Re: Whats the %=C:% environment variable used for ?

Frank Slootweg wrote:
> R.Wieser <[email protected]> wrote:
>> Hello all,
>> 
>> A couple of days ago I was looking at the envronment variables, and noticed 
>> a variable named "=C:", containing the current path of that drive.
> 
>   Current working directory, to be precise.
> 
>> Can anyone tell me what it was/is used for ?
> 
>   Google, which you apparently don't want to use, says (amongst others):
> 
> "The %=C:% environment variable in Windows is a hidden, special
>  system-wide variable that stores the current working directory for the
>  C: drive. 
> 
>  It is part of a legacy mechanism (dating back to MS-DOS) that allows
>  the command processor (cmd.exe) to track separate current directories
>  for every drive letter, rather than having one single global current
>  directory for the whole system. 
> 
>  Here are the key details about %=C:%:
> 
>  * Function: It keeps track of where you are in the C: drive if you are
>    currently working on a different drive (e.g., if you are in D:\Data
>    but type cd C:\Users, %=C:% will be updated to C:\Users).
> 
>  * Automatic Management: This variable is managed automatically by
>    cmd.exe when you use the cd (chdir) command."
> 
>> Remarks: Using the command-prompt it doesn't appear in the "set" list, and 
>> it can be read, but not written.
> 
>   Google confirm that behaviour.
> 
>   It also gives a comparison with other variables. such ad %CD%
> %SystemDrive% and mentions the somwhat obvious:
> 
> "If you change drives, other variables like %=D:%, %=E:%, etc., are
>  created for those respective drives.".

Windows contains secrets normally kept hidden under the floorboards.

Following on what Frank said, it turns out Windows creates "drive-specific
current directory" environment variables which look funnily like this.
     =C:
     =D:
     =Z:  
Each one stores the current working directory (cwd) for that drive.

Try this:
     C:\> cd \Windows
     C:\Windows> D:
     D:\> cd \Games
     D:\Games> C:
     C:\Windows>
Notice how switching back to C: returns you to C:\Windows automatically?
That's because Windows stored it in that =X: stuff.

They're maintained internally by the OS, not by the shell so you can't read
them (echo %=C:%) or see them with set or even write to them.

Try these proof-of-concept examples:
     C:\> cd \Windows
     C:\Windows> echo %=C:%
     C:\Windows

Switch drives:
     C:\Windows> D:
     D:\> echo %=C:%
     C:\Windows
It stays the same until you change directories on C:.

It's mysterious. Much like the %CD% vs. %__CD__% split
 %__CD__% === an internal variable used by cmd.exe to track the raw current
directory path including UNC paths (\\server\share), extended path lengths
(\\?\C:\very\long\path), temp dirs, exact casing, setc. It's the true
working directory as the command interpreter sees it.

You might not notice the difference unless you're in a UNC or long path:
    C:\> echo %__CD__%
Just like the =C: variables, it's protected.

Don't even get me started on the bizarre CONIN$/CONOUT$ handles!
-- 
Windows contains secrets normally kept hidden under the floorboards.