Developer(s) | Microsoft, IBM, Digital Research, ReactOS Contributors |
---|---|
Initial release | 1985, 38–39 years ago |
Operating system | MS-DOS, PC DOS, SISNE plus, OS/2, eComStation, ArcaOS, DR DOS, ROM-DOS, PTS-DOS, FreeDOS, Windows, ReactOS |
Platform | Cross-platform |
Type | Command |
Website | docs |
![]() | This article contains instructions, advice, or how-to content .(January 2021) |
In computing, SUBST
is a command on the DOS, IBM OS/2, [1] Microsoft Windows and ReactOS [2] operating systems used for substituting paths on physical and logical drives as virtual drives.
In MS-DOS, the SUBST
command was added with the release of MS-DOS 3.1. [3] The command is similar to floating drives, a more general concept in operating systems of Digital Research origin, including CP/M-86 2.x, Personal CP/M-86 2.x, Concurrent DOS, Multiuser DOS, System Manager 7, REAL/32, as well as DOS Plus and DR DOS (up to 6.0). DR DOS 6.0 includes an implementation of the SUBST
command. [4] The command is also available in FreeDOS [5] and PTS-DOS. [6] The Windows SUBST
command is available in supported versions of the command line interpreter cmd.exe. [7] In Windows NT, SUBST
uses DefineDosDevice()
to create the disk mappings.
The JOIN
command is the "opposite" of SUBST
, because JOIN
will take a drive letter and make it appear as a directory.
Some versions of MS-DOS COMMAND.COM
support the undocumented internal TRUENAME
command which can display the "true name" of a file, i.e. the fully qualified name with drive, path, and extension, which is found possibly by name only via the PATH environment variable, or through SUBST
, JOIN
and ASSIGN
filesystem mappings.
This is the command syntax in Windows XP to associate a path with a drive letter:
SUBST [drive1: [drive2:]path] SUBST drive1: /D
drive1:
– Specify a virtual drive to which to assign a path.[drive2:]path
– Specify a physical drive and path to assign to a virtual drive./D
– Delete a substituted (virtual) drive.This means that, for example, to map C:'s root to X:, the following command would be used at the command-line interface:
C:\>SUBST X: C:\
Upon doing this, a new drive called X: would appear under the My Computer virtual folder in Windows Explorer.
To unmap drive X: again, the following command needs to by typed at the command prompt:
C:\>SUBST X: /D
A custom label can[ citation needed ] be assigned to a drive letter created in this way by way of a registry key, which can be created by renaming (select "rename" from the drive letter context menu or press F2 ) the SUBST drive in Windows Explorer/My Computer.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\DriveIcons\M\DefaultLabel\
(DefaultValue) = Your Drive Label
"M" represents the drive letter to assign a custom label to.
However, labels created for SUBST drives in this manner are overridden by the label of the host drive/partition: the custom labels are only used if the host drive has no label. One may then:
Note that the LABEL command is NOT able to change the label name of a drive letter created using subst. [8] LABEL is one of several commands that only work on physical drives. [9]
Drive letters mapped with the command are not available during system startup for services nor do they persist across a reboot.
The DOS Devices mechanism that underlies subst can be set in registry. This way, the mapped drives are usable immediately during startup.
Create a new registry entry "String Value" in the following key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices
The name should be "X:
" where X is the drive letter.
The value should be the path in one of the following NT Object Manager forms:
\??\C:\some\directory
(preferred)\DosDevices\C:\some\directory
(long version of the former)\Device\Mup\127.0.0.1\C$\some\directory
(discouraged, as this goes through SMB share)There are tools available to make the necessary changes for the user, including psubst
.
The relative to this thematic registry key is HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
. It defines mapping of drive letters into particular hard disk partitions, similar to /etc/fstab on a Unix system. It also can be edited manually, but only at that time while that particular installed Window operation system is "inactive". So that, for example, if you currently boot from "D:\Windows
" then you can edit the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
key of Windows that is installed in a "C:\Windows
" folder, for an instance by doing the following actions:
reg load hklm\$system C:\WINDOWS\system32\config\system
regedit.exe
HKEY_LOCAL_MACHINE\$system\MountedDevices
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
registry key of the Windows instance that is installed in C:\WINDOWS
)regedit.exe
reg unload hklm\$system
(to complete the editing procedure)The easiest way to do this is to create a registry file (.reg), and double click the file to import the settings into the registry.
Here is an example registry file.
REGEDIT4[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices]"F:"="\\??\\D:\\Mount\\db""G:"="\\??\\D:\\Mount\\log"
After configuring the registry the computer must be rebooted for the changes to take effect.
Create a batch file to run the built-in SUBST
command to create a virtual drive letter for the existing mount points and place it in the user accounts startup folder.
This is not preferred, as the mapping only appears at the end of user logon.
Here is an example:
@ECHO off SUBST f: d:\mount\db SUBST g: d:\mount\log
The user must log off and back on or the service must be restarted for the changes to take effect.
Edit the registry to run the built-in subst command during computer startup or user logon by leveraging the appropriate Run registry key. The easiest way to do this is to create a registry file (.reg), and double click the file to import the settings into the registry.
This is not preferred, as the mapping only appears at the end of bootup.
Example to run during computer boot
REGEDIT4[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]"F Drive"="SUBST f: d:\\mount\\db""G Drive"="SUBST g: d:\\mount\\log"
The computer must be rebooted for the changes to take effect.
Example of user logon
REGEDIT4[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Run]"F Drive"="SUBST f: d:\\mount\\db""G Drive"="SUBST g: d:\\mount\\log"
The user must log off and back on for the changes to take effect.
autorun.inf
files present in the new drive letter are ignored; thus, AutoRun/AutoPlay does not work on drive letters created in this way. In addition, assigning a custom icon or label to the drive letter created this way via autorun.inf
does not work. A custom label assigned to the drive letter created with subst
only appears if the source drive/volume does not have a volume label set (check and set with the label
command).In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, drive letter assignment allows multiple highest-level namespaces. Drive letter assignment is thus a process of using letters to name the roots of the "forest" representing the file system; each volume holds an independent "tree".
In computing, a symbolic link is a file whose purpose is to point to a file or directory by specifying a path thereto.
A path is a string of characters used to uniquely identify a location in a directory structure. It is composed by following the directory tree hierarchy in which components, separated by a delimiting character, represent each directory. The delimiting character is most commonly the slash ("/"), the backslash character ("\"), or colon (":"), though some operating systems may use a different delimiter. Paths are used extensively in computer science to represent the directory/file relationships common in modern operating systems and are essential in the construction of Uniform Resource Locators (URLs). Resources can be represented by either absolute or relative paths.
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in subsequent versions. The first version in the 9x series was Windows 95, which was succeeded by Windows 98 and then Windows Me, which was the third and last version of Windows on the 9x line, until the series was superseded by Windows XP.
The Windows Registry is a hierarchical database that stores low-level settings for the Microsoft Windows operating system and for applications that opt to use the registry. The kernel, device drivers, services, Security Accounts Manager, and user interfaces can all use the registry. The registry also allows access to counters for profiling system performance.
AutoRun and the companion feature AutoPlay are components of the Microsoft Windows operating system that dictate what actions the system takes when a drive is mounted.
AutoPlay, a feature introduced in Windows 98, examines newly discovered removable media and devices and, based on content such as pictures, music or video files, launches an appropriate application to play or display the content. It is closely related to the AutoRun operating system feature. AutoPlay was created in order to simplify the use of peripheral devices – MP3 players, memory cards, USB storage devices and others – by automatically starting the software needed to access and view the content on these devices. AutoPlay can be enhanced by AutoPlay-compatible software and hardware. It can be configured by the user to associate favourite applications with AutoPlay events and actions.
An autorun.inf
file is a text file that can be used by the AutoRun and AutoPlay components of Microsoft Windows operating systems. For the file to be discovered and used by these component, it must be located in the root directory of a volume. As Windows has a case-insensitive view of filenames, the autorun.inf file can be stored as AutoRun.inf or Autorun.INF or any other case combination.
In computer data storage, a volume or logical drive is a single accessible storage area with a single file system, typically resident on a single partition of a hard disk. Although a volume might be different from a physical disk drive, it can still be accessed with an operating system's logical interface. However, a volume differs from a partition.
As the next version of Windows NT after Windows 2000, as well as the successor to Windows Me, Windows XP introduced many new features but it also removed some others.
Drive mapping is how MS-DOS and Microsoft Windows associate a local drive letter with a shared storage area to another computer over a network. After a drive has been mapped, a software application on a client's computer can read and write files from the shared storage area by accessing that drive, just as if that drive represented a local physical hard disk drive.
The Session Manager Subsystem, or smss.exe
, is a component of the Microsoft Windows NT family of operating systems, starting in Windows NT 3.1. It is executed during the startup process of those operating systems.
Contig is a command line defragmentation utility for Windows developed by Microsoft as part of the Sysinternals Suite.
The booting process of Microsoft Windows varies between different releases.
NTBackup is the first built-in backup utility of the Windows NT family. It was introduced with Windows NT 3.51. NTBackup comprises a GUI (wizard-style) and a command-line utility to create, customize, and manage backups. It takes advantage of Shadow Copy and Task Scheduler. NTBackup stores backups in the BKF file format on external sources, e.g., floppy disks, hard drives, tape drives, and Zip drives. When used with tape drives, NTBackup uses the Microsoft Tape Format (MTF), which is also used by BackupAssist, Backup Exec, and Veeam Backup & Replication and is compatible with BKF.
A batch file is a script file in DOS, OS/2 and Microsoft Windows. It consists of a series of commands to be executed by the command-line interpreter, stored in a plain text file. A batch file may contain any command the interpreter accepts interactively and use constructs that enable conditional branching and looping within the batch file, such as IF
, FOR
, and GOTO
labels. The term "batch" is from batch processing, meaning "non-interactive execution", though a batch file might not process a batch of multiple data.
In computing, label
is a command included with some operating systems. It is used to create, change, or delete a volume label on a logical drive, such as a hard disk partition or a floppy disk. Used without parameters, label
changes the current volume label or deletes the existing label.
The Windows 9x series of operating systems refers to the monolithic kernel which powers these operating systems. The basic code is similar in function to MS-DOS. As a 16-/32-bit hybrid, it requires support from MS-DOS to operate.
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executable. Service processes interact with SCM through a well-defined API, and the same API is used internally by the interactive Windows service management tools such as the MMC snap-in Services.msc
and the command-line Service Control utility sc.exe
. Terminating this file is used as a method of causing the Blue Screen of Death.