Windows environment variables

Four types of environment variables

There are four types of Windows environment variables

System	Found in the registry under `HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment`
User	Found in the registry under `HKCU\Environment`
Volatile	Found in the registry under `HKCU\Volatile Environment`
Process	The value of process variables is stored «within» the process and only for the duration of the process.

Querying environment variables (.NET)

The currently set environment variables are exposed in the static methods GetEnvironmentVariable() and GetEnvironmentVariables in the .NET class System.Environment.

When used in PowerShell, the method returns a System.Collections.Hashtable (see also hash tables in PowerShell):

PS C:\> [System.Environment]::GetEnvironmentVariables()
Name                           Value
----                           -----
SESSIONNAME                    Console
ProgramFiles(x86)              C:\Program Files (x86)
USERDNSDOMAIN                  TQ84.LOCAL
…

The value of a particular environment variable can be queried with GetEnvironmentVariable(varName), again in PowerShell:

PS C:\> [System.Environment]::GetEnvironmentVariable('SystemDrive')
C:

Permanently setting an environment variable

Command line approach with PowerShell

The following example creates the local user variable USER_VAR with the value foo and a system wide (global) variable GLOBAL_VAR with the value bar:

[environment]::setEnvironmentVariable('USER_VAR'  , 'foo', 'user'   )
[environment]::setEnvironmentVariable('GLOBAL_VAR', 'bar', 'machine')

It is also possible to set a variable only for the current process:

[environment]::setEnvironmentVariable('PROC_VAR'  , 'baz', 'process')

GUI based approach with control panel

The advanced tab of the System Properties control panel (Control panel -> System -> Change Settings) has a button Environment Variables that allows to view and modify user and system environment variables.

From the command line (cmd.exe), this tab can be opened like so:

control sysdm.cpl,,3

When opening this control panel with PowerShell, the arguments need to be quoted:

control 'sysdm.cpl,,3'

Common variables

Variable	Possible value		registry
ALLUSERSPROFILE	C:\ProgramData	Set by `wininit.exe`, same value as `%ProgramData%` (at least on Windows 10)
APPDATA	C:\Users\Rene\AppData\Roaming		`HKCU\Volatile Environment`
CLIENTNAME			`HKCU\Volatile Environment\n`
CommonProgramFiles	C:\Program Files\Common Files
CommonProgramFiles(x86)	C:\Program Files (x86)\Common Files
CommonProgramW6432	C:\Program Files\Common Files
COMPUTERNAME	DESKTOP-HELLO
ComSpec	C:\Windows\system32\cmd.exe		`HKLM\…SessMgr\Env…`
DriverData	C:\Windows\System32\Drivers\DriverData		`HKLM\…SessMgr\Env…`
FPS_BROWSER_APP_PROFILE_STRING	Internet Explorer
FPS_BROWSER_USER_PROFILE_STRING	Default
HOMEDRIVE	C:	The combination of `HOMEDRIVE` and `HOMEPATH` points to a location where personal files should be stored. It might, but needs not to, be the same directory as `%USERPROFILE%`.	`HKCU\Volatile Environment`
HOMEPATH	\Users\Rene	Rene corresponds to `%USERNAME%`. Compare with `%USERPROFILE%`	`HKCU\Volatile Environment`
HOMESHARE	`\\alpha.foo\some\path`	If `HOMEDRIVE` is on a UNC path, `HOMESHARE` contains the its UNC path	`HKCU\Volatile Environment`
LOCALAPPDATA	C:\Users\Rene\AppData\Local		`HKCU\Volatile Environment`
LOGONSERVER	\\DESKTOP-HELLO		`HKCU\Volatile Environment`
NUMBER_OF_PROCESSORS	2		`HKLM\…SessMgr\Env…`
OneDrive	C:\Users\Rene\OneDrive	See also OneDrive	`HKCU\Environment`
OS	Windows_NT		`HKLM\…SessMgr\Env…`
PATH	…	…	…
PATHEXT	.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC		`HKLM\…SessMgr\Env…`
PROCESSOR_ARCHITECTURE	AMD64		`HKLM\…SessMgr\Env…`
PROCESSOR_IDENTIFIER	AMD64 Family 21 Model 16 Stepping 1, AuthenticAMD		`HKLM\…SessMgr\Env…`
PROCESSOR_LEVEL	21		`HKLM\…SessMgr\Env…`
PROCESSOR_REVISION	1001		`HKLM\…SessMgr\Env…`
ProgramData	C:\ProgramData	Set by `wininit.exe`, same value as `%ALLUSERSPROFILE` (at least on Windows 10)
ProgramFiles	C:\Program Files
ProgramFiles(x86)	C:\Program Files (x86)
ProgramW6432	C:\Program Files
PUBLIC	C:\Users\Public	Set by `wininit.exe`
SESSIONNAME	Console		`HKCU\Volatile Environment\n`
SystemDrive	C:
SystemRoot	C:\Windows
TEMP	C:\Users\Rene\AppData\Local\Temp		`HKCU\Environment` overwrites `HKLM\…SessMgr\Env…`
TMP	C:\Users\Rene\AppData\Local\Temp		`HKCU\Environment` overwrites `HKLM\…SessMgr\Env…`
USERDOMAIN	DESKTOP-HELLO		`HKCU\Volatile Environment`
USERDOMAIN_ROAMINGPROFILE	DESKTOP-HELLO		`HKCU\Volatile Environment`
USERNAME	Rene		`HKCU\Volatile Environment` overwrites `HKLM\…SessMgr\Env…`.
USERPROFILE	C:\Users\Rene	Compare with `%HOMEDRIVE%%HOMEPATH%`	`HKCU\Volatile Environment`
windir	C:\Windows

Unclear is where the values are defined for

CommonProgramFiles
CommonProgramFiles(x86)
CommonProgramW6432
FPS_BROWSER_APP_PROFILE_STRING
FPS_BROWSER_USER_PROFILE_STRING
ProgramFiles
ProgramFiles(x86)
ProgramW6432
SystemDrive
SystemRoot

Less common variables

__COMPAT_LAYER Setting __COMPAT_LAYER to RunAsInvoker prevents elevation and thus also suppresses UAC prompts.

PSModulePath

$env:PSModulePath is used by PowerShell to locate modules and consists of the following default paths

In order to load a module that is not located in one of the directories at which $env:PSModulePath points, the import-module cmdlet must be used.