Special filenames
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## Special filenames

### DOS devices

Filenames invalid under Win32 are not necessarily invalid under Cygwin since release 1.7.0. There are a couple of rules which apply to Windows filenames. First of all, DOS device names like `AUX`, `COM1`, `LPT1` or `PRN` (to name a few) cannot be used in a native Win32 application, even with an extension (`prn.txt`). Cygwin can handle files with these names just fine.

### Special characters in filenames

Win32 filenames can't contain trailing dots and spaces for backward compatibility. When trying to create files with trailing dots or spaces, all of them are removed before the file is created. This restriction does only affect native Win32 applications. Cygwin applications can create and access files with trailing dots and spaces without problems.

Some characters are disallowed in filenames on Windows filesystems:

      "   *   :   <   >   ?   |   \

Cygwin can't fix this, but it has a method to workaround this restriction. All of the above characters, except for the backslash, are converted to special UNICODE characters in the range 0xf000 to 0xf0ff (the "Private use area") when creating or accessing files.

### Case sensitive filenames

In the Win32 subsystem filenames are only case-preserved, but not case-sensitive. You can't access two files in the same directory which only differ by case, like `Abc` and `aBc`. While NTFS (and some remote filesystems) support case-sensitivity, the NT kernel starting with Windows XP does not support it by default. Rather, you have to tweak a registry setting and reboot. For that reason, case-sensitivity is not supported by Cygwin, unless you change that registry value.

If you really want case-sensitivity in Cygwin, you can switch it on by setting the registry value

    HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\kernel\obcaseinsensitive

to 0 and reboot the machine. For least surprise, Cygwin expects this registry value also on Windows NT4 and Windows 2000, which usually both don't know this registry key. If you want case-sensitivity on these systems, create that registry value and set it to 0. On these systems (and *only* on these systems) you don't have to reboot to bring it into effect.

### Note

Note that when installing Microsoft's Services For Unix (SFU), you're asked if you want to use case-sensitive filenames. If you answer "yes" at this point, the installer will change the aforementioned registry value to 0, too. So, if you have SFU installed, there's some chance that the registry value is already set to case sensitivity.

After you set this registry value to 0, Cygwin will be case-sensitive by default on NTFS and NFS filesystems. Be aware that using two filenames which only differ by case might result in some weird interoperability issues with native Win32 applications. You're using case-sensitivity at your own risk. You have been warned!

Even if you use case-sensitivity, it might be feasible to switch to case-insensitivity for certain paths for better interoperability with native Win32 applications (even if it's just Windows Explorer). You can do this on a per-mount point base, by using the "posix=0" mount option in /etc/fstab, or your /etc/fstab.d/$USER file.

For a start, it might be best to switch the cygdrive path to case-insensitivity, because the default Windows $PATH variable is not always using the correct case by default. As a result, your shell will claim that it can't find Windows commands like attrib or net. Here's an example how you can switch the cygdrive prefix to case-insensitivity:

Example 3.2. Example mount point to enforce case-insensitivity on cygdrive paths

    none /cygdrive cygdrive binary,posix=0 0 0

Note that mount points as well as device names and virtual paths like /proc are always case-sensitive! The only exception are the subdirs and filenames under /proc/registry, /proc/registry32 and /proc/registry64. Registry access is always case-insensitive. Read on for more information.

### POSIX devices

There is no need to create a POSIX `/dev` directory as Cygwin automatically simulates it internally. These devices cannot be seen with the command ls /dev/ although commands such as ls /dev/tty work fine. If you want to be able to see all devices in `/dev/`, you can use Igor Pechtchanski's create_devices.sh script.

Cygwin supports the following character devices commonly found on POSIX systems:

    /dev/null
    /dev/zero
    /dev/full

    /dev/console	Pseudo device name for the standard console window created
    		by Windows.  Same as the one used for cmd.exe.  Every one
    		of them has this name.  It's not quite comparable with the
    		console device on UNIX machines.

    /dev/tty	The current tty of a session running in a pseudo tty.
    /dev/ptmx	Pseudo tty master device.
    /dev/ttym

    /dev/tty0	Pseudo ttys are numbered from /dev/tty0 upwards as they are
    /dev/tty1	requested.
    ...

    /dev/ttyS0	Serial communication devices.  ttyS0 == Win32 COM1,
    /dev/ttyS1	ttyS1 == COM2, etc.
    ...

    /dev/pipe
    /dev/fifo

    /dev/mem	The physical memory of the machine.  Note that access to the
    /dev/port	physical memory has been restricted with Windows Server 2003.
    /dev/kmem	Since this OS, you can't access physical memory from user space.

    /dev/kmsg	Kernel message pipe, for usage with sys logger services.

    /dev/random	Random number generator.
    /dev/urandom

    /dev/dsp	Default sound device of the system.

Cygwin also has several Windows-specific devices:

    /dev/com1	The serial ports, starting with COM1 which is the same as ttyS0.
    /dev/com2	Please use /dev/ttySx instead.
    ...

    /dev/conin	Same as Windows CONIN$.
    /dev/conout	Same as Windows CONOUT$.
    /dev/clipboard	The Windows clipboard, text only
    /dev/windows	The Windows message queue.

