How to query the value of %SystemDrive% in an empty environment?

[Corinna Vinschen]

On Aug  7 14:41, Michael Haubenwallner wrote:
> On 8/7/19 1:19 PM, Corinna Vinschen wrote:
> > On Aug  7 13:08, Michael Haubenwallner wrote:
> >> On 8/7/19 4:33 AM, Brian Inglis wrote:
> >>> On 2019-08-06 09:20, Michael Haubenwallner wrote:
> >>>> using 'env -i' to create an empty environment, the SYSTEMROOT and WINDIR
> >>>> environment variables are preserved (or recreated):
> >>>>  $ /usr/bin/env -i /usr/bin/env
> >>>>  SYSTEMROOT=C:\Windows
> >>>>  WINDIR=C:\Windows
> >>>> And with cygpath, there is the -A, -D, -H, -O, -P, -S, -W and even -F flags
> >>>> to query the values for various directories.
> >>>> Now what I've failed to find is how to query the value for the "SystemDrive"
> >>>> environment variable.
> >>>> The problem behind is that I'm using "vswhere.exe" to locate some Visual Studio
> >>>> environment from within some scripts run via 'env -i', causing vswhere.exe to
> >>>> create a directory named "%SystemDrive%" in the current working directory:
> >>>> So I better ensure the SystemDrive environment variable is set for vswhere.exe.
> >>>> Any ideas?
> >>>
> >>> There is a reg entry:
> >>>
> >>> $ head
> >>> /proc/registry/HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/SystemBootDevice;
> >>> echo
> >>> multi(0)disk(0)rdisk(1)partition(3)
> >>>
> >>> but how do you convert that to a device letter?
> >>>
> >>> SYSTEMDRIVE is a dynamic env var created at startup pointing to the boot drive
> >>> letter. It is not instantiated anywhere else as far I could find. A number of
> >>> low level reg entries use that env var. Only option is to pass it through:
> >>
> >> Heck, even CreateEnvironmentBlock() relies on SYSTEMROOT env var being set,
> >> otherwise returning things like ProgramData="%SystemDrive%\ProgramData".
> > 
> > So, what does this have to do with Cygwin in case you clean out the
> > environment?  This is nothing you want to do if you plan to start
> > a non-Cygwin executable.
> 
> I do prefer to have full control over the environment, recreating the needed
> vars from the registry or similar, because I did have too much troubles with
> polluted environment already.  This also applies to setting up some wrapper
> around the MSVC toolchain, that provide the vswhere.exe helper these days.
> 
> Actually I would have been fine it was obvious enough to locate SYSTEMDRIVE,
> much like SYSTEMROOT and WINDIR are available via cygpath, or reading other
> specific environment variable values from /proc/registry/.
> 
> As SYSTEMROOT and WINDIR are preserved already, even if they are available
> via cygpath as well, also preserving SYSTEMDRIVE simply feels suitable here.
> And even more since I found this commit, which seems to intent the same:
> https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=commitdiff;h=1f99dd3ecf3252517363ec8f0fec4b0a95706f31
> 
> Anyway: If it is possible to map above SystemBootDevice registry value to
> SYSTEMDRIVE, I would be fine as well.

You can just as easily get it from SYSTEMROOT, as outlined before.
SYSTEMROOT is the system directory located on the system boot drive
so just strip off the path and keep the drive.


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
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