- Windows Environment Variables Tutorial
- What’s Environment Variables
- View Environment Variables
- Sample Values of Environment Variable
- Types of Environment Variable
- Environment Variables
- How-to: Windows Environment Variables
- Array variables
- Standard (built-in) Environment Variables
- Shell variables
- Undocumented Dynamic variables (read only)
- Undocumented Dynamic variables (read/write)
- Pass variables between batch scripts
Windows Environment Variables Tutorial
What’s Environment Variables
Environment variables are system-wide global variables. They are config parameters and is used by processes. For example, apps need to know the path of your Windows kernal, path of your home dir, paths to search for shell programs, etc.
Windows environment variable names are not case sensitive
View Environment Variables
Press ❖ Window key, then type “environment”
Type in shell c:\Windows\System32\SystemPropertiesAdvanced.exe to launch the GUI app directly.
Windows 10 SystemPropertiesAdvanced 2021-02-04
Sample Values of Environment Variable
Here’s example of env var values as it exists on my system.
One of the most important environment variable is PATH . Here’s a sample value of path (with line break added for easy reading):
value of the environment variable PATH
Types of Environment Variable
There are 3 types of environment variable:
Process Per session. It is temporary. It exist only for the current session in cmd.exe or PowerShell. After you exit the shell, the variables you created are gone. When a shell starts, it gets environment variable from the Windows Registry. User Permanent, stored in Windows Registry. Per user. Each user may have different values.
Examples: home folder HOMEPATH , temp dir ( TEMP and or TMP ).
In Registry, they are at: HKEY_CURRENT_USER\Environment . Machine Permanent, stored in Windows Registry. Per machine. Each machine may have different values.
Examples:
- OS kernal path ( WINDIR )
- processor info ( PROCESSOR_ARCHITECTURE , NUMBER_OF_PROCESSORS , etc)
- application paths ( PATH )
- executable file name extensions ( PATHEXT )
- OS type ( OS )
- current user name ( USERNAME )
In Registry, they are at:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment
[see Microsoft Windows Registry Tutorial]
Process environment variable is also known as Local Environment Variable.
User environment variable and Machine environment variable are together also known as System Environment Variable.
Note that you can set any new env vars in any category. Programs have access to all your env vars, but which ones are meaningful to the program is up to the program.
Environment Variables
Every process has an environment block that contains a set of environment variables and their values. There are two types of environment variables: user environment variables (set for each user) and system environment variables (set for everyone).
By default, a child process inherits the environment variables of its parent process. Programs started by the command processor inherit the command processor’s environment variables. To specify a different environment for a child process, create a new environment block and pass a pointer to it as a parameter to the CreateProcess function.
The command processor provides the set command to display its environment block or to create new environment variables. You can also view or modify the environment variables by selecting System from the Control Panel, selecting Advanced system settings, and clicking Environment Variables.
Each environment block contains the environment variables in the following format: Var1=Value1\0
Var2=Value2\0
Var3=Value3\0
.
VarN=ValueN\0\0
The name of an environment variable cannot include an equal sign (=).
The GetEnvironmentStrings function returns a pointer to the environment block of the calling process. This should be treated as a read-only block; do not modify it directly. Instead, use the SetEnvironmentVariable function to change an environment variable. When you are finished with the environment block obtained from GetEnvironmentStrings, call the FreeEnvironmentStrings function to free the block.
Calling SetEnvironmentVariable has no effect on the system environment variables. To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParam set to the string «Environment». This allows applications, such as the shell, to pick up your updates.
The maximum size of a user-defined environment variable is 32,767 characters. There is no technical limitation on the size of the environment block. However, there are practical limits depending on the mechanism used to access the block. For example, a batch file cannot set a variable that is longer than the maximum command line length.
Windows ServerВ 2003 and WindowsВ XP: The maximum size of the environment block for the process is 32,767 characters. Starting with WindowsВ Vista and Windows ServerВ 2008, there is no technical limitation on the size of the environment block.
The GetEnvironmentVariable function determines whether a specified variable is defined in the environment of the calling process, and, if so, what its value is.
To retrieve a copy of the environment block for a given user, use the CreateEnvironmentBlock function.
To expand environment-variable strings, use the ExpandEnvironmentStrings function.
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 (Read-Only) | Default value assuming the system drive is C: |
|---|---|---|
| ALLUSERSPROFILE | C:\ProgramData | |
| APPDATA | C:\Users\ \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 | ||
| 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. |
| FPS_BROWSER_APP_PROFILE_STRING FPS_BROWSER_USER_PROFILE_STRING |
Default
These are undocumented variables for the Edge browser in Windows 10.
System
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.
C:>
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
Under XP this was \ \Local Settings\Temp
User Variable
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.
Shell variables
Variables for current shell session are created with the SET command and are available only to the current CMD shell. These are only stored in memory not the registry. Shell variables are destroyed when the current shell (CMD) exits.
Running the SET command with no options will display all shell variables but also all User and System environment variables.
If the SET command is used to modify the PATH, or if it is removed completely with PATH ; that will not affect any other programs or CMD sessions opened before or after the current one.
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:
%CD% %DATE% %TIME% %RANDOM% %ERRORLEVEL% %CMDEXTVERSION% %CMDCMDLINE% %HIGHESTNUMANODENUMBER%
“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”
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.
