be a path or an instance of Gem::Package.  options is a Hash with the following keys:
Where to put a bin wrapper if needed.
Whether or not development dependencies should be installed.
Use /usr/bin/env in bin wrappers.
Overrides all version checks and security policy checks, except for a signed-gems-only policy.
Format the executable the same as the Ruby executable. If your Ruby is ruby18, foo_exec will be installed as foo_exec18.
Don't raise if a dependency is missing.
The directory to install the gem into.
Use the specified security policy.  See Gem::Security
Indicate that the gem should be unpacked into the users personal gem directory.
Only validate dependencies against what is in the install_dir
Install wrappers if true, symlinks if false.
An Array of arguments to pass to the extension builder process. If not set, then Gem::Command.build_args is used
Print gem post install message if true
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 179
def initialize(package, options={})
  require 'fileutils'
  @options = options
  @package = package
  process_options
  @package.dir_mode = options[:dir_mode]
  @package.prog_mode = options[:prog_mode]
  @package.data_mode = options[:data_mode]
  if options[:user_install]
    @gem_home = Gem.user_dir
    @bin_dir = Gem.bindir gem_home unless options[:bin_dir]
    @plugins_dir = Gem.plugindir(gem_home)
    check_that_user_bin_dir_is_in_path
  end
end
             
            Return the text for an application file.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 758
  def app_script_text(bin_file_name)
    # note that the `load` lines cannot be indented, as old RG versions match
    # against the beginning of the line
    return <<-TEXT
#{shebang bin_file_name}
#
# This file was generated by RubyGems.
#
# The application '#{spec.name}' is installed as part of a gem, and
# this file is here to facilitate running it.
#
require 'rubygems'
#{gemdeps_load(spec.name)}
version = "#{Gem::Requirement.default_prerelease}"
str = ARGV.first
if str
  str = str.b[/\\A_(.*)_\\z/, 1]
  if str and Gem::Version.correct?(str)
    #{explicit_version_requirement(spec.name)}
    ARGV.shift
  end
end
if Gem.respond_to?(:activate_bin_path)
load Gem.activate_bin_path('#{spec.name}', '#{bin_file_name}', version)
else
gem #{spec.name.dump}, version
load Gem.bin_path(#{spec.name.dump}, #{bin_file_name.dump}, version)
end
TEXT
  end
             
            Builds extensions. Valid types of extensions are extconf.rb files, configure scripts and rakefiles or mkrf_conf files.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 850
def build_extensions
  builder = Gem::Ext::Builder.new spec, build_args
  builder.build_extensions
end
             
            The location of the default spec file for default gems.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 450
def default_spec_file
  File.join gem_home, "specifications", "default", "#{spec.full_name}.gemspec"
end
             
            Return the target directory where the gem is to be installed. This directory is not guaranteed to be populated.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 891
def dir
  gem_dir.to_s
end
             
            Ensure that the dependency is satisfied by the current installation of gem. If it is not an exception is raised.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 412
def ensure_dependency(spec, dependency)
  unless installation_satisfies_dependency? dependency
    raise Gem::InstallError, "#{spec.name} requires #{dependency}"
  end
  true
end
             
            Ensures the Gem::Specification written out for this gem is loadable upon installation.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 639
def ensure_loadable_spec
  ruby = spec.to_ruby_for_cache
  ruby.tap(&Gem::UNTAINT)
  begin
    eval ruby
  rescue StandardError, SyntaxError => e
    raise Gem::InstallError,
          "The specification for #{spec.full_name} is corrupt (#{e.class})"
  end
end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 801
  def explicit_version_requirement(name)
    code = "version = str"
    return code unless name == "bundler"
    code += <<-TEXT
    ENV['BUNDLER_VERSION'] = str
TEXT
  end
             
            Extracts only the bin/ files from the gem into the gem directory. This is used by default gems to allow a gem-aware stub to function without the full gem installed.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 870
def extract_bin
  @package.extract_files gem_dir, "#{spec.bindir}/*"
end
             
            Reads the file index and extracts each file into the gem directory.
Ensures that files can't be installed outside the gem directory.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 861
def extract_files
  @package.extract_files gem_dir
end
             
            Prefix and suffix the program filename the same as ruby.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 877
def formatted_program_filename(filename)
  if @format_executable
    self.class.exec_format % File.basename(filename)
  else
    filename
  end
end
             
            Filename of the gem being installed.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 898
def gem
  @package.gem.path
end
             
            Lazy accessor for the spec's gem directory.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 279
def gem_dir
  @gem_dir ||= File.join(gem_home, "gems", spec.full_name)
end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 792
  def gemdeps_load(name)
    return '' if name == "bundler"
    <<-TEXT
Gem.use_gemdeps
TEXT
  end
             
            Creates the scripts to run the applications in the gem.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 543
def generate_bin_script(filename, bindir)
  bin_script_path = File.join bindir, formatted_program_filename(filename)
  require 'fileutils'
  FileUtils.rm_f bin_script_path # prior install may have been --no-wrappers
  File.open bin_script_path, 'wb', 0755 do |file|
    file.print app_script_text(filename)
    file.chmod(options[:prog_mode] || 0755)
  end
  verbose bin_script_path
  generate_windows_script filename, bindir
end
             
