diff --git a/bin/magic_frozen_string_literal b/bin/magic_frozen_string_literal
index d3f8504..a15c53c 100755
--- a/bin/magic_frozen_string_literal
+++ b/bin/magic_frozen_string_literal
@@ -3,5 +3,19 @@
 # A simple tool to prepend magic '# frozen_string_literal: true' comments to multiple ".rb" files
 
 require_relative '../lib/add_magic_comment'
+require "optparse"
 
-AddMagicComment.process(ARGV)
+process_files = false
+OptionParser.new do |opts|
+  opts.banner = "Usage: magic_frozen_string_literal app/ lib/"
+
+  opts.on("-f", "--files", "Apply to specific files instead of files in directories") do |v|
+    process_files = true
+  end
+end.parse!
+
+if process_files
+  AddMagicComment.process_files_at(ARGV)
+else
+  AddMagicComment.process(ARGV)
+end
diff --git a/lib/add_magic_comment.rb b/lib/add_magic_comment.rb
index 713ad15..759b551 100644
--- a/lib/add_magic_comment.rb
+++ b/lib/add_magic_comment.rb
@@ -18,6 +18,7 @@ module AddMagicComment
     "*.gemspec"   => "# #{MAGIC_COMMENT}\n\n",
     "*.rabl"      => "# #{MAGIC_COMMENT}\n\n",
     "*.jbuilder"  => "# #{MAGIC_COMMENT}\n\n",
+    "*.pbbuilder" => "# #{MAGIC_COMMENT}\n\n",
     "*.haml"      => "-# #{MAGIC_COMMENT}\n",
     "*.slim"      => "-# #{MAGIC_COMMENT}\n"
   }
@@ -25,41 +26,59 @@ module AddMagicComment
   def self.process(argv)
     directory = argv.first || Dir.pwd
 
-    count = 0
+
+    touched_paths = []
     EXTENSION_COMMENTS.each do |pattern, comment|
-      filename_pattern = File.join(directory, "**", "#{pattern}")
-      Dir.glob(filename_pattern).each do |filename|
-        File.open(filename, "rb+") do |file|
-          lines = file.readlines
-          newline = detect_newline(lines.first)
-          next unless lines.any?
-          count += 1
-
-          if lines.first =~ SHEBANG_PATTERN
-            shebang = lines.shift
-          end
-
-          # remove current magic comment(s)
-          while lines.first && (lines.first.match(MAGIC_COMMENT_PATTERN) || lines.first.match(EMPTY_LINE_PATTERN))
-            lines.shift
-          end
-
-          # add magic comment as the first line
-          lines.unshift(comment.gsub("\n", newline))
-
-          # put shebang back
-          if shebang
-            lines.unshift(shebang)
-          end
-
-          file.pos = 0
-          file.print(*lines)
-          file.truncate(file.pos)
-        end
+      file_path_pattern = File.join(directory, "**", "#{pattern}")
+      Dir.glob(file_path_pattern).each do |file_path|
+        process_file_at(file_path, comment, touched_paths)
+      end
+    end
+
+    puts "Magic comments added to #{touched_paths.size} source file(s)"
+  end
+
+  def self.process_file_at(path, comment, touched_paths)
+    File.open(path, "rb+") do |file|
+      lines = file.readlines
+      newline = detect_newline(lines.first)
+      return unless lines.any?
+
+      if lines.first =~ SHEBANG_PATTERN
+        shebang = lines.shift
+      end
+
+      # remove current magic comment(s)
+      while lines.first && (lines.first.match(MAGIC_COMMENT_PATTERN) || lines.first.match(EMPTY_LINE_PATTERN))
+        lines.shift
+      end
+
+      # add magic comment as the first line
+      lines.unshift(comment.gsub("\n", newline))
+
+      # put shebang back
+      if shebang
+        lines.unshift(shebang)
+      end
+
+      file.pos = 0
+      file.print(*lines)
+      file.truncate(file.pos)
+    end
+    touched_paths << path
+  end
+
+  def self.process_files_at(paths)
+    touched_paths = []
+    paths.each do |path|
+      matching_pattern_and_comment = EXTENSION_COMMENTS.find do |(glob_pattern, _comment)|
+        File.fnmatch(glob_pattern, path)
       end
+      _, comment = matching_pattern_and_comment
+      process_file_at(path, comment, touched_paths) if comment
     end
 
-    puts "Magic comments added to #{count} source file(s)"
+    puts "Magic comments added to #{touched_paths.size} source file(s)"
   end
 
   def self.detect_newline(line)
diff --git a/spec/acceptance_spec.rb b/spec/acceptance_spec.rb
index 2bea2ab..df7f7ff 100644
--- a/spec/acceptance_spec.rb
+++ b/spec/acceptance_spec.rb
@@ -14,7 +14,7 @@
     }
   end
 
-  it "inserts magic comments where expected", :aggregate_failures do
+  it "inserts magic comments across directories", :aggregate_failures do
     AddMagicComment.process([directories[:test]])
 
