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

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

[Michael Haubenwallner]

Hi,

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?

Thanks!
/haubi/

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

[Corinna Vinschen]

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On Aug  6 17:20, Michael Haubenwallner wrote:
> Hi,
> 
> 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.

Just strip it off SYSTEMROOT?


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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

[Bill Stewart]

On Tue, Aug 6, 2019 at 9:42 AM Corinna Vinschen wrote:
>
> On Aug  6 17:20, Michael Haubenwallner wrote:
> > Now what I've failed to find is how to query the value for the "SystemDrive"
> > environment variable.
>
> Just strip it off SYSTEMROOT?

IIRC, that will not give the expected value if Windows is installed on
a different drive.

Bill

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

[Corinna Vinschen]

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On Aug  6 09:47, Bill Stewart wrote:
> On Tue, Aug 6, 2019 at 9:42 AM Corinna Vinschen wrote:
> >
> > On Aug  6 17:20, Michael Haubenwallner wrote:
> > > Now what I've failed to find is how to query the value for the "SystemDrive"
> > > environment variable.
> >
> > Just strip it off SYSTEMROOT?
> 
> IIRC, that will not give the expected value if Windows is installed on
> a different drive.

How so?  SYSTEMDRIVE is the drive SYSTEMROOT is installed on, no?


Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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

[Bill Stewart]

On Tue, Aug 6, 2019 at 9:53 AM Corinna Vinschen wrote:

> How so?  SYSTEMDRIVE is the drive SYSTEMROOT is installed on, no?

https://support.microsoft.com/en-us/help/314470/

"The system volume refers to the disk volume that contains the
hardware-specific files that are needed to start Windows, such as
Ntldr, Boot.ini, and Ntdetect.com."

"The boot volume refers to the disk volume that contains the Windows
operating system files and the supporting files. By default, the
Windows operating system files are in the WINDOWS folder, and the
supporting files are in the WINDOWS\System32 folder."

"The boot volume can be the same volume as the system volume. However,
this configuration is not required. There is only one system volume.
However, there is one boot volume for each operating system in a
multiboot system."

Regards,

Bill

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

[Corinna Vinschen]

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On Aug  6 10:10, Bill Stewart wrote:
> On Tue, Aug 6, 2019 at 9:53 AM Corinna Vinschen wrote:
> 
> > How so?  SYSTEMDRIVE is the drive SYSTEMROOT is installed on, no?
> 
> https://support.microsoft.com/en-us/help/314470/
> 
> "The system volume refers to the disk volume that contains the
> hardware-specific files that are needed to start Windows, such as
> Ntldr, Boot.ini, and Ntdetect.com."
> 
> "The boot volume refers to the disk volume that contains the Windows
> operating system files and the supporting files. By default, the
> Windows operating system files are in the WINDOWS folder, and the
> supporting files are in the WINDOWS\System32 folder."
> 
> "The boot volume can be the same volume as the system volume. However,
> this configuration is not required. There is only one system volume.
> However, there is one boot volume for each operating system in a
> multiboot system."

Nothing in there says that SYSTEMDRIVE == "system volume".

https://en.wikipedia.org/wiki/Environment_variable#Windows claims that
"The %SystemDrive% variable is a special system-wide environment
variable found on Windows NT and its derivatives. Its value is the drive
upon which the system directory was placed."


Corinna

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Corinna Vinschen
Cygwin Maintainer

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

[Bill Stewart]

On Tue, Aug 6, 2019 at 10:25 AM Corinna Vinschen wrote:

> https://en.wikipedia.org/wiki/Environment_variable#Windows claims that
> "The %SystemDrive% variable is a special system-wide environment
> variable found on Windows NT and its derivatives. Its value is the drive
> upon which the system directory was placed."

That may be correct - I seem to remember in the past that they could
differ. My recollection may be incorrect.

Bill

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

[Brian Inglis]

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:

$ /usr/bin/env -i SYSTEMDRIVE="$SYSTEMDRIVE" /usr/bin/env
SYSTEMDRIVE=C:
SYSTEMROOT=C:\WINDOWS
WINDIR=C:\WINDOWS

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

This email may be disturbing to some readers as it contains
too much technical detail. Reader discretion is advised.

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

[Michael Haubenwallner]

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".

> 
> $ /usr/bin/env -i SYSTEMDRIVE="$SYSTEMDRIVE" /usr/bin/env
> SYSTEMDRIVE=C:
> SYSTEMROOT=C:\WINDOWS
> WINDIR=C:\WINDOWS

Feels like the Cygwin DLL better should try to keep SYSTEMDRIVE being set, no?

/haubi/ (about to prepare some patch)

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

[Corinna Vinschen]

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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.


Corinna

-- 
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Cygwin Maintainer

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

[Michael Haubenwallner]

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.

Thanks!
/haubi/

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

[Corinna Vinschen]

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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 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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