How to write BOM marker to a file in Ruby

Question

I have some working code with a crutch to add BOM marker to a new file.

  #writing
  File.open name, 'w', 0644 do |file|
    file.write "\uFEFF"
    file.write @data
  end

  #reading
  File.open name, 'r:bom|utf-8' do |file|
    file.read
  end

Is there any way to automatically add the marker without writing cryptic "\uFEFF" before the data? Something like File.open name, 'w:bom' # this mode has no effect maybe?

Answer 1

**** This answer lead to a new gem: file_with_bom ****

I had the similar problem in the past and I extended File.open with additional encoding variants for the w-mode:

class File
  BOM_LIST_hex = {
      Encoding::UTF_8      => "\xEF\xBB\xBF", #"\uEFBBBF"
      Encoding::UTF_16BE => "\xFE\xFF", #"\uFEFF",
      Encoding::UTF_16LE => "\xFF\xFE",
      Encoding::UTF_32BE => "\x00\x00\xFE\xFF",
      Encoding::UTF_32LE => "\xFE\xFF\x00\x00",
    }
  BOM_LIST_hex.freeze
  def utf_bom_hex(encoding = external_encoding)
    BOM_LIST_hex[encoding]
  end

class << self
  alias :open_old :open
  def open(filename, mode_string = 'r', options = {}, &block)
    #check for bom-flag in mode_string
    options[:bom] = true if mode_string.sub!(/-bom/i,'')

    f = open_old(filename, mode_string, options)
    if options[:bom]
      case mode_string
        #r|bom already standard since 1.9.2
        when /\Ar/   #read mode -> remove BOM
          #remove BOM
          bom = f.read(f.utf_bom_hex.bytesize) 
          #check, if it was really a bom
          if bom != f.utf_bom_hex.force_encoding(bom.encoding)
            f.rewind  #return to position 0 if BOM was no BOM
          end
        when /\Aw/  #write mode -> attach BOM
          f = open_old(filename, mode_string, options)
          f << f.utf_bom_hex.force_encoding(f.external_encoding)
        end #mode_string
    end

    if block_given?
      yield f 
      f.close
    end
  end
  end
end #File

Testcode:

EXAMPLE_TEXT = 'some content öäü'
File.open("file_utf16le.txt", "w:utf-16le|bom"){|f| f << EXAMPLE_TEXT }
File.open("file_utf16le.txt", "r:utf-16le|bom:utf-8"){|f| p f.read }
File.open("file_utf16le.txt", "r:utf-16le:utf-8",  :bom => true ){|f| p f.read }
File.open("file_utf16le.txt", "r:utf-16le:utf-8"){|f| p f.read }

File.open("file_utf8.txt", "w:utf-8", :bom => true ){|f| f << EXAMPLE_TEXT }
File.open("file_utf8.txt", "r:utf-8", :bom => true ){|f| p f.read }
File.open("file_utf8.txt", "r:utf-8|bom",              ){|f| p f.read }
File.open("file_utf8.txt", "r:utf-8",                     ){|f| p f.read }

Some remarks:

• The code is from pre 1.9-times (but it still works).
• I used -bom as a bom indicator (ruby 1.9 uses |bom.

Some needed fixes to be better:

• use |bom instead -bom
• use the standard r|bom for reading
• make it ruby 1.8 and 1.9 enabled

Perhaps I will find some time tomorrow to refactor my code and provide it as a gem.