     Dir["#{directories[:test]}/*"].each do |path|
@@ -22,6 +22,18 @@
     end
   end
 
+  it "inserts magic comments just into specific files", :aggregate_failures do
+    paths = Dir.glob(directories[:test] + "/*.rb")
+    AddMagicComment.process_files_at(paths)
+
+    rb_paths = Dir["#{directories[:test]}/*.rb"]
+    rb_paths.each do |path|
+      assert_file_matches_expected(File.basename(path))
+    end
+
+    assert_file_not_touched("Rakefile")
+  end
+
   def setup_test_files
     FileUtils.mkdir_p(File.dirname(directories[:test]))
     FileUtils.cp_r(directories[:input], directories[:test])
@@ -31,6 +43,12 @@ def teardown_test_files
     FileUtils.rm_rf(directories[:test])
   end
 
+  def assert_file_not_touched(filename)
+    processed_path = File.join(directories[:test], filename)
+    expected_path = File.join(directories[:expected], filename)
+    expect(FileUtils).not_to be_identical(processed_path, expected_path)
+  end
+
   def assert_file_matches_expected(filename)
     paths = {
       test: File.join(directories[:test], filename),
diff --git a/spec/fixtures/expected/shebang_already_commented.rb b/spec/fixtures/expected/shebang_already_commented.rb
new file mode 100755
index 0000000..3795ca2
--- /dev/null
+++ b/spec/fixtures/expected/shebang_already_commented.rb
@@ -0,0 +1,4 @@
+#!/usr/bin/ruby
+# frozen_string_literal: true
+
+puts "hello"
diff --git a/spec/fixtures/input/Gemfile b/spec/fixtures/input/Gemfile
index 25115d0..c036588 100644
--- a/spec/fixtures/input/Gemfile
+++ b/spec/fixtures/input/Gemfile
@@ -1,5 +1,3 @@
-# frozen_string_literal: true
-
 source 'https://rubygems.org'
 
 gem 'magic_frozen_string_literal'
diff --git a/spec/fixtures/input/Rakefile b/spec/fixtures/input/Rakefile
index f039a8b..1737b57 100644
--- a/spec/fixtures/input/Rakefile
+++ b/spec/fixtures/input/Rakefile
@@ -1,5 +1,3 @@
-# frozen_string_literal: true
-
 require "bundler/gem_tasks"
 
 task :test do
diff --git a/spec/fixtures/input/blank_line.rb b/spec/fixtures/input/blank_line.rb
index ae917d5..bbad01b 100644
--- a/spec/fixtures/input/blank_line.rb
+++ b/spec/fixtures/input/blank_line.rb
@@ -1,3 +1 @@
-# frozen_string_literal: true
-
 puts "hello"
diff --git a/spec/fixtures/input/config.ru b/spec/fixtures/input/config.ru
index 842bccc..f7ba0b5 100644
--- a/spec/fixtures/input/config.ru
+++ b/spec/fixtures/input/config.ru
@@ -1,5 +1,3 @@
-# frozen_string_literal: true
-
 # This file is used by Rack-based servers to start the application.
 
 require_relative 'config/environment'
diff --git a/spec/fixtures/input/shebang.rb b/spec/fixtures/input/shebang.rb
old mode 100644
new mode 100755
index 3795ca2..e69af1e
--- a/spec/fixtures/input/shebang.rb
+++ b/spec/fixtures/input/shebang.rb
@@ -1,4 +1,3 @@
 #!/usr/bin/ruby
-# frozen_string_literal: true
 
 puts "hello"
diff --git a/spec/fixtures/input/shebang_already_commented.rb b/spec/fixtures/input/shebang_already_commented.rb
new file mode 100755
index 0000000..3795ca2
--- /dev/null
+++ b/spec/fixtures/input/shebang_already_commented.rb
@@ -0,0 +1,4 @@
+#!/usr/bin/ruby
+# frozen_string_literal: true
+
+puts "hello"
diff --git a/spec/fixtures/input/shebang_blank_line.rb b/spec/fixtures/input/shebang_blank_line.rb
old mode 100644
new mode 100755
index 3795ca2..e69af1e
--- a/spec/fixtures/input/shebang_blank_line.rb
+++ b/spec/fixtures/input/shebang_blank_line.rb
@@ -1,4 +1,3 @@
 #!/usr/bin/ruby
-# frozen_string_literal: true
 
 puts "hello"
diff --git a/spec/fixtures/input/t1.haml b/spec/fixtures/input/t1.haml
index aeaeb94..d1ded0b 100644
--- a/spec/fixtures/input/t1.haml
+++ b/spec/fixtures/input/t1.haml
@@ -1,2 +1 @@
--# frozen_string_literal: true
 <b>Hello!</b>
diff --git a/spec/fixtures/input/t1.rb b/spec/fixtures/input/t1.rb
index ae917d5..bbad01b 100644
--- a/spec/fixtures/input/t1.rb
+++ b/spec/fixtures/input/t1.rb
@@ -1,3 +1 @@
-# frozen_string_literal: true
-
 puts "hello"
diff --git a/spec/fixtures/input/t1.slim b/spec/fixtures/input/t1.slim
index aeaeb94..d1ded0b 100644
--- a/spec/fixtures/input/t1.slim
+++ b/spec/fixtures/input/t1.slim
@@ -1,2 +1 @@
--# frozen_string_literal: true
 <b>Hello!</b>
diff --git a/spec/fixtures/input/utf8.rb b/spec/fixtures/input/utf8.rb
index 904b2ba..0010968 100644
--- a/spec/fixtures/input/utf8.rb
+++ b/spec/fixtures/input/utf8.rb
@@ -1,3 +1 @@
-# frozen_string_literal: true
-
 puts " "