making cygwin use native symlinks

cygwin-symlinks.md

How to make Cygwin use native symlinks

I stumbled upon this cool Cygwin option that allows you to use native Windows (NTFS) symlinks (like CreateSymbolicLinkW in the Win32 API, or mklink via cmd shell). Apparently there's a CYGWIN environment variable that "is used to configure many global settings for the Cygwin runtime system".

One of those options is winsymlinks:{lnk,native,nativestrict}.

if set to just winsymlinks or winsymlinks:lnk, Cygwin creates symlinks as Windows shortcuts with a special header and 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.

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.

For more information on symbolic links, see the section called “Symbolic links”.

So basically, setting the winsymlinks option to nativestrict forces Cygwin to use native symlinks!

Normally, when you make symlinks in Cygwin, it creates plain files containing a magic cookie, followed by the path to which the link points to. You can read up on this in the Cygwin documentation.

But with this option, we can create symlinks using ln and it will create NTFS-style symlinks automatically!

CYGWIN=winsymlinks:nativestrict
ln -sf /source/file /new/path

Cool! No need to open cmd and manually symlink my files anymore.

Note: You still need Administrator privileges, or must have the SeCreateSymbolicLinkPrivilege privilege for this to work.

Now 2 is a .symlink file and not a native symlink! Even nativestrict does NOT fail here. You get silent corruption.

Perhaps there is something I'm not observing, but your example seems like it works fine:

mlovell@COL-U513277L001:~/test
% export CYGWIN=winsymlinks:nativestrict

mlovell@COL-U513277L001:~/test
% mkdir adir

mlovell@COL-U513277L001:~/test
% ln -s adir 1

mlovell@COL-U513277L001:~/test
% ln -s 1 2

mlovell@COL-U513277L001:~/test
% ll
total 0
lrwxrwxrwx  1 mlovell Domain Users 4 Jun 11 13:09 1 -> adir
lrwxrwxrwx  1 mlovell Domain Users 1 Jun 11 13:09 2 -> 1
drwxr-xr-x+ 1 mlovell Domain Users 0 Jun 11 13:09 adir

mlovell@COL-U513277L001:~/test
% touch adir/test_file

mlovell@COL-U513277L001:~/test
% ll 2
lrwxrwxrwx 1 mlovell Domain Users 1 Jun 11 13:09 2 -> 1
mlovell@COL-U513277L001:~/test
% cd 2

mlovell@COL-U513277L001:~/test/2
% ll
total 0
-rw-r--r-- 1 mlovell Domain Users 0 Jun 11 13:09 test_file

Could you clarify what's broken?

EDIT: Just wanted to clarify that the above commands were run in a cygwin shell with administrator privileges. Trying the nativestrict setting in a non-privileged shell just gets one an error:

ln: failed to create symbolic link '1': Operation not permitted

Creating symbolic links is a privileged operation in Windows (sic!). That means you need to give that permission to the user. If the user has admin rights, then you need to run "as administrator". To add the permission to a unprivileged user:
Start the "Local Security Policy" app (Run "secpol.msc")
Under Security Settings/Local Policies/User Rights Assignment, select "Create symbolic links"
Right-click: Properties
Add Users or Group
enter the usernames, then click "Check Names"
OK

You can only observe the problem in Windows Filesystem. Within Cygwin all works fine.
Look at the screenshot of Windows Explorer.

one advanced options to allow mklink and ln -sf works on windows 10 home + UAC
described here
https://www.joshkel.com/2018/01/18/symlinks-in-windows/

Enable developer mode: Go under Settings, under Update & Security, under For developers, and set “Use developer features” to “Developer mode.” This is how you enable the Windows 10 Creators Update option of allowing users to create symlinks without UAC elevation.