File Format

개요

'fileformat' ('ff' for short) controls the way that Vim handles different line-ending sequences.
Vim recognizes three file formats: unix, dos, and mac.
Each of these file formats differs in the line-ending character sequences used on disk.

Assuming that the fileformat has been properly detected (see below),
it is very simple to convert to a different format.
Simply set 'ff' to the desired format name; for example,
if you are editing a dos file and want to convert to unix, just do :set ff=unix.
That's it. The next time the file is written, unix line ends will be used.
Things are a little tougher when your file has different ending sequences on different lines;
this situation is discussed below

As absolon was kind enough to point out in #vim, you can do mass conversion from the shell like so
```
vim +"argdo set ff=<format>" +wqa <files>
```
This will set 'fileformat' for each file in the argument list, then quit, saving all files.
See :help argdo and :help -c if you need more detail on what this does and why.

Basically, Vim tries to find a line-ending sequence which appears at the end of every line,
and then uses that for 'ff'.
This process can get fooled in two ways:
1. The true fileformat is not found in 'fileformats',
  eg a Mac file on a Unix platform using the defaults
2. There is a mix of line-ending sequences.
  This can happen when mixing tools made for different platforms.

When 'ff' is not detected correctly,
you will usually see part or all of the line ending sequences in the editor window.
Here is a short illustration of what you see when a mismatch happens.

If you have extra leading or trailing characters (^M in unix or ^J in mac), use :%s/\r// to remove them.
If you have long lines (mac line-ends with ff=dos or unix, unix line ends with ff=mac), use :%s/\r/\r/g to replace the wrong line-ends with the correct line-ends.
If you are curious, the reasoning behind this methodology appears below.

:substitute can be tricky when dealing with CR and NL.
The problem stems from the fact that Vim uses NL in memory to represent a Nul character (see :help NL-used-for-Nul).
The result of this is that unexpected things tend to happen when you try to modify line-endings using :s.
Vim help says that \r matches CR and \n matches an end-of-line.
In fact, when you use :s, \r will match CR or LF, depending on the fileformat!
Basically, it matches whichever control character (^M or ^J) you see in the editor window.
Furthermore, in the replace pattern, \n will expand to a LF, which then gets interpreted as… a Nul! (see :help sub-replace-special).
Fortunately, when \r is used in the replace pattern, it is interpreted as a line-end, which will then display (and get written) as desired.