How-to: Windows Environment Variables

Environment variables are mainly used within batch files, they can be created, modified and deleted for a session using the SET command. To make permanent changes, use SETX

Variables can be displayed using either SET or ECHO.

Variables have a percent sign on both sides: %ThisIsAVariable%
The variable name can include spaces, punctuation and mixed case: %_Another Ex.ample%
(This is unlike Parameter variables which only have one % sign and are always one character long: %A )

A variable name may include any of the following characters:
A-Z, a-z, 0-9, # $ ' ( ) * + , - . ? @ [ ] _ ` { } ~
The first character of the name must not be numeric.

Array variables

Unlike PowerShell, which fully supports arrays, there is no built in support for array variables within the CMD shell. However with some effort you can replicate this functionality using a series of separate variables, named to represent the array:

Set elem[1]=First element
Set elem[2]=Second one
Set elem[3]=The third one

To perform array indexing operations with these, use EnableDelayedExpansion and a reference like !elem[%var%]!
this is explained fully in this StackOverflow Q/A.

Standard (built-in) Environment Variables

Variable Volatile
Default value assuming the system drive is C:
APPDATA   C:\Users\{username}\AppData\Roaming
CD Y The current directory (string).
ClientName Y Terminal servers only - the ComputerName of a remote host.
CMDEXTVERSION Y The current Command Processor Extensions version number. (NT = "1", Win2000+ = "2".)
CMDCMDLINE Y The original command line that invoked the Command Processor.
CommonProgramFiles   C:\Program Files\Common Files
COMMONPROGRAMFILES(x86)   C:\Program Files (x86)\Common Files
COMPUTERNAME   {computername}
COMSPEC   C:\Windows\System32\cmd.exe or if running a 32 bit WOW - C:\Windows\SysWOW64\cmd.exe
DATE Y The current date using same region specific format as DATE.
ERRORLEVEL Y The current ERRORLEVEL value, automatically set when a program exits.

  Internet Explorer
These are undocumented variables for the Edge browser in Windows 10.
HighestNumaNodeNumber Y (hidden) The highest NUMA node number on this computer.
HOMEPATH Y \Users\{username}
LOCALAPPDATA   C:\Users\{username}\AppData\Local
LOGONSERVER   \\{domain_logon_server}
NUMBER_OF_PROCESSORS Y The Number of processors running on the machine.
OS Y Operating system on the user's workstation.
PATH User and
C:\Windows\System32\;C:\Windows\;C:\Windows\System32\Wbem;{plus program paths}
.COM; .EXE; .BAT; .CMD; .VBS; .VBE; .JS ; .WSF; .WSH; .MSC
Determine the default executable file extensions to search for and use, and in which order, left to right.
The syntax is like the PATH variable - semicolon separators.
PROCESSOR_ARCHITECTURE Y AMD64/IA64/x86 This doesn't tell you the architecture of the processor but only of the current process, so it returns "x86" for a 32 bit WOW process running on 64 bit Windows. See detecting OS 32/64 bit
PROCESSOR_ARCHITEW6432   =%PROCESSOR_ARCHITECTURE% (but only available to 64 bit processes)
PROCESSOR_IDENTIFIER Y Processor ID of the user's workstation.
PROCESSOR_LEVEL Y Processor level of the user's workstation.
PROCESSOR_REVISION Y Processor version of the user's workstation.
ProgramW6432   =%ProgramFiles%(but only available when running under a 64 bit OS)
ProgramData   C:\ProgramData
ProgramFiles   C:\Program Files or C:\Program Files (x86)
ProgramFiles(x86) 1   C:\Program Files (x86)   (but only available when running under a 64 bit OS)
PROMPT   Code for current command prompt format,usually $P$G
PSModulePath   %SystemRoot%\system32\WindowsPowerShell\v1.0\Modules\
Public   C:\Users\Public
RANDOM Y A random integer number, anything from 0 to 32,767 (inclusive).
%SessionName%   Terminal servers only - for a terminal server session, SessionName is a combination of the connection name, followed by #SessionNumber. For a console session, SessionName returns "Console".
SYSTEMROOT   By default, Windows is installed to C:\Windows but there's no guarantee of that, Windows can be installed to a different folder, or a different drive letter.
systemroot is a read-only system variable that will resolve to the correct location.
Defaults in early Windows versions are C:\WINNT, C:\WINNT35 and C:\WTSRV
TEMP and TMP User Variable C:\Users\{Username}\AppData\Local\Temp
Under XP this was \{username}\Local Settings\Temp
TIME Y The current time using same format as TIME.
UserDnsDomain Y
User Variable
Set if a user is a logged on to a domain and returns the fully qualified DNS domain that the currently logged on user's account belongs to.
USERDOMAIN   {userdomain}
USERDOMAIN_roamingprofile   The user domain for RDS or standard roaming profile paths. Windows 8/10/2012 (or Windows 7/2008 with Q2664408)
USERNAME   {username}
USERPROFILE   %SystemDrive%\Users\{username}
This is equivalent to the $HOME environment variable in Unix/Linux

