cygwinmingwsymlinkntfsln

Enable native NTFS symbolic links for Cygwin

Recent NTFS and Windows implement symlinks:

But on Cygwin 1.7 (installed on Windows 7), ln -s creates a text file.

on Cygwin:

$ ln -s -v target mylink
`mylink' -> `target'

on MinGW (or your favorite editor):

$ cat mylink
!<symlink>ÿþt a r g e t

Is it possible to tell Cygwing to use NTFS junction point or NTFS symbolic link?

other question: Is this available on MinGW?

Solution

  • ⸻⸻  Short answer  ⸻⸻

    Define environment variable:

    CYGWIN=winsymlinks:nativestrict

    As pointed out by mwm you may also have to go to the settings or to run as Administrator. See the Notes section.

    ⸻⸻  Long answer  ⸻⸻

    Default Cygwin symlinks are just regular files

    By default Cygwin creates text files as workaround for Windows symlink flaw. These files are not really symlinks. Almost all Windows programs do not considers these files as symlinks.

    Native symlinks are available on recent Windows versions

    Recent NTFS and Windows implement symlinks:

    Cygwin can create native NTFS symlinks

    Simplified extract of the Cygwin documentation:

    Symbolic links

    [...]

    Cygwin creates symbolic links potentially in multiple different ways:

    • The default symlinks are plain files containing a magic cookie followed by the path to which the link points. [...]

    • The shortcut style symlinks are Windows .lnk [...] created if the environment variable CYGWIN [...] is set to contain the string winsymlinks or winsymlinks:lnk. [...]

    • Native Windows symlinks are only created on Windows Vista/2008 and later, and only on filesystems supporting reparse points. Due to to their weird restrictions and behaviour, they are only created if the user explicitely requests creating them. This is done by setting the environment variable CYGWIN to contain the string winsymlinks:native or winsymlinks:nativestrict. [...]

    • On the NFS filesystem, Cygwin always creates real NFS symlinks.

    Configuring Cygwin

    Cygwin User's Guide presents variable CYGWIN and option winsymlinks:

    The CYGWIN environment variable is used to configure many global settings [...]. It contains the options listed below, separated by blank characters. [...]

    • [...]

    • [...]

    • [...]

    • [...]

    • winsymlinks:{lnk,native,nativestrict} - if set to just winsymlinks or winsymlinks:lnk, Cygwin creates symlinks as Windows shortcuts with a special headerand the R/O attribute set.

      If set to winsymlinks:native or winsymlinks:nativestrict, Cygwin creates symlinks as native Windows symlinks on filesystems and OS versions supporting them. If the OS is known not to support native symlinks (Windows XP, Windows Server 2003), a warning message is produced once per session.

      The difference between winsymlinks:native and winsymlinks:nativestrict is this: If the filesystem supports native symlinks and Cygwin fails to create a native symlink for some reason, it will fall back to creating Cygwin default symlinks with winsymlinks:native, while with winsymlinks:nativestrict the symlink(2) system call will immediately fail.

    CYGWIN=winsymlinks:native
    always creates a link but uses a Cygwin fall-back when target does not exists

    on Cygwin:

    $ export CYGWIN="winsymlinks:native"
$ ln -s -v target mylink
`mylink' -> `target'
$ echo content > target

    on MinGW:

    $ cat mylink
content

    People using both Windows and Cygwin programs may have issues when a symlink is created as a dummy file (Cygwin fallback when target is missing)...

    CYGWIN=winsymlinks:nativestrict
    always uses native-Windows symlink but fails when target does not exist

    on Cygwin:

    $ export CYGWIN="winsymlinks:nativestrict"
$ rm -f  a b
$ ln -sv a b
ln: failed to create symbolic link `b': No such file or directory
$ touch    b
$ ln -sv a b
ln: failed to create symbolic link `b': File exists
$ rm b
$ touch a
$ ln -sv a b
`b' -> `a'

    Because nativestrict requires the target exists before the symlink creation, some commands/scripts may fail when creating a link.

    Notes

    Since Windows 10 build 14972, native NTFS symlinks are available in a non-elevated shell by enabling the Developer Mode in the Developer Settings. Reference: https://blogs.windows.com/windowsdeveloper/2016/12/02/symlinks-windows-10/

    In older versions, only administrators have the ability to create native NT symlinks so under Windows UAC, the Cygwin terminal emulator (mintty) should be run with elevated privileges (right-click the shortcut and choose Run as Administrator or set the mintty shortcut property, Advanced → Run as Administrator).

    Special thanks to Guria, Spooky and Gene Pavlovsky for their contributions.