            Creates the symlinks to run the applications in the gem. Moves the symlink if the gem being installed has a newer version.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 563
def generate_bin_symlink(filename, bindir)
  src = File.join gem_dir, spec.bindir, filename
  dst = File.join bindir, formatted_program_filename(filename)
  if File.exist? dst
    if File.symlink? dst
      link = File.readlink(dst).split File::SEPARATOR
      cur_version = Gem::Version.create(link[-3].sub(/^.*-/, ''))
      return if spec.version < cur_version
    end
    File.unlink dst
  end
  FileUtils.symlink src, dst, :verbose => Gem.configuration.really_verbose
rescue NotImplementedError, SystemCallError
  alert_warning "Unable to use symlinks, installing wrapper"
  generate_bin_script filename, bindir
end
             
            Creates windows .bat files for easy running of commands
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 475
def generate_windows_script(filename, bindir)
  if Gem.win_platform?
    script_name = formatted_program_filename(filename) + ".bat"
    script_path = File.join bindir, File.basename(script_name)
    File.open script_path, 'w' do |file|
      file.puts windows_stub_script(bindir, filename)
    end
    verbose script_path
  end
end
             
            Installs the gem and returns a loaded Gem::Specification for the installed gem.
The gem will be installed with the following structure:
@gem_home/ cache/<gem-version>.gem #=> a cached copy of the installed gem gems/<gem-version>/... #=> extracted files specifications/<gem-version>.gemspec #=> the Gem::Specification
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 303
def install
  pre_install_checks
  run_pre_install_hooks
  # Set loaded_from to ensure extension_dir is correct
  if @options[:install_as_default]
    spec.loaded_from = default_spec_file
  else
    spec.loaded_from = spec_file
  end
  # Completely remove any previous gem files
  FileUtils.rm_rf gem_dir
  FileUtils.rm_rf spec.extension_dir
  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p gem_dir, :mode => dir_mode && 0755
  if @options[:install_as_default]
    extract_bin
    write_default_spec
  else
    extract_files
    build_extensions
    write_build_info_file
    run_post_build_hooks
  end
  generate_bin
  generate_plugins
  unless @options[:install_as_default]
    write_spec
    write_cache_file
  end
  File.chmod(dir_mode, gem_dir) if dir_mode
  say spec.post_install_message if options[:post_install_message] && !spec.post_install_message.nil?
  Gem::Specification.reset
  run_post_install_hooks
  spec
# TODO This rescue is in the wrong place. What is raising this exception?
# move this rescue to around the code that actually might raise it.
rescue Zlib::GzipFile::Error
  raise Gem::InstallError, "gzip error installing #{gem}"
end
             
            True if the gems in the system satisfy dependency.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 422
def installation_satisfies_dependency?(dependency)
  return true if @options[:development] and dependency.type == :development
  return true if installed_specs.detect {|s| dependency.matches_spec? s }
  return false if @only_install_dir
  not dependency.matching_specs.empty?
end
             
            Return an Array of Specifications contained within the gem_home we'll be installing into.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 392
def installed_specs
  @specs ||= begin
    specs = []
    Gem::Util.glob_files_in_dir("*.gemspec", File.join(gem_home, "specifications")).each do |path|
      spec = Gem::Specification.load path.tap(&Gem::UNTAINT)
      specs << spec if spec
    end
    specs
  end
end
             
            Performs various checks before installing the gem such as the install repository is writable and its directories exist, required Ruby and rubygems versions are met and that dependencies are installed.
Version and dependency checks are skipped if this install is forced.
The dependent check will be skipped if the install is ignoring dependencies.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 911
def pre_install_checks
  verify_gem_home
  # The name and require_paths must be verified first, since it could contain
  # ruby code that would be eval'ed in #ensure_loadable_spec
  verify_spec
  ensure_loadable_spec
  if options[:install_as_default]
    Gem.ensure_default_gem_subdirectories gem_home
  else
    Gem.ensure_gem_subdirectories gem_home
  end
  return true if @force
  ensure_dependencies_met unless @ignore_dependencies
  true
end
             
            Generates a #! line for bin_file_name's wrapper copying arguments if necessary.
If the :custom_shebang config is set, then it is used as a template for how to create the shebang used for to run a gem's executables.
The template supports 4 expansions:
$env the path to the unix env utility $ruby the path to the currently running ruby interpreter $exec the path to the gem's executable $name the name of the gem the executable is for
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 597
def shebang(bin_file_name)
  path = File.join gem_dir, spec.bindir, bin_file_name
  first_line = File.open(path, "rb") {|file| file.gets } || ""
  if first_line.start_with?("#!")
    # Preserve extra words on shebang line, like "-w".  Thanks RPA.
    shebang = first_line.sub(/\A\#!.*?ruby\S*((\s+\S+)+)/, "#!#{Gem.ruby}")
    opts = $1
    shebang.strip! # Avoid nasty ^M issues.
  end
  if which = Gem.configuration[:custom_shebang]
    # replace bin_file_name with "ruby" to avoid endless loops
    which = which.gsub(/ #{bin_file_name}$/," #{ruby_install_name}")
    which = which.gsub(/\$(\w+)/) do
      case $1
      when "env"
        @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
      when "ruby"
        "#{Gem.ruby}#{opts}"
      when "exec"
        bin_file_name
      when "name"
        spec.name
      end
    end
    "#!#{which}"
  elsif @env_shebang
    # Create a plain shebang line.
    @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
    "#!#{@env_path} #{ruby_install_name}"
  else
    "#{bash_prolog_script}#!#{Gem.ruby}#{opts}"
  end
end
             
