(emacs.info) MS-DOS File Names

 
 File Names on MS-DOS
 ====================
 
    MS-DOS normally uses a backslash, `\', to separate name units within
 a file name, instead of the slash used on other systems.  Emacs on
 MS-DOS permits use of either slash or backslash, and also knows about
 drive letters in file names.
 
    On MS-DOS, file names are case-insensitive and limited to eight
 characters, plus optionally a period and three more characters.  Emacs
 knows enough about these limitations to handle file names that were
 meant for other operating systems.  For instance, leading dots `.' in
 file names are invalid in MS-DOS, so Emacs transparently converts them
 to underscores `_'; thus your default init file ( Init File) is
 called `_emacs' on MS-DOS.  Excess characters before or after the
 period are generally ignored by MS-DOS itself; thus, if you visit the
 file `LongFileName.EvenLongerExtension', you will silently get
 `longfile.eve', but Emacs will still display the long file name on the
 mode line.  Other than that, it's up to you to specify file names which
 are valid under MS-DOS; the transparent conversion as described above
 only works on file names built into Emacs.
 
    The above restrictions on the file names on MS-DOS make it almost
 impossible to construct the name of a backup file ( Backup
 Names) without losing some of the original file name characters.  For
 example, the name of a backup file for `docs.txt' is `docs.tx~' even if
 single backup is used.
 
    If you run Emacs as a DOS application under Windows 9X, you can turn
 on support for long file names.  If you do that, Emacs doesn't truncate
 file names or convert them to lower case; instead, it uses the file
 names that you specify, verbatim.  To enable long file name support,
 set the environment variable `LFN' to `y' before starting Emacs.
 Unfortunately, Windows NT doesn't allow DOS programs to access long
 file names, so Emacs built for MS-DOS will only see their short 8+3
 aliases.
 
    MS-DOS has no notion of home directory, so Emacs on MS-DOS pretends
 that the directory where it is installed is the value of `HOME'
 environment variable.  That is, if your Emacs binary, `emacs.exe', is
 in the directory `c:/utils/emacs/bin', then Emacs acts as if `HOME'
 were set to `c:/utils/emacs'.  In particular, that is where Emacs looks
 for the init file `_emacs'.  With this in mind, you can use `~' in file
 names as an alias for the home directory, as you would in Unix.  You
 can also set `HOME' variable in the environment before starting Emacs;
 its value will then override the above default behavior.
 
    Emacs on MS-DOS handles the directory name `/dev' specially, because
 of a feature in the emulator libraries of DJGPP that pretends I/O
 devices have names in that directory.  We recommend that you avoid
 using an actual directory named `/dev' on any disk.
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