Information about dos2unix' implementation choices.

1. Smart conversion
===================

  There are some dos2unix implementations that automatically convert all type of
line breaks. For instance converting both DOS and Mac linebreaks to Unix line
breaks at once. Or automatically detect the line break type and convert to the
other side.

  Smart conversions could lead to unexpected behaviour. For instance when a
dos2unix is run on a file with only Unix line breaks and the line breaks are
flipped to the other side. This dos2unix implementation does exactly what you
tell it to do. When you run 'dos2unix' only DOS line breaks are converted to
Unix line breaks. Unix line breaks stay in the file. Seen from a DOS or Unix
perspective, a Mac line break is not a line break, so also Mac line breaks stay
untouched.  The same applies for mac2unix. Mac2unix leaves Unix and DOS line
breaks untouched.


2. Unix filter
==============

  When a standard Unix filter, e.g. sed or sort, reads input from a file it sends
its output by default to standard out. This implementation of dos2unix does by
default in-place conversion (overwriting the input file), which seems not in line.

  Dos2unix is not part of the Unix standard. Most Unixes have their
own implementation of dos2unix. There is a lot of variation in command names,
options, and behavior. The SunOS version of dos2unix, after which this version was
modeled, does by default paired conversion.
  This implementation of dos2unix has too much legacy to change the current
default behaviour.  Changing it would have more disadvantages than advantages.
Most people expect dos2unix to do in-place conversion. The majority of other
open source implementations also convert by default in-place. In-place
conversion has the advantage that it is very easy to convert multiple files by
using wild cards. Note that dos2unix/unix2dos is also used a lot on non-Unix
operating systems where the filter idea is less known.
  Since version 7.5.0 dos2unix has option -O to write to standard output like
a Unix filter.


3. Recursive conversion of files
================================

  There are implementations that have builtin functionality to do recursive
conversion of all files in a directory tree.

  This functionality is not needed in dos2unix. By using an external program,
like Unix 'find', you can do recursive conversion of directory trees. There is
no need to duplicate this.


4. Encoding conversion
======================

  Dos2unix can do several encoding conversions. First there are the conversions
of several DOS code pages to and from ISO-8859-1. These conversions are also
part of the SunOS dos2unix implementation after which this implementation has
been modeled. Although these conversions are not much used these days they have
been added for the sake of completeness. Conversion of CP1252 was added, because
it is used a lot in the Western world. It's almost identical to ISO-8859-1. There
is no intention to add other conversions to and from ISO-8859-1.

  Conversion from UTF-16 was added, because the world is moving towards
Unicode.  Microsoft Windows uses by default UTF-16 format for Unicode. UTF-16
is part of Windows' core design for historical reasons. Microsoft standardized
on UCS-2, a predecessor of UTF-16, in a time when UTF-8 did not exist yet.
However a lot of Windows software is able to read UTF-8 files. In Windows
"Unicode" means usually UTF-16. For instance saving a file with Notepad in
"Unicode" encoding means in UTF-16 encoding. When you work in PowerShell and
echo some text to a file you get an UTF-16 encoded text file. UTF-16 is there
to stay, although many people would like to see otherwise and are dreaming of
an UTF-8 only world. The Unix/Linux world is moving towards UTF-8 encoding,
because it's backwards compatible with ASCII. Unix programs typically do not
support UTF-16.

  One end of the encoding spectrum is an ASCII only world, where the only
differences between DOS and Unix text files are line breaks. In English
speaking regions this is a good working environment, because ASCII is in
practice sufficient for English language. Diacritics are hardly used and can be
omitted. The other end of the spectrum is an Unicode only world. All languages
of the world are supported. Dos2unix aims to support these two ends of the
spectrum: ASCII and Unicode. The Chinese GB18030 encoding is also seen as an
Unicode transformation format. UTF-32 is not supported, because this is
practically only used as an internal format.  Other encoding transformations
are left to specialized programs like iconv and recode. The few conversion
modes to and from ISO-8859-1 are only there for legacy reasons.

  In the ASCII days DOS to Unix text file conversion, and vice versa, was only
converting line breaks.  In the Unicode era it is not only line break
conversion, but also Unicode transformation format conversion (e.g. UTF-16 to
UTF-8), and Byte Order Mark (BOM) removal or addition.

  Conversion towards UTF-16 is not supported and there is no intention to support
it in the future. UTF-8 encoded files are well supported on Windows, so
conversion to UTF-16 is not needed. And we keep on dreaming of an UTF-8 only
world...
