Question

If I start a new CMD shell from an existing shell, the new shell inherits the existing environment. Is there a way to start a new shell but have it initialized to the system defaults and not inherit?

Current result:

B:\>set _test=blooharky

B:\>cmd
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

B:\>set _
_test=blooharky

Desired result:

B:\>set _test=blooharky

B:\>cmd /env=default
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

B:\>set _
Environment variable _ not defined

[update] Solution for this is start /i cmd, as shared by dbenham, below. However it doesn’t help in the situation where the current shell is already second generation. Example:

d:\>set _
Environment variable _ not defined

d:\>set _test=blooharky

d:\>cmd /k

:: some work done using _test here...
:: ...but after we need a new clean shell:

d:\>start /i cmd

d:\>set _
_test=blooharky

:: uhoh, our shell isn't clean!

Answer 1

Some of the variables are initialized at logon and are not stored with the other registry entries so if you want a environment that is equal to the initial explorer.exe environment you need to white-list those items and hope they have not been changed by anyone:

@echo off
setlocal ENABLEEXTENSIONS DISABLEDELAYEDEXPANSION
goto main

:SetFromReg
FOR /F "tokens=2,*" %%A IN ('REG query "%~1" /v "%~2"^|find /I "REG_"') DO (
    call set %~3=%%B
)
goto :EOF

:GetRegEnv
FOR /F %%A IN ('REG query "%~1" /s^|find /I "REG_"') DO (
    if /I not "%%~A"=="Path" call :SetFromReg "%~1" "%%~A" "%%~A"
)
goto :EOF

:InheritOrDelete
for %%A in (save_TEMP Path SystemRoot SystemDrive ProgramFiles CommonProgramFiles ALLUSERSPROFILE COMPUTERNAME LOGONSERVER USERNAME USERDOMAIN HOMEDRIVE HOMEPATH USERPROFILE APPDATA) do if /I "%%~A"=="%~1" goto :EOF
set %~1=
goto :EOF

:main
REM Save temp
set save_TEMP=%temp%
if not defined save_TEMP set save_TEMP=%tmp%

for /F "delims==" %%A in ('set') do call :InheritOrDelete "%%~A"
call :GetRegEnv "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment"
call :GetRegEnv "HKCU\Environment"

REM Special handling for Path
call :SetFromReg "HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment" Path Path
setlocal
set u=
call :SetFromReg "HKCU\Environment" Path u
endlocal&if not "%Path%"=="" if not "%u%"=="" set Path=%Path%;%u%

REM Restore TEMP/TMP
set TEMP=%save_TEMP%
set save_TEMP=
set TMP=%TEMP%

REM start some command...
start cmd /d /k set

The output from reg.exe is not the same on every windows version so you would have to make sure that it works on your target system (It will also have problems if the name of a variable contains a space, you could fix that by replacing "tokens=2,*" with "tokens=2,* delims= " (delims equals tab) but before you do that make sure that the reg.exe output always uses tab as the separator)

You can work around these issues by using a windows scripting host script instead of a batch file:

'orgenvshell.vbs:
Set WShl = CreateObject( "WScript.Shell" )
Set FSO = CreateObject("Scripting.FileSystemObject")

Function CanInherit(n)
    CanInherit = False
    w = Split("SystemRoot SystemDrive ProgramFiles CommonProgramFiles ALLUSERSPROFILE COMPUTERNAME LOGONSERVER USERNAME USERDOMAIN HOMEDRIVE HOMEPATH USERPROFILE APPDATA")
    For Each i In w
        If 0 = StrComp(i,n,1) Then
            CanInherit = True
            Exit Function
        End If
    Next
End Function

Function GetShortFolderPath(p)
    GetShortFolderPath = p
    On Error Resume Next
    GetShortFolderPath = FSO.GetFolder(p).ShortPath
End Function

Sub E(dst,src)
    set envs = WShl.Environment(src)
    For Each i In envs
        t = Split(i,"=")
        n = t(0)
        If n = "" Then n = "="&t(1)
        If IsNull(dst) Then
            If not CanInherit(n) Then envs.Remove n
        Else
            v = Mid(i,Len(n)+2)
            envd = dst
            If "X" = dst Then
                v = WShl.ExpandEnvironmentStrings(v)
                envd = src
                If 0 = StrComp(n,"TMP",1) Then v = GetShortFolderPath(v)
                If 0 = StrComp(n,"TEMP",1) Then v = GetShortFolderPath(v)
            End If
            WShl.Environment(envd)(n) = v
        End If
    Next
End Sub

E Null,"PROCESS"
E "PROCESS","SYSTEM"
E "PROCESS","USER"
E "X","PROCESS"

'Special hack for Path
s = WShl.Environment("SYSTEM")("Path")
u = WShl.Environment("USER")("Path")
If Len(u) Then s = s&";"&u
WShl.Environment("PROCESS")("Path") = WShl.ExpandEnvironmentStrings(s)

'Test a command
WShl.Run "cmd /d /k set ",1

You could probably remove a lot of the white-listed items by querying WMI and using other WSH methods…