            Lazy accessor for the installer's spec.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 286
def spec
  @package.spec
rescue Gem::Package::Error => e
  raise Gem::InstallError, "invalid gem: #{e.message}"
end
             
            The location of the spec file that is installed.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 442
def spec_file
  File.join gem_home, "specifications", "#{spec.full_name}.gemspec"
end
             
            Unpacks the gem into the given directory.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 432
def unpack(directory)
  @gem_dir = directory
  extract_files
end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 725
def verify_spec
  unless spec.name =~ Gem::Specification::VALID_NAME_PATTERN
    raise Gem::InstallError, "#{spec} has an invalid name"
  end
  if spec.raw_require_paths.any?{|path| path =~ /\R/ }
    raise Gem::InstallError, "#{spec} has an invalid require_paths"
  end
  if spec.extensions.any?{|ext| ext =~ /\R/ }
    raise Gem::InstallError, "#{spec} has an invalid extensions"
  end
  if spec.platform.to_s =~ /\R/
    raise Gem::InstallError, "#{spec.platform} is an invalid platform"
  end
  unless spec.specification_version.to_s =~ /\A\d+\z/
    raise Gem::InstallError, "#{spec} has an invalid specification_version"
  end
  if spec.dependencies.any? {|dep| dep.type != :runtime && dep.type != :development }
    raise Gem::InstallError, "#{spec} has an invalid dependencies"
  end
  if spec.dependencies.any? {|dep| dep.name =~ /(?:\R|[<>])/ }
    raise Gem::InstallError, "#{spec} has an invalid dependencies"
  end
end
             
            return the stub script text used to launch the true Ruby script
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 814
  def windows_stub_script(bindir, bin_file_name)
    rb_topdir = RbConfig::TOPDIR || File.dirname(rb_config["bindir"])
    # get ruby executable file name from RbConfig
    ruby_exe = "#{rb_config['RUBY_INSTALL_NAME']}#{rb_config['EXEEXT']}"
    ruby_exe = "ruby.exe" if ruby_exe.empty?
    if File.exist?(File.join bindir, ruby_exe)
      # stub & ruby.exe within same folder.  Portable
      <<-TEXT
@ECHO OFF
@"%~dp0#{ruby_exe}" "%~dpn0" %*
      TEXT
    elsif bindir.downcase.start_with? rb_topdir.downcase
      # stub within ruby folder, but not standard bin.  Portable
      require 'pathname'
      from = Pathname.new bindir
      to   = Pathname.new "#{rb_topdir}/bin"
      rel  = to.relative_path_from from
      <<-TEXT
@ECHO OFF
@"%~dp0#{rel}/#{ruby_exe}" "%~dpn0" %*
      TEXT
    else
      # outside ruby folder, maybe -user-install or bundler.  Portable, but ruby
      # is dependent on PATH
      <<-TEXT
@ECHO OFF
@#{ruby_exe} "%~dpn0" %*
      TEXT
    end
  end
             
            Writes the file containing the arguments for building this gem's extensions.
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 937
def write_build_info_file
  return if build_args.empty?
  build_info_dir = File.join gem_home, 'build_info'
  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p build_info_dir, :mode => dir_mode && 0755
  build_info_file = File.join build_info_dir, "#{spec.full_name}.info"
  File.open build_info_file, 'w' do |io|
    build_args.each do |arg|
      io.puts arg
    end
  end
  File.chmod(dir_mode, build_info_dir) if dir_mode
end
             
            Writes the .gem file to the cache directory
 
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 959
def write_cache_file
  cache_file = File.join gem_home, 'cache', spec.file_name
  @package.copy_to cache_file
end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 995
  def bash_prolog_script
    if load_relative_enabled?
      script = +<<~EOS
        bindir="${0%/*}"
      EOS
      script << %Q(exec "$bindir/#{ruby_install_name}" "-x" "$0" "$@"\n)
      <<~EOS
        #!/bin/sh
        # -*- ruby -*-
        _=_\\
        =begin
        #{script.chomp}
        =end
      EOS
    else
      ""
    end
  end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 976
def build_args
  @build_args ||= begin
                    require_relative "command"
                    Gem::Command.build_args
                  end
end
             
             
               # File ruby-3.1.2/lib/rubygems/installer.rb, line 991
def load_relative_enabled?
  rb_config["LIBRUBY_RELATIVE"] == 'yes'
end