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X-HELO:	mout.kundenserver.de
Date:	Wed, 7 Aug 2019 16:31:21 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: How to query the value of %SystemDrive% in an empty environment?
Message-ID:	<20190807143121.GD11632@calimero.vinschen.de>
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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 WI=
NDIR
> >>>> environment variables are preserved (or recreated):
> >>>>  $ /usr/bin/env -i /usr/bin/env
> >>>>  SYSTEMROOT=3DC:\Windows
> >>>>  WINDIR=3DC:\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 "Syst=
emDrive"
> >>>> environment variable.
> >>>> The problem behind is that I'm using "vswhere.exe" to locate some Vi=
sual Studio
> >>>> environment from within some scripts run via 'env -i', causing vswhe=
re.exe to
> >>>> create a directory named "%SystemDrive%" in the current working dire=
ctory:
> >>>> So I better ensure the SystemDrive environment variable is set for v=
swhere.exe.
> >>>> Any ideas?
> >>>
> >>> There is a reg entry:
> >>>
> >>> $ head
> >>> /proc/registry/HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/Sy=
stemBootDevice;
> >>> 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 b=
oot drive
> >>> letter. It is not instantiated anywhere else as far I could find. A n=
umber of
> >>> low level reg entries use that env var. Only option is to pass it thr=
ough:
> >>
> >> Heck, even CreateEnvironmentBlock() relies on SYSTEMROOT env var being=
 set,
> >> otherwise returning things like ProgramData=3D"%SystemDrive%\ProgramDa=
ta".
> >=20
> > 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.
>=20
> I do prefer to have full control over the environment, recreating the nee=
ded
> vars from the registry or similar, because I did have too much troubles w=
ith
> polluted environment already.  This also applies to setting up some wrapp=
er
> around the MSVC toolchain, that provide the vswhere.exe helper these days.
>=20
> Actually I would have been fine it was obvious enough to locate SYSTEMDRI=
VE,
> much like SYSTEMROOT and WINDIR are available via cygpath, or reading oth=
er
> specific environment variable values from /proc/registry/.
>=20
> As SYSTEMROOT and WINDIR are preserved already, even if they are available
> via cygpath as well, also preserving SYSTEMDRIVE simply feels suitable he=
re.
> And even more since I found this commit, which seems to intent the same:
> https://cygwin.com/git/gitweb.cgi?p=3Dnewlib-cygwin.git;a=3Dcommitdiff;h=
=3D1f99dd3ecf3252517363ec8f0fec4b0a95706f31
>=20
> 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

--=20
Corinna Vinschen
Cygwin Maintainer

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