Block devices are accessible by Cygwin processes using fixed POSIX device names. These POSIX device names are generated using a direct conversion from the POSIX namespace to the internal NT namespace. E.g. the first harddisk is the NT internal device \device\harddisk0\partition0 or the first partition on the third harddisk is \device\harddisk2\partition1. The first floppy in the system is \device\floppy0, the first CD-ROM is \device\cdrom0 and the first tape drive is \device\tape0. The mapping to the POSIX /dev namespace is as follows:

    /dev/st0	\device\tape0, rewind
    /dev/nst0	\device\tape0, no-rewind
    /dev/st1	\device\tape1
    /dev/nst1	\device\tape1
    ...
    /dev/st15
    /dev/nst15

    /dev/fd0	\device\floppy0
    /dev/fd1	\device\floppy1
    ...
    /dev/fd15

    /dev/sr0	\device\cdrom0
    /dev/sr1	\device\cdrom1
    ...
    /dev/sr15

    /dev/scd0	\device\cdrom0
    /dev/scd1	\device\cdrom1
    ...
    /dev/scd15

    /dev/sda	\device\harddisk0\partition0	(whole disk)
    /dev/sda1	\device\harddisk0\partition1	(first partition)
    ...
    /dev/sda15	\device\harddisk0\partition15	(fifteenth partition)

    /dev/sdb	\device\harddisk1\partition0
    /dev/sdb1	\device\harddisk1\partition1

    [up to]

    /dev/sddx	\device\harddisk127\partition0
    /dev/sddx1	\device\harddisk127\partition1
    ...
    /dev/sddx15	\device\harddisk127\partition15

if you don't like these device names, feel free to create symbolic links as they are created on Linux systems for convenience:

    ln -s /dev/sr0 /dev/cdrom
    ln -s /dev/nst0 /dev/tape
    ...

### The .exe extension

Win32 executable filenames end with `.exe` but the `.exe` need not be included in the command, so that traditional UNIX names can be used. However, for programs that end in `.bat` and `.com`, you cannot omit the extension.

As a side effect, the ls filename gives information about `filename.exe` if `filename.exe` exists and `filename` does not. In the same situation the function call `stat("filename",..)` gives information about `filename.exe`. The two files can be distinguished by examining their inodes, as demonstrated below.

    C:/> ls *
    a      a.exe     b.exe
    C:/> ls -i a a.exe
    445885548 a       435996602 a.exe
    C:/> ls -i b b.exe
    432961010 b       432961010 b.exe

If a shell script `myprog` and a program `myprog.exe` coexist in a directory, the shell script has precedence and is selected for execution of myprog. Note that this was quite the reverse up to Cygwin 1.5.19. It has been changed for consistency with the rest of Cygwin.

The gcc compiler produces an executable named `filename.exe` when asked to produce `filename`. This allows many makefiles written for UNIX systems to work well under Cygwin.

### The /proc filesystem

Cygwin, like Linux and other similar operating systems, supports the `/proc` virtual filesystem. The files in this directory are representations of various aspects of your system, for example the command `cat /proc/cpuinfo` displays information such as what model and speed processor you have.

One unique aspect of the Cygwin `/proc` filesystem is `/proc/registry`, see next section.

The Cygwin `/proc` is not as complete as the one in Linux, but it provides significant capabilities. The `procps` package contains several utilities that use it.

### The /proc/registry filesystem

The `/proc/registry` filesystem provides read-only access to the Windows registry. It displays each `KEY` as a directory and each `VALUE` as a file. As anytime you deal with the Windows registry, use caution since changes may result in an unstable or broken system. There are additionally subdirectories called `/proc/registry32` and `/proc/registry64`. They are identical to `/proc/registry` on 32 bit host OSes. On 64 bit host OSes, `/proc/registry32` opens the 32 bit processes view on the registry, while `/proc/registry64` opens the 64 bit processes view.

Reserved characters ('/', '\', ':', and '%') or reserved names (`.` and `..`) are converted by percent-encoding:

    bash$ regtool list -v '\HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices'
    ...
    \DosDevices\C: (REG_BINARY) = cf a8 97 e8 00 08 fe f7
    ...
    bash$ cd /proc/registry/HKEY_LOCAL_MACHINE/SYSTEM
    bash$ ls -l MountedDevices
    ...
    -r--r----- 1 Admin SYSTEM  12 Dec 10 11:20 %5CDosDevices%5CC%3A
    ...
    bash$ od -t x1 MountedDevices/%5CDosDevices%5CC%3A
    0000000 cf a8 97 e8 00 08 fe f7 01 00 00 00

The unnamed (default) value of a key can be accessed using the filename `@`.

If a registry key contains a subkey and a value with the same name `foo`, Cygwin displays the subkey as `foo` and the value as `foo%val`.

### The @pathnames

To circumvent the limitations on shell line length in the native Windows command shells, Cygwin programs expand their arguments starting with "@" in a special way. If a file `pathname` exists, the argument `@pathname` expands recursively to the content of `pathname`. Double quotes can be used inside the file to delimit strings containing blank space. Embedded double quotes must be repeated. In the following example compare the behaviors of the bash built-in echo and of the program /bin/echo.

Example 3.3. Using @pathname

    bash$ echo  'This   is   "a     long"  line' > mylist
    bash$ echo @mylist
    @mylist
    c:\> c:\cygwin\bin\echo @mylist
    This is a     long line

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