- Persistent SUBST command
- Abstract
- Installation
- Divide and power
- Do we need it?
- How does it work?
- Shortcomings
- Overview of the new features
- Related links
Associates a path with a drive letter and extends the standard SUBST command allowing to create persistent substituted drives between startups.
Download the archive following by the download link, unpack it or checkout the source and put the single file to comfortable place in your hard disk.
Time ago I was asked to publish the tool as the Chocolatey package (see issue 14). If you use Chocolatey actively, you can install it as follows:
choco install psubst
Also you can find it by this link: https://chocolatey.org/packages/psubst.
Since oldest times in Windows there is admirable feature to map some path with name of a virtual drive using the SUBST command. This feature simplifies an access to objects on a disk. It means a usage of name of a virtual drive instead of a long path. For example, the following command is used to create virtual drive Z for the path C:\Documents and Settings\All Users\Shared Documents:
subst Z: "C:\Documents and Settings\All Users\Shared Documents"
So to reach targets in this folder it does not need to type the full path or go over a tree of folders in the Explorer window. To select the Z: drive is enough.
There are several certain examples when this feature is useful:
- Temporary stub when the physical drive is missing;
- Operational system limitation for the size of filename (for example, 256 characters);
- Working of some application within own space;
- Emulating other operational systems.
Print the list of existing drives:
subst
Create new virtual drive:
subst Z: "C:\Documents and Settings\All Users\Shared Documents"
Delete the virtual drive:
subst Z: /D
There are strict conventions on correct typing the substituted path:
- the path should not be trailed by a backslash;
- the root path should be ended by a backslash.
For example, these are correct
subst Z: "C:\Documents and Settings\All Users\Shared Documents"
subst Z: C:\
But these are incorrect:
subst Z: "C:\Documents and Settings\All Users\Shared Documents\"
subst Z: C:
However restart of the system destroys the virtual disk. What to do? A disk can be created after startup. But what to do, when the disk is required on early steps of the startup? For example, to run services? There is system feature to establish virtual disks from the system registry:
REGEDIT4
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices]
"Z:"="\\??\\C:\\Documents and Settings\\All Users\\Shared Documents"
It is enough to create a text file with the extension .REG and run it. When the next system starting, the virtual disk will be established. It needs to define the name of the disk and path. Note that each backslash in the path is doubled.
Answer is yes! This article is the result of the work devoted to availability of joining both features. The batch script was developed to cover problems described earlier. Also it adds a lot of owned features.
As well as the standard SUBST command this script named as PSUBST.BAT implements all standard features of the command.
Print all virtual drives:
psubst
Create the virtual drive:
psubst drive1: drive2:path
Delete the virtual drive:
psubst drive1: /D
Typing the /P or /PF argument you run the tool with the extended features to work with persistent virtual disks:
/Pstands for creating, deleting or displaying persistent drives;/PFstands for creating and deleting persistent drives using elevated privileges; it can be useful for managing persistent drives by non-administrative users.
Print all virtual persistent drives (read from the registry)
psubst /P
Restore a virtual drive from the persistent drive, if any:
psubst drive1: /P
In the following commands the option /P can be replaced with the option /PF to elevate privileges.
Create the persistent virtual drive with saving its persistency in the registry:
psubst drive1: drive2:path /P
Delete the persistent drive from the registry:
psubst drive1: /D /P
Great advantage of the tool is independency of existence or lack of the trailing backslashes. It means that incorrect examples described earlier in this article will work always – incorrect input arguments will be transformed to the required format and the command will execute substitution successfully. Nevertheless the standard command works with the slashes in a path correctly, the script transforms these to backslashes usual in Windows.
Are there own shortcomings? Yes, a bit of them! There are:
- this is batch script and it works a bit slower than binary analog;
there is quite weak probability to run the script twice with different arguments and disturb results of both. In fact, it's not this script issue. It's the issue of any tools having access to any resources at the same time;there is weak probability to break the script execution when disk is already created but the registry is not updated yet. To be honest, it's not lack of this script because managing the drives and updating the registry are two separate independednt actions. And it's not the real issue of this script. Every tool implementing a complicated functionality that consists of two ore more steps can be stopped and not complete its work.
