Add custom_shebang gemrc option for shebang alteration

lib/rubygems/installer.rb

@@ -358,6 +358,17 @@ def generate_bin_symlink(filename, bindir)
   ##
   # 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
+  #
 
   def shebang(bin_file_name)
     ruby_name = Gem::ConfigMap[:ruby_install_name] if @env_shebang
@@ -371,6 +382,25 @@ def shebang(bin_file_name)
       shebang.strip! # Avoid nasty ^M issues.
     end
 
+    if which = Gem.configuration[:custom_shebang]
+      which = which.gsub(/\$(\w+)/) do
+        case $1
+        when "env"
+          @env_path ||= ENV_PATHS.find do |env_path|
+                          File.executable? env_path
+                        end
+        when "ruby"
+          "#{Gem.ruby}#{opts}"
+        when "exec"
+          bin_file_name
+        when "name"
+          spec.name
+        end
+      end
+
+      return "#!#{which}"
+    end
+
     if not ruby_name then
       "#!#{Gem.ruby}#{opts}"
     elsif opts then

test/rubygems/test_gem_installer.rb

@@ -18,8 +18,14 @@ def setup
       @user_installer.spec = @user_spec
       @user_installer.gem_home = @installer_tmp
     end
+
+    @config = Gem.configuration
   end
 
+  def teardown
+    super
+    Gem.configuration = @config
+  end
 
   def test_app_script_text
     @spec.version = 2
@@ -994,6 +1000,45 @@ def test_shebang_version_env_arguments
     assert_equal "#!#{Gem.ruby} -ws", shebang
   end
 
+  def test_shebang_custom
+    conf = Gem::ConfigFile.new []
+    conf[:custom_shebang] = 'test'
+
+    Gem.configuration = conf
+
+    util_make_exec @spec, "#!/usr/bin/ruby"
+
+    shebang = @installer.shebang 'executable'
+
+    assert_equal "#!test", shebang
+  end
+
+  def test_shebang_custom_with_expands
+    conf = Gem::ConfigFile.new []
+    conf[:custom_shebang] = '1 $env 2 $ruby 3 $exec 4 $name'
+
+    Gem.configuration = conf
+
+    util_make_exec @spec, "#!/usr/bin/ruby"
+
+    shebang = @installer.shebang 'executable'
+
+    assert_equal "#!1 /usr/bin/env 2 #{Gem.ruby} 3 executable 4 a", shebang
+  end
+
+  def test_shebang_custom_with_expands_and_arguments
+    conf = Gem::ConfigFile.new []
+    conf[:custom_shebang] = '1 $env 2 $ruby 3 $exec'
+
+    Gem.configuration = conf
+
+    util_make_exec @spec, "#!/usr/bin/ruby -ws"
+
+    shebang = @installer.shebang 'executable'
+
+    assert_equal "#!1 /usr/bin/env 2 #{Gem.ruby} -ws 3 executable", shebang
+  end
+
   def test_unpack
     util_setup_gem