%windir% is a regular User variable and can be changed, which makes it less robust than %SystemRoot%
Set by default as windir=%SystemRoot%
%WinDir% pre-dates Windows NT, its use in many places has been replaced by the system variable: %SystemRoot%

1 Only on 64 bit systems, is used to store 32 bit programs.

Unless stated otherwise, all the variables above are System variables

Environment variables are stored in the registry:

User Variables: HKEY_CURRENT_USER\Environment
System Variables: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

By default, files stored under Local Settings do not roam with a roaming profile.

Dynamic environment variables are read-only and are computed each time the variable is expanded. When all variables are listed with SET, these will not appear in the list. Do not attempt to directly SET a dynamic variable. 

Undocumented Dynamic variables (read only)

%__APPDIR__%   The directory path to the current application .exe, terminated with a trailing backslash. (Global) - discuss
%__CD__%   The current directory, terminated with a trailing backslash. (Global)
%=C:%   The current directory of the C: drive. ( See Raymond Chen's explanation of this.)
%=D:%   The current directory of the D: drive if drive D: has been accessed in the current CMD session.
%DPATH%   Related to the (deprecated) DPATH command.
%=ExitCode%   The most recent exit code returned by an external command, such as CMD /C EXIT n, converted to hex.
%=ExitCodeAscii%   The most recent exit code returned by an external command, as ASCII. (Values 0-32 do not display because those map to ASCII control codes.)
%FIRMWARE_TYPE% The boot type of the system: Legacy ,UEFI,Not implemented ,Unknown Windows 8/2012.
%KEYS%   Related to the (deprecated) KEYS command.
More detail on these undocumented variables can be found in this stackoverflow answer from Dave Benham.

Undocumented Dynamic variables (read/write)

%__COMPAT_LAYER%   Set the ExecutionLevel to either RunAsInvoker (asInvoker), RunAsHighest(highestAvailable) or RunAsAdmin(requireAdministrator) for more see elevation and Q286705 / Application Compatibility Toolkit for other Compatibility Layers (colours,themes etc).

Pass variables between batch scripts

There are several ways to pass values between batch files, or between a batch file and the command line, see the CALL and SETLOCAL pages for full details.

A child process by default inherits a copy of all environment variables from its parent, this makes environment variables unsuitable for storing secret information such as API keys or user passwords, especially in rare occasions like crashes where a crash log will often include the full OS environment at the time of the crash. PowerShell/Get-Credential is a more secure approach.

If Command Extensions are disabled, the following dynamic variables will be not accessible:

“Men may be convinced, but they cannot be pleased against their will. But though taste is obstinate, it is very variable, and time often prevails when arguments have failed” ~ Samuel Johnson


How the environment-building process works - Raymond Chen [MSFT]
PowerShell - Working with Environment variables
User Shell Folders - Profile, Start Menu - Location of user profile folders.
Detecting 32 vs 64 bit Windows
CALL - Evaluate environment variables.
SET - View environment variables, set local variables.
SETX - Set environment variables.
Q100843 - The four types of environment variable.
Q286705 - Set compatibility variables in a batch file.
Q242557 - Registry Settings for Folder Redirection.
StackOverflow - Storing a Newline in a